MANY A MICRITE-FILLED MOLLUSK...BUT ARE THERE ANY MICROBES?

The Silurian-aged limestones and dolomites of the Brassfield Formation at Oakes Quarry (formerly Reed North Quarry), Fairborn, Ohio, are typically tan-pink in color and contain an abundance of echinoderms, brachiopods, bryozoans, corals, and stromatoporoids. This regularity, however, is interrupted by an orange-brown, mollusk-rich grainstone that is bounded on the top and bottom by shales. The fossil content of this marker bed is also distinct, dominated by gastropods and straight-shelled nautiloids. These mollusk shells contain micrite that is not present as matrix in the unit. Fossils in the grainstone appear to be thinly coated with pyrite. The bed has been sampled to produce thin-sections that cross-cut mollusk chambers in order to gather data about the micritic components. Recent research has revealed the presence of calcifying microbes such as Girvanella within other units at the quarry (Schmidt and Ausich, 2004). Petrographic analysis suggests that microbial activity contributed to fine-grained sediments within this unit as well.

Fieldwork is also currently underway to better delineate the macroscopic characteristics of the marker bed. The unique composition and distinct appearance of this unit could have implications for the identification of Silurian rocks locally and regionally. It is clear that this unit recorded a change in the environment between the deposition of those below and above it. The units below the marker bed and bounding shale are grainstones dominated by crinoids and the units above the marker bed are diverse fossiliferous grainstones and packstones rich in stromatoporoids, corals, bryozoans, and crinoids. This investigation aims to improve understanding of the depositional and post-depositional processes responsible for the cephalopod marker bed's composition and contribute to the geologic history of the Silurian Period in southwestern Ohio.