DIVERSITY AND SPECIES ABUNDANCE PATTERNS IN LATE CENOMANIAN BLACK SHALE BIOFACIES, WESTERN INTERIOR, US

Questions concerning the application of established biofacies models t o mid-Cretaceous black shales prompted a study of diversity characteri stics in a fauna from the Late Cenomanian. Hartland Shale Member, West ern Interior basin. Numerical faunal data are used to assess species a bundance patterns, and a new method of analyzing diversity is introduc ed that incorporates species richness, Shannon index, and equitability into a single plot. In addition, numerical simulations designed to em ulate the sampling of species-abundance distributions are used to impr ove data interpretation. The study illustrates how measured diversity results from the combination of primary ecological controls and sampli ng effects. Proximal offshore assemblages are characterized by high di versity and log series species-abundance patterns, interpreted as trun cated (incompletely sampled) log-normal distributions. Primary ecologi cal controls include variable physical/chemical parameters, biological factors such as predation. and competition, and intermediate disturba nce frequency. Distal offshore assemblages are characterized by low di versity with patterns of species abundance resembling geometric series . These are interpreted as truncated log-series distributions (samplin g effect) that reflect dominance of multiple opportunists, abundant re sources in a dysoxic environment, and high disturbance frequency. The data are used to develop an ecological model for diversity levels in b asinal black shale facies based on the interplay of recruitment, growt h rate, tolerance to low oxygen. and sulphide, and disturbance frequen cy (due to fluctuations of the redox boundary). Although certain, taxa (chiefly Inoceramidae) evolved highly opportunistic life strategies t o exploit basinal paleoenvironments, it was the unpredictable interact ion of these four factors that determined diversity patterns. Analysis of Hartland Shale biofacies illustrates the difficulties in applying a strictly linear relationship between paleo-oxygen levels and diversi ty.