Siliciclastic sediment across the north queensland margin (Australia): A Holocene perspective on reciprocal versus coeval deposition in tropical mixed siliciclastic-carbonate systems

According to conventional models of reciprocal sedimentation for tropical mixed siliciclastic-carbonate systems, shedding of carbonate material dominates slope and basin sedimentation during transgression and highstand while siliciclastic deposition dominates during lowstand. This understanding permeates the stratigraphic literature and is the accepted depositional model on the Great Barrier Reef (GBR) margin. The results of this study, however, document coeval carbonate and siliciclastic highstand deposition on the GBR slope and basin. Seafloor sediment from Queensland Trough slopes and basin floor cast of the GBR contains 20% to 50% terrigenous siliciclastic material. One hundred and twenty-six sediment samples were obtained from core tops and sediments grabs previously retrieved from shelf, slope, and basin environments of the North Queensland Margin. These samples were analyzed for their carbonate content, mineralogy, and major-element composition. The amount of terrigenous siliciclastic material in sediment from the North Queensland Margin can be approximated by three independent methods: (1) the noncarbonate residual, (2) the sum of quartz, clays, and feldspars, and (3) the sum of SiO2, Al2O3, Fe2O3, and TiO2. All three tracers show a distinct distribution of siliciclastic sediment across the margin. Shelf siliciclastic content is highest on the inner shelf, decreasing eastward toward the GBR, with the exception of relatively high siliciclastic content in several interreef passages. Queensland Trough siliciclastic content is highest on the slope and basin between 15 degrees and 17 degrees S latitude, decreasing to the south and east. Although the relative abundances of quartz and clay minerals vary across the margin, the chemistry suggests a similar origin for the siliciclastic material. Moreover, the zone of high siliciclastic abundance in Queensland Trough reflects enhanced siliciclastic accumulation. Siliciclastic material escapes the outer shelf to Queensland Trough through interreef passages between 15 degrees and 17 degrees S latitude. It is then focused to the south by ocean currents. Siliciclastic sediment is likely sourced from a combination of rivers transporting Holocene sediment to the GBR shelf and late Pleistocene sediment eroded from the outer shelf and reworked to Queensland Trough. Siliciclastic material crosses the shelf, although the mechanisms (e.g., river plumes, nepheloid layers, tidal currents, cyclones, mushroom jets) remain poorly constrained. However, shelf width is clearly an important factor in allowing cross-shelf transport to occur. Mass-balance calculations indicate that up to 13% of the late Holocene annual riverine sediment output may be delivered to Queensland Trough. Modern and ancient mixed system depositional models must be reevaluated to allow for the possibility of coeval siliciclastic and carbonate deposition.