Insights into the Origins of First- and Second-Order Depositional Sequences in the North Slope of Alaska

A. D. Donovan, J. O. Byrd, C. J. Warrner, G. J. Nolet, and K. A. Knabe

Delineation of the major first- and second-order depositional sequences of the North Slope of Alaska, and comparison of the timing of these sequences with long-term eustatic curves from the Exxon Cycle Chart, provides insights into the origins of the major depositional sequences of the North Slope.

The first-order depositional sequences of the North Slope are bounded by the 3 major regional angular unconformities the: Pre-Upper Devonian, Pre-Upper Permian, and Lower Cretaceous unconformities. These unconformities, which represent periods of maximum erosion and minimum accommodation within the basin, define the Exxon Ellesmerian, Beaufortian, and Brookian Sequences of the North Slope. The Endicott, Echooka, and Kemik overlie these major unconformities. The maximum flooding surfaces within the first-order sequences correspond to major downlap surfaces, and a change from retrogradational to progradational stacking of second-order sequences within the first-order sequences. These maximum flooding surfaces represent periods of maximum accommodation within the first-order sequences, add are used to subdivide the first-order sequences of the North Slope. The base of the Lisburne, Kingak, and HRZ overlie these first-order maximum flooding surfaces.

Comparison of the timing of first-order depositional sequences of the North Slope, with the Exxon Cycle Chart, shows no direct correlation between the first-order North Slope sequences and long-term eustatic trends. In fact, the Alaskan sedimentary cycles are, in general, 180 degree out of phase with the eustatic cycles suggesting that regional tectonics control the flrst-order cyclicity of the North Slope. However, analysis of the 7 Beaufortian and 4 Brookian second- order sequences of the North Slope, indicates that 4 of the 7 Beaufortian, and 3 of the 4 Brookian second-order sequences, correlate with long-term eustatic trends on the Cycle Chart. This correlation suggests that some of the second-order sequences of the North Slope may be influenced by eustacy.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California