FLOOD-DOMINATED MOUTH BARS OF THE BURDEKIN RIVER DELTA, NORTH QUEENSLAND, AUSTRALIA

The Burdekin River Delta of northeastern Australia has been previously described as a wave-tide-fluvial interactive delta, and a published vertical sequence model shows a continuously coarsening-upward delta front/mouth bar succession tens of metres thick. However, the lower delta plain on air photos seems to be dominated by broadly triangular areas that resemble in all respects the modern mouth bars of the delta, raising the possibility that a large part of the lower delta plain area (and hence Holocene sediment volume) is made up of such bodies. Our recent drilling has established that these areas are underlain by 5-8 m thick, sharp-based bodies of moderately sorted, coarse-grained sand with either no vertical grain-size trend, or a fining-upward. These bodies are texturally inseparable from those of the modern mouth bar, where the immediate subsurface shows weakly-defined flat and low-angle lamination. The only modification of this pattern occurs at the seaward boundary of the mouth bars, where reworking by waves and currents produces a wave-dominated foreshore of well-sorted, fine-medium-grained sand. We suggest that the Holocene Burdekin Delta is flood-dominated in the sense that it has prograded through the rapid construction of mouth bars during major flow events of the river (most of which last for <1 to a few days). Inspection of serial air photographs confirms that mouth bar construction, stabilisation and abandonment of associated channels takes place on a 10’s of years timeframe. The fining-upward middle ground mouth bar sand bodies are a product of the homopycnal to possibly hyperpycnal nature of outflow onto a shallow, low gradient delta front. Caution is therefore needed in the interpretation of ancient deltaic successions, where in certain situations sharply-based, fining-upward mouth bar deposits may also be preserved.