The development of Early Paleozoic shell concentrations: Evidence from the Basin and Range province of the western United States (California, Nevada, and Utah)

The primary objective of this thesis is to examine and document the impact of the Cambrian and Ordovician evolutionary radiations on the development of shell concentrations. Results show that shell concentrations (shell beds) are abundant and diverse in the Lower Paleozoic strata of the Basin and Range Province (California, Nevada, and Utah). They range from mm-thick pavements composed of exclusively of trilobites or brachiopods to well-developed composite monotaxic and polytaxic shell beds meters in thickness. There are primarily four types (small shelly accumulations, trilobite-dominated, echinoderm-dominated, and brachiopod-dominated) shell beds in the Cambrian and Ordovician strata. The small shelly deposits are limited in the Pre-trilobite Lower Cambrian strata. The Cambrian and Lower Ordovician (lbexian) shell beds are primarily trilobite- and echinoderm-dominated. In contrast, brachiopod, ostracod, and gastropod shell beds dominate Middle Ordovician (Whiterockian) strata. Overall, the physical dimension, abundance, and the diversity of taxonomic types of shell beds increased from Early Cambrian to Middle Ordovician times. This temporal trend is coursed primarily by diversification of different clades such as trilobites, echinoderms, and brachiopods and the appearance of new shell-producing clades.; Shell beds are found throughout the Basin and Range Lower Paleozoic strata in nearly all lithofacies. However, the stratigraphic, taphonomic, and paleontologic features of shell beds vary across lithofacies. These environmental variations in shell-rich deposits are coursed primarily by physical processes such as storm events and current reworking. The early Middle Ordovician (Whiterockian) is a critical time in the development of Lower Paleozoic shell concentrations; the impact of the Ordovician radiation significantly changed the biofabric of the sedimentary rocks at this time. Brachiopod-dominated shell beds along with ostracod and gastropod shell concentrations replaced the trilobite-dominated shell beds as the dominant type of shell beds in shallow marine carbonate and mixed carbonate and siliciclastic settings in the early Middle Ordovician (Whiterockian). This study additionally suggests that shell beds are useful proxies for recognizing the changing dominant components of paleocommunities through time.