DEPARTMENT OF THE INTERIOR
UNITED STATES GEOLOGICAL SURVEY
TO ACCOMPANY MAP 1-455
GEOLOGIC MAP OF THE
FAIRBANKS QUADRANGLE, ALASKA
By Troy L. Pe'we, Clyde Wahrhaftig,
and Florence R. Weber
DESCRIPTION OF MAP UNITS
UN CONSOLIDATED SEDIMENTARY DEPOSITS
Qa Flood-plain alluvium.—Well-stratified layers and lenses of
unconsolidated gray silt, sand, pebbles, cobbles, and
boulders; occurs as two facies: (1) gravelly facies on
Tanana River east of mouth of Chena River and on other
streams in and near Alaska Range; (2) silt and silty sand
facies on Tanana River west of mouth of Chena River,
'on streams north of the Tanana, and on lower courses
of some streams draining Alaska Range. Gravelly facies
contains pebbles, cobbles, and boulders, from 'A inch to
3 feet in diameter, of resistant rocks from neighboring
uplands, and is locally perennially frozen. Ice content
low. Sand and silty sand facies contains swale and
slough deposits consisting of poorly stratified lenses and
layers of fairly well-sorted stream-laid silt and silty sand,
composed mostly of angular to subrounded grains of
quartz, mica, and feldspar, 10 to 30 percent clay, and
some organic material. This facies is colored mottled
brown to grayish blue by carbonaceous material and iron
staining. Swale and slough deposits are perennially
frozen and have moderate to high ice content.
Qs Swamp deposits.—Dark-brown to black peat and silt more
than 5 feet thick in areas of impeded drainage in which
standing water is present throughout the year. Peren-
nially frozen and contain lenses and veinlets of clear ice.
Many small swamps and seasonally flooded areas are in-
cluded in map units Qa, Qab, Qaf, Qtf, Qsu, and Qho.
Ql Landslide debris.—Chaotic masses of angular fragments of
shale, sandstone, conglomerate, clay, sand, gravel, and
schist embedded in a matrix of silt, clay, and gravel, re-
sulting from slumps and flows along cuesta and terrace
escarpments and other oversteepened slopes. Character-
istic irregular hummock and hollow topography with
cracks, areas of water-saturated clay, and ponds. Some
landslides are active; others would be reactivated by de-
gradation of permafrost within them.
Qg Reworked creek gravel.—Placer-mine dredge tailings de-
rived from buried creek gravels. Creek gravel from which
the dredge tailings were derived underlies large areas of
undifferentiated perennially frozen silt (Qsu), rests on
Birch Creek Schist and intrusive rocks (Mzi) and, where
exposed in walls of excavations, consists of well-stratified
layers and lenses of poorly sorted angular to subrounded
brown to buff, locally heavily iron-stained, auriferous
sandy gravel, containing boulders of quartz, gneiss, and
schist as much as 24 inches in diameter. Locally peren-
nially frozen, with little ground ice.
Qbc Basaltic cinders.—Cinder cone from late Pleistocene or
Recent eruption at north base of mountain southwest of
Buzzard Creek, a headwater of Totatlanika River. Un-
consolidated poorly sorted aggregate of irregular to sub-
rounded fragments of scoriaceous to dense basalt. Frag-
ments range from 3 inches or larger at crater rim to 0.5 to
1 inch 1.5 miles northeast of crater. Basalt is dark gray
to black and composed of about 20 percent olivine in well-
formed unembayed phenocrysts 0.25 mm in diameter,
20 to 25 percent calcic labradorite or sodic bytownite
laths 0.1 mm long and 0.02 mm thick, and 5 percent clino-
pyroxene, set in a largely glassy brownish-gray ground-
mass containing fine opaque particles. Scattered broken
crystals of orthoclase and quartz, apparently picked up
from Totatlanika Schist, are present. Deposit is 100 to
200 feet thick at crater rim, thinning to a feather edge
1 mile northeast. No cinders on mountain to southwest.
Cinders rest on youngest terraces on Buzzard Creek,
which are correlated with the Riley Creek glaciation.
