ABSTRACT

Observation of shelf sediments collected immediately before and after Cyclone Winifred crossed the central Great Barrier Reef shelf (1 February 1986) confirmed that the storm produced a normally graded, mixed terrigenous-carbonate storm layer extending 30 km offshore in water up to 43 m deep. Distinct post-Winifred changes in the cross-shelf distribution of organic carbon and carbonate in the mud-fraction of the sediment suggest that suspended sediment transport was extensive and that the storm layer had multiple sediment sources.

On a shelf-wide scale, the storm layer is composed almost entirely of reworked shelf sediment. Erosion depths were greater on the mid-shelf (20-40 m water depth) than on the inner shelf(< 20 m water depth), averaging > 6.9 cm and 5.1 cm, respectively. Particles finer than medium sand were eroded and transported out of the mid-shelf.

The inner-shelf portion of the storm layer formed by the combination of three sediment sources, including 1) seaward transport of terrigenous sediment in buoyant freshwater flood plumes, 2) resuspension and settling of inner-shelf sediment, and 3) resuspension and shoreward transport of mid-shelf sediment. Mass-balance calculations predict that at least 10-30% of the inner-shelf storm layer is composed of mid-shelf mud. Combined wave and wind-forced currents probably resuspended mid-shelf material and drove the suspended fraction alongshelf and shoreward a minimum distance of 15 km.

The results suggest that tropical cyclones are capable of sporadic but efficient cross-shelf transport of suspended sediment. On shallow cyclone-prone shelves, suspended sediment may easily be exchanged between adjacent sedimentary facies. In ancient shelf sequences, the transport history of the mud will be complex, and stratigraphically equivalent facies may have similar mud types but completely different sands.