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Carbonate Submarine Fans and Aprons in Frontier Petroleum Region--Carboniferous Calico Bluff Formation, East-Central Alaska

Harry E. Cook, Leslie B. Magoon, Richard G. Stanley, Arturo Casas

The Calico Bluff Formation (CBF) is a spectacularly exposed 500-m thick sequence of carbonate sediment-gravity-flow deposits and inter-bedded fetid, organically rich lime muds and siliceous shales. The area of its location (Yukon-Kandik region), which contains the most complete Proterozoic and Phanerozoic sedimentary sequence in Alaska, is traditionally considered to have formed on the western edge of the North American passive continental margin.

CBF facies record an overall seaward-prograding, shoaling-upward section from noncyclic turbidites deposited in basin plain settings to numerous thickening- and coarsening-upward cycles of turbidites and debris flows representing outer-fan lobes or base-of-slope aprons. These sediment-gravity-flow deposits contain abundant crinoid and bryozoan fragments and lesser amounts of calcareous green algae, other biotic components, and sand-sized detrital quartz. We interpret the flows were derived from a shallow water platform rimmed with bryozoan and crinoid meadows. The presence of detrital quartz in the mass-flow deposits indicates that this platform was probably attached to a continental landmass and was not an isolated offshore carbonate platform.

Paleocurrent flow directions, in present-day coordinates, flow toward the south-southeast, toward the craton. These anomalous flow directions may be explained by block rotation or by derivation from a nearby, yet-to-be-discovered platform margin in Lisburne Group carbonates to the north and northwest.

The organic content of the CBF lime muds and siliceous shales averages 5 wt. %. TAI and Tmax data indicate the organic matter is well into oil generation but not overmature, and S2/S3 ratio suggests that the CBF contains mixed gas/oil prone source rocks. These source-rock data, in conjunction with possible carbonate apron, fan, and platform margin reservoir facies, indicate the CBF is a new potential exploration target in this frontier area of Alaska.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.