Qab Abandoned flood-plain al luvium.—Well-stratified layers
and lenses of unconsolidated gray silt, sand, pebbles,
cobbles, and boulders. Occurs in two facies: (1) coarse
gravelly alluvium, indicated by pattern of small circles,
which was deposited close to the Alaska Range and grades
northward into (2) silty alluvium (shown without pattern)
containing 95 percent silt and 5 percent sand, mantled
by eolian silt and sand. The silty alluvium ranges in
color from light to dark gray through buff to brown; it
includes much organic material and grades into swamp
deposits in poorly drained low areas; it also forms the
natural levees. Many narrow discontinuous abandoned
stream-channel deposits, consisting chiefly of washed
medium- to fine-grained sand and minor amounts of
gravel, cross the silty alluvium from south to north in
the area between the Teklanika and Wood Rivers. The
top of permafrost is 2 to 4 feet below the surface through-1
out areas of silty alluvium between abandoned stream
channels, but is more than 10 feet deep beneath the
channels. Ice content of silty alluvium is moderate to
high.
Qaf Alluvial-fan deposits.—Well-stratified layers and lenses of
gray to brown coarse sand and pebble, cobble, and boulder
gravel. Deposits range in coarseness from sand in the
area north and east of Blair Lakes (shown unpatterned)
to cobble gravel close to the Alaska Range (shown with
pattern of circles). Boulders 18 inches in diameter com-
mon within 10 miles of mountains. Cobbles and boulders
are of granite, granodiorite, conglomerate, sandstone,
schist, gneiss, quartz, and gabbro. Locally perennially
frozen; ice content low. Areas within the Alaska Range
shown by this pattern are alluvial terraces correlated
with the Riley Creek Glaciation.
Qdr Dune Sand, reworked.—Organic silty sand underlying un-
drained depressions and old lake or pond beds; derived
from nearby sand dunes. Silt and organic content vary
considerably, and locally this unit grades into swamp
deposits. Permafrost and considerable ground ice prob-
ably present at shallow depths.
Qrm Morainol deposits of Riley Creek Glaciation.—Unsorted and
unstratified glacial till with silt cover V6 foot thick. Silt,
sand, and gravel with boulders more than 3 feet in dia-
meter. Coarse fragments angular to rounded. Perma-
frost present, ice content low to moderate. May contain
large masses of glacially deformed Nenana Gravel. Map-
ping based largely on topographic expression. Riley
Creek Glaciation probably correlates with Wisconsin
Glaciation of mid-Continental United States (Wahrhaftig,
1958, p. 56).
Qro Out-wash of Riley Creek Glaciation.—Well-stratified well-
sorted porous and very permeable gray to pale yellowish-
brown sandy gravel containing lenses of coarse well-sorted
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sand. Average pebble diameter 1 to 6 inches, maximum
4 to 15 inches. Pebbles and boulders are of sandstone,
conglomerate, schist, gabbro, granite, volcanic rocks,
quartz, chert, and other rock types present in Alaska
Range. Silt cover, which may be as much as 4 feet thick,
indicated by stippled pattern. Locally perennially frozen,
ice content low.
Qtf Torrential fan deposits.—Interfingering cobble and pebble
gravel, mudflow deposits, and minor amounts of silt and
sand, bordering the foothills. Average coarseness de-
creases away from foothills. Surface mantled with wind-
blown and waterlaid sand and silt, but generally well
drained. Seasonally swampy where fans join. Depth to
permafrost and ice content of deposits unknown except
.between the Totatlanika and Nenana Rivers, where top
of permafrost is at least 4 feet, and probably 15 feet, be-
neath surface (Kachadoorian, 1960).
Qf Fairbanks Loess.—Massive, homogeneous eolian silt on up-
per slopes and hilltops. Well sorted, less than 10 percent
clay; grains angular, consist mostly of quartz, feldspar,
and mica; locally cemented by iron oxide; locally calcar-
eous. Thickness north of Tanana River ranges from 3
feet on upper hillslopes to a maximum of 200 feet on lower
hillslopes; south of Tanana River thickness ranges from 3
to 50 feet. Not mapped on hilltops or slopes where less
than 3 feet thick. Color buff to tannish-gray when dry,
brown when wet. Locally mottled by iron staining and
carbonaceous material. Contains a few thin (%- to 6-inch)
white volcanic ash beds in the Fairbanks area. Remains
of middle to late Quaternary land mammals present. No
permafrost except in southwestern corner of quadrangle
where loess covers flat, poorly drained surfaces.
