Tectono-sedimentary history of uppermost Cretaceous through Oligocene sequences from the Tasmanian region: A temperate Antarctic margin

Abstract

The opening of the Tasmanian Gateway between Australia and Antarctica at the Eocene/Oligocene boundary (∼33.5 Ma) was a profoundly important event that affected global oceanographic circulation and climate. Ocean Drilling Program Leg 189 (in the gateway), together with other geoscience information, has increased our understanding of the Late Cretaceous and younger tectonic and depositional history of the region. From the mid-Cretaceous until the latest Eocene, Australia and Antarctica faced each other across an ever-widening Australo-Australian Gulf, terminated to the east by a Tasmanian land bridge (Tasmania and South Tasman Rise [STR]). Siliciclastic sediments poured into the rifts from Antarctica, Australia and parts of the land bridge, forming deltas in a low-oxygen environment. Sedimentation kept up with subsidence, except on oceanic crust in the spreading Tasman Sea. Until the Paleocene/Eocene boundary (∼55 Ma), Australia moved northwestward from Antarctica along a fracture west of the land bridge. Thereafter, Australia-Antarctic motion changed to N-S along the Tasman Fracture Zone west of STR, and an oceanic basin opened south of eastern STR. In the middle Eocene (∼43 Ma), spreading rates increased between Australia and Antarctica, and Tasman Sea spreading ceased. By the latest Eocene, the STR had subsided until parts of it were current swept, and winnowing reduced sedimentation rates. Rapid subsidence marked the final (earliest Oligocene) separation of STR and Antarctica. In the Pacific Ocean (STR and East Tasman Plateau), strong currents eroded the shelves and opening straits, and a latest Eocene to early Oligocene hiatus was followed by deposition of bathyal carbonate oozes. The Indian Ocean was different. Along the southern margin of mainland Australia the siliciclastic-carbonate transition came largely in the late Eocene and early Oligocene. The west Tasmanian margin ODP site had gradual increases in carbonate content through the Oligocene. In nearby areas of Antarctica, non-marine and shelfal siliciclastic sedimentation gave way to glacigene detrital or diatomaceous sedimentation at the Eocene-Oligocene transition.