Volcanic graywacke and conglomerate

Unit symbol: Kvgc
Age range Lower Cretaceous, Hauterivian and older (145 to 129.4 Ma)
Lithology: Sedimentary
Group name: Volcanic graywacke and conglomerate
Consists of a variety of lithologies including thin-bedded tuffaceous chert, massive graywacke, conglomerate, argillite, a few volcanic flows and impure limestone beds, and massive, coarse-grained crystal-lithic tuff (Hoare and Coonrad, 1983) in southwest Alaska in the Bethel and Goodnews Bay quadrangles. Rocks range widely in color, mostly green and gray but also red, yellow, brown, or black (Hoare and Coonrad, 1983). Most distinctive rock type in unit in southwest Alaska is massive andesitic crystal-lithic tuff, which is at least 1,000 m thick (Hoare and Coonrad, 1983). Fine-grained tuff and some graywacke is commonly laumontized. Unit also includes turbidites composed of highly calcareous sandstone interbedded with non-calcareous micaceous siltstone and shale. Hoare and Coonrad (1983) inferred the unit to be coarsening upward from sandstone and shale to conglomerate. Generally thick-bedded to massive, with alternating sandstone and shale intervals 5- to 20-m-thick, unit locally has thin-bedded sections (Hoare and Coonrad, 1983). Base of unit was not recognized, and, as such, Hoare and Coonrad (1983) suggested that rocks of Jurassic age may be present in the unit. Rocks are strongly folded and commonly overturned (Hoare and Coonrad, 1983). Box and others (1993) reported the presence of lithic clasts likely derived from the Kanektok metamorphic complex (Xio), as well as other local units. Buchia crassicollis of Early Cretaceous Valanginian age is found in calcareous graywacke, conglomerate, and impure limestone. In cherty tuff, Radiolaria of Early Cretaceous age have been found at 3 localities (Hoare and Coonrad, 1983). Presence of Buchia crassicollis indicates these rocks are, in part, coeval with the graywacke of Buchia Ridge (unit Kqcs)

Source map information

Source map Patton, W.W., Jr., Moll, E.J., Dutro, J.T., Jr., Silberman, M.L., and Chapman, R.M., 1980, Preliminary geologic map of the Medfra quadrangle, Alaska: U.S. Geological Survey Open-File Report 80-811A, 1 sheet, scale 1:250,000.
Symbol Kvg
Unit name Volcanic graywacke and conglomerate
Description Poorly sorted fine- to coarse-grained graywacke, sandstone, grit, and pebble to cobble conglomerate composed chiefly of volcanic rock and chert detritus; interbedded dark mudstone.
Lithology Sedimentary

Correlated geologic units

Label Klvm
Description Earliest Cretaceous volcanic conglomerate
Geologic age Early-Cretaceous to Hauterivian
Geologic setting Sedimentary, shallow-marine-siliciclastic
Lithology Form Importance
Conglomerate < Clastic < Sedimentary Bed Major