GEOLOGY of the SLEETMUTE A-5, A-6, B-5, and B-6 QUADRANGLES, SOUTHWESTERN ALASKA by John Decker, R.R

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GEOLOGY OF THE SLEETMUTE A-5, A-6, B-5, AND B-6 QUADRANGLES, SOUTHWESTERN ALASKA by John Decker, R.R. Reifenstuhl, M.S. Robinson, C.F. Waythomas, and J.G. Clough

Professional Report 99 1995

Published by Alaska Department of State of Alaska Department of Natural Resources NATURAL DIVISION OF GEOLOGICAL & GEOPHYSICAL SURVEYS GEOLOGY OF THE SLEETMUTE A-5, Ad, B-5, AND B-6 QUADRANGLES, SOUTHWESTERN ALASKA by John Decker, R.R. Reifenstuhl, M.S. Robinson, C.F. Waythomas, and J.G. Clough

Professional Report 99 Division of Geological & Geophysical Surveys

Cover photo: Northwest-vergent isocline of veryfine-grained sandstone of the Lower(?) and Upper Cretaceous Kuskokwim Group. The outcrop of medium- bedded sandstone is 1.5 miles west of Kiokluk Lake in the Sleetmute B-6 Quadrangle (map unit Kkm). The hammer handle in the photo is 46 cm Fairbanks, Alaska long. Photo by R.R. Reifenstuhl. 1995 STATE OF ALASKA Tony Knowles, Governor

DEPARTMENT OF NATURAL RESOURCES John Shively, Commissioner

DIVISION OF GEOLOGICAL & GEOPHYSICAL SURVEYS Milton A. Wiltse, Acting Director and State Geologist

Division of Geological & Geophysical Surveys publications may be in- spected at the following locations. Address mail orders to the Fairbanks office.

Alaska Division of Geological University of Alaska Anchorage Library & Geophysical Surveys 321 1 Providence Drive 794 University Avenue, Suite 200 Anchorage, Alaska 99508 Fairbanks, Alaska 99709-3645

Elmer E. Rasmuson Library Alaska Resource Library University of Alaska Fairbanks 222 W. 7th Avenue Fairbanks, Alaska 99775-1005 Anchorage, Alaska 995 13-7589

Alaska State Library State Office Building, 8th Floor 333 Willoughby Avenue Juneau, Alaska 998 11-057 1 This publication released by the Division of Geological & Geophysical Surveys, text was produced and printed in Fairbanks, Alaska by Graphics North and maps were printed in Colorado Springs, Colorado by Pikes Peak Lithographing Co., at a cost of $9.50 per copy. Publication is required by Alaska Statute 41, "to determine the potential of Alaskan land for production of metals, minerals, fuels, and geothermal resources; the location and supplies of groundwater and construction materials; the potential geologic hazards to buildings, roads, bridges, and other installations and structures; and shall conduct such other surveys and investigations as will advance knowledge of the geology of Alaska." CONTENTS

Page

Introduction ...... Stratigraphy. age and structure ...... Mineral resources ...... Description of map units ...... Unconsolidated deposits ...... Volcanic rocks ...... Hypabyssal rocks ...... Holokuk basalt ...... Sedimentary and metamorphic rocks ...... Kuskokwim Group ...... Holitna group ...... Intrusive rocks ...... Acknowledgments ...... References cited ......

FIGURE

Figure 1. Map showing paleocurrent data for the Sleetmute A.5. A.6. B.5. and B-6 Quadrangles ...... 10

TABLES

Table 1. Major oxide geochemical analysis of rock samples ...... 12 1a . Normative mineralogy ...... 13 2 . Trace element geochemical analyses of rock samples ...... 15 3 . K-Ar age analyses ...... 16 4 . Fossil information ...... 16

MAPS [in pocket]

Sheet 1. Geologic map of the Sleetmute A.5. A.6. B.5. and B-6 Quadrangles. southwestern Alaska 2 . Stratigraphic sections in the Sleetmute A-6 and B-6 Quadrangles. southwestern Alaska GEOLOGY OF THE SLEETMUTE A-5, A-6, B-5, AND B-6 QUADRANGLES, SOUTHWESTERN ALASKA

by John Decker,' R.R. Reifenstuhl? M.S. Robinson? C.F. Waythomast and J.G. Clough2

