May 1983
PHYSICAL CHARACTERISTICS AND SEDIMENT BUDGET
FOR BOTTOM SEDIMENTS IN THE MARYLAND PORTION
OF CHESAPEAKE BAY
by
Randall T. Kerhin, Jeff-ey P. Halka, E. Lamere Hennessee,
Patricia J. Blakeslee, Darlene V. Wells, Nicholas Zoltan,
and Robert H. Cuthbertson
Maryland Geological Survey
Baltimore, Maryland 21218
Contract No. EPA R805965
Project Officer
Duane Wilding
Chesapeake Bay Program
U.S. Environmental Protection Agency
Region III
6th and Walnut Streets
Philadelphia, Pennsylvania 19106
OFFICE OF RESEARCH AND DEVELOPMENT
U.S. ENVIRONMENTAL PROTECTION AGENCY
WASHINGTON, D.C. 20460
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CONTENTS
Abstract i
Conclusions ii
Acknowledgements iv
List of Figures v
List of Tables • viii
List of Plates x
List of Appendices xi
Introduction 1
Previous studies 2
Chesapeake Bay Geological System .... 4
Basin Evolution 6
Basin geometry 8
Sedimentary Parameters 12
Grain size characteristics of the sediments 12
Water content and bulk density 16
Carbon and sulfur contents 16
Heavy mineral assemblages 19
Bathymetric comparison 19
Methods 20
Field 20
Laboratory 22
Water content 22
Grain size analysis 22
Carbon 28
Sulfur 29
Heavy minerals : 29
Bathymetric comparisons 30
Sediment budget
Quality Assurance 43
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Sediment Characteristics 52
Granulometric classification and distribution 52
Ternary diagram 52
Sediment distribution 54
Relationship between depth and sediment type 55
Moment parameters 55
Averaged sediment sample 63
Sediment distribution by segments 63
Eastern Bay 74
Choptank River 74
Water content 77
Carbon distribution 77
Results 77
Discussion 83
Sulfur distribution 88
Sulfur versus organic carbon 94
Sulfur versus percent clay and organic carbon 94
Heavy mineral variability 100
Regional variability 103
Local variability 106
Patterns of Deposition and Erosion 107
Comparisons with subbottom profiles Ill
Pb210 sedimentation rates versus bathymetric comparison 112
Sediment Budget 119
Sediment sources 120
Fluvial sediments 120
Shoreline erosion 120
Dredging and overboard emplacement 122
Depositional-erosional patterns 124
Sediment volume 124
Sediment mass 126
Sediment mass versus depth 127
Sediment mass by segments 129
Mass - Sand 130
Mass - Silt/Clay 131
References 133
Appendices 141
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ABSTRACT
The characterization of the bottom sediments and identification of
patterns of erosion and deposition were integral tasks of the major
study areas outlined by the Environmental Protection Agency's Chesapeake
Bay Program. As part of the major study area, the Toxics Program, the
physical and chemical characteristics of the bottom sediments and the
patterns of erosion and deposition were measured and mapped.
Using the sediment classification defined by Shepard (1954), 75%
of the sediments sampled were sand and silty clay, the two dominant end
member populations. Relatively few of the sediments were mapped as mixed
sediments, clay or silt. The general distribution of the sand and silty
clay was along the nearshore margins and axial channel, respectively.
Zones of mixed sediments between the sand and silty clay fields were
generally absent, occurring as isolated pockets. These isolated pockets
of mixed sediments in some cases, have been identified as exposures of
pre-Holocene sediments.
The total carbon content was totally dominated by the organic carbon
fraction of the sediments. The organic carbon content was strongly assoc-
iated with the fine grained sediment, ranging to a high of 10.5%. Highest
organic carbon content was found in the northern Bay, decreasing downbay.
Sulfur content ranged from 0.01 to 2.00%, averaging 0.58%. Sulfur content
generally increased downbay.
Several distinct patterns of erosion and deposition were identified:
1) extensive areas of Bay floor erosion, 2) deposition and erosion exceed-
ing 2.4 meters/century in the axial channel and 3) variable patterns at
the confluence of major tributaries. The northern Bay was primarily
depositional changing to erosional in the upper middle Bay. The lower
middle Bay was depositonal.
A sediment budget for the Maryland Bay showed a net deficit of
sediment deposited on the Bay floor. For sand, 521xl06 mtons/century |was
contributed from the various sources with 473xl06 mtons/century recorded
as accumulation. This was a net deficit of sand accumulation at -47xl06
mtons/century. For the silt/clay fraction, the sources contributed
387xl06 mtons/century with accumulation at 273xl06 mtons/century, a net
deficit of silt/clay accumulation at -114xl06 mtons/century.
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CONCLUSIONS
1) Based on Shepard's Classification, all ten sediment classes are
represented in the Bay, but a majority (>75%) of the sediments are sands
and silty clays. Mixing of these two sediment end-member populations
yields the sediments defined as clayey sand, silty sand, and sand-silt-
clay. Clay dominates over silt, accounting for the low frequency of clayey
silt sediments.
2) Overall, the sediments in the Maryland portion of the Bay are
finer-grained than those in Virginia, where the predominance of silt over
clay yields clayey silt as opposed to silty clay in Maryland.
3) Sand is generally restricted to nearshore margins, with silty
clays in the axial channel. Zones of mixed sediment intermediate between
the sand and silty clay fields are conspicuously absent. Rather, they
occur as isolated pockets, most prevalent in the Middle Bay, Segments
4 and 5.
4) In the northern Bay, Segments 1-3, the sediments become increas-
ingly finer downbay, grading from sand-silt-clay to clayey silt, to silty
clay. This fining trend is attributed to hydraulic fractionation of the
Susquehanna River-derived sediments. Susquehanna River sediments are
deposited as far south as Segment 3, south of the zone of maximum tur-
bidity.
5) In Segments 4 and 5, sand and silty clay dominate, with pockets of
mixed and clay sediment mapped along a broad terrace separating the near-
shore and axial channel. Based on field evidence, patterns of erosion-
depositon, and subbottom profiling, many of these isolated pockets of
mixed and clayey sediments are subaqueous exposures of pre-Holocene
sediments.
. 6) The total carbon content of Bay sediments essentially equals
organic carbon content. Organic carbon content ranges from 0 to 10.5
percent, averaging 2.2 percent. The extremely high organic carbon contents
are concentrated in Segment 2, with organic carbon content decreasing
southward. Laterally, organic carbon is highest in the axial channel and
decreases toward the shorelines.
7) The sulfur content of the sediment ranges from 0.01 to 2.00
percent, averaging 0.58 percent. Sulfur content correlates strongly with
clay content and varies exponentially with organic carbon content. Two
distinct sulfur regimes are present in the Maryland Bay. Sulfate reduc-
tion in the northern Bay appears to be limited by low sulfate concentra-
tions in the overlying water. As sulfate concentrations increase so does
i i
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the sulfur content of the sediments.
8) Detailed comparisons of time-separated bathymetric surveys reveal
several distinct erosion-deposition patterns. The most notable is the
extensive areas of Bay floor erosion in Segments 4 and 5. The vast extent
of the erosional areas suggests that the Bay floor is a significant source
of sediments. Areas of high deposition and erosion exceeding 2.4 meters/
century are generally confined to the axial channel. Two small lobes of
high deposition (>2.4 meters/century) are associated with major accretion-
ary features; i.e., Flag Ponds and Cove Point. The patterns at the
confluence of major tributaries are quite variable suggesting a hydraulic
interaction between the tributaries and the main Bay.
9) The patterns of deposition and erosion relate to the geographic
location as well as the geomorphologic character of the Bay. The
northern Bay Segments 2 and 3 are characterized by depositional patterns
except in areas maintained by dredging. The patterns coupled with the
sediment distribution indicate that the northern Bay is a repository for
Susquehanna River-derived sediments. Segment 4 is primarily erosional,
particularly along the broad terraces and the inner nearshore margins.
Deposition is confined to the axial channel and the outer nearshore
margin. High erosion along the eastern wall of the channel is attributed
to slumping. Segment 5 is primarily depositional, with some erosional
areas along the inner nearshore and along the steep channel wall.
10) The total mass of the inorganic fraction of the sediment
deposited was 764.2x10s mtons/century, while 646.8xl06 mtons/century were
eroded from the Bay floor. Each component - sand, silt, and clay -
exhibited a net surplus of sediment deposited. Total deposition of silt
and clay (273xl06 mtons/century) was significantly higher than previously
reported. The net deposition (84.3xl06 mtons/century) was lower.
11) Sediment budget calculations for the Maryland portion of the Bay
show a net deficit of sediment deposited for both sand and silt/clay.
Erosion of the Bay floor is assumed to be the major source of sediment.
The net deficit of silt/clay deposited (-114xl06 mtons/century) indicates
a loss of silt/clay to the tributaries and the Virginia portion of the Bay.
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ACKNOWLEDGEMENTS
A project of such large dimensions could not be completed without
the efforts and contributions of many talented individuals. We would
like to acknowledge K. Weaver and E. Cleaves of the Maryland Geological
Survey for guidance and administrative support and J. Glaser and K.
Schwarz for valuable discussion. J. Hill, R. Conkwright, E. Reinharz
and A. O'Connell of the Maryland Geological Survey assisted in field
operations and in general discussion. S. Jones provided excellent
secretarial support. F. Askew and L. Zeni of the Department of Natural
Resources provided financial support. 0. Bricker, formerly of the
Maryland Geological Survey and now with the U.S. Geological Survey,
assisted in the initial phases of the project. R. Byrne, C. Hobbs, and
M. Nichols of the Virginia Institute of Marine Sciences provided valuable
discussions and assisted in technical development. The EPA Chesapeake
Bay Program staff, in particular D. Wilding, W. Cook, J. Klein, C.
Strobel, and T. DeMoss, provided project guidance. The captains and
crews of the tyV Tawes, M/V Tar Bay, and R/V Discovery, particularly
Captain G. Cox, piloted the field crews to the sampling areas. We want
to acknowledge the following individuals for their assistance in various
project operations: J. Watson, W. Skinner, R. Gelblat, G. Gunzelman,
R. McGrain, W. McCafferty, D. Vanko, P. Marx, N. Weintraub, P. Newman,
M. Lester, B. Rose, C. Hilmoe, K. McDonald, A. McDonald, A. Bricker,
L. Hart, M. Chattey, and J. Holman.
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FIGURES
Number Page
1 Areas covered by previous sedimentological studies 3
2 Generalized geologic map of Maryland 5
3 Trend of paleochannel under Poplar Island Group into Dorchester
landmass 7
4 Segmentation of Chesapeake Bay developed by Chesapeake Bay
Program, EPA 9
5 Sediment classification as defined by Shepard's (1954)
nomenclature 15
6 Sulfur cycle 18
7 Laboratory flow chart 27
8 Areas surveyed between 1896 and 1902 31
9 Areas surveyed between 1932 and 1956 32
10 Water content with depth, 1 meter cores 38
11 Core locations for one meter cores 39
12 Water content with depth, Bay Bridge borings 40
13A Distribution curves for Coulter Counter and pipette analysis 45
13B Distribution curves for Coulter Counter and pipette analysis 46
13C Distribution curves for Coulter Counter and pipette analysis 47
14 Duplicate samples plotted on tertiary diagram 48
15 Tertiary plot of all samples 53
16A Sediment class with depth; 0-5 and 5-10 meters 56
16B Sediment class with depth; 10-15 and 15-20 meters 57
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Number Page
16C Sediment class with depth; >20 meters 58
17 Mean grain size versus sorting 60
18 Mean grain size versus skewness 61
19 Shepard's plot, Segments 1 and 2 65
20 Shepard's plot, Segment 3 66
21 Shepard's plot, Segment 4 68
22 Shepard's plot, Segment 5 71
23 Shepard's plot, Eastern Bay 73
24 Shepard's plot, Choptank River 75
25 Water content versus clay content 76
26 Organic carbon versus total carbon 79
27 Carbon versus sediment type 82
28 Organic carbon versus depth 85
29 C13 ratios, after Hunt, 1966 87
30 Sulfur versus Shepard's class 89
31 Sulfur versus clay 91
32 Mean sulfur versus clay 92
33 Mean sulfur versus mud 92
34 Sulfur versus mud 93
35 Sulfur versus organic carbon, all samples 95
36 Sulfur versus organic carbon, Segments 1-3 97
37 Salinity distribution in Chesapeake Bay 99
38 Heavy mineral locations, regional variability study 101
39 Heavy mineral locations, local variability study 102
40 Time differentials, bathymetric comparison 108
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Number
Page
41 General location of areas where subbottom profiles were com-
pared to erosion-deposition maps, Pb210 derived sedimenta-
tion rates. Numbers are core designation and location for
Pb210 measurements 113
42 Seismic profile showing infilled channel. Stippled pattern
is deposition; dashed line is erosion 114
43 Seismic profile showing unstable slope configuration over
buried sedimentary horizon. Stippled pattern is deposition;
dashed line is erosion 115
44A Seismic profile showing slump scar on the east wall of the
axial channel 116
44B Seismic profile showing unstable slope configuration on east
wall of the axial channel 117
45 Area of shoreline erosion 123
46 Mass of sediments at depth intervals 128
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TABLES
Number Page
I Phi sizes, equivalent diameter (mm), classification nomen-
clature and method of analysis for sediment analyzed by MGS... 23
II Categories for bathymetric change 34
III Linear regression equations for water content in percent (y)
versus depth in centimeters (x) 41
IV Idealized strati graphic section - Calvert County 43
V Maximum dry densities of Coastal Pal in sediment 43
VI Sediment samples analyzed by both MGS and VIMS 50
VII Areas of each sediment type in Maryland portion of Bay 59
VIII Grain size characteristics for composite sample from Maryland
portion of the Bay 64
IX Summary of organic carbon statistics for the Maryland Bay and
its segments 80
X Correlation coefficients for organic carbon versus various
sediment parameters, baywide (Maryland portion) and by
segment 81
XI Carbon contents of some sediment samples from segments 1 and 2;
expressed in percent dry weight 86
XII Correlation matrix showing relationships between sulfur and
other sedimentological factors 96
XIII Percentages of species within non-opaque mineral assemblage
and percent of non-opaque grains with the heavy minerals
for samples on the western shore of the Bay 104
XIV Percentages of species within non-opaque mineral assemblage
and percent of non-opaque grains with the heavy minerals
for samples on the eastern shore of the Bay 105
vi i i
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Number Page
XV Regression analysis of mean grain size versus the percentage
of mineral observed in the non-opaque suite. The linear
trend was tested at the 5% level of significance 106
XVI Regression analysis of percent of opaque grains within the
sample versus the mineral percent observed in the non-opaque
suite. The linear trend was tested at the 5% level of
significance 107
XVII Comparison of sedimentation rates, bathymetric comparison
versus Pb210 118
XVIII Various estimates of suspended sediment discharge 121
XIX Estimates of input of sediment from shoreline erosion 121
XX Mass of shoreline sediment eroded 122
XXI Estimated volume of dredged sediments 124
XXII Volume of sediments deposited and eroded 125
XXIII Mass of sediment deposited and eroded 126
XXIV Comparison of silt-clay mass 127
XXV Sediment budget, Maryland portion of Chesapeake Bay 130
ix
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PLATES
Number Page
1 Bathymetry In pocket
2 Sediment Distribution in pocket
3 Water Content in pocket
4 Organic Carbon Content in pocket
5 Sulfur Content in pocket
6 Patterns of Erosion-Deposition in pocket
7 Patterns of Erosion-Deposition, >2.4 meters per century in pocket
x
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APPENDICES
Number Page
I Geodetic to Raydist conversion program 141
II NOAA hydrographic survey numbers used in bathymetric
comparison of the Bay 147
III Percentages of species within the non-opaque mineral assem-
blage, percent of non-opaque grains within the heavy minerals,
and weight percent of all heavy minerals within the smaple 150
IV Mass by depth increment 153
V Volume and mass computations for shoreline erosion 156
xi
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INTRODUCTION
The Chesapeake Bay and its tributaries constitute one of the major
estuarine systems of the United States. This system represents a unique
feature of the earth's surface. The Chesapeake Bay is one of Maryland's
most valuable natural and economic resources. Historically, it has had
major cultural.and economic impacts on the state and on the entire eastern
seaboard. Today, it serves as a vital link in the nation's transportation
system, supports a major seafood industry, serves as a hatchery and nursery
ground for many marine organisms and as a habitat for waterfowl, and
provides a recreational outlet for a large portion of the mid-Atlantic
population. In addition, the Bay and its tributaries are used as a source
of water for industry, a site for industrial and municipal waste disposal,
and a heat exchange for power generating plants. Many of these uses
conflict, and some threaten the environmental health of the Bay. The
continuing increase in population in the Bay area, together with the
growing number, size and complexity of industrial and public utility
operations, is placing ever-expanding demands on an environmental resource
of finite capacity.
In recent years, many of the demands placed on the Bay are beginning
to exceed the capacity of the resource, leading to deterioration of the
quality of this unique environment. In light of this, the states of
Maryland and Virginia through the Environmental Protection Agency (EPA)
authorized a multi-disciplined research effort to provide answers to
difficult managerial and environmental problems. One important segment
of this effort was the Toxics Program, designed to address the problems
of toxic substances in the Bay on a regional scale (Environmental Protec-
tion Agency, 1979).
Four major study areas were outlined; 1) a baseline inventory of the
spatial distribution of toxic materials and the estuarine components that
affect these substances, 2) an assessment of the sources of toxic sub-
stances, 3) an assessment of the mechanisms and routes of transport for
toxic substances, and 4) an integration of the results of the Toxics
Program with those of the other Chesapeake Bay system studies.
This particular study has been designed to provide information
relevant to study areas one and three. The baseline inventory involved
identifying the spatial distribution of the following physical character-
istics of modern Bay sediments:
• grain size
• water content
• carbon content
1
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• sulfur content.
The potential transport pathways of toxic substances can be extrapolated
from the results of the following tasks:
• identification of patterns of sediment deposition and erosion in
the estuary,
• evaluation of sediment sources,
•. formulation of a sediment budget, incorporating the sedimentologi-
cal elements of the first objective.
Characterization of the bottom sediments and identification of pat-
terns of erosion and deposition were accomplished by a cooperative effort
between the Maryland Geological Survey (MGS) and the Virginia Institute
of Marine Science (VIMS). Each agency was responsible for sampling and
analyzing sediments deposited in the waters of their respective states.
Sampling plans and laboratory techniques were coordinated so that the
findings of the two institutions would be comparable.
Data generated by this study are stored on computer at EPA. Summar-
ized, they are also available as maps, a more useful format for resource
management.
PREVIOUS STUDIES
One of the first systematic Baywide sedimentological studies was
conducted by Ryan (1953), in which over two-hundred cores and grab samples
were collected. The results of this reconnaissance study showed that
very fine to medium grained sands occurred along the Bay margins and
clayey silts in the channels. Sedimentological studies following Ryan's
work were more detailed, both in terms of sampling density and the
characteristics studied, but were limited in areal coverage. Kofoed and
Gorsline (1965) studied the textural characteristics of the sediments at
the mouth of the Choptank River. Biggs (1967) examined the color, general
texture, sulfate, organic carbon, and sedimentary structures of cores
collected from the area around Cove Point and the mouth of the Patuxent
River. Knebel and others (1981) conducted sedimentological and geophysical
studies in the lower Potomac River estuary. A multi-disciplined effort
focusing on the Chester River estuary included a textural and mineralogical
analysis of the bottom sediments (Palmer, 1972). Another multi-disci piined
study, patterned after the Chester River Program, was conducted for the
northern Bay from its head to Baltimore Harbor and included a textural
analysis of the bottom sediments (Palmer, 1975). Folger (1972) used the
organic carbon content of bottom sediments in the Maryland portion of the
Bay as a pollution indicator. Figure 1 details the areas covered by these
investigations. Aside from Ryan's and Folger's studies, the areas of focus
for most of the previous works have been the tributaries. Relatively
little sedimentological work has been done on the mainstem of the Bay.
The hydrography of Chesapeake Bay and its adjacent waters was first
2
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Figure 1. Areas covered
by previous
sedimentologi-
cal studies.
1) Koroed and
Gorsline
(1966)
2) Biggs (1967)
3) Palmer
(1972)
4) Palmer et
al. (1975)
5) Knebel et
al. (1981)
*6) Ryan (1953)
*7) Folger
(1972)
* Study
covered the
entire Mary-
land portion
of the Bay.
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surveyed in the early 18401s by the United States Coast and Geodetic Survey
to improve navigation and transportation. As a result of advancements in
methodologies and surveying techniques, a second survey of the Bay was
authorized in the 1890's. This survey differed from the previous one by
having a greater density of soundings and improved navigational accuracy.
With the advent of electronic navigational positioning and depth recording
equipment in the 1930's, a third complete survey of the Bay was conducted.
Although these three hydrographic surveys cover the entire Chesapeake Bay,
a few high usage areas such as Baltimore and Annapolis Harbors were sur-
veyed as many as five times since the 1840's.
Although not directly addressed by these surveys, a record of changes
through time brought about by natural (erosion and deposition) and cultural
(dredging and spoil disposal) processes can be compiled from their review.
This method has been used to study the patterns of deposition and erosion
in limited areas of the Chesapeake Bay. One of the first uses of this
method was Hunter's (1914) study of the mouth of the Choptank River, which
documented the bathymetric changes that occurred between 1847 and 1901.
Jordan (1961) studied approximately the same area but was able to compare
a 100 year span. Schubel and others (1972) constructed longitudinal
profile plots from the bathymetric data for the period 1847-48 to 1944-45
along a section of shoreline in Calvert County.
These studies of changes in the bathymetry through historical time
were examined from a geomorphic point of view with the intent of under-
standing the processes responsible for the changes. They did not address
the question of developing an overall budget for the sediments on the Bay
floor. Indeed, they could not due to their limited areal extent within
the larger geomorphic system. Previous attempts to define a sediment
budget for the Bay have concerned the input, transport, and fate of
materials suspended within the water column (Biggs, 1970; Schubel and
Carter, 1976; Yarbo, et al., 1981).
CHESAPEAKE BAY GEOLOGICAL SYSTEM
The accumulation and resultant sedimentary fabric of Bay bottom
sediments are governed by a complex interaction of physical, chemical
and biological conditions, acting singly or in combination. The sedimen-
tary environment reflects the influence of the basin geometry and sediment
availability, as well as the physical, chemical and biological processes.
These, in turn, control depositional processes and patterns.
Though modern depositional environments are defined and interpreted by
their physical, chemical, and biological characteristics, the geological
history of the Bay is of primary importance in their development. In the
Bay's geologically short life span, the depositional history of the region
has varied from fluvial to estuarine and marginal marine systems. Each of
these distinct depositional systems imprinted characteristic features on
the modern Bay environment. These features may influence the present
bathymetric configuration and the sediment available for erosion, trans-
port, and deposition.
4
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Figure 2. Generalized geologic map
of Maryland.
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BASIN EVOLUTION
The Chesapeake Bay, located in the embayed section of the Atlantic
Coastal Plain Province, is an estuary formed by the post-Wisconsin rise
in sea level which drowned the lower valley of the Susquehanna River
(Ryan, 1953; Hack, 1956; Harrison and others, 1965; Kerhin and Halka,
in preparation). Prior to submergence, the Susquehanna River had developed
an extensive drainage network in unconsolidated to weakly consolidated
sediments of Cretaceous, Tertiary and Quaternary Age. These sedimentary
units become progressively younger southward along the Bay axis, from the
Cretaceous Potomac Group in the northern Bay to undifferentiated Quaternary
sediments in the southern Bay (Figure 2). An excellent summary of the
pre-Pleistocene Bay history is reviewed by Byrne and others (1982).
Although the Bay's present configuration was inherited from the
Wisconsin-Holocene regressive-transgressive cycle, there is evidence that,
prior to the Wisconsin glaciation, a broader, more extensive Chesapeake
Bay was in existence (Owens and Denny, 1979). The development of the
ancestral Chesapeake Bay was very similar to that of the modern Chesapeake
Bay; i.e., channelling during a low sea stand followed by inundation of
the basin during a rise in sea level. Evidence of downcutting and
channelling during the Illinoian regression has been reported by Schubel
and Zabawa (1973) and by Kerhin and others (1980). Schubel and Zabawa
discovered a major paleochannel entering the Chester River and trending
east of present-day Kent Island. They postulated a southeastern trend,
possibly connecting with the Naylor Mills paleochannel (Hansen, 1966) near
Salisbury, Maryland. Recent seismic profiles along the Eastern Shore near
Talbot County reveal another major paleochannel network, trending south
under the Poplar Island Group and into Dorchester County (Kerhin et al.,
1980) (Figure 3). This paleochannel may be the southern extension of the
Chester River paleochannel reported by Schubel and Zabawa, despite its
location west of their postulated extension. The complex bathymetry at
the entrances of Tangier and Pocomoke Sounds and seismic evidence of buried
Tangier Sound and Pocomoke Sound paleochannels strongly suggest pre-
Wisconsin channelling in that region (Halka and Kerhin, 1982, in prepara-
tion).
Channelling beneath the Maryland Eastern Shore margins has been tenta-
tively dated as Illinoian, based on stratigraphic relationships, channel
geometry, and depths of thalweg. With the onset of the Sangamon trans-
gression, channel infilling occurred. Sea level rose as much as +12
meters above its present level, flooding the basin. Although the physical
dimensions of the ancestral Chesapeake Bay are uncertain, recent geologic
mapping on the Delmarva Peninsula indicates an eastern boundary along an
escarpment near Easton, Maryland (Owens and Denny, 1979). The western
boundary is much more difficult to fix, because of the erosion of Tertiary
and Cretaceous sediments during both the Sangamon and Holocene trans-
gressions. Sediments of the Kent Island Formation along the Eastern
Shore, interpreted as an estuarine sequence of the ancestral Chesapeake
Bay (Owens and Denny, 1979), are age-equivalent to the Talbot Formation
(Glaser, 1969) on the western shore. The Talbot Formation is a sequence
of fluvial sands and gravels capped by fine-grained silts and clays. The
6
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Figure 3. Trend of paleochannel under Poplar Island Group into Dorchester
1andmass.
7
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silt-clay unit is probably of estuarine origin and may delineate the
western extent of the ancestral Chesapeake Bay.
The northern boundary of the Bay coincided with the present one, as
indicated by the entrenchment of the Susquehanna River in the Piedmont
Province. The regional geology of the Upper Bay suggests that the
Illinoian river valley took a sharp turn to the southwest upon entering
the Coastal Plain Province. The river probably reoccupied this older
valley during the Wisconsin channelling cycle.
The southern boundary of the estuary, or ancestral Bay mouth complex,
was probably located near the lower Delmarva Peninsula, in the vicinity
of Dorchester County. Jordan (1974) postulated that during Sangamon time,
a barrier island complex trended northeast-southwest across Delmarva
Peninsula. This barrier island complex, along with a major paleochannel
and an estuarine strati graphic sequence, strongly suggests that the mouth
of the ancestral Chesapeake Bay was located near the head of present day
Tangier Sound. Carron (1979), using shallow seismic methods, also con-
cluded that the ancestral Susquehanna must have flowed through the present
Tangier Sound region.
BASIN GEOMETRY
An imposing body of water trending in a north-south direction, the
Chesapeake Bay is 315 kilometers long from the mouth of the Susquehanna
River to the Cape Charles-Cape Henry entrance. It varies in width from
5 to 56 kilometers, the greatest expanse of open water being in Virginia,
near the confluence of Tangier and Pocomoke Sounds with the Bay. The Bay
is narrowest at its head. Numerous small ambayments and subestuaries
indent the shoreline, which totals over 12,900 kilometers in length. The
extreme irregularity of the shoreline is a remnant of the early configura-
tion of the Bay - a drowned upland drainage system which is being modified
slowly to straight 'secondary' shorelines (Rosen, 1976).
The Bay is exceptionally shallow. Water depths in the mainstem
average 8 to 10 meters; in the major tributaries, mean depth is about 6
meters. The width:depth ratio is 3000:1, remarkably high (Wolman, 1968).
The deep axial channel, a relict of Pleistocene channelling (Hack, 1957),
is incised into this shallow basin, extending most of the length of the
Maryland Bay (Plate 1).
In spite of its complexities, the Bay can be subdivided into segments
by sediment type, physiography or circulation patterns. For purposes of
conformity and discussion, the segmentation scheme developed by the
Chesapeake Bay Program staff, based on general physiography and circula-
tion patterns, will be used throughout this report (Figure 4). The
remainder of this section will be devoted to a discussion of the geomorphic
form of each of these segments, as shown in Plate 1.
Segment 1, extending from the mouth of the Susquehanna River near
Havre de Grace to Turkey Point, encompasses broad, shoal head waters of the
8
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PATASCPO
RIVER
CB 2
CB 3
CB 5
TANGIER
SOUND
Figure 4. Segmentation of Chesapeake Bay developed by Chesapeake Bay
Program, EPA.
9
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Chesapeake Bay, known locally as the Susquehanna Flats. The Flats region
is a shallow, tidally influenced, dish-shaped basin dominated by the flow
of the Susquehanna River. Four channels, or curvilinear troughs, effec-
tively ring the basin, with the majority of the area averaging 0.5-1.5
meters in depth.
The northern-most channel trends east from the mouth of the Susquehan-
na River to Carpenter Point by the Northeast River. Another channel runs
along the western edge of the basin, from the Susquehanna River mouth to
Spesutie Island, while a maintained channel of approximately three meters
depth extends from near Havre de Grace almost due south to Spesutie Island.
The fourth channel trends to the eastern edge of the basin, continuing
southward into the Bay. This channel, up to ten meters deep in this
region, is more or less continuous throughout the Bay.
Only a limited number of samples were collected in Segment 1 due to
its shallowness. Therefore throughout the remainder of this report the
characteristics of the sediments in Segment 1 will not be discussed
separately. These samples will instead be included in the discussion of
Segment 2. For the physical sedimentology task this effectively moves
the boundary separating Segments 1 and 2 north to a line connecting
Spesutie Island with Turkey Point.
Segment 2 extends south from an east-west line connecting Turkey
Point with Spesutie Island. Within this area mean water depth is quite
shallow, generally averaging less than five meters (Plate 1). Along the
Eastern Shore, the remnants of a channel downcut during the Wisconsin
glaciation are locally evident; in most places, though, the channel is
maintained by dredging to provide access to the Chesapeake and Delaware
Canal. The narrow eastern flank of the channel drops abruptly along the
steep channel wall. A broad, irregular shoal extending to the west is
interrupted by Pooles Island and an adjacent relict channel deeper than
five meters. Within Segment 2 the Gunpowder, Bush, and Middle Rivers
drain the Western Shore, and the Elk, Sassafras, and Bohemia Rivers drain
the Eastern Shore.
