Lower and Middle Ordovician Shell Beds from the Basin and Range Province of the Western United States (California, Nevada, and Utah)

Abstract

A study of shell concentrations (shell beds) from Basin and Range (California, Nevada, and Utah) Lower and Middle Ordovician strata demonstrates that they are taxonomically diverse and very abundant locally in this region. Ordovician shell beds range from mm-thick pavements, composed exclusively of trilobites or brachiopods, to well-developed composite monotaxic and polytaxic shell beds, meters in thickness. Lower Ordovician (Ibexian) shell beds are primarily trilobiteand echinoderm-dominated and are taphonomically similar to Cambrian shell beds. In contrast, brachiopod, ostracod, and gastropod shell beds dominate Middle Ordovician (Whiterockian) strata. Shell beds are found throughout Lower and Middle Ordovician strata in nearly all lithofacies of the Basin and Range. However, frequency of occurrence and the stratigraphic, taphonomic, and paleontologic features of shell beds vary across lithofacies. These environmental variations are caused primarily by physical processes, such as storm events and current reworking. The early 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. Overall, 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 and gastropods replaced the trilobite-dominated beds as the dominant type in shallow marine-carbonate settings in the early Middle Ordovician. This study additionally suggests that shell beds are useful proxies for recognizing the changing dominant components ofpaleocommunities through time.