The Maastrichtian and early Tertiary record of the Great Australian Bight Basin and its onshore equivalents on the Australian southern margin: a nannofossil study

Samples dredged during BMR Survey 66 by KV, Rig Seismic in the central Great Australian Bight Basin are examined and their calcareous nannofossils are recorded. The Maastrichtian, Eocene and Oligocene assemblages are compared with those known from the onshore southern Australian sequence, allowing a better understanding of the history of the southern margin of Australia. The Maastrichtian assemblages, the first found in southern Australia, probably represent a marine ingression encompassing three discernible phases. The Eocene record includes assemblages older than any from onshore and is also older than the base of the Eocene section on the Naturaliste Plateau. An offset parallelism with the onshore record is evident: in the offshore (Great Australian Bight) sequence, early Eocene ingressions preceded a middle Eocene transgression, while in the onshore Otway Basin (to the east) middle Eocene ingressions preceded a late Eocene transgression. In both sequences there are earlier Tertiary ingressions which were suited for calcareous foraminiferids but apparently not coccolith-forming nannoplankton. The previously reported excursion of the low-latitude Sphenolithus ciperoensis into southern Australia in the Oligocene is confirmed, being a result of a short warm episode, Surface waters along the southern margin of Australia were warmer in the west than in the east during much of the Eocene and Oligocene, This is attributed to a warm intermittent proto-Leeuwin Current, beginning in the middle Eocene, which brought warm surface waters from northwestern Australia into southern Australia. Dilution of the currents effects on the surface waters of southern Australia would be expected in an easterly direction. Nannofossil evidence, supported by palynological and lithological data, suggests that the seafloor in the Great Australian Bight Basin has subsided considerably since the Late Cretaceous. The onset of the increase in rate of subsidence in the middle Eocene (as reflected by the nannofossil assemblages) marked the end of a stage of very slow subsidence initiated at about 90 Ma ago. The assemblages provide strong evidence for a marked fall in sea level during the latest late Eocene, at a rate considerably higher than that of subsidence, resulting in shoaling well into the Oligocene.