Segment 3 of the Bay encompasses the broadened area lying between
the Patapsco and Chester Rivers, extending southward to a line projected
west from the northern tip of Kent Island. Depths within this segment
are shallow, generally averaging less than eight meters, except to the
south and within the relict axial channel cut during the Wisconsin glacia-
tion. Depths in the channel approach 20 meters (Plate 1). Approach
channels to the Baltimore Harbor area are maintained by yearly dredging
operations. The main tributaries emptying into Segment 3 are the Patapsco,
Magothy and Back Rivers draining the Western Shore, and the Chester River
draining the Eastern Shore.
Segment 4 includes the mainstem of the Bay from Love Point on the
northern tip of Kent Island to a line extending east from Cove Point on
the Western Shore. The most distinctive morphological feature of Segment
4 is a single deep axial channel, offset slightly to the east of the
centerline and running the entire length of the segment (Plate 1). Thalweg
10
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depths within this channel are generally greater than 30 meters and, in
places, surpass 50 meters. In cross section the bathymetric profile is
asymmetrical, skewed toward the western shore. In general, proceeding
from west to east, the nearshore margin slopes down to the 4 to 8 meter
depth contour. Beyond this marginal slope the profile flattens, forming a
broad terrace which borders most of the Western Shore. The platform
becomes narrower and eventually pinches out at the northern and southern
ends of this segment. Beginning at approximately 16 meters depth, the
slope increases, and the depths fall off rapidly to the thalweg of the
axial channel. The eastern wall of the channel is characterized by very
steep slopes. The bottom rises rapidly to an approximate depth of 8
meters, whereupon the gradient decreases, and depths gradually become
shallower to the eastern shoreline. Flat, shallow-water platforms occur
adjacent to the Eastern Shore, most notably in the vicinity of the Poplar
Islands and flanking the north and south margins of the Choptank and
Little Choptank embayments. Holocene sedimentation has differentially
infilled portions of the Wisconsin-cut channel and smoothed its original
geomorphic form. Nonetheless, the existing bathymetry is largely a relict
of the original channel shape (Ryan, 1953) and probably reflects the
eastward migration of the channel during downcuttipg.
The basin form of Segment 5 between Cove Point and the Maryland-
Virginia border, is very similar to that of Segment 4. Along the western
shore, depths increase fairly rapidly to the 8 meter contour, east of
which a broad, flatter shelf is present. This marginal shelf, interrupted
by the major rivers of the western shore, the Patuxent and Potomac, extends
to approximately the 16 meter depth contour. The axial channel generally
exceeds 30 meters in depth in this segment, and locally surpasses 50
meters. It shallows somewhat to the south of Point Lookout, where the
maximum depth contour is 20 meters. As in Segment 4, the eastern wall of
the channel is quite steep, and depths rise rapidly to the eight meter
contour, east of which a quite broad, generally flat shelf is present
between the 4 and 8 meter contours. The greatest expanse of the shelf
lies to the south of the Hooper Islands and approaches 12 kilometers in
width west of the Smith Island-South Marsh Island group.
Longitudinally along the axial channel in Segments 4 and 5, three
elongate troughs separated by shallower rises occur in areas where the
Bay narrows or near the confluence of major tributaries. The northern
trough is located south of the Bay Bridge across from the Severn and South
Rivers. The middle trough is located at the confluence of the Patuxent
River, and the southern trough is just south of the Potomac River mouth.
The juxtaposition of the elongated troughs and the tributaries or shore-
line constriction may relate to Pleistocene channelization and the conver-
gence of the tributary and ancestral Susquehanna River valleys.
Eastern Bay and the Choptank River mouth are broad embayments on
the eastern shore, bordering Segment 4 of the main Bay. Eastern Bay drains
the Miles and Wye Rivers and several small creeks. The Eastern Bay channel
issues from the Miles River and enters the main Bay south of Kent Island.
The Choptank River channel extends into the main Bay, merging with the
main Bay channel south of the Sharps Island shoal. The axial channels of
11
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both embayments exceed twelve meters in depth, incising broad zones less
than eight meters deep (Plate 1). Mater depths in Eastern Bay are some-
what greater than those in the Choptank River. Both of these embayments
were formed by flooding of the river and tributary stream valleys cut in
the unconsolidated materials of the Kent Island Formation during the
latter stages of the Holocene sea level rise.
SEDIMENTARY PARAMETERS
A principal objective of this study was to provide a Baywide data
base on the distribution patterns of selected physical and geochemical
characteristics of the bottom sediments. The parameters measured to
accomplish this objective were: 1) physical size and proportions of the
individual sediment particles, 2) water content of the sediments, 3) carbon
and sulfur content and 4) "heavy" mineral assemblages of the sand-sized
fraction. .
The results of these investigations are stored in the EPA's STORET
data base system. Although all of these parameters are not specifically
addressed in this report, their environmental significance and the ration-
ale for their study are briefly outlined below.
GRAIN SIZE CHARACTERISTICS OF THE SEDIMENTS
The size distribution of sediment particles has long been used by
geologists as a valuable tool for interpreting sedimentary environments
and processes. Particles transported by a fluid medium respond differently
to processes active in that medium based in large part on their physical
mass. It is accepted that the resultant size distribution of particles
reflects the dynamics of the fluid processes that led to the deposition
of the sediments.
Grain size is, in part, a function of the mineralogic components of
the sediments. Stable minerals such as quartz may predominate in the sand-
sized range; whereas, clay minerals, the end-product of the weathering
of feldspars and micas, invariably exist as finer particles. The size
distribution can also provide clues concerning the depositional history
of the sediments; e.g. whether a particular sediment has been recently
weathered and deposited or eroded from ancient sediments and redeposited.
The grain size distribution of the sediments can affect other compon-
ents of the subaqueous environment. The individual sediment grains may act
as either a source or a sink for dissolved chemical species, depending upon
the reactions which take place between the sediment grains and interstitial
(pore) waters with which they are in contact. The ratio of surface area of
the grains (determined by size) to volume of interstitial water determines,
in part, the degree to which the mineralogical composition of the grains
will affect the aqueous solution. The physical and chemical components of
the sediments affect the benthic biota which, in turn, have an important
modifying role (i.e. bioturbation) on the characteristics of the sediments
12
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and the chemical nature of the interstitial waters.
The modifying roles of the physical, chemical and biological processes
are best observed in the formation of composite particles. Agglomeration,
flocculation, and pelletization characteristically form composite parti-
cles larger in size than the individual grains. These "natural" particles
will behave differently in a fluid medium than the individual particles
comprising the composite particles. Observations of "natural" particles
in an estuarine environment have been made by Kranck (1981) in suspended
sediments of Miramichi estuary and by Haven and Morales-Alamo (1969) in
Chesapeake Bay. In each observation, the natural particles were fragile
composites subjected to disaggregation by natural processes and experimen-
tal design. Although "natural" particles appear to be characteristic of
estuarine sediments, the transport processes by suspension or bedload is
poorly understood, and the analytical techniques to measure the "natural"
size are not developed.
In this study, the individual grain size data has been reduced to
four statistical parameters: mean, standard deviation (sorting), skewness,
and kurtosis. These parameters were calculated by two different methods
and are expressed in phi (<|>) units, where phi is a logarithmic transforma-
tion of the grain size diameter in millimeters (mm):
= -log2(mm).
The first, and mathematically simplest, uses the graphic method and
equations described by Folk and Ward (1957):
Graphic median Md = <(>50
16 + <(>50 + <|>84
Graphic mean Mz = 3 —
Inclusive Graphic Standard Deviation ar = (l>84 ~ ^ + 4*^"^
. . 84 - 2<(>50 j. 5 + <}>95 - 250
Inclusive Graphic Skewness SkT = 2(<(>84-16) 2(95-<(,5)
<(>95 - <(>5
Graphic Kurtosis Kg = 2.44 (<(,75-<(,25)
In these equations the phi value refers to that number determined
from a graphic plot of the cumulative distribution curve on normal
probability paper.
The al ternative method for calculating these statistical parameters
is the method of moments (Krumbein and Pettijohn, 1938). The equations
for this method are:
Mean X =
100
Standard <• = Ifm2 _ 72
Deviation + 100
13
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Skewness
Sk
Efm3 - 3 • Ifm2 • Xt + 2(X(j))3
100 100
100
W3"
Kurtosis
- 4 • Xt • Zfm3 + 6(Xf)2 • Sfm2 - "3
100 TOO 100
where: f = the weight percent in each phi class interval
m = the midpoint of the grain size interval in phi units.
The method of moments calculations incorporates the distribution data from
each sediment size class and is particularly responsive to the information
contained in the tails of the distributions (Koldjick, 1968; Isphording,
1972; Jaquet and Vernet, 1976; Swan et al., 1978). Although many previous
studies have attempted to make environmental discriminations based upon
these parameters their effectiveness for this purpose has been inconclu-
sive. Their usefulness in providing insight into the processes that have
produced a particular sedimentary deposit cannot, however, be completely
discounted. The values calculated by both methodologies have been included
in this study to facilitate comparisons with previous sedimentological work
which may have utilized either method of calculation.
The values for the calculated parameters vary depending on the method
used. The mean value for a sample is indicative of the central tendency
of the grain size distribution, while the standard deviation describes
the degree of dispersion of the distribution around the mean. The values
of mean and standard deviation calculated by both the graphic and moment
techniques are usually similar.
Each method does, however, measure different properties of the distri-
bution with respect to skewness and kurtosis. Skewness indicates the
degree of asymmetry of a curve. In the moment calculations, the skewness
value is divided by the third power of the standard deviation in an attempt
to render the parameter dimensionless and make it independent of the
standard deviation. In the graphic calculations, this manipulation is not
performed. Davis and Ehrlich (1970) point out in a description of the
moment calculations that this mathematical technique is effective only
for distributions which approach a normal curve and in general makes the
skewness parameter inversely proportional to the standard deviation. In
practice, however, the moment and graphic calculated values for the skew-
ness of sediment grain size distributions are generally in close agreement
(Isphording, 1972; Jaquet and Vernet, .1976).
In contrast to the skewness, the two methods for calculating kurtosis
produce widely divergent values (Davis and Ehrlich, 1970; Isphording, 1972;
Jaquet and Vernet, 1976). This is because the graphic kurtosis measures
the ratio of the sorting in the tails of the distribution to the sorting in
the center of the distribution, relative to a normal distribution with the
same standard deviation. Both values of kurtosis give an indication of the
degree of mixing of two populations of sedimentary particles (Cronan, 1972;
14
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15
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Thomas et al., 1972, 1973), but the graphic kurtosis indicates the degree
of sorting each of these populations has undergone (Folk and Ward, 1957).
The distribution of sedimentary particles in a samples is also ex-
pressed in the more general terms of the major size categories present.
Thus the percentages of gravel, sand, silt, and clay in each sample have
been reported to EPA. A textural classification based upon the percentages
of sand, silt and clay present was also developed for each sample. This
classification scheme (Figure 5) utilized Shepard's (1954) ternary diagram
and nomenclature. Gravel, representing only a minor proportion of the
sedimentary particles in the Maryland portion of the Bay was excluded from
this classification. There are several advantages to this ternary class-
ification system. Each of the fields within it is roughly equivalent in
size, thus ensuring an equal probability of representation within the
ten sediment types. The diagram conveys the maximum amount of information
about the particulate grains in the sediment in a simple and accurate
manner.
WATER CONTENT AND BULK DENSITY
As sediment grains accumulate on the Bay bottom, water is trapped in
the voids between the individual particles. The physical and mineralogical
nature of the sediment determines its capacity to accommodate this water
and its ability to retain it upon further burial. Water content is an
important characteristic of the physical nature of a sedimentary deposit.
It has been shown to be roughly inversely proportional to the grain size of
the particles and directly proportional to the porosity and organic carbon
content (Harrison et al., 1964; Keller, 1974). Evaluated in conjunction
with the grain size distribution, it also provides information about the
cohesiveness and erodability of sediments. Several studies have shown that
there is a strong correlation between critical erosive velocities of clay-
rich sediments and their water content (Postma, 1967; Southard, 1974).
Water content also reflects the compaction history of sediments and can be
instrumental in differentiating between recent materials and older
deposits.
Pore water plus solids define the volume occupied by materials depos-
ited on the Bay floor. Water content must therefore be considered in any
conversions made between the volume a sediment occupies and its mass.
This is most readily accomplished by determining the unit weight or bulk
density of the sediment. The bulk density values for the sediments were
utilized for calculating sediment masses in the budget analysis.
CARBON AND SULFUR CONTENTS
Many chemical reactions occurring in estuaries such as the Chesapeake
Bay are dependent on the availability of carbon and sulfur. The sediment
oxygen demand and the behavior of a host of toxic metals and synthetic
organic compounds are directly determined by these elements. Because the
concentrations of these elements can serve as pollution level indicators,
16
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aiding in the location of potential "sinks" for heavy metals and other
polluting substances, a knowledge of the distribution and chemistry of
carbon and sulfur in the sediments is essential to the management of the
Chesapeake Bay. For these reasons the concentrations of total carbon,
organic carbon, and total solid phase reduced sulfur have been determined
for the surficial sediments, and are recorded in the STORET data base
management system.
Carbon is introduced into the Bay from a multitude of sources and
accumulates in the sediments. It is transported by rivers in the form of
terrestrial plant matter, dissolved organic substances and, to minor
extents, particulate carbonate minerals and coal particles. Rain and dust
fallout also contribute organic matter. Marine derived organic detritus
is carried into the Bay with seawater during the normal tidal flux. The
most important natural contribution of organic matter, to the sediments is
the estuarine biota whose remains settle to the bottom after death. In
addition to these sources, human activities are contributing an ever
increasing proportion of carbon in the.form of sewage and industrial
wastes.
High organic carbon content is associated with fine-grained sediments
(Baker-Blocker et al., 1975; Schafer et al., 1980). Low energy deposi-
tional environments allow both fine-grained sediments and low density
organic debris to settle to the bottom. Also, fine-grained sediments tend
to be composed predominantly of clay minerals which have an affinity for
organic materials.
Upon deposition, the organic matter is oxidized (or decayed) by
bacterially mediated reactions utilizing the most energetically favorable
oxidants available, in the following order:
1) oxidation using oxygen
2) oxidation using nitrate and nitrite
3) oxidation using sulfate.
In the first reaction, aerobic decay, as carbon is oxidized, oxygen is
reduced - the reverse of the photosynthetic process. If the oxygen is
depleted, the second most favored oxidants are nitrate and nitrite. In
organic rich estuarine sediments oxygen is usually depleted a short dis-
tance below the sediment water interface. However, in the Bay bottom
environment, nitrates and nitrites are usually so low that NO," reduction
is not a major decomposition process (Hill and Conkwright, 1981). The next
most favored decay reaction utilizes sulfate (SO^-) as the oxidant. In the
estuarine environment this may result in the accumulation of reduced sulfur
species in the bottom sediments.
Jorgensen (1977) suggests several reasons for examining the sulfur
distribution in estuarine sediments. The processes comprising the sulfur
cycle influence (1) the chemical environment of the sediment; (2) the flow
of energy in food chains involving anaerobic decomposition; (3) the distri-
bution of benthic organisms, insofar as it is controlled by sulfide
tolerance; and (4) early diagenetic changes in anoxic sediments.
17
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V
Fe S2
(PYRITE)
Figure 6. Sulfur cycle.
18
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The sulfur found in Bay sediments is largely, though not exclusively,
a by-product of the metabolic processes of sulfate-reducing bacteria.
Anaerobic bacteria (DesuTfovibrio), utilizing consumable organic matter for
energy, transform sulfate (S0it"z) derived from seawater to hydrogen sulfide
(H2S). Hydrogen sulfide reacts with iron, present in the sediment as
ferric oxide grain coatings, to form iron sulfide minerals, mainly mackin-
awite (FeS) and greigite (Fe3Sh). These acid-soluble phases are converted
to pyrite by the addition of elemental sulfur. Extraneous hydrogen sulfide
and elemental sulfur are oxidized to sulfate by aerobic bacteria (Thiobac-
illus). This process is depicted schematically in Figure 6. Prerequisites
of the production of reduced sulfur species are, therefore, available iron
and sulfate and an environment conducive to the luxuriant growth of sul-
fate-reducing bacteria; i.e., abundant organic carbon and absence of
oxygen.
HEAVY MINERAL ASSEMBLAGES
Another component of the physical sedimentology task consisted of
the separation of the light and heavy mineral assemblages in the coarse-
grained (sand sized) fraction of the sediments. Light and heavy minerals
have been used to identify source areas and to determine transport path-
ways and the depositional history of sediments in many geological studies.
Because of the tedious and time consuming nature of this task and the
specific guidelines of the EPA Chesapeake Bay Program, only the heavy
minerals were identified in selected samples restricted to the nearshore
vicinity. This data has not been transferred to the STORET data base due
to the limited number of analyses conducted. The data is reported in its
entirety, in this report.
BATHYMETRIC COMPARISON
The Bay is a highly dynamic system, not simply a receiving basin for
sediments. Sediments are transported into the basin, sedimented, resus-
pended or eroded, and resedimented. Many factors, acting in concert,
produce morphologic changes in the Bay. River discharge and salt water
inflow affect sediment movement and deposition through current velocity,
density and chemical differences. Storm events, varying in magnitude and
frequency, shift subaqueous sediments around, cause shore erosion, and
increase river discharge and sediment influx (suspended and bedload). Wind
and wave activity randomly affect the basin, while semidiurnal tides con-
tinually modify the basin hydrography.
The comparison of changes in the bathymetry of the estuary within
historical time provides two critical pieces of information. First, the
determination of sites of erosion and deposition and the magnitude of
changes at these sites is needed to evaluate the efficacy of dredging and
overboard dredge spoil disposal operations and the specific processes
leading to shoreline erosion i'p the Bay. That information is also instru-
mental in identifying sites for the establishment of oyster beds and
19
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harvest of soft clams. Secondly, in combination with the sediment
particulate and organic matter composition and bulk density, it is
utilized in the computation of the sediment budget, thus providing
insight into the sources, transport pathways, and ultimate depositional
locations of sedimentary materials that are internally derived in the Bay.
The information derived from the bathymetric comparisons study is
available only in map form. Due to the computational method and fine
detail it has not, at this time, been transferred to the data base
management system.
METHODS
FIELD
To provide information on the physical characteristics of the Bay
sediments two types of samples were collected, surficial samples and
gravity cores. Surficial samples consisted of the top five centimeters
of the sediment and supplied information on the Baywide distribution of
characteristics. Gravity cores consisted of vertical plugs of the Bay
bottom sediments, approximately one meter in length, providing information
on the depth related changes in these characteristics at selected locales
in the Bay.
The surficial sampling program was divided into two major tasks,
referred to as nearshore and mid-Bay sampling. This separation was
necessitated by vessel size and capabilities. A shallow draft, limited
range 17 foot Boston Whaler was utilized for sample collection in near-
shore waters of less than three meters depth. A variety of deeper draft
generally more seaworthy vessels with extended ranges were used for
sample collecting in waters greater than three meters deep. These varied
in size from the lapstrake design wooden hulled 24 foot Tar Bay to the
aluminum hulled 46 foot Discovery. Each of these tasks had its own
specific sampling plan, navigation techniques and sampling equipment that
were dictated by the vessel capabilities and sampling needs of that
specific task.
Nearshore samples were collected along transects oriented perpendicu-
lar to shore which were located approximately one kilometer apart around
the margin of the Bay. Navigation was achieved by the azimuth-range
method using a shore based compass and transit or theodolite. Along the
predetermined azimuth a time calibrated transect at a specific boat speed
was made out to the limiting depth of three meters. Bottom samples were
then collected at specific time interval markers with a Dietz-Lafond
sampler and packaged in Whirl-Pac bags. The date, location, sample number,
water depth and description of the sample were recorded. In addition, a
bathymetric profile along the transect was recorded to aid in the interpre-
tation of nearshore processes, and the shoreline morphology was described
in a field notebook. This description provided additional information to
aid in the interpretation of nearshore processes and supplied information
necessary for calculating the input of material from shoreline erosion in
20
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the sediment budget task. Accuracy of sample location in the nearshore
zone is estimated to be ±30 meters.
Mid-Bay samples, located in waters of greater than three meters depth,
were collected with a Van Veen or Petersen grab sampler, and location was
determined with a Teledyne Hastings-Raydist radionavigational system. At
each location two samples were collected from the top five centimeters of
the sediment. One, for grain size analysis was placed in a Whirl-Pac
bag. The other, used for water content, and carbon and sulfur determina-
tions, was placed in a thirty dram opaque plastic vial and refrigerated.
The date, sample number, water depth, and physical description were
recorded for each sample.
The theoretical locational accuracy of the Raydist navigational system
is reported to be ±0.5 meters (Teledyne Hastings-Raydist, 1970). To ensure
accuracy, calibration checks were conducted at least two times during
each sampling day. From these checks it was determined that operational
accuracy was on the order of ±1.5 meters. Due to the factors of vessel
orientation, wind, and tidal currents sampling accuracy was estimated
to be ±10 meters.
The majority of the samples for this study were collected at locations
on a one kilometer grid developed from a generation point located at
76°38'00"W and 38°00'00"N, and based upon a Universal Transverse Mercator
Projection. These locations in latitude and longitude were supplied by
EPA and were converted to Raydist values. The collecting vessel then
occupied these stations as closely as possible and the actual Raydist
values at the time of sample collection were recorded. These radiolocation
values were then converted to the actual latitude and longitude of the
collection point. The computer program used to convert from Raydist
location to latitude and longitude, and vice versa, are contained in
Appendix I.
In the early stages of the study navigational limitations precluded
occupying the orthogonal grid points. At that time Raydist 'lanes'
located one kilometer apart were followed and samples collected every
kilometer along these 'lanes'. This resulted in the sampling locations
falling along the hyperbolic lines described by the Raydist signals in
the area between the Bay Bridge and Holland Point, including Eastern Bay.
The program for conversion from Raydist location to latitude and longitude
is shown in Appendix I.
Gravity cores were collected at locations chosen for good latitudinal
and longitudinal coverage of the Bay, and were located using a LORAN-C
radionavigational system (Hill and Conkwright, 1981). A Benthos-type
gravity corer (Model #2171) with plastic core liners was used for sample
collection. The cores were photographed, the lithology described, and
samples collected at specified depth intervals for water content measure-
ments. The locations, lithologic descriptions and water content values
can be found in Hill and Conkwright (1981).
21
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LABORATORY
Water Content
In the characterization of surficial sediments of the Chesapeake Bay
bottom, the sedimentary enviornment was defined as consisting of both
particulate matter (inorganic and organic) and water. It was assumed that
the surficial sediments were saturated with free water - water that is not
bound up in the crystalline lattice of clay minerals. The procedure for
measuring water content consisted simply of weighing, then drying at 65°C
and reweighing a 25-50 gram split of the homogenized sample. Water content
of the sediments, in percent, was calculated as:
weiqht of water (grams)
percent water = ^"weight of sediment (grams) * 100
where water weight is the difference between the wet and the dry weights
of the sample.
Bulk density was also calculated from these weights. Bennett and
Lambert (1971) developed a mathematical equation for determining bulk
density from water content measurements:
bulk density (g/cm^) = wet weight of sediment (grams)
3 dry weiqht of sediment (grams + .
2.72 (grams/cm*)
weight of water(gm)
The use of this equation assumes that the average grain density of the
particulate matter is 2.72 g/cm3 and that all voids in the sediment are
completely saturated with water.
The determination of the water content was an important first step in
further processing of samples for grain size and carbon and sulfur
analysis. It has been noted in many previous sedimentological studies that
the water content of a sediment is strongly related to its grain size
distribution and concentration of organic materials. In the early stages
of the study it was determined that a 25% water content provided a reason-
able first approximation of the grain size distribution, carbon and sulfur
contents and was thenceforth utilized as a boundary to determine the
routing of each sample for further preparation and analysis.
Grain Size Analysis
The objectives of particle size analysis are threefold: to describe
the sediment, to compare sedimentary deposits, and to interpret the
depositional history of a particular sedimentary unit. These objectives
can be achieved only if sample preparation and analytical procedures are
standardized. Comparing sediment distributions is meaningless unless
laboratory, techniques are similar for all samples. Furthermore, if the
procedures used are common arid widespread, the results can be compared to
analyses performed by other operators studying other sedimentary
22
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TABLE I: PHI SIZES, EQUIVALENT DIAMETER (mm), CLASSIFICATION NOMENCLATURE
AND METHOD OF ANALYSIS FOR SEDIMENTS ANALYZED BY MGS
Phi Class
Diameter (mm)
Method of Analysis
-1.0
-0.75
-0.5
-0.25
0.0
0,25
0.5
0.75
1.0
1.25
1.5
1.75
2.0
2.25
2.5
2.75
3.0
3.25
3.5
3.75
4.0
4.33
4.67
5.0
5.33
5.67
6.0
6.33
6.67
7.0
7.33
7.67
8.0
8.33
8.67
9.0
9.33
9.67
10.0
11
12
13
1 4
2.00 —
1.68
1.41
1.19
1.00 —
0.84
0.71
0.59
0.50 —
0.42
0.35
0.30
0.25 —
0.21
0.177
0.149
0.125 —
0.105
0.088
0.074
0.0625—
0.0497
0.0393
0.0312 -
0.0249
0.0196
0.0156 -
0.0124
0.0098
0.0078 -
0.0062
0.0049
0.0039 -
0.0031
0.0025
0.0020
0.0016
0.0012
0.00098
0.00049
0.00024
0.00012
Q-nnnofi
gravel
very
coarse
sand
coarse
sand
medium
sand
fine
sand
very
fine
sand
coarse
silt
medium
silt
fine
silt
very fine
silt
clay
sieve
RSA
-sieve/pipette
Coulter Counter
pi pette
23
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environments.
The particle size distribution was determined for the -1 to 10 phi
range (2 mm to 0.00098 mm diameter). This range of sizes represents the
sand, silt and coarse clay portions of the grain size scale for sediments.
No single analytical technique can effectively measure this extent of
particle sizes, so the results of several procedures were combined. A
rapid sediment analyzer (RSA) was utilized for the sand fraction of the
samples with the results reported at one quarter phi intervals. The silts
and coarse clay particles were analyzed by Coulter Counter at one third
phi intervals (Table 1). The separation of the sediment into size frac-
tions appropriate for each analytical technique was effected by a combina-
tion of sieve and pipette techniques as outlined below.
The initial steps in grain size analysis, termed preparation, entail
disaggregation and dispersion of the sediment. Samples are treated with
mild chemical agents to separate aggregates into individual grains and to
prevent particles from coagulating during the subsequent analyses. The
preparatory procedure utilized by MGS is a composite of several well-
established techniques used in sediment analysis (Krumbein and Pettijohn,
1938; Griffiths, 1967; Carver, 1971; and Folk, 1974). Insofar as possible
it was standardized with the companion project conducted in the Virginia
portion of the Bay by the Virginia Institute of Marine Science (Byrne,
et al., 1982).
The first step in sample preparation was digestion of the wet, homo-
genized sample in a 10% hydrochloric acid solution to eliminate carbonate
materials. Following rinsing and decanting, the sample was treated with
hydrogen peroxide to remove organic matter. The water content of the
sample, calculated earlier, determined the concentration of hydrogen
peroxide utilized in this step. Those samples containing less than 25%
water were found to be composed predominantly of sand sized materials and
to be low in organic matter. These samples were treated with a 6% solution
of hydrogen peroxide. Samples containing greater than 25% water, being
finer grained and more organic rich, were treated with a stronger solution
of 15% hydrogen peroxide. The sample was then rinsed and decanted to
remove the soluble humic acids formed from the interaction between organic
matter and the hydrogen peroxide.
If, in the operator's estimation, the silt/clay portion of the
sample exceeded five percent, the sample was treated with a 0.26% solution
of the dispersant sodium hexametaphosphate ((NaP03)13) to ensure that the
individual grains did not reaggregate. After soaking in the dispersant,
the sample was agitated ultrasonlcally with a Bronwill model Biosonik IV
probe. If the silt/clay fraction of the sample was judged to be less than
five percent, this procedure was omitted.
At this point the separation of the sand and silt/clay portions of
the sample was accomplished by wet sieving through a 4 phi mesh sieve.
If the sample was greater than 95% sand, it was processed through the
sieve using deionized water. Both the portion retained on the sieve (sand)
and the portion that passed through it (silt/clay) were dried, then
24
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weighed to determine the relative amounts of each. The sand fraction was
sieved to remove any particles coarser than -1, phi and routed to the RSA
for further analysis. If the silt/clay fraction constituted more than
five percent of the sample, the fine particles were washed through the
sieve using the dispersant - sodium hexametaphosphate. The silt/clay
suspension, which passed through the sieve, was introduced into a 1000
milliliter cylinder, and analysis proceeded by pipette and Coulter Counter
techniques. The sand fraction was washed with dionized water to remove
the dispersant, dried, and weighed. If it represented more than five
percent of the sample, it was passed through a -1 phi mesh sieve and
analyzed with the RSA.
Standard pipette techniques (Carver, 1971; Folk, 1974) were utilized
to determine the proportion of silt and clay sized materials present and
to obtain a subsample of this material for further analysis with the
Coulter Counter. Three 20 milliliter withdrawals were taken from the
suspension in the 1000 milliliter cylinder. Two of these were "4 phi
withdrawals' and contained particles 4«j> in size and smaller; hence, they
represented the entire silt/clay fraction of the sample. One of these
withdrawals was routed to the Coulter Counter for a detailed analysis of
the silt/clay fraction. The other was deposited in an evaporating dish,
dried, and weighed to determine the proportion of silt and clay in the
sample.
The third, or '10 phi withdrawal', contained particles finer than 10
phi in size. This aliquot was dried in an evaporating dish and sub-
sequently weighed to determine the proportion of the clay fraction which
was finer than the detection limit of the Coulter Counter.
The rapid sediment analyzer utilized to determine the grain size
distribution of the sand sized portion of the sample is similar in design
to that shown in Gibbs (1974). Significant improvements over that design
included modifications to reduce large scale weight oscillations produced
by coarse particles striking the balance pan, improvements in the timing
of sample introduction, and interfacing to a dedicated microcomputer for
data reduction. The fall times of particles were converted to equivalent
grain diameters by applying the fall velocity equation of Gibbs and
others (1971), where the input is the density and viscosity of the water
in the analyzer and the distance the particles fall. All computations
were performed by the microcomputer. Details of the design, calibration
and operation are described more fully in Halka and others (1980).
