ABSTRACT: Relationship of Sedimentary Cycles to Tectonic Events and Subsidence: An Example from North Alaska
K. W. Rudolph, B. A. Rassman, M. A. Abrams, C. M. Ando, J. B. Carter, K. J. Ferdinand, P. M. Lorber
The fundamental concept in interpreting sedimentary cyclicity is accommodation--the space available for deposition. If the sedimentation rate exceeds the rate of accommodation increase, a progradational pattern results. Conversely, if the sedimentation rate is less than the rate of accommodation increase, a retrogradational pattern results. Although eustasy has been emphasized as a key factor in accommodation, in basins with active tectonism subsidence and uplift also are very important.
On the North Slope, the gross sedimentation patterns were controlled by the structural and subsidence history. Extensional events that commenced in the Upper Devonian, Lower Permian, and Neocomian are characterized by an initial uplift that caused widespread erosion and a downward shift in facies.
Following this uplift, a rapid increase in subsidence occurred (i.e., accommodation). This subsidence caused an overall retrogradation of lithofacies. As subsidence decreased due to thermal cooling, a progradational pattern resulted.
Compressional events that commenced in the Jurassic, Neocomian/Aptian, Cenomanian, and lower Eocene are characterized by rapid subsidence due to flexural loading in the orogenic belt (Brooks Range). In the distal Colville trough and Barrow arch areas of north Alaska, this event is expressed as an overall retrogradation of facies. As loading ceased, subsidence decreased and a progradational pattern resulted.
These retrogradational-progradational sedimentary cycles have the same characteristics as eustatic cycles, but are of much longer duration. Eustatic cycles (sequences) stack within the larger scale tectonic/subsidence cycles to produce the composite stratigraphy of north Alaska.
AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990