Sedimentology of the South Tasman Rise, south of Tasmania, from ‘groundtruthed’ acoustic facies mapping

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Abstract
Acoustic facies mapping using 3.5 kHz profiles and swath imagery allows rapid mapping of sea‐floor sediment type over large areas. A groundtruthed acoustic facies map for the South Tasman Rise has been compiled using 200 000 km2 of swath data and 74 core and dredge samples. Results indicate that widespread current winnowing has blanketed much of the South Tasman Rise with foraminiferal sand of indeterminate thickness. Modern sediment waves on the western flank of the South Tasman Rise are significantly smaller than buried examples (with wavelengths of several kilometres and up to 100 m amplitude), suggesting that the Antarctic Circumpolar Current has abated. This finding supports oceanographic evidence that the Subantarctic Front of the Antarctic Circumpolar Current lies south of the South Tasman Rise and has migrated north during glacial periods. The flanks of the South Tasman Rise are characterised by a range of dominantly pelagic deposits that are reworked into mappable units interpreted as slumps, mass‐flow and gravity‐flow deposits. Submarine canyons, debris flows and manganiferous deposits can be identified from the acoustic facies map. Discriminating facies that contain manganese nodules and crusts from those composed of coarse pelagic sediment is tentatively possible as the manganiferous varieties have a higher acoustic reflectivity. Comparison of facies from the South Tasman Rise with those of other regions indicate that CaCO3 content—and (perhaps) more importantly very high water content in beds of foraminiferal sand—is producing echo types normally associated with abundant coarse sandy sediment.