Intercalibration'of this RSA with the one utilized by VIMS in the
comparison study was accomplished to insure compatability of study results.
Details of this procedure may be found in Byrne and others (1982;
pp. 52-56).
A Coulter Counter Model TAII was used to analyze particle sizes in
the four to ten phi range. Because this size range is too large for a
single analysis on the Coulter Counter each sample had to be run twice,
utilizing 140y and 30y aperture tubes. The Coulter Counter was calibrated
according to procedures outlined in the instruction manual (Coulter
Electronics, 1975), with the exception that the output was adjusted to
25
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correspond to the one third phi intervals shown in Table I. This calibra-
tion was checked every five days. Nine of the data output channels from
the 140y and 30y aperture tubes overlapped and allowed merging of the
data by the volume percent method outlined in Coulter Electronics (1975).
The 4 phi pipette withdrawal sample routed to the Coulter Counter was
resuspended prior to analysis by placement in a sonic bath to insure
dispersion of the individual sediment particles. The sample was first
analyzed using the 140y aperture tube at a concentration index of four to
five percent, recommended for minimum coincidence counting (Coulter
Electronics, 1975). The commercially available electrolyte, Isoton II,
was used in the analysis. After introduction of the sediment sample, the
stirring speed was adjusted to 500-600 rpms using a hand-held tachometer
(Behrens, 1978), and analyzed following the procedure outlined in the
instruction manual until a standard total particle volume was accumulated.
Time, total particle count, and differential percent volume for the
analysis were printed out. The sample, along with the electrolyte, was
then sieved through a 20p mesh to remove larger particles that would inter-
fere with the 30y aperture analysis. The sieved sample was stored in a
clean beaker until it could be analyzed using the 30y aperture tube.
This analysis followed the same procedure as outlined for the 140y tube
and was completed within two hours of the first analysis with the 140y
aperture tube, before flocculation of the fine particles could occur.
The data generated by sieving and pipetting, and from the Coulter
Counter and the RSA were computer merged to yield a cumulative grain size
distribution for each sample. The cumulative weight percentages for each
of the quarter phi intervals comprising the sand component were converted
to percentages of the total sample weight by the following formula:
The sand weight was calculated by subtracting the weight of gravel retained
on the -1 <#> sieve from the weight of material retained on the 4 sieve. The
size distribution of the gravel fraction was not determined in this study
because in the great majority of the samples it represented only a very
minor component of the particulate matter. In the few cases (69) where 1t
exceeded five percent of the sample weight, it was present as cobble sized
materials obviously representing a lag deposit. The results from each
Coulter Counter one third phi interval were multiplied by:
to adjust the percentage occurring in that interval so that it represented
a fraction of the entire sample.
The distribution of the material finer than 10 phi was estimated by
arithmetically extrapolating from the cumulative percent at 10 phi, to
100 percent at 14 phi (Folk, 1974). Although the justification for this
technique has largely been based upon empirical assumptions, recent work by
k phi interval wt
final RSA wt
4 phi weight - 10 phi weight
total weight
26
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< sand/gravelV
fraction Jj
wei gh
1
dry sieve
2
mm
,K2 mm
sand fraction
<
i
RSA
>0.062 mm
samp
prepar
le
ation
*
<
wet sieve
0.062 mm
<
, <0.062
>2 mm
[gravel fract
pipette
weigh
<4 phi
I
Coulter
Counter
data reduction
and
interpretation
<10 phi "N
~~i—;
weigh
Figure 7: Laboratory flow chart
27
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Behrens (1978) has added additional credibility to its use. In determin-
ing the grain size distribution for samples from the Maryland portion of
the Bay extrapolation by this technique was a sound decision. Of the
1848 samples comprised of at least 5% clay, 97% contained particles finer
than 10 phi. In about three-fourths of those samples, the 10-14 phi
interval included at least 10%, and as much as 50% by weight of the entire
grain size distribution. At this time no analytical technique exists
which can adequately and conveniently define the distribution of particles
within this size range. Because they constituted such a large proportion
of the sedimentary particulates encountered in this study, the extrapola-
tion technique was the only available method for accounting for their
distribution.
If either the sand or the silt/clay fraction of the sample constituted
less than 5% of the total sample weight, a detailed analysis of the grain
size distribution within that fraction was not performed. The percentage
of the sample represented was recorded, however, in the cumulative
distribution data. A generalized flow chart of the sample preparation and
analysis procedures is shown in Figure 7.
Carbon
Total and organic carbon content was determined for approximately
25% of all mid-Bay samples by combustion in a Leco induction furnace
(Model 521-0000) and analysis using a Leco gasometric analyzer (Model
572-100). The extent of baywide sample coverage for carbon analysis
varied with time. Initially, every mid-Bay and some nearshore samples
were analyzed for carbon to provide coverage on a 1 km grid. This
coverage was reduced to one quarter of all mid-Bay samples when it became
apparent that carbon analysis could not keep pace with other phases of
the project. Predominantly sandy nearshore samples with low water contents
were excluded from analysis from this point in time. Results from the
early analyses showed that in 85% of these sandy samples the carbon content
was zero, obviating the need for continued analysis. Similarly, samples
from every fourth mid-Bay station were selected for analysis only if their
water contents exceeded twenty five percent. Samples with less than 25%
water were avoided and replaced with samples from adjacent stations, unless
coverage on the map required their inclusion.
Each oven dried sample was mechanically powdered with a ball mill to
insure homogeneity and to minimize problems associated with obtaining
subsamples for anlaysis. Two splits were collected, one was analyzed
directly for total carbon content, the other was treated with 10% hydro-
chloric acid to remove mineralic carbon, oven dried, repowdered, and
analyzed for organic carbon content.
Operating procedures recommended by Leco were followed for the
analyses. Approximately 0.1 grams of dried sample plus one charge each
of iron and copper accelerator were used for each analysis. A minimum of
two analyses was done for each/sample. Barium carbonate standards were
analyzed between every 10-12 sediment analyses to insure machine calibra-
tion. Carbon values are judged accurate to ±.10 weight percent except in a
28
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few cases, usually with low carbon values, where split problems prevailed
despite multiple analyses.
Sulfur
Percent sulfur, by weight, was determined for each of the samples
selected for carbon analysis. Duplicate 0.1 g samples were analyzed on
a Leco automatic sulfur titrator (Model 532-000) operated in conjunction
with the Leco 521-000 induction furnace. Only sulfur converted to S02
during combustion was measured by this process.
Operating procedures recommended by Leco were followed, with these
amendments. Portland cement containing 0.92% sulfur (NBS standard refer-
ence material #636) was used in machine calibration. One scoop (0.3 ml)
each of tin chips and low sulfur iron powder was added to a sample to
accelerate combustion. Samples were burned for ten minutes. Sodium
azide (5.0 g/1) was added to the \% HC1 solution to block interference
by nitrogen.
If percent difference between the two runs exceeded five percent, a
third split was processed and the results of the three runs were averaged.
Otherwise, the mean of the duplicate runs was reported. Percent difference
was calculated as follows:
where Sx and S2 are the percent sulfur values obtained from the first two
runs, and 5 is their average.
Heavy Minerals
The heavy mineral assemblage of the sand-sized fraction was separated
from the lighter grains by fractionating in the heavy liquid tetrabromoe-
thane ($.G.=2.97), following the procedures outlined in Carver (1971). All
heavy mineral grains between 1 and 4 phi in size were mounted on glass
slides in Caedex (refractive index = 1.55). Grains coarser than 1 phi
were excluded from the sample because they physically interfered with the
mounting procedure. Usually, these grains represented only a small portion
of the total heavy mineral assemblage.
The percentage of non-opaque grains to total grains was determined by
counting 200 grains in ribbon traverses. Then the proportion of each
non-micaceous mineral species within the non-opaque assemblage was deter-
mined by counting an additional 200 non-opaque grains in ribbon traverses.
This technique increased the precision with which the non-opaque suite was
determined. Non-opaque grains constituted from 5 to 78 percent of the
total mineral assemblage. Therefore, counts represented from about 250
to over 3,000 grains.
percent difference =
29
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BATHYMETRIC COMPARISONS
The objective of this study was to determine the pattern and magnitude
of changes in the bottom topography through a comparison of bathymetric
surveys for the Maryland portion of the Chesapeake Bay. Two time-differ-
entiated bathymetric images, each representing the Bay morphology at a
single point in time, were compared. Ideally, the entire study area would
be covered by two complete end points, enabling the comparison of two
separate, stop-action images of the Bay.
The theory behind the bathymetric comparison project is most easily
explained through an analogy to a motion picture. Each frame of a motion
picture depicts a segment of a continuous motion. Viewed in sequence,
frame after frame, the picture 'moves'. Assume that a geostationary
satellite camera could instantaneously and continuously monitor the
Chesapeake Bay floor. By viewing a series of these photos, frame after
frame, changes in bottom morphology would be revealed.
Assume, for example, that the satellite camera is governed by the
tidal cycle, which is more regular than other environmental stresses.
Once every six hours the tide changes in the'Chesapeake Bay. The Bay
floor and shoreline may change throughout a flooding or ebbing tide, but
for simplicity's sake only the end point, or peak tide, conditions will be
used in this model. For each of these end points the camera will develop
one photographic image. Therefore, in one day (24 hours) four frames will
be taken, 1461 frames for a given calendar year.
In the course of one century - the time span covered in the bathy-
metric comparison map project - almost one hundred fifty thousand pictures
would be taken. If a researcher were to compare the first frame against
the last, he would see an image of change. This image would not take into
account the thousands of changes that had previously occurred; rather, 1t
would show only the differences between the two end points. By adjusting
the time interval at which comparisons were made, various images could be
derived: tidal changes (every other frame); seasonal changes (approx-
imately every 465 frames); yearly changes (every 1461 frames); or changes
caused by specific storm events, spring or neap tides, or cultural events
(before and after dredging or shore nourishment projects).
Since no such ideal photographic records exist, the researcher 1s
compelled to use available surveys. The bathymetry of the main Bay has
been surveyed three times: total coverage with a series of 13 surveys
from 1845 to 1849; about 80% coverage with a series of 17 surveys between
1896 and 1902; and about 80% coverage with a series of 63 surveys from
1932 and 1956 (see Figures 8 and 9). In total, some 99 comparisons of
these surveys were done to determine bathymetric changes in the Maryland
portion of the Bay. Each comparative survey unit was comprised of two end
points, at least one of which was distinct from those used in adjacent
comparisons (see Appendix II).
Sallenger and others (1&75) described three methods for comparative
analysis of bathymetric charts: contour lines, data point and grid point
30
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-------
Figure 9. Areas surveyed between 1932 and 1956.
32
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methods. Each method has its advantages and disadvantages, and the selec-
tion of one over another depends on the objective of the study, the avail-
ability of bathymetric charts, and the areal extent of the region.
The contour line method compares the changes or shifts 1n contour
lines between two bathymetric charts surveyed at different times. The
shifts in contour lines represent either erosion or deposition, depending
on the direction of the shifts relative to the coastline. This method
accurately determines the pattern of erosion and deposition but 1s not
conducive to producing volume calculations. The data point method involves
the construction of longitudinal profile plots and the comparison of
cross-sectional areas. This method is similar to the graphic data presen-
tation used for beach profiles. It quantifies the areal losses and gains
but does not necessarily display regional patterns. The third method,
the grid point, consists of replotting the depth soundings to a user-
defined grid system. Although this technique is limited by the density
of the depth sounding data and the regional coverage of the bathymetric
charts, pattern and volume are easier to obtain. The grid point method
was, therefore, deemed most suitable for this Investigation, because it
produced a quantitative image of regional patterns, useful in later
volumetric studies.
The grid patterns used in this study were referred to whole minute
values of latitude and longitude and were formed by dividing each linear
minute into ten equal parts. Constructed in this manner, the grid network
consisted of cells six seconds on a side. The reference to the whole
minute values of latitude and longitude accommodated the warping of the
Mercator projection along the length of the Bay. This 1n-phase relation
with the Mercator projection manifested slight differences in areal
coverage of the arid cells from south to north. These differences, on
the order of 10'^km2, are thought to be insignificant in relation to the
average areal coverage of 3xl0"2km2 per grid cell.
Every survey and chart used in this program was digitized within this
grid network. Surveys post-dating 1930 were obtained in digitized format
from NOAA, while those pre-dating 1930 were digitized at MGS. All sounding
data falling within a given grid cell were averaged, and the result was
plotted. All measurements were converted from English to metric units.
Seen in three dimensions, this digitizing process transformed the smooth,
contoured bathymetric charts into plots of stepped plateaus, equal in
area but varying in height.
Digitized surveys were compared by subtracting recent from historic
data, on a cell by cell basis. The results of the comparisons denote
changes in the height of the water column over a given time interval.
Since mean-low-water (MLW) was used as the sounding datum for all surveys
used, the results are assumed to imply changes in the bottom depth. To
account for the increase in the water column through time due to recent
sea level rise, a correction factor of one millimeter per year was applied
to all results (Rusnak, 1967). The result for each cell was then normal-
ized to a 100 year time interval. Compared and adjusted in this manner,
areas of sedimentation exhibited positive values, and areas of erosion
33
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displayed negative values. Other sea level data presented by Hicks and
Crosby (1972) for the Chesapeake Bay region, in particular, is stated at
2.5 millimeters per year, based on examinations of tidal records collected
from the 1900's to 1920's. This time period represents approximately
half of the period adjustment .for bathymetric comparison and it was
decided that the worldwide sea level rise of 1.0 millimeter per year would
best fit the total time period of comparison.
The numeric results were grouped into five categories, and the
associated patterns were mapped. For a given 100 year span, an interval
of -40 to +20 centimeters, about zero centimeters change, was judged a
negligible change category. By adding or subtracting two hundred centi*
meters from either end of the negligible change interval, five categories
were formed (Table II).
TABLE II: CATEGORIES FOR BATHYMETRIC CHANGE
Amount of Change
(cms/100 years)
»
Change
Pattern
+220«-h-oo
second order positive
++
+20^+220
deposition
+
-4CK-H-20
negligible change
0
-240^-40
erosion
-
-ax—>-240
second order erosion
Adjustments of the bathymetric comparison data due to compaction of
the sedimentary column was not applied to the original data sets. Compac-
tion of the sedimentary column resulting from dewatering and overburden
pressure would in effect produce a downward or decrease in the apparent
thickness measured by the comparative analysis. Since the thickness is a
measurement of change between two time periods it is not critical that
adjustments due to compaction be made in the original data. It is
critical in determining sedimentation rates or mass of sediment accumula-
tion that compaction of the sediments be taken into account. In the
sediment budget analysis, the mass data was adjusted for compaction of
the sedimentary column.
Implicit in the bathymetric comparison project are several assumptions
pertaining to data integrity and viability. Neither the original nor the
copy was drafted onto stable media, making them susceptible to warping
with changes in humidity and temperature. Presumably the digitizing
process was able to correct for these problems. Rounding and metric
34
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conversion(s) led to some loss of precision, though not necessarily loss
in accuracy.
The Mercator projection used to locate positions in latitude and
longitude has shifted through modern history. The projection used in the
mid-1800's is not directly (linearly) related to that projection standard
in 1927 (N.A. 1927 datum). The repositioning of the modern grid onto
older maps by triangulation techniques was assumed to be as accurate as
possi ble.
Sounding data used is this project, both recent and historic, were
measured in reference to mean low water (MLW). This implies that accurate
tidal measurements were kept throughout all survey projects for use in
accurate water datum corrections.
Bathymetric surveys predominantly chart the morphology of a basin's
navigable waters. This bias tends to lessen data density near shorelines
and, consequently, available comparative results. In essence, the shore-
line region is a grey area of suspected changes, with few actual measure-
ments.
A similar limitation arises from the loss of above-water land mass.
Land topography is not included among the available data for the compar-
isons. Therefore, the inundation of land gives no historic datum compar-
ison. The only results are qualitative, with no supporting quantitative
or numeric measurements.
With the advent of electronic sounding and navigational equipment,
accurate measurements became readily attainable. It must be assumed that
positioning and depth sounding techniques of the past gave compatible
results. Lukin (personal correspondence) has found that the techniques
and working theories used in the past are not necessarily as accurate as
desired.
The working hypothesis for the bathymetric comparison project assumes
that the patterns determined from the comparisons are representative of
long-term averages and document long-term net changes. It does not
reveal episodic or subtle changes in the sedimentary processes, which may
render patterns different from those mapped. The patterns delineated by
this method represent comparisons of fixed moments in the depositlonal
history of the Bay. This record may or may not reflect processes that are
presently active. No matter what the time interval covered, the compar-
isons show a stop action Image or pattern of change, which cannot be
equated to a rate of change. "Bathymetric changes should be expressed as
displacement over a discrete observational period and NOT as a rate
extrapolated over a period of time. As far as we know, the rates of
sedimentation have fluctuated considerably over the last couple of cen-
turies and any attempt to smooth over these fluctuations by reporting
bathymetric changes as rates is misleading at the very least" (Lukin,
personal coimuni cation, 1981).
"Normalizing" data to conform to a given time span leads, in Itself,
35
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to spurious application of results. Normalizing assumes that the amount
of change observed for a given time interval is a viable datum to be
used for different spans of time. As catastrophic (storms) and non-.
catastrophic changes (everyday, normal events) are quite different 1n
nature and effect, the interpolation of a given change, of unknown cause,
would result in the presentation of a distorted image. The normalizing
done for this project did not affect the numeric results, but was used
to adjust the contour intervals drawn for various time spans.
Sediment Budget
Calculation of a sediment budget for Chesapeake Bay required deter-
mining the mass of sediment introduced into the Bay system and the mass
accumulated on the Bay bottom. The sources of sediments to Chesapeake
Bay have been defined as external (fluvial), internal (primary production),
and marginal (shoreline erosion). A secondary Internal source that has
been overlooked is the Bay floor - erosion and subsequent resedimentation
of "Bay-derived" sediments. In addition to these natural sources are
man's contributions, principally overboard disposal of dredge spoils.
In the calculation of the sediment budget, data on fluvial, primary
production and dredge spoil input were extracted from existing informa-
tion. The contributions from Bay bottom and shoreline erosion were
calculated as part of this study.
The sources and sinks of sediments on the Bay bottom were developed
from the bathymetric comparison maps. This provided the areal extent
of deposltional and erosional surfaces within the basin and the thickness
of the depositional sequence or the erosional cut. The physical character-
istics of the sediments including the grain size distribution, water
content and organic carbon were then used in combination with the bathy-
metric comparisons information to calculate the mass of sediments deposited
on or eroded from the Bay floor.
A unit volume of Bay sediment is composed of sedimentary particulates,
interstitial water, and biogenic matter. The sedimentary particulates
consist of sand, silt, clay, and organic matter. Interstitial, or free,
water fills the voids between grains of sediment; it does not include the
water bound up in the crystalline structure of clay minerals. Four assump-
tions regarding these sedimentary constituents were made in converting
volume to sedimentary mass.
The first assumption is that the sediments are 100? water saturated.
That is, all void spaces between the sediment grains are occupied by water.
Second, the grain specific gravity of the inorganic particulates is
assumed to be 2.72 g/cm3. This is based upon published Information con-
cerning sediments of the Bay (Supp, 1949; Harrison et al., 1974). Third,
the density of the interstitial water is 1.00 g/cm3. Although the salinity
of the interstitial waters ranges from 2 to 25 parts per thousand in the
Maryland portion of the Bay (^111 and Conkwright, 1981), the water content
was not corrected for salts 1n the laboratory procedure. Preliminary
calculations in the sediment budget analysis showed that the error
36
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introduced by not correcting for dissolved salts was negligible relative
to the overall sediment mass. Fourth, the lithology of the sediments at
the surface remains constant across the depth of change observed from the
bathymetric comparisons. This assumption is born out by examination of
Hthologlc descriptions of short cores (one meter or less) presented 1n
Ryan (1952), Biggs (1970) and Hill and Conkwright (1981). Borings from
the Chesapeake Bay Bridge (Supp, 1949) and the channel entrances to
Baltimore Harbor supply the only available sedimentary information for
depths exceeding one meter. Examination of these data showed that,
although Hthologles vary with depth, they are generally uniform within
ten meters of the surface, the maximum depth of change observed 1n the
bathymetric comparisons task.
Upon burial, sediments compact, expressing interstitial water. This
compaction results in a decrease 1n water content and a concomitant
increase in the sediment mass per unit thickness. Although uniform
lithology is assumed throughout the vertical sequence, 1t 1s thus necessary
to compensate for decreasing water content and thereby compaction of
the sediments 1n the volume to mass conversion.
Information regarding the relationship between water content and
depth of burial is sparse for sediments of the Chesapeake Bay. The only
sources of such information are the Bay Bridge borings (Supp, 1955) and
one-meter gravity cores (H111 and Conkwright, 1981).
For depths between zero and one meter in the sediments the data from
the short cores presented 1n Hill and Conkwright (1981) was used to
calculate the change in water content with depth (Figure 10). These
cores were located along the axial channel of the Bay and consisted of
fine-grained silts and clays (Figure 11). Regression lines were fitted
to the data and are shown for each core station in Table III. For each
location the negative correlation between water content and depth 1s quite
strong. In converting from volume to mass within the top one meter of
sediments these formulas were utilized and assumed to be representative
of the fine grained sediments 1n the vicinity of the coring location.
The data from the Bay Bridge borings (Supp, 1955) were used to
calculate the changes 1n water content which occurred at depths greater
than one meter below the sediment-water interface. A plot of water
content versus thickness (Figure 12) shows a linear decrease over the
one to ten meter range. Below ten meters the water content remains
fairly constant. A regression line was fitted to the upper ten meters
of sediments because the greatest bathymetric change did not exceed that
thickness. As in the 0-1 meter thickness interval a strong negative
correlation (r=-0.94) was found to exist between the two variables.
These equations for determining the change in water content with
thickness in the sediment are valid only for fine grained sediments
composed predominantly of silts and clays. For coarser grained sediments
composed predominantly of sands the surface water content was extended
throughout the thickness of change. In these sediments grain to grain
contact supports a framework around the voids in which the Interstitial
37
-------
Wate r
ST 00
2l° 4l° 6i° 8i°
2 0-
40-
60-
80-
0
m 100'
©
Content (percent)
ST 103
20 40 ^3 80
ST 115
, 2i° 4,° V.8.0
7
I
ST 114
ST 115
ST 14
o
O
20
a
« 4 0
a
60
80
100
2
-------
Figure 11. Core locations for one meter cores,
39
-------
WATER CONTENT", PERCENT
20
¦
40
60
_j
80
-Wf-
100
I.
r
/
•/
t
/
J
/
Boy Bridge Cores
D-28
D-30
D-32
D-33
Figure 12. Water content
with depth, Bay
Bridge borings
30J
40
-------
waters reside. This framework is not appreciably stressed at the shallow
depths observed in the bathymetric changes. Thus it is reasonable to
project the surface water content of the sandier sediments throughout
the thickness of change.
TABLE III: LINEAR REGRESSION EQUATIONS FOR WATER CONTENT IN PERCENT (y)
VS. THICKNESS IN CENTIMETERS (x). Core data from Hill and
Conkwright (1981)
Station Cruise
00
3
y
63.05-.15x
r
=
-.84
103
3
y
=
58.15-.15x
r
=
-.91
113
3
y
-
68.00-.23x
r
=
-.95
114
3
y
=
73.70-.33x
r
=
-.91
115
3
y
=
76.62-.23x
r
-.96
14
1. 2, 3
y
=
71.75-.15x
r
=
-.83
The calculation of the sediment mass accumulating on or eroding from
the bottom of the Bay proceeded as follows. To facilitate the computa-
tional process the Bay was divided into 2.5 minute latitude strips from
its head to the Maryland-Virginia boundary. Within each strip each
different sediment type was delineated from the sediment distribution maps,
and the areas of high erosion or deposition, low erosion or deposition, and
no change for each of these sediment types was determined from the bathy-
metric comparison maps. In addition, the average water content, organic
carbon content and percentages of sand, silt, and clay were calculated for
each sediment type within each erosional or depositional field. The
average thickness of deposition or erosion was then calculated. For areas
of less than one meter of change a weighted average thickness method was
utilized. In areas of erosion or deposition exceeding one meter the actual
values were used to determine the average change within that area. The
volume of material was then calculated by multiplying the area by the
averaged depth change.
The water content within the finer grained sediments was then extra-
polated to the water content at depths using the regression equations
presented previously. In the zero to one meter interval the water content
was estimated from the equation derived from the nearest gravity core
(Figure 10). The thickness entered into the equation was the intermediate
value of the averaged thickness change within that sediment type. In areas
41
-------
where changes exceeded one meter water contents within each one meter
interval down to the total thickness were calculated from the regression
equation derived from the Bay Bridge borings. The changes in water content
thus in effect takes into account the compaction of the fine-grained
sediments.
Using the equations presented in Bennett and Lambert (1971) the bulk
density and porosity of the sediment could then be calculated from the
water content data assuming a grain density of 2.72 g/cm3. These calcula-
tions were performed for each one meter interval and/or fraction thereof
within the sediment type. The density and porosity values were then
entered into the equation of Van Andel and others (1975) to determine the
mass of solids within each given volume of change. The mass contributed
by organic matter was then deleted by subtracting the averaged organic
carbon content for the sediment multiplied by 1.82 (Bezrukov et al., 1977).
Using the percentages of sand, silt, and clay observed at the sediment
surface the mass of each component within the erosional or depositional
volume was then calculated.
To calculate the contribution of sediment'from shoreline changes, the
data collected by Singewald and Slaughter (1949) and transformed into maps
by Conkwright (1975), were digitized to determine the areas of erosion and
deposition along shore. The conversion to volume required a knowledge of
vertical heights along the reaches of shoreline. Bank heights at 395 sites
were measured along the Maryland Bay as part of the nearshore sampling
program. Heights between measured points were extrapolated from topograph-
ic contour quadrangles.
Estimates of the sand, silt, and clay components of the mass contri-
buted by shoreline erosion were made from stratigraphic descriptions
reported in the literature. Because each bank may be composed of more
than one stratigraphic horizon, the average composition for the bank was
determined by a modification of the procedure outlined by Schubel and
others (1976). The thickness of each horizon was multiplied by the
associated percent sand to yield an adjusted sand thickness. Summing the
adjusted sand thicknesses and dividing by the bank height provided the
average sand composition of the bank, the remainder consisting of silt
and clay. This procedure 1s illustrated for an idealized section in
Table IV.
Using these estimates of the sand and silt/clay components of each
bank and the volume of bank eroded, the mass of material can then be
calculated given the bulk density of the sediments. The State Highway
Administration has made estimates of the bulk densities of Coastal Plain
sediments within Maryland (Table V). These values were consistent with
those used by Byrne and others (1982) in calculating the sediment budget
for the Virginia portion of the Bay and were utilized in this analysis.
It was assumed from field observation that shoreline deposition took
the form of beach formation at the base of a fastland. In determining the
volume of shoreline deposition, it was further assumed that the vertical
42
-------
TABLE IV: IDEALIZED STRATIGRAPHIC SECTION - CALVERT COUNTY
(after Schubel et al., 1976)
Section
Thickness of
% Sand per
Adjusted Sand
Units ft'
Interval ft'
Interval
Thickness
100
19
76
14.4
81
5
50
2.5
76
13
12
1.5
63
11
80
8.8
52
30
8
2.4
22
22
75
16.5
0
Total 46.1
Total Adjusted Sand Thickness x 100 . % Sand 1n u(l1t
Total Section Thickness
46.1
100
x 100 = 46.U
TABLE V: MAXIMUM DRY DENSITIES OF COASTAL PLAIN SEDIMENTS
(from Maryland State Highway Administration)
SILT-CLAY
1.67 mtons/m
SAND-SILT-CLAY
1.76
SAND
1.92
SAND (Beach)
2.08
height was two meters and that the shape of the deposit was rectangular.
In reality, these deposits are wedge-shaped, thickening landward. Thus,
the volume of sediment deposited along the shoreline 1s overestimated, and,
as a result, the excess mass is excluded from the sediment budget of the
Bay proper. Conversion to mass was made using the bulk density value for
beach sand from the Highway Administration.
QUALITY ASSURANCE
The quality assurance program of the grain size analysis task involved
43
-------
three major components: 1) calibration of the individual analytical
instruments, 2) intercallbration of specially designed instruments with
similar devices at the Virginia Institute of Marine Science, and 3) dupli-
cate analyses at both MGS and VIMS of sediments collected in both Maryland
and Virginia waters.
Calibration of the RSA was accomplished through the use of glass beads
of known size, sphericity and density. It was determined that the fall
velocity equation presented in Gibbs and others (1971) was suitable for
determining the size distribution of particles analyzed in the RSA. De-
tails of the procedure may be found in Halka and others (1980). Inter-
calibration of the RSA with the one utilized by VIMS was accomplished by
42 replicate analyses of the same sample conducted on each instrument.
Results from this study indicated that the precision, or repeatability, of
the two instruments was quite good and that results obtained from each
could be used interchangeably. Details of the statistical method of
comparison may be found in Byrne and others (1982).
Calibration of the Coulter Counter was accomplished through the
procedures recommended in the TAII operating manual (Coulter Electronics,
1975). Intercalibration with VIMS was deemed unnecessary because these
are stock instruments with standardized calibration procedures.
The great majority of sedimentologlcal studies have utilized the
pipette technique rather than the Coulter Counter to determine the size
distribution of fine grained materials. Therefore, it Is advantageous to
make a comparison of results obtained from the two methods to determine
what differences may exist. Shideler (1976) and Behrens (1978) have both
presented work on this subject. Shideler (1976) determined that the grain
size characteristics determined by Coulter Counter tended to be coarser
than those produced by the pipette analysis. He attributed this, to one
or more of the following factors: 1) coincidence counting, wherein two or
more particles register simultaneously on the Coulter Counter and are
counted as a single, larger particle, 2) an omission artifact due to the
fact that the Coulter Counter is incapable of registering and counting
particles finer than approximately 10 phi, 3) effects due to particle
shape. Behrens (1978) attempted to isolate the omission artifact by
comparing Coulter Counter data to pipette data modified to exclude that
portion of the fine fraction omitted by the Coulter Counter analysis. He
concluded that the apparent coarsening was due entirely to this omission
artifact and that once corrected the Coulter Counter actually resulted
in a finer meausrement of grain size. The only explanation offered for
this difference was operator bias (Behrens, 1978).
