Depositional environments and paleoecology of the lower Ordovician Pogonip group, Opd Unit, Arrow Canyon Range, Clark County, Nevada
The Opd Unit of the Pogonip Group is a slope-forming carbonate sequence that crops out in the Arrow Canyon Range 75 kilometers north of Las Vegas, Nevada. It is composed mainly of grainstones, packstones, and intraformational conglomerates and also contains Calathium-bearing bioherms composed predominantly of bioclastic wackestones. These rocks were deposited in a shallow-water, high-energy, subtidal, tropical marine environment. Nuia and Nuia-crinoid shoals developed on this shallow ramp and the area was frequently disturbed by storms; Calathium-dominated carbonate mudmounds emerged on the Nuia and Nuia-crinoid shoals. Even though Calathium dominated the mounds on a macroscopic scale, the mounds were built by unpreserved microorganisms. Calathium simply lived in the mounds and may have acted as sediment baffles. Other mound dwellers included trilobites, crinoids, gastropods, nautiloids, brachiopods, and bryozoa; The bioherms of the Opd Unit represent the transition from bioconstructions dominated by microorganisms during the Middle and Late Cambrian to those dominated by sessile benthonic macroorganisms during the Middle Ordovician.