Felsic volcanic rocks of southwest Alaska

Unit symbol: Tev
Age range Tertiary, Oligocene? and Eocene (56 to 33.9 Ma)
Lithology: Igneous - Volcanic
Group name: Older volcanic rocks, undivided
Primarily felsic tuff and subordinate flows and hypabyssal intrusive rocks exposed across southwest Alaska. In the Iliamna quadrangle, unit consists of cream, light-gray, green, and purple bedded lithic, crystal, and vitric tuff (Detterman and Reed, 1980). Also includes distinctive light-colored horizons of felsic tuff interlayered with olivine basalt in the northeast Dillingham quadrangle (Wilson and others, 2003). A sample from the northeast Dillingham quadrangle yielded a 40Ar/39Ar age determination of 45.1±0.19 Ma, whereas a sample from the southern Dillingham quadrangle yielded a 40Ar/39Ar age of 41.62±0.18 Ma, consistent with the age of the Meshik Volcanics (unit Tmv). Includes two areas of massive columnar-jointed olivine basalt flows in the northeast Dillingham quadrangle (Wilson and others, 2003). The younger of these flows range from 3 to 10 m thick, are interbedded with felsic tuff, and yielded a 40Ar/39Ar age of 44.47±0.41 Ma. The older olivine basalt may, in part, be a sill—locally it is overlain by contact-metamorphosed rocks of unit KTrs, marine volcaniclastic sandstone, conglomerate, and argillite; it yields a 40Ar/39Ar isochron age of 53.71±0.61 Ma. Similar tuff and subordinate flows and hypabyssal intrusive rocks of apparently felsic composition occur in small exposures in the eastern Lime Hills and the Melozitna quadrangles, as well as in local accumulations of volcaniclastic sedimentary rocks. In some areas of the Lime Hills quadrangle, rocks range from andesite to dacite in composition, but in general the composition was not reported (Gamble and others, 2013). Farther west in the Bethel quadrangle, Box and others (1993) mapped moderately altered rhyolite flows, domes, and ash-flow tuff. Rhyolite of the main volcanic field contains phenocrysts of sanidine, riebeckitic amphibole, and rare quartz. A 40Ar/39Ar total-fusion age on riebeckite was 54.7±1.6 Ma. Also includes andesite to basalt plugs, volcanic rubble and breccia, including some agglomerate; may include deposits of lahars (Detterman and Reed, 1980). Many of the rocks of this map unit are associated with eruptive centers. An isolated andesitic plug in northeast Dillingham quadrangle yielded a 40Ar/39Ar isochron age of 47.95±1.39 Ma. Overall, dates on rocks of this map unit range from the 54.7±1.6 Ma age mentioned above to 33.6±3.4 Ma

Source map information

Source map Patton, W.W., Jr., Wilson, F.H., and Labay, K.A., in press, Reconnaissance geologic map of the lower Yukon River Region, Alaska: U.S. Geological Survey Scientific Investigations Series Map SIM-3015, scale 1:500,000.
Symbol TKa
Unit name Andesite and basalt flows and volcanicclasitc rocks
Description Composed primarily of andesite and basalt lava flows and volcaniclastic rocks. Flows are generally porphyritic and composed of phenocrysts of plagioclase and pyroxene in a groundmass of plagioclase microlites. Volcaniclastic rocks include breccia, tuffs, and agglomerates. The andesitic and basaltic rocks commonly are interlayered with or intruded by small bodies of dacite and rhyolite belonging to units TKi and TKd
Lithology Igneous

Correlated geologic units

Label Tvbs
Description Andesite and basalt, southwest Alaska
Geologic age Eocene
Geologic setting Extrusive, mafic
Lithology Form Importance
Andesite < Mafic-volcanic < Volcanic < Igneous Major
Basalt < Mafic-volcanic < Volcanic < Igneous Major
Basaltic-andesite < Mafic-volcanic < Volcanic < Igneous Indeterminate, major
Felsic-volcanic < Volcanic < Igneous Incidental