INTRODUCTION as Triassic are also present within their Holitna Group. The oldest strata recognized in the unit, based on regional The Sleetmute A-5, A-6, B-5, and B-6 Quadrangles correlation with the Lone Mountain area of the McGrath are located 450 km (280 mi) west of Anchorage in the Quadrangle, are of probable Late Proterozoic age (R.B. Kuskokwim Mountains of southwesternAlaska.Elevation Blodgett, written commun., 1994). Adrain and others (in in the map area ranges from about 1,300 m (4,265 ft) in press) have suggested that the designation "Holitna the pluton-cored Kiokluk and Chuilnuk Mountains to Group" be ultimately abandoned, because it is too broadly 100 m (328 ft) along the Holitna River and Chineekluk defined. We agree with this, and use Holitna group as an Creek. Except for the glaciated peaks of the Chuilnuk informal unit designation within our report area. and Kiokluk Mountains, the physiography of the area is Most of the area is underlain by the Kuskokwim dominated by rounded, tundra-covered hills of low to Group, a >12-km-thick (39,370 ft) marine turbidite moderate relief. sequence of thick- to medium-bedded sandstone and Geologic mapping utilized 1:40,000-scale topo- shale. The age of the Kuskokwim Group has been graphic base maps, 1:63,360-scale aerial photographs, and generally considered Early Cretaceous and Late field notes and maps from unpublished U.S. Geological Cretaceous (Albian to Coniacian) (Moore and Wallace, Survey field studies. A reconnaissance study by Cady and 1985; Decker and others, 1994). The inclusion, by these others (1955) included the quadrangles mapped in this authors, of the Early Cretaceous (Albian) age-range is report. based on the rare occurrence of one species of arenaceous Preliminary 1:40,000-scale geologic maps of the foraminifera of probable Albian age from the Mulchatna Sleetmute A-6, B-5, and B-6 Quadrangles have been River area. This occurrence was reported in an in-house published, respectively, by Decker, Robinson, Murphy, ARC0 Alaska fossil report (Moore and Wallace, 1985). Reifenstuhl, and Albanese (l984), Reifenstuhl, Robinson, Numerous published reports on Kuskokwim Group Smith, Albanese, and Allegro (1984), and Robinson and macrofossils indicate a Late Cretaceous (Cenomanian to others (1984). Rock, pan-concentrate, and stream- Santonian) age and none reports ages other than Late sediment geochemistry maps of the Sleetmute A-6, B-5, Cretaceous (Cady and others, 1955; Elder and Box, and B-6 Quadrangles were compiled by Robinson 1992; Box and Elder, 1992; Elder and Miller, 1991). (1984a, b, c respectively). Compilations of previously Consequently, we will refer to the Kuskokwim Group unpublished data, mainly from U.S. Geological Survey age as Early(?) and Late Cretaceous (Albian(?) to field studies, are available in Alaska Division of Santonian). Geological & Geophysical Surveys Public-data File maps A far less extensive rock unit, superficially (Decker, Reifenstuhl, and Coonrad, 1984; Reifenstuhl, resembling the Kuskokwim Group, is the Inoceramus- Decker, Murphy, and Coonrad, 1984; and Murphy and bearing calcareous sandstone, siltstone, and shale of others, 1984). Cretaceous to Jurassic(?) age. Contacts between the calcareous deposits and other bedrock units are covered STRATIGRAPHY. AGE by surficial deposits. The Holokuk Basalt (Cady and others, 1955) consists of calc-alkalic volcanic rocks (table 1) that The oldest rock unit in the region is the Holitna Group unconformably overlie the Kuskokwim Group and range of Cady and others (1955). These authors reported in composition from basaltic andesite to rhyolite. K-Ar Silurian and Devonian fossils, but inferred that Ordovician ages range from 64.3 to 74.5 Ma.This formation is >1 km strata may also be present. More recent field investigations (3,280 ft) thick and consists of subaerial flows and have shown that strata as old as Cambrian and as young interlayered agglomerate, tuff breccia, basalt-matrix conglomerate, and lahar deposits; it is spatially and 'ARC0 Alaska, Inc., P.O.Box 100360,Anchorage, Alaska 995 10-0360. 2Alaska Division of Geological & Geophysical Surveys, 794 University chrOnO1Ogicall~associated with plutonic rocks' Avenue, Suite 200, Fairbanks, Alaska 99709-3645. Low initial SrS7/SrS6ratios in the rhyolitic rocks preclude 3Wrangle Shipyard Inc., Box 1348, Wrangle, Alaska 99929. involvement of radiogenic continental crust in their 4U.S. Geological Survey, Water Resources Division, Grace Hall, 4230 (table 2). University Drive, Suite 200, Anchorage, Alaska 99508. Two granodiorite stocks of latest Cretaceous age (mean and sericite selvages border most of the veins. Ore of K-Ar ages = 68.2 Ma; table 3) occur in the Chuilnuk and minerals in the veins include gold, scheelite, wolframite, Kiokluk Mountains. The Holokuk Basalt is locally arsenopyrite, pyrite, jamesonite, stibnite, argentite, and hornfelsed in the western Kiokluk Mountains by one of these rare gold-silver tellurides. Gangue minerals include plutons. Latest Cretaceous to earliest Tertiary volcano-plu- quartz, carbonate, white mica, tourmaline, and sericite. tonic complexes of similar size, age, and composition are Gold and scheelite have been recovered from placers common throughout southwestern Alaska (Bundtzen and (Cobb, 1976), but there is no record of production. Laird, 1982; Bundtzen and Laird, 1983; Bundtzen and Laird, Several mercury and antimony deposits are known 1992; Bundtzen and others, 1992; Decker, 1994; Miller and in the map area. The Mountain Top deposit (lat Bundtzen, 1994; Moll-Stalcup, 1994). 61'23'47" N., long 157'58'23" W.), the best exposed of Hypabyssal rocks include mafic to felsic dikes and the mercury deposits, is located about 16 km (9.9 mi) small stocks of Tertiary and Cretaceous age. The young- north of Kiokluk Lake and has produced an unknown est volcanic rock is columnar-jointed olivine basalt, dated amount of mercury; visible reserves are estimated at at 38 Ma (sample 20, table 3). 