U.S. DEPARTMENT OF THE INTERIOR TO ACCOMPANY MAP I–2677 U.S. GEOLOGICAL SURVEY GEOLOGIC MAP OF THE NULATO QUADRANGLE, WEST-CENTRAL ALASKA By W.W. Patton, Jr. and E.J. Moll-Stalcup DESCRIPTION OF MAP UNITS Volcanic and intrusive rocks OVERLAP ASSEMBLAGES Ta Andesite and basalt (Eocene)—Form a Surficial deposits large area of columnar-jointed flows Qfy Younger flood-plain deposits (Holo- overlying marine and nonmarine deltaic cene)—Chiefly light-gray micaceous silt deposits (Ks) of the Yukon-Koyukuk along Yukon and Koyukuk Rivers. basin in the southwestern corner of the Silt, sand, and gravel along streams that Nulato quadrangle and small flows drain bedrock uplands. Locally include overlying rocks of the basalt, gabbro, some terrace and fan deposits along and chert unit (ၣMb) and the quartzite, narrow tributary valleys. Characterized phyllite, and limestone unit (ၣMq) of physiographically by bars, oxbow lakes, the Angayucham-Tozitna terrane in the meander scrolls, abandoned channels, southeastern corner of the Nulato quad- and other evidence of recent flood-plain rangle. Basalt is much less common than building. Mapped almost entirely from andesite and is distinguished solely on aerial photographs. the basis of chemical composition. In Qfo Older flood-plain deposits (Holocene or southwest corner of the quadrangle Pleistocene)—Chiefly light-gray and andesite and basalt flows are composed grayish-orange micaceous silt. Sub- of 5 to 10 percent phenocrysts of plagio- ordinate lenses of sand and peat, clase, clinopyroxene, and less commonly reworked eolian sand, and gravel. orthopyroxene set in a groundmass of Deposits are at or near river level but plagioclase microlites, granules of youthful flood-plain features such as orthopyroxene, and opaque oxides. Some oxbow lakes, meander scrolls, and flows contain abundant dark, partly abandoned channels are much modified devitrified glass subophitically enclosing or absent. Contact with younger flood- phenocrysts. Textures typical of calc- plain deposits (Qfy) locally gradational alkalic rocks and indicative of magma and poorly defined. Mapped almost mixing are common, including glomero- entirely from aerial photographs. porphyritic clots and strongly zoned and Qhs High-terrace and slope deposits resorbed plagioclase phenocrysts. One (Pleistocene)—Light-gray and grayish- andesite flow (81Pa321, table 4) contains orange micaceous silt deposits. a disequilibrium assemblage of quartz Subordinate lenses of sand and peat. and strongly zoned plagioclase pheno- Deposits form a broad terrace 10 to 100 crysts in a groundmass of granular clino- m above the floodplain of the Koyukuk pyroxene, opaque oxides, plagioclase Flats and extend as much as 250 m up microlites, and interstitial glass. Unit the slopes of the bedrock hills locally includes tuffs, breccias, and surrounding the Flats. Terrace deposits agglomerates in subordinate amounts. characterized by thermokarst topography The basalt and andesite flows appear marked by many small lakes. Unit also to underlie or are intruded by the rhyo- forms terrace and slope deposits along lite and dacite unit (Tr). The circular streams that drain bedrock uplands and drainage pattern of Poison Creek in the locally include coarse-grained colluvial Nulato quadrangle and Stink Creek in and fan deposits. the adjoining Norton Bay quadrangle 1 appears to outline a volcanic depression clase, biotite, and hornblende in a finer or “caldera” about 12 km in diameter crystalline groundmass of plagioclase, within the area covered by these flows. quartz, and potassium feldspar(?). The In the southeastern part of the quad- domal intrusive bodies typically are rangle the unit is represented by two deeply weathered and in several places small columnar jointed andesite por- form prominent reddish-orange limonite phyry flows composed of 5 to 7 percent gossans. Three samples of the rhyolite phenocrysts of ortho- and clinopyroxene and dacite unit were analyzed chem- and plagioclase set in a groundmass of ically: two from the southwest corner plagioclase microlites, granular ortho- of the quadrangle (81Pa319 and 82Pa76, pyroxene, opaque oxides, and dark table 3) and one from the southeast part partially devitrified glass. of the quadrangle (79Pa519, table 3). The basalt and andesite unit is The rhyolite and dacite unit is assigned an Eocene age on the basis of assigned an Eocene age on the basis of three whole rock K-Ar ages ranging two K-Ar mineral-separate ages obtained from 52.4±1.6 to 56.5±1.7 Ma from ex- from dacite exposures along the west posures on the west bank of the Yukon bank of the Yukon River (table 1; fig. River (no. 1, table 1 and fig. 22). 