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ABSTRACT: A Theoretical Model for Sequence Geometry: Simulation of the U.S. Western Interior Cretaceous Foreland

J. Thorne, D. J. P. Swift

A two-dimensional numerical model of basin formation and sedimentation is developed to simulate the characteristic sequence geometry of sediment fill in a foreland basin setting. We describe a simple model for sediment dispersion on a wave-dominated shelf. In this approximation, waves and bottom currents are treated as random variables, producing dominantly longshore transport and a net offshore diffusive transport dependent on water depth and grain size.

The model is used to investigate the controlling factors on sequence geometry of the U.S. Western Interior Cretaceous basin. Initial results show that the sedimentation history of the Cretaceous basin has developed in response to (1) major thrusting events at 110, 100, and 90 Ma and (2) basement movements relating to the start of Laramide tectonics at 80 Ma. Each thrust-related sequence can be described by two stages of sedimentation. In the first, alluvial-fan conglomerates and sandstones grade rapidly seaward into dominantly shaly, relatively deep-water, strike-fed deposits. In the second stage, progradation of a low-relief shelf edge forms a thickening sandy clastic wedge. In contrast, the response of sedimentation to vertical tectonics is illustrated by a sequence of prograding cl noforms developed during the Campanian. As simulated by the model, changes in progradational style develop in response to relative changes in sea level associated with the regional rate of subsidence/uplift in the Wyoming area.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990