Reports: AC8

48462-AC8 Oligocene-Miocene Eustatic Record of the Northeastern Margin: Linking the Timing and Magnitude of Sea-Level Changes with the Stratigraphic Response and Paleoceanography of the Subtropical Pacific

R. Mark Leckie, University of Massachusetts (Amherst)

Eustasy is a key parameter to understand sedimentary sequences on continental margin, but its timing and magnitude remain controversial, especially for the Miocene period. In this paper, we present new results from carbonate sediments drilled during Ocean Drilling Program Leg 194 on the Marion Plateau, offshore northeastern Australia. The goal of our research was to better understand the mechanisms and timing of sequence formation on the Marion Plateau, and to assess the suitability of this stratigraphic record to constrain the global sea-level curve. We initially focused on the most distal drill sites, and we significantly revised the existing age model using a new biostratigraphic analysis based on calcareous nannofossils and planktic foraminifers. We characterize the distal slope using the sediment coarse fraction (>63µm) and digitally scanned core sections, and investigated the temporal evolution of the planktic to benthic foraminifer ratio (P/B ratio, a proxy for paleo water-depth) and amount of glauconite in slope sediment.

Results show that lowstands in the distal slope are characterized by condensed facies rich in glauconite, and by a lower P/B ratio. We propose that sediment condensation resulted from exposure of the carbonate factory, and we interpret the base of the lowstand glauconitic layers as distal expressions of sequence boundaries. We further demonstrate that sequences on the Marion Plateau are controlled by glacio-eustasy: each sequence boundary (condensed interval) is marked by an increase in d18O (increased ice volume on Antarctica). The Marion Plateau record comprises all of the sequences identified on the nearby Queensland Plateau, Great Bahamas Banks, New Jersey Margin, and the Exxon Production Research curve. Age control on the Marion Plateau curve is excellent, and the stratigraphic record of eustatic changes appears complete. We conclude that the Marion Plateau provides one of the best records of the timing of eustasy between 15.4 to 10.8 Ma, and has the potential to provide independent amplitude estimates for this time period. Crucially, this is the geologic period for which the New Jersey margin sea-level record is the least well constrained.