Tectonics and landscape evolution in Southeast Australia

Abstract

In SE Australia the history of river development basin sedimentation and the evolution of major divides can all be correlated. The region has a basement of Palaeozoic rocks eroded to a palaeoplain on which lie two sedimentary basins separated by a system of warp axes. 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. Rivers flowing North and West across southeast Australia are older than the formation of the eastern continental margin the eastern highlands and the Murray Basin. 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. This drainage pattern is older than the formation of the continental margin, the eastern highlands, and the Murray Basin, and traces survive today. The catchment of Jurassic drainage was bounded to the E by the Tasman Divide. Later downwarping of the palaeoplain formed the Victoria Divide and the Gippsland Basin in which Cretaceous sediments accumulated. Rifting and seafloor spreading formed the Tasman Sea, starting about 80 Ma ago. The paleoplain was downwarped, creating the Great Divide and a new continental shelf on which marine sediments accumulated. The palaeoplain is thus equated with the breakup unconformity. Drainage from the Victoria Divide and the Great Divide continued to flow to the GAB until the Murray Basin started to subside starting in Paleocene times. A new warp axis, the Conobolas Divide, appeared between the GAB and the Murray Basin. Basically west-flowing drainage developed in the Murray Basin, Cenozoic sediment accumutated, and sediment supply to the GAB was further depleted . Ancilliary features consistent with this morphotectonic history include: Old channels with gravels cross the Victoria, Great and Conobolas 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.