Etivluk Group, undivided

Unit symbol: JIPe
Age range Middle Jurassic to Pennsylvanian (323.2 to 168.3 Ma)
Lithology: Sedimentary
Group name: Etivluk Group, undivided
Includes Otuk and Siksikpuk Formations and Imnaitchiak Chert; consists of maroon, red, green, gray, black, and variegated chert and siliceous argillite, minor maroon calcareous siltstone and argillite, and rare maroon or gray limestone lenses (Karl and others, 1989a; Mayfield and others, 1987). Original definition suggested Etivluk Group is present only on allochthonous sheets in the Brooks Range, and not present on autochthonous rock units (Mull and others, 1982). Locally, the Otuk and Siksikpuk Formations are mapped separately as units JTro and TrIPeg

Source map information

Source map Patton, W.W., Jr., Wilson, F.H., and Taylor, T.A., 2011, Geologic map of Saint Lawrence Island, Alaska: U.S. Geological Survey Scientific Investigations Map SIM-3146, pamphlet, 13 p., 1 sheet, scale 1:250,000, and database.
Symbol TrPs
Unit name Graywacke, grit, and shale, Tomnamie terrane
Description Unit composed of dark fine-grained to gritty carbonaceous graywacke, dark carbonaceous shale, and subordinate bedded chert and siliceous mudstone. The graywacke clasts consist mainly of quartz but also include plagioclase, K-feldspar, shale, chert, granitic rocks, quartzite, schist, and limestone; the matrix is made up of finely divided siliceous and carbonaceous debris. Highly calcareous graywacke and impure clastic limestone containing abraded echinoderm and coral debris occur in the lower part of the unit. Cone-in-cone structures are common. Unit is intensely deformed and locally altered to hornfels along contacts with granitic plutons. Unit assigned a Permian and Triassic age on the basis of: (1) the occurrence of radiolaria of probable Triassic age in chert beds (D.L. Jones, written commun, 1978), and (2) correlation with a lithologically similar assemblage in nearby parts of the Chukotsk Peninsula, Russia that contains Early Triassic and Permian fossils (Arkavyi and others, 1975). Location-Unit is best exposed along streams draining into Tomname and Ongoveyuk lagoons in the eastern part of the island. Scattered rubble exposures of the unit also occur in the western part of the island
Lithology Sedimentary

Correlated geologic units

Label Pzeg
Description Etivluk Group, undivided: Includes Siksikpuk and Otuk Formations and underlying Immaitchiak Chert
Geologic age Pennsylvanian to Bajocian
Geologic setting Sedimentary, slope-and-deep-water
Lithology Form Importance
Chert < Chemical < Sedimentary Bed Major
Graywacke < Sandstone < Clastic < Sedimentary Carbonaceous Indeterminate, major
Argillite < Metaclastic < Metasedimentary < Metamorphic Zeolitic (prehnite-pumpellyite) Minor
Shale < Mudstone < Clastic < Sedimentary Carbonaceous Minor
Limestone < Carbonate < Sedimentary Bed Incidental