MICROBIAL CARBONATE MEDIATION IN THE LOWER SILURIAN BRASSFIELD FORMATION OF SOUTHWESTERN OHIO

Well-preserved Silurian limestones within the Brassfield Formation (Llandovery) at the Oakes Quarry Park near Fairborn, Ohio contain a lithofacies that developed via the influence of benthic microbial communities. Fossils of calcifying cyanobacteria exist with a range of preservation and include the coccoidal Renalcis and the filamentous Girvanella. Well-defined Girvanella fossils consist of intertwined filaments that were preserved by the calcitic impregnation of the cyanabacteria’s mucilagenous sheaths. Poorly-preserved Girvanella fossils occur as scattered filaments within amorphous masses of micrite, indicating that the micritic matrix precipitated as a byproduct of bacterial degradation of mucilage derived from Girvanella’s polysaccharide sheaths.

Decomposition of soft organics by heterotrophic bacteria produced 20 µm-diameter micropeloids within gastropod shells and in other cryptic settings. Other products of microbial carbonate meditation include micritic halos around skeletal grains and stromatolitic and thrombolitic microtextures. In addition, the occurrence of microbial fabrics adjacent to penecontemporaneous crystals of fibrous calcite indicates that the cement precipitated via several possible pathways of microbial metabolism that elevated microenvironmental alkalinity and pH.

Stromatactis-like fenestrae occur as cavities with flat bottoms and irregular ceilings, indicating that portions of the microbial lithofacies underwent rapid syndepositional lithification and subsequent cavity formation via the winnowing of loose material. Voids commonly contain veneers and pendants of microstromatolitic and micropeloidal micrite, isopachous fibrous calcite of a probable microbial origin, and internal geopetal micrite. Cavities were initially formed during direct connection to freely-circulating water; however, the production of microbial coatings and the internal sedimentation of micrite gradually decreased water circulation and produced anoxic conditions that prompted the precipitation of pyritic laminae on the fenestral walls.