Abstract

As large, deep rivers enter their coastal reaches, they are usually affected by a lengthy backwater zone. Recent studies have demonstrated pronounced geomorphic effects, such as channel deepening and decreased lateral migration rates. These observations appear to be linked to variable fluxes of sediment in rivers that characteristically vary their discharge across their backwater segments. How do such a variable sediment transport regime and its depositional effects manifest as stratigraphic architecture over geologic time? To address this question, we mapped and analyzed over 700 channel belts from a large (70,000 km²) seismic survey of the Mungaroo Formation, Northern Carnarvon Basin, Australia. Following analysis of measurements of channel-belt patterns, we propose that the Mungaroo stratigraphy was constructed by a continental-scale paleodelta and that it reveals a record of backwater-influenced deposition over a length scale of 400 km. Width measurements of the Mungaroo channel belts decay only weakly down system, in a manner similar to the lowermost reaches of the late Holocene Mississippi River, here invoked as an analogous system. In addition, significant downstream fining in the Mungaroo system is interpreted from wireline-log and lithologic core data, a trend that is also seen in the Mississippi channel belt, where bank-attached bars fine downstream due to reduced contributions of bedload. By comparing the two systems, we show that the depositional trends expressed in the 10³ year record of the Mississippi River and the 10⁶ year record of the Mungaroo paleodelta are similar, each having generated channel belts that only slightly narrow down system but transition from sand-rich to heterolithic or mud-dominated along the lower 30 to 50 percent of the backwater reach. We present a transtemporal backwater-influenced depositional model that leverages quantitative data from an analogous modern transport system for characterization of an ancient stratigraphic system.