Abstract

Computer modeling suggests that thrusting order can be discriminated on the basis of foreland basin geometry and parasequence stacking patterns. The position of sequential thrust sheets relative to the basin affects the amount and distribution of subsidence, source terrain elevation and distance from the shoreline, and sediment transport into the basin. For this study, a forward numerical model was constructed to quantitatively simulate the interaction of geologic processes active in the foreland basin setting, including thrusting and orogenic development, lithospheric flexure, orogenic denudation, sediment transport across the coastal plain, and nearshore deposition. In order to isolate the effects of tectonics and sediment supply on nearshore deposition, eustatic sea level is held constant. Model runs simulate two different thrusting sequences: foreland stepping (in sequence) and hinterland stepping (out of sequence). When in-sequence thrusting is modeled, sediment is trapped in intra-orogen basins. Because sediment is prevented from reaching the shoreline, the basin is underfilled. When out-of-sequence thrusting is modeled, sediment is transported from the orogen to the shoreline. Episodes of thrusting result in subsidence and marine flooding surfaces, and resulting accommodation space allows for parasequence development. Comparing model output to stratigraphy observed in eastern Utah suggests that Late Cretaceous out-of-sequence uplift and thrust loading resulted in aggradational and progradational parasequence stacking in the Campanian Blackhawk Formation. This result supports the theory that the Pavant thrust system was active prior to deposition of the Blackhawk Formation and that the location of thrust shortening shifted toward the hinterland prior to or during Blackhawk Formation deposition.

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