Tectonics and landscape evolution in southeast Australia

Abstract

In southeast Australia the history of river development, basin sedimentation and the evolution of major divides can all be related. The region has a basement of Palaeozoic rocks eroded to a palaeoplain. Two sedimentary basins are separated by a system of tectonic warp axes that correspond closely to drainage divides. The Great Artesian Basin (GAB) is Mesozoic; the Murray Basin is Cenozoic. The Cretaceous-Cenozoic Gippsland-Otway Basin lies to the south, and a Cenozoic sedimentary wedge on the continental shelf to the east.In the Jurassic, before the breakup of Gondwana, Australia extended further east and south. Rivers from the south and east provided coarse sediment to the GAB.The catchment of Jurassic drainage was bounded to the east by the Tasman Divide. Downwarping of the palaeoplain formed the east-west Victoria Divide and the Gippsland Basin in which Cretaceous sediments accumulated. Rifting and seafloor spreading formed the Tasman Sea, starting about 80 m.y. ago. The palaeoplain was downwarped, creating the Great Divide and a new continental shelf on which marine sediments accumulated. Drainage from the Victoria Divide and the Great Divide continued to flow to the GAB until the Murray Basin started to subside in Paleocene times. A new warp axis, the Canobolas Divide, appeared between the GAB and the Murray Basin. Basically west-flowing drainage developed across the Murray Basin, Cenozoic sediments accumulated, and sediment supply to the GAB was further depleted.Ancillary features consistent with this morphotectonic history include: Ancient channels with gravels cross the Victoria, Great and Canobolas Divides. Volcanicity follows the warp axes. Reversed rivers are found on the coastal side of the Victoria and Great Divides. Deposition on the continental shelf is roughly equal to erosion on land. The change from coarse to fine sediment which gives the GAB its artesian character fits with the shrinkage of its catchment. The Divides are in different stages of erosion consistent with their ages.The morphotectonic development of southeast Australia, with responses to non-cyclic unique events on the time scale of global tectonics, is an example of evolutionary geomorphology.