DIFFERENTIATING BETWEEN BURROWS AND MICROBIALLY-INDUCED MESOSTRUCTURES IN LATE ORDOVICIAN CARBONATES (Invited Presentation)

The Late Ordovician Bighorn Dolomite is a massive bedded, cliff forming unit that unconformably overlies upper Cambrian strata across Wyoming and in surrounding states. One of the defining features of the Bighorn Dolomite is a conspicuous mottled fabric that is recognized in upper Ordovician strata across western Laurentia. In the Teton Range, the Steamboat Point Member contains unusual dark, cm-scale, vertical structures that are often attributed to bioturbation. Here we investigate the textural characteristics of the mottled fabrics of the Bighorn Dolomite using core observations from Sheridan, Wyoming, northeast of the Bighorn Mountains. Slabbed, core material from the Sheridan region, reveal three dominant mesofabric types. The basal-most unit contains vertical, parallel mesostructures similar to those reported from the Steamboat Point Member of the Teton Range. The second massive-bedded mottled fabric is composed of cm-scale micritic mesoclots surrounded by a wackestone fill. The third and topmost mottled fabric contains 1-3 mm diameter round to ovoid structures that are surrounded by laminated micrite that are reminiscent of Tubiphytes. Our observations suggest that much of the Bighorn Dolomite lacks clear evidence for a preponderance of bioturbation and instead exhibits constructional microbial features deposited in a subtidal environment. These results have implications for understanding the paleoecology, functional morphology, and taxonomic affinity of biosediments immediately preceding the end-Ordovician glaciation and for interpreting similar mesostructures during other times in Earth history.