Prydz Bay is located at the downstream end of a drainage system that originates in the Gamburtszev Mountains of central East Antarctica (see Fig. F1 in the "Leg Summary" chapter). Modeling studies of ice-sheet development indicate that the first glaciers in Antarctica would develop over these mountains, if the mountains were high in the Paleogene (e.g., Huybrechts, 1993). Glacier ice—and entrained debris—flowing down the drainage system would give the earliest sedimentary evidence of a large-scale ice sheet in East Antarctica. The early development and growth of the Cenozoic Antarctic Ice Sheet is believed to have started by early middle Eocene to early Oligocene time (Abreu and Anderson, 1998; Barron et al., 1991); however, to date, drilling on the continent and the continental margin has not sampled a stratigraphic section that clearly spans and includes the transition period from preglacial to glacial conditions. Glacial deposits sampled at ODP Site 742 in eastern Prydz Bay were interpreted by Barron et al. (1991) as late middle Eocene to early Oligocene age. Cooper et al. (1991) estimated that these Paleogene sediments continued for 100 m below the total depth of Site 742 and rested above an erosion surface on older sediments. By drilling into the 100 m below Site 742 and into the sediments underlying the unconformity, the older age limit for large-scale glaciation of Prydz Bay could be derived. Sediments in the lower section of the hole would contain paleontological and sedimentological evidence of the preglacial environment.
Site 1166 is situated on the southwestern flank of Four Ladies Bank, ~40 km southwest of Site 742 in the zone of iceberg-scoured seabed mapped by O'Brien and Leitchenkov (1997) (Fig. F1). This site was chosen during Leg 188 after pack ice prevented access to preferred drill sites in western Prydz Bay. Site 1166 lies close to shotpoint 2050 on seismic line BMR 33-23P3. The expected section consists of 150- to 200-m-thick horizontally bedded glacigene diamictons of late Miocene to Pleistocene age, unconformably overlying ~150-m-thick Paleogene-age sediment, which in turn overlies Early Cretaceous-age nonmarine sediments (Fig. F2). The seismic line through Site 1166 shows three erosion surfaces (Fig. F2). The uppermost is the base of the upper horizontal sequence and truncates underlying units at a low angle. The second is an undulating surface in the probable Paleogene section that displays pronounced channeling toward north of the site (Fig. F2). The third surface delineates the top of the Lower Cretaceous sediments of unit PS.2B (Cooper et al., 1991) that were cored at Site 741 (ODP Leg 119). Line BMR 33-23P3 does not show truncation of underlying reflectors at the top of unit PS.2B near Site 1166; however, low-angle truncations can be seen on seismic lines from other parts of the basin.