Ice Breaker Support, 1-5 February 1993.

The USCGC Polar Star was used to collect approximately 450 nautical miles of 3.5 Khz bottom profile data off the south Victoria Land coast during 1-5 February 1993 (figure 1). This data was collected for two purposes.

Offshore Cape Roberts 11 east-west lines between 14 and 17 nautical miles long and spaced 1and 2 nautical miles apart were run in the area of future drilling (Cape Roberts Project). A detailed bathymetry map will be prepared from this data to better define potential drill sites.Over most of the area of potential drill sites very little sub sea floor structure was imaged indicating that no soft (gravel?) sediment is present and in many areas "hard sea floor" reflections indicate that the sedimentary basement outcrops at the sea floor.

The second purpose was to look in the deep basins of Granite Harbour and the Nordenskjold Ice Tongue for Holocene sediments deposited by ice retreat since the Last Glaciation (Figure 2). In Granite Harbour the presence of fast ice and time constraints restricted the planned survey and a line to the MacKay Glacier Tongue was not completed. Good data was recovered from the previously known basin of Avalanche Bay about 800 m deep where several metres of soft sediments drape over hummocky sea floor.

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Offshore of the Nordenskjold Ice Tongue a (granitic?) basement high (570 m) has a "hard" sea floor reflection and a "glaciated" shape probably cut by northwards moving ice. Closer towards the ice tongue a high area averaging 500 m deep and about the same width (10 km) as the present tongue is present. This structure in contrast has a hummocky "soft" surface that is interpreted to be grounding zone sediments deposited beneath the ice tongue probably when sea level was 120 m lower during the last glaciation. On the south side this structure a east-west trending basin up to 1050 m deep is present and contains layered sediments several tens of metres thick (figure 2).

The profiles in Granite Harbour and at the Nordenskjold Ice Tongue will be used to identify sites for future sea floor sediment coring (vibracoring).