The Holocene Mitchell Megafan, Gulf of Carpentaria, Australia

AbstractAs the world’s lowest and flattest continent, Australia exhibits numerous gently inclined topographic basins that have generated a large number of megafans. One of the most extensive contiguous arrays occurs on the western flank of Cape York Peninsula. These megafans are derived from a complex network of modern and ancient distributary channels that have debouched into the epicontinental seaway of the Gulf of Carpentaria since at least Pliocene times. The Mitchell megafan-delta represents the largest of these fans. In this case study we examine the modern processes and resulting sedimentary facies and architecture of a type example. Mapping, field exploration and chronologic investigations were used to characterise these channel-belt facies and to link them to specific forcing conditions. These Holocene surficial deposits are associated with a network of modern streams and recent palaeochannels overlying, and sometimes incised into, older, fine-grained, partially indurated Quaternary alluvium. Where present, the partially indurated alluvium restricts channel migration, thereby promoting anabranching as a means of channel adjustment. Additionally, channel-belt distribution, morphology and sediment infilling are strongly affected by marine backwater dynamics operating in the downstream reaches. This has had an overriding influence on the position of avulsion nodes and channel-belt distribution in this zone.