Patterns of mixed siliciclastic–carbonate sedimentation adjacent to a large dry-tropics river on the central Great Barrier Reef shelf, Australia

The Great Barrier Reef represents the largest modern example of a mixed siliciclastic–carbonate system. The Burdekin River is the largest source of terrigenous sediment to the lagoon and is therefore an ideal location to investigate regional patterns of mixed sedimentation. Sediments become coarser grained and more poorly sorted away from the protection of eastern headlands, with mud accumulation focused in localised ‘hot spots’ in the eastern portion of embayments protected from southeast trade winds. The middle shelf has a variable facies distribution but is dominated by coarse carbonate sand. North of Bowling Green Bay, modern coarse carbonate sand and relict quartzose sand occur. Shore-normal compositional changes show Ca-enrichment and Al-dilution seawards towards the reef, and shore-parallel trends show Al-dilution westwards (across bays) along a Ca-depleted mixing line. Intermediate siliciclastic–carbonate sediment compositions occur on the middle shelf due to the abundance of relict terrigenous sand, a pattern that is less developed on the narrow northern Great Barrier Reef shelf. Rates of sediment deposition from seismic evidence and radiochemical tracers suggest that despite the magnitude of riverine input, 80–90% of the Burdekin-derived sediment is effectively captured in Bowling Green Bay. Over millennial time-scales, stratigraphic controls suggest that sediment is being preferentially accreted back to the coast