In this study a method similar to Behrens' was utilized to compare
the Coulter Counter and pipette results. However, because the grain size
results for this project were calculated to the 14 phi size, cumulative
grain size distribution curves (4$ to 14<|>) derived from the pipette
technique were compared to Coulter Counter distribution curves modified to
include those particles (10$ to 14<(>) falling below the calibrated lower
limit of the Coulter Counter. The results of comparison made on three
separate samples are shown in Figure 13 (A-C).
44
-------
99199, SAMPLE A
PIPETTE TECHNIQUE
COULTER COUNTER TECHNIQUE
.?
£
¦/
y
'A'
-7
aoii
40 50 BP 7 P BP BP 100 110 120 OP HP
DIAMETER (0)
Figure 13A. Distribution curves for Coulter Counter and pipette analysis.
45
-------
DIAMETER <0)
Figure 13B. Distribution curves for Coulter Counter and pipette analysis.
-------
DIAMETER it)
Figure 13C. Distribution curves for Coulter Counter and pipette analysis.
-------
CLH Y
41
12-
*2. IV
1(W®
,5»
in
r
Figure 14. Puplicate samples plotted on tertiary diagram.
48
-------
In each case, the curves converge toward the fine end of the distri-
bution because the Coulter Counter cumulative frequency distribution was
normalized to include the 10<{> weight percent of sample obtained by the
pipette method (I.e., both curves are based on the same cumulative percent
at 10 to 14<|>, the distribution was extrapolated as a straight
line on an arithmetic scale (Folk, 1974).
In all of the cases the grain size distribution resulting from the
Coulter Counter analysis 1s finer than that determined by the pipette
analysis. The differences 1n the mean grain sizes calculated by each
technique range from a low of 0.07 for sample B to a high of 0.29<|> for
sample C. It Is also instructive to note that the envelope encompassing
the Coulter Counter data for each sample is much smaller than that for
the pipette analysis. Thus the precision of the Coulter Counter technique
1s superior to the pipette technique.
While it has not been possible to isolate the causes for the finer
results from the Coulter Counter analysis two possibilities exist; 1) par-
ticle density or shape effects, 2) a machine 'noise' effect. It Is felt
that noise is the most likely source because it has been noted by the
operator that machine noise tends to accumulate 1n the output channels
representing the finer end of the distribution. In any case the differ-
ences between the two techniques are quite small and the greater degree of
precision of the Coulter Counter technique is a strong argument 1n favor of
Its use.
Twenty sediment samples were processed by both MGS and VIMS to deter-
mine if the results from both laboratories were similar. The results of
this comparison are presented 1n Table VI. Samples 1-8 were obtained In
the Virginia portion of the Bay and the remainder (samples 9-20) from the
Maryland portion.
A graphic comparison of percentages of sand, silt and clay determined
by each institution for each sample 1s presented on a ternary diagram 1n
Figure 14. Maryland's determinations are represented by circles, Virgin-
ia's by crosses. Without exception, each of the paired samples matches
on Shepard's class. Thus, based on percentages of sand, silt, and clay,
the grain size analyses performed by the two groups agree closely.
For 14 of the 20 samples sufficient data existed to calculate the
mean grain size determined by each institution (Table VI). This data
permitted a comparison of the results utilizing a t-test for dependent
samples:
EDi
* t = ^nZDi* - (EDI)*
1 n2
/n-1
where Di = difference between mean grain size, pairs and n = number of
pairs of samples (Kirk, 1978).
49
-------
TABLE VI; SEDIMENT SAMPLES ANALYZED BY BOTH MGS (M) AND VIMS (V)
Mean Grain Size Sorting
Sample % % % % Graphic Graphic
Number Gravel Sand S1lt Clay $ $
1M
-
39.3
31.5
29.2
6.3
3.3
V
-
33.5
36.5
29.9
5.7
2.3
2M
42.3
57.7
8.7
2.7
V
0.5
4.0
44.9
50.6
7.5
1.8
3M
41.4
58.6
8.9
2.8
V
-
1.5
42.0
56.5
7.5
1.8
4M
92.2
3.3
4.5
2.2
1.1
V
-
87.9
4.8
7.2
2.3
1.3
5M
97.5
2.0
0.6
1.9
0.6
V
-
97.3
1.3
1.3
1.9
0.5
6M
7.3
88.2
6.3
5.5
1.3
1.9
V
82.3
6.2
4.2
1.2
1.7
7M
•
74.0
14.2
11.8
4.3
2.0
V
0.4
72.9
17.8
8.9
3.9
1.3
8M
67.2
23.0
9.8
4.2
2.0
V
1.2
57.0
31.6
10.1
4.1
1.5
9M
_
2.1
23.0
74.9
9.9
2.6
V
4.7
1.8
24.8
68.7
8.1
1.7
10M
-
0.5
23.7
75.8
9.9
2.5
V
-
0.4
24.6
75.0
8.1
1.1
11M
_
6.1
23.7
70.2
V
1.0
1.7
24.6
72.7
8.3
1.5
12M
-
15.5
29.4
55.1
V
-
11.0
39.5
49.5
7.2
2.1
13M
7.8
39.7
52.5
V
-
6.6
44.5
48.9
7.4
2.1
14M
5.3
31.6
63.1
V
-
4.2
37.3
58.5
7.8
1.7
15M
•
78.;9.
16.0
5.2
3.7
1.2
V
-
83.1
9.8
7.1
3.8
0.7
50
-------
TABLE VI: SEDIMENT SAMPLES ANALYZED BY BOTH MGS (M) AND VIMS (V) (cont.)
Sample
Number
%
Gravel
*
Sand
%
Silt
%
Clay
Mean Grain Size
Graphic
*
Sorting
Graphic
16M
83.7
5.8
10.5
2.8
2.0
V
-
83.4
6.4
10.1
3.0
1.5
17M
—
89.9
•
10.1
V
-
90.4
4.1
5.6
2.1
0.7
18M
•
94.0
.»
6.0
_
V
-
93.5
2.1
4.3
2.2
0.4
19M
95.8
2.1
2.1
3.0
0.4
V
-
90.7
6.2
3.2
3.0
0.5
20M
-
99.2
0.4
0.4
1.9
0.3
V
•
97.9
0.7
1.3
1.9
0.3
51
-------
Results indicated that there was no difference, at the five percent level
of significance, between the results obtained by the two institutions, over
this wide range of sample types.
SEDIMENT CHARACTERISTICS
GRANULOMETRIC CLASSIFICATION AND DISTRIBUTION
Ternary Diagram
The results from a grain size analysis can be discussed in a variety
of ways. One of the simplest and yet most revealing is to combine the
detailed results of the sieve and pipette, and RSA and Coulter Counter
analyses to determine the relative proportions of gravel, sand, silt and
clay size particles in the sediment. Using this information, the ternary
classification system of Shepard (1954) was used to describe and map the
distribution of sediments (Figure 5). An obvious problem of this classifi-
cation is the exclusion of gravel. An examination of field descriptions
made at the time of sample collection together with knowledge of the
present bathymetry and the patterns of erosion or deposition indicates
that a majority of the sediments with a gravel fraction are lag deposits.
These sediments are located on existing shoal areas, 1n the vicinity of
pre-existing islands and adjacent to projecting landmasses. Also, some
of the gravel has been identified as coal and slag, indicative of humian
activities. It has therefore been assumed that gravel is not presently
active in or representative of the sedimentation processes occurring 1n
the immediate area. They have therefore been excluded from the classifica-
tion system.
In a plot of all the sediment samples collected in Maryland, all ten
sediment types are represented on the ternary diagram (Figure 15). The
majority (>75 percent) fall into the sand (1594) and the silty clay (.776)
classifications and represent the two dominant populations in the Maryland
portion of the Bay. Mixing of these two sediment types in various propor-
tions yields the classification of sediment defined by the central section
of the diagram; clayey sand (69), silty sand fl04), and sand-silt-clay
(179). Relatively few of the sediments are typed by the clay (96) or
silt (1) end members and therefore only a few of the sediments are sandy
silt (28) and sandy clay (8). The predominance of clay over silt accounts
for the relatively low number of sediments in the clayey silt class (199)
in comparison to the dominance of the silty clay class.
The trend of the sediments 1n Figure 15 is from the sand population
across the entire central section of mixed sediment to the silty clay
population, and is widely dispersed across the diagram. In contrast,
Byrne and others (1982) reported for the Virginia Bay sediments a trend
from the sand population to the clayey silt population. Furthermore, the
trend in the central section of the ternary diagram 1s more narrowly
defined and skewed toward the silt end member. Byrne also points out
that the sediments are sandier than previously mapped for Virginia. The
52
-------
-------
significant difference between the sediments In the two sections of the
Bay is the coarser grained nature including the dominance of silt over
clay that occurs in Virginia waters.
Sediment Distribution
A map of the sediment distribution based upon the ternary diagram 1s
illustrated by Plate 2. The sediment data depicted on this plate have
been compiled and reduced from more detailed sediment distribution maps
(Chesapeake Bay Atlases) now in the process of publication. In the
reduction process, sediment types represented by a single sample have been
deleted. Also, in certain locales, a "representative" classification was
mapped to generalize a variety of single sample types found 1n close
proximity.
The sediment distribution map (Plate 2) clearly Illustrates the dom-
inance of sand and silty clay sediment types 1n the Maryland portion of the
Bay. The sands are generally located adjacent to the shoreline and on
large platforms around the peninsulas and islands of the Eastern Shore.
The silty clays are generally confined to the axial channel regions. An
interesting feature of the sediment distribution map 1s the general lack
of transitional classifications between the two dominant sediment types.
In most areas the sand directly abuts the silty clay, and sediment
populations intermediate to the two end member populations are absent.
These mixed sediment types could, of course, be present In zones narrower
than the one kilometer grid, and therefore, not been sampled. If so, their
distribution is extremely restricted. Those mixed sediment types which are
found 1n the Bay tend to occur as isolated pockets surrounded by sediments
of either the sand or silty clay types, and not as narrow zones separating
these two sediment populations.
Sediments that plot across the center of the Shepard's Diagram from
sand to the finer silty clay are generally assumed to represent a gradual
diminishing of energy. The sand end member represents high energy wave
environments in which movement of materials occurs by traction or salta-
tion along the bed. Due to the energy impinging upon the sediment bed,
finer material is either not deposited or 1s actively removed from the
sediments. In contrast, the silty clay end member represents deposition of
fine materials from suspension in lower energy environments into which sand
sized materials cannot be carried. Between these two end members, the
formation of a zone of mixed sediment should be present to reflect the
gradual decrease of energy associated with the transition from traction to
suspension transport. Additionally, these mixed sediments could be the
result of alternating high and low energy events creating intercalated
sand and silty clay layers that are subsequently reworked by biogenic
processes. In either case a zone of "mixed" sediment types would occur
between areas characterized by a zone of mixed sediment separating the
sand and silty clay. The two exceptions are the extreme northern Bay and
the Choptank River mouth where the dominant sediments are clayey silts,
adjacent to the nearshore sands. In both areas, the accumulation of
clayey silts reflect a diminishing energy regime and the local geology or
sediment availability. Instead, the distribution of mixed sediments occur
54
-------
as isolated pockets located in the midst of larger sand or silty clay
fields. Field evidence suggests that many of these Isolated pockets of
mixed sediments actually represent subaqueous exposures of pre-Holocene
deposits, particularly in areas where current or wave activity is assumed
sufficiently high to prevent accumulation of modern sediments. Of course,
the effects of anthropogenic activities, particularly dredging and spoil
disposal is not discounted as a possible mechanism for development of mixed
sediments but is confined to segments 2 and 3. Because the textura] char-
acteristics of these sediments do not reflect modern processes, environ-
mental interpretations based upon their grain size characteristics would
be misleading.
The areal extent of each of the mapped sediment types is shown in
Table VII. Here, again, the dominance of the sand and silty clay types
is evident. These two sediment types cover over eighty percent of the Bay
floor. Silt is the most subordinate sediment size 1n the Bay, all silts
together covering only 1.94x10® square meters - less than seven percent
of the Bay. The 'mixed' sediment types that fall between the sand and
silty clay fields on the ternary diagram (Figure 15), range from a low of
0.1 percent for sandy clays to a high of approximately seven percent for
the clayey silts.
Relationship between Depth and Sediment Type
The presence of sand along the margins of the Bay and predominantly
silty clays in the deeper axial sections suggests that a linear relation-
ship exists between water depth and the sediment types. A plot of the
types on a series of ternary dlagrmas, each representing a different depth
interval attempts to Illustrate this relationship (Figure 16). In each
of the depth intervals the trend of the samples plots across the center
of the diagram from the sand apex to the silty clay field, the majority of
the fields being represented at all depth intervals. However, the propor-
tion of samples falling in the finer sediments Increases with depth at
the expense of sand and silt. In the 0-5 meter depth Interval, there is a
wide spread of sediment samples plotting across the diagram from the sand
field through sand-silt-clay to the silty clays and along the bottom
through silty sand-sandy silt to the clayey silt field. In the 5-10 and
10-15 meter depth Intervals the dominant trend is towards the silty clay
with a decreasing frequency of silty sediments and by the 15-20 meter
interval only a few sediments are located near the silt apex plotting 1n
the clayey silt field. An interesting aspect of these plots is the
occurrence of the two dominant sediment populations, sand and silty clay
at all water depths with a lack of silty sediment in the deeper waters.
It appears that in a shallow, recently drowned embayment such as Chesa-
peake Bay, the natural association of sediment type with water depth 1s
complicated by the basin morphology, localized sediment availability, and
transport processes.
Moment Parameters
The detailed analysis provided by the RSA and Coulter Counter produces
grain size distribution curves that can be simplified into four statistical
55
-------
Figure 16A. Sediment class with depth; 0-5 and 5-10 meters.
56
-------
Figure 16B. Sediment class with depth; 10-15 and 15-20 meters.
57
-------
Figure 16C. Sediment class with depth; >20 meters.
58
-------
parameters. Assuming the grain size distributions curves approximate a
normal distribution, the statistical parameters of mean, standard deviation
(sorting), skewness, and kurtosis can be calculated using the methods of
moments. Bivariate plots of these moment parameters can provide insights
into the sediment populations which are not readily apparent from the
sand-silt-clay plots on a tertiary diagram and are useful in further
describing the textural characteristics of the sediments.
TABLE VII: AREAS OF EACH SEDIMENT TYPE IN MARYLAND PORTION OF BAY
SEDIMENT TYPE
AREA (sq. m.)
SAND
1.21xl09
SILTY SAND
1.82xl07
CLAYEY SAND
4.19xl07
(SANDS)
(1.27xl09)
SILT
not present
SANDY SILT
3.40xl06
CLAYEY SILT
1.91xl08
(SILTS)
(1.94xl08)
CLAY
6.89xl07
SANDY CLAY
2.86xl06
SILTY CLAY
1.17xl09 1.17x10
(CLAYS)
(1.24xl09)
SAND-SILT-CLAY
1.50xl08
A plot of moment mean grain size (X^) and standard deviation (S^)
shows two populations clustered near the ends of the mean grain size axis
(Figure 17). One sediment population is located in the sand range with
mean grain size centered around 2.1 phi and the other population in the
clay range, centered around 9.0 phi. The medium to fine sands, with mean
grain size of 1.0 to 3.0 phi, are characterized by good sorting with a
standard deviation below 0.75 phi. As mean grain size decreases to approx-
imately 7.0 phi, the standard deviation increases indicating poorer sort-
ing. At this point in the plot, the trend reverses. The standard devia-
tion decreases as the mean grain size continues into the clay range.
Sediments with a mean grain size of 8.5 phi and finer have standard
deviations centered around 2.5 phi. Between these two dominant popula-
tions, a wider scatter of points reflects the various.admixtures of these
two sediment populations and the presence of sediments composed primarily
of silt. Sorting is very poor in this area of the plot, reflecting the
wide range of particle sizes. Within this general area of the plot,
between the sand and clay populations, those sediments that are slightly
better sorted would tend to be composed of a greater percentage of silt
59
-------
t -
-i . ..
• " t . •
•'"•'•A'v
I >;'<*
H h
H 1 I 1-
s - w n
d in
— N F1 X W ID
MERN ERR IN 5 I ZE
Figure 17. Mean grain size versus sorting.
60
-------
3.0
2.5
2.0
1 .5
1 .0
0.5
0.0
-0.5
> /
• -Vi'v''
' : *!#V* V.
• ' ••• ••
£>*>*•'•....
i. •
-1 .0
-1 .5
-2.0
1 1 1 1 1 1 1 1 1 • 1 1
B - N
H-NPlJWUlhDDl- — —
METRN ERR IN 5 I ZE
Figure 18. Mean grain size versus skewness.
61
-------
sized grains while those sediments with poorer sorting result from a mixing
of the two dominant end member populations. This plot further demonstrates
the presence of the two dominant sediment types, sand and silty clay.
A plot of the mean grain size and skewness (Figure 18) again illus-
trates the dominance of the two sediment populations. The medium to fine
sands (1 to 3 phi) show skewness values centered at zero ranging from
approximately +0.6 to -0.6. Finer grained sediments have either been added
to the distribution, resulting in a positive skewness, or winnowed from it,
producing a negative skewness (Friedman, 1961; Duane, 1964; Valia et al.,
1977).
Sediments with a mean grain size falling in the very fine sand to
coarse silt range, 3.0 to 4.5 phi, show large positive skewness values.
These sediments contain a significant component of fine sediments in the
distribution. This fine fraction shifts the value of the mean grain size
towards the finer end, and imparts to the calculated skewness parameter a
very strong positive value. For example, sediments with extreme skewness
values, greater than 2.0, represent fine sands that are generally well
sorted with a low standard deviation, but that contain a silt and clay
fraction. Since the formula for skewness involves division by the third
power of the standard deviation in order to render it dimenslonless,
these grain size distributions have resulting large values of skewness
indicative of some mixing process of relatively well sorted sand with a
small proportion of finer sediments.
From these high skewness values in the very fine sand to silt range,
the plot falls off as the mean grain size decreases until, at the finer
mean grain sizes, the skewness again is centered around zero. The positive
values for skewness across this portion of the plot indicate that there
still exists a significant fraction of finer grained sediments. The pro-
portion of finer grained sediments decreases with the mean grain size as
the distributions become more normal in shape. This fraction of finer
grained sediments exists even when mean grain size falls in the fine silt
to clay size range. Only in the very finest portion of the plot do any of
the sediments show negative skewness values, indicating a tail toward the
coarser side of the distribution. Even in this portion of the plot,
sediments with negative skewness are subordinate to those with positive
values in the same area of the plot. The reason for a persistent tall of
material toward the finer grained fraction even in the fine silt to clay
size sediments is most likely related to the processes that lead to the
deposition of fine particles in the estuarine environment.
It has been widely noted that fine silts and clays are incapable of
being deposited as Individual particles due to their very low settling
velocities (Meade, 1972; Middleton and Southard, 1977). Other mechanisms
have been called upon to account for the accumulation of particles with
such low settling velocities that, singly, cannot be deposited. Most of
the bottom sediments with mean grain sizes in the very fine silt to clay
size range have settled out of suspension by agglomeration of particles,
whether the agglomeration be caused by flocculation (Ippen, 1966; Van
Olphen, 1977; Krank, 1978, 1980), by biogenic ingestion and pelletization
62
-------
(Haven and Morales-Alamo, 1972), or by coagulation (Zabawa, 1978). Ex-
cepting selective feeding and,pelletization, the distribution of particle
sizes within each agglomerate should be dependent upon random processes
resulting from chance encounters in the water column as the particles
accumulate and settle toward
-------
Sand is restricted to a narrow zone along the Eastern Shore and in CB1,
the Susquehanna Flats, whereas silty clays are confined to the western
edge of Segment 2.
TABLE VIII: GRAIN SIZE CHARACTERISTICS FOR COMPOSITE SAMPLE FROM THE
MARYLAND PORTION OF THE BAY
Mainstem
Eastern Bay
Choptank River
SAND
MEAN
44.9
64.1
59.4
MINIMUM
0.0
0.6
0.9
MAXIMUM
100.0
100.0
99.6
SILT
MEAN
20.8
16.9
24.3
MINIMUM
0.0
0.0
0.2
MAXIMUM
63.5
62.0
69.8
CLAY
MEAN
33.8
18.9
15.8
MINIMUM
0.0
0.0
0.2
MAXIMUM
98.8
76.5
77.7
Within Segment 2, between the Susquehanna Flats and the mouth of the
Sassafras River, sands contributed from the Susquehanna Flats are incor-
porated with the silts and to some extent clays derived from the Susque-
hanna River. The resultant sediments plot in the sand-silt-clay field with
some areas of silty sands and sandy silts. South of the Sassafras River
mouth to Segment 3, clayey silt is dominant on the Bay floor with the
admixture of sand-silt-clay localized along the shoreline.
In Segment 3, the braod area lying between the Patapsco and Chester
Rivers, silty clays have replaced clayey silts as the major sediment type.
The trend of the sediments on a ternary diagram passes from the sand field
through the sand-silt-clay field in the center of the diagram to the silty
clays (Figure 20). The general trend is shifted strongly toward the clay
apex of the diagram, distinctly different than the trend which occurred 1n
Segment 2.
Within Segment 3, the silty clays accumulate in the central portion,
while sands occur adjacent to the margin (Plate 2). Along the Eastern
Shore, these sand fields are quite broad in contrast to the narrow fields
along the western shore. The mixed sediments, sand-silt-clay, occur as
isolated patches adjacent to and within the approach channels to Baltimore
Harbor and are probably due to open-water dredge disposal. A few areas of
clayey silts are prevalent in the extreme northeast section of this seg-
ment, a continuation of the clayey silts from Segment 2.
64
-------
Figure 19. Shepard's plot, Segments 1 and 2.
65
-------
Figure 20. Shepard's plot, Segment 3.
66
-------
The Susquehanna River is the dominant sediment source for this section
of Chesapeake Bay, with a secondary source from shoreline erosion. An
estimated 50 percent of the sediment transported by the Susquehanna River
falls in the silt size range with 10 percent as sand and 40 percent as clay
(Williams and Reed, 1972). Palmer (1975) reported for the spring freshet
in 1974, that half of the suspended sediments introduced to the water
column west of Turkey Point were in the silt size range. Presumably the
remaining suspended sediments were composed of clay sized materials. The
coarser-grained sands are trapped either behind the dams upstream or 1n
the Susquehanna Flats. The silts and clays bypass these areas and are
introduced to the head of the Bay.
Schubel (1971, 1975) found no evidence of hydraulic fractionation of
the suspended sediments either temporally or spatially within the upper
Bay but recognized two distinct populations. The first, composed of fine-
grained clay size particles is present at all depths and at all locations.
The second composed of larger particles and clusters of small particles is
present only during times of high tidal current velocities, and show
increasing concentration with depth. This population 1s resuspended during
maximum tide and redeposited during slack water. When in the water column,
these larger clustered particles are capable of accumulating additional
clay size particles from the background suspended population. The proces-
ses of flocculation, organic binding, and bacterial attachment provide the
mechanisms for removal (Zabawa, 1978).
Although mechanisms exist for fine particle agglomeration and removal
from suspension, it 1s the circulation patterns that dictate the distribu-
tion of these particles throughout the northern Bay. The discharge of the
Susquehanna River governs this circulation pattern. During normal flow
conditions the intersection of fresh water from the Susquehanna River and
the more saline water of Chesapeake Bay produce a zone of maximum turbidity
that extends from the mouth of the Sassafras River to TolChester. Under
extreme discharges, the zone of maximum turbidity extends further south.
Within the zone of maximum turbidity, resuspension and resedlmentation of
particles are the dominant sedimentation processes. South of the turbidity
maximum, the influence of the Susquehanna River flow is diminished and
estuarine circulation is established.
Some observations and inferences can be made relative to the distri-
bution of bottom sediments and the general circulatory pattern. North of
the zone of the maximum turbidity, sand-silt-clays, silty sands and sandy
silts occur where the Susquehanna River debouches into the head of the Bay.
In this area, current velocities diminish and coarser-grained sediments are
deposited. Within the zone of maximum turbidity the sediments become finer
with the deposition of clayey silts. Through the processes of resuspension
and particle attachment, sediments from the suspended populations combining
with the coarser grained materials from the head of the Bay are mixed and
redeposited. The resultant sediments are clayey silt.
South of the zone of maximum turbidity, within the broad area of
Segment 3, silty clays are dominant. To promote deposition of silty clay,
current velocities must be greatly reduced with possibly the establishment
67
-------
CLRY
•IV*
•*v
•/V
Figure 21. Shepard's plot, Segment 4.
68
-------
of a null zone. Fine-grained sediments that have bypassed Segment 2 are
transported into this null zone, agglomerated and subsequently deposited.
This segment probably represents the southern extent of the sediments
introduced by the Susquehanna River as a significant percentage of the
suspended load bypasses Segment 2 and is deposited in Segment 3. The
silty clay in Segment 3 are more representative of the Bay in the distri-
bution of fine-grained sediment which suggests diminishing influence of
the Susquehanna River and the beginning of estuarine sedimentation.
The ternary plot for Segment 4 shows a wide distribution of sediment
types (Figure 21). All sediment classes, except silt are represented. The
majority of the sediments fall in the sand and silty clay fields with the
remaining sediments scattered throughout the central portion of the
diagram. This distribution differs significantly from the plot of sediment
types for Segment 3 in that a broader range of sediment types are repre-r
sented.
Two noticable.differences are evident in a comparison between the
sediments of Segment 4 and Segment 3. The most apparent is the Increase
of silt size particles in Segment 4 resulting in a distribution of sed-
iments plotting toward the silt apex. The second significant difference,
although more subtle, is also an increase in the clay size particles
shifting the silty clays towards the clay apex. In Segment 4, the majority
of sediments within the silty clay field are located in the upper portion
of the diagram (Figure 21), close to the silty clay-clay boundary, and form
a continuum with the sediments in the clay field. A few of the sediments
plot near the clay apex. In contrast, the majority of silty clays in
Segment 3 plot closer to the center of the silty clay field with very
few sediments in the clay field (Figure 20). The distinct difference
between the silty clay in the two segments is the increased percentage of
clay-size particles in Segment 4 relative to the silt-size particles.
It is apparent from the ternary diagrams for Segments 3 and 4 that
in Segment 4 there is an increased percentage of both clay and silt
sized particles. Two distinct trends apparently exist across the diagram
from the sand end member to the finer side. One trend heads across the
diagram through the upper portion of the sancNsilt-clay field to the silty
clay and clay fields. The second trend, although represented by a lesser
number of sediment samples, crosses the lower section of the sand-silt-clay
field and the silty sand and sandy silt field, to the clayey silt field.
The silts derived from the Susquehanna River are effectively trapped in
Segment 2 and 3 and are not significantly transported into Segment 4. A
more proximal source of silt such as shoreline erosion can supply these
grain sizes in this segment of the Bay. As silt sized particles are
derived from shoreline erosion, the silts are reworked and transported
across the shallow nearshore and deposited in deeper waters in zones of
lower wave and current energies. The resultant sediment types plot
through the lower section of the ternary diagram.
Clay-size particles apparently are not completely trapped in the
northern Bay and bypass Segments 2 and 3 possibly as the background popula-
tion of the suspended sediments. The clay size particles derived from
69
-------
the northern Bay coupled with more proximal sources of clay and silt
result in sediments that plot through the central and upper portions of the
sand-silt-clay field to the si.lty-clay field.
Plate 2 provides a generalized view of the areal distribution of
sediment types. The actual distribution in Segment 4 is more complex
than seen on Plate 2. As previously mentioned, occurrences of various
sediment types in close proximity were generalized into a representative
sediment type, and Isolated single samples deleted at this scale. Many
of these isolated pockets occur within the larger sand and sllty clay
fields, primarily in the southern half of Segment 4.
In Segment 4, a marginal sand field is generally wider on the eastern
shore than on the western shore. This marginal sand field is particularly
wide around Poplar and Tilghman Islands. The sands are derived, in large
part, from the erosion of the weakly consolidated Pleistocene sediemnts
of the Kent Island Formation (Owens and Denny, 1979). Interpretation of
the Kent Island Formation as an ancestral estuarlne deposit and the
presence of a major paleochannel system under Poplar Island (Kerhln et al,,
1980) indicate that the sands on this shield represent relict deposits that
are being presently reworked by modern processes, forming a thin sheet over
the Kent Island Formation. Locally, where the sand.sheet thins, the Kent
Island Formation 1s exposed on the Bay floor. Reworking of these exposed
sediments results In the occurrence of sediments with admixture of silts
and clays in a larger sand field.
The finer grained sllty clays predominate in the deeper portions of
Segment 4. In contrast to Segment 3, however, the silty clays are
Interrupted by a wide variety of sediment types ranging from the very
finest clays to sands. The abrupt occurrence of these sediment types
appears to bear no direct relation to the bathymetry or expected circula-
tion patterns, but appears to show an association with the rates of
sedimentation. In the northern sector of Segment 4, sedimentation rates
are quite high with a reported value of 17.8 mm/yr (Helz et al., 1981).
The silty clays in this area form a continuous blanket on the Bay floor,
possibly a continuation of the depositional pattern discussed for Segment
3. Southward, in the lower section of Segment 4, the sedimentation rates
are lower with reported values of 0.7 mm/yr (Helz et al., 1981). In this
entire lower section Isolated pockets of sediments varying from clay
to sand within the predominant silty clay are commonplace. The majority
of occurrences are along the flanks of the axial channel, necessarily
occurring in the deeper sections. The apparent isolation of these
sediment types strongly suggests a localized source which most conven-
iently can be shoreline erosion. The lack of transitional mixed sed-
iments from the shoreline to the deeper axial channel and the isolated
form of these deposits argues for an even more localized source, such as
subaqueous exposure of pre-Holocene sediments.
The exposure of pre-Holocene sediments on the Bay floor appears
to be fairly common within Segment 4. Fine grained sediment pockets
within the sand sheets of the eastern and western shores must be inter-
preted as exposures of Coastal Plain sediments. With sedimentation very
70
-------
CL.RY
* + i
• • • > |
'ic:
r%. •••
Figure 22. Shepard's plot, Segment 5.
71
-------
low in the deeper portions of Segment 4, the silty clays form a thin veneer
of sediment over the Coastal Plain deposits. Tidal currents and wind waves
apparently create velocities high enough to promote non-deposition or
active erosion on the bottom, exposing the Coastal Plain sediments. Once
exposed, these deposits are subjected to modern estuarine processes of
erosion and biogenic reworking. Their surface expression creates the
patchy nature of sediment distribution patterns.
The plot of sediment types for Segment 5 on the ternary diagram
(Figure 22) shows a fairly narrow spread of points across the center of
the diagram connecting the sand apex with the silty clay field. , In gen-
eral, the distribution of points is similar to the major trend observed 1n
Segment 4, passing throughout the upper portion of the sand-silt-clay to
the finer end of the silty clay field. S1lt-s1zed particles are scarce
with very few sediments plotting toward the silt end of the diagram.
