Qac
Alluvial, coluvial, and glacial drift deposits, undivided
Quaternary
Alluvial and colluvial deposits consist mainly of silt, sandy silt, and peat. Alluvium is chiefly fluvial in origin, but locally includes reworked windblown deposits. Alluvium deposits cover wide areas of the Yukon and Kuskokwim lowlands and delta plain.
Patton, W.W., Jr., Wilson, F.H., and Labay, K.A. , in prep.,  Reconnaissance geologic map of  the lower Yukon River Region, Alaska:  U.S. Geological survey Scientific Investigations Serives Map SIM-xxxxx, scale 1:500,000.
Qf
Floodplain, tidal flats, and estuarine deposits
Unit composed chiefly of silt, fine sand, and peat. Some gravel and coarse sand deposits occur locally along the tributary streams draining upland areas.  The tidal flats and estuarine deposits consist of fluvial silt and sandy silt that have been rework
Qbl
Beach and lagoonal deposits
Silt and sandy silt.  Unit includes active offshore bars, spits, and beaches ridges and former beach ridges that have been recently stabilized by a thin vegetation cover. Scattered localities along the Norton Sound and Bering Sea coast in the St. Michael
Qw
Wind-blown deposits
Silt and sandy silt deposits form broad partly dissected stabilized dune fields in the lowlands of the Yukon and Kuskokwim Rivers.  They compose sheet-like bodies as much as 100 feet thick that are characterized by modified dunes and blowouts.  The dune
Qb
Young scoriaceous tholeiitic basalt lava flows and cones
Young scoriaceous tholeiitic basalt lava flows and cones. Mapped separately on the western half of Stuart Island in the St Michael quadrangle.  The flows and cones are unmodified by erosion suggesting a Holocene or late Pleistocene age
QTb
Basalt lava flows
Quaternary  and latest Tertiary
Vesicular tholeiitic and alkali basalt lava flows, cones and maar craters.  The flows are interlayered with thinner pyroclastic deposits that range from fine ash to coarse breccia.  The lava flows are overlain locally by younger cinder cones and short fl
Tb
late Tertiary
Flat-lying vesicular basalt yielding whole-rock K/Ar ages of 19.35 Ma and 6.19 Ma (table 1).  Basalt flows located in Three small areas in the Russian Mission quadrangle: two on the Yukon River in the northwestern part of the quadrangle and one on the Ow
TKad
Dacite, rhyolite, andesite, and basalt lava flows, and volcaniclastic and hypabyssal rocks, undivided
Tertiary and latest Cretaceous
Dacite, rhyolite, andesite, and basalt lava flows, and volcaniclastic and hypabyssal rocks, undivided. Confined to the Holy Cross quadrangle and northern part of the Russian Mission quadrangle where field data are insufficient to map units TKa and TKd se
TKi
Shallow intrusive rocks of intermediate and silicic composition
early Tertiary and latest Cretaceous
Rhyolite, dacite and lesser amounts of trachyte and andesite. Typical rock is composed of a fine-grained felsic groundmass with phenocrysts of plagioclase, quartz, biotite, and hornblende.  Unit forms numerous large sills, dikes, and domes intruding Cret
TKd
Dacite and rhyolite lows, domes, ash-flow tuffs, and hypabyssal rocks
early Teritiary and latest Cretaceous
Dacite, rhyolite, and subordinate trachyandesite lava flows, sills, dikes, and interlayered tuffs, ash-flow tuffs, and breccias.  Flows generally are porphyritic and composed of phenocrysts of plagioclase, sanidine, quartz, and biotite in a fine-grained
TKa
Andesite and basalt flows and volcanicclasitc rocks
Andesite and basalt lava flows and volcaniclastic rocks widely exposed in the central part of the Holy Cross quadrangle and in a small area in the north-central part of the Russian Mission quadrangle between the Yukon and Kuskokwim Rivers. Flows are gene
TKg
Granite, granodiorite, syenite, and monzonite
Unit is composed of plutonic rocks that have been assigned various petrographic names based largely on field inspection and examination of only a few thin sections.  The most common rock type appears to be biotite granite, but other rock types include gr
TJm
Altered mafic plutonic rocks
early Tertiary? to Jurassic?