200 flasks (Cobb, 1976). The deposit is hosted in highly Quaternary surficial deposits, subdivided into 24 sheared and altered sedimentary and volcanic rocks. Most units, define four glacial advances: (1) Oskawalik Creek of the known mercury and antimony mineralization is drift (early Pleistocene); (2) Chuilnuk drift (middle late localized in probable tectonic breccias near the contacts Pleistocene); (3) Buckstock drift (late Pleistocene); and between brecciated and unbrecciated basaltic andesite. (4) Kiokluk peak drift (Holocene). The chronology of The source of mercury is not known, but some mineral- these surficial deposits was determined on the basis of ized breccias at Mountain Top may represent vent relative dating. breccias, and mineralization may originate from hot The Kuskokwim Group is variably deformed by over- springs. turned and isoclinal folds, broad open folds, and local Tin occurs in quartz-tourmaline veins and stockworks polyphase deformation. Layered Holokuk Basalt depos- in the Chuilnuk and Kiokluk plutons and in hornfelsed its were gently folded after 65 Ma. The Boss Creek and Kuskokwim Group sedimentary rocks near pluton contacts. Holitna faults, which offset late Pleistocene deposits, are All known mineralized zones in the area appear to splays of the Denali-Farewell fault system and have un- have downstream or downslope geochemical signatures. certain Cretaceous and Tertiary histories. Stream-sediment geochemistry may therefore be useful in delineating new mineralized areas. MINERAL RESOURCES DESCRIPTION OF MAP UNITS Dickite-bearing altered dikes such as those in the andesite dikes map unit (Kim) are commonly associated UNCONSOLIDATED DEPOSITS with lode occurrences of cinnabar and antimony in the lower Kuskokwim region (Sainsbury and MacKevett, Qa MODERN FLOOD-PLAIN ALLUVIUM- 1965). Moderately to well-sorted cobble and pebble Rocks of the lower Kuskokwim River region host gravel, sand, and overbank silt along the chan- placer deposits of gold and tungsten, vein deposits of mer- nels of modern streams and rivers. Abundant cury, antimony and other base and precious metals, and organic debris locally and good primary strati- greisen deposits containing tin and other base and pre- fication. May be mantled by thin (<1 cm) cious metals. colluvium or loess or both Gold and scheelite occur in placer deposits along Fortyseven Creek (lat 61°02'40" N., long 158O09' W.) and Qaf ALLUVIAL-FAN DEPOSITS-Variously originate from a series of mineralized quartz veins that sorted, weakly stratified cobble and pebble cut hornfelsed Kuskokwim Group where Fortyseven gravel in a sand and silt matrix. Associated with Creek heads. The hornfelsed sedimentary rocks weather cone- or fan-shaped alluvial deposits along the a distinctive brown and contain secondary biotite; they bases of mountain fronts and valley slopes. May were probably produced by intrusion of a small rhyolite contain lenses of angular colluvial gravel plug near the head of Fortyseven Creek. The quartz veins are typical hydrothermal vein deposits, ranging in width Qp PEAT, PEATY SILT, AND OTHER PALUDAL from a few centimeters to more than a meter, and can be DEPOSITS-Generally thick accumulations of traced discontinuously for tens of meters along strike; peat and organic silt in fens and swamps; locally they contain minor calcite and white mica, which yielded interbedded with loess; usually capping fluvial a K-Ar age of 60.9 k 1.8 Ma. (sample 21, table 3). Chlorite sediments. Includes palsas, fens, and string fens UNDIFFERENTIATED COLLUVIAL DE- GLACIAL DRIFT OF CHUILNUK GLACIA- POSITS-Angular, poorly sorted, unstratified TION-Diamicton of subdued morainal deposits gravel in a silty matrix on slopes of 45" or less. and outwash, forming isolated remnants close Derived principally from frost-shattered bed- to modern valley floors. Poorly to moderately rock. May contain solifluction deposits and sorted silty gravel with variable amounts of sand minor alluvium and boulders; moraines exhibit local hummocky topography with kettles SOLIFLUCTION DEPOSITS-Poorly sorted, angular gravel in a silty matrix. Associated GLACIAL DRIFT OF OSKAWALIK CREEK with lobate or tongue-like microrelief forms on GLACIATION-Diamicton of very subdued colluvial slopes drift bodies partially capping ridges and knolls; includes minor outwash, retransported FINE-GRAINED COLLUVIUM-Retrans- drift and solifluction deposits. Composed of ported silt or muck, locally interbedded with poorly sorted silty gravel, locally frozen at sand and gravel lenses, mainly where it overlies depths of 2-3 m alluvium on lower slopes and valley bottoms BLANKET COLLUVIUM-Blocky rock TALUS-Frost-rived, blocky rock rubble rubble with variable amounts of fine-grained forming cones, fans, and aprons along base of material as blankets. Includes stone stripes, solif- steep mountain slopes. Includes avalanche- luction debris, stone-banked terraces, and boulder tongues, boulder trains and runs, and protalus lobes rockfall debris LACUSTRINE DELTAIC DEPOSITS-Well- LANDSLIDE DEPOSITS-Hummocky, het- sorted sands and silts deposited in standing erogeneous to chaotic debris deposited by slope bodies of water failures, including slump, slides, and slips UNDIFFERENTIATED FLUVIAL DE- RETRANSPORTED DRIFT-Unsorted, un- POSITS-Moderately to well-sorted cobble and stratified silty gravel with variable amounts of pebble gravel, sand, and overbank silt. Com- sand and boulders on glaciated valley slopes; monly with well-developed primary fluvial gradational with undisturbed drift sedimentary structures, and associated with in- active flood plains and stream terraces ROCK-GLACIER DEPOSITS-Lobate or tongue-shaped accumulationsof bouldery rubble UNDIFFERENTIATED COLLUVIUM AND below talus and scree slopes, showing evidence ALLUVIUM-Complexly mixed colluvial such as multiple, crescentic ridges, of downslope and fluvial deposits forming debris-rich fans, movement on most mountain slopes or at mouth of steep bedrock walls, couliers, and valleys. Consists UNDIFFERENTIATED GLACIAL DEPOS- of poorly stratified, unsorted gravel to silt, ITS-Mixtures of till, outwash, and minor often with crude clast imbrication and colluviated drift or other glacial diamicton not bouldery zones; includes deposits of mud- readily separated. Poorly sorted, generally un- flows, debris flows, slushflows, snow stratified, gravelly sediment with a sand to silt avalanches, and floods matrix UNDIFFERENTIATED OUTWASH-Well- GLACIAL DRIFT OF KIOKLUK PEAK AD- sorted cobble to small-boulder gravel, usually VANCE--Poorly sorted bouldery drift in highest, exhibiting obvious primary stratification. north-facing cirque of Kiokluk Mountains Contains rounded to subrounded clasts. Locally capped by 1-2 m of eolian silt or sand or both. GLACIAL DRIFT OF BUCKSTOCK GLA- Age uncertain CIATION-Diamicton associated with unmodified knob-and-kettle moraines. Poorly OUTWASH OF BUCKSTOCK GLACIA- sorted bouldery gravels to well-sorted sandy TION-Well-sorted, stratified, cobble and gravel. May include some outwash gravel, flow pebble gravel and sand; locally mantled by 1-2 m till, and loess of eolian silt. Outwash valley trains or outwash heads related to Buckstock terminal moraines and feldspar fragments are common. Locally and ice limits contains exotic clasts of shale, sandstone, and rhyolitic to basaltic volcanic rock. Unit locally Qdo, OUTWASH OF CHUILNUK GLACIATION- underlain and cut by thin (15 m) zones of dark- Moderately to well-sorted, stratified, cobble and gray to black fragmental volcanic rock small-boulder gravel; locally mantled by 1-2 m composed of shale, sandstone, rhyolite, and ba- of eolian sediment, colluvium or both. Most salt clasts in obsidian-like matrix. Unit appears outwash valley trains or outwash heads related to be locally discordant; may represent dikes or to Chuilnuk terminal moraines and ice limits sills

