Summary
Bathymetric, sidescan sonar and high-resolution seismic data indicate that the shelf off Mac. Robertson Land, East Antarctica is deeply eroded by glaciers and currents. Basement is exposed over wide areas. Data available include MCS and 3.5 kHz obtained by Stagg (1985) in 1982 (see OBrien et al., this volume, for details). In 1995 R.V. Aurora Australis collected bathymetric data using a Simrad EA200 12.5 KHz echo sounder and seismic data were obtained using a Seismic Systems Inc. generator-injector (GI) gun (150 cubic inches) with four groups (four channels) of ten hydrophones. Analogue (uncorrected) sidescan sonar data were recorded using an EG&G 960 tow fish and an EG&G 996 digital modem connected to an EPS 9800 graphic recorder. The sidescan sonar was operated at ranges of 375 to 500 m. Figure 1. Map showing location of geophysical survey tracks on the Mac. Robertson shelf and geomorphic provinces (inset). The shelf exhibits four major geomorphic zones (see Figure 1): (1 ) high-relief, ridge and valley topography in depths of 200 to 1,400 m; (2) smooth seafloors associated with low-energy, depositional shelf valleys and basins in depths of 400-800 m; (3) low-relief, planated bank-tops in depths of 100 to 200 m; and (4) iceberg gouged and current reworked seaward-bank margins and upper slope in 200 and up to 630 m depth. Bathymetric data reveal a series of shelf deeps (Iceberg Alley, Nielsen Basin and Burton Basin; Figure 1) up to 1,400m deep, joined to the adjacent continental slope by U-shaped shelf valleys (Figure 2). Sill depths connecting the basins to the slope average about 375 m. The basin morphology is extremely complex, containing elongate ridges and arcuate, deep basins. Shelf banks in 110 to 150 m water depth are cut by the shelf valley systems.