Drilling on the Wilkes Land Margin

DSDP Leg 28 drilled Sites 268 and 269 on the continental rise and abyssal plain, respectively, to determine the geologic and climate history of Antarctica and the Southern Ocean (Hayes and Frakes, 1975). The drill cores document that extensive Antarctic glaciation began at least by Oligocene to early Miocene time, and that water temperatures were cool-temperate in the late Oligocene and early Miocene and cooled during the Neogene, presumably as glaciation intensified.

DSDP Site 268 was drilled to a subbottom depth of 474.5m in 3,544m water depth with total core recovery of 14% (Hayes and Frakes, 1975). Three units were described, based on lithologies and amounts of diatoms, nannofossil ooze and ice-rafted pebbles and granules (Hayes and Frakes, 1975). Piper and Brisco (1975) interpreted the two deeper units to be contourites, based on the character of silt laminae. The shallowest of the three units, dated as Pliocene and Quaternary, was interpreted as turbidites, based on a high content of silty clay with common silt laminae and fine-sand beds 2-20 cm thick. Hayes and Frakes (1975) infer that the deepest lower Miocene and Oligocene unit was deposited when the ice sheet first advanced onto the shelf. Water at that time was warm enough to support calcareous biogenic sedimentation, but ice-rafting and contourites provide evidence for nearby ice on East Antarctica and for bottom currents, possibly generated by cold bottom water production associated with a limited ice shelf or tongue (Hayes and Frakes, 1975).

DSDP Site 269 was drilled to a subbottom depth of 958 m in a water depth of 4,285 m and with 42% recovery of Eocene to recent rocks (Hayes and Frakes, 1975). The section consists predominantly of silts and clays with variable amounts of microfossils. Diatom oozes and diatom mud dominate the upper half of the section, which is dated as Quaternary to Late Miocene in age (Hayes and Frakes, 1975). In the lower half, which is late Miocene to early Miocene and Oligocene in age, diatoms are absent but calcareous nannofossils are found in trace amounts. Similar to DSDP Site 268, there is a transition in facies at DSDP Site 269 from more distal facies in the lower part of the core to more proximal facies near the surface. Piper and Brisco (1975) interpret this facies change as resulting from substantial increased supply of sand and coarse silt and clay from the Antarctic continent, possibly in response to prograding of the continental margin.

5.3.3. The Inferred Long-Term Record of Glaciations

Investigators interpret the WL-U3 unconformity as having been eroded during the first grounding of an ice sheet on the continental shelf (Tanahashi et al., 1994; Eittreim et al., 1995; Escutia et al., 1997; Escutia et al., 2005), either about 40m.y. ago (Eittreim et al., 1995) or 33.4-30 Ma (Escutia et al., 2005) (Table WL-2). Above WL-U3, early glacial strata (e.g. likely glacial outwash deposits) were provided by fluctuating temperate glaciers, and were deposited as low-dip-angle prograding foresets. The increase in stratal dips across unconformity WL-U8 in the prograding wedge at the shelf edge is interpreted to record a glacier-regime change from intermittent fluctuating glaciers to persistent oscillatory ice sheets, either on the Late Miocene (Escutia et al., 2005) or about 3 Ma (Rebesco et al., 2006) (Table WL-2). The steep foresets above WL-U8 likely consist of ice proximal (i.e. waterlain till and debris flows) and open-water sediments deposited as grounded ice sheets

Table WL-2: Continental shelf and rise stratigraphy and inferred East

Antarctic Ice Sheet evolution in the Wilkes Land margin and timing of events.

Regional Timing of glacial events Wilkes Land glacial unconformities/ sesmic units

WL-S9 Pliocene-Pleistocece latest Miocene (?) Pliocene (3 Ma) to

WL-U8- mid-late Miocece -

WL-U6- Dynamic ice sheet

WL-S6 middle Miocene (?)

WL-S4 late Oligocene-early Miocene

WL-U3- early Oligocene-

(33.5-30 Ma) (?) early Oligocene to late Cretaceous

Persistent but oscillatory? Ice sheet

Transition from a dynamic to a persistent ice sheet

First arrival of an ice sheet — to the coast-

Ice Free

Source: Modified from Escutia et al. (2005).

extended intermittently onto the outer shelf - similar to sediments recovered from ODP Site 1167 on the Prydz Trough fan (O'Brien et al., 2001).

On the continental rise, the up-section increase in the energy of the depositional environment in units WL-S5 to WL-S7 (i.e. seismic facies indicative of proximal turbidites and of bottom-contour-current deposition) likely resulted from enhanced shelf progradation. Maximum rates of sediment delivery to the rise appear to have occurred during the development of units WL-S6 and WL-S7, which is inferred to have been during the Miocene (Hayes and Frakes, 1975; De Santis et al., 2003; Escutia et al., 2005). During deposition of WL-S8 and WL-S9, sediment supply to the lower continental rise decreased and depocentres shifted landward to the base of the slope and outer shelf (Escutia et al., 2002; De Santis et al., 2003; Donda et al., 2003; Escutia et al., 2005; Rebesco et al., 2006). Inferred age for Units WL-S8 and WL-S9 is Pliocene to Recent (De Santis et al., 2003). Sequence WL-S9 was deposited under a polar regime with a persistent ice sheet during the Pliocene-Pleistocene. At that time, most sediment delivered to the margin was trapped on the outer shelf and slope, forming steep prograding wedges, with some sediment bypassing the slope in channelized turbidity currents (Escutia et al., 2002; De Santis et al., 2003; Escutia et al., 2005).

During the Holocene open-water interglacial thick sections of diatom mud and oozes are deposited in deep inner shelf basins (Costa et al., 2007). These sediments hold an ultra-high-resolution record of climate variability likely by solar and ENSO forcing.

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