The similarities between Segments 4 and 5 in terms of the sediment
types and their areal distribution are also apparent on Plate 2. Sands
characterize the margins of this segment with silty clays in the deeper
portions and axial channel. As in Segment 4, there are many isolated
pockets of differing types occurring within larger sedimentary environ-
ments. A sand environment adjacent to the eastern shore 1s quite broad In
areal extent, forming a platform between the 4 and 8 meter depth contours.
Here the sediments are derived 1n large part from the erosion of the
weakly consolidated Pleistocene sediments of the eastern shore. Sand-
sized materials, left behind as a lag deposit, form a thin sheet overlying
pre-Holocene deposits. Locally, where the sand sheet thins, these pre-
Holocene deposits are exposed on the Bay bottom. This Is evidenced by
the occurrence of sand-sllt-clays and clayey sands within the larger
sand environment. Extensive shoreline erosion during the Holocene rise in
sea level supplied these sand-sized materials. The prevailing winter
northwesterly winds and the long fetch 1n this section of the Bay combine
to move this material 1n a southerly direction. The broadening of the
sand field to the south 1n Segment 5 and the large depositlonal areas
noted at the distal portion of this field In Virginia (Byrne et al., 1982)
are the result of this southerly movement.
The deeper portions of Segment 5 are largely sites of accumulation of
fine grained silty clays, accumulating in the axial channel and west onto
a broad terrace bordering St. Mary's County. Sedimentation rates range
from 1.6 mm/yr in the northern sector to 12.6 mm/yr near the southern
boundary of the segment (Helz et al., 1981). Present within the silty
clays is a narrow, linear zone of mixed sediments ranging from sand to
silty sand and sand-silt-clay. The greatest proportion of these mixed
sediments are in the northern sector of the segment on this broad terrace
where sedimentation rates are reported to be low. In this area, current
activity and low sedimentation rates combine resulting 1n the exposure
and reworking of pre-Holocene sediments. To the south, where sedimentation
rates are higher, the number of these exposures decreases. This area 1s
characterized by a broad expanse of silty clay sediments. Of particular
note Is the broad platform, bounded by the 12 and 16 meter depth contours,
located off the mouth of the Potomac (Plate 1). This platform 1s composed
72
-------
Figure 23. Shepard's plot, Eastern Bay.
73
-------
entirely of silty clays. This area is apparently the site of high deposi-
tion of fine grained sediments that have infilled the relict Potomac River
channel at its confluence with the Chesapeake Bay axial channel. This area
is probably the location of a current gyre and/or a null zone of tidal
circulation (Klein, personal communication). These estuarine circulation
patterns are apparently effective in entrapping and concentrating fine
grained suspended materials and reducing turbulence enough to enable them
to settle from suspension. Over the period of time of the Holocene rise in
sea level these fine grained silty clays have accumulated rapidly 1n this
area infilling the confluence of the Potomac River and main Bay.
Eastern Bay
The distribution of sediment types within Eastern Bay tends to mirror
that of the main Bay in Segment 4 (Plate 2, Figure 23). On the Shepard's
Classification Diagram the samples plot across the center of the field
from the sand apex to the silty clay field, with one sample lying just
over the boundary into the clay field and a few scattered In the clayey
silt and sandy silt sediment types. A sand sheet, locally interrupted by
other sediment types, outlines the margins of the Bay at depths of less
than 8 meters. As in Segments 4 and 5 of the main Bay, the Isolated
patches of other sediment types, predominantly clayey silts, sandy silts,
silty sands, and sand-sllt-clays, are presumed to represent outcrops of
the Pleistocene Kent Island Formation or older Coastal Plain deposits,
that project through the generally thin, sandy veneer. Silty clays occupy
the deeper, central portions of Eastern Bay where low current velocities
and turbulence permit the accumulation of fine-grained sediments. Most
of these fine-grained materials are derived from the erosion of the
shorelines and the shallow bottoms. Fine-grained fluvial sediments are
undoubtedly trapped upstream of the Bay, in the vicinity of the turbtdtty
maxima of the Miles, Wye and East Rivers.
The Choptank River
The distribution of sediment types within the Choptank River mouth
is quite different from that of Eastern Bay 1n that the samples are
strongly shifted to the silt apex of the Shepard's Classification Diagram
(Figure 24). The largest number of samples plot 1n the clayey silt, sandy
silt, and silty sand fields. The marginal sand sheet occupies the
shallower terraces of the Choptank River, as occurs throughout the Bay,
but the axial channel is accumulating clayey silt as opposed to silty
clays (Plate 2). Apparently, the marginal deposits of the Kent Island
and Choptank Formations, which are being eroded as sea level rises,
contain much more silt-sized material in this area than 1n the vicinity of
Eastern Bay. These materials are winnowed from the marginal sand fields
but are largely retained 1n the central, deeper portions of the Choptank
embayment. Some clayey silts are transported out of the embayment 1n the
Choptank River channel and are deposited adjacent to Its confluence with
the main Bay axial channel in Segment 4 (Plate 2).
74
-------
Figure 24. Shepard's plot, Choptank River.
75
-------
Id
q:
u
a.
30 " ~.
V ¦
IB
•/
I 0
30 M0 K0 B0 70
PERCENT CLRY
Figure 25. Water content versus clay content.
-------
WATER CONTENT
The mean water content of sediments in the Maryland portion of the
Bay is 47 percent, wet sediment weight, with a minimum value of 16 percent
for a sand and a maximum value of 83 percent for a si 1ty clay. Generally,
water content increases as the amount of fine-grained material in the
sediment increases, with the sequence from lowest to highest being sand,
si 1ty sand, sandy silt, clayey silt, sand-silt-clay, clayey sand, sandy
clay, clay and silty clay. Increasing amounts of silt relative to sand
results in an increase in water content but lower than the increase 1n
water content with increasing clay relative to sand. Clay seems to have
the dominant influence on water content as clayey sands have higher
water contents than silty sand.
A plot of water content with clay shows the direct relationship of
increasing water content with increasing clay content (Figure 25). At
the higher end of clay content with clay greater than 75%, there is a
suggestion in the plot of decreasing water content; clay having lower
water content than silty clay. The distribution of clay-rich sediment
occurs west of the axial channel in Segment 4. These clay-rich sediments
are tentatively interpreted as exposed pre-Holocene sediments. Similar
clay-rich sediments exposed in the nearshore zone have been traced
onshore to pre-Holocene deposits. These sediments when exposed on the Bay
floor are resaturated with water, developing a very soft high water content
surface but very firm at depth. Resaturation of indurated sediments
increases the water content but not to the level of recently deposited
fine-grained sediments.
CARBON DISTRIBUTION
Carbon is an essential constituent in the geochemical cycle of
estuarine sediments. Organic carbon is the primary source of food for
most benthic organisms. Once buried, organic carbon plays a critical
role in determining the redox potential of the sediments. This makes
it an important variable in determining how chemical species behave in
the natural environment (Sillen, 1967). Finally, trace metals and other
toxic compounds such as pesticides are commonly associated with particulate
organic carbon. Areas where fine-grained, organic-rich sediments are
known to accumulate may prove important sites to monitor for pollution.
Sediments deposited in the Maryland portion of Chesapeake Bay are an
admixture of land-derived detritus (both organic and inorganic), organic
fallout, and redistributed pre-Holocene sediments. The distribution of
carbon in the Bay 1s a function of the interaction of physical, chemical
and biological processes. Because this interaction is complex and poorly
understood, it is unwise to make absolute statements on the mechanisms
responsible for the existing distribution. The data will, therefore, be
discussed in terms of possible or probable controls.
Results
The total carbon content of Bay sediments is comprised of organic
77
-------
carbon and inorganic carbon. Inorganic carbon in the Bay is derived mainly
from shell material, though bones and teeth of vertebrates do contribute
minor amounts. Inorganic sources contribute little to the total carbon
content of sediments in the Maryland portion of the Bay. The dominance of
organic carbon is demonstrated by the linear relationship that exists
between it and total carbon (Figure 26). Sources of organic carbon
include organic fallout of phytoplankton and zooplankton, terrestrial
plant detritus, coal and, to a minor degree, decayed material from
macrobenthos and marine flora. Not all organic carbon being incorporated
into Bay sediments is readily reactive, i.e. in the short-chain molecular
form required by most microorganisms. This is especially true of land-
derived organic material. It is, therefore, essential to evaluate the
type of carbon available to the system before using organic carbon as an
indicator of chemical potential. The relative contribution of terrestrial
and marine sources of organic carbon in Chesapeake Bay sediments is not
yet clearly defined.
High carbon values are generally associated with fine-grained sed-
iments (silts and clays), while low carbon values closely correspond with
coarser, usually sandy, deposits. The strength of this generalization is
born out by the area! distribution of carbon in the Bay (Plate 4). For
the most part, low carbon values parallel the shoreline and are restricted
to sandy shallow water deposits. In such environments the winnowing action
of the waves promotes aerobic decomposition and/or removes much of the
fine-grained material, including carbon, to deeper waters. Consequently,
carbon values tend to increase toward the central, deeper waters where
fine-grained sediments accumulate. In some areas of the Bay, this general
trend is complicated by other factors; e.g., the exposure of pre-Holocene
sediments, open-water disposal of dredged material and particle agglomera-
tion processes.
Organic carbon values in the Maryland portion of the Bay range from
0-10.5 percent, dry weight, around a mean value of 2.0 percent (Table IX),
A cluster of extremely high values is concentrated in a discrete lobe at
the head of the Bay near the entrance of the Susquehanna River (Plate 4).
A secondary lobe of high, but less extreme values stretches south into
Segment 3. This second lobe is cored with carbon contents in the 5.0-6.0
percent range, which occur in the deeps of the channel east of Pooles
Island. The mean carbon content of sediments in Segments 1 and 2 is nearly
twice that of the collective sample population (4.1 percent versus 2.0
percent, respectively). The interpolation of the contours in this area
of the Bay may be somewhat compromised by the absence of samples from the
restricted waters of Aberdeen Proving Grounds.
In Segment 3, the mean carbon content falls to 2.7 percent. The
central portion of this segment is characterized by a laterally continuous
distribution of carbon contents in the 3.0-4.0 percent range. The only
occurrence of carbon contents greater than 4.0 percent, other than those
associated with the lobe described above, is located in a small area just
north of the Bay Bridge. The location of these few high values corresponds
with an area of mixed sediments (see Plates 2 and 4) and a former dredge
disposal site.
78
-------
I M
a
m
o:
nz
a;
i—
~
f—¦
I 3
i a
t i
t a
3
2
I
• ¦
« W"1
1 •>*
~ZT
I B9 II I 2 13
550R13F1N 1 C CRRBDN
Figure 26. Organic carbon versus total carbon.
-------
TABLE IX: SUMMARY OF ORGANIC CARBON STATISTICS FOR THE MARYLAND BAY
AND ITS SEGMENTS
Segment
Mean
[% dry wt.) a
Maximum
Minimum.
Sample
Size
Maryland Bay
2.0
1.5
10.5
0
727
Segments 1 & 2
4.1
2.2
10.5
0.5
65
Segment 3
2.7
1.2
5.0
0.1
80
Segment 4
1.9
1.1
7.1*
0
296
Segment 5
1.4
1.0
3.1
0
223
Eastern Bay
1.5
0.8
2.6
0.2
18
Choptank River
0.9
0.4
1.8
0
45
* This value is anomalously high for samples in this segment and is known
to have a significant coal component. 4.8% is probably more representa-
tive of the sample population.
South of the Bay Bridge, carbon contents greater than 3.0 percent
become laterally discontinuous but are still confined to the central,
deeper waters. Carbon values continue to diminish south of the Bay Bridge.
The area between the bridge and South River is rimmed with low carbon
values and the center is dominated by values in the 2-3 percent range.
South of South River, large, discontinuous areas with carbon contents
greater than 3 percent prevail in the center. These areas roughly corres-
pond with clay areas defined on the sediment distribution map (see Plates 2
and 4). Their location also corresponds with an erosional surface outlined
on the erosion-deposition map (Plate 6).
In Segment 5, the occurrences of sediments with greater than 3 percent
carbon become extremely localized and, again, contents between 2 and 3
percent dominate the central portion.
Organic carbon is commonly associated with fine-grained sediments;
i.e. silts and clays. In the Maryland Bay, the strength of the correla-
tion between fine-grained sediments and organic carbon is diminished by
the occurrence of anomalously high carbon contents in relatively coarse
sediments in the northernmost reaches of the Bay (Segments 1 and 2).
Graphical representation of the mean carbon values within sand, silt, clay
and mixed sediment populations, for the entire Maryland Bay and for each .
section, shows that sands tend to have low carbon contents while silty,
clayey and mixed sediments tend to have moderate to high carbon values
80
-------
(Figure 27). Figure 27 also shows the downbay decline of the mean carbon
content of Bay sediments. This corresponds with the increasing areal
extent of sands, a trend which continues into the Virginia portion of the
Bay (Byrnes and others, 1982).
Baywide correlations of organic carbon with percent silt, percent
clay, percent sand and mean grain size are in the 0.6-0.7 percent range.
If the correlation between carbon and each of these variables is assessed
on a segment by segment basis (Table X) the anomalous behavior of northern
Bay sediments stands distinct from the overall Baywide trends. In general,
organic carbon correlates most strongly with the clay fraction. In
Segment 3, a fairly strong correlation exists between organic carbon and
clay (r=0.783), but its correlation with the silt fraction is actually
stronger (r=0.914). The correlation between carbon and mean grain size
is fairly strong throughout the Bay (Segments 1 and 2 excepted). This
correlation strengthens downbay.
TABLE X: CORRELATION COEFFICIENTS FOR ORGANIC CARBON VS. VARIOUS
SEDIMENT PARAMETERS, BAYWIDE (MARYLAND PORTION) AND BY
SEGMENT
Mean
Segment Depth % Sand % Silt % Clay % Mud Grain Size % Water
Maryland Bay
0.083
-0.703
0.634
0.631
i
0.702
0.683
0.701
Segments 1 & 2
-0.084
-0.144
0.388
-0.114"
0.094
0.127
0.449
Segment 3
0.226
-0.897
0.914
0.783_
0.849
0.856
Segment 4
0.370
-0.847
0.542
0.871"
0.868
0.859
0.894
Segment 5
0.506
-0.947
0.887
0.953_
0.954
0.971
Eastern Bay
0.739
-0.817
0.621
0.790
0.801
0.820
Choptank River
0.479
-0.887
0.769
0.932
0.925
0.875
Though no direct correlation is evident between organic carbon content
and depth (Table X), visual comparison of bathymetric and carbon contour
maps (Plates 1 and 4) recommends further consideration of their relation-
ship. Figure 28 shows the frequency distribution of organic carbon concen-
trations within various depth intervals for two broad areas of the Bay, the
area north of the Bay Bridge (Segments 1-3) and the mainstem area south of
the Bay Bridge (Segments 4 and 5). In Segments 1 and 2 the extreme values
that characterize the northern Bay are associated with the shallowest
interval (0-6 meters), as are the more expected low values. Carbon
81
-------
5 0
4-0
O
OQ
tr
<
oo O
ro
O
z
<
o
cc
o
55
30
2-0
10
0
M SANDY SEDIMENTS
Q SILTY SEDIMENTS
£ CLAYEY SEDIMENTS
PI SAND/SILT/CLAY
S EASTERN BAY CH0PTANK R. BAYWIDE
C B SECTION
27. Carbon versus sediment type.
-------
contents within the 0-6 meter interval range from 0.2-10.5 percent around
a mean of 3.3 percent. At the 6-10 and 10-14 meter intervals the mean
persists around 3.4 percent, but the range of values decreases.
South of the Bay Bridge (Segments 4 and 5), the mean carbon content
of the sediments is significantly lower, x = 1.9 versus x = 3.3. Carbon
values greater than 3.6 percent are rare and are restricted to sediments
deposited in greater than 10 meters of water. Carbon contents within the
0-6 meter interval range from 0-3.6 percent with a mean of 0.9 percent.
Carbon values within this interval are concentrated at the low end of the
range, with two-thirds of the samples in this interval containing less
than 1.0 percent carbon. The 6-10 meter interval seems to be transitional.
The range of values remains the same, but the mean increases to 1.7 percent
and carbon values are much more evenly distributed throughout the range.
In the 14-16 meter depth interval, the mean increases to 2.4 percent, and
the distribution of values begins to favor the higher end of the range.
This pattern persists with increasing depth. Overall, the trend for the
distribution of carbon in the middle Bay shows a greater frequency of low
carbon values in shallow water sediments and a greater frequency of higher
carbon values in deeper water sediments. Distribution in the northern Bay
is complicated by the occurrence of a few very high values in shallow
water. It can be said, however, that, even here, low carbon values tend
to be associated with shallow water sediments.
Carbon distribution in the Eastern Bay and Choptank River subestuarles
conforms to the general Baywide trends. As previously reported, low
carbon values tend to be associated with shallow water, sandy deposits
while higher carbon values are associated with deeper water, fine-grained
sediments. In Eastern Bay, the strength of the correlations between
carbon and grain-size parameters are diminished by the occurrence of some
fairly high carbon values in the mixed sediments (sand-silt-clay).
Discussion
In the previous section the distribution of organic carbon was dis-
cussed in terms of various sediment parameters. Here, the aim Is to relate
the observed distribution to the processes which control that distribution.
The organic carbon content of sediments is controlled by its availa-
bility relative to the supply of inorganic constituents and, once depos-
ited, by the rate at which it is metabolized by microorganisms residing
within the sediments. Carbon is preserved when the rate of supply exceeds
the rate at which it can be utilized. Rate of utilization differs from
simple demand in that it encompasses other controls on the preservation
potential of carbon; e.g., the chemical environment of the sediments and
the nature of the organic carbon.
Preservation of carbon is enhanced by rapid burial, especially tn
fine-grained sediments. The platy structure of clays reduces the permea-
bility of the sediments and promotes reducing conditions. It is generally
believed that anaerobic decay of carbon proceeds more slowly than aerobic
decay.
83
-------
Some types of organic carbon are more reactive, i.e. more easily
metabolized, than others. In general, carbon derived from terrestrial
plant material contains less reactive carbon than an equivalent amount
derived from marine sources. Thus, the type of carbon supplied to the
sediment influences the rate of utilization.
In the mainstem portion of the Maryland Bay, the distribution of
organic carbon can be divided into two main sub-environments, the Bay
north of the Bay Bridge (northern Bay) and the Bay south of the Bay Bridge
(middle Bay). The northern Bay (Segments 1-3) is characterized by extreme-
ly high carbon contents at its head, with a downbay decrease. Organic
carbon in northern Bay sediments is derived from both terrestrial and
marine sources (Hunt, 1966; Spiker, 1982). South of the Bridge, carbon
values continue to decrease, and moderate carbon concentrations predom-
inate. Carbon, here, is primarily algal (Spiker et al., 1982).
The Northern Bay is an area of rapid deposition with high detrital
input from the Susquehanna River. Rapid burial can not account for the
high preservation of organic carbon in surface sediments. Rather, low
demand, limitations on the accessibility of carbon (i.e., a high non-
reactive component), or by some combination of these factors must contri-
bute to these elevated carbon concentrations. There is no evidence to
recommend low demand as an explanation; thus, high preservation is best
explained by limited accessibility. Northern Bay sediments are enriched
in terrestrial detritus. C13 data supports the premise that organic
carbon in these sediments is primarily land-derived (Hunt, 1966; Spiker
et al., 1982). Terrestrial plant detritus is composed primarily of
cellulose and lignin (Tissot and Welte, 1978), materials which are
resistant to decomposition by virtue of their complex molecular struc-
ture (Krauskopf, 1967). Cellulose can be decomposed by certain types of
bacteria (Manskaya and Drozdova, 1968), but whether this occurs in Bay
sediments is not yet known.
Preliminary efforts to assess the relative importance of marine and
terrestrial inputs indicate that much of the organic carbon in northern
Bay sediments is highly resistant to chemical degradation. Eleven samples
from Segments 1 and 2 were treated with 15$ hydrogen peroxide until reac-
tion ceased. The samples were then rinsed, dried, reground and analyzed
for carbon content. The carbon that survived this treatment is deemed
"non-reactive" and that lost in the digestion is considered "reactive".
For all but one sample, at least fifty percent of the organic carbon
component is "non-reactive" (Table XI). Samples with a non-reactive com-
ponent greater than 4.0 percent were dark in color. Since coal has been
previously identified in northern Bay sediments (Ryan, 1953), it is assumed
that its presence explains these high values. It is .important to note that
the mean value of the "reactive" carbon component is approximately equiv-
alent to the Baywide mean for total organic carbon. Spiker and others
(1982) maintain that organic carbon south of the turbidity maximum is
primarily algal. If this is so, one would expect an asymptotic decline in
the ratio of non-reactive to reactive carbon in the sediments diminishing
downbay from Segment 1 and leveling off as the effect of fluvial input
lessens.
84
-------
points Northern Bay (Segments 1-3)
10.0
1UI
9.0
X
8.0
"—
7.0
X
a
o
6.0
XX
XX
.0
S-
5.0
xxxxx
XXXXX X
fO
u
4.0
xxxxxxx x21
XXXXXXXX11 XXX
o
3.0
«xxxxxxx33 txxxxxxxie «xx
c
2D
xxxxxxxxis
XX
14
depth (meters)
* One sample
~ Group mean; coincides with sample value
e Group mean; does not coincide with sample.value
Note: Numbers at end of columns represent total number
of samples within range.
mid-
points
Middle Bay (Segments 4-5)
X
X
X
X XX
X
xxxxxxxxxx
XXXXXXXX20
XXXXXXXX47
XXXXXXXXXX xxxxxxx xxxxxxxx
«xxx
XX
#xxxxxx
XXX
XXXXXXXXX
«xxxxxxxi9
«xxxx«xx17
xxxxxxxxll
©
e
«xxxxxxx
xxxxxxxx21
xxxxxxxxx
X XXXXXX XXX
XXX
xxxxi(xxx3S
X XXXXXXX11
XXX
XXX XXX
X
X
0-6
6-10
10-14
14-16 16-20 20-24
24-28
28:32
32-36
>36
depth (meters)
c
7.0
o
-O
6.0
I.
(0
5.0
u
4.0
u
•r—
3.0
c
(0
20
cn
i-
1.0
o
OO
Wt
Figure 28. Organic carbon versus depth.
-------
TABLE XI: CARBON CONTENTS OF SOME SEDIMENT SAMPLES FROM SEGMENTS 1 & 2;
EXPRESSED IN PERCENT DRY WEIGHT
Sample
Number
Organic
Carbon
"Reactive"
Carbon
"Non-reactive"
Carbon
1
10.13
3.12
7.01
2
4.13
1.89
2.24
3
4.54
1.94
2.61
4
4.07
1.85
2.22
5
9.51
1.71
7.80
6
0.99
0.64
0.35
7
10.04
1.78
8.26
8
4.63
2.23
2.31
9
2.86
1.79
1.07
10
6.46
1.91
4.55
11
4.17
2.15
2.02
mean
1.91
3.68
standard
deviation
0.58
2.79
It is generally assumed that organic carbon is most strongly associat-
ed with fine-grained sediments. In open marine environments fine-grained
sediments accumulate in deep water below wave base. In estuarine environ-
ments such as Chesapeake Bay, much of the supply of fine-grained sediment
is trapped near the interface of fresh and salt waters as a result of
estuarine circulation patterns and particle agglomeration (Schubel, 1971).
For this reason, most of the fine-grained sediment introduced into the Bay
by the Susquehanna River is deposited north of the Bay Bridge in Segments
1-3. It is, therefore, not surprising that carbon concentrations in
northern Bay sediments tend to be rather high despite the shallowness of
the water, especially since this area receives organic material from marine
as well as terrestrial sources. Even optimum conditions and dual source
are not sufficient to explain the occurrence of extreme carbon concentra-
tions in Segments 1 and 2.
86
-------
Increasing Ci3
— CRUDE OIL
COAL
PLANTS
LIPIDS
"CRimr nn
PLANTS
NONMARINE
30 -25 -20 -15 -10
C13/C12per mil relative to
Peedee belemnite
-212 y
t
Figure 29: CJ3 ratios, after Hunt, 1966
87
-------
Comparison of carbon and sediment distribution maps (Plates 4 and 2)
show that the previously discussed adjacent lobes of high concentrations
which stretch from the head of the Bay into the northern part of Segment 3
correspond coarser grained materials, i.e., sand-silt-clays and clayey
silts. This correlation combined with previously cited evidence of the
occurrence of coal in northern Bay sediments suggests that hydraulic
fractionation may concentrate coal in the mixed sediments at the head of
the Bay and disperse it in decreasing concentration.
Detrital input from the Susquehanna diminishes greatly in the middle
Bay. This is reflected in the C13 ratios, which show this area to be in
transition from river-dominated to marine-dominated in terms of carbon
sources (Hunt, 1966; see Figure 29). Terrestrial carbon south of the
bridge is probably derived from erosion of coastal plain sediments. Sea-
ward transport of the lag deposits of shoreline erosion and the diminshed
influx of fine grained sediments from the Susquehanna results in an overall
continued gradual decline of mean carbon values south of the bridge.
Subaqueous exposures of indurated pre-Holocene sediments may affect
the distribution of carbon south of the bridge. This is especially true
in Segment 4, where high carbon values roughly correspond to clay areas
on the sediment distribution map. Preliminary investigations suggest that
these clay areas represent pre-Holocene outcrops.
SULFUR DISTRIBUTION
Sulfur is but one constituent of the sedimentary environment - a
complex assemblage of mineral grains, interstitial water, and organisms,
living and dead. Interactions among these variables control the distri-
bution of reduced sulfur in Bay sediments, the following combination
providing the ideal environment for iron sulfide formation: clay-size
particles coated with ferric oxide; interstitial water containing sulfate,
but not dissolved oxygen; and a flourishing community of bacteria feeding
on a plentiful supply of organic matter.
Sulfur content of sediments in the Maryland portion of the Chesapeake
Bay ranges from 0.00 to 2.00%, averaging 0.56%. Approximately half of the
samples contain less than 0.4% sulfur. The range of values is identical to
that found by VIMS for the Virginia section of the Bay. Mean percent
sulfur, however, is greater than the 0.35% average for Virginia.
The relationship between Shepard's Class categories and mean percent
sulfur is summarized in Figure 30, for each segment and for the entire Bay.
The Shepard's Class categories are grouped; for example, sandy sediments
include not only sands, but also silty sands and clayey sands. The sand-
silt-clay class is reported separately. For each subsection except 1 and
2, where no end member clays were analyzed for sulfur, mean percent sulfur
increases dramatically as grain size diminishes. Highest sulfur values
are associated with clayey sediments. A dichotomous sulfur distribution
between the northern and middle Bay is also clearly shown on the graph.
Although mean percent sulfur is nearly constant for sandy samples, the
88
-------
lH SANOY SEOIMENTS
ffZl SILTV SEDIMENTS
^ CLAYEY SEOIMENTS
| | SAND / SILT / CLAY
182
EASTERN BAY CHOPTANK R.
BAY WIDE
C B SECTION
Figure 30. Sulfur versus Shepard's class.
-------
mean percent sulfur of silty sediments is 0.36% in the northern Bay and
0.76% in the middle Bay. For clayey sediments, the difference between
the two sections of the Bay is even greater; mean percent sulfur is 0.35%
and 0.98% for north and middle respectively.
The association between sulfur concentration and clay content demon-
strated above and the comparatively strong correlation between those
variables baywide (r = 0.75) warrants a closer look at their relationship.
Figure 31 is a bivariate plot of percent sulfur versus percent clay for
all samples analyzed for sulfur. Besides the direct proportionality be-
tween sulfur and clay, the graph also shows that as clay content increases,
the absolute range of sulfur values widens. There is more scatter about
the regression line at the higher end of the clay scale. For samples
containing less than 20% clay, sulfur varies from 0.00 to 0.68%. The
range increases from 1.20% for samples consisting of 20-50% clay, to
1.92% for predominantly clayey sediments. Thus, although high sulfur
concentrations are invariably associated with high clay content, the
reverse is not true. Given a large clay fraction, it is virtually impos-
sible to predict accurately the amount of sulfur in the sediment.
A plot of mean percent sulfur versus percent clay (Figure 32)
corroborates the sulfur-clay relationship. For samples containing between
20-70 percent, there is a gradual, linear increase in mean percent sulfur
as the clay fraction of the sediment increases. A natural break in the
linear trend occurs at the point corresponding to 70 percent clay. Beyond
that cut-off, a more steeply sloping line best fits the data. For each
incremental change in clay content there is a correspondingly larger
change in sulfur concentration. Unfortunately, only four samples of the
727 analyzed for sulfur contain more than 80 percent clay, casting doubt
on generalizations based on this part of the graph. The points represent-
ing samples with less than 20 percent clay may constitute yet a third sub-
set. Separating those samples seems justifiable on the basis of variations
in range discussed earlier.
A similar plot (Figure 33) of mean percent sulfur versus percent mud
reveals the same relationship between sulfur and fine grain size, though
not as strikingly. Sulfur increases in a stepwise fashion with percent
mud, in contrast to the continual increase in sulfur with percent clay.
A bivariate plot of percent sulfur versus percent mud (Figure 34)
seems to show an upper limit on sulfur concentrations, dependent on the
mud component of the sediment. Thus, samples consisting entirely of mud
may contain a maximum of 2 percent sulfur; whereas, the sulfur content
of sediments consisting of equal amounts of sand and mud ought to be less
than 1 percent.
Table XII summarizes the correlations between sulfur and several
other sedimentological factors, by segment and by segment groups. Statis-
tics computed for all samples, effectively averaging conditions baywide,
conceal the pronounced differences between northern and middle Bay sedi-
ments. Correlations between variables are strengthened or weakened when
the Bay segments are examined individually.
90
-------
l£>
DC
Z]
Ll.
Lil
*-
U
cr
LJ
CL
• • •
I • • • • V
• *
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f • •• • #1
• • •
• • » • t •
• ... •. •
• A i A • • •
• \* >
• • • • • % • • •
• j» •_ • • • •
• *• * •
•I' '.'i V" '
•.*: * .* 2* • •>.
. . • * . • • • • • * %<»
. • • . • - *... • • •*. . • • •
• * # rT 'T' • •* • • ' # • * •
/ • • • >ij •• * • • ® («
* • • • • ^ • • • • ¦ • • • ••
l.'.*-."?:, •• •• •. • .• *.
