Quantifying the stratigraphic and spatial facies distribution in an ancient mixed-influence delta

Abstract

Analysis of the sedimentology and stratigraphic architecture of tightly spaced three dimensional outcrops reveals that the Turonian (Upper Cretaceous) Wall Creek Member of the Frontier Formation in the western Powder River Basin, Wyoming, USA, is not composed of one continuous coarsening upward succession but of a complex stacked delta system containing three distinct sequences (S1-S3), each with a unique facies distribution and architectural heterogeneity. The basal sequence S1 consists of a fluvial dominated delta with two distinct lobes. These lobes are spatially constrained to the northeastern study area and show a rapid facies transition from trough crossbedded mouthbar deposits to lower delta front turbidites. Low angle clinoforms suggest a low accommodation setting with main sediment transport to the south. The middle S2 sequence is common throughout the study area and contains an abundance of storm-derived deposits, including hummocky cross stratification, suggesting the transition to a wave and storm-dominated delta setting. Sediment transport is largely to the south controlled by wind induced shear stresses. Lastly, heterolithic trough crossbedded sandstones with flaser bedding and abundant thin mudstones and rip-up clasts are characteristic for sequence S3. These deposits are interpreted as tidal bars in a tidal influenced delta. Quantitative evaluation of facies in the Wall Creek Member sequences shows that the dimensions and connectivity (baffle or barrier competence) of fine-grained thin beds varies systematically within the three delta types. The S1 fluvial delta is largely composed of laterally continuous delta front turbidites with continuous fine-grained thin beds (mean length 21.1 m or 69.2 ft, max length 83.9 m or 275.2 ft) separating individual sandstone beds. Conversely, abundant bioturbation and intense scouring by storms results in high amalgamation of sandy beds in sequence S2 and a limited length of fine-grained thin beds (mean 8.5 m or 27.9 ft) in the wave-dominated delta sequence. Tidally influenced deposits of sequence S3 are largely composed of heterolithic trough crossbedded sandstones and mudstones with low bioturbation, resulting in an intermediate fine-grained thin bed deposit (mean 12.1 m or 39.7 ft).