Evidence for the initial opening of the Tasmanian Gateway during the Early to Middle Eocene (51.9-49.0 Ma) based on shallow water stratigraphy from South Tasman Rise ODP Leg 189 Site 1171

Abstract:

Foraminiferal biofacies, P/B ratios, grain size, physical properties and downhole log records spanning the early to Middle Eocene (54-41 Ma) from the South Tasman Rise ODP Leg 189 Site 1171 provides evidence that the commencement of surface water flow through the Tasman Seaway occurred at ~51.9 Ma. Seven foraminiferal biofacies were identified and planktonic/ benthic (P/B) foraminiferal ratios were calculated to estimate water-depth changes and evaluate paleoceanographic conditions. The biofacies are characterized by a change from exclusively agglutinated to more calcareous forms, with the transition starting at 51.9 Ma. Foraminiferal abundances, nannofossil and CaCO3 content are low for the interval between 54.0-51.5 Ma. Above 51 Ma, calcareous foraminifers, such as Dentalina, Nodosaria, Lenticulina, Cibicides, Elphidium, and Bulimina spp. become abundant within many intervals as well as a long-term increase in nanofossil and CaCO3 content occurs. Higher P/B ratios occur in samples < 51 Ma although they vary throughout the early and middle Eocene.

Our data indicate a gradual change from: 1) shallow, poorly ventilated waters before 51.9 Ma; 2) slightly deeper and better ventilated waters occurred between 51.9 and 49.0 Ma; and 3) a transition from inner and middle/ outer neritic environments, with generally better ventilated conditions after 49.0 Ma. This is concomitant with a change in sedimentation rates from 164 m/myr between 52.7 and 51.7 Ma to 18 m/myr at 41.0 Ma. As shallow-water depths occur throughout this section, sedimentation rates from Site 1171 can be used as a proxy for subsidence for the eastern side of the nascent Tasman Seaway. Taken together these data suggest an early episode of high subsidence (e.g., possible crustal thinning) occurred between 52.7 and 51.7 Ma, with final rifting resulting in deep-water flow starting during the Late Eocene to earliest Oligocene.