BACKWATER HYDRODYNAMIC INFLUENCE ON AVULSION STYLE AND FLOODPLAIN CHARACTER: CASE STUDY FROM MUNGAROO FORMATION, NORTHERN CARNARVON BASIN

Understanding controls on river form and scale is important for sustainably managing modern rivers and floodplains, reconstructing ancient surface conditions on Earth and other planets, and predicting the distribution and connectivity of subsurface aquifers and hydrocarbon reservoirs. Recent advances have improved understanding of large, network-scale trends in channel size and form due to, for example, fluvial fan development near sediment sources or backwater hydrodynamics near shorelines. The Triassic Mungaroo Formation – deposits of a continental scale, low-slope fluvio-deltaic system (Northern Carnarvon Basin, offshore Northwest Australia) – provides a unique opportunity to observe and evaluate upstream-to-downstream changes in a paleo-channel network.

Using publicly available seismic and well-log data, we documented changes in channel and floodplain deposits from upstream to downstream in the Mungaroo paleo-network. Upstream of the backwater influence in the proximal zone of the paleo-delta, there is an abundance of small, highly sinuous channels and abundant cutoffs that then transition to high width, low sinuosity channels and channel-belts in the distal zone. Seismic facies contrast shift from low in the upstream reflecting a high degree of coarse material in the floodplain to low distally, reflecting finer grained floodplains which contrast relatively sand rich channels. Medially, facies exhibit the effect of backwater hydrodynamics as sand facies peak at the paleo-apex, before sharply declining, distally. These results indicate that the coupling between channel and floodplain sedimentation changes downstream as channels encounter the influence of backwater hydrodynamics. This connection is an important control on channel scale, form, and architecture of subsurface channel accumulations.