Lower and Middle Ordovician shell beds from the Basin and Range province of the western United States (California, Nevada, and Utah)

A study of shell concentrations (shell beds) from Basin and Range (Californ ia, Nevada, and Utah) Lower and Middle Ordovician strata demonstrates that they are taxonomically diverse and very abundant locally in this region. Or dovician shell beds range from mm-thick pavements, composed exclusively of trilobites or brachiopods, to well-developed composite monotaxic and polyta xic shell beds, meters in thickness. Lower Ordovician (Ibexian) shell beds are primarily trilobite- and echinoderm-dominated and are taphonomically si milar to Cambrian shell beets. In contrast, brachiopod, ostracod, and gastr opod shell beds dominate Middle Ordovician (Whiterockian) strata. Shell beds are found throughout Lower and Middle Ordovician strata in nearl y all lithofacies of the Basin and Range. However, frequency of occurrence and the stratigraphic, taphonomic, and paleontologic features of shelf beds vary across Lithofacies. These environmental variations are caused primari ly by physical processes, such as storm events and current reworking. The e arly Middle Ordovician (Whiterockian) is a critical time in the development of Paleozoic shell concentrations; the impact of the Ordovician radiation significantly changed the biofabric of the sedimentary rock at this time. O verall, the physical dimension, abundance, and diversity of taxonomic types of shell beds increased from Ibexian to Whiterockian time. Significantly, brachiopod-dominated shell beds along with those dominated by ostracodes an d gastropods replaced the trilobite-dominated beds as the dominant type in shallow marine-carbonate settings in the early Middle Ordovician. This stud y additionally suggests that shell beds are useful proxies for recognizing the changing dominant components of paleocommunities through time.