The Pliocene record around the Prydz Bay margin: review and questions (Invited)

In situ Pliocene sediments have been known from the Vestfold Hills since 1980. The original site discovered by John Pickard at Marine Plain, is now recognised as being deposited at 4.5-4.0 Ma. This horizontal, 7.5 m thick sequence (the Sørsdal Formation of poorly consolidated, uniform diatomaceous sandstone) has no glacial features until the top 0.5 m (the Graveyard Sandstone member). The unit covers some 10 km2 and is horizontal but with rotated blocks due to some past expansion of the Sørsdal Glacier. It is most noteworthy for its fauna of dolphins and whales that are under study at present. Only Australodelphis mirus has been formally described to date and several other species of cetaceans await formal description. It also contains abundant molluscs and a few other invertebrates, including foraminifera and a crustacean, which are very poorly preserved. Much of the Marine Plain section was deposited in a quiet embayment under non-glacial conditions. A second Pliocene section was identified in Heidemann Valley in 1989 but its age was disputed until recently when an age of 3.5-2.6 Ma was determined on the basis of amino-acid stratigraphy. It is about 4 m thick but not exposed and seen only in excavated pits and a trench. It is diatom-poor, of coarse sand and boulders to 2 m, containing a modest foraminiferid and mollusc fauna that has been described. The Heidemann Valley sequence accumulated under ice in a straight glacial valley. Significant areas of the northern Vestfold Hills have the potential to contain further sections. In the Larsemann Hills, some 100 km south of the Vestfold Hills, is a thin veneer (40 cm thick) of re-deposited Pliocene coarse sediment with abundant fragments of at least two species of bivalves. It has been dated as contemporaneous with the Marine Plain section but contains a good foraminiferid fauna with abundant Ammoelphidiella. It is hoped that carbon and oxygen isotope data will be available for the meeting. Ammoelphidiella and Cibicides from Larsemann Hills are currently being analysed as is Cibicides from Heidemann Valley. Fragments of the bivalve Cyclocardia and pectenids from Marine Plain, and of hiatellid and pectenids from Larsemann Hills have been sent for analysis in an attempt to get further refinement. It may be possible to get fragments of bivalves (hiatellid, pectenid and possible Laternula elliptica) from Heidemann Valley. Peculiar features are the presence of oolites in the Heidemann Valley section and abundant leiospheres of unknown affinities at Marine Plain. Issues to be addressed include the evolution of the Southern Hemisphere biota during the Pliocene, including the origin of the current seal fauna, dramatic changes to the cetacean fauna in the mid-Pliocene, changes in krill and the microbiota - in short, the ecosystem. How resilient or otherwise were elements of the ecosystem during change and how do we determine this?