Qsu Perennially frozen silt, undifferentiated.—Massive, homo-
geneous unconsolidated well-sorted silt of eolian origin
with less than 10 percent clay, locally rich in organic silt
and larger organic fragments, retransported from orig-
inal hillside sites of eolian deposition to lower slopes and
valley bottoms by mudflows, gullying, and slopewash.
Inorganic components are angular grains of quartz, feld-
spar, and mica, locally cemented by iron oxides. Inor-
ganic silt is buff to brown or gray, locally mottled by
organic matter and iron staining. Organic silt is brown
to grayish black. Deposit incorporates large quantities
of plant and animal remains in valley bottoms (including
abundant remains of late Quaternary land mammals),
and includes large oval areas of frozen peat chiefly com-
posed of sphagnum mosses, as well as large masses of
undecomposed or partially de'composed leaves, twigs,,
stems, and branches of higher plants. Permafrost 3 to
200 feet thick in valley bottoms creates poor drainage.
Ground ice is abundant as horizontal and vertical sheets,
vertical wedges, and saucer-shaped and irregular masses
1 to 50 feet in diameter. Thaw lakes and polygonal
ground common.
Qd Dune sand.—Well-sorted, angular to round, moderate yel-
lowish-brown eolian sand consisting mainly of yellowish-
white, clear to opaque quartz grains (65 to 85 percent),
but including some dark-gray to black rock fragments,
chert, and mica (common in some areas), and traces of
feldspar and light-colored rock fragments. West of the
Teklanika and south of the Tanana, dune sand covers a
broad area to a depth of 5 to 200 feet and occurs chiefly
in longitudinal but locally parabolic dunes whose form and
structure indicate deposition by northeasterly winds.
Dunes are covered by eolian silt as much as 3 feet thick.
Isolated dunes on alluvial and outwash plains east of the
Teklanika and north of the Tanana. Permafrost absent
or at considerable depth.
Qhm Morainal deposits ofHealy Glaciation.—Unstratif ied glacial
till with silt cover M> foot to 3 feet thick. Silt, sand, and
gravel with boulders more than 3 feet in diameter.
Coarse fragments angular to rounded. Mapped from
aerial photographs on basis of topographic expression;
lithology based on comparison with morainal deposits of
Healy age in the adjacent Healy quadrangle. The Healy
Glaciation may correlate with the Illinoian Glaciation of
mid-continental United States.
Qho Outwash ofHealy Glaciation.—Course, clean, well-sorted,
well-stratified gray- to yellowish-brown porous and very
permeable gravel with layers and lenses of coarse clean
sand. Gravel along Nenana River averages % to 3 inches,
contains boulders as much as 1 or 2 feet across, and con-
sists of sandstone, conglomerate, gabbro, granite, quartz-
sericite schist, orthoclase-quartz-sericite schist, and gneiss.
Gravel is locally perennially frozen and has low ice con-
tent, but is overlain by 2 to 6 feet of silt which is gener-
ally perennially frozen and has moderate ice content.
West of Blair Lakes this unit contains cobbles as much as
6 inches in diameter consisting of gray granitic rocks,
gneiss, volcanic rocks, and quartz, and is overlain by as
much as 4 feet of silt, indicated by stipple pattern. Loc-
ally it is perennially frozen but contains little ground ice.
Qha Alluvial fan deposits contemporaneous with Healy Glaci-
ation.—On the Dry Creek fan these deposits consist of
well-stratified gray to brown pebble, cobble, and boulder
gravel with layers and lenses of coarse sand, and range
from fine clean gravel with pebbles '/£ inch to 3 inches in
diameter at north end of fan east of Blair Lakes to
gravel with cobbles 8 to 12 inches in diameter south of
Blair Lakes; boulders as much as 18 inches in diameter
common close to mountains. Pebbles, cobbles, and boul-
ders composed of gray granitic rocks, conglomerate,
schist, gneiss, diorite, and quartz. Gravel locally peren-
nially frozen; ice content low. In valleys in Alaska Range
these deposits consist of interbedded gravel, sand, silt,
and mudflow deposits derived from nearby mountains of
Nenana Gravel and Totatlanika Schist. Locally they may
be perennially frozen and contain little ground ice. Silt
cover more than 4 feet thick is indicated by stippled
pattern.
