High Resolution Outcrop Stratigraphy and Surface-to- Subsurface Correlation of Upper Triassic Rocks, Northern Alaska

By

M.T. Whalen, L.N. Kelly (University of Alaska, Fairbanks), C.G. Mull (Alaska Division of Oil and Gas), M.B. Mickey (Micropaleo Consultants, Inc.), A.C. Banet (Bureau of Land Management), and E.J. Hulm (BP Sunbury)

Sequence stratigraphic, biostratigraphic, and geochemical analyses of Upper Triassic rocks in the northern Brooks Range have provided new insight into correlation and deposition of Triassic source rocks in Arctic Alaska. Detailed outcrop studies of the Shublik, Otuk and adjacent formations in the northeastern (Fire Creek, Shublik Mts.) and central Brooks Range (Atigun Gorge, Tiglukpuk Creek) provide stratigraphic data comparable to recent subsurface analyses of coeval units near Prudhoe Bay. Facies stacking patterns in outcrop and the subsurface display similar stratigraphic discontinuities and landward and seaward stepping patterns that permit detailed surface-to-subsurface correlations. Two complete third order sequences and part of a third sequence are recorded by these units. All outcrop sections represent relatively distal depositional settings. A significant erosional unconformity is recorded only at the base of Sequence 1 at Fire Creek. The base of Sequences 2 and 3 are characterized by pebbly phosphorites or bedded cherts that indicate significant transgressive surfaces that subdivide these units into additional genetic sequences. Outcrop gamma-ray and scintilometer data facilitated correlation between the surface and subsurface. Biostratigraphic data from the subsurface and Atigun Gorge indicate that all three sequences fall within the Carnian and Norian stages of the LateTriassic. Organic geochemical data show >5%TOC in some Otuk samples and imply that the richest source rocks accumulated during mid-sequence maximum flooding. These Triassic oil-prone source rocks were deposited across much of eastern Arctic Alaska and may be a hydrocarbon source in the Brooks Range thrust belt and southern foothills of the Colville basin.

AAPG Search and Discovery Article #90008©2002 AAPG Pacific Section/SPE Western Region Joint Conference of Geoscientists and Petroleum Engineers, Anchorage, Alaska, May 18–23, 2002.