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The AAPG/Datapages Combined Publications Database

AAPG Bulletin

Abstract


Volume: 69 (1985)

Issue: 4. (April)

First Page: 657

Last Page: 657

Title: Age and Correlation of the Otuk Formation, North-Central Brooks Range, Alaska: ABSTRACT

Author(s): Dirk A. Bodnar, C. G. Mull

Article Type: Meeting abstract

Abstract:

Allochthonous Triassic rocks of the north-central Brooks Range thrust belt were originally mapped as part of the Middle to Upper Triassic Shublik Formation. Recently, these strata were named the Otuk formation. Detailed paleontologic studies of 11 measured sections more precisely document the age of the Otuk and show that its base is older than the base of the Shublik and that its top is younger than the top of the Shublik. Megafossils (pelecypods and ammonites) and microfossils (radiolaria, conodonts, and foraminifers) indicate an age range of Early Triassic (Dienerian-Smithian or older) to Middle Jurassic (Bajocian). The lithology consists of 120 m (390 ft) of interbedded, very fine-grained rocks (shale, limestone, and chert) representative of very slow deposition, belo wave base in an open marine environment. The Otuk formation does not contain suitable reservoir rocks, but organic geochemical data indicate that the shales are possible oil source rocks. The Otuk formation is disconformable with both the underlying Permian (Wolfcampian-Guadalupian) Siksikpuk Formation and overlying Lower Cretaceous (Valanginian) coquinoid limestone and shale. These unconformities are correlative with similar unconformities in the northeastern Brooks Range and subsurface of the North Slope. Thus, the Otuk formation is a condensed, deeper water, more distal equivalent of the Ivishak and Shublik Formations, Karen Creek Sandstone, and lower Kingak Shale of the northeastern Brooks Range and equivalent subsurface units of the North Slope.

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