Qdm Morainal deposits of Dry Creek Glaciation.—Mapped from
aerial photographs on basis of topographic expression.
Probably similar to morainal deposits of Healy Glaciation
but somewhat more weathered and eroded.
Qdo Outwash of Dry Creek Glaciation.— Similar to outwash of
Healy Glaciation on same stream but more weathered
and eroded, and overlain by thicker deposit of windblown
silt.
Qda Alluvial fan deposits contemporaneous with Dry Creek
Glaciation.—Similar to alluvial fan deposits contempo-
raneous with Healy Glaciation on same stream but more
weathered and eroded, and overlain by thicker deposit
of windblown silt.
Qbm Morainal deposits of Browne Glaciation.—Coarse sand and
gravel, with abundant blocks of granite, gabbro, and con-
glomerate several feet on a side. Small areas at altitudes
of 2,200 feet west of Windy Creek and 5,000 feet southeast
of Gold King Creek consist largely of boulders and blocks.
Includes erratics (Qbe)—isolated angular blocks and
groups of blocks of granite, gabbro, and conglomerate, 3
to 30 feet in diameter, resting on present topography.
Apparently derived from headwater regions of Nenana
and Wood Rivers in Alaska Range and deposited by
ancient glaciers that flowed north over former valley
floors much higher than present river beds. Ancient val-
ley floor of Nenana River is 500 feet above present river
level at Rex and 1,000 feet above river level at Ferry
(Wahrhaftig, 1958, p. 22-28); ancient Wood River followed
approximately the course of Gold King Creek in valley
now about 2,000 feet above present river level at south
border of quadrangle.
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CONSOLIDATED SEDIMENTARY AND METAMORPHIC ROCKS
Tn Nenana Gravel.—Buff to reddish-brown poorly consolidated
pebble to boulder conglomerate and coarse sandstone,
with interbedded mudflow deposits, thin claystone layers
and local thin lignite beds. Maximum thickness in Fair-
banks quadrangle about 2,000 feet. West of California
Creek includes several hundred feet of coarse sandstone
at base; east of California Greek entire formation is con-
glomerate. Thickness and pebble size decrease northward;
imbrication and crossbedding indicate deposition by north-
flowing streams. West of Totatlanika River pebbles and
boulders consist of sandstone and conglomerate from Cant-
well Formation (Lower Cretaceous), quartz, chert, quartz-
ite, volcanic rocks, granite, and granodiorite from plutons
in Alaska Range to the south; and dark-green altered
ophitic ilmenite-bearing gabbro from belt of Triassic
greenstone in southern part of central Alaska Range.
Between Totatlanika River and Gold King Creek Nenana
Gravel consists entirely of clasts of Birch Creek and
Totatlanika Schists, derived from mountains less than 15
miles to south. East of Gold King Creek clasts include
Birch Creek and Totatlanika Schists, granitic rocks, sand-
stone and conglomerate of Cantwell Formation, and
Triassic gabbro from head of Wood River. Unit peren-
nially frozen, but contains little ground ice. According
to Jack A. Wolf (written communication, March 19,1964),
age of Nenana Gravel, based on plant remains, is Plio-
cene^).
Structure contours on base of Nenana Gravel show the
inferred configuration of the surface (in part restored)
on which it was deposited. In constructing contours east
of California Creek and south of Buzzard and Boulder
Creeks, thickness of coal-bearing formation beneath
Nenana Gravel was assumed to be 500 to 700 feet. A
synclinal basin of coal-bearing rocks with maximum
thickness of 1,000 feet is believed to havfe underlain
Nenana Gravel in area immediately north of Rex Dome,
as inferred from single patch of coal-bearing formation
on ridgetop north of mouth of Rex Creek.
Tcb Coal-bearing formation.—Poorly consolidated readily eroded
pebbly sandstone, claystone, and subbituminous coal, in
synclinal basins in the foothills of the Alaska Range be-
tween Nenana River and Dry Creek. Pinches out north-
ward and is overlapped unconformably by Nenana Gravel.
