Geologic Map of Alaska: Otuk Formation

Otuk Formation

Unit symbol:	JTro
Age range	Middle Jurassic to Triassic (251.2 to 168.3 Ma)
Lithology:	Sedimentary
Group name:	Etivluk Group, undivided

Interbedded fossiliferous black chert, limestone, and shale in four lithogenetic units: (1) a basal poorly exposed black organic shale, (2) a cherty member of black silicified mudstone, chert and shale, (3) a thinly interbedded shale and thin-bedded black- and light-gray banded limestone and silicified limestone member, and (4) the formally defined Blankenship Member, which is organic-rich black shale and thin bedded chert. The Blankenship Member is thought to represent condensed deposition of Early and Middle Jurassic age (Mull and others, 1982). Bedding surfaces in the silicified limestone member weather cream-colored or light-brown to green, and it has a few beds that contain Monotis fossils. Chert member is well-bedded and contains Halobia fossils in shaly layers. Lower black shale member, which is only locally present, contains Early Triassic conodonts (Curtis and others, 1990; Ellersieck and others, 1990; Mayfield and others, 1990). Unit is less than 100 m thick. In the past, some maps assigned rocks of this unit to the Shublik Formation (unit Trgs)—see, for example, Campbell (1967), Grybeck and others (1977), Sable and others (1984a, b, c), or Sable and Mangus (1984). The coeval Shublik Formation is confined to the autochthonous part of northern Alaska in northeastern Alaska and the subsurface of the North Slope; the Otuk—in particular the Blankenship Member—is also coeval with the lower part of the Kingak Shale

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	Trs
Unit name	Shale, limestone, and chert, Ongoveyuk terrane
Description	Unit consists of an upper member, 50m thick, composed of black shale, dark thin-bedded limestone, and chert; and a lower member, 70m thick, composed of thin-bedded chert and dark siltstone. The upper member contains flat clams, Daonella, Halobia, and Monotis indicating a condensed section ranging in age from Middle (Ladinian) to Late (Karnian and Norian) Triassic (Patton and Dutro, 1969) (Zhang and Blodgett, 2007). The age of the lower member is uncertain but, for reasons explained in the accompanying text, is presumed to be Middle or early Triassic. Location-Unit is best exposed in the eastern part of the island along small incised streams that drain into Ongoveyuk and Tomname lagoons. Rubble patches of this unit also have been identified on the Seknak River near Myghapowit Mountain and in the western part of the island on Yaghmelngak Mountain
Lithology	Sedimentary

Correlated geologic units

Label	Trcs
Description	Otuk Formation, Triassic
Geologic age	Olenekian to Bajocian
Geologic setting	Sedimentary, slope-and-deep-water

Lithology	Form	Importance
Black-shale < Shale < Mudstone < Clastic < Sedimentary	Bed	Major
Limestone < Carbonate < Sedimentary	Bed	Indeterminate, major
Chert < Chemical < Sedimentary	Bed	Indeterminate, major
Siltstone < Clastic < Sedimentary	Bed	Minor