BENTHIC MICROBIAL CALCIFICATION OF COLUMNAR STRUCTURES IN THE LATE ORDOVICIAN BIGHORN DOLOMITE OF WESTERN AND NORTHERN WYOMING

The Late Ordovician Bighorn Dolomite of western and northern Wyoming is a massive dolostone composed of cryptic, cm-scale columnar structures that were historically attributed to Thalassinoides: the burrows of an unidentified shrimp-like crustacean. At the mesoscale (cm-scale) the column texture is dark, crystalline, and relatively resistant to weathering. The interstitial matrix is lighter in color, fine grained, and relatively more weathered. On bedding plane exposures, the structures exhibit 1-2 cm of relief, reflecting differential weathering between the vertical, anastomotic structures and the surrounding sediment matrix. The cm-scale columns are composed of micritic mesoclots surrounded by interlocking crystals of spar. The mesoclots contain diffuse to shrub-like micritic textures, whereas the intercolumnar matrix is composed of dolomicrite grains, skeletal clasts, and intraclasts, indicating gravity-driven deposition. We interpret the microscopic shrub texture to be a product of benthic microbial calcification. Paragenetic analyses demonstrate evidence for early lithified microbial biofilms despite dolomite replacement and extensive aggrading neomorphism. These observations indicate the columnar structures were deposited in highly supersaturated seawater with respect to calcium carbonate and are consistent with other studies that suggest deposition the during greenhouse conditions prior to the end-Ordovician glaciation.