Course of the lower River Murray in South Australia: effects of underprinting and neotectonics?

Abstract

The change in direction of the River Murray from westerly to southerly at North West Bend has been attributed to faulting or warping, but no appropriate structure has been located in the country rock coincident with the river course. Yet the angularity and the straightness of major sectors argue structural control. The plan course of the Murray downstream from Morgan is attributed to underprinting from basement fractures following the Middle Miocene but prior to the Late Pliocene. Uplift of the Marmon Jabuk structure superimposed on the effects of underprinting accounts for major departures from the SSW trend downstream from North West Bend, as well as the impounding of Lake Bungunnia. The upper shallow section of the valley-in-valley form was shaped at a time of higher baselevel in the Middle-Late Tertiary. The lower section is the present Gorge. The valley floor was lowered probably by subterranean solution and flow followed by collapse of the cavern roofs. Regression of the River at times of lower sea level caused the breach of the Marmon Jabuk blockage and the draining of Lake Bungunnia.