Geoscience Australia Survey 234, Post-cruise Report: Cross-shelf Sediment Transport in the Torres Strait - Gulf of Papua Region. RV Franklin Cruise 01/02, January - February 2004

The RV Franklin sailed from Brisbane on 17th January 2002 and returned to Cairns on 9th February, 2002. The cruise discovered that a zone of strong tidal currents at the northern end of the Great Barrier Reef prevents the southward advance of sediment that would otherwise bury the coral reefs. The Fly River, located in close proximity to the northern end of the Great Barrier Reef, discharges about 120 million tonnes/yr of sediment. This sediment does not penetrate as far south into the reef area as might be expected because, over glacial-interglacial cycles of sea level change, the southward-prograding deposits are eroded by tidal currents. Deployment of an instrumented current meter and suspended sediment measurement frame on the seabed, offshore from the Fly River Delta, recorded a net sediment advection southwards. Sediment transport was greatest following a northerly wind event, which caused high bottom stress and increased turbidity levels. Swath sonar mapping and underwater video equipment were used to map a series of channels up to 220 m deep extending from eastern Torres Strait across the northern end of the Great Barrier Reef. Channels in the north are clearly relict fluvial channels, exhibiting lateral accretion surfaces and incised channels that intersect and truncate underlying strata. Over-deepened channels in the south, however, appear to have formed by tidal current scour. They exhibit closed bathymetric contours at both ends and are floored with well-sorted carbonate gravely sand. Oceanographic observations indicate that the channels provide a conduit onto the shelf for up-welled Coral Sea water. The deepest channels form isolated depressions, and possibly were the sites of lakes during the last ice age. Preliminary modelling indicates that the strongest tidal currents occur when sea level is about 40m below present, suggesting that the channels are Pleistocene or older in age and of relict origin.