Qdo, OUTWASH OF OSKAWALIK CREEK GLA- Tvrl RHYOLITE-Tan to olive-green, fine-grained, CIATION-Moderately well sorted cobble porphyritic rhyolite with abundant quartz phe- gravel forming terrace remnants not directly nocrysts to 3 mm diam and alkali feldspar traceable to equivalent terminal moraines. Prob- (sanidine) phenocrysts to 5 mm long; locally, able remnants of extensive valley-fill deposits phenocrysts constitute as much as 20 percent of that have been buried, redistributed, and dis- rock. Rhyolite occurs as several thick (-30 m) sected layers which exhibit no perceptible change in composition. The contact between this unit and Qat UNDIFFERENTIATED STREAM-TERRACE the overlying black rhyolite (Tvr*) appears lo- DEPOSITS-Cobble to pebble gravel and sand, cally conformable. Exact nature of lower contact typically well sorted and stratified in stream with basalt is unknown terraces >2 m high. Commonly mantled by eolian silt HYPABYSSAL ROCKS

VOLCANIC ROCKS Holokuk Basalt The Holokuk Basalt was named by Cady and others Tvb COLUMNAR BASALT-Rubble of fresh oliv- (1955). It is a gently dipping sequence of ine basalt exposed locally on densely vegetated predominantly basaltic volcanic rock, containing minor hillside 10 km northeast of Fortyseven Creek. rhyolite, vitric tuff, and breccia, in the western One whole-rock sample yielded K-Ar age of Sleetmute Quadrangle. The formation is at least 1 km 38.2 + 1.1 Ma. (table 3) thick and unconformably overlies the Kuskokwim Group. Cady and others (1955) assigned aTertiary (late Tvr, GRAY RHYOLITE-Light- to medium-gray Eocene to early Miocene) age, mainly on the basis of and buff, fine-grained, well-layered to massive structural arguments. New radiometric age data, rhyolite and rhyolite tuff breccia. Rhyolite however, suggest that Holokuk volcanism took place contains irregular clots of hematite to 5 mm between 64.3 and 74.5 Ma, or during latest Cretaceous diam; weathered rhyolite is distinctively to very earliestTertiary time. Whole-rock major-oxide hematite-stained and spotted; exact nature of data (table 1) indicate a basaltic andesite to rhyolite contact with underlying black rhyolite unit (Tvr2) compositional range (Peccerillo and Taylor, 1976). is unclear; one sample yielded K-Ar age of 43.8 Seven samples from the Holokuk Basalt yielded K-Ar Ma (table 3). Intermixed with hematite-spotted, ages ranging from 64.3 to 74.5 Ma (mean of K-Ar ages gray rhyolite are zones of light-gray rhyolite tuff = 70.0 Ma; table 3); the Holokuk Basalt is therefore breccia composed of angular and subangular probably correlative with the Upper Cretaceous clasts of shale, sandstone, and volcanic rocks in Iditarod Basalt (Cady and others, 1955), and possibly a very fine grained scoriaceous matrix. Tuff the undated Getmuna Rhyolite Group (Cady and others, breccia occurs in well-layered sequence near 1955) in the northern Sleetmute and southern Iditarod core of volcanic complex Quadrangles. In the Kiokluk and Chuilnuk Mountains, rocks previously mapped by Cady and others (1955) Tvr, BLACK RHYOLITE-Very dark gray to black, as the Holokuk Basalt are here subdivided into fine-grained, porphyritic rhyolite; includes mi- 10 informal units. nor basaltic andesite. Rhyolite contains alkali feldspar (sanidine) phenocrysts to 10 mm long Khap BASALTIC ANDESITE-Dark-gray to brown and beta-quartz phenocrysts to 3 mm diam in a and green, very fine grained, porphyritic basaltic very fine grained, glassy groundmass. Quartz andesite. These rocks contain plagioclase phenocrysts to 3 mm long, clinopyroxene containing quartz phenocrysts to 4 mm diam, (augite?) to 2 mm diam, olivine in anhedral with laminated, swirled, and contorted flow masses to 2 mm diam, and orthopyroxene as sub- banding hedral grains to 3 mm long. Olivine and orthopyroxene are altered to serpentine minerals. Khf INTERMEDIATETO FELSIC FLOWS, TUFF, The basaltic andesite occurs as thick flow(?) AND AGGLOMERATE-Textures and com- units which unconformably overlie rocks of positions vary greatly within this unit, and only Kuskokwim Group. Six basaltic andesites in the thick, distinct layers can be correlated laterally. map area yielded K-Ar ages of 64.3,68.3,68.0, Prominent lithologies are: (1) dark-green to gray, 70.7, 70.8, and 74.5 Ma (mean of K-Ar ages = platy-weathering, thinly laminated aphanitic 69.4 Ma; table 3) andesite containing aligned plagioclase laths, relict hornblende(?), and minor secondary chlo- Khlb INTERLAYERED LAHAR DEPOSITS AND rite with local, small-scale isoclinal flow(?) BASALTIC ANDESITE-Grayish-green, folds; (2) light-green to medium-gray felsic- to brownish-red, dark-green, and black interlayed intermediate aphanite; (3) tan-weathering inter- lahar deposits and basaltic andesite. Basal se- mediate agglomerate containing subangular, quence depositionally overlies Upper Cretaceous intermediate lithic clasts to 2.5 cm long, with Kuskokwim Group and is locally crosscut by epidote and calcite along fractures and to 3 per- Cretaceous andesite dikes (Kim). Basaltic andes- cent disseminated pyrite; (4) finely laminated ite occurs as massive flows and dikes and amygdaloidal basalt with secondary calcite and contains plagioclase, olivine, or pyroxene phe- chlorite; and (5) finely laminated, light-green nocrysts. Olivine is moderately to highly altered lithic tuff with clasts to 1.5 cm long, a black to serpentine minerals and magnetite. One K-Ar matrix, and locally graded bedding. Conform- whole-rock sample gave a minimum age of ably overlies massive and amygdaloidal basaltic 73.2 * 2.2 Ma (table 3). This unit is 426 m thick andesite unit (Khaa). Unit is hornfelsed by Kiok- in a measured section 5 km east of Holokuk luk pluton in western Kiokluk Mountains Mountain (Reifenstuhl, Robinson, Smith, Albanese, and Allegro, 1984; sheet 2) Khaa MASSIVE AND AMYGDALOIDAL BA- SALTIC ANDESITE-Sequence of basaltic Khl RED AND GRAY LAHAR DEPOSITS- andesite and andesite flows with thick massive Brownish-red, matrix supported deposit; locally bases and thin amygdaloidal tops. Massive flows contains angular to rounded pyroxene basalt are dark-gray, olivine-augite andesites with fine- clasts from 0.5 to 70 cm diam. Muddy grained trachytic groundmass. Olivine is volcaniclastic matrix locally forms up to subhedral, generally 0.2 mm diam, and consti- 60 percent of the rock. Gray lahar deposit is tutes 1 percent of rock. Augite is euhedral to distinctive grayish green, unstratified, and subhedral, 1.5 mm long, and 2 percent of the composed of angular, very poorly sorted mafic rock. Amygdaloidal andesites are gray to black volcanic clasts in weakly indurated groundmass. where fresh, and weather to maroon and green. Locally interbedded with graded muddy Amygdules are 2.5 cm long, irregularly shaped, sandstone and mudstone; individual beds range rounded or flattened, commonly lined with con- in thickness from 10 to 80 cm; may be tightly centric chalcedony, and filled with one or more folded; contacts with volcanic rocks are sharp of the following: quartz, calcite, epidote, or chlo- and irregular rite. Amygdaloidal zones range from 0.6 to 8.0 mm thick; amygdule size increases from base Kha RED BASALTIC ANDESITE-Massive, to top. Basalt-matrix conglomerate occurs at the brownish-dark-red basaltic andesite with pla- base of the unit and contains subrounded shale gioclase phenocrysts to 4 mm long in matrix of and sandstone fragments derived from the under- plagioclase laths ~0.2mm long, brown volca- lying Kuskokwim Group. These clastic nic glass, and chalcedony. Locally, subhedral fragments are up to 2.5 cm long with little or no orthopyroxene is 1.2 mm long and constitutes evidence of thermal metamorphism. This unit is up to 8 percent of rock. Occurs above and be- overlain by the intermediate to felsic flows, tuff, low red and gray lahar deposits unit (Khl) and agglomerate unit (Khf). Unit is hornfelsed by Kiokluk pluton in western Kiokluk Moun- Khr WHITE RHYOLITE-White aphanitic rhyolite tains. Unit is 258 m thick at Peak 3770 and consists of at least six basalt flows, each with The Kuskokwim Group in the Sleetmute A-5, A-6, thick massive base and thin amygdaloidal top B-5, and B-6 Quadrangles is poorly exposed; it typically (Decker, Robinson, Murphy, Reifenstuhl, and forms low, rolling hills mantled with vegetation and frost- Albanese, 1984; sheet 2) shattered bedrock. There are few outcrops where bedding can be determined with certainty and stratigraphic rela- Khtb TUFF BRECCIA-White to green, felsic to tions observed; some occur along cutbanks of major siliceous, aphanitic to sandy-matrix tuff breccia. rivers, in steep gulleys, and in hornfelsed rock adjacent Local flow