The Antarctic shelf differs in many ways from continental shelves of mid and low latitudes and even from shelves of high northern latitudes. The Antarctic shelves in water depths between 300 and 1000 m are overdeepened and slope landward, principally because of the effect of glacial erosion and flexural loading by grounded ice (Ten Brink and Cooper, 1992; Barker et al., 1998). Sedimentary sequences exhibit two principal geometries in seismic reflection profiles collected across the West Antarctic Peninsula shelf: shelf topsets and slope foresets form the prograding wedge (Larter et al., 1994; Larter and Barker, 1989, 1991). In most areas of the middle and inner shelf, the topsets and underlying foresets are separated by a prominent regional unconformity (Larter et al., 1997). This unconformity marks a major change in the style of deposition from progradational to aggradational and progradational, adding large sediment-retaining capacities to the shelf (Fig. F1). Prograding sedimentary sequences on the continental shelf record changes in West Antarctic ice sheet volume, sea level, climate, ice and sediment-induced isostatic change, and tectonic and thermal effects.
The aim of drilling the shelf-transect Sites 1100, 1102, and 1103 during Leg 178 was to characterize the age and depositional environment of seismostratigraphic Units S1 (topsets) and S2, S3, and S4 (foresets of the prograding wedge). Unfortunately, drilling through the topset sequences was very difficult with the available rotary core barrel (RCB). Unsorted crystalline clasts (up to headsized) in an unindurated sand/silt/clay matrix prevented rapid penetration and resulted in minimal core recovery, primarily as a result of clogging of the central opening of the rotary drill bit and the core catcher with stones. Despite drilling difficulties, Site 1103 penetrated to 362.7 mbsf with mixed recoveries. The upper 247 m of cored sediment, belonging for the most part to seismic Unit S1, yielded only 2.3% recovery. The lower 116 m of cored material with a cemented matrix belonging to seismic Unit S3 yielded 34% recovery (Fig. F1). A hole blockage prevented the collection of logging data below 244 mbsf (Shipboard Scientific Party, 1999a). Thus, no or only very limited comparisons and cross-checks of log and laboratory data are possible. In our investigation, we compiled all available data sources and performed quality checks and nonstandard processing techniques with the logging data obtained to arrive at a reliable and continuous depth velocity profile presented in this paper.
Reliable velocity profiles of the shelf for time-depth conversions of multichannel seismic reflection profiles are needed for all further geological interpretations and models of shelf sedimentation in seismic sections (Camerlenghi et al., in press). Even though the shelf sediment record is less continuous and age constraints are less confined compared to all other depositional environments drilled during Leg 178 (i.e., inner continental shelf deep basins and continental rise drift deposits), the best possible depth control of the shelf sequences is essential for regional stratigraphic correlations across the Antarctic Peninsula continental shelf and between shelf and rise.