Cenozoic volcanism, tectonism and stream derangement in the Snowy Mountains and northern Monaro of New South Wales

Abstract

Studies of Cenozoic lavas and associated sediments in the Kiandra–Cabramurra and Adaminaby–Cooma areas identify and date tectonic deformations responsible for differential uplift and drainage development of the region. Volcanic activity on the northern Monaro was mainly Eocene–Oligocene but in the extreme north there are Early Miocene sediments and lavas. Volcanic activity and folding began to rearrange the drainage in the Eocene–Oligocene. The headwaters of the Murrumbidgee River originally flowed south into the Eucumbene River but Early Miocene folding and faulting uplifted the Monaro Range and created a large lake near Adaminaby. Lake overtopping rerouted the drainage east and then south along the basalt‐filled valley of an old north‐flowing tributary, the 'Adaminaby River', forming the present‐day Murrumbidgee River. The folding also produced a 300 m height difference between the Berridale and Adaminaby Plateaus and formed a section of the Great Divide. This fold displacement ranks with the largest Cenozoic fault displacements. In the Kiandra area tectonism associated with Early Miocene volcanism rearranged the drainage and tilted the Kiandra area and Kosciuszko Block to the north.