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 Mull, C.G., Moore, T.E., Harris, E.E., and Tailleur, I.L., 1994, Geologic map of the Killik River quadrangle, Brooks Range, Alaska: U.S. Geological Survey Open-File Report 94-679, 1 sheet, scale 1:125,000.
Symbol JTro
Unit name Otuk Formation; Endicott Mountains Allochthon, Ivotuk Hills Sequence
Description Interbedded fossiliferous black chert, limestone, and shale in four lithogenetic units: a basal poorly exposed shale member--black organic shale (Early and Middle Triassic); the 'chert' member--black silicified mudstone, chert and shale (Middle and Late Triassic); the limestone member--thinly interbedded shale and thin-bedded black- and light-gray banded limestone and silicified limestone; and Blankenship member--organic-rich black shale and thin bedded chert, represents condensed deposition (Early and Middle Jurassic). In the Ivotuk Hills sequence, unit is generally not well exposed, commonly exposed only as yellow-gray weathering rubble of the limestone member or in a few well exposed but structurally complicated partial sections in stream cuts. Type locality is on east bank of Otuk Creek, 6.3 km north of Ivotuk Hills, where thickness <50 m and shale member is mostly covered.
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