Bathymetric expression of the Fitzroy River palaeochannel, northeast Australia: Response of a major river to sea-level change on a semi-rimmed, mixed siliciclastic-carbonate shelf

Palaeo-river channels record the response of alluvial and estuarine depositional environments to sea-level change, and are also important geological indicators of former subaerial unconformities. Earlier studies have characterised lowstand palaeochannels on the central Great Barrier Reef (GBR) shelf of northeastern Australia, reporting channel entrenchment, shelf aggradation, and topographic control of rivers by the outer shelf reef barrier. We report the bathymetric expression of a palaeochannel corresponding to the last glacial lowstand path of the Fitzroy River, a high sediment yield river in the southern GBR. During regression and lowstand, the Fitzroy River palaeochannel entrenched into the low-gradient middle shelf, and may have begun to incise the steeper outer shelf between the exposed reef platforms. The wide spacing of outer shelf reefs and the continuous seabed slope may have enabled the palaeo-Fitzroy to maintain connection with the lowstand shoreline without becoming trapped behind shelf edge reefs. The palaeochannel now forms a prominent depression across the middle and outer shelf. Evidently, the sloping inter-reefal seabed of the southern GBR, coupled with tidal processes and wave exposure, has limited shelf aggradation since the transgression. This finding provides evidence that modern bathymetry can reflect palaeochannel location on the GBR shelf. Our results offer new insights into the evolution of the southern GBR shelf, and show that no single existing model is able to adequately explain the behaviour of lowstand palaeochannels on rimmed siliciclastic/carbonate shelves.