Volcanic graywacke and mudstone

Unit symbol: Kyg
Age range Lower Cretaceous (113 to 100.5 Ma)
Lithology: Sedimentary
Group name: Volcanic graywacke and mudstone
Hard, well indurated, fine- to medium-grained, locally tuffaceous sandstone and micaceous mudstone. Some sandstone beds have a distinctly mottled appearance owing to the presence of laumontite, which commonly occurs in tuffaceous layers (Hoare and Condon, 1966). Unit widely exposed in a broad belt that extends from the southeastern corner of the Saint Michael quadrangle to the south-central part of the Kwiguk quadrangle. Also includes fine-grained graywacke thinly interbedded with siliceous argillite (Patton and others, 1975) on Saint Matthew Island. Graywacke composed of angular to subrounded feldspar, volcanic lithic, and quartz grains in an altered argillaceous matrix (Patton and others, 1975). Argillite is siliceous, dense, breaks with conchoidal fracture, shows small-scale crossbedding and convolute laminations, and is interpreted as a distal facies of a marine turbidite (Patton and others, in press LY). In the Yukon-Koyukuk Basin, unit has yielded marine mollusks of late Early Cretaceous age (Patton and others, 2009). Unit correlates with unit Kygv of Patton and others (2009; Wilson and others, 2013)

Source map information

Source map Patton, W.W., Jr., 2000, Unpublished data.
Symbol Kvc
Unit name Volcanic conglomerate and breccia
Description Massive, poorly stratified pebble to boulder conglomerate, interbedded with dark graywacke and dark finely laminated mudstone. Unit composed of debris flows and alluvial fan deposits. Consists chiefly of andesitic volcanic rock detritus, but locally includes notable amounts of granitic and fine-grained tuffaceous material. Unit overlies volcanic breccia and agglomerate of unit Kv of the Koyukuk terrane
Lithology Sedimentary

Correlated geologic units

Label Kvcc
Description Conglomeratic facies, Volcanic conglomerate and breccia
Geologic age Cretaceous
Geologic setting Sedimentary, deltaic-and-nearshore
Lithology Form Importance
Conglomerate < Clastic < Sedimentary Bed Major