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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