THE LATE-HOLOCENE ORIGIN OF THE MODERN MURRAY RIVER COURSE, SOUTHEASTERN AUSTRALIA

Abstract

The modern course of the Murray River flows south around the Cadell Tilt Block in southeastern Australia. The avulsion that took the river in this direction formed the Barmah Choke, a reach with an unusually straight planform. This morphology has been used to argue Holocene inertia of the modern river system. The avulsion was initially placed at ? 10 ka on the basis of a single charcoal age. Optically stimulated luminescence (OSL) and AMS 14C dating of deposits along the Barmah Choke demonstrates that the Murray has not been inert during this period. A digitate delta deposited on the floor of a palaeolake by the avulsion is only ? 550 years old. Point bar deposition along the Choke also began around this time. The Murray River probably followed the course of Bullatale Creek for most of the Holocene. It formed large, inset meander scrolls of Pleistocene ‘Kotupna-type’ along this creek course but without source-bordering dunes. The persistence of Kotupna-type bedload along this course suggests that there was no simple switch to suspended load channels at the Pleistocene/Holocene transition. The Barmah Choke avulsion trebled the discharge of the Goulburn River causing morphological adjustment and formation of new reaches downstream. The extensive Barmah and Gunbower-Koondrook red gum forests probably arose from the increased flood flows. Pollen and spores from lacustrine sediments beneath the Choke show that the vegetation ? 4000 years ago was very different. The lake and hinterland began to dry from this time and the lake became a swamp. A pedogenic interval shows that the swamp was dry for lengthy periods prior to deposition of the digitate delta.