banding may be swirled or contorted. to the Kiokluk and Chuilnuk stocks. Quartz phenocrysts to 4 mm diam occur locally. The Kuskokwim Group in and around the Kiokluk Pyrite blebs to 1 cm diam constitute to 3 percent and Chuilnuk Mountains consists entirely of marine tur- of rock bidites, subdivided here into two informal units: medium-bedded sandstone and shale (Kkm), and thick- Khb FELSITE BRECCIA-White to light-gray, ver- bedded sandstone and shale (Kkt). Bouma (1962) tically jointed, matrix-supported breccia intervals for individual turbidite beds are indicated by containing subangular felsite clasts to 0.6 m diam subscript: (Tabcde).Groups of beds are assigned to facies in a felsite matrix. Locally, north of Peak 3770, according to the model of Mutti and Ricci Lucchi (1972): breccia matrix contains to 20 percent green to B, C, D, E, F, G. It was not possible to consistently dis- brown pleochroic tourmaline as radiating clus- tinguish between shale related to turbidity currents (T,) ters with individual crystals to several and that of normal hemipelagic regime (facies G).Thin centimeters long. Contact relationships with ad- shale beds interbedded with sandstone and siltstone tur- jacent units unclear bidite are referred to arbitrarily as T,, and thick shaly intervals are assigned to facies G. Khc BASALT-MATRIX CONGLOMERATE- Brown-weathering sandstone and shale, pebble Kkm MEDIUM-BEDDED SANDSTONE AND to granule conglomerate in a light-green volca- SHALE-Light- to dark-gray, medium- to very nic matrix. Contains subrounded shale and fine grained, massive, graded, plane-laminated sandstone fragments derived from the underlying and crossbedded lithic sandstone turbidites Kuskokwim Group. Clasts are up to 2.5 cm long interbedded with medium-gray to black, mas- and indicate only minor thermal metamorphism. sive, plane-laminated and crossbedded siltstone, In western Kiokluk Mountains, unit is thermally and black shale. Sandstone beds range in thick- metamorphosed by Kiokluk stock. In the north- ness from 1 cm to 4 m; mean values range from west part of map area, this basal unit is mapped 15 to 20 cm. Beds are laterally continuous and as tuff breccia, which contains basalt and shale have sharp bases and sharp or gradational tops. clasts in a sandy matrix Amalgamated beds and shale rip-up layers are rare, but well-preserved flute casts are common. SEDIMENTARY AND METAMORPHIC ROCKS This unit is characterized by alternating sand- and shale-dominated sequences that range to Kuskokwim Group 25 m and 3 m, respectively. The sand to shale The Kuskokwim Group (Cady and others, 1955) ratio is as high as 50: 1, averages about 4: 1, and consists primarily of marine turbidites and subordinate locally is less than 1: 1. Both thickening- and fluvial and shallow marine strata deposited in an elon- coarsening-upward, and thinning- and fining- gate, southwest-trending basin covering over upward megacycles occur. Bouma (1962) 70,000 km2 in southwestern Alaska. The Kuskokwim intervals in sandstones include massive or graded Group is thickest (>I2 km) in the central part of the Tab,Tae, Tabcde, Tbcde, Tcde, Tee, andT&. Bouma basin between the villages of McGrath and Aniak. interval Tce is dominant, and these rocks show Mixed marine and nonmarine sections are relatively internal cross-lamination, sharp bases, and sharp thin (<3 km) and are restricted to basin margins and tops. Bouma intervals in shaly sections com- local basement highs. The Kuskokwim Group is posed of fine- to very fine grained sands and Early(?) and Late Cretaceous (Albian(?) to Santonian) shale include Tbcde,Tcde, and Tde; Tde is domi- age, on the basis of sparse age-diagnostic megafossil nant. Shaly sections are composed of repeated collections (40) and a few collections of fossil plants, 1- to 3-cm-thick turbidite beds interlayered with palynomorphs, and dinoflagellates (table 4; Decker and black shale. Thin sands have either sharp or gra- others, 1994). dational tops and are interpreted to be facies D and E deposits; medium-bedded sandstone se- reflects alteration of sandstone and shale; quences assigned to facies C, D, and E; suggests however, structural deformation, including middle- and outer-fan deposition faulting, foliation, and isoclinal and open folding, masks lateral continuity of bedding Kkt THICK-BEDDED SANDSTONE AND SHALE-Light- to dark-gray, very coarse to very Kkq MINERALIZED QUAR'IZ VEINS AND fine grained, massive, graded, plane-laminated QUARTZ-VEIN STOCKWORK AND HORN- and cross-laminated lithic sandstone turbidites FELS-Near Fortyseven Creek: T. 11 N., interbedded with black, massive, plane-laminated R. 50 W.; (secs. 8, 17, and 18) brown-weather- and cross-laminated siltstone and silty shale. ing hornfelsed clastic sediments with Sandy, pebbly, and chaotic mudstones also oc- mineralized quartz veins and quartz-vein cur locally. Unit characterized by thick alternating stockwork. Hornfelsed zone is adjacent to small sequences of sandstone and shale with sand to rhyolite bodies. The brown-weathering charac- shale ratios of 100:1 to 1: 1,000(commonly about ter and fine-grained mineral laths distinguish this 10:1) hornfels zone from nonhornfelsed rocks and from other nonmineralized hornfels. Southwest Sandstone beds from c1 cm to >5.5 m thick of Fortyseven Creek, a hornfelsed zone similar (mean thickness = 0.4 m) occur in massive shale to the zone at Fortyseven Creek may represent sections, either as thin, laterally discontinuous an extension of the Fortyseven Creek trend. beds with sharp bases and tops, or as thick, Quartz veins and their alteration selvages con- commonly amalgamated beds with scoured tain native gold, scheelite, and arsenopyrite.The bases and gradational tops; contain diffuse lami- Fortyseven Creek occurrence also contains wol- nations composed of shale-chip intraclasts (see framite, arsenopyrite, pyrite, jamesonite, stibnite, Kuskokwim Group stratigraphic section, argentite, and traces of gold-silver tellurides; sheet 2). Other bedding features include load, tourmaline and sericite accompany quartz flute, and groove casts. Paleocurrent indicators gangue (Cady and others, 1955). On Fortyseven suggest a southwesterly transport direction Creek downstream from the main mineralized (fig. 1). Shale-chip intraclasts range in long shear zone, two distinct mineralized horizons dimension from microscopic to 7 cm and may occur in placer deposits: gold concentrations on occur at any level in the enclosing sandstone bed. bedrock and small amounts of gold distributed Bouma intervals in sandstone-dominated in gravels above bedrock, and gold and scheelite sections include T,, Tae, Tab, and Tabe and concentrations on a yellow clay false bedrock subordinate Tce and Tcde. Bouma intervals in shale-dominated sections include parallel- Kkh HORNFELSED SANDSTONEAND SHALE- laminatedTde and subordinate, thin T,, and Tcde; Hornfelsed clastic rocks of the Kuskokwim sandstone and siltstone intervals all have sharp Group occur adjacent to plutons, stocks, and bases and tops. Some shale-dominated sections dikes. Intensely hornfelsed rock contains are composed of poorly sorted sandy and pebbly poikiloblastic biotite; very fine grained, (sometimes chaotic) mudstones, which are subhedral, granular zoisite(?); sparse euhedral matrix-supported and contain clasts of angular pyrite; black-tourmaline rosettes; and fibrous to nebulitic shale and sandstone intraclasts and sillimanite. These inner-zone minerals are Inoceramus sp. fragments. Pebbly mudstones enclosed in completely recrystallized detrital correspond to facies F chaotic deposits, whereas grains and, rarely, by myrmekite; visible biotite sandstone-dominated sections correspond found adjacent to large igneous bodies mainly to facies B, and the shale-dominated sections to facies D and G. Facies associations KJs CALCAREOUS SANDSTONE, SILTSTONE, indicate inner-fan deposition. AND SHALE-Medium-brown and dark-gray, light-reddish-brown-weathering sandstone, silt- Unit also characterized by moderately to well- stone, and shale; locally crosscut by quartz and developed foliation, structurally dismembered carbonate veinlets; sandstone is generally sandstone phacoids in deformed shale, moderate medium grained and poorly sorted; contains recrystallization, grain elongation up to 2:1, abundant carbonate fossil fragements; larger highly shattered and flaggy outcrops, and local fragments west of Kiokluk Mountains have been white quartz veins. Swale and ridge topography identified as Inoceramus sp. (of probable Cretaceous age, table 4, map no. VI). Contact apparent intrusive relationship into Kuskokwim relationships with units in report area obscured