Can be divided into five units, locally separated by un-
conformities: (1) At the base, a discontinuous unit 600
feet thick in Rex Creek and MacAdam Creek basins, 160
feet thick at the head of Tatlanika Creek, and less than
100 feet thick elsewhere, of interfingering lenticular
poorly sorted clayey pebbly sandstone, sandy micaceous
claystone, and lenticular and locally bony coal. Clastic
rocks were derived locally, and unit was deposited on ir-
regular land surface. Prominent coal bed 24 feet in
maximum thickness at top. (2) A 100-foot unit of yel-
lowish-brown-weathering laminated gray claystone.
(3) A 500- to 1,000-foot unit of clean white-weathering
cross-bedded sandstone, coarse and pebbly at base, grading
upward into greenish-gray silt and clay and topped by a
laterally persistent coal bed 3 to 20 feet thick. Pebbles
in sandstone are chiefly quartz, chert, and quartzite.
(4) A 500- to 800-foot unit of clean buff cross-bedded
pebbly sandstone, grading upward into a section of green-
ish-gray siltsone and claystone with several thin lenticular
coal beds, generally less than 5 feet thick (one bed on
Tatlanika Creek is 17 feet thick). Pebbles of granitic,
gabbroic, and volcanic rocks are common to abundant.
This unit grades northwestward into non-coal-bearing
facies as much as 1,300 feet thick, consisting of sandstone
and conglomerate; it overlaps units 1 to 3, apparently with
local unconformity, and rests directly on schist west of
California Creek and north of Buzzard and Hearst Creeks.
(5) At the top, interbedded dark carbonaceous siltstone
and claystone and dark sandstone, locally pebbly, totaling
1,000 to 1,500 feet in thickness between Tatlanika Creek
and Wood River; pinches out abruptly westward between
Tatlanika Creek and Totatlanika River. Near base of
unit 5 on Tatlanika Creek are one or two discontinuous
rhyolitic ash beds 0 to 25 feet thick. All units are peren-
nially frozen to depths of 5 to 200 feet and have low to
moderate ice content. Units 1 to 4 are thicker and con-
tain more coal in adjacent parts of Healy Quadrangle to
south. Areas of outcrop of non-coal-bearing facies of unit
4 and all of unit 5 are shown by stipple pattern.
Mt Totatlanika Schist.— Predominantly quartz-microcline-
sericite schist and augen gneiss, gray in color, consisting
of a coarse facies with large deformed phenocrysts of
microcline, quartz, and rare albite 0.1-1 inch in diameter,
in schistose groundmass of sericite, chlorite, quartz, feld-
spar, and calcite; interbedded with a fine facies consisting
of angular grains of feldspar and quartz 0.01 to 1.0 inch
in diameter, in a dark-gray to yellowish-gray schistose
groundmass of sericite and chlorite. Contacts between
the two facies are generally sharp. Augen are locally
deformed into spindles, parallel to schistosity, elsewhere
original crystalline outlines are at a sharp angle to schis-
tosity. Locally contains layers of black carbonaceous
schist 10 to 20 feet thick. In mountains between the
head of Buzzard Creek and Wood River, the coarse facies
(schist and gneiss) is separated by a thrust fault from
an underlying third facies consisting of pale-yellow
quartz-feldspar gneiss, with infaulted and interbedded
black carbonaceous schist and dark-green chloritic schist,
Foliation of Totatlanika Schist is due to parallel orien-
tation of mica and chlorite flakes and fine layers of dif-
ferent mineral composition; lineation is commonly a
linear streaking and elongation of some of the minerals,
particularly the microcline augen, in a single direction in
the plane of foliation. The permafrost is cut by two or
three sets of joints, trending roughly N to NW, NE, and
due E, spaced 1 to 5 feet apart and perpendicular to foli-
ation. Totatlanika Schist is considered to be Mississip-
pian(?) in age, on the basis of Syringapora (Wahrhaftig,
1958, p. 12).
DSt Tolovana(?) Limestone.—Thick-bedded to massive, fine-
crystalline to lithographic, medium dark bluish-gray
(rarely light bluish-gray) limestone of unknown thickness.
