Chuck Ciampaglio (Committee Member), David Dominic (Committee Co-chair), David Schmidt (Committee Co-chair)
Master of Science (MS)
The Brassfield Formation is an early Silurian (Aeronian, Llandoverian) unit of limestone and dolostone with excellent preservation of early reef communities and a diverse invertebrate fauna. A well-preserved outcrop occurs in southwestern Ohio at the Oakes Quarry Park in Fairborn; it contains abundant corals and stromatoporoids that are co-dominant in reef and reef-affected settings. This research provided an enhanced perspective of the Brassfield Formation to better reconstruct its paleoenvironment and better understand its diagenetic features. The depositional and diagenetic features of the Brassfield are described and interpreted using histologic methods and light microscopy. Selected thin sections were treated with carbonate stains to reveal the mineral interactions and possible microbial influence that led to the dolomitization present. These stains also revealed banding patterns that may indicate seasonal events. Features visible in the thin sections provided evidence for dolomite crystallization mediated by microbial activity and for both oxic and anoxic environments. Oxic environments are indicated by increased hematite precipitate and little organic carbon. Anoxic environments are indicated by increased pyrite precipitate and micritic or microbial associations. Dolomite associated with oxic characteristics appears unstable, with patches of dedolomitization. Dolomite associated with anoxic characteristics appears more intact, with larger mosaics and with larger crystal size. Growth patterns for all coral specimens have detectable high density (HD) and low density (LD) banding, most have growth interruption bands, and some have absent or breakage bands. Patterns indicate seasonal events (HD banding) that led to disruption of the coral viability, increased internal sedimentation, increased carbonate accumulations, and increased precipitates (namely pyrite and dolomite). Some events were marked by sediment infills and breakage bands at the probable point of organism death, suggesting a period of major environmental stress on the coral. Periods between events (LD banding) showed marked cementation, porosity, ferrous precipitation, and dolomite instability. This research suggested an important link between reducing environments and dolomite formation. Additionally, a relationship is probable between increased organic content, pyrite precipitation, and dolomite nucleation. Given that microbial evidence has been detected in past research examining carbonate and pyrite deposits, a link between microbial activity and dolomite formation is likely. Continued research into the geologic record may prove that indicators for specific microbial forms are marked by distinct phases of dolomite formation and degradation.
Department or Program
Department of Earth and Environmental Sciences
Year Degree Awarded
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