CSIRO Marine and Atmospheric Research
Past Seminars

Wednesday 26 November 2003, 11.30 am (Tas time)

CSIRO Auditorium, Hobart

Dr Peter Harris
Geoscience Australia, Coastal and Marine Environment Group

Multi-beam sonar mapping of submarine shelf valleys and coral reefs in Torres Strait and the Gulf of Carpentaria

Multi-beam sonar and seismic mapping at the northern end of Australia's Great Barrier Reef in the Gulf of Papua, has revealed a shelf valley system up to 220 metres deep that extends for more than 80km across the continental shelf.

Two valley types are evident. Relict fluvial valleys located in the Gulf of Papua exhibit lateral accretion surfaces in seismic profiles and incised margins that intersect and truncate underlying strata. Over-deepened valleys located adjacent to the northern Great Barrier Reef exhibit closed bathymetric contours and are floored with well-sorted carbonate gravely sand containing a large relict fraction. The deepest valley segments were probably the sites of lakes during the last ice age, when Torres Strait formed an emergent land-bridge between Australia and Papua New Guinea. Post-glacial transgression of the shelf transformed the relict fluvial valleys into estuaries where terrigenous and marine sediments were deposited, as documented by cores and seismic data. Radiocarbon ages of mangrove peat samples range from 8,597 to 11,733 years BP in water depths of from 35 to 81 m. The southern valleys, however, would have formed estuarine embayments, whose origin owed nothing to fluvial erosion processes, and where minimal terrigenous deposition occurred.

Numerical modelling indicates that the strongest tidal currents occur over the deepest, outer shelf segment of the valleys when sea level is about 40 to 50 metres below its present position, suggesting the channels are Pleistocene in age and of relict origin. At such low sea-level times, strong tidal currents at the northern end of the Great Barrier Reef prevents the deposition of sediment derived from Papua New Guinea's rivers that would otherwise bury coral reefs growing along the valley margins. Submerged reefs found in Torres Strait have analogues in other parts of the world, including those recently discovered in the southern Gulf of Carpentaria.

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