LINKING THE SURFACE TO THE SUBSURFACE: ENVIRONMENTAL VARIABILITY OF THE FRONTIER FORMATION, POWDER RIVER BASIN, WYOMING, USA

The Wall Creek Member of the Frontier Formation is an Upper Cretaceous siliciclastic package that outcrops in areas associated with the Bighorn Uplift and is found in the subsurface in Wyoming’s Powder River Basin. We hypothesize that these strata represent an assortment of depositional environments, from near shore to mixed river-wave-dominated deltaic. An asymmetrical delta and an array of near shore lithofacies were preserved with greater accommodation in the south and a sediment source near Kaycee, Wyoming.

Strata in the Wall Creek Member contain five facies that are represented in coarsening upwards parasequences. Data from measured sections and core descriptions demonstrate the existence of one to four coarsening upwards parasequences that grade up from shales, to interbedded sands and shales, to amalgamated sandstone packages. Variability in the ichnofacies assemblages, number of parasequences, and degree of bioturbation at each outcrop indicate a complex depositional environment. Evidence for increased accommodation to the south includes an increase in the total thickness to the south relative to the north. The sediment source is hypothesized to have been located near Kaycee, Wyoming due to the presence of channelized sand bodies and the increase in sand-shale ratio to the north. Sandstone packages contain a variety of sediment transport and environmental indicators that demonstrate a southerly paleo-transport direction, pointing to the presence of a strong longshore current that led to the development of an asymmetrical delta. Log analysis shows a definite northeastern limit to Wall Creek deposition, an eastern limit to coarse sediment dispersal, and shore parallel sediment dispersal patterns. Log correlation assists in the determination of spatial variations in parasequence stacking patterns which lead to a greater understanding of depositional processes on local and basin-wide scales.

From near shore to mixed river-wave-dominated deltaic, the Wall Creek strata represent a range of depositional environments. We interpret the non-systematic variation in bioturbation, thickness, parasequence stacking patterns, accommodation, sedimentary structures, and lithology to arise from autogenic processes in a complex depositional system.