SIGNIFICANCE OF MICROBIAL FACIES IN THE LATE ORDOVICIAN BIGHORN DOLOMITE

The Late Ordovician Bighorn Dolomite Formation is a massive, cliff forming, carbonate unit exposed in Wyoming and adjacent states. This work interprets the morphology of vertical structures observed in the Steamboat Point Member of the western Teton Range. The semi-parallel structures are 0.5-1.0 cm in diameter, anastomotic, display 1-2 cm of relief on the bedding plane surface, and resemble the thrombolite Favosamaceria cooperi: a Cambrian, benthic microbialite from the Great Basin. Thin section analyses of the structures, moreover, reveal dendroidal contours, interpreted as microbially-induced microstructures resulting from calcification of microbial communities. The micritic texture of the structures, along with the surrounding intraclastic, fossiliferous, microsparic matrix, confirm previous studies indicating the Bighorn Dolomite was deposited in waters supersaturated with calcium carbonate. In the interplay between constructive and destructive forces, infaunal bioturbation traces are largely absent, leaving sedimentation rate, carbonate-saturation levels, and microbial growth rate as the primary controls on morphology. Additional formational mechanisms involving sediment displacement, such as fluid escape, are ruled out using an original ternary diagram that identifies the genesis of cryptic, cm-scale structures. This reinterpretation of the seafloor of the warm, shallow, epeiric sea of Katian Laurentia, from heavily bioturbated to extensively colonized by microbial communities, revises our understanding of the impact of metazoans and microbes on early Paleozoic biogeochemical cycles prior to the End-Ordovician mass extinction.