MULTISCALE FACIES ANALYSIS OF MICROBIALITES FROM THE LATE ORDOVICIAN BIGHORN DOLOMITE, WESTERN WYOMING

The Late Ordovician Bighorn Dolomite is a massive cliff forming unit in the Green River Basin. The focus of this work is on meter-scale microbialite deposits (up to 2 meters thick) that occur in the Steamboat Point Member of the western Teton range. In cross section, the m-scale microbialite domes exhibit roughly parallel, vertical columns (approximately 1cm in diameter) that anastamose and contain decimeter-scale vertical continuity. The spacing of the columns is moderately regular and they display approximately 1-2 cm of synoptic relief. At the cm-scale, the column texture is composed of closely packed grains, while the adjacent sediment fill is darker in color, more weathered, and grainy. The columns sometimes display stromatolitic laminations and proto-bridges that connect adjacent columns. Microscopically, the columns are composed of interlocking crystals of spar that are approximately 100-300 micrometers in diameter with loosely dispersed clotted, shrub-like micritic textures, whereas the sediment fill is composed of smaller grains that range from 10 to 50 micrometers in diameter. Metazoan skeletal fossils and intraclasts are abundant in the sedimentary fill, and barren in the columns. Together, these observations bear striking similarity to Cambrian thrombolites from the Great Basin (e.g., Favosamaceria cooperi) where the columns described herein are analogous to maceria, a morphology interpreted to be of microbial affinity. We interpret the shrub texture to be a product of benthic microbial calcification and the proto-bridges as a part of a larger microbial mat structure that lithified quickly due to high calcium carbonate saturation in an open marine subtidal setting. Our observations are consistent with other studies that suggest the lower Bighorn Dolomite was deposited in marine waters of high calcium carbonate saturation during greenhouse climate conditions prior to the end-Ordovician glaciation.