The benthic boundary biofacies model: Hartland Shale member, greenhorn formation (cenomanian), western interior, North America

Well-laminated, predominantly non-burrowed strata of the mid-Cretaceous Western Interior basin contain in situ epifaunal body fossils indicative of dysaerobic benthic oxygen levels. A gradient of increasing ecological complexity in these dysaerobic faunas is identifiable and reflects improvements in benthic conditions. Using these faunal assemblages, a Benthic Boundary Biofacies Model is developed in which the critical redox boundary is in dynamic equilibrium at the sediment-water interface. Variations in bottom water oxygenation, as well as substrate characteristics, are cited as the controlling factors in the position of this boundary, and thus control benthic colonization. The model describes a background of low-oxygen tolerant flat-clam communities which were predominant during deposition of the late Cenomanian Hartland Shale Member. Superimposed on this background, a history of brief episodic colonization and sedimentation events reveals a dynamic climate-ocean system in the Western Interior sea which maintained a delicate balance of oxygen deficiency at the sediment-water interface.