Evolution and architectural styles of a forced-regressive Holocene delta and megafan, Mitchell River, Gulf of Carpentaria, Australia

The monsoonal Mitchell River delta and megafan shows minimal anthropogenic disturbance and thus provides a unique opportunity to examine the autogenic and allogenic controls on the evolution of deltas and fluvial megafans. Detailed mapping, vibracoring, trenching and topographic surveying were used to characterize the palaeodistributary channel belts on the megafan and the depositional elements that comprise the delta. Chronological analyses of these data facilitated the reconstruction of the megafan and delta evolution and enabled the identification of discrete periods of delta progradation within the last 6000 years. These results indicate that sediment distribution is controlled primarily by two types of avulsion: (1) delta avulsions, which are frequent (>16/1000 years), typically backwater-mediated and associated with local shifts in sedimentation loci; and (2) megafan avulsions, which are less frequent (>3/1000 years), but which are also associated with more significant shifts in depositional loci. These links between megafan and delta processes and geomorphology in the Mitchell River region were integrated to develop a new model of channel belt facies associated with fluvial (F), fluvial backwater-affected (F_BW), fluvial-dominated, tide-influenced (Ft) and tide-dominated, fluvial-influenced (Tf ) channels. This model enables improved predictions of channel belt composition in modern and ancient marginal-marine systems by providing sedimentological and ichnological criteria for distinguishing between channel types.