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 Camb rian strata of the Basin and Range Province (California, Nevada, and U tah). These beds occur in many lithofacies and vary from mm-thick pave ments composed exclusively of trilobites to well-developed composite b eds, meters in thickness, composed of a variety of taxa. Overall, the physical dimensions, abundance, and diversity of shell accumulations i ncreased from Early to Late Cambrian times in shallow-marine, mixed ca rbonate and siliciclastic facies. This temporal trend is likely contro lled primarily by the diversification of trilobites and the appearance of new clades. A decrease in the abundance and an associated rapid in crease in thickness of fossil accumulations in the latest Cambrian is likely tied to lithologic changes associated with changes in accommoda tion space. Sedimentary rocks from inner detrital, carbonate platform, and outer detrital belts contain taphonomically and stratigraphically distinct suites of shell concentrations. Relatively thin storm-genera ted event concentrations are most common in the inner and outer detrit al belts, whereas composite and condensed shell beds predominate in ca rbonate platform facies. The environmental variations in shell accumul ations 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.