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., and Miller, T.P., 1966, Regional geologic map of the Hughes quadrangle, Alaska: U.S. Geological Survey Miscellaneous Geologic Investigations Series Map I-459, 1 sheet, scale 1:250,000.
Symbol Ktg
Unit name Tuff, volcanic graywacke, and mudstone
Description Andesitic and latitic volcaniclastic rocks including crystal -lithic tuff, lithic tuff, volcanic graywacke, and mudstone
Lithology Sedimentary

Correlated geologic units

Label Ktg
Description Tuff, volcanic graywacke, and mudstone
Geologic age Early-Cretaceous
Geologic setting Extrusive
Lithology Form Importance
Andesite < Mafic-volcanic < Volcanic < Igneous Pyroclastic, tuff Major
Dacite < Felsic-volcanic < Volcanic < Igneous Pyroclastic, tuff Indeterminate, major
Graywacke < Sandstone < Clastic < Sedimentary Bed Minor
Andesite < Mafic-volcanic < Volcanic < Igneous Flow Incidental