LATE CRETACEOUS TO LATE PLEISTOCENE DRAINAGE EVOLUTION OF COBAR UPLAND, N.S.W., AUSTRALIA

Late Cretaceous to late Pleistocene drainage evolution of Cobar upland, N.S.W., Australia.

The Cobar Basin, was formed in Devonian time. A major structural feature occupies the axes of a Cretaceous marine basin to the north. Deformation in Cobar includes overturned anticlines along faults trending generally north-northwest to south-southeast with diagonal secondary faults. Most faults and fractures in the region have been formed in the same direction. Even modern structural features are being formed by reactivation of the Devonian structural framework. The Devonian to Cretaceous sedimentation record has been denudated almost completely. A few remnants survived. Important mineralization was formed along these structures, Ag, Zc, Cu, Au between them.

An interior sea basin was formed (Eromanga) during the Cretaceous that lasted until late Cretaceous. This event triggered a new cycle of denudation and drainage down cutting. In the last 62 million years the Cobar landscape evolution is defined by diversion of drainage caused by tectonic reactivation along Devonian structures. Volcanic plugs were also formed along the same structures developed from mid to late Tertiary. Some of the basalts covered Tertiary conglomerates giving an estimate for the age of these deposits.

Cobar can be separated in two major regions were there is general migration of drainage to the south in the eastern area and to the north in the western area. Changes in drainage were established from Late Cretaceous to Pliocene time. Migration of drainage in the eastern area is indicative of uplift from north to south while the western area the opposite, from south to north. The two blocks moved around a pivot point located in the center of the basin. Drainage captured by regressive erosion was also present at the headwater of major diversions.

The Cobar drainage evolved from south-north in Late Cretaceous to progressively east-west trend in late Tertiary. Further, it evolved from a high drainage density associated with a much wetter climatic conditions to a drainage containing sporadic or intermittent water indicative of a drier and warmer conditions.

Paleo-channel deposits containing gold has been found. Changes in drainage patterns will be used for mineral exploration in the Cobar Basin.