Northern sequences contain abundant organic matter and calcareous temperate microfossil assemblages.
Southern sequences contain siliceous microfossils of colder-water character, and varves in some intervals, suggesting strong seasonality perhaps related to the onset of glacial conditions in the Antarctic region. The middle Eocene to upper Oligocene sequences are crucial to understanding the opening of the Tasmanian Seaway, initially in shallow and later in deep water. Before the Oligocene, sequences on either side of the STR should have distinctive biogeographic characters. The opening of this gateway was such a profound event that biotic, sedimentologic, and geochemical parameters would almost certainly have undergone distinct changes. When studied in detail and in unison, changes in these parameters are expected to provide the crucially needed evolutionary information on this gateway.