• b . ib •
1 B SB
30 MB KB
SB 7B BB BB
PERCENT CLRY
Figure 31. Sulfur versus clay.
-------
10
~T~
20
I
SO
I
40
I
SO
—f-
60
-r~
70
~r*
eo
~r
90
—1
100
% CLAY
Figure 32. Mean sulfur versus clay.
B
-i r~
10 20
-J-
40
—i r~
so eo
~~r~
T0
"T"
•0
~I
100
I
so
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V. MUD
Figure 33. Mean sulfur versus mud.
92
-------
CJ
rr
zi
Lx.
-J
Zl
LTI
Id
V
IX.
UJ
tL
1 la . la
• V
. •••£••
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• • • • > •*' \ ' si
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• * > , i
• • ••
i m aa
MB KB
*783
PERCENT MUD
Figure 34. Sulfur versus mud.
-------
Sulfur Versus Organic Carbon
A bivariate plot of sulfur versus organic carbon for all samples
(n = 727), Including subestuaries, is shown in Figure 35. The dominant
trend is parabolic about the ordinate, sulfur content increasing exponen-
tially with organic carbon. Subordinate to the deviating from that pre-
dominant pattern is an ill-defined scatter of points representing samples
with high carbon content and corresponding low sulfur values. A comparable
plot (Figure 36) based only on measurements for segments 1-3, establishes
that these points are restricted to the head of the Bay. Thus, Figure 35
is a composite of two geographically distinct organic carbon/sulfur
regimes, separated by the boundary between segments 3 and 4. The dissim-
ilarities between the two regions are reflected in the correlations
between organic carbon and sulfur: r = 0.14 1n Sections 1-3 and r = 0.79
in Sections 4 and 5.
Sulfur Versus Percent Clay and Organic Carbon
The results of a stepwise regression in which percent clay and percent
organic carbon were the independent variables indicate that these two
factors account for slightly more than half of the variance in sulfur bay-
wise (r2 = 0.57). If percent clay is known, r2 is increased by only
0.02 when organic carbon is entered into the regression equation.
In the northern Bay, the correlation between sulfur and percent clay
is poor (r = 0.39); the correlation between sulfur and organic carbon is
almost nonexistent (r = 0.14). Combined, clay and organic carbon account
for just 16 percent of the variance in sulfur. South of Kent Island, the
correlations between sulfur and clay (r = 0.85) and sulfur and organic
carbon (r = 0.79) are moderately strong. Taken together, these two varia-
bles account for 71 percent of the variance in sulfur in this part of the
Bay.
Sulfur in Bay sediments is largely derived from the bacterially
mediated reduction of sulfate in seawater to sulfide. Anthropogenic
sources, e.g., combustion of fossil fuels, leaching/erosion of fertilized
soil, and sulfur contributed by the decomposition of protein in organic
detritus, are probably of minor importance, except in special cases.
Likewise, the presumed flux of sulfur out of nearby or underlying Miocene
sediments, which may be the predominant source of clay sediment in segment
4, is restricted to the vicinity of the Choptank River (Kerhin et al.,
1982).
Baywide, the most striking observation concerning sulfur distribution
is the association between percent sulfur and percent clay. The relation-
ship is not surprising. Quiet environments are conducive to the deposition
of clay and low-density organic matter and to the concomitant development
of anoxic conditions. Nutrients supplied by organic debris foster the
growth of anaerobic sulfate-reducing bacteria.
The joint occurrence of clay and organic detritus is not fortuitous.
Taken singly, fine particles - mineral and organic - are sufficiently
94
-------
2.0
~
L
J
~
in
h
Z
UJ
V
o:
y
Q.
1 .5
I .0
0.5
• • •
• *
~. *•
i • • »«
•• • •
• % *v
% •
*
. .• - •
•• * • •. ••: *
• • i *
. .*•» : .*
• s :T"'
/
• *• •
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»••• >•. ••
• ,» 'x • •
^ . • . « 4 /
•
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• «•« • • • t •
I • « • •
£ * ••
TOT
Q
N n J Id . ID I" D DI
B
PERCENT DRERNIC CRRBDN
Figure 35. Sulfur versus organic carbon, all samples.
95
-------
TABLE XII: CORRELATIONS BETWEEN SULFUR AND OTHER SEDIMENTOLOGICAL FACTORS, BAYWIDE AND
BY SEGMENT
Segment
% Organic
Carbon
% Sand
% Silt
% Clay
% Mud
Mean
Grain Size
Sorting
Water
Depth
% Wai
1 & 2
0.22
-0.41
0.58
0.14
0.42
0.30
0.11
0.56
0.24
3
0.17
-0.42
0.30
0.46
0.42
0.44
0.22
0.16
0.35
1-3
0.14
-0.42
0.33
0.39
0.42
0.42
0.17
0.27
0.33
4
0.69
-0.74
0.43
0.79
0.74
0.78
0.30
0.34
0.69
5
0.91
-0.91
0.84
0.91
0.90
0.91
0.71
0.36
0.89
4-5
(middle Bay)
0.79
-0.82
0.64
0.85
0.82
0.84
0.52
0.34
0.78
1-5
(mainstem)
0.36
-0.68
0.42
0.75
0.68
0.72
0.41
0.42
0.67
Eastern Bay
0.94
-0.77
0.50
0.80
0.77
0.78
0.59
0.66
0.80
Choptank River 0.59
-0.57
0.42
0.70
0.57
0.64
0.41
0.50
0.66
Baywide
0.37
-0.67
0.39
0.75
0.67
0.72
0.41
0.42
0.67
-------
2.0
o:
~
L
j
D
in
h
z
u
V
q:
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(L
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I . 0
0.5
• •
• • • «.
• » | • * •
• ••
« •
« •
1 I :-H 1 h
"ETTET
Q
NPlJUUhOlin
B
PERCENT DRGRNIC CRRBDN
Figure 36. Sulfur versus organic carbon, Segments 1-3.
97
-------
buoyant to remain suspended indefinitely in the water column, since
vertical upward mixing velocities exceed the settling velocities of these
particles (Schubel, 1971). Only by aggregation - effectively enlarging
suspended particles - are fine sediments deposited. Flocculatton and/or
ingestion/expulsion by filter-feeding organisms result in the clustering
of clay particles, organic detritus, and microorganisms. Thus, by the
very nature of the processes responsible for deposition of fine sediments,
food 1s made available to the microorganisms residing in them.
Theoretical considerations might lead one to postulate a stronger
correlation between sulfur and organic carbon than between sulfur and
percent clay. Organic carbon, after all, supports microbial life. This
discrepancy may be explained, in part, by the fact that organic carbon
is consumed in the transformation of sulfate to sulfide. The amount of
organic matter originally deposited in the sediment is unknown, as are the
rates of its consumption and replenishment. Alternatively, not all of
the organic carbon present may be utilized by sulfate-reducing bacteria,
which require short-chain organic molecules. In measuring organic carbon,
no distinction is made between reactive and non-reactive compounds.
There are two distinct sulfur regimes in the Maryland portion of
Chesapeake Bay - revealed both in map pattern and in the statistical
characterization of the sediments. The reactions comprising the sulfur
cycle proceed readily in the sedimentary environments of the middle Bay.
All of the necessary components - sulfate, reactive organic carbon, and
iron - are available. Bottom water is sufficiently saline so that the
rate of sulfate reduction is unaffected by sulfate concentration. The redox
conditions here promote the formation of iron sulfides. Furthermore,
smaller average grain size favors the development of anoxic conditions. In
contrast, the requirement? for sulfate reduction are not satisfied in the
northern Bay. Sulfate concentrations are significantly lower due to the
influx of fresh water from the Susquehanna River. Consequently, the amount
of sulfur found there is considerably lower.
The concentration of sulfate in bottom water diminishes as salinity
decreases from the mouth to the head of the Bay. Figure 37, taken from
Pritchard, shows average summer salinity (ppt) along a vertical section
through the axis of the Bay. The dashed line at latitude 39°03' separates
segments 1-3 from 4-5. Bottom water salinity is 15 ppt at that latitude,
varying little seasonally.
The rate of sulfate reduction is independent of sulfate concentration
at concentrations greater than 10 milllmoles (mM). Below that amount, the
growth of sulfate-reducing bacteria is restricted, limited by the availa-
bility of sulfate. This, in turn, controls the rate at which sulfate is
converted to sulfide (Postgate, 1951; Goldhaber and Kaplan, 1975). The
salinity of seawater corresponding to a concentration to 10 mM sulfate,
assuming simple dilution, is 12.7 ppt. The calculated salinity nearly
coincides with measured salinity (15 ppt) at the boundary. Thus, lower
levels of sulfur in upper Bay sediments may be due, in part, to insufficient
sulfate to satisfy the respiratory needs of sulfate-reducing bacteria.
98
-------
SALINITY
SPRING AVERAGE-VERTICAL SECTION ALONG AXIS OF BAY
Figure 37. Salinity distribution in Chesapeake Bay.
-------
Another factor contributing to the lower concentration of sulfur in
northern Bay sediments may be the nature of the organic matter available
for decomposition. Terrestrial inputs of organic matter are highest in
the northern Bay (Hunt, 1966; Spiker, 1982). Goldhaber and Kaplan (1975)
suggest that land-derived organic matter may not be as easily assimilated
by bacteria as plankton-derived detritus. Preliminary efforts to assess
the availability of "reactive" carbon in the northern Bay indicate that it
is probably not a limiting factor in sulfate reduction (see section on
Carbon distribution). The mean "reactive" carbon concentration of 1.9
percent is in perfect agreement with the baywide mean. Admittedly, the
anomalously high organic carbon concentrations in northern Bay sediments
Reflect the high influx of land-derived organic detritus from the Susque-
hanna River, but inputs from marine sources are also very high here
(Flemer, 1970) and cannot be dismissed.
Finally, redox conditions in upper Bay sediments may dictate that
the iron present combine preferentially with carbonate or phosphate
rather than with sulfide. Bray and others (1973) found evidence of in
situ formation of siderlte (FeC03) and vivianite (Fe3(P0lt)) in Bay sed-
iments. If the sulfide produced by sulfate reduction does not combine
with iron, it reverts to sulfate through the intervention of aerobic
bacteria. No sulfide was detected in the pore water extracted from cores
collected north of the Bay bridge (Hill and Conkwright, 1981). Hence,
though sulfide might be produced, it does not survive in a measurable form.
HEAVY MINERAL VARIABILITY
Heavy mineral suites in nearshore sandy sediments have been extensively
studied with the aim to provide insight into the provenance of the sediment
and hydraulic conditions, subsequently leading to determination of sediment
transport and pathways. In the initial stages of this study, the heavy
minerals suites were identified for the nearshore sands to assess the
regional and local variability as a possible tool in sediment transport
studies.
Separation and identification of the heavy mineral assemblages in
representative sands reveal that while the mineral constituents of all
assemblages are similar, the relative proportions of each constituent
vary significantly. To gain familiarity with the variety of heavy minerals
present and to assess the regional variability in the non-opaque fraction,
the heavy minerals were separated from 22 beach samples (Figure 38) and the
relative proportion of each mineral constituent determined. Forty minerals
were identified; six of these were opaque species. The results for Western
and Eastern Shore beaches were tabluated separately (Tables XIII and XIV).
The heavy mineral fraction of fifty additional sand samples were also
examined to assess the effects of local source variability and processes on
the percentages of various mineral species. These samples were collected
from 9 transects located along an 11 kilometer stretch of shoreline on Kent
Island (Figure 39). Percentages of each mineral species, percent of non-
opaque grains, and the weight percent of the heavy mineral fraction for
each sample are shwon in Appendix III. The percentages of mineral species
100
-------
Figure 38. Heavy mineral locations, regional variability study.
101
-------
co
Uj
Ul
a
""l/i
09
Uj
3:
i
O
^v.J-
«?
Figure 39. Heavy mineral locations, local variability study.
102
-------
within each sample do not always sum to 100 because species with low
percentages and highly altered opaque grains are not listed in the table.
Tourmaline species were divided into two major subgroups based on informa-
tion derived from the regional variability study.
The high degree of local variability demands that some attention be
paid to the importance of hydraulic fractionation processes in controlling
the distribution of heavy minerals within a sediment population. Conse-
quently, the percentage of certain minerals in the non-opaque suite was
plotted against mean grain size of the sample to determine whether these
two variables were directly related. Mean grain size for this test was
adjusted to include only those size classes examined for the heavy mineral
study (1 to 4 phi). Additionally, the percentage of certain species in
the non-opaque assemblage was plotted against the percent of opaque grains
within the heavy mineral fraction. This provided some measure of the
dependence of species variability on specific gravity of individual mineral
constituents (Table XVI).
Regional Variability
While each sample contained the same suite of heavy minerals, the
relative proportion of each component varied considerably with most of the
common species ranging from a few percent to as high as 57 percent. The
Coastal Plain sediments surrounding the Bay have been subjected to a number
of cycles of erosion and deposition which have probably included some
regional transport. The resultant redistribution of sediments may have
modified or obliterated regionally distinctive assemblages or trends within
modern Bay sediments. Nonetheless, a few tentative observations can be
offered.
Along the west shore of the Bay (Table XIII) the sediments from Balti-
more County (BA8, BA10) northern Anne Arundel County (AA3), and, to a.lesser
extent, St. Mary's County (SM8, SM10) contain higher percentages of the
amphibole minerals (the hornblendes, and the tremoltte-actinolite series)
than the sediments from southern Anne Arundel (AA6, AA1) and Calvert
Counties (CA8, CA15, CA4). This may be explained by the proximity of
Baltimore County sediments to Piedmont source rocks and by the emergence
of the Potomac River, which drains the Piedmont Region, adjacent to St.
Mary's County. Baltimore County sediments contain high percentages of
kyanite as does one sediment sample from Kent County (KE 13), directly
across the Bay. The Wissahickon Schist lying to the northwest of Balti-
more County contains kyanite and may be an ultimate source of sediment to
these localities in Calvert County (CA8, CA15).
Sediments from eastern shore localities (Table XIV) show few systematic
changes in their heavy mineral content. Pleistocene sediments which mantle
the older Coastal Plain deposits in this area have been reworked by succes-
sive cycles of erosion and deposition. These processes have eliminated any
regional trends in mineral composition. Although not systematically record-
ed during the mineral counts, the distribution of the various tourmaline
species seem to show an overall trend. Indicolite is the dominant species
in sediments from the northern part of the Bay. It is gradually replaced
1 03
-------
TABLE XIII: PERCENTAGES OF SPECIES WITHIN THE NON-OPAQUE MINERAL ASSEMBLAGE
AND PERCENT OF NON-OPAQUE GRAINS WITHIN THE HEAVY MINERALS FOR
SAMPLES ON THE WESTERN SHORE OF THE BAY
Mineral
BA10
BA8
AA3 ¦
AA6
AA1
CA8
CA15
CA4
SM8
SM10
Staurolite
47
23
7
33
7
51
19
11
40
32
Tourmaline
10
13
30
27
30
14
25
17
12
15
Epidote
10
7 .
5
2
5
6
9
9
8
9
Zoisite
0
1
1
4
1
0
2
3
2
4
Kyanite
16
13
4
7
4
9
3
6
7
6
Sillimanite
1
2
2
2
2
1
9
4
3
5
Andalusite
0
0
0
2
0
0
0
1
0
0
Hornblende
6
13
5
0
5
0
0
2
6
1
Remolite-
Actinolite
2
7
4
0
4
1
0
1
2
1
Augite-
Diopside
3
2
1
0
1
0
1
0
2
1
Hypersthene-
Enstatite
0
0
2
2
2
0
0
1
1
0
Zi rcon
0
2
18
1
18
2
10
21
8
12
Garnet
2
2
3
2
3
7
8
2
2
3
Rutile
0
0
3
0
3
1
3
3
0
1
% non-opaque
grains
78
38
14
15
22
47
19
21
30
35
104
-------
TABLE XIV: PERCENTAGES OF SPECIES WITHIN THE NON-OPAQUE MINERAL ASSEMBLAGE
AND PERCENT OF NON-OPAQUE GRAINS WITHIN THE HEAVY MINERALS FOR
SAMPLES ON THE EASTERN SHORE OF THE BAY
Mineral
CE3
CE2
KE13
KE 10
KE2
QA10
QA11
TA1
DO 6
S03
S02
SO-JB
Staurolite
8
8
53
9
8
20
10
28
57
12
20
30
Tourmaline
34
21
9
27
19
12
23
22
11
24
21
18
Epidote
0
6
0
19
7
8
5
3
5
5
9
2
Zoisite
3
1
0
.6
5
4
3
2
0
6
4
2
Kyanite
1
4
18
2
3
2
7
14
3
3
10
7
Sillimanite
1
5
2
1
0
2
3
1
0
10
7
3
Andalusite
0
0
0
0
0
1
2
1
1
1
0
0
Hornblende
4
4
0
9
3
4
18
6
0.
6
3
6
Tremolite-
Actinolite
3
8
1
4
3
0
5
1
0
9
5
10
Augite-
Diopside
1
3
1
0
0
0
2
2
0
0
1
0
Hypersthene-
Enstatite
0
5
1
4
0
1
3
4
0
0
1
0
Zi rcon
7
11
2
2
27
26
1
0
3
7
3
3
Garnet
2
3
1
2
2
4
0
2
11
1
2
1
Rutile
1
3
0
0
3
4
1
1
1
0
1
0
% non-opaque
grains
6
16
50
50
27
21
62
27
-
47
53
69
105
-------
downbay by achlorite and dravite. In a few isolated samples, the tourmaline
consists predominantly of elaite.
Local Variability
The heavy mineral fraction generally constituted between a quarter of
one percent to 2 percent by weight of each sediment sample. A few contained
higher percentages, most notably, R66-G136 #1, in which heavy minerals
comprised 25 percent of the sediment. This sample was collected on the
beach foreshore from a heavy mineral lag deposit. Staurolite, the chlorite
and dravite species of tourmaline, epidote, hornblende and zircon were the
dominant non-opaque heavy minerals.
The variation of heavy mineral species within this nearshore zone was
quite large - as broad as the variability found in the Baywide samples.
Local variability may be large enough to mask regional populations or
trends. Local variability may be partly dependent upon very localized
sources for some mineral species, the relative enrichment of some species
in specific size fractions, or differential transport of species with
different densities.
Regression analysis of mean grain size versus mineral percentage
indicates that the distribution of minerals within heavy mineral assem-
blages are not directly related to grain size parameters. Staurolite and
epidote were the only minerals that showed any direct correlation with
grain size, r = -0.357 and r = 0.534 respectively and these correlations
are not strong (Table XV).
TABLE XV: REGRESSION ANALYSIS OF MEAN GRAIN SIZE VERSUS THE PERCENTAGE
OF MINERAL OBSERVED IN THE NON-OPAQUE SUITE. THE LINEAR
TREND WAS TESTED AT THE 5% LEVEL OF SIGNIFICANCE.
Mineral
r2
Significance
Equation
Staurolite
-.357
Yes
28.33-7.03 X
All Tourmalines
-.216
No
24.09-2.87 I
Epidote
.534
Yes
0.387+3.83 X
Hornblende
.198
No
5.62+2.04 X
Tremoli te-Acti nolite
.239
No
.86+1.09 K
ATI Amphiboles
. .266
No
6.30+3.21 X
All Pyroxenes
-.232
No
5.14-0.95 X
Zi rcon
.238
No
9.25+7.29 X
Garnet
-.152
Nd
7.16-1.08 X
The percentages of the various species in the non-opaque assemblage
were also tested against the percent of opaque grains within the heavy
106
-------
minerals (Table XVI). The opaque grains consist almost entirely of species
with specific gravities greater than 4.5 (ilmenite - 4.6 to 4.9; magnetite -
5.17). If density is a factor in determining the distribution of mineral
species, then those species with densities close to those of the opaque
grains should show a positive' correlation with the percent of opaque grains.
Those with the lowest densities should have an inverse correlation. The two
non-opaque mineral species with the highest and lowest specific gravities
(zircon and all tourmalines) show positive and negative correlation with
the percentages of opaque grains, with the linear trends tested at the 5%
level of significance. Staurolite, with an intermediate density, also
shows a strong negative correlation with the percent of opaque grains. This
apparent anomaly may be explained by the fact that, within any one sample,
the staurolite grains tend to be larger than the mean size of the heavy
minerals. They are, therefore, more easily entrained than the remainder of
the heavy minerals and tend to be removed. The denser opaque minerals
remain in place. The remainder of the non-opaque minerals do not show a
significant trend relative to the percent of opaque grains.
TABLE XVI: REGRESSION ANALYSIS OF PERCENT OF OPAQUE GRAINS WITHIN THE
SAMPLE VERSUS THE MINERAL PERCENT OBSERVED IN THE NON-OPAQUE
SUITE. THE LINEAR TREND WAS TESTED AT THE 5% LEVEL OF
SIGNIFICANCE.
Speci fic
Correlation
Significant
Regression
Mineral
Gravity
Coefficient
Linear Trend
1 Equation
All Amphiboles
2.9-3.5
-.015
No
13.93-0.01% opaques
Epidote
3.36
-.07
No
10.87-0.03T opaques
Garnet
3.5-4.3
.075
No
2.97+0.03% opaques
Pyroxenes
3.3
-.01
No
3.1-0.01% opaques
Staurolite
3.7
-.430
Yes
42.69-0.44% opaques
All Tourmalines
3.1
-.397
Yes
36.5-0.27% opaques
Zi rcon
4.6
.500
Yes
30.5+0.8% opaques
PATTERNS OF DEPOSITION AND EROSION
Detailed comparisons of time-separated bathymetric surveys were
constructed to delineate patterns of deposition and erosion. Surveys
separated by the greatest available timespan were compared to offset the
possibility of measuring short-term, yet large and erratic, changes caused
by episodic events. Inadequate density of available sounding data some-
times forced the comparison of more closely time-related surveys. Conse-
quently, time differentials in the comparison varied from around forty to
over one hundred years Baywide (Figure 40). To compensate for the patchwork
of comparisons required for total Baywide coverage, all comparisons were
proportionally adjusted to one hundred years for uniformity. The patterns
1 07
-------
Figure 40. Time differentials, bathymetric comparison.
108
-------
of deposition and erosion are displayed on Plates 6 and 7. Plate 6 delin-
eates areas of deposition, erosion and no change without regard to the
magnitude of change. Plate 7 only delineates those general areas where
deposition and erosion exceeds ±2.4 meters per century.
From Plates 6 and 7, several distinctive Baywide patterns are apparent.
The most notable features, best seen on Plate 6, are the rather extensive
erosional areas, in addition to the expected depositional ones. The Bay
has long been viewed as a depositional system, while erosion of the Bay
floor has been overlooked. The extent of these erosional areas suggests
that the Bay floor may serve as a significant source of sediment. A second
feature, shown on Plate 7, is that the areas of deposition and erosion
exceeding 2.4 meters per century are generally confined to the main axial
channel. Deposition here is.far more common than erosion. Thirdly, al-
though not as obvious as the first two features, the patterns at the con-
fluences of major rivers with the Chesapeake Bay are quite variable, a
mixture of deposition and erdsion. This is best illustrated on the
Chesapeake Bay atlases now in press. These very chaotic patterns at the
confluences seem to indicate hydraulic interaction between the river
systems and the main Bay, particularly at the mouths of the Patuxent and
Choptank Rivers. The exact nature of this interaction or process is not
known.
The patterns of deposition and erosion are not random occurrences;
rather they are associated with the geographic location and geomorphologic
character of Chesapeake Bay. In the northern Bay, Segment 1-3, the pattern
is predominantly depositional interspersed with areas of mixed patterns
and or erosion. Segment 1, the Susquehanna Flats, and the northern section
of Segment 2 (mouth of the Sassafras River) are depositional with high
deposition restricted to a channel exiting Segment 1. The sediments are
sand and sand-silt-clay (Plate 2). Though this sediment distribution
continues into Segment 2, patterns of deposition and erosion become
highly variable. In this section of Segment 2 the width of the Bay is
narrow, restricting circulation.
Dredging and overboard disposal, distinguished by linear zones of
deposition and erosion in the middle section of Segment 2, have complicated
the patterns of erosion and deposition. High erosion is confined to the
maintained channel leading to the Chesapeake-Delaware Canal. High depos-
ition marks the sites of overboard disposal. Initial dredging of the
channel commenced in 1937, a year before the completion of the bathymetric
survey.
Southward, Segment 2 broadens and the patterns become dominantly
depositional, linked with high deposition in the channel area. The depos-
itional pattern continues into Segment 3, particularly in the broad expanse
between the Patapsco and Chester Rivers. Net erosion in Segment 3 is
limited to the extreme southern sector, essentially the dredged Baltimore
Harbor entrance channel and to the nearshore platform surrounding Love
Point.
The northern Bay (Segment 1-3) is primarily a depositional environment
1 09
-------
complicated by dredging and overboard disposal activities. The sediments
become progressively finer, with sand and sand-silt-clay grading into
clayey -.11 ts in Segment 2 and; silty clays in Segnent 3. The continuation
of a net depositional pattern and progressive fining of the sediments, even
south of the turbidity maximum imply the direct influence of the Susquehanna
River on the entire northern Bay region, including Segment 3.
The net depositional pattern mapped for Segment 3 extends into the
northern section of Segment 4. The largest areal extent of high deposition
(Plate 7) and the greatest magnitude of net change lie within the axial
channel, along the length of Kent Island. Net deposition of approximately
10 meters has been measured. High erosion occurs locally within the
channel. The only area of net erosional change is west of the axial channel
as a narrow zone associated with a break in slope at the 10 meter contour.
The erosional pattern parallels the slope break, suggesting a modification
of the Bay bottom to an equilibrium configuration.
While the extreme northern section of Segment 4 1s dominated by
depositional patterns, erosion predominates in the southern section of
Segment 4. It is in this southern section of Segment 4, that a wide, near-
planar terrace between the 16 and 30 meter contours (Plate 1) is developed
between the shallow nearshore and the deep axial channel. Along this
terrace unit, net erosion is predominant. The linear zone of net erosion
in the northern section of Segment 4 is an extension of this terrace.
In the southern section of Segment 4, low deposition and erosion
characterize this stretch of the axial channel. High deposition and high
erosion are very limited in areal extent. The sediments in the channel
are silty clays with isolated areas of mixed sediments. Mixed sediments
are associated with areas of high deposition or high erosion, particularly
along the steep eastern channel wall. These mixed sediments probably
constitute a mixture of two distinct sediment types; coarse-grained
nearshore and fine-grained channel sediments.
Throughout Segment 4, net erosion of the inner nearshore appears to be
associated with shoreline erosion. A less extensive area of net deposition
lies immediately seaward of the net erosional pattern. Slaughter (1967)
suggested a correspondence between erosion of shoreline (fastland) and
changes in the nearshore for an area off of Tilghman Island, but concluded
that scour in front of a vertical protective structure by reflective wave
patterns was the apparent cause. Reflective wave patterns off of vertical
banks may be the same process producing net erosion of the nearshore.
Shoreline and associated nearshore changes are also seen in areas of high
deposition in the nearshore. Only two such areas were mapped, and both are
associated with major accretionary landforms - the cuspate forelands of
Flag Ponds and Cove Point.
The erosional trend seen in Segment 4 continues in Segment 5, parti-
cularly in the extreme northern section of Segment .5, where the Bay narrows
considerably. Just east of the Patuxent River mouth along the main axial
channel, an area of high deposition and high erosion is delineated (Plate
7). The sediments in the thalweg of the channel are silty clays with mixed
110
-------
sediments along either side (Plate 2). This sediment distribution suggests
mixing of the two dominant sediment types. The high erosion along the
channel wall recommends slumping as the mechanism that generates this patr
tern.
As Segment 5 stretches southward toward the Maryland and Virginia
border, the Bay widens and a net depositional pattern predominates- The
net depositional pattern continues into Virginia, particularly in the axial
channel. High depositional and high erosional areas are limited to the
axial channel and are directly adjacent to one another. The close proximity
of high deposition and erosion indicates mechanisms other than normal
estuarine sedimentation as possible sedimentation processes.
The nearshore zones display the same continuous patterns evident in
Segment 4. Along both the eastern and western shores, the inner nearshore
is characterized by net erosion, with net deposition immediately seaward.
This pattern is best illustrated along the Smith-Tangier Shields extending
into Virginia and terminating at Tangier Sound channel (Byrne et al., 1981).
Two major embayments, Eastern Bay and the mouth of the Choptank River,
differ with respect to both sediment types and patterns of deposition and
erosion. In Eastern Bay the dominant pattern is erosional, with localized
deposition. Areas of high deposition and erosion are located where the
Eastern Bay channel issues into the main Bay (Plate 1). Here, the steep
northern flank of the Eastern Bay channel outlines the nearshore platform
of Kent Island. Areas of high deposition on the platform and at the top of
the channel wall lie adjacent to areas of high erosion at the base and in
the thalweg. In the Choptank River, a net depositional pattern extends, from
the point where the river enters the broad expanse of the embayment,
throughout the channel area. Net erosion is prevalent along the shoreline
platform.
COMPARISONS WITH SUBBOTTOM PROFILES
During the course of this study, the Maryland Geological Survey con-
ducted a separate proejct investigating the subbottom characteristics of
Chesapeake Bay. Several cross-Bay transects in Segments 4 and 5, made with
a Raytheon RTT/1000 system with a 3,5/7.0 kilohertz transducer, provided a
cross-sectional profile of the near-surface subbottom, down to a depth of
approximately 10 meters. Subbottom profiles were compared to detailed
erosion-deposition maps in an effort to improve our understanding of sed-
iment distribution patterns in the Bay. This comparison showed that in
several cases there was good aqreement between the interpretation of the
subbottom profiles and the deposition-irosion data. However, just as
often, the comparisons were inconclusive. Agreement was best where the
subbottom profiles intersected areas of dramatic depositional or erosional
change. The following discussion examines three cases where interpretations
drawn from the erosion-deposition maps are supported by information from
the subbottom profiles. The results of these comparisons are preliminary
and should be regarded as such.
Ill
-------
Figure 41 shows the general location of the two areas of comparison.
All are near the eastern flank of the main Bay axial channel. These will
be discussed from north to south.
The first area is located on the Kent Island platform. Here the gently
sloping nearshore is bisected by a narrow trough. Bathymetric comparison
data indicate that this trough is an area of high deposition. Where the
subbottom profile crosses the trough, a relict channel is infilled with
sediment (Figure 42).
Offshore of Poplar Island, bathymetric comparisons show that the
upper slope of the eastern channel waTl is a site of high deposition, while
the base of this slope is highly erosional (Figure 43). The corresponding
subbottom profile shows a slight break in slope near the crest of the wall.
