ABSTRACT

Shelf sediments were collected immediately before (8-20 January 1986), immediatly after (9-10 February 1986), and one year after (27 February 1987) Cyclone Winifred crossed the central Great Barrier Reef shelf on 1 February 1986. The storm produced a normally graded, mixed terrigenous-carbonate storm bed more than 11 cm thick, covering an area at least 1,200 km², and extending more than 30 km offshore to water depths greater than 40 m.

The storm layer encompasses two lithologically distinct, shelf-parallel facies belts. The nearshore (0-25 m depth) storm bed is 311 cm thick, thins offshore, and grades upward from medium sand to silty clay. Offshore (25-43 m depth) the storm bed thickens to more than 11 cm and consists of normally graded, well sorted, relict quartz and skeletal, gravelly sands capped by a thin mud veneer.

The cross-shelf distributions of grain size, mineralogy, carbonate content, and carbon isotope ratios were very similar before and after Winifred, suggesting the storm bed did not result from offshore-directed storm currents but, rather, in situ resuspension and settling of the shelf sand with shoreward transport of mud. In contrast to storm sedimentation models which predict thinning and fining offshore, Winifred created a layer that becomes thicker and coarser grained in deeper water in response to cross-shelf substrate changes.

After one year the Winifred storm layer was well preserved nearshore (< 20 m water depth), but completely bioturbated offshore (> 30 m water depth) below fairweather wave base. Preservation may not be a matter of insulating the layer from physical reworking below fairweather wave base but of burying it where rates of sediment accumulation outpace bioturbation. The effects of storms similar in size to Winifred may be well concealed in ancient shelf sequences subject to rapid biological remixing of storm layers.