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Downstream Changes in Architecture and Fill of Distributary Channels on a Delta Fed by a Fluvial Megafan: A Case Study From the Mitchell River, Gulf of Carpentaria, Australia


The Mitchell River megafan comprises a distributive network of modern and palaeo-distributary channel belts. This monsoonal catchment supplies sediment to a mixed-process marginal marine system within the low accommodation Gulf of Carpentaria. Detailed mapping, sonic coring, trenching and topographic surveying have been used to characterise the palaeo-distributary channel belts; the bathymetry and bottom composition of their modern counterparts on the lower delta plain have also been surveyed. These data have revealed systematic downstream and lateral changes in palaeo-channel belt architecture, with implications for similar subsurface reservoirs. The deepest channel belts occur proximal to the apex of the fan and are characterised by coarse fills. Their planform geometries are generally straight and single thread. Laterised alluvium has restricted their lateral migration and promoted aggradation and avulsion on the fan. These channel belts transition into laterally accreted scrolled systems on the lower fan, which are shallower and are comprised of finer grained material. The highest concentration of avulsion nodes occurs in this downstream zone, which coincides with the landward limit of the backwater effect. These channel belts are not directly influenced by tidal currents. Further downstream, channel belts on the delta plain have been influenced by tidal currents and are classified along the continuum of fluvial- dominated, tide-influenced (Ft), to tide-dominated fluvial-influenced (Tf), to tide dominated (T). The majority of active channels that comprise these belts show a landward tapering, “funnel shaped” morphology. Ft channels are sinuous and are characterised tidally influenced inclined heterolithic stratification (IHS) overlying fluvial sand, and drawdown effects near their mouths determine that their channel depths typically reduce upstream. In contrast, Tf fills are composed of a higher proportion of mud than Ft channels, and often exhibit a straight-meandering-straight pattern, typical of estuaries. T channels are almost entirely mud filled. This continuum of channel types is also observed laterally within channel belts on the delta, as upstream avulsions cause channel belts to become progressively abandoned and influenced by changing combinations of processes. The geometry and composition of the channel fills are thus intimately linked to the palaeogeographic evolution of entire distributive system and are the subject of ongoing research.