Three time lines through the neritic stratigraphic record distributed around the northern margin of the Australo-Antarctic Gulf (AAG) mark three fundamental shifts in global environments collectively comprising the Auversian facies shift. The three lines are: (1) the beginning: the Khirthar transgression and the onset of neritic carbonate accumulation in the Bartonian Age (preceding onset of the Middle Eocene climatic optimum [MECO]); (2) the midlife change (Bartonian-Priabonian transition): the shift from carbonate-rich to carbonate-poor, higher-nutrient environments under estuarine circulation, causing widespread dysaerobia culminating in opaline silicas; and (3) the Eocene-Oligocene = Priabonian-Rupelian boundary and glaciation during oxygen isotope event Oi-1, with return of improved ventilation in neritic environments and resumption of carbonate accumulation. Meanwhile, it was warm and very wet at ~60°S. In developing a scenario for the death of the AAG, the birth of the Southern Ocean, and the transition from Paleogene greenhouse Earth to Neogene icehouse Earth, the neritic record of the northern margin is more in accord with the “Dinocyst biogeographic hypothesis” than with the “Tasman gateway hypothesis.”

Abstract The Port Willunga Formation is a cool–water, marine, quartzose, clay–rich, biosiliceous, and calcareous sedimentary succession of Early Oligocene age that accumulated in a series of proximal estuarine paleoenvironments along the eastern side of the St. Vincent Basin, South Australia. Coeval strata in two of the paleo–embayments are interpreted to record deposition during one ~ 3.5 My–long eustatic sea–level fluctuation. Transgressive facies above a ravinement surface comprise quartzose sands (subaqueous marine tidal dunes) that grade upward into fossiliferous floatstones and mudstones (shoreface to shallow basin–floor environments) that accumulated in a protected embayment. Highstand sediments are distinctly cyclic at the meter scale and consist of epifaunal bryozoan–pecten–echinoid clay–rich floatstones that become less fossiliferous but more spiculitic and chert–rich upward in each cycle. Whereas cyclic sediments in one embayment (Willunga) are interpreted to have accumulated on a current–swept, illuminated seafloor, those in the other (Noarlunga) are thought to have been deposited in a lower–energy, sub–photic setting. Cyclicity is interpreted to record the increasing influence of fluvial fresh water in the system during each sea–level fluctuation. Comparison with underlying strata reveals a striking similarity in depositional style and stratigraphic packaging between Late Eocene and Early Oligocene deposits; both are interpreted as paleoestuarine. Differences between the dark, organic–rich, biosiliceous, and low–diversity Eocene highstand deposits and the light, more calcareous, and more diverse Oligocene highstand deposits are interpreted to be due to local depositional controls. An important implication of local controls is that several postulated unconformities in the succession are not due to global eustatic changes but are ravinement surfaces related to estuarine sedimentation dynamics. Such controls, specifically terrestrial climate, hydrodynamic energy, and trophic resource levels were more important in determining sediment composition than eustasy and Southern Ocean cooling. Similar biosiliceous–carbonate sedimentary facies are a recurring feature of cool–water deposition throughout the Phanerozoic.

Abstract This section includes abstracts presented in the Stratigraphy & Evolution Session at the Ninth International Conference on Fossil and Modern Dinoflagellates held between 28 August and 2 September 2011 at the University of Liverpool, UK.