SEARCH FOR MICROBIAL CARBONATES IN THE SILURIAN OF THE CINCINNATI ARCH REGION

Although the carbonate rocks within the Silurian of the Cincinnati Arch region have commonly undergone extensive dolomitization and recrystallization, they contain evidence of the sediment-binding and/or mineral-precipitating activities of benthic microbes. Fossil microbes occur with varying degrees of preservation, and microbial activity influenced the development and lithification of micrite, as indicated by micritic coatings around skeletal grains, laminated, clotted, and micropeloidal fabrics, and micrite with gravity-defying features.

Fossils of the calcifying microbe Girvanella occur in well-preserved rocks of the Brassfield Formation (Llandovery) cropping out at the Reed North Quarry near Fairborn, Ohio. The quarry also contains mottled mudstones and wackestones surrounding the enigmatic structure Stromatactis. The enclosure of Stromatactis by micritic veneers suggests that microbial calcification may have stabilized the walls of cavities that underwent later infilling by sparry calcite.

Bioherms in the Waldron Shale (Wenlock) near Hartsville, Indiana consist of fossils of macroinvertebrates that encrusted, and were encrusted by, micrite having clotted and laminated microtextures and steep-angled margins. The microfabrics and overall domical forms of the Waldron Shale bioherms strongly suggest that they developed via synsedimentary lithification of micrite produced by calcifying microbes.

Stromatolites occur within rocks of the Salina Group (?Ludlow) that crop out in the Duff Quarry in Huntsville, Ohio. Dark, asphaltic laminae, interspersed with light layers, show an upward gradation in geometry from planar laminated to interconnected discrete domes. The stromatolites developed in intertidal zones, and, like modern stromatolites, are interpreted as having formed via the baffling and binding activities of filamentous microbes.

Analysis of the Silurian rocks within the Cincinnati Arch region indicates that despite the apparent global reduction in microbialite development during the post-Ordovician, communities of calcifying microbes continued to serve as framebuilders and sediment contributors throughout the Silurian.