Distal Facies Variability within the Upper Triassic Part of the Otuk Formation in Northern Alaska

The Triassic-Jurassic Otuk Formation is a potentially important source rock in allochthonous structural positions in the northern foothills of the Brooks Range in the north slope of Alaska. It consists of four members, in ascending order: the shale, chert, and limestone members and the Blankenship Member. The lower three members are distal equivalents of the Shublik Formation, which generated significant hydrocarbons that have migrated to Prudhoe Bay Field. This study focused on three localities of the Upper Triassic (Norian) limestone member, which form a present-day east-west transect, 110 km long, in the central Brooks Range. All three sections are within the structurally lowest Endicott Mountain allochthon and are interpreted to have been deposited along an outer shelf margin.

One- to two- meter sections of the limestone member at each locality were measured and sampled in detail to capture fine scale features. The Atigun Gorge section in the east consists of dark gray calcareous shale/black calcareous shale/ dark gray limestone triplets (20-30 cm thick). The Welcome Creek outcrop to the west displays couplets (20-30 cm thick) of gray calcareous shale and dark gray limestone. The character of this outcrop varies on a multimeter scale, from medium gray, medium bedded limestone and shale to dark gray, thinly bedded limestone and shale. The Tiglukpuk Creek outcrop farther west consists of tan limestone/gray shale couplets (10-15 cm thick), with rare black shale intervals ~2 cm thick. Many of the limestones are bioturbated. The darkest gray to black limestone and calcareous shale at Welcome Creek and particularly at Atigun Gorge are interpreted to represent periods of increased anoxia, thus enhancing source rock development, that did not occur during deposition of the limestone member in the Tiglukpuk area.

The Otuk is interpreted to have been deposited outboard of a southwest-facing basin margin, with the three outcrops likely in relatively close proximity during deposition. This suggests that the different character of the limestone-shale units and variable levels of anoxia are due to topographic irregularities along the outer shelf, potentially structurally controlled, within the Late Triassic depositional basin. Understanding spatial variability in the basin configuration will significantly aid in predicting source rock quality within the Otuk and Shublik Formations.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California