Abstract

Questions concerning the application of established biofacies models to mid-Cretaceous black shales prompted a study of diversity characteristics in a fauna from the Late Cenomanian Hartland Shale Member, Western Interior basin. Numerical faunal data are used to assess species abundance patterns, and a new method of analyzing diversity is introduced that incorporates species richness, Shannon index, and equitability into a single plot. In addition, numerical simulations designed to emulate the sampling of species-abundance distributions are used to improve data interpretation. The study illustrates how measured diversity results from the combination of primary ecological controls and sampling effects. Proximal offshore assemblages are characterized by high diversity and log series species-abundance patterns, interpreted as truncated (incompletely sampled) log-normal distributions. Primary ecological controls include variable physical/chemical parameters, biological factors such as predation and competition, and intermediate disturbance frequency. Distal offshore assemblages are characterized by low diversity with patterns of species abundance resembling geometric series. These are interpreted as truncated log-series distributions (sampling effect) that reflect dominance of multiple opportunists, abundant resources in a dysoxic environment, and high disturbance frequency. The data are used to develop an ecological model for diversity levels in basinal black shale facies based on the interplay of recruitment, growth rate, tolerance to low oxygen and sulphide, and disturbance frequency (due to fluctuations of the redox boundary). Although certain taxa (chiefly Inoceramidae) evolved highly opportunistic life strategies to exploit basinal paleoenvironments, it was the unpredictable interaction 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 diversity.

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