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

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 margins and to reconstruct continental ice volume in the Cenozoic, but timing and magnitude of global sea level changes remain controversial, especially for the Miocene Epoch. We analyzed sediment cores recovered from the Marion Plateau, offshore northeastern Australia, during Ocean Drilling Program (ODP) Leg 194 to define the mechanisms and timing of sequence formation on mixed carbonate-siliciclastic margins, and to estimate the amplitude of Miocene eustatic adjustments. We identified sequence boundaries on seismic reflection lines, significantly revised the existing biostratigraphic age models, and investigated the sedimentary response to sea-level changes across the Marion Plateau. We subdivided the Miocene sediments into three sequence sets comprising a set of prograding clinoforms, a muddy prograding carbonate ramp evolving into an aggrading platform, and a lowstand ramp evolving into a backstepping ramp. We recognized eight individual sequences dated at 18.0 Ma, 17.2 Ma, 16.5 Ma, 15.4 Ma, 14.7 Ma, 13.9 Ma, 13.0 Ma, and 11.9 Ma (Ma = mega-annums or millions of years). We demonstrate that sequences on the Marion Plateau are controlled by glacio-eustasy since sequence boundaries are marked by increases in _18O (deep-sea Miocene isotope events Mi1b, Mbi-3, Mi2, Mi2a, Mi3a, Mi3, Mi4, and Mi5, respectively), which reflects increased ice volume primarily on Antarctica. Our backstripping estimates suggest that sea-level fell by 26-28 m at 16.5 Ma, 26-29 m at 15.4 Ma, 29-38 m at 14.7 Ma, and 53-81 m at 13.9 Ma. Combining backstripping with _18O estimates yields sea-level fall amplitudes of 27±1m at 16.5 Ma, 27±1m at 15.4 Ma, 33±3m at 14.7 Ma, and 59±6m at 13.9 Ma. We use a similar approach to estimate eustatic rises of 19±1m between 16.5-15.4 Ma, 23±3 m between 15.4-14.7 Ma, and 33±3 m between 14.7-13.9 Ma. These estimates can be combined into a eustatic curve that suggest sea-level fell by 53-69 m between 16.5 to 13.9 Ma. This implies that at least 90% of the East Antarctic Icesheet was formed during the middle Miocene. The new independent amplitude estimates are crucial as the Miocene is the geologic Epoch for which the New Jersey margin sea-level record is least constrained.