Nature and distribution of Cambrian shell concentrations: Evidence from the Basin and Range Province of the Western United States (California, Nevada, and Utah)

Shell concentrations (shell beds) are abundant and diverse in the Cambrian strata of the Basin and Range Province (California, Nevada, and Utah). These beds occur in many lithofacies and vary from mm-thick pavements composed exclusively of trilobites to well-developed composite beds, meters in thickness, composed of a variety of taxa. Overall, the physical dimensions, abundance, and diversity of shell accumulations increased from Early to Late Cambrian times in shallow-marine, mixed carbonate and siliciclastic facies. This temporal trend is likely controlled primarily by the diversification of trilobites and the appearance of new clades. A decrease in the abundance and an associated rapid increase in thickness of fossil accumulations in the latest Cambrian is likely tied to lithologic changes associated with changes in accommodation space. Sedimentary rocks from inner detrital, carbonate platform, and outer detrital belts contain taphonomically and stratigraphically distinct suites of shell concentrations. Relatively thin storm-generated event concentrations are most common in the inner and outer detrital belts, whereas composite and condensed shell beds predominate in carbonate platform facies. The environmental variations in shell accumulations across the Cambrian shelf are controlled primarily by physical processes, such as storm events and current and/or wave agitation. The common association of shell beds with hiatal surfaces and meter-scale stratigraphic cycles indicates that many Cambrian trilobite-dominated shell beds can be used as hey marker beds for stratigraphic analysis.