LEGS 119 and 120

Kerguelen Plateau and Prydz Bay

Legs 119 and 120 completed a latitudinal transect in the Southern Ocean between Kerguelen Island (49 deg S) and Prydz Bay, East Antarctica (67 deg S), a region critical to the understanding Cenozoic climate and oceanographic development. The Kerguelen Plateau lies south of the present-day Polar Front (Antarctic Convergence) and beneath the main flow of the Antarctic Circumpolar Current. These two oceanographic features have a major effect on global climate and surface-water circulation. During Leg 119, five sites were drilled in Prydz Bay (Site 739 to Site 743) and six sites were drilled on the Kerguelen Plateau (Sites 736 and 737 on the Northern Kerguelen Plateau and Sites 738, 744, 745, and 746 on the Southern Kerguelen Plateau) with the objective of studying the paleoceanographic history of the southern Indian Ocean, the geological development of the Kerguelen Plateau, and the development of glaciation in East Antarctica. During Leg 120, five sites were drilled on the central part of the plateau (Site 747 to Site 751) to test the conflicting hypotheses about the origin of the Kerguelen Plateau and to determine when and how the plateau formed.

The oldest preglacial sequence above acoustic basement in Prydz Bay consists of "red bed" type sediments of continental origin, suggesting deposition within proximal reaches of a river system. Climatic conditions were probably warm and characterized by seasonal rainfall. Although the paucity of microfossils inhibits age determination, these red beds may be equivalent to deposits in the Lambert Graben of possible Permian or later age. Changes in the sediment composition indicate that Antarctic glaciation proceeded in phases with the presence of significant late/middle-late Eocene glaciation in east Antarctica (~ 42 Ma), and the presence of a continental-size ice sheet in east Antarctica during the earliest Oligocene. The sedimentological evidence indicates that the outer limit of the ice front during the earliest Oligocene was beyond that of the present day by at least 140 km. This suggests that early Oligocene glaciation recorded from the Ross Sea in west Antarctica was more than a local event, and extended beyond the Trans-Antarctic mountains. The occurrence of marine diatom-rich sediment for intervals of the upper Miocene, lower Pliocene, and upper Pliocene indicates fluctuation in the extent of the ice sheet and the waxing and waning of glaciers across the Prydz Bay shelf during the latter part of the late Miocene and Pliocene.

The Kerguelen Plateau is divided into two distinct domains, the Northern Kerguelen Plateau, which includes the plateau's only features above water, and the Southern Kerguelen Plateau, which is characterized by a subdued topography.

Sediments recovered from the Northern Kerguelen Plateau consist of middle Eocene to lowermost Miocene calcareous claystone overlain by middle Miocene to Quaternary diatom and calcareous oozes; a major hiatus of 8 m.y. Volcanic glass and minerals are superimposed on the general pelagic biogenic character of the sediments throughout the sequence, but distinct ash layers are few. The northern plateau sites are situated directly below a major surface-water boundary, the Antarctic Convergence, which separates more temperate surface waters from polar waters. An abrupt change in the diatom assemblage from species typical of temperate regions to species endemic to the Southern Ocean suggests that present-day oceanographic circulation patterns were established during the late Miocene or early Pliocene, probably as a result of climatic cooling.

Basement beneath the central and Southern Kerguelen Plateau is predominantly silica-saturated transitional tholeiite that erupted before the early Late Cretaceous above, or just below, the sea surface. Early sedimentation denotes fluvial conditions with wood fragments indicative of soil and vegetation. During the Late Cretaceous, open-marine conditions prevailed in the eastern part of the Raggatt Basin and the eastern margin of the plateau slowly subsided to a water depth of about 2,000 m, while to the west, the plateau subsided slowly and remained shallow (50-200 mbsl). A major tectonic episode affected the plateau at about 75 Ma on the eastern margin and 65 Ma on the western margin. Normal listric faults developed to the east, possibly related to the break between the Southern Kerguelen Plateau and Broken Ridge-Diamantina Fracture Zone. To the west , this event corresponds to the emplacement of the 77 E Graben and to a large northwest-southeast uplift that abuts the southern end of this graben. At this time, the western margin of the Raggatt Basin subsided rapidly to 1,000 mbsl. From the late Maastrichtian to the middle Eocene, sedimentation was continuous over the entire slowly-subsiding plateau. A hiatus of at least 2 m.y. occurred during the middle Eocene. In the transition zone between the Northern and Southern Kerguelen plateaus, this hiatus, which extends over 15 m.y., is accompanied by a subsidence of about 500 m and is related to the separation by seafloor spreading of the Northern Kerguelen Plateau and Broken Ridge. From the middle Eocene to the Pliocene, sedimentation continued. The transition from calcareous to siliceous deposition commenced during the earliest Oligocene, and was completed during the late Miocene and sediments on the plateau record glacial-interglacial cycles during the last 10 m.y. Increased deposition of gneissic/granitic ice-rafted debris of probable Antarctic derivation took place during the late Pliocene to Holocene, signaling intensification of glaciation in Antarctica. Erosion affected the whole Southern Kerguelen Plateau until ~ 100 ka.