The Ice Sheet Development Mid Early Miocene to Early Pliocene

This period includes seismic sequences RSS-4, -5 and -6 (Fig. RS-3 and Foldouts RS-1 and RS-2). Sediment from these sequences was recovered at DSDP Sites 271, 272 and 273, MSSTS-1 and DVDP 10/11.

The early-middle Miocene is postulated to have been a time of major ice buildup of ice in the Ross Sea region, and the carving of the first deep troughs, similar in size to the present ice streams, across the continental shelf (Anderson and Bartek, 1992). Bart (2003) and Chow and Bart (2003) recognize at least two major WAIS expansions during the early part of the middle Miocene and five in the entire Miocene. These expansions suggest that either portions of the West Antarctic land elevation were above sea-level and/or the air and water temperatures were sufficiently cold to support a marine-based ice sheet.

Drill cores from the middle Miocene have been recovered at DSDP Sites 272 and 273 (Hayes and Frakes, 1975; Savage and Ciesielsky, 1983; Leckie and Webb, 1986), and consist of diatom-bearing sediments interpreted as waterlain tills and proximal- to distal-glacimarine deposits (Hambrey and Barrett, 1993).

Upper Miocene rocks are missing from all continental shelf drill cores, except in the McMurdo Sound region (MSSTS-1, DVDP-10 and -11 drill sites), where glaciomarine diamictites (tillites) and terrestrial strata are found. These deposits are interpreted as having originated from glaciers flowing out of the Transantarctic Mountains (Powell, 1981; Barrett, 1986; Ishman and Webb, 1988; McKelvey, 1991).

The recent ANDRILL drilling on the Ross Ice shelf, near McMurdo Sound (MIS project; Naish et al., 2007), recovered a 1,284m long core that records Antarctica's history over the last 14 million years. The core indicates periods of ice-sheet growth, advancing over the drill site and then retreating again to allow the open-marine conditions to return. More than 60 of these advance-retreat cycles are present in the core.

On the outer continental shelf and upper slope, well-stratified seismic sequences inferred to be of late Miocene age (i.e. RSS-6) are present in the

Northern and Eastern basins, but the sequences are thin or absent on the inner shelf (Cooper et al., 1995). In both basins, these sequences are characterized by steeply prograding clinoforms with relatively thin or eroded topset beds and a major seaward shift of the palaeo-shelf edge (Foldouts RS-1 and RS-2). The sequences are thought to have been deposited by intermittent grounded ice sheets carrying sediment to the continental shelf edge (Bartek et al., 1991; Cooper et al., 1991b; Anderson and Bartek, 1992).

0 0

Post a comment