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A detailed outcrop and sub-surface analysis was completed on the Peay Member sandstone (Frontier Formation) in the northeast Bighorn Basin, Wyoming, building on previous work by Clark (2010) and Hutsky (2011). Regional correlations reveal the sandstone body to be digitate in planform geometry, elongate along depositional dip, and restricted across depositional strike. It is interpreted to be the product of south-southeastward progradation from a fluvially-dominated delta lobe into the Cretaceous Western Interior Seaway (KWIS) Basin. This shore-parallel progradation direction suggests a southward-deflected delta lobe, facilitated by a counter-clockwise gyre circulation in the KWIS. Outcrop investigations concentrated on evaluating lateral variations in sedimentological and ichnological characteristics across a single river-dominated deltaic sandstone body. The Peay Member grades laterally from a thick, high-energy, fluvial mouth bar facies (axial core), to a tide- & wave-influenced proximal-medial delta flank facies, and finally to a thin, low-energy, storm- and wave-influenced prodelta-distal delta flank facies at the peripheries of the delta lobe. The axial core contains a vertical succession of prodelta-mouth bar facies displaying generally low and sporadic bioturbation intensities, reflecting depositional conditions that were stressful to bottom-dwelling organisms. Flankward, the medial delta flank facies reflects more tidal-, wave-, and storm-influences resulting in an impoverished expression of the proximal Cruziana Ichnofacies with elements of distal Skolithos Ichnofacies; bioturbation intensities fluctuate within facies. At the delta lobe peripheries, the prodelta-distal delta flank facies contains an archetypal expression of the Cruziana Ichnofacies with abundant bioturbation. Similar cross-sectional and planform geometries reveal a relationship between specific geomorphic zones and recurring facies distribution patterns. This relationship is used to develop a three-dimensional model that could potentially predict sandstone body geometry, regional facies distribution, and sandstone body dispersal patterns in the Bighorn Basin in this and other sandstone bodies. This study emphasizes the point that most deltas are dynamic systems and depositional influences may fluctuate both temporally and spatially throughout their existence.
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