USING STRATAL PATTERNS TO UNDERSTAND THE INTERACTION BETWEEN THE SEVIER FORELAND BASIN AND THE CRETACEOUS WESTERN INTERIOR SEAWAY

Strata within the Cretaceous Sevier Foreland Basin record a complex history of depositional events, with sediment dispersal and sedimentary facies varying considerably between times of highstand and lowstand conditions. The Sevier Foreland Basin (FLB) was joined with the Cretaceous Western Interior Seaway (KWIS) through much of the Cretaceous, and formed a major physiographic feature in North America during this time. Fluvial, strand plain and shoreface strata within the basin transition to marine shales to the east. During highstand events, wave dominated shorelines along the western margin prograded into the basin in basinward-stepping stratal packages, reflecting thrust activity. The wide areal extent of the sea produced an expansive fetch length and large wave heights, which formed the thick hummocky and trough cross stratification characteristic of shoreface strata from the region. Comparison of these strata with hydrodynamic studies on modern shorefaces allows for interpretation of the dynamics of depositional conditions, such as daily wave and tide regime, and size and frequency of storms. Trace fossil assemblages allow for more refined interpretations of shoreface conditions: for example, concentrations of large-scale, thick-walled Ophiomorpha are used to interpret barred vs. non-barred shorefaces.

During lowstand events, depositional controls changed and the geometry of the FLB had a more dominant influence on sediment distribution along the western margin of the seaway. In extreme lowstand events, the FLB and the KWIS may have been decoupled, with sediment dispersal and drainage patterns changing to a north-south trend, indicating a shift in sediment transport parallel to the axis of the FLB. A number of lowstand deposits and incised valley fills record such events. Analysis of facies changes and paleocurrent directions of the incised valley fill of the Late Cretaceous Castlegate Sandstone supports this trend, and illustrates the effect of the basin topography on the drainage patterns of the FLB. A decoupling of the FLB from the KWIS would have resulted in a much restricted fetch length and lower wave energy. Resulting lowstand deposits within the FLB often lack the strong wave influence present in the highstand strata.