Group. Similar albite rhyolite on Barometer by Quaternary deposits Mountain (near Sleetmute, northeast of map area) yielded one minimum whole-rock K-Ar Holitna Group age of 67.9 + 2.0 Ma and one K-Ar age on bi- DSh LIMESTONE AND DOLOMITEGVery light to otite of 70.5 * 2.1 Ma (DGGS unpublished data, medium-gray, thin- to massive-bedded, finely 1984); relationship with this unit is unclear laminated, limy and dolomitic mudstone to wackestone with interbeds of grainstone. Tird-Greenish medium-gray, fine-grained, al- Represents shallow, subtidal to peritidal cyclic tered, porphyritic quartz-plagioclase feldspar carbonate sedimentation (Clough and others, rhyolite phase. These non-biotite rocks are 1984). Commonly fossiliferous in subtidal deeply weathered, with green clots of dickite(?) wackestone to grainstone facies. Locally recrystallized and brecciated in areas adjacent Tirg-Medium-gray, fine-grained, porphyritic to faults. Estimated thickness of Holitna group garnet-biotite rhyolite. Medium-grained garnet in the central Kuskokwim region is 1,500 to and biotite in fine-grained, potassium-feldspar- 3,000 m (Cady and others, 1955). Age in map rich matrix area is Silurian to Devonian, on the basis of our lithologic correlation with equivalent, TKii HYPABYSSAL INTERMEDIATE INTRU- megafossil-bearing strata in the Kulukbuk Hills, SIVE ROCKS-Medium-gray and light-gray approximately 14 km east of map area porphyritic quartz diorite to granodiorite with medium-grained hornblende and plagioclase INTRUSIVE ROCKS phenocrysts (mean length = 3 mm) in fine- grained matrix of randomly to slightly oriented Tirl ALBITE RHYOLITE AND ALBITE RHYO- plagioclase laths. Intrudes Upper Cretaceous Tird/ LITE PORPHYRY-Medium-light-gray to buff, Kuskokwim Group. Forms dikes and small stocks. Tirg fine- to medium-grained, porphyritic, biotite-al- Locally, quartz monzonite to granite occur, con- bite rhyolite and white mica-biotite-albite taining potassium feldspar phenocrysts to 3.5 cm rhyolite porphyry. Tertiary age assigned on the long and medium-grained quartz and biotite phe- basis of intrusive relationship with Upper Cre- nocrysts. Extensive hydrothermal alteration taceous Kuskokwim Group and three K-Ar ages products include calcite, chlorite, epidote, opaque on white mica (61.7,63.6, and 61.5 Ma, table 3). minerals, and fine-grained white mica Biotite-albite rhyolite is characterized by euhedral and subhedral grains of quartz, albite, TKi APLITE-Light- to dark-gray and buff, fine- to biotite and sanidine in a matrix of very fine medium-grained aplite dikes composed of grained quartz, feldspar, and altered glass. quartz, potassium feldspar, and plagioclase feld- Geochemistry indicates a rhyolite to rhyodacite spar, with minor white mica, biotite, and range (table 1). Quartz phenocrysts contain re- hornblende sorbed rims and resorption channels. Sanidine and albite are altered to fine-grained white mica TKif FELSITE DIKES-Light-gray to buff, fine- and clay minerals locally. Subhedral grains of grained felsite dikes containing disseminated, biotite to 8 mm across are fresh and appear to fresh or oxidized pyrite cubes, or euhedral be primary phase of rhyolite. Minor white mica garnet (muscovite?) occurring as subhedral grains to 5 mm across appears also to be primary. Scat- Kkgd GRANODIORITE TO GRANITE OF THE tered subhedral to euhedral grains of dark-blue KIOKLUK MOUNTAINS-Medium- to dark- tourmaline occur throughout rhyolite. Near west- gray, nonfoliate hypidiomorphic-granular, ern flank of Holokuk Mountain, a breccia body medium- and coarse-grained hornblende-biotite contains angular to rounded fragments of and biotite-hornblende granodiorite to granite. tourmalinized rhyolite and rhyolite porphyry set Geochemistry indicates a granite to quartz in matrix of quartz and tourmaline. Where phe- monzonite composition (table 1). The color nocrysts of quartz and feldspar become dominant index is approximately 18. The Cretaceous age constituent, unit is rhyolite porphyry. No strati- of Kiokluk pluton is based on the K-Ar age of graphic evidence for age exists other than one biotite separate: 68.7 + 2.1 Ma (table 3). Plagioclase (An35) is subhedral to euhedral and where granite is hydrothermally altered. Two is slightly altered to fine-grained white mica, K-Ar ages on biotite separates from the granite locally to epidote. Quartz is anhedral and phase (Kcg) are 68.7 k 2.1 Ma and 67.5 k 2.0 Ma medium to fine grained. Potassium feldspar is (table 3) typically medium grained, occurs as subhedral and euhedral grains to 7 mm long, and locally Kgi GRANODORITEcSmall granodiorite intrusion forms rosettes. Biotite occurs as 0.5- to 3.0-mm in northeastern part of Sleetmute B-5 Quad- subhedral to anhedral grains that constitute 5 to rangle; age and relationship to other granodiorite 8 percent of the rock and may surround intrusions uncertain hornblende, producing mafic mineral clots. Hornblende occurs as 2- to 4-mm-long subhedral Kii INTERMEDIATE TO MAFIC STOCKS AND grains that form 6 to 12 percent of the rock and DIKES-Medium- to dark-gray, medium- is moderately altered to white mica, chlorite, grained, granodiorite and biotite, quartz-bearing epidote, and opaque minerals; accessory apatite gabbro. Intrudes Upper Cretaceous Kuskokwim is common. Quartz-tourmaline veins and minor Group. A minimum K-Ar age on one hornblende greisens occur locally separate is 63.8 k 1.9 Ma (table 3); unit crosscut by andesite dike unit (Kim), which is dated Kcgdl GRANODIORITE AND GRANITE OF THE at 69.8 * 2.1 Ma. Textures range from Kcg CHUILNUK MOUNTAINS-Medium- to dark- equigranular in stocks to porphyritic in dikes. gray, nonfoliate hypidiomorphic-granular, Plagioclase (An50) occurs as grains to 3 mm long medium-grained, hornblende- biotite grano- in stocks and to 4.5 mm long in porphyritic rocks diorite. 'Ibo geochemical analyses indicate a Augite occurs locally, forming to 15 percent of granodiorite composition (table 1). Textures rock in a mafic border phase of stocks and to range from equigranular to seriate and 5 percent in one dike. Alteration is moderate; porphyritic. Plagioclase feldspar (An3,) is the products include fine-grained white mica, chlo- predominant feldspar and occurs as subhedral rite, carbonate and opaque minerals to euhedral grains to 10 mm long. Potassium feldspar usually occurs as interstitial, anhedral Kim ANDESITE DIKES-Medium- to dark-gray, grains. Biotite occurs as subhedral grains from fine- to medium-grained andesite containing 1 to 3 mm across and constitutes to 11 percent subhedral plagioclase as dominant feldspar and of rock. Hornblende occurs as subhedral grains and locally minor quartz and potassium feldspar. anhedral masses to 4 mm long and constitutes 2 to Geochemical analysis yields an andesitic com- 10 percent of rock. Quartz occurs as anhedral position (table 1). Andesite dikes crosscut masses that constitute >8 percent of rock. lower(?) part of interlayered lahar deposits and Accessory minerals include zircon, apatite, and basaltic andesite unit (Khlb) which has been ra- opaques. Locally, granodiorite is hydrothermally diometrically dated at 73.2 k 2.2 Ma. One K-Ar altered to chlorite, white mica, epidote, and minor age is 69.8 * 2.1 Ma (table 3). Andesite dikes calcite. Crosscutting quartz-tourmalineveins occur also crosscut the intermediate to felsic flows, locally and are characterized by thin selvages of tuff, and agglomerate unit (Khf), the felsite brec- tourmaline surrounding quartz-dominant vein cia unit (Khb), and the intermediate to mafic centers. Granodiorite yielded K-Ar ages on biotite stocks and dikes unit (Kii). Mafic-mineral con- of 67.5 ~t2.0 Ma and 68.4 ~t2.1 Ma (table 3). The tent variable; includes biotite and hornblende. border phase, near the eastern and northern Andesite dikes altered locally; plagioclase al- boundaries of Chuilnuk pluton (Kcg), is light- to tered to clay minerals, some of which are green medium-gray, nonfoliate, seriate to porphyritic, (dickite?), biotite to chlorite, and hornblende to medium- to coarse-grained granite. One biotite and epidote geochemical analysis indicates a granite composition (table 1). Potassium feldspar occurs ACKNOWLEDGMENTS as euhedral and subhedral phenocrysts to 10 mm long and as subhedral masses in groundmass. We thank T.K. Bundtzen, DGGS, and S.E. Box, Biotite occurs as subhedral grains that constitute U.S. Geological Survey, for their thoughtful reviews, to 10 percent of rock; minor hornblende is also and Robert Blodgett, U.S. Geological Survey, for his present. Accessory zircon and apatite are comments. common, and epidote, sericite, and chlorite occur Six groove casts: one crossbed

