Sedimentation and physiography of the sea floor south of Australia

Abstract

Carbonate sedimentation has characterized the sea floor off the mainly arid and low-lying southern Australian continent during the Tertiary and Quaternary. The 30–100 mile wide and 600 mile long shelf of the Great Australian Bight lies seaward of the Tertirary Eucla Basin and is covered by coarse calcareous and mainly relic Pleistocene sands that are constantly shifted and rippled by bottom currents. Bryozoans, molluscs, sponges and other calcareous species live on the shelf and also comprise the bulk of the relic sands. Vittaticellid bryozoans, foraminifera and sponge spicules are abundant near the shelf and upper parts of the continental slope, while foraminiferal and coccolith oozes characterize the slope sediments. The shelf to the east and west of the Great Australian Bight is generally more narrow and flanked by steep continental slopes incised by the Albany Canyons lying seaward of the crystalline shield in the west, and the Murray Canyons lying seaward of the folded Adelaide geosyncline rocks to the east. The 350 mile-long Ceduna Plateau bounded by the 500–1,000 fathom isobaths flanks the eastern part of the Great Australian Bight and probably represents a downwarped extension of the Eucla Basin. The continental slope merges into a wide continental rise which extends the complete length of the continental margin and merges seawards around depths of 2,900 fathoms into the Great Australian Bight Abyssal Plain which has maximum depths around 3,100 fathoms. Eight piston cores from the rise and plain consist of greater than 90% light-coloured calcareous graded beds of clay, silt and sand interbedded with dark-coloured, burrow-mottled, calcareous-poor, pelagic or hemipelagic silts and clays. The calcareous sands consist of bryozoans, molluscs, foraminifera, algae, sponge spicules, beachrock, reworked Pleistocene to Tertiary organisms and a variety of other organisms derived from the continental shelf and slope and transported up to 700 miles across the sea floor by turbidity currents. The 1–2 km of poorly consolidated sediments that comprise the continental rise and abyssal plain may have been mainly deposited during the Quaternary.