Downslope of this break, the bulging surface configuration overlies a
steeper sedimentary horizon, possibly the former channel wall. Deposition-
erosion data suggest that the base of the slope is erosional, but the
seismic profile provides no evidence for this. Nevertheless, the break in
slope, combined with its bulging slope configuration, are suggestive of a
depositional sequence as indicated by the deposition-erosion data.
Immediately southeast of the Patuxent River mouth, high deposition in
the axial channel is associated with high erosion along the channel wall.
A slump scar (Figure 44A) suggests that the high erosion along the channel
wall is the result of slope failure. The scar lies above a buried horizon,
possibly the infilled channel. Thus, channel infilling as seen on the
seismic record corroborates the high deposition mapped in this area. The
slope configuration in the second transect (Figure 44B) also indicates that
slumping occurred along the channel wall.
None of the evidence thus far discussed is conclusive, but, if used
cautiously, this kind of information can help to explain sediment distribu-
tion patterns. Erosion-deposition data provide a measure of the mobility
of the sediments. Seismic profiles provide clues to the mechanisms involved
in transport; e.g., slumping, infilling, and undercutting. When this type
of information is related to sediment type, a clearer picture of sediment
distribution may develop. For example, in Segment 4 this type of comparison
was attempted for a small area between Tilghman Island and Holland Point,
where large areas of clay-rich sediment occur in relatively shallow water.
According to the erosion-deposition map, this area is an erosional plat-
form. Seismic records explain this unlikely combination. An indurated,
pre-Holocene surface is exposed here.
Pb210 Sedimentation Rates Versus Bathymetric Comparison
In the course of the EPA-Bay Program studies, estimates of sedimenta-
tion rates were obtained by applying Pb210 techniques (Helz, 1981) to
selected cores and by the method of bathymetric comparisons. Care must be
exercised in judging the comparison, since both estimates have particular,
and different, limitations,v The comparison is given in Table XVII for
nine core locations proceeding down the Bay (see Figure 41 for location).
Since any given core location might not fall exactly in a six second grid
112
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General location of
areas where subbottom
profiles were compared
to erosion-deposition
maps, Pb210 derived
sedimentation, rates.
Numbers are core
designation and loca-
tion for Pb210
measurements.
-------
Figure 42. Seismic profile showing infilled channel. Stippled pattern
is deposition; dashed line is erosion.
114
-------
Figure 43. Seismic profile showing unstable slope configuration over
buried sedimentary horizon. Stippled pattern is deposition;
dashed line is erosion.
115
-------
Figure 44A. Seismic profile showing slump scar on the east wall of the axial channel.
-------
•7*30rV "jj QpPPH
Figure 44B. Seismic profile showing unstable slope configuration on
east wall of the axial channel.
117
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TABLE XVII: COMPARISON OF SEDIMENTATION RATES, BATHYMETRIC COMPARISON
VERSUS Pb210
Sedimentation Rate averaged within:
4 nearest
time between
Pb210 rate
Core
six second % km \ km
surveys
(from G. Helz
Number
cells radius radius
(years)
1981)
4
1.21
cm/yr
1.33
cm/yr
1.24
cm/yr
40
0.31
cm/yr
6
-.42
cm/yr
-.21
cm/yr
-.23
cm/yr
52
0.87
cm/yr
14
-.31
cm/yr
-.21
cm/yr
.79
cm/yr
97
0.07
cm/yr
18
.58
cm/yr
.31
cm/yr
.42
cm/yr
98
0.36
cm/yr
24
0
0
-.20
cm/yr
106
1.22
cm/yr
55
1.50
cm/yr
1.11
cm/yr
1.40
cm/yr
30
1.79
cm/yr
60
.03
cm/yr
.10
cm/yr
.10
cm/yr
98
0.13
cm/yr
62
1.82
cm/yr
1.82
cm/yr
2.10
cm/yr
98
1.26
cm/yr
63
.27
cm/yr
.27
cm/yr
.14
cm/yr
94
0.66
cm/yr
having comparative depth values, three different integration areas were
used. The most critical of these is the h km radius circle. The direction
of the change is consistent for six of the nine comparisons; In three cases
the bathymetric changes indicate erosion; whereas, the Pb210 indicates
deposition. Some of these differences have reasonable explanations. Core 6
is very near, if not in, a spoil disposal area for Baltimore Harbor approach
channels. The bathymetric comparisons are for the period 1845-1897 (52
years), and the first major channel deepening occurred in 1881. The dif-
ference could thus be due to disposal either in 1881 or later. There is
very little bathymetric data in the vicinity of Core 24. While there is no
apparent explanation for the discrepancy at Core 14, the sign of the bathy-
metric change switches from minus to plus as the integration area increases.
Moreover, the Pb210 value itself is very low (0.07 cm/yr). Core 4 is loca-
ted near the approach channel to the C & 0 Canal and the Pooles Island
overboard disposal area. Dredging and spoil disposal in the region may
have influenced the results.
Aside from the core locations discussed above, the correspondence
between the estimates range from a factor of 1.2 to 5. The length of time
covered by a bathymetric comparison seems to correspond directly to the
118
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degree of similarity between the bathymetric comparisons and Pb210 tech-
niques. Cores 18, 60, 62, and 63 all give results compatible with Pb210
findings, spanning an average of 96 years. Core 24, with a 106-year span,
doesn't relate well at all, though this is most probably due to Insufficient
data density in the comparison. Core 55 matches well with Pb210 data,
though only spanning 30 years.
SEDIMENT BUDGET
Any attempt to construct and reconcile a sediment budget for the
Maryland portion of Chesapeake Bay requires an evaluation of existing
sources of sediments to the Bay and this development of the patterns of
changes (depositional or erosional). The sources of sediments to the Bay
vary but can be summarized as follows:
1) Fluvial, mainly suspended sediments from the Susquehanna River
and other tributaries;
2) Shoreline erosion;
3) Primary production, including carbonate shell formation;
4) Atmospheric, dry fall;
5) Bay floor, through erosional processes or dredging procedures;
6) Man modification, placement of dredged spoil from outside the
Main Bay system, and;
7) Northward transport from the Virginia section of the Bay.
Existing information provided the basic framework to evaluate the contribu-
tion provided by each of these sources and, when necessary, new calculations
of the contributions were made, specifically in the case of shoreline
erosion. Although this list constitutes the major sources of sediment to
the Bay, information is not always available to thoroughly evaluate all
of these. Detailed information on the input of atmospheric dry fall is
inadequate. Primary production, particularly the formation of shell
deposits, and northward transport of sediment from Virginia needs further
investi gation.
Although the organic constituents of the suspended sediments trans-
ported into the Bay may constitute as much as 45£ of the total suspended
system (Biggs, 1970; Biggs and Flema, 1972; Yarbo et al., 1981), the ensuing
discussion is limited to the inorganic fraction of the total sediment
system. Wherever possible the organic fraction of the sediments were
subtracted out.
119
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SEDIMENT SOURCES
Fluvial Sediments
In previous discussions of fluvial sources of sediments to Chesapeake
Bay, the input of the suspended sediment load of the Susquehanna River to
the northern Bay has been emphasized. Since the Susquehanna River is the
dominant source of fresh water to the Bay, it is reasonable to expect that
the corresponding suspended sediment load would also be high.
The importance of the remaining tributaries as sources of fluvial
sediment remains a subject of debate. Are the lesser tributaries sources
or sinks of sediments? For example, Yarbo and others (1981) cite the
Choptank River as a significant source of sediment to the middle Bay.
However, Schubel and Carter (1976) and Palmer (1972) suggest that the
tributaries are sinks for Bay-derived sediments. The question of the flux
of sediment, both suspended and bedload, across the tributary mouths remains
to be investigated.
The suspended sediment discharge of the Susquehanna River has received
considerable attention during the past fifteen years. Various estimates of
the mass of sediment discharge for the Susquehanna and other tributaries
are available. These are summarized in Table XVIII and represent average
discharge under 'normal' flow conditions. Schubel and Zabawa (1974)
documented sediment discharge during a major flood event, tropical storm
Agnes in 1972, reporting discharges of 30xl06 metric tons. Thus, storm
conditions and flooding may represent a major contribution. Major flood
events in the 1930's may have been major contributors during the period
of the bathymetric comparisons. Unfortunately, estimates of these dis-
charges are not available.
The construction of the Safe Harbor, Holtwood, and Conowingo dams
along the Susquehanna River during the period of the bathymetric compari-
sons demands some consideration. The dams, their construction dates and
locations are as follows (Williams and Reed, 1972):
1) Safe Harbor 1931 33 miles upstream
2) Holtwood 1910 25 miles upstream
3) Conowingo 1928 10 miles upstream
Schubel and Carter (1976) evaluated the trapping efficiency of these dams.
However, within the time period of the bathymetric comparisons, the
effectiveness of the dams was not considered.
Shoreline Erosion
Schubel (1968) and Biggs (1970) provide estimates of the sediment
contribution from shoreline erosion for the northern Bay and middle Bay,
respectively (Table XIX). Recalculation of the volume and mass of sediments
120
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TABLE XVIII: VARIOUS ESTIMATES OF SUSPENDED SEDIMENT DISCHARGE
Investigations
Discharge (metric tons/year)
Schubel (1968)
Biggs (1970)
Will aims and Reed (1972)
Schubel (1972)
Palmer, Schubel and Cronin (1975)
Schubel and Carter (1976)
Schubel and Carter (1976)
Yarbo et al. (1981)
Biggs (1970)
0.6x10s
0.46xl06 (0.084xl06 tons/yr Corg)
1.8xl06
0.3xl06
0.8xl06
1.07x106
0.16x106*
41xl03+ (includes organic matter)
10.2x106
§
Estimates of sediment deposited into tributaries
^Estimates of seston from Choptank River to Bay
Estimates of primary production (ash weight) in upper and middle Bay
TABLE XIX: ESTIMATES OF INPUT OF SEDIMENT FROM SHORELINE EROSION FOR MAIN
CHESAPEAKE BAY (EXCLUDING EASTERN BAY AND CHOPTANK RIVER)
Area
Total
(mtons/yr)
Sand
(mtons/yr)
Silt and Clay
(mtons/yr)
Schubel (1968)
northern Bay
0.3xl06
0.18xl06
0.12xl06
Biggs (1970)
middle Bay
1.3x106
1.02xl06
0.28xl06
Combined Schubel
and Biggs
Maryland Bay
1.6xl06
1.2x106
0.4x106
This study
expanded
Maryland Bay
2.11x106
0.74xl06
1.37x106
121
-------
contributed by shoreline erosion to the Maryland section of the Bay was
required because this study included the shorelines of the Susquehanna
Flats, Eastern Bay and the Choptank River estuary. Figure 45 shows the
shoreline area covered by Schubel (1968), Biggs (1970) and this study.
The mass of sediments provided by shoreline erosion calculated from
this study was 2.11x10s mtons/yr. The sand fraction was calculated at
0.74xl0e mtons/yr and the silt-clay fraction at 1.37xl06 mtons/yr.
Table XX lists the mass contributions for contiguous reaches of shore-
line. It should be noted that these values and subsequent values of mass
will be reported as mtons/century. This is done to provide units compar-
able to those derived for the bathymetric comparison.
TABLE XX: MASS OF SHORELINE SEDIMENT ERODED BY SEGMENT (xlO6 mtons/century)
Net
Sand
Silt-Clay
Segment 2
39.0
20.8
18.2
Segment 3,
16.7
6.4
10.3
Segment 4
92.2
34.8
57.4
Segment 5
62.8
12.0
50.8
Total
210.7
74.0
136.7
~Segment 1
8.1
4.5
3.6
~Eastern Bay
14.6
5.8
8.8
~Choptank River
15.5
2.9
12.6
~Not included in sediment budget
Dredging and Overboard Emplacement
An accurate assessment of the amount of sediment contributed or removed
by dredging and overboard emplacement operations, particularly in the
northern Bay, is difficult. Historical dredging records are scant; thus,
an element of conjecture enters any attempt to evaluate the amount of
material dredged and the amount and location of sediment emplaced. The
dredging projects of particular importance to the sediment budget are the
Baltimore Harbor approach and connecting channels and the approach channel
to the Chesapeake-Delaware Canal. Schubel and Williams (1976) examined
122
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Figure 45. Area of shoreline erosion
123
-------
historical records and estimated the total volume'of sediment dredged from
these channels and disposed of overboard. The geographic location of the
dredging areas coincides with the channels themselves, but sites of dredged
spoil emplacement are much more difficult to identify. It seems likely
that emplacement occurred in the northern Bay, particularly in Segments 2
and 3. The volumes indicated in Table XXI correspond to the time periods
of the bathymetrlc comparison in the northern Bay, for Segment 2 in 1938,
and for Segment 3 in 1933. Overboard disposal in the Kent Island dump site
in the southern section of Segment 3 could not be estimated. Removal of
sediment by dredging should show up as net erosion on the bathymetrtc
comparison; whereas, overboard disposal would result in net deposition. An
overlay of the dredged channels with the patterns of deposition and erosion
shows that high erosion is confined to the channels and that high deposition
occurs in areas designated as sites of overboard disposal.
TABLE XXI: ESTIMATED VOLUME (MASS) OF DREDGED SEDIMENTS. DATA FROM
SCHUBEL AND WILLIAMS, 1976. (ASSUME SEDIMENT MASS OF 0.45
mtoris/m3)
Dredged
Emplaced
Approach Channel to Chesapeake-
Delaware Canal
Segment 2
19xl0em3(8.5xl05mtons)
Baltimore Harbor Approach
Channel and Connecting Channel
Segment 3
Total for Segments
32xl0%3
32xl0%3
51xl0€m3(22.9xl06mtons) 32xl06m3
(14.4xl06fttons)
DEPOSITIONAL-EROSIONAL PATTERNS
Sediment Volume
The volumes of sediment deposited and eroded were calculated by
sediment type. The total surface area of the Maryland Bay measured in
these calculations was 2710 square kilometers, of which 1414 km2 (523)
was depositional, 1146 km2 (42%) was erosional, and 150 km2 \b%) showed no
measurable change. Volumetrically, the total accumulation of sediment for
a 100-year period was +901.3xl06 cubic meters, while erosion for the same
period was measured at -682.5xl06 cubic meters. Sand and silty clays
accounted for 84% of the total volumetric changes measured. Silty clays
alone constituted 45£ of the total volumetric change. Table XXII lists the
volumetric changes {+ and -) by sediment type.
1 24
-------
TABLE XXII: VOLUME OF SEDIMENTS DEPOSITED AND ERODED (xlO6)
PO
CJ1
Shepard
Class
Deposit Erosion
Volume Volume
cubic meters
No Change
Deposit
Area
square
Erosion
Area
meters
Average Thickness
Change (m/100 yrs)
+
Total
Volume
m3
Total
Area
+/-
*
Sand
302.21
-306.98
37.62
551.04
560.49
0.55
0.55
609.19
1111.
52
Clay
7.35
- 28.05
11.53
13.57
43.84
0.54
0.64
35.40
57.
41
SandSiltClay
55.13
- 30.48
17.24
95.35
56.62
0.58
0.54
83.61
151.
97
Clayey Sand
17.70
- 10.48
5.45
29.74
17.31
0.60
0.61
28.18
47.
05
Clayey Silt
64.51
- 13.78
7.32
86.81
26.74
0.74
0.52
78.28
113.
55
Silty Clay
427.29
-287.19
70.18
596.33
394.76
0.72
0.73
714.47
991.
08
Si 1ty Sand
11.33
- 4.01
0.04
17.12
6.75
0.66
0.59
15.34
23.
88
Sandy Clay
15.79
- 1.57
0.14
23.75
39.23
0.66
0.04
17.36
62.
98
Total
901.31
-682.54
149.52
1413.71
1145.74
1584.82
2559.
44
*Does not include No Change areas
-------
The average thickness of change per 100 years (volume/surface area) for
all sediment types was .64 meters/100 years (.64 cm/yr) deposition and
-.59 meters/100 years (-.59 cm/yr) erosion, or a net depositional change of
.05 cm/year. "Sedimentation rates" based on Pb210 (Helz et al., 1981)
compare favorably with those provided by the sediment budget analysis. The
average mean sedimentation rate derived from Pb210 analyses is .71 cm/year;
whereas, the mean thickness of deposition is .64 cm/year. These values are
comparable because rates based on Pb210 cannot take erosion into account.
The mean thickness of change includes the thickness changes in both fine-
grained and coarse-grained sediments. Pb210 measurements were generally
restricted to fine-grained sediments. The mean thickness change for only
fine-grained sediments (silty clays) was figured at .72 cm/year.
Sediment Mass
Using the procedures described earlier, sedimentary volumes were
converted to mass, reported as million metric tons per century. For the
Maryland section of Chesapeake Bay, excluding Eastern Bay and the Choptank
River, which is excluded throughout the entire discussion, the mass of the
inorganic sediment fraction deposited was +747.15xl06 mtons/century, with.
-635.56xl06 mtons/century eroded from the Bay floor. The net or residual
indicates deposition of +111.59xl06 mtons/century. Based on the distribu-
tion of sediments, the net mass of the inorganic sediment fraction breaks
down to +27.28xl06 mtons/century of sand, +44.62xl06 mtons/century of silt,
and +39.69xl06 mtons/century of clay (Table XXIII).
TABLE XXIII: TOTAL MASS OF SEDIMENTS DEPOSITED AND ERODED (106 mtons/
century)
North Latitude
South Latitude
39.2630
37.5300
Total
Org
Inorg
Sand
Silt
Clay
Deposition
+764.17
+17.02
+747.15
+473.86
+117.10
+156.21
Erosion
-646.82
-11.25
-635 .'5 6
-446.58
- 72.48
-116.51
Net
+117.35
+ 5.76
+111.59
+ 27.28
+ 44.62
+ 39.69
Although this is the first attempt to use the sediment record to
determine the mass of sediments deposited in the Maryland portion of the
Bay, two other studies have provided estimates of sediment mass using
another technique (Biggs, 1970, and Schubel and Carter, 1976). Both
studies calculated sediment mass by balancing the suspended sediments and
sediment sources. By subtracting the masses of the various components of
126
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the suspended sediments, the investigators reasoned that the residual ,
indicated deposition of the f1ne-gra1ned, inorganic fraction. Extrapolation
of these estimates over 100 years yields the following figures: +83.8xl06
mtons/century, from Biggs, and +141xl06 mtons/century, from Schubel and
Carter (fable XXIV). Schubel and Carter's original data included the
TABLE XXIV: COMPARISON OF SILT-CLAY MASS
Biggs (1970)
Schubel-Carter (1976)
This study - Net
This study - Total
Byrne (Virginia) - Net
+83.8xl06 mtons/century*-
+141.0xl06 mtons/century*
+84.31xl06 mtons/century
+273.3xl06 mtons/century
+490xl06 mtons/century
~Includes shoreline erosion contributions
Virginia portion of the Bay. By using their reasoning for input-output,
+141xl06 mtons/century was estimated for only the Maryland portion of the
Bay. For this study, the net mass of silt-clay deposited on the Bay floor
was calculated to be +84.31 mtons/century. Because the suspended sediment
approach cannot take erosion of silt-clay from the Bay floor Into account,
the total mass of silt-clay accumulation in the Maryland Bay is considerably
higher than the suspended sediment estimates; i.e., +273.3xl06 mtons/century
versus +141xl06 mtons/century of Schubel and Carter (1976).
Sediment Mass Versus Depth
Four sediment components; total organic sand, silt, and clay were
plotted against water depth (Figure 46). In the nearshore areas, between
the 0 and 4 meters interval, the erosional mass of the inorganic fraction
exceeded the depositional mass resulting in a net erosional mass of -39.97
xlO6 mtons/century. Between the 4 and 8 meter interval, a net depositional
mass (+37.23xl06 mtons/century) nearly balanced the net erosional mass of
the 0-4 meters interval. Deeper than 8 meters, the net mass was deposition-
al, with the greatest net change in mass between the 8-12 Intervals. Below
12 meters the total net depositional mass was less than recorded for the
8-12 meter interval.
Closer examination of the total inorganic fraction shows that the net
erosional mass in the 0 to 4 meter interval consists primarily of the net
erosion of sand (-43.89xl06 mtons/century), offset slightly by the net
deposition of silt and clay (+3.91xl06 mtons/century). In the 4 to 8 meter
intervals, the depositional-erosional mass of sand nearly balanced (+230.39
versus -230.48xl06 mtons/century) while the silt and clay showed a net
depositional mass of +37.31xl06 mtons/century. The net erosional mass of
the total inorganic fraction (-2.75xl06 mtons/century) for the 0 to 8 meter
interval consisted of net erosional mass of sand In the 0 to 4 meter
127
-------
MASS
(10 mtona/cantury)
128
-------
interval with a net depositional mass of silt and clay in the 4 to 8 meter
intervals. The net depositional mass of the silt and clay in the 4 to 8
meter interval reflects the deposition in Segment 2 and 3.
The net erosion of sand between 0 and 8 meters depth is offset bv net
deposition in the 8 to 12 depth interval, -43.97xl05 versus +50.07xl(P
mtons/century, respectively, resulting in a net depositional mass of sand
of +6.Q9xl06 mtons/century. The greatest concentrations of naturally
forming oyster and clam beds are located at depths shallower than 12 meters.
Shell beds, particularly oyster beds, effectively reduce the total mass of
sand, both erosional and depositional. How shell beds figure into esti-
mates of sediment is still uncertain. In Virginia, Byrne and others (1981)
allowed for a 10% shell contribution but did not state whether this compen-
sation was applied to the total sediment mass or only to the net mass of
sand. Shell concentration and density per unit area is currently being
investigated by the state of Maryland.
In depths greater than 12 meters, the net mass of sand diminishes
approaching zero at depths greater than 30 meters. Correspondingly, there
is a net depositional mass of silt and clay. It is interesting to note,
that the net mass of silt decreases from a net depositional mass of +6.19
xlO6 mtons/century within the 12 to 16 meter interval to +3.03xl06 mtons/
century at depths between 20 and 30 meters. In the same depth intervals,
the net depositional mass of clay increases with depth from +2.57xl06 mtons/
century to +7.34xl06 mtons/century at 16-20 meters and to +5.25xl06 mtons/
century at the 20-30 meter interval.
SEDIMENT MASS BY SEGMENTS
In the balancing of terms - sources and sinks - the mass of sediment
introduced to the Maryland section of the Bay exceeds the mass accumulation
of sediment determined from bathymetric comparisons. More sand and silt/
clay are introduced into the Bay than are deposited. Bay floor erosion
proved to be a significant source of sediment to the Bay. Without its
inclusion in the sediment budget, the mass accumulation in the Bay would
exceed the contributions from the other sources; i.e. Susquehanna River,
shoreline erosion, and primary production.
Table XXV lists the mass terms for the Maryland section of the Bay.
For the sand fraction the total mass of sand contributed from the various
sources was calculated at 521xl06 mtons/century; whereas, the total mass
accumulation was 474.11xl06 mtons/century. This represents a deficit of
sand accumulation of -47xl06 mtons/century. In the silt/clay fraction,
the total mass of sediment introduced was 287xl06 mtons/century with a
calculated total mass accumulation of 273xl06 mtons/century. These terms
result in a deficit of silt/clay deposited of -114.2xl06 mtons/century.
Baywide, the data show a net deficit of sediment deposited throughout
Maryland. It is unlikely that sand is transported across tributary mouths.
Rather, it is confined to contiguous reaches of shoreline. Therefore, a
balancing of terms for sand must be within these reaches of shoreline,
hereby described by the Bay segmentation scheme. The silt/clay fraction
129
-------
can be transported as suspended sediments throughout the Bay system via
estuarlne circulation. The silt/clay sediment mass will also be discussed
by Bay segments.
TABLE XXV: BALANCE SHEET FOR SEDIMENT MASS
Sources
Sinks
+/- (mtons/century)
Sand
Silt/Clay
521xl06
387x106
473xl06
273xl06
- 47xl06
-114xl06
For northern Bay Segments 2 and 3, the mass data was corrected to
compensate for dredging and overboard disposal activities. Because it
was difficult to gather information on the percentage of sand sized material
involved in the dredging activities, it was assumed that all dredging
activities involved the silt/clay sediment types. No adjustments were made
to the sand fraction.
Mass-Sand
In Segment 2, the mass of sand from the sources balances the mass of
sand accumulated on the Bay floor. From the various sources, 41.9xl06
mtons/century were introduced to Segment 2, with 41.5xl06 mtons/century
measured as accumulation, for a deficit of sand deposited of only -0.4xl0e
mtons/century.
In Segment 3, the data show a surplus of sand deposited of +13.0xl06
mtons/century; 32.9xl06 mtons/century was contributed from the various
sources while accumulation was measured at +45.8x106 mtons/century. The
surplus of sand deposited cannot be explained by regional transport and
must, therefore, be due to an unexplained localized source, possibly sand
contributed by dredging activities, not taken into account previously.
Secondly, the net bathymetric changes, affected by shell beds, may result
in the overestimation of the mass of sand accumulation relative to the
actual deposition of sand.
Segment 4 is described as net erosional in the comparative analysis.
This interpretation is supported by mass of sand. The sources of sand
contributed a total mass of 249.4xl06 mtons/century which included 34.8xl06
mtons/century from shoreline erosion and 214.4xl06 tons/century eroded from
the Bay floor. The total mass of sand accumulation was meausred at 130.3
xlO6 mtons/century; a net deficit of -119.1xl06 mtons/century. Approxi-
mately 525K of the sand Introduced into Segment 4 was deposited on the Bay
floor.
130
-------
For Segment 5, the data show a net surplus of sand accumulation. The
sand mass from the sources was 196.4xl06 mtons/century, of which 184.4xl06
mtons/century were derived from Bay floor erosion. The net mass accumula-
tion of sand was calculated at 256.1xl06 mtons/century, with a net surplus
of sand accumulation of +59.7xl06mtons/century.
The net deficit in Segment 4 and the net surplus in Segment 5 suggest
transport of sand from Segment 4 to Segment 5. The boundary between
Segments 4 and 5 dividies the nearshore along a contiguous reach of shore-
line and not at a natural break, such as the confluence of the Patuxent
River with the main Bay. On the western shore, as shown by Plate 7, an
area of high sand deposition occurred at the boundary of Segments 4 and 5.
The mass of this deposit was recorded for Segment 5, but transport paths
indicated transport from Segment 4, around Cove Point. On the eastern
shore, at the boundary between segments, Plate 7 indicated a nearshore
erosional area at the boundary, changing into depositional areas north and
south of the segment boundary. Again, transport of sand from the erosional
area in Segment 4 to the depositional area in Segment 5 would add to the
net mass results calculated.
Mass - Silt/Clay
Similar to the net mass of sand, the net mass of the silt/clay showed
a net deficit of silt/clay accumulation on the Bay floor (Table XXV). The
sources, including Bay floor erosion, exceeded accumulation. In contrast to
sand, the silt/clay fraction is easily transported from segment to segment
as suspended sediment. A net deficit of silt/clay accumulation indicates
export from the Maryland portion of the Bay.
In Segments 2 and 3, silt/clay contribution was calculated at 137.8xl06
mtons/century, with a measured mass accumulation of 99.2x106 mtons/century.
Compensating for dredging activities (-8.5xl06 mtons), the resultant net
deficit of silt/clay accumulation was -30.3xl06 mtons/century. This deficit
seems to indicate that silt/clay bypass Segments 2 and 3 to other segments
of the Bay. Palmer (1972a) and Schubel and Carter (1976) suggest that the
Bay provides a source of sediments to the tributaries. An excess of silt/
clay contributed to the Bay may act as that source and accumulate in the
tributaries. It cannot be stated conclusively that this is the case. An
alternate explanation is that the silt/clay bypassing Segments 2 and 3 is
deposited in Segment 4. Some of the highest depositional changes found in
the Bay were reported in the upper portion of Segment 4, particularly in
the axial channel. It seems plausible that a portion of the excess silt/
clay was transported into the northern tributaries and downbay into Segment
4.
The majority of Segment 4 was erosional. The sources of silt/clay
contributed 160.9xl06 mtons/century, with accumulation calculated at
79.7xl06 mtons/century, for a net deficit silt/clay accumulation at
-81.2xl06 mtons/century. In Segment 5, the sources and sinks nearly
balanced; 96.9xl06 mtons/century were derived from the sources and 94.4xl06
mtons/century accumulated on the Bay floor, for a net deficit of silt/clay
accumulation at -2.5xl06 mtons/century. Combining the data from Segment 4,
131
-------
Segments 4 and 5 showed a net deficit of -83.2xl06 mtons/century. This,
combined with the net deficit from Segments 2 and 3, results in a total net
deficit of silt/clay accumulation of -113.5xl06 mtons/century.
In order to balance the terms, a total net deficit of silt/clay accum-
ulation requires export of silt/clay from the main stem of the Maryland
Chesapeake Bay. Two primary pathways for export of silt/clay are into the
tributaries (Patuxent and Potomac Rivers) and into Virginia. For the
Potomac River, Schubel and Carter (1976) estimated that 0.03xl06 mtons/year
were exported from the main Bay and deposited in the Potomac River. Recent-
ly, Knebel and others (1981) calculated the mass of silt/clay accumulation
at the Potomac River mouth at .154xl06 mtons/year or 15.4xl06 mtons/century.
This estimate is five times greater than that of Schubel and Carter's
(1976). It seems possible to derive the mass accumulation of silt/clay
at the Potomac River from the excess silt/clay calculated for the main Bay.
Byrne and others (1981) calculated a net surplus of silt/clay for
Virginia Chesapeake Bay at +490xl06 mtons/century, significantly greater
than the net deficit of silt/clay in Maryland Chesapeake Bay. An examina-
tion of Byrne's data from the area of the axial channel in the northern
Virginia Chesapeake Bay showed a net surplus of silt/clay accumulation at
+79.2xl06 mtons/century, within range of the net deficit in Maryland. The
net surplus in Virginia and the net deficit in Maryland indicates an export
of silt/clay from Maryland into Virginia Chesapeake Bay.
132
-------
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140
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APPENDIX I
The following programs are for conversion from geodetic coordinates
(latitude and longitude) based upon a Universal Transverse Mercator Projec-
tion to Raydist radionavigation locations (red and green lanes) and back
again. The programs are written for and were executed on a Hewlett-Packard
9825T desktop domputer. The programs utilize the geodetic locations.in
degrees and degree fractions in the calculations but are reported in
degrees, minutes, and seconds. The value constants refer to the Raydist
Net within which the sample is located with nets one and two covering the
Maryland portion of the Bay. Although net three falls entirely within the
Virginia portion of the Bay the constants are included here for complete
ness.
