INDICATORS OF MICROBIAL MAT DEPOSITION IN PLAYA AND SABKHA LIKE CONDITIONS IN THE EARLY ORDOVICIAN COTTER DOLOMITE OF BRANSON MISSOURI

Microbial communities are ubiquitous in any continually moist environment where grazers are low or not present. Mats stabilize the sedimentary features and improve the probability of preservations after lithofication. Sedimentary structures, as well as geochemical traces, imply the presence of microbial communities during the deposition of the Cotter Dolomite during the early Ordovician (Ibexian). Three groups of microbial communities are inferred, based on observed differences in lithofaces: a.) oceanic, b.) mudflats or sabkha, c.) laucustrine playa deposited in alkaline salt lakes. Eustatic fluxuations during the Ibexian were extreme. During maximum transgression, the Ozark Highland harbored shallow marine communities. During maximum regression, waters would drain back below the continental shelf. As ocean waters receded below the continental margins of the Ozark Dome, large bodies of water remained in basins across the highland. The remaining waters formed massive playa lakes and/or sabkha, if not fully separated from the ocean. During the Ibexian, CO² levels were the highest on record during the Phanerzoic, reaching Precambrian levels. This caused extremes in temperature and eustatic fluxuations. During low water levels, playa lakes would form. Exposed to the sun and CO², these conditions favored the growth of archeobacteria, which release C¹/C² hydrocarbons like methycloroform and chloroform into the atmosphere. During the Ordovician, massive release of these compounds could have depleted Ozone (O³) and caused increased surface radiation of ultraviolet. Increased aridization, attributed to the release of gases by archeobacteria, occurs today, primarily along the 20º and 40º north and south latitudes (L. Weissflog, 2005), a similar position for the Branson area during the early Ordovician. This cynobacterial-dominated community persisted throughout the early and middle Ordovician, resembling that of the mat structures of the Precambrian and Pahrump Supergroups. This resulted in Snowball Earth at the end of the Precambrian and may have contributed to the glaciations at the end of the Ordovician.