A continental review of Australia's Cenozoic megafans and associated large accretionary fluvial systems

Australia is the world's lowest and flattest continent with the most negative water balance. Gentle Cenozoic tectonism has formed numerous moderately depressed basins, mostly on the eastern half of the continent, that have accumulated substantial alluvium in diverse climatic zones. While many can be defined as megafans, a minority are not fan-shaped but are confined in axial valleys. Some in Western Australia have formed with alluvium eroded from ancient cratonic lithologies. We propose the generic term large accretionary fluvial systems (LAFS) to describe all alluvial accumulations &gt;1000&nbsp;km2&nbsp;that adopt these varied planforms and retain the term megafan for those that are fan shaped. Identified are 61 LAFS totalling 480,000&nbsp;km2, or 6.3&nbsp;% of mainland Australia that are supplied from catchments totalling 1,828,000&nbsp;km2, or 24&nbsp;% of the mainland's area. LAFS divide into two primary types: Type 1 - Unconfined (<em>n</em>&nbsp;=&nbsp;51) have formed in unconstrained settings downstream of valley confinements, and expand into megafan planforms of anabranching channels with distributive fluvial networks (DFN). Type 1 total 400,000&nbsp;km2, and by area represent 83&nbsp;% of Australia's LAFS. Type 2 - Confined (<em>n</em>&nbsp;=&nbsp;10) have formed where, after an initial expansion, they are constrained by the semi-parallel sides of axial valleys and anabranching channels display axial fluvial networks (AFN). Type 2 cover 81,000&nbsp;km2&nbsp;and by area represent 17&nbsp;% of Australian LAFS. Due to downstream variations in valley width, Type 2 switch surface-channel networks between DFN, AFN and, when further confined downvalley, to contributive fluvial networks (CFN). Types 1 and 2 can be further differentiated from their distal components into three subtypes: a. terrestrial; b. coastal delta; c. playa delta. Type 1 have the most variable gradients and those in arid central regions are the steepest. Type 2 have some of the lowest gradients, even in arid regions. With Type 1, the largest catchments tend to form the largest LAFS whereas, because of confinement, this does not apply to Type 2. The largest LAFS form in more humid regions and the smallest in arid regions. Australia's LAFS have been primarily formed by overloaded, avulsing, low-sinuosity anabranching and braided rivers, mostly during enhanced fluvial episodes of the Quaternary. Meandering, as an essentially incisional process, has made a minimal contribution except in tidal reaches. In the monsoon north, LAFS remain weakly aggradational and are continuing to prograde. In arid to semiarid areas, and in the humid to sub-humid Murray-Darling basin, LAFS are largely relict and yield chronostratigraphic evidence of pronounced growth during Pleistocene hydroclimatic episodes. Their surfaces often feature slightly entrenched erosional meandering channels. On such a low gradient dry continent with low rates of sediment production, the largest LAFS are caused by minor tectonic deformations and gradual basin and axial valley subsidence. Australian LAFS often feature aeolian dunes and lunette lakes. Ranging from thin alluvial sequences to multistorey with depths of several hundred metres, most LAFS are potential sources of near-surface groundwater.

<b>Citation:</b> Gerald C. Nanson, Rachel A. Nanson, A continental review of Australia's Cenozoic megafans and associated large accretionary fluvial systems, <i>Earth-Science Reviews</i>, Volume 266, 2025, 105084, ISSN 0012-8252, https://doi.org/10.1016/j.earscirev.2025.105084.

Tags: Australia-wide geoscience data, Cenozoic, Darling River, Geomorphology and earth surface processes, Hamersley Basin, Karumba Basin, Lake Eyre Basin, Murray Basin, Onshore National Geology, Pilbara Craton, Resourcing Australia’s Prosperity, channel patterns, coastal delta, facies model, fluvial stratigraphy, lunette lakes, megafan, neotectonism, shallow groundwater
Organization: Geoscience Australia Data