Geodetic to Raydist Conversion Program
ent "LATITUDE?", rl; fxd 6;prt "LATITUDE*", rl
1+C;gsb "IDMS"
ent "LONGITUDE?",r2;prt "LONGITUDE3",r2;spc 1
2+C;gsb "IDMS"
atn((rl9/rl4)tan(rl))-»-r3
rl4cos(r3)cos(r2)1000+r5
rl4cos(r3)sin(r2)1000»-r6
rl9sin(r3)1000->-r7
0->-r8^r9;10^X;5->-Y;gsb "R"
r&*r9
20+X;gsb "R"
r9-r8-»-r9
l&*Y;gsb "R"
r9+r8-»-r9
r9*rl3+A
141
-------
Geodetic to Raydist Conversion Program (cont.)
(Kr&>r9
15+X;5->-Y;gsb "R"
r8->-r9
25->-X;gsb "R"
r9-r8-*-r9
1&+Y;gsb "R"
r9+r8->r9
r9*rl3-*B
fxd 2;prt "RAYDIST RED=",A,1"RAYDIST GREEN=",B;spc 2
END
"R":(rX-rY)(rX-rY)+(r(X+l)-r(Y+l))(r(X+l)-r(Y+l)Kr8
/(r8+(r(X+2)-r(Y+2))(r(X+2)-r(Y+2)))-Hr8
asn(r8/r23)-*r8
ret
"IDMS" :rC-(1nt(rC)-*r3)->-X
X*100-(int(X*100)+X)-*-Y
(Y*100+X*60)/3600->X
r3+X-»-rC
ret
142
-------
Constants
Variable
Net 1
Net 2
Net 3
rlO
1159654.193
1195801.169
1195801.170
rll
4811547.405
4914917.802
4914917.840
rl2
4009510.561
3872077.376
3872077.330
rl3
2452.520540
2453.363202
2433.11099
rl4
6378.206
6378.206
6378.206
rl5
1187364.638
1241981.617
1241987.685
rl6
4787152.507
4882569.171
4882569.281
rl7
4030403.273
3898199.140
3898199.097
rl8
2452.297814
2453.140635
2432.888
rl9
6356.584
6356.584
6356.584
r20
1196337.353
1196337.260
1230615.880
r21
4846345.497
4846345.541
4955373.564
r22
3956833.734
3956833.703
3809523.928
r23
12747339.23
12745868.22
12744773.45
r24
-
-
-
r25
1160922.163
1160922.611
1201883.53
r26
4846485.777
4846485.293
4963648.50
r27
3967127.069
3967127.011
3807934.77
Raydist to Geodetic Conversion Program
"ENTER":ent "RAYDIST RED",A,"RAYDIST GREEN",B
gsb "CALC"
prt MRAY-RED=",A,"RAY-GREEN=",B;spc
prt " LATITUDE","DEGREES*",r71
prt "MINUTES=",r72,"SEC0NDS=",r73;spc
prt " LONGITUDE","DEGREES=",r74
prt "MINUTES=",r75 ,"SEC0NDS=",r76;spc 3
gto "ENTER"
end
"CALC" :0+r21-*r22
A*r9-rl0-»-r23
B*rll-rl2->-r24
0->r25
143
-------
Raydist to Geodetic Conversion Program
(cont.)
"LP":0*C
if C>3;gto +5
/((r(l+C)-r21)(r(l+C)-r21)+(r(5+C)-r22)(r(5+C)-r22))-*r(30+C)
(r(l+C)-r21)/r(30+C)-»r(34+C)
(r(5+C)-r22)/r(30+C)-r(38+C)
C+l-»-C;gto -4
(r30-r31-r23)/(r39-r38)->r44
(r35-r34)/(r39-r38)->-r45
(r32-r33-r24)/(r41-r40)-»r46
(r37-r36)/(r41-r40Kr47
r44-r46-»-r48
r45-r47->r49
r21-r48/ r49-*r21
r48/r49*r47-r46-»-r50
r22+r50*r22
r25+l-»-r25
if r25>100;gto +3
if abs(r50)>l;gto "LP"
if abs(r48)>l;gto "LP"
l-»-r54
- (rl3-/(rl3*rl3-r21*r21-r22*r22) )-ht55
rl4+X;gsb "RECT"
r64-*r56 ;r65-»r57
rl5+X'gsb "RECT"
r64->r58;r65-»-r59
144
-------
Ra.ydist to Geodetic Conversion Program
(cont.>
r59*r56*r55-r21*r58-r59*r57*r22+rl6->r60
r56*r58*r55-r57*r58*r22+r21*r59+rl7->-r61
gsb "POLR"
atn( (r22*r56+r55*r57+rl8)/(rl9*r54))->r63
r63+X;gsb "DEG"
r66->-r71 ;r6&*r72 ;r6£*r73
r62-»-X;gsb "DEG"
r66-»-r71 ;r68->-r72 ;r69-»-r73
r62->X;gsb "DEG"
r66->r74;r6&*r75 ;r69->-r76
ret
"RECT":r54*cos(Xhr64
r54*sin(X)+r65
ret
"POLR" :atn(r60/r61)-+r62
/(r61*r61+r60*r60)->-r54
ret
"DEG" :int(Xhr66
(X-r66)*60*r67
int(r67)-»-r68
int((r67-r68)*60)+r69
ret
145
-------
Variable
Net 1
rl
-10018
r2
17405
r3
22672
r4
-17040
r5
22414
r6
-45256
r7
49448
r8
-32077
r9
90.7156
rlO
73017
rll
90.7239
rl2
90684
rl3
6373669
rl4
39
rl5
76.3333
rl6
4822923
rl7
1172730
rl8
3992121
rl9
0.993213
Constants
Net 2
Net 3
-12560
-14031
4336
10101
40009
38544
-30087
-19777
-64177
32015
43529
-46593
-31043
65138
56764
-48563
90.6745
91.4213
109025
82230
90.6827
91.4297
112356
127788
6372934.11
6372387
38.2
37.3333
76.1833
76.1667
4873636
4930489.57
1198583
1214086.33
3922719
3846679.53
0.9932313
0.9932313
Using these programs geodetic grid coordinate sampling stations
supplied by EPA were converted to Raydist locations, sampling occurred as
close as practicable to this location, the actual Raydist location was
recorded, and finally the actual geodetic location recomputed. The results
of three example stations are indicated in the table below.
Ideal Geodetic Calculated Actual Raydist Actual Geodetic
Coordinates Raydist Values Values Coordinates
lat.=38°18'40" red = 1639.04 1639.80 38°18'39"
=7C°91I
long.=76"21'40" green = 1501.06 1500.80 76021'39"
lat.=38019'12" red = 1659.26 1659.30 38019'12"
long.=76021'41" green = 1518.54 1518.36 76°21'40"
lat.=38°19'45" red = 1679.35 1679.37 38°19'44"
long.=76 21'41" green = 1536.06 1536.10 76°21'41"
146
-------
APPENDIX 11
Due to the Irregular pattern and uneven coverage af bathymetrlc
surveys available for comparisons a mosaic of 98 comparative survey units
were used to attain Baywide coverage. In addition, for two areas no
bathyraetrfc data was available to make comparisons (see figure and table).
Choice of end member surveys was contingent on the age difference between
surveys and density of available data wfthtn the surveys. The numbers in
the table refer to DOAA Hydrographfc Survey numbers.
147
-------
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
Hydroqraphic Survey Comparisons
2432-6364
26
166-5416
2432-6362
27
166-6598
2432-6365
28
175-6598
2432-6368
29
2345-5797
2432-6363
30
NO DATA
2393-6365
31
2345-5237
2393-6368
32
175-2375
2393-6363
33
2402-5197
2393-6366
34
5198-8860
2393-6371
35
2402-8860
2393-6370
36
167-2402
2393-6367
37
2402-5237
2393-6368
38
2402-8522
2399-6367
39
2652-5237
2335-6373
40
2667-5197
2399-6373
41
188-5501
2399-6375
42
2667-5432
2399-6372
43
2667-5501
166-2335
44
177-6603
166-2399
45
177-6605
166-6374
46
2464-5328
166-2345
47
177-5328
166-6957
48
188-5328
166-6367
49
2652-5327
166-6597
50
188-5327
2629-5374
76
199-6683
2629-5501
77
199-7094
2631-5327
78
209-6956
188-5374
79
209-6683
188-6954
80
209-7091
188-6952
81
209-7154
188-6958
82
209-6876
199-6954
83
209-7094
199-6952
84
209-7092
199-6958
85
209-7155
199-7075
86
209-6775
199-6955
87
209-7156
199-7009
88
209-7093
199-6953
89
209-6779
199-7065
90
211-7093
201-7047
91
211-8279
201-7001
92
211-8283
201-6958
93
211-6775
201-7032
94
211-6779
201-7075
95
211-6776
201-7043
96
211-7781
2681-7075
97
211-8435
2681-7065
98
NO DATA
2681-7064
99
211-7943
199-6956
100
211-8278
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
148
-------
149
-------
APPENDIX III
Percentages of species within the non-opaque mineral assemblage, percent of non-opaque grains within
the heavy minerals, and weight percent of all heavy minerals within the sample. Samples taken from
an 11 km length of shoreline on Kent Island shown in Figure
Location R57-G122 R59-G125 R59-G126
Sample No. .1236 2345678 12345
Staurolite
4.5
7.0
19.0
12.0
7.0
4.5
6.0
13.5
8.5
9.5
8.5
32.5
11.5
9.5
10.5
12.0
Tourmaline(l) 28.0
8.0
14.5
13.0
12.5
14.5
16.0
17.5
15.0
8.0
13.5
24.5
14.0
19.0
13.5
15.5
Tourmaline(2)
1.5
0.5
4.5
2.0
6.5
1.5
-
2.5
3.5
2.5
1.5
3.0
2.5
1.0
3.0
1.5
Epidote
7.5
•4.5
5.5
7.0
10.0
11.5
12.0
5.5
15.0
14.0
7.0
9.0
8.0
11.0
10.0
16.0
Zoisite
2.0
1.5
3.5
3.0
6.5
2.0
4.5
2.0
6.5
3.0
3.0
1.0
2.5
2.0
2.0
1.0
Kyanite
0.0
-
1.5
4.0
1.5
0.5
1.5
1.0
0.5
0.5
1.5
3.0
1.5
-
3.0
1.5
Si 11 imam* te
3.5
1.0
4.5
3.0
2.0
2.5
3.5
2.5
3.5
2.0
5.0
2.0
2.5
3.0
3.5
2.0
Hornblende
10.5
7.5
12.0
10.0
8.0
7.5
11.0
11.5
8.5
6.0
13.0
7.0
17.5
12.5
8.5
7.0
Tremolite-
2.0
3.0
3.5
4.0
5.0
5.5
5.5
3.0
7.5
1.5
6.5
1.0
3.0
5.0
3.0
2.5
Actinolite
Augite-
7.5
0.5
3.0
2.0
0.5
2.5
-
1.5
-
1.5
1.5
1.0
0.5
-
1.0
1.5
Diopsi de
Hypersthene-
1.0
2.5
2.0
3.0
1.0
2.5
-
3.0
0.5
1.5
2.5
2.0
1.5
2.5
1.0
2.0
Enstatite
Zi rcon
11.0
48.5
16.0
21.0
27.5
34.0
28.0
24.5
20.5
39.0
17.5
2.5
27.5
23.0
30.5
27.5
Garnet
6.5
5.0
2.0
11.0
3.5
2.0
2.0
7.0
2.5
0.5
8.0
6.5
6.5
3.5
3.0
6.0
Rutile
1.5
-
-
-
0.5
1.5
1.5
1.0
-
0.1
0.5
0.5
0.5
0.5
1.0
0.5
% non-opaque 30 21 40.5 28 28 29.5 28.5 26 40.5 27.5 35.5 49.0 32 38 26 33.5
grains
Weight % 1.6 0.78 0.46 0.84 0.42 0.41 0.52 0.52 0.72 0.36 0.74 2.24 0.59 0.62 0.26 0.55
heavy minerals
(1) schlorite and dravite species of tourmaline
(2) indicolite and elbaite species of tourmaline
-------
APPENDIX III
Percentages of species within the non-opaque mineral assemblage, percent of non-opaque grains within
the heavy minerals, and weight percent of all heavy minerals within the sample. Samples taken from
an 11 km length of shoreline on Kent Island shown in Figure (cont.).
Location R61-G129 R65-G134 R66-G136
Sample No. 12345 12 345 12345 6
Staurolite
25.0
18.0
13.0
23.5
5.5
36.5
15.5
10.5
17.0
9.0
11.0
4.5
6.5
12.5
10.5
9.5
Tourmaline(l)
15.5
8.0
17.0
19.5
13.0
20.0
15.0
15.0
20.0
15.0
3.3
8.5
12.5
11.5
16.5
10.0
Tourmaline(2)
2.5
3.5
1.5
5.5
4.5
3.5
5.5
6.5
2.5
3.0
2.2
8.0
3.5
1.5
2.0
4.5
Epidote
6.5
4.0
8.5
7.0
9.5
6.5
8.5
8.0
7.0
7.5
3.3
6.5
11.0
7.5
8.0
9.0
Zoisite
2.0
2.0
0.5
3.0
1.5
1.5
2.0
4.0
1.0
1.5
-
1.0
3.0
3.0
3.0
2.5
Kya,nite
2.0
-
4.5
2.0
-
3.0
2.5
2.5
2.5
1.0
0.6
0.5
1.5
0.5
2.0
-
Si 1limanite
1.5
1.0
4.0
4.0
4.0
5.0
2.5
2.5
2.5
2.0
' 0.6
0.5
2.0
2.0
5.0
4.5
Hornblende
8.5
3.0
5.5
4.5
6.5
4.5
8.0
10.0
10.0
12.5
2.8
9.5
10.0
12.5
10.5
17.0
Tremolite-
3.0
2.0
1.5
3.5
3.0
1.5
1.5
2.0
4.0
3.5
0.6
4.0
5.0
3.0
6.0
5.0
Actinolite
Augite-
-
-
0.5
0.5
1.0
3.0
-
-
-
-
1.7
2.0
0.5
-
2.0
0.5
Diopside
Hypersthene-
1.5
1.5
1.0
-
3.0
2.0
2.5
2.0
2.5
3.0
0.6
0.5
in
•
o
0.5
2.5
4.0
Enstatite
Zi rcon
19.5
44.0
31.0
16.0
42.5
1.5
24.0
26.5
19.0
26.0
54.0
48.0
32.0
34.0
20.0
19.5
G arnet
3.0
7.5
4.5
4.5
2.5
3.5
2.0
3.0
8.0
12.5
12.0
3.0
4.5
6.5
5.5
9.0
R utile
1.5
-
2.0
2.0
1.0
-
3.0
3.0
1.5
0.5
6.0
1.0
2.0
1.5
2.0
0.5
% non-opaque 40.0 37.5 33.5 30.0 43.5 54.0 25.5 32.0 31.5 38 10.5 30.0 28.5 26.0 30.5 38
grains
Weight % 0.38 0.49 0.71 0.59 1.27 3.14 0.57 0.65 0.42 1.20 25.51 0.74 1.00 0.44 0.14 0.45
heavy minerals
(1) schlorite and dravit species of tourmaline
(2) indicolite and elbaite species of tourmaline
-------
APPENDIX III
Percentages of species within the non-opaque mineral assemblage, percent of non-opaque grains within
the heavy minerals, and weight percent of all heavy minerals.within the sample. Samples taken from
an 11 km length of shoreline on Kent Island shown in Figure (cont.).
Location R68-G138 R71-G141 R76-G146
Sample No.
1
2
3
4
5
6
7
1
2
3
4
5
1
2
3
4
5
6
Staurolite
11.5
16.5
11.0
5.0
10.0
11.5
3.5
18.5
16.5
12.5
10.5
8.0
14.0
17.5
13.5
9.0
40.0
15.5
Tourmaline(l)
9.5
12.0
24.0
10.0
14.0
14.5
12.5
25.5
12.5
11.5
16.0
15.5
24.5
14.0
19.0
9.5
18.5
11.5
Tourmaline(2)
4.5
6.0
6.5
1.0
3.0
2.5
1.5
4.0
4.0
1.0
3.5
1.0
1.5
3.0
3.0
3.0
1.5
1.5
Epi dote
7.0
6.0
10.0
8.0
9.5
13.5
6.0
6.0
9.5
11.5
13.0
12.0
7.5
9.0
7.0
10.0
9.0
11.5
Zoisite
3.5
4.0
3.5
2.0
3.5
1.5
3.0
3.5
3.5
3.5
3.0
4.0
0.5
2.0
4.5
3.5
1.5
1.5
Kyai\-i te
1.0
-
2.0
0.5
-
0.5
1.0
10.0
0.5
2.0
1.5
1.5
9.0
2.0
1.5
5.5
5.0
1.5
Sillimanite
3.0
2.5
2.0
-
1.5
4.5
3.0
5.0
3.0
2.5
5.0
3.0
4.0
6.5
4.5
2.0
2.0
4.5
Hornblende
8.5
13.5
12.0
7.5
13.0
17.5
10.0
6.5
8.5
12.5
16.5
11.5
9.5
9.5
11.5
21.0
3.5
17.5
Tremolite-
8.0
3.5
1.0
4.0
2.0
5.0
" 3.5
1.5
2.0
4.5
3.5
2.0
4.0
1.5
1.0
1.0
0.5
3.0
Actinolite
Augite-
0.5
1.5
1.0
2.5
0.5
1.0
-
0.5
-
-
0.5
-
0.5
0.5
-
1.0
0.5
1.0
Diopside
Hypersthene-
2.0
1.0
5.0
3.5
3.5
2.0
3.0
2.0
2.5
3.0
2.5
3.5
4.0
3.0
2.0
3.0
2.5
2.0
Enstatite
Zi rcon
22.5
24.0
12.0
46.5
31.0
18.5
39.0
4.0
25.0
27.5
16.0
31.0
5.0
26.0
22.5
21.0
8.5
21.5
G arnet
8.5
4.0
4.0
3.5
2.5
4.5
8.0
2.5
7.0
3.0
2.5
6.0
2.0
2.5
3.0
2.5
2.5
5.0
R utile
2.0
0.5
0.5
2.5
1.0
1.5
1.5
1.0
1.0
1.5
2.0
0.5
-
2.0
1.5
3.5
1.5
1.5
% non-opaque 36.0 18.0 23.0 21.0 30.0 38.5 33.0 43.0 30.0 25.0 28.5 31.5 30.0 24.5 36.0 33.5 34.0 35.0
grains
Weight % 1.11 0.44 0.26 0.27 0.39 0.37 0.31 0.48 1.02 0.46 0.32 0.8 0.23 0.9 0.86 0.57 0.45. 1.28
heavy minerals
(1) schlorite and dravite species of tourmaline
(2) indicolite and elbaite species of tourmaline
-------
APPENDIX IV: MASS BY DEPTH INCREMENT (xlO6 tons/century)
Upper Depth 4.0
Lower Depth 0.0
Total
Org
Inorg
Sand
Silt
Clay
Deposition
Low
124.47
1.39
123.08
108.82
7.62
6.64
High
14.60
0.08
14.52
13.73
0.44
0.35
Erosion
Low
-171.90
-0.80
-171.10
-160.33
-6.32
-4.45
High
-6.50
-0.03
-6.47
-6.10
-0.18
-0.19
Deposition
139.07
1.47
137.60
122.55
8.07
6.99
Erosion
-178.40
-0.82
-177.58
-166.43
-6.51
-4.64
Net
-39.33
0.64
-39.97
-43.89
1.56
2.35
Upper Depth
Lower Depth
Deposition
Low
Hi gh
Erosion
Low
High
Deposition
Erosion
Net
8.0
4.0
Total
284.55
35.50
-264.93
-14.98
320.05
-279.91
40.13
Org
5.70
0.84
-3.20
-0.44
6.54
-3.64
2.91
Inorg
278.84
34.66
-261.73
-14.54
313.50
-276.28
37.23
Sand
210.21
20.18
-220.54
-9.94
230.39
-230.48
-0.09
Silt
33.71
7.66
-18.18
-2.10
41.38
-20.28
21.10
Clay
34.92
6.82
-23.02
-2.50
41.74
-25.52
16.21
153
-------
APPENDIX IV: MASS BY DEPTH INCREMENT (xlO6 tons/century)
(cont.)
Upper Depth 12.0
Lower Depth 8.0
Total
Org
Inorg
Sand
Silt
Clay
Deposition
Low
124.45
2.77
121.68
74.33
19.97
27.38
High
12.39
0.30
12.09
6.27
2.72
3.10
Erosion
Low
-54.69
-1.64
-53.05
-19.31
-12.49
-21.26
High
-16.95
-0.26
-16.69
-11.23
-2.25
-3.21
Deposition
136.84
3.07
133.77
80.60
22.69
30.48
Erosion
-71.64
-1.90
-69.74
-30.54
-14.73
-24.47
Net
65.20
1.17
64.03
50.07
7.96
6.01
Upper Depth 16.0
Lower Depth 12.0
Total
Org
Inorg
Sand
Silt
Clay
Deposition
Low
60.47
1.98
58.49
17.48
15.44
25.56
High
5.75
0.08
5.67
3.31
1.04
1.32
Erosion
Low
-41.86
-1.82
-40.04
-7.43
-9.51
-23.10
High
-3.12
-0.09
-3.04
-1.04
-0.78
-1.21
Deposition
66.22
2.06
64.16
20.80
16.48
26.88
Erosion
-44.98
-1.91
-43.07
-8.47
-10.29
-24.31
Net
21.24
0.15
21.08
12.33
6.19
2.57
Upper Depth 20.0
Lower Depth 16.0
Total
Org
Inorg
Sand
Silt
Clay
Deposition
Low
27.23
0.87
26.37
9.07
6.36
10.94
High
24.08
0.73
23.35
7.86
5.73
9.77
Erosion
Low
-19.11
-0.77
-18.34
-3.39
-5.16
-9.79
Hi gh
-9.26
-0.28
-8.98
-3.27
-2.13
-3.58
Deposition
51.32
1.60
49.72
16.93
12.09
20.71
Erosion
-28.37
-1.05
-27.32
-6.66
-7.29
-13.37
Net
22.95
0.55
22.40
10.27
4.80
7.34
154
-------
APPENDIX IV: MASS BY DEPTH INCREMENT (xlO6 tons/century)
(cont.)
Upper Depth 30.0
Lower Depth 20.0
Total
Org
Inorg
Sand
Silt
CI ay
Deposition
Low.
24.07
1.08
22.99
1.44
7.69
13.86
High
26.24
1.19
25.05
0.97
8.65
15.44
Erosion
Low
-19.27
-0.83
-18.44
-2.88
-5.44
-10.12
High
-23.88
-1.11
-22.77
-0.97
-7.86
-13.94
Deposition
50.32
2.27
48.04
2.41
16.34
29.30
Erosion
-43.15
-1.94
-41.21
-3.85
-13.31
-24.06
Net
7.17
0.33
6.84
-1.44
3.03
5.25
Upper Depth 99.9
Lower Depth 30.0
Total
Org
Inorg
Sand
Silt
Clay
Deposition
Low
0.14
0.00
0.14 .
0.07
0.03
0.05
High
0.21
0.00
0.21.
0.12
0.04
0.06
Erosion
Low
-0.27
-0.00
-0.27
-0.11
-0.06
-0.10
High
-0.10
-0.00
-0.10
-0.05
-0.02
-0.03
Deposition
0.36
0.00
0.35
0.18
0.06
0.11
Erosion
-0.37
-0.00
-0.37
-0.16
-0.08
-0.14
Net
-0.01
0.00
-0.02
0.03
-0.01
-0.03
155
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AP?!N3IX V: VOLUME AND MASS COMPUTATIONS FOR SHORELINE EROSION
Erosion Deposition
m3 tons Sand SfFt/Clay % Sand Bulk Density
Name volume mass mass mass Volume Mass (g/cm3)
(xlO6 tons/century)
Abbey Pt.-Plum
Pt., including
Spesutie Island
1.10
1.94
1.16
0.77
0.01
0.03
60
1.76
Elk Neck
3.70
6.51
3.91
2.60
0.40
0.83
60
1.76
Havre de Grace
0.83
1.46
0.88
0.58
0.70
1.46
60
1.76
Back River Neck
0.29
0.48
0.19
0.29
0.02
0.03
40
1.67
Aberdeen Proving
Ground
0.70
1.17
0.47
0.70
0.02
0.05
40
1.67
Spry-Pooles Island
0.53
0.89
0.35
0.53
-
-
40
1.67
Bodkin Neck
0.66
1.16
0.41
0.76
0.01
0.03
35
1.76
Black Marsh
0.23
0.41
0.14
0.26
-
35
1.76
Hart-Miller Islands
0.71
1.25
0.48
0.81
-
35
1.76
Bodkin Neck-
Gibson Island
1.30
2.29
0.80
1.49
-
35
1.76
Sandy Point
3.30
5.81
2.03
3.78
0.04
0.08
35
1.76
Greenbury Point
0.93
1.64
0.33
1.31
0.01
0.02
20
1.76
Tolley Point
1.40
2.46
0.49
1.97
0.57
1.19
20
1:76
-------
APPENDIX V: VOLUME AND MASS COMPUTATIONS FOR SHORELINE EROSION
(cont.)
m3 tons Sand Silt/Clay Deposition % Sand Bulk Density
Name volume mass mass mass Volume Mass (g/cm3)
(xlO6 tons/century)
Saundner Point
0.29
0.51
0.10
0.41
-
-
20
1.76
Shady Side
2.60
4.34
0.87
3.47
0.05
0.10
20
1.67
Rose Haven
1.40
2.34
0.47
1.87
0.04
0.08
20
1.67
North Beach
6.10
10.70
4.51
6.23
0.14
0.29
42
1.76
Parker Creek
7.30
12.20
1.83
10.40
0.07
0.14
15
1.67
Kenwood-
Calvert Beach
1.40
2.46
1.03
1.43
-
-
42
1.76
Calvert Beach-
Cove Point
12.00
21.10
12.20
8.87
0.06
1.25
58
1.76
Little Cove Pt.-
Drum Point
3.50
6.16
2.96
3.20
0.07
0.15
48
1.76
Biscue Pt. -
St. Jerome Cr.
2.10
3.70
1.11
2.59
0.06
0.13
30
1.76
Wise Marsh,
Turkey Neck
0.42
0.74
0.22
0.52
0.02
0.03
30
1.76
St. Jerome Cr.-
Cornfield
1.50
2.51
0.50
2.00
0.11
0.23
20
1.76
-------
APPENDIX V: VOLUME AND MASS COMPUTATIONS FOR SHORELINE EROSION
(cont.)
m3 tons Sand Silt/Clay Deposition % Sand Bulk Density
Name volume mass mass mass Volume Mass (g/cm3)
(xlO5 tons/century)
Grove Neck to
Bohemia River
2.90
5.10
3.06
2.04
0.15
0.31
60
1.76
Fairlee Creek
5.80
9.69
3.87
5.81
-
40
1.67
Fairlee Neck
6.80
12.00
7.18
4.79
0.04
0.08
60
1.76
Worton Point
2.40
4.22
2.53
1.69
-
-
60
1.76
Still pond Neck
1.18
2.11
1.27
0.85
-
-
60
1.76
Swan Point
2.13
3.70
1.70
2.00
0.03
0.06
46
1.76
Tolchester Beach
1.20
2.11
0.97
1.14
-
-
46
1.76
Love Point
5.20
9.15
4.21
4.94
-
-
46
1.76
Wilson Point-
Huntingfield Pt.
2.30
4.05
1.86
2.19
-
-
46
1.76
Kent Island
2.70
4.75
2.33
2.42
0.20
0.42
49
1.76
Crab Alley Bay
1.00
1.76
0.53
1.23
0.09
0.19
30
1.76
Piney Neck
0.41
0.72
0.22
0.51
-
-
30
1.76
Bennett Point
0.30
0.53
0.16
0.38
0.02
0.03
30
1.76
-------
APPENDIX V: VOLUME AND MASS COMPUTATIONS FOR SHORELINE EROSION
(cont.)
m3 tons Sand Silt/Clay Deposition % Sand Bulk Density
Name volume mass mass mass Volume Mass (g/cm3)
(xlO6 tons/century)
Lower Kent Island
2.10
3.70
0.89
2.81
0.12
0.25
24
1.76
Poplar Islands
2.80
4.68
0.94
3.74
0.04
0.10
20
1.67
Lowes Point
1.20
2.30
1.29
1.01
-
-
56
1.76
CI airborne
3.80
7.30
4.09
3.21
-
-
56
1.76
Bennett Point
1.10
1.94
0.58
1.36
0.02
0.05
30
1.76
Tilghman Island
2.50
4.18
0.84
3.34
0.12
0.25
20
1.76
Nevitt
7.70
12.90
2.57
10.30
0.02
0.04
20
1.76
Deep Neck
0.26
0.43
0.09
0.35
-
-
20
1.67
Ferry Neck
0.44
0.74
0.15
0.59
0.05
0.10
20
1.67
Oxford
0.07
0.12
0.02
0.09
-
-
20
1.67
Chlora Point
0.23
0.38
0.04
0.35
-
-
20
1.67
James Island
6.40
10.70
1.60
9.08
0.03
0.06
15
1.67
Sharp Island
5.30
8.85
1.33
7.52
-
-
15
1.67
Cook-Ragged Pts.
2.90
4.84
0.73
4.12
0.14
0.29
15
1.67
-------
APPENDIX V: VOLUME AND MASS COMPUTATIONS FOR SHORELINE EROSION
(cont.)
m3 tons Sand Silt/Clay Deposition % Sand Bulk Density
Name volume mass mass mass Volume Mass (g/cm3)
(xlO6 tons/century)
Hooper Neck
0.59
0.98
0.15
0.84
0.02
0.04
15
1.67
Taylor Island
7.30
12.20
1.83
10.40
0.06
0.12
15
1.67
Fishing Point
1.08
1.80
0.27
1.53
0.02
0.04
15
1.67
Barren Island
2.40
4.01
0.60
3.41
0.10
0.20
15
1.67
Upper Hooper Is.
0.34
0.57
0.09
0.48
0.02 .
0.04
15
1.67
Middle Hooper-
Lower Hooper Is.
0.39
0.65
0.23
0.42
-
-
15
1.67
Bloodsworth
0.67
1.12
0.39
0.73
-
-
15
1.67
Pone
0.18
0.30
0.11
0.19
-
-
15
1.67
Adam
0.13
0.22
0.07
0.14
-
-
15
1.67.
Hoi land
0.22
0.37
0.13
0.24
15
1.67
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