Forms prominent ridge in extreme northwest corner
of quadrangle. Weathers buff to yellowish brown.
Contains crushed zones recemented by white calcite and
quartz. Chert rare or absent. Correlated on basis of
lithology and geographic trend with the Tolovana Lime-
stone in the White Mountains 90 miles to the northeast,
which is 4,000 feet thick at the type locality (Church and
Durfee, 1961), and is Middle Silurian to Middle or early
Late Devonian in age (Mertie, 1937, p. 89; Helen Duncan
written communication, July 10, 1961; W. A. Oliver,
written communication, February 13, 1961 and July 6,
1961).
Nilkoka Group.—The group consists of the following units,
which are believed to be progressively older in the order
given:
nc Chert and siliceous shale.—Chert is hard, thin bedded to1
blocky, locally cut by shear planes, medium light gray or
rarely nearly white or black with thin color layering;
commonly stained yellow or red by iron oxide from
weathering; forms reddish soil. Interbedded siliceous
shale is hard, thin bedded, medium light gray weathering1
to light yellowish gray.
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ng Grit, argillite, quartzite, and limestone.—Interbedded grit _
and variegated clay slate, quartzite, and phyllite, with
rare siltstone beds and a few fine-grained lenticular lime-
stone beds as much as 5 feet thick. Color of fine-grained
rocks predominantly green but mottled by red through
pale olive green, dusky yellow, and light yellowish gray
areas produced by leaching and oxidation along joints,
on exposed surfaces, and in more porous parts. Siltstone
green or tan; limestone medium gray to dark gray-brown,
or creamy where weathered. Grit composed predomin-
antly of rounded grains, >/4 inch in maximum diameter, of
clear bluish quartz, light-colored microcline and plagio-
clase, and rare light-colored chert, embedded in a siliceous
matrix; locally grades into coarse quartzite. Grit is gray
but stained rusty brown on weathered surfaces. Locally
the rock is foliated, generally parallel to bedding. The
fine-grained rocks range from shale and argillite to
phyllite.
In the Nilkoka Group the chert and shale (nc) alternate
in bands several miles wide with grit and phyllite
(ng). The Nilkoka is the underlying bedrock in most of
the area west of Tolovana River and north of the Tanana;
type locality is west bank of Tanana downstream from
mouth of Tolovana (Brooks, 1900, p. 472). Unit nc is cor-
related with Mertie's (1937, p. 65-76) unit B, which may
be Cambrian or Ordovician, on basis of fossils found in
upper part of unit (Mertie, 1937, p. 73); unit ng is corre-
lated with Mertie's pre-Ordovician unit C and possibly
part of his late Precambrian unit D. Age of Nilkoka un-
certain, but is believed to be Precambrian or early
Paleozoic.
be Birch Creek Schist.—Light- to dark-gray, reddish-brown to
tan-weathering schists, predominantly quartz-sericite
schist and micaceous quartzite. Includes muscovite-bio-
tite schist, garnet-mica schist, calcite- and dolomite-bear-
ing schist, dark-gray to black chloritic and graphitic schist,
amphibolite, light-gray to white impure marble, and light-
brown (light-gray-weathering) gneiss that has been af-
fected by retrograde metamorphism. Forms basement
rock east of Minto Flats north of Tanana River and un-
derlies Clear Creek Butte and hills around Blair Lakes
south of Tanana River. A small area of quartz-sericite
schist and graphitic schist on upper Totatlanika River
and Bonanza Creek at south border of area mapped as
Birch Creek Schist, but may be younger. Essential min-
erals in Birch Creek Schist are quartz, sericite, biotite,
plagioclase (commonly albite, some albite-oligoclase), po-
tassium feldspar, graphite, calcite, dolomite, and amphi-
boles (actinolite and tremolite); accessory minerals include
chlorite, garnet, epidote, clinozoisite, hornblende, phlogo-
pite, iron oxides and hydroxides, and rarely staurolite and
andalusite. Birch Creek Schist formed by synkinematic
metamorphism of shale, siltstone, sandstone, and lime-
stone. Age uncertain but considered to be early Precam-
brian by Mertie (1937, p. 55) and others. Recent isotopic
measurements give dates ranging from 120 m.y. to 1,170
m.y. (Wasserburg, Eberlein, and Lanphere, 1963); on
basis of these dates, the age of the Birch Creek Schist is
considered to be Precambrian or early Paleozoic. Be-
cause of complicated structure and intermixed rock types,
the gross thickness of the Birch Creek Schist is unknown,
but must be many thousands of feet (Mertie, 1937, p. 73).
