DISTAL FACIES VARIABILITY WITHIN THE UPPER TRIASSIC PART OF THE OTUK FORMATION IN NORTHERN ALASKA

K.J. WHIDDEN
US Geological Survey, Central Energy Resources Science Center, Denver Federal Center MS 939, Denver, Colorado 80225 USA
e-mail: kwhidden@usgs.gov

J.A. DUMOULIN
US Geological Survey, Alaska Science Center, 4210 University Drive, MS 259, Anchorage, Alaska 99508 USA

M.T. WHALEN, E. HUTTON
University of Alaska, Department of Geology and Geophysics, Reichardt Building, 900 Yukon Drive, Fairbanks, Alaska 99775 USA

T.E. MOORE
US Geological Survey, Geology, Minerals, Energy and Geophysics Science Center, 345 Middlefield Road, MS 901, Menlo Park, California 94025 USA

AND

S.B. GASWIRTH
US Geological Survey, Central Energy Resources Science Center, Denver Federal Center MS 939, Denver, Colorado 80225 USA

ABSTRACT: 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. This study focuses on three localities of the Upper Triassic (Norian) limestone member, which form a present-day, 110-km-long, east–west transect in the central Brooks Range. All three sections are within the structurally lowest Endicott Mountain allochthon and are interpreted to have been deposited along a marine outer shelf with a ramp geometry.

The uppermost limestone member of the Otuk was chosen for this study in order to better understand lateral and vertical variability within carbonate source rocks, to aid prediction of organic richness, and ultimately, to evaluate the potential for these units to act as continuous (or unconventional) reservoirs. At each locality, 1 to 4 m sections of the limestone member were measured and sampled in detail to capture fine-scale features. Hand sample and thin section descriptions reveal four major microfacies in the study area, and one diagenetically recrystallized microfacies. Microfacies 1 and 2 are interpreted to represent redeposition of material by downslope transport, whereas microfacies 3 and 4 have high total organic carbon (TOC) values and are classified as primary depositional organofacies. Microfacies 3 is interpreted to have been deposited under primarily high productivity conditions, with high concentrations of radiolarian tests. Microfacies 4 was deposited under the lowest relative-oxygen conditions, but abundant thin bivalve shells indicate that the sediment–water interface was probably not anoxic.

The Otuk Formation is interpreted to have been deposited outboard of a southwest-facing ramp margin, with the location of the three limestone outcrops likely in relatively close proximity during deposition. All three sections have evidence of transported material, implying that the Triassic Alaskan Basin was not a low-energy, deep-water setting, but rather a dynamic system with intermittent, yet significant, downslope flow. Upwelling played an important role in the small-scale vertical variability in microfacies. The zone of upwelling and resultant oxygen-minimum zone may have migrated across the ramp during fourth- or fifth-order sea-level changes.

KEY WORDS: Otuk, Triassic, Alaska, microfacies, upwelling

Whidden KJ, Dumoulin JA, Whalen MT, Hutton E, Moore TE, Gaswirth SB. Distal facies variability with the Upper Triassic part of the Otuk Formation in northern Alaska. In Verwer K, Playton TE, Harris PM (Editors). Deposits, Architecture, and Controls of Carbonate Margin, Slope, and Basinal Settings, Special Publication 105: SEPM (Society for Sedimentary Geology), Tulsa, OK. doi: 10.2110/sepmsp.105.16.