Andesite and basalt

Unit symbol: Jab
Age range Jurassic (174.1 to 152.1 Ma)
Lithology: Igneous - Volcanic
Group name: Andesite and basalt
Massive andesitic and basaltic flows; minor trachyte, trachyandesite, and dacite; some interbedded tuffaceous sedimentary rocks; and, locally, several hundred feet of volcanic breccia at or near base (Box and others, 1993) are primarily exposed in the Bethel and Russian Mission quadrangles of southwest Alaska. An estimated 25 percent of the unit is interbedded graywacke, siltstone, impure limestone, and pebble conglomerate (Hoare and Coonrad, 1959a, b). The rock unit forms a belt in low rolling hills on the west flank of the Kuskokwim Mountains from the Kwethluk River northward (Hoare and Coonrad, 1959a). Marine pelecypods of Bajocian age (Middle Jurassic) occur in sedimentary strata of this unit (Hoare and Coonrad, 1959a; Box and others, 1993). Unit provisionally also includes an isolated outcrop in the north-central Dillingham quadrangle that is predominantly diabase, which may intrude rocks provisionally assigned to map unit K^s, thus suggesting that its age may be Triassic or younger. Volcanic rock outcrops to the northeast within the Dillingham quadrangle were inferred to be Triassic by Wallace and others (1989); in neither case does true age control exist

Source map information

Source map Box, S.E., Moll-Stalcup, E.J., Frost, T.P., and Murphy, J.M., 1993, Preliminary geologic map of the Bethel and southern Russian Mission quadrangles, southwestern Alaska: U.S. Geological Survey Miscellaneous Field Studies Map MF-2226-A, 1 sheet, scale 1:250,000.
Symbol Jab
Unit name Andesitic and Basaltic lava flows and marine epiclastic rocks, Nyac terrane
Description Andesitic and basaltic lava flows, flow breccias, and tuffaceous marine sedimentary rocks
Lithology Sedimentary

Correlated geologic units

Label Jvab
Description Andesitic and basaltic flows, flow breccias and epiclastic marine rocks; younger than Talkeetna
Geologic age Middle-Jurassic to Kimmeridgian
Geologic setting Extrusive
Lithology Form Importance
Andesite < Mafic-volcanic < Volcanic < Igneous Flow Major
Basalt < Mafic-volcanic < Volcanic < Igneous Flow Indeterminate, major
Conglomerate-sandstone < Mixed-clastic < Clastic < Sedimentary Tuffaceous Indeterminate, major