Print this pageExploration and Production Models for Ancient Tide-Influenced Lowstand Deltas: Cretaceous Frewens Castle Sandstone, Wyoming
BHATTACHARYA, J.R, B.J. WILLIS, S.L. GABEL, C.A.M. BROQUET, and C. SWEZEY
Conventional models suggest that prograding tide-influenced depositional systems have numerous tidal channels and that tidal deposits may form during transgression of incised valleys. We show examples of prograding tidal systems, with no tidal channels that are interpreted to form during falling relative sea level. Similar sand bodies have been interpreted elsewhere as offshore shelf bars.
The Upper Cretaceous (Cenomanian) Frewens Castle Sandstone, Belle Fourche Member, Frontier Formation, is exposed in the western margin of the Powder River Basin. Data, including 90 measured sections, several hundred well logs, and photomosaics of more than 5 km of outcrop, show that the Frewens Castle Sandstone comprises several stacked, overlapping, elongate to lobate parasequences that gradationally overlie marine mudstones containing extensive bentonites. Older parasequences are elongate and show tidal features, rare burrows, and clinoform internal bed geometries but no channels. Thick sandstones pass laterally into thin heterolithic beds capped by pebbly lags. Younger parasequences show an overall Skolithos ichnofacies, comprise flat to hummocky cross-stratified sandstones passing up into tidally reworked sandstones, and are interpreted as being deposited in a shallow-marine, high-energy setting.
We interpret these sand bodies as prograding, tide-influenced deltaic deposits that become successively more marine upward. The lack of a basal lag or truncation of bentonite markers implies that the elongate sand bodies lie within a tectonic low rather than in an incised valley. The lack of channels is typical of sandstones deposited during forced regression. Subsequent transgression reworks the thin delta top to produce the capping lags. The lack of tidal-channel, delta-plain, and nonmarine deposits is common in other interpreted reworked prograding lowstand tidal systems in Wyoming such as in the Cretaceous Haystack Mountains Formation and the Shannon Sandstone.