Cyclone pumping, sediment partitioning and the development of the Great Barrier Reef shelf system: a review

Abstract

The modern Great Barrier Reef (GBR) is part of the world's largest and best known mixed terrigenous-carbonate continental margin. The GBR shelf contains three shore-parallel sedimentary belts: an inner shelf zone of terrigenous sedimentation at depths of 0–22 m; a middle shelf zone of sediment starvation at depths of 22–40 m; and an outer shelf reef tract with its inner edge at ca. 40 m depth. These zones are controlled by the dynamics of northward, fair-weather, along-shelf drift, driven by southeasterly trade winds, and by the regular passage of tropical cyclones. Cyclones cause wind-driven north-directed middle shelf flows in excess of 130 cm/s, which erode the seabed, concentrate the sparse mobile sediment into sand ribbons, and advect suspended load onto the outer part of the nearshore terrigenous sediment prism and into inter-reef depocentres within the outer shelf reef tract. Cyclones largely control the input of new sediment into the GBR system, via river flooding, seabed erosion or reef breakage. They also help to control the partitioning and dispersion of the three main shore-parallel belts of sediment, and hence stratigraphic accumulation. Acting as a sediment pump, especially during interglacial highstands, cyclones have exerted great control on the development of the modern GBR province and its sediments by maintaining a broad shelf-parallel zone of episodically mobilised sediment and scoured seabed upon which coral reefs have been unable to form. Cyclones may also have partly controlled the timing of initiation of the first GBR at ∼0.6 mybp. Contrary to current models, GBR storm beds are most likely to be preserved intact close to the shoreline, and become coarser-grained away from the shoreline. For the central GBR, “highstand shedding” only applies to carbonate sediment at the scale of local reefs; system-wide, oceanographic controls cause high rates of carbonate sedimentation on the slope during both sea-level rise and highstand; concomitantly, terrigenous sediment accumulates fastest on the slope during sea-level rise, and slowest during sea-level lowstand and highstand.