Depostional [sic] Environments of the Rock Springs Formation, Southwest Flank of the Rock Springs Uplift, Wyoming

Abstract

A record at an ancient delta system Is preserved in outcrops along the flanks of the Rock Springs Uplift, southwestern Wyoming. The deltaic Upper Cretaceous Rock Springs Formation of the Mesaverde Group was deposited during early Campanian time near the end of the regressive phase of the Niobrara cyclothem. On the southwest end of the Uplift, part of the delta system is exposed near the seaward edge of a series of transgressive/regressive sequences, which consist of intertonguing prodelta, delta-front, and delta-plain deposits. Eight major delta-front sandstones are vertically stacked and laterally continuous throughout the main study area, intertonguing with prodefta deposits of the Blair Formation and Black Butte and Coulson Shale Tongues bf the Rock Springs Formation. Delta-front sandstones consist mainly of interdistributary shoreface deposits composed of sand redistributed from point sources by longshore currents. Preservation of distributary-channel and mouth-bar deposits is rare due to intense wave reworking. The shorelines were modified by large-scale storm events. Distributary-channel, crevasse-splay, overbank, crevasse-subdelta, interdistributary-bay, marsh, swamp, flood-tidal delta, and washover deposits accumulated behind delta-front sandstones, mostly in lower delta-plain environments. In this lower delta-plain setting, peat was deposited in freshwater forest swamps as isolated pods and linear belts behind delta-front sands. The resulting coal beds are thin, pinching and splitting to the southeast of the study area. In contrast, in more landward settings north of the study area, non-marine sequences contain thicker and more continuous coal beds that accumulated in the lower parts of upper delta-plain environments.