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Detailed ichnological and petrographic analyses were conducted on two biostratigraphically-constrained intervals (11.9-11.3 Ma and 10.7-9.4 Ma) from the Middle-Upper Miocene stratigraphy of the Great Bahama Bank’s leeward margin to assess spatiotemporal trends in paleoenvironmental conditions and reservoir properties in carbonate slope deposits. Six ichnofacies, varied in expression, are documented: the Skolithos Ichnofacies (distal and impoverished expressions) the Cruziana Ichnofacies (proximal, archetypal, distal and impoverished expressions), the Zoophycos Ichnofacies, the Nereites Ichnofacies, the Glossifungites Ichnofacies, and the Trypanites Ichnofacies. An analysis of spatiotemporal ichnofacies trends reveals two distinct responses of the benthic community to significant environmental perturbations on the slope. An abrupt lateral expansion of impoverished Cruziana suites c. 11.6 Ma coincides with a period of highstand shedding and increased depositional stress on the slope. Additionally, an ichnofacies change c. 10.5 Ma, recorded by lateral dominance of the archetypal Cruziana ichnofacies on the slope, represents a prolonged period of quiescent conditions, characteristic of a sea-level lowstand. Petrographic analysis reveals that porosity is largely controlled by bioturbation in these deposits. Bioturbation characteristically results in porosity heterogeneity, as burrow fills, linings, spreiten, or halos differ significantly in character to host sediment. Porosity heterogeneity is highest in non-impoverished Cruziana and Glossifungites fabrics and lowest in impoverished Cruziana fabrics, while Zoophycos and Nereites fabrics show intermediate porosity contrasts. Measurements indicate that distal Cruziana fabrics have the highest average porosities at 17.7 ± 5.9%, followed by Zoophycos fabrics at 14.8 ± 4.3%. Impoverished proximal and distal Cruziana fabrics possess comparable, but lower porosities at 13.3 ± 4.2% and 11.0 ± 4.2% respectively. Nereites and Glossifungites fabrics have intermediate porosities at 10.2 ± 3.6% and 11.3 ± 3.2% respectively. This study demonstrates the utility of trace fossil analysis in highlighting physiochemical change in carbonate slope environments and expands upon previous observations concerning impacts of bioturbation on porosity distribution in carbonates.
Advisor: Tracy D. Frank