Figure 1. Paleocurrent data for the Sleetmute A-5, A-6, B-5, and B-6 Quadrangles. Data have been corrected for tectonic dip.

REFERENCES CITED in Alaska: U.S. Geological Survey, Bulletin 1999, p. 8-16. Bundtzen, T.K. and Laird, G.M., 1982, Geologic map of the Adrain, J.M., Chatterton, B.D.E., and Blodgett, Iditarod D-2 and eastern D-3 Quadrangles, Alaska: Alaska R.B., in press, Silurian trilobites from Division of Geological & Geophysical Surveys Geologic southwestern Alaska: Journal of Report 72, 1 sheet, scale 1:63,360. Paleontology. 1983, Geologic map of the McGrath D-6 Quadrangle: Bouma, A.H., 1962, Sedimentology of some Alaska Division of Geological & Geophysical Surveys flysch deposits: Amsterdam, Elsevier, Professional Report 79, 1 sheet, scale 1:63,360. 168 p. 1991, Geology and mineral resources of the Russian Box, S.E. and Elder, W.P., 1992, Depositional Mission C-1 Quadrangle, southwest Alaska: Alaska Divi- and biostratigraphic framework of the sion of Geological & Geophysical Surveys Professional Upper Cretaceous Kuskokwim Group, Report 109, 24 p., 2 sheets, scale 1:63,360. southwestern Alaska, in Bradley, D.C., 1992, Geology and mineral resources of Iditarod min- and Ford, A.B. (eds.), Geologic studies ing district, Iditarod D-4 and eastern B-5 Quadrangles, west-central Alaska: Alaska Division of Geological & Geo- America, v. G-1, p. 589-619, plates 1, 2, 3, 5, scale physical Surveys Professional Report 97, 88 p., 2 sheets, 1:2,500,000. scale 1:63,360. Moore, T.E., and Wallace, W.K., 1985, Submarine-fan facies Cady, W.M., Wallace, R.E., Hoare, J.M., andwebber, E.J., 1955, of the Kuskokwim Group, Cairn Mountain area, south- The central Kuskokwim region, Alaska: U.S. Geological western Alaska (abs.), Geological Society of America Survey Professional Paper 268, 132 p. Abstract with Programs, v. 17, no. 6, p. 371. Clough, J.G., Blodgett, R.B., and Smith, T.N., 1984, Middle Murphy, J.M., Decker, John, Reifenstuhl, R.R., and Coonrad, Paleozoic subtidal to tidal flat carbonate sedimentation in W.L., 1984, Compilation of geologic data from the Sleet- southwest Alaska [abs.]: Geological Society of America mute B-6 Quadrangle, southwestern Alaska: Alaska Abstracts with Programs, v. 16, no. 5, p. 275. Division of Geological & Geophysical Surveys Public- Cobb, E.H., 1976, Summary of references to mineral occur- data File 84-6, 1 sheet, scale 1:63,360. rences (other than mineral fuels and construction materials) Mutti, Emiliano, and Ricci Lucchi, Franco, 1972, Le torbiditi in the Dillingham, Sleetmute, and Taylor Mountains Quad- dell'Appenino settentrional-introdusione all'analisi rangles, Alaska: U.S. Geological Survey Open-File Report de facies [Turbidites of the northern Apennines-introduc- 76-606, 92 p. tion to facies analysis]-with English summary: Memoire Decker, John, Bergman, S.C., Blodgett, R.B., Box, S.E., del la Societa Geologic Italiana, v. 11, p. 161-199. Bundtzen, T.K., Clough, J.G., Coonrad, W.L., Gilbert, Peccerillo, Angelo, and Taylor, S.R., 1976, Geochemistry of W.G., Miller, M.L., Murphy, J.M., Robinson, M.S., and Eocene calc-alkaline volcanic rocks from the Kastamonu Wallace, W.K., 1994, Geology of southwestern Alaska, in area, northern Turkey: Contributions to Mineralogy and Plafker, G., and Berg, H.C., eds., The Geology of Alaska: Petrology, v. 58, no. 1, p. 63-81. Boulder, Colorado, Geological Society of America, The Reifenstuhl, R.R., Decker, John, Murphy, J.M., and Coonrad, Geology of North America, v. G-1, p. 285-310, plate 3, W.L., 1984, Compilation of geologic data from the scale 1:2,500,000. Sleetmute A-6 Quadrangle, southwestern Alaska: Alaska Decker, John, Reifenstuhl, R.R., and Coonrad, W.L., 1984, Division of Geological & Geophysical Surveys Public- Compilation of geologic data from the Sleetmute B-5 data File 84-43, 1 sheet, scale 1:63,360. Quadrangle, southwesternAlaska: Alaska Division of Geo- Reifenstuhl, R.R., Robinson, M.S., Smith,T.E.,Albanese, M.D., logical & Geophysical Surveys Public-data File 84-44, and Allegro, G.A., 1984, Geologic map of the Sleetmute 1 sheet, scale 1:63,360. B-6 Quadrangle, Alaska: Alaska Division of Geological Decker, John, Robinson, M.S., Murphy, J.M., Reifenstuhl, R.R., & Geophysical Surveys Report of Investigations 84-12, andAlbanese, M.D., 1984, Geologic map of the Sleetmute 1 sheet, scale 1:40,000. A-6 Quadrangle, Alaska: Alaska Division of Geological Robinson, MS., 1984a, Rock, pan-concentrate and stream-sedi- & Geophysical Surveys Report of Investigations 84-8, ment geochemistry, Sleetmute A-6 Quadrangle, Alaska: 1 sheet, scale 1:40,000. Alaska Division of Geological & Geophysical Surveys Elder, W.P., and Box, S.E., 1992, Late Cretaceous inoceramid Report of Investigations 84-7, 1 sheet, scale 1:40,000. bivalves of the Kuskokwim basin, southwestern Alaska, 1984b, Rock, pan-concentrate and stream-sediment and their implications for basin evolution, Journal of Pa- geochemistry, Sleetmute B-5 Quadrangle, Alaska: Alaska leontology Memoir 26, 39 p. Division of Geological & Geophysical Surveys Report of Elder, W.P., and Miller, J.W., 1991, Maps showing fossil Investigations 84-9, 1 sheet, scale 1:40,000. localities and checklists of Jurassic and Cretaceous 1984c, Rock, pan-concentrate and stream-sediment macrofauna of western Alaska, U.S. Geological Survey geochemistry, Sleetmute B-6 Quadrangle, Alaska: Alaska Open-File Report 91-629,72 p., 3 sheets, scale 1:500,000. Division of Geological & Geophysical Surveys Report of Miller, M.L. and Bundtzen, T.K., 1994, Generalized geologic Investigations 84-11, 1 sheet, scale 1:40,000. map of the Iditarod Quadrangle, Alaska, showing Robinson, M.S., Decker, John, Reifenstuhl, R.R., Murphy, J.M., potassium-argon, major-oxide trace-element, fossil, and Box, S.E., 1984, Geologic map of the Sleetmute B-5 paleocurrent, and archaeological sample localities: U.S. Quadrangle, Alaska: Alaska Division of Geological & Geo- Geological Survey, Miscellaneous Field Studies Map, physical Surveys Report of Investigations 84-10, 1 sheet, 48 p., 1 sheet, scale 1:250,000. scale 1:40,000. Moll-Stalcup, E.J., 1994, Latest Cretaceous and Cenozoic Sainsbury, C.L., and MacKevett, E.M., Jr., 1965, Quicksilver magmatism in mainland Alaska in Plafker, G., and Berg, deposits of southwestern Alaska: U.S. Geological Survey H.C., eds., The Geology of Alaska: Boulder, Colorado, Bulletin 1187, 89 p. Geological Society of America, The Geology of North Table 1. Major oxide geochemical analysis of rock samplesa