At least two stages of deformation and recrystallization
are indicated, the first probably of higher metamorphic
grade than the second. Metamorphic grade is middle
(biotite and garnet zones) near Fairbanks, and decreases
northwestward to lower (chlorite zone). Well-developed
zones of contact metamorphism border intrusive bodies.
Earliest folds appear to have northwesterly trend, but
later structural trends northeasterly (Forbes and Brown,
1961), parallel to trends of younger rocks in Livengood
quadrangle to north. Mineralogic layering and planar
parallelism of platy minerals, as well as rare relict bed-
ding, folded into closely spaced isoclinal folds with ampli-
tudes of tens of feet. Only structure mappable on a
regional scale is strain-slip cleavage parallel to axial
planes of minor folds,
INTRUSIVE IGNEO'JS ROCKS
Mzi Intrusive rocks.—Dikes and stocks of granite, granodiorite,
dacite, and granite porphyry, intruded into schist forma-
tions of the Yukon-Tanana upland and the Alaska Range.
Unconformably overlain in the Alaska Range by the coal-
bearing formation. Age probably Mesozoic, although
some may be early Tertiary. Intrusive mass east of Fox
is gray to yellowish-brown coarse porphyritic biotite
granite or quartz monzonite with microcline phenocrysts.
Intrusion northeast of Fort Wainwright (Ladd Air Force
Base) is spheroidally weathering medium-grained hypid-
iomorphic pyroxene-bearing granodiorite, gray where
fresh and brown on weathered surfaces. Dikes north and
west of Fairbanks are gray to yellowish-brown medium-
grained quartz-bearing porphyritic granitic rocks. Dikes
near Liberty Bell Mine in Alaska Range are granite por-
phyry (Overbeck, 1918, p. 355). Intrusion west of Totat-
lanika River at south edge of map area is gray porphyritic
dacite, intruded at shallow depth, with phenocrysts of
beta-quartz, calcic andesine, biotite, and hornblende 0.5-3
mm across in a groundmass averaging 0.01-0.1 mm of
quartz, andesine, orthoclase, and chlorite. This body has
closely spaced nearly vertical joints.
Dmu Mafic and ultramafic rocks.—Ophitic diorite and serpen-
tinized peridotite make up Wood River Buttes. Diorite
contains 45 percent strongly zoned plagioclase laths 1 to
3 mm long and 0.2 to 1 mm thick, with slightly sericitized
cores of sodic labradorite (An58) and rims of sodic ande-
sine (An^), 5 to 20 percent pigeonitic augite, and 30 to 40
percent amphibole with variable pleochroism, either pale
green to colorless or deep brown to pale yellow. Most
augite occurs as cores in amphibole crystals. Mafic min-
erals locally altered to chlorite(?). Ore minerals consti-
tute about 5 percent of the diorite.
Serpentinized rocks consist of masses of bastite, antig-
orite, and chrysotile that make up 30 to 80 percent of the
rock surrounding relict grains of olivine, bronzite, and,
locally, edenite (iron-free hornblende). Picotite is rarely
present. Wood River Buttes are on projection of belt of
mafic and ultramafic rocks between Chena and Salcha
Rivers in Big Delta quadrangle to east, mapped by Mertie
(1937, p. 203-205), to which he assigned a Devonian age.
An intense linear positive magnetic anomaly extends
across the alluviated plain between Wood River Buttes
and belt of mafic and ultramafic rocks to northeast, in-
dicating that they are parts of a single belt (Andreasen,
Wahrhaftig, and Zeitz, 1964).
EXTRUSIVE IGNEOUS ROCKS
Tb Basalt. — Dark-gray and black or brownish olivine basalt,
closely jointed and deeply weathered, with local pillow
or columnar structure; palagonite present in pillow lavas.
On lower slopes of hills in the vicinity of Fort Wainwright
(Ladd Air Force Base).
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