Abstract:
The middle Miocene to early late Miocene ( approximately 14-10.5 Ma) was a time of several rapid (<1 myr) cooling events that lead to the step-wise growth of Antarctic ice sheets and a corresponding fall in global sea level. The Marion Plateau contains a submerged carbonate platform that is composed of mixed carbonate/clastic sediments and is located on the Queensland margin, northeast Australia. This tectonically stable margin provides a unique opportunity to compare eustatic records from a mixed carbonate/clastic margin to those from a siliciclastic margin, such as the well-studied New Jersey margin. ODP Leg 194 drilled at eight locations on the Marion Plateau, both on the submerged platform and in the surrounding basinal slope deposits. A number of studies have attempted to reconstruct sea level change using a variety of proxies. In previous studies Cedric John and colleagues constrained the range of sea level fall for Mi-3 ( approximately 13.9 Ma) and Mi-4 ( approximately 13.2 Ma) to between 45 and 55 m, showed evidence that the final drowning of the carbonate platforms on the Marion Plateau was linked to changes in ocean currents shifting sediment deposition centers, and correlated sediment flux to sea level change. With events lasting <1 Myr a precise time scale is essential. Unfortunately, prior to our study, no shore-based research had been done on the microfossils from Leg 194, and as a result age control remained poor. After sampling and processing the cores using standard techniques, tropical/subtropical foraminiferal and calcareous nannofossil biozones were identified based the first and last occurrences (FO, LO) of key taxa, including the FO of Orbulina universa (14.74 Ma), LO of Globorotalia fohsi (11.79 Ma), and FO Neogloboquadrina acostaensis (9.83 Ma). This study has allowed us to refine the shipboard age model and test previous interpretations of the sedimentology and eustatic history of the Marion Plateau based on redeposited platform debris, quartz, and reworked glauconite in relation to benthic foraminiferal stable isotope data from ODP Sites 1192 and 1195. Specifically, we were able to improve intra-basinal correlation and better correlate the timing of the middle Miocene Mi3 and Mi4 events, as well as the latest middle Miocene Mi5, and perhaps early late Miocene Mi6 events.