Holocene storage of siliciclastic sediment around islands on the middle shelf of the Great Barrier Reef Platform, north-east Australia

Thick sequences of sediment surround the Whitsunday Islands on the middle shelf of the Great Barrier Reef (GBR) Platform. Much of this sediment is siliciclastic material deposited since the sea-level highstand at around 6.5 ka. This raises a mass balance dilemma because modern terrigenous discharge to the GBR Platform is restricted to the inner shelf. Shallow seismic profiles and sediment samples were collected over 450 km 2 around the Whitsunday Islands to quantify the mass of siliciclastic sediment for a dynamic model of the shelf. The sea floor and pre-Holocene surfaces were mapped using 4584 stations along the seismic profiles and a graphical computer program. The total volume of sediment between these two surfaces is 3.67 ± 0.45 × 10 9 m 3. This volume is composed of buried reefs (0.13 ± 0.01 × 10 9 m 3), medium- (0.70 ± 0.30 × 10 9 m 3) and fine-grained shoals (2.84 ± 0.35 × 10 9 m 3). The volume estimates combined with measurements of carbonate concentration, density and porosity indicate that 1850 ± 380 Mt of Holocene siliciclastic sediment surround the Whitsunday Islands in medium- (510 ± 225 Mt) and fine-grained shoals (1340 ± 155 Mt). The total mass of siliciclastic material is 1.7-2.6 times that stored in Cleveland Bay, a similar sized repository on the inner shelf. A simple numerical model has been constructed to explain this large quantity of Holocene siliciclastic sediment. The model results in the appropriate siliciclastic mass next to the Whitsunday Islands by integrating regional shelf processes over time. Unlike the present day, rivers discharged sediment to the middle shelf during the early Holocene. This material was subsequently focused by northward transport into the vicinity of the islands, a geomorphologically complex region that serves as a sediment trap. Although direct riverine inputs to the middle shelf have stopped during the present sea-level highstand, previously deposited siliciclastic sediment is continually being winnowed from the middle shelf and redeposited next to the Whitsunday Islands. The transport and distribution of siliciclastic sediment on the GBR Platform is thus influenced significantly by storage around islands on the middle shelf.