INSIGHTS INTO PALEOENVIRONMENTAL CONDITIONS, RELATIVE SEA-LEVEL, AND CARBONATE RESERVOIR QUALITY USING ICHNOLOGY: MIDDLE-UPPER MIOCENE SLOPE DEPOSITS FROM THE GREAT BAHAMA BANK

Detailed ichnological and petrographic analyses were conducted on two biostratigraphically-constrained time intervals (11.9-.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. Four ichnofacies are identified: the Cruziana Ichnofacies (archetypal, distal and stressed expressions), the Zoophycos Ichnofacies, the Nereites Ichnofacies, and the Glossifungites Ichnofacies. An analysis of spatiotemporal trends in ichnofacies reveals two distinct responses of the benthic community to significant environmental perturbations on the slope. An abrupt lateral expansion of the stressed Cruziana ichnofacies at approximately 11.5 Ma coincides with a period of highstand shedding and increased depositional stress on the slope. Additionally, an ichnofacies change at roughly 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 the host sediment. Porosity heterogeneity is highest in archetypal Cruziana and Glossifungites fabrics and lowest in stressed Cruziana fabrics, while distal Cruziana, Zoophycos, and Nereites fabrics show intermediate porosity contrasts. Measurements indicate that distal Cruziana fabrics have the highest average porosities at 17.3 ± 2.5%, followed by Zoophycos fabrics at 13.6 ± 2.0%. Stressed and archetypal Cruziana fabrics possess comparable, but lower porosities at 11.3 ± 2.2% and 10.9 ± 2.5% respectively. Nereites fabrics show intermediate porosities at 9.9 ± 1.2%, while Glossifungites fabrics display lowest average porosities at 7.3 ± 2.5%. This study demonstrates the utility of trace fossil analysis in highlighting physiochemical change in carbonate slope environments, and expands upon previous observations concerning the impact of bioturbation on porosity distribution in carbonates.