Woodchopper Volcanics, and Schwatka unit of Weber and others (1992)

Unit symbol: Dvec
Age range Devonian (419.2 to 358.9 Ma)
Lithology: Igneous - Volcanic
Group name: Woodchopper Volcanics and Schwatka unit of Weber and others (1992)
Woodchopper Volcanics are described as amygdaloidal basalt, pillow basalt, and aquagene tuff, subordinate interbedded chert, argillite, quartzite, and limestone of Late(?) to Early Devonian age in the Charley River quadrangle (Brabb and Churkin, 1969). Brabb and Churkin (1969) also mapped several hundred feet of dark-gray laminated shale that contains minor interbeds of limestone associated with the Woodchopper Volcanics, which we have included here; Dover and Miyaoka (1988) included these sedimentary rocks in their redefined Nation River Formation, which we do not believe is an appropriate assignment, as the Nation River Formation is a plant- and coal-bearing clastic unit. Schwatka unit of Weber and others (1992) is included here; it is mapped in Livengood and Circle quadrangles, offset from the Woodchopper Volcanics by the Tintina Fault System. The Schwatka consists of massive basalt flows, agglomerate, tuff, fine-grained volcaniclastic rocks, minor thin lenses of laminated platy impure limestone, and local lenticular bodies of calcite-cemented conglomerate interbedded with Devonian limestone (Weber and others, 1992). The Schwatka unit is metamorphosed to greenschist facies

Source map information

Source map Weber, F.R., Wheeler, K.L., Rinehart, C.D., Chapman, R.M., and Blodgett, R.B., 1992, Geologic map of the Livengood quadrangle, Alaska: U.S. Geological Survey Open-File Report 92-562, 19 p., scale 1:250,000.
Symbol Dsv
Unit name Schwatka unit, mafic metavolcanic rocks and minor clastic sedimentary rocks
Description Schwatka-Rampart area. Massive greenschist-grade basalt flows, agglomerate, tuff, fine-grained volcaniclastic rocks, minor thin lenses of laminated platy impure limestone, and local lenticular bodies of calcite-cemented conglomerate. Basalt contains coarse grains, or aggregates, of albite after calcic plagioclase, augite and coarse porphyroblasts of chlorite have relict amygdaloidal texture. Matrix of basalt is chlorite, white mica, and epidote
Lithology Metamorphic

Correlated geologic units

Label Dsv
Description Schwatka unit, mafic volcanic rocks, Middle and Early Devonian
Geologic age Early-Devonian to Middle-Devonian
Geologic setting Undivided
Lithology Form Importance
Mafic-volcanic < Volcanic < Igneous Major
Mudstone < Clastic < Sedimentary Bed Minor