Altered gabbro and diorite bodies crops out as small scattered bodies in the Russian Mission and southern Holy Cross quadrangles. Intrudes units Jab, Jvc, KJv, and KMu. Age of unit is uncertain, but not older than Jurassic.  Some or the entire unit may b
Kym
Yukon-Koyukuk Basin, marginal shelf and slope deposits, polymict conglomerate, sandstone, siltstone, and shale
Cretaceous
Pebble and cobble conglomerate interbedded with sandstone, siltstone, and shale.  Conglomerate contains clasts of chert, greenstone, quartz, and schist in variable amounts.  Sandstone varies from poorly-sorted graywacke turbidite to well-washed quartz-ri
Kks
Kuskokwim Basin, deltaic and submarine fan deposits, undivided, sandstone, siltstone, and shale
Deltaic and submarine fan deposits. The deltaic deposits consist of well-sorted, quartz-rich, cross-bedded sandstone interbedded with siltstone and shale.  The submarine fan deposits are composed of turbiditic sequences that grade from fine- to medium-gr
KJv
Koyukuk Terrane, Andesitic and basaltic volcaniclastic rocks and lava flows, chert, and mudstone
Early Cretaceous and Jurassic
Highly disrupted assemblage of mafic to intermediate tuff, breccia, agglomerate, volcanic-rich conglomerate and graywacke, cherty tuff, chert, mudstone, and andesitic and basaltic pillow lava.  Unit locally intruded by sills and dikes of diabase, diorite
Kys
Yukon-Koyukuk Basin, Deltaic Deposits, sandstone and siltstone
Fluvial and shallow marine deltaic deposits of sandstone, siltstone, and shale.  Locally crossbedded and ripple marked.  Unit contains abundant plant debris and fresh- and brackish-water mollusks. Unit is widely exposed in a broad belt that extends from
Dot pattern where Kys is intruded by numerous small hypabyssal bodies of rhyolite and dacite (TKi)
Undivided areas where numerous small hypabyssal bodies of rhyolite and dacite (TKi) intrude the Kys unit.  Kys-fluvial and shallow marine deltaic deposits of sandstone, siltstone, and shale.  Locally crossbedded and ripple marked.  Unit contains abundant
Kygc
Yukon-Koyukuk Basin, submarine fan deposits, highly calcareous graywacke sandstone and mudstone turbidites
Turbidites composed of highly calcareous sandstone interbedded with noncalcareous micaeous siltstone and shale. Unit is infolded with and locally intertongues with unit Kygv. Unit extends in a broad belt from the southeast corner of St. Michael quadrangl
Kygv
Yukon-Koyukuk Basin, submarine fan deposits, graywacke sandstone and mudstone turbidites containing abundant laumontitized volcanic debris
Hard, well-indurated, fine- to medium-grained, locally tuffaceous, sandstone and micaceous mudstone.  Some sandstone beds have a distinctly mottled appearance owing to the presence of laumontite, most commonly in tuffaceous-rich layers. Unit widely expos
Kyg
Yukon-Koyukuk Basin, submarine fan deposits, graywacke sandstone and mudstone turbidites, undivided
Sparse field data on this sedimentary rock unit suggests that this unit is composed largely of graywacke turbidites including both highly calcareous graywacke as in unit Kygc and graywacke containing abundant volcanic debris as in unit Kygv. Unit poorly
Kg
Granodiorite and granite
Late and Early Cretaceous
Biotite granite and hornblende-biotite granodiorite forming two large plutons that intrude the Nyac terrane in Kilbuck Mountains of the southern Russian Mission quadrangle and a large pluton that forms the Askinuk Mountains in the Hooper Bay quadrangle.
KJb
Nyak Terrane, Basalt and andesite lava flows
Early Cretaceous and/or Late Jurassic
Variably altered basalt and andesite.  Conformably(?) overlies unit Jab and unconformably(?) overlain by unit TJb. Unit confined to Kilbuck Mountains, southeastern Russian Mission quadrangle. Assigned an Early Cretaceous to latest Jurassic age.
TJb
Whitefish Lake Volcanic Field
Altered basaltic lava flows and subordinate altered andesite, dacite, rhyolite, and gabbro.  Inferred to be the extrusive equivalent of the mafic intrusive rocks of unit TJm. Unit confined to Kilbuck Mountains, southeastern Russian Mission quadrangle. Re
TJi
Hyabyssal felsic intrusive rocks
Altered rhyolite and dacite porphyries confined to a single body in the Kilbuck Mountains, southeastern Russian Mission quadrangle. Age of unit is uncertain, but not older than Jurassic.  May be as young as Tertiary.
Jvc
Nyak Terrane, volcaniclastic conglomerate, sandstone, and shale
Late and Middle Jurassic
Tuffaceous marine sandstone, shale, and conglomerate.  Minor basaltic and andesitic lava flows.  Felsic pyroclastic rocks locally near base of unit.  Unit rests conformably(?) on unit Jab and is conformably(?) overlain by unit KJb. Mollusks  collected fr
Jt
Koyukuk Terrane, Trondhjemite, tonalite, and gabbro
Jurassic
Unit is composed chiefly of trondjhemite and tonalite plutonic bodies that locally have been metasomatized by potassium-rich fluids to a pink granite and granonodiorite.  Unit also includes subordinate amounts of quartz diorite, diorite, and gabbro.  Som
Jab
Nyak Terrane, andesitic and basaltic lava flows and marine volcaniclastic rocks
Middle Jurassic
Andesitic and basaltic lava flows and flow breccias.  Tuffaceous sandstone and conglomerate containing abundant clasts of mafic and intermediate volcanic and intrusive rocks.  Unit altered to low-grade metamorphic facies by variable replacement of primar
MzPzum
Koyukuk Terrane, serpentinized ulramafic rocks
Mesozoic and Paleozoic?
Unit presumed to be composed of partly serpentinized harzburgbite and dunite.  May also include some gabbro. Unit confined to four small bodies distributed along the eastern side of a fault-bounded belt composed of units KMu and Jt in the north-central p
KMu
Angayucham-Tozitna Terrane, andesitic and basaltic volcaniclastic rocks, lava flows, chert, and mudstone (KJv) and altered basalt, chert, chert breccia, argillite, tuff, volcaniclastic graywacke, and limestone (TrMv), undivided
Cretaceous to Mississippian
Andesitic and basaltic volcaniclastic rocks, lava flows, chert, and mudstone (KJv) and altered basalt, chert, chert breccia, argillite, tuff, volcaniclastic graywacke, and limestone (TrMv), undivided.
TrMv
Angayucham-Tozitna Terrane, complexly deformed assemblage of altered basalt, chert, chert breccia, argillite, tuff, volcaniclastic graywacke, and limestone
Triassic to Mississippian
Unit composed of a wide variety of mafic and intermediate volcanic and intrusive rocks including basalt, andesite, diabase, gabbro, and volcaniclastic rocks that are weakly metamorphosed to prehnite-pumpellyite and locally to greenschist facies assemblag