22). Sample 81APa319 (no. 3) yielded Tr Rhyolite and dacite (Eocene)—Unit ages of 50.6±1.5 (biotite) and 47.6±1.4 overlies and intrudes nonmarine and (hornblende) Ma and sample DT 83-15A marine deltaic deposits (Ks) of the (no. 2) yielded ages of 53.3±1.6 (biotite) Yukon-Koyukuk basin in the southwest and 49.1±1.5 (hornblende) Ma. corner of the Nulato quadrangle. East TKi Shallow intrusive rocks of silicic and of the Yukon River in the south-central intermediate composition (early and southeastern part of the quadrangle Tertiary and Late Cretaceous)— it forms small sparsely distributed bodies Includes a wide variety of shallow that overlie and intrude the schist and intrusive rocks including rhyolite, dacite, quartzite unit (ၦၠs) of the Ruby terrane and andesite plugs, domes, sills, and and the basalt, gabbro, and chert unit dikes and larger more coarsely crys- (ၣMb) of the Angayucham-Tozitna talline bodies of granite, granodiorite, terrane. The rhyolite and dacite unit is tonalite, and monzonite porphyry. Unit spatially associated with the andesite and is confined to the northeastern part of basalt map unit (Ta) and forms a com- the quadrangle where it intrudes sedi- positionally continuous suite with that mentary deposits of the Yukon-Koyukuk unit. The rhyolite and dacite unit is basin (map units Ks, Kg) and andesitic composed of finely banded flows, tuffs, volcanic rocks (map unit Kv) of the breccias, and hypabyssal domal intrusive Koyukuk terrane. The unit is poorly bodies. The rhyolite consists of 2 to 5 exposed and has not been studied in percent fine-grained phenocrysts of detail. Thin-section examination was plagioclase, biotite, magnetite, and, in limited to four samples from widely places, quartz and anorthoclase in a separated localities. The coarser grained groundmass of devitrified glass. The bodies typically have a porphyritic dacite consists of 2 to 25 percent pheno- texture consisting of phenocrysts of feld- crysts of oligoclase, magnetite, and spar and biotite or hornblende set in a either granular pyroxene or green horn- finer crystalline groundmass of feldspar, blende and biotite in a groundmass of quartz, and subordinate mafic minerals. plagioclase microlites and glass. Spher- A thin section from a columnar-jointed ulitic growths, including some litho- plug-like dacite body forming Meuller physae, are common. The degree of Mountain, 25 km east of Galena, con- devitrification varies: some samples have sists of 7 percent phenocrysts of strongly retained abundant glass and display a zoned plagioclase and a greenish-brown vitrophyric texture; others are composed alteration mineral, probably replacing almost entirely of aphyric perlitic glass. amphibole, in a fine-grained groundmass The hypabyssal domal intrusive bodies of plagioclase laths and minor interstitial are composed of phenocrysts of plagio- quartz. 2 The unit is provisionally assigned gray micaceous shale, quartz-chert- an early Tertiary and Late Cretaceous pebble conglomerate, and thin seams of age. No isotopic age data are available bituminous coal. The unit is composed for these rocks in the Nulato quadrangle, of nonmarine, fluvial, delta-plain de- but to the northeast, in the adjoining posits that grade downward into marine Melozitna quadrangle, similar intrusive delta-front deposits. It appears to have rocks yield a Late Cretaceous isotopic had its source to the east and to fine age (Patton and others, 1978) and to westward; paleocurrent directions are the southwest, in the adjoining Una- westward (Nilsen, 1989). The total lakleet quadrangle, a similar assemblage thickness of the unit along the west bank of shallow intrusive rocks yield early of the Yukon River between the mouth Tertiary and latest Cretaceous isotopic of the Koyukuk River and the Kaltag ages (Patton and Moll-Stalcup, 1996). Fault is estimated to be 750 m (Patton and Bickel, 1956). Along the western Complex of small intrusive bodies and thermally altered boundary of the Nulato quadrangle the sedimentary deposits thickness of the unit is estimated to be TKis Shallow intrusive rocks of silicic and 2,500 m (Bickel and Patton, 1957). intermediate composition (early The nonmarine fluvial deposits are Tertiary and Late Cretaceous), characterized by fine-grained, locally marine and nonmarine sandstone, cross-bedded quartzose sandstone that shale, and conglomerate deltaic characteristically weathers to a rusty deposits (Late and Early yellow orange. The sandstone is inter- Cretaceous), and volcaniclastic bedded with dark-grayish-olive to gray graywacke and mudstone turbidite micaceous shale and siltstone. Near the deposits (Early Cretaceous), base, the fluvial beds are composed of undivided—Small bodies of rhyolite, fine- to coarse-grained, lenticular, cross- dacite, and andesite porphyry and larger bedded, friable sandstone and conglo- more coarsely crystalline bodies of merate containing