Map Map b no. Rock type unit Si02 A1203 Fe203 FeO MgO CaO Na20 K20 Ti02 Pz05 MnO LO^ Total

Basaltic andesite nap Basaltic andesite Gap Rhyolite Tir Rhyolite Tir Dacite Khap Basaltic andesite Khap Basaltic andesite Tvr2 Rhyolite Tvr3 Rhyolite Tvr3 Rhyolite Tvr3 Basaltic andesite map Basaltic andesite Khap Quartz latite Khlb Quartz latite Khl Basaltic andesite Khlb Basaltic andesite Khlb Andesite Khlb Rhyolite Tir Rhyolite Tir Rhyolite Tir Rhyolite Tir Rhyolite Tir Rhyolite Tir Rhyodacite Tir Rhyolite Tir Basaltic andesite Kha Basaltic andesite Kha Basaltic andesite Khap Basaltic andesite Khap Granite Keg Granodiorite Kcgd Granodiorite Kcgd Basaltic andesite Khaa Andesite Kim Andesite Khaa Andesite Kh f Quartz monzonite Kkgd Quartz monzonite Kkgd Quartz monzonite Kkgd Hornfels, Kkh Basaltic andesite Khaa Dacite TKif Aplite TKif Dacite Tkif a~-rayfluorescence analyses by the DGGS Geochemical Lab, Fairbanks, Alaska. Major oxide values are in percent. b~eccerilloand Taylor (1976) classification system. bsson ignition. - - = Not analyzed. Table la. Nonnative mineralogy (calculated using U.S. Geological Survey Petcal program)

CIPW Norms Map no. Quartz Corundum Orthoclase Albite Anorthite Hypersthene Wollastonite Diopside Magnetite Hematite Illmenite Apatite Total

- - = Not present.

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Table 2. Trace element geocliemical analyses of rock samples

Map no.

"Instrumental neutron activation analysis by Becquerel Labs, Mississauga, Ontario, Canada b~nalysisby X-ray fluorescence by Becquerel Labs, Mississauga, Ontario, Canada. *Calculated initial ratio by Dr. Teny Davis, University of Califomia, Davis, California dMeasured initial ratio by Dr. Terry Davis, University of California, Davis, Califomia - - =Not analyzed.