Eocene and Oligocene calcareous nannofossils from the Great Australian Bight: evidence of significant reworking episodes and surface-water temperature changes

Eocene and Oligocene assemblages identified from carbonates dredged from the outer margin of the Ceduna Terrace in the Great Australian Bight, southern Australia, document two major Palaeogene reworking episodes with probable counterparts in the western central Pacific Ocean. The first episode (mid Eocene, ~43 .5 Ma) coincides with and is related to events affecting deposition on the Australian southern margin, which were initially controlled by a sudden change in seafloor spreading rate between Australia and Antarctica. In this reworking episode, nannofossils of mainly late Cretaceous age (from the Naturaliste Plateau and/or the Eyre Terrace in the Great Australian Bight) were carried eastward and deposited by short-lived bottom currents flowing along the Australian southern margin. In the second episode, nannofossils of mainly Eocene and early Oligocene age were included in mid-upper Oligocene carbonates on the Australian western margin (Carnarvon Terrace and Perth Canyon) and in the Great Australian Bight. There was almost no supply of Eocene or lower Oligocene carbonate debris from onshore areas to the shelf (and deep sea) , due to the aridity of the Australian continent during the Oligocene and the lack of diagenetically suitable sediments onshore. Reworked nannofossils in the upper Palaeogene shelf carbonates of western and southern Australia are therefore interpreted as coming from nearby shallow and intermediate oceanic sites during large-scale, late Palaeogene erosion in the southeast Indian Ocean. The late Palaeogene erosive events, although intimately related to the Oligocene cooling, were probably tectonically triggered. Some Oligocene assemblages from the Ceduna Terrace suggest cool surface waters. Others contain low-latitude species and suggest short episodes of warming during the mid to late Oligocene. Warm surface water from northwestern Australia was introduced to southern Australia by an intermittent proto-Leeuwin Current starting in the mid Eocene; the effects of this surface current varied in intensity but generally petered out eastward along the southern margin . The warm-water influences were pronounced during at least two distinct short Oligocene episodes.