Micaceous graywacke

Unit symbol: Jms
Age range Lower Jurassic (201.3 to 174.1 Ma)
Lithology: Sedimentary
Group name: Volcanic and sedimentary rocks of southwest Alaska
Marine unit of micaceous, fine-grained black graywacke, siltstone, and slate, and gritty limestone known only from the Taylor Mountains quadrangle of southwest Alaska. Contains Weyla pelecypods of Early Jurassic age. Crops out near Lake Chauekuktuli, these rocks occur between exposures of Paleozoic rocks on the adjacent ridges and are structurally lower than the surrounding Paleozoic rocks

Source map information

Source map Hoare, J.M., and Coonrad, W.L, 1978, Geologic map of the Goodnews and Hagemeister Island quadrangles region, southwestern Alaska: U.S. Geological Survey Open-File Report 78-9-B, 2 sheets, scale 1:250,000.
Symbol Jlvs
Unit name Volcanic and sedimentary rocks
Description Marine unit of mafic flows, some pillow structure, volcanic breccias, and massive fine- to medium-grained volcanogenic sedimentary rocks. Fractures commonly coated with laumontite
Lithology Sedimentary

Correlated geologic units

Label Jms
Description Micaceous, fine-grained black graywacke, siltstone, and slate, and gritty limestone.
Geologic age Early-Jurassic
Geologic setting Extrusive
Lithology Form Importance
Basalt < Mafic-volcanic < Volcanic < Igneous Flow Major
Graywacke < Sandstone < Clastic < Sedimentary Bed Major
Slate < Metaclastic < Metasedimentary < Metamorphic Indeterminate, major
Limestone < Carbonate < Sedimentary Bed Indeterminate, major
Siltstone < Clastic < Sedimentary Bed Indeterminate, major