Abstract

Incorporation of ichnological (trace fossil) data into facies models for deltaic systems and their deposits has yielded useful new criteria for discriminating between alternative interpretations. Nevertheless, despite the recognition that trace-fossil suites may vary spatially across an individual delta system, few spatially resolved datasets are currently available to inform these interpretations. We present a new sedimentological and ichnological analysis of the Upper Cretaceous (Cenomanian) Peay Sandstone Member of the Frontier Formation in the northern Bighorn Basin, Wyoming, U.S.A., based on three-dimensional mapping of surface exposures and subsurface drilling data. Both down-depositional-dip and across-depositional-strike trends can be determined and quantified prima facie from this dataset.

The Peay Sandstone Member is a crudely coarsening-upward sandstone body that has an elongate planform shape, projecting southeastward into the basin. Based on a facies analysis, the vertical facies succession in the core of the body represents, from base to top, deposits of prodelta (stratified siltstone), distal delta front (thinly interbedded siltstone and sandstone), through medial–proximal delta front (thickly interbedded siltstone and sandstone to bioturbated fine-grained sandstone), to mouth bar (amalgamated, largely unbioturbated, fine- to medium-grained sandstone) environments. This complete progradational succession persists for at least 40 km in the depositional dip direction, thinning and fining abruptly over a few kilometers south of the town of Greybull, Wyoming. Transects from the delta axial core to the peripheral flanks reveal changes in facies reflecting a transition from a high-energy, river-dominated mouth-bar environment to a tide- and wave-influenced medial–proximal delta flank, and ultimately, to a low-energy, storm- and wave-influenced distal delta flank setting on the peripheries of the delta lobe. The trace assemblage in the axial core is interpreted as a highly stressed expression of the Skolithos Ichnofacies. This passes gradationally down depositional dip into stressed expressions of the Cruziana Ichnofacies, and along depositional strike into an archetypal (unstressed) expression of the Cruziana Ichnofacies on the delta flanks. This spatial variation in bioturbation intensities and trace-fossil assemblages is interpreted to reflect dissipation of physico-chemical stresses from the axial core to peripheral flanks. The outcomes of this study emphasize the point that data acquired on an ancient deltaic deposit in isolation of their paleogeographic context may give rise to a misleading impression of that system, and potentially a less than parsimonious interpretation of its process balance and planform geometry.