SEDIMENTOLOGICAL EVIDENCE FOR THE FORMATION OF AN EAST ANTARCTIC ICE-SHEET IN EOCENE OLIGOCENE TIME

Middle Eocene to late Oligocene sediments recovered at Ocean Drilling Program Sites 689 and 690 on Maud Rise in the southernmost Atlantic Ocean and at Sites 738 and 744 on Kerguelen Plateau in the southernmost Indian Ocean were analysed in order to study the depositional environment and the palaeoclimate. Special emphasis was placed on the reconstruction of the formation of a cryosphere on the Antarctic continent. The investigations include quantifications of carbonate and opal contents, grain size analyses and studies of clay mineral assemblages. The sedimentary sequence at all sites under investigation is highly pelagic, with nannofossil oozes and chalks dominant. The first indication of probable glacierization at sea level is in the form of isolated gravel and terrigenous sand grains, which indicate ice-rafting from middle Eocene time at c. 45.5 Ma. This is supported by enhanced concentrations of detrital chlorite and reworked kaolinite. Probably, some glaciers reached the sea, while most of the continent remained under the influence of a humid and warm to temperate climate. The growth of the inland ice resulted in enhanced physical weathering and in increased contents of detrital chlorite and kaolinite from about 40 Ma. A strengthening of the glacial conditions and the onset of continental East Antarctic glaciation is recorded in early Oligocene sediments with an age of 36 Ma. All major sediment parameters document this event. The clay mineralogy changed between c. 36.3 Ma and c. 35.5 Ma from smectite-dominated assemblages to illite- and chlorite-dominated assemblages, the latter being indicative of physical weathering under cooler climates. Large quantities of ice-rafted gravel and sand accumulated on the Kerguelen Plateau between 36.0 Ma and 35.8 Ma. At the same time, an 1 increase in opal content occurred as well as a decrease in carbonate. The sediment parameters imply that the East Antarctic continent was more or less totally buried beneath the ice during Oligocene time. The ice sheet, however, probably did not have a polar but a temperate character, as indicated by the occurrence of Nothofagus and relatively warm surface water temperatures prohibiting the long-distance transport of debris by icebergs.