Andesite and basalt flows

Unit symbol: Tepv
Age range Tertiary, Eocene and Paleocene (66 to 33.9 Ma)
Lithology: Igneous - Volcanic
Group name: Volcanic rocks, undivided
Composed of columnar-jointed andesite and basalt lavas that locally contain interbedded tuff, breccia, and agglomerate, this unit is exposed in two general areas, the largest of which is a belt in west-central Alaska. In the Takhakhdona Hills of the Melozitna quadrangle, dark-green to black vesicular basalt flows formerly mapped by Patton and others (1978) in their unit TKv are now known to be of Tertiary age. Isolated exposures include the Roundabout Mountain volcanic field in the Kateel River quadrangle and olivine basalt flows on the eastern part of the boundary between the Bettles and Tanana quadrangles. The second area of exposure is in the Talkeetna Mountains quadrangle, where there are outcrops of predominantly reddish-brown-weathering, aphanitic to porphyritic basaltic andesite and basalt lavas, often columnar-jointed, and including lesser tuff, and local cinder deposits (Csejtey, 1974, Anderson, 1969, Oswald, 2006). Individual flows and tuff beds are tens of meters thick; as a whole, unit may be several hundred meters thick. A small area of basalt flows is also mapped in the western Lime Hills quadrangle (Wilson and others, 1998). K/Ar isotopic cooling ages range from 65.2±3.9 to 38.6±2.4 Ma

Source map information

Source map Patton, W.W., Jr., Wilson, F.H., Labay, K.A., and Shew, Nora, 2009, Geologic map of the Yukon-Koyukuk basin, Alaska: U.S. Geological Survey Scientific Investigations Map SIM-2909, 26 p., 2 sheets, scale 1:500,000.
Symbol Tad
Unit name Andesite and basalt lava flows and volcaniclastic rocks (Ta) and dacite and rhyolite lava flows, domes, and volcaniclastic rocks (Td), undivided
Description Confined to west-central part of Melozitna quadrangle where field data are insufficient to map units Ta and Td separately.
Lithology Igneous

Correlated geologic units

Label Tvab
Description Andesite and basalt flows
Geologic age Paleocene to Eocene
Geologic setting Extrusive
Lithology Form Importance
Andesite < Mafic-volcanic < Volcanic < Igneous Flow Major
Basalt < Mafic-volcanic < Volcanic < Igneous Flow Major