Thermochronological and Geochronological Constraints on the Origin and Evolution of the Cretaceous Ceduna Sub-Basin, Great Australian Bight

Abstract

The Ceduna Sub-basin is the main depocentre of the frontier Bight Basin, which formed as a result of the late Jurassic-Cenozoic separation of Australia and Antarctica. The sedimentary fill of the Ceduna Sub-basin is dominated by two structurally distinct deltaic lobes of Cenomanian and Santonian-Maastrichtian age with combined thickness >12 km. This region is the focus of growing exploration interest, and thus improved knowledge of its origin and evolution is essential for reducing exploration uncertainty. However, because the Ceduna sub-basin is located completely offshore in water depths up to 5 km, to date there has been little exploratory drilling in this region. With primary data from the sub-basin itself lacking, we have collected a variety of new thermochronological and geochronological datasets from the onshore margins and hinterland to the Ceduna sub-basin, which have a bearing on the evolution of the offshore region. These datasets include: 1. Zircon U-Pb ages from several samples of drillcore from the Lower Cretaceous Loongana Formation, which is preserved in the Denman Basin, a shallow depression that underlies the onshore Eucla Basin. Age populations within these data suggest that sediment input at this time was predominantly from the N and W. 2. Zircon U-Pb ages from several samples of drillcore from the Winton Formation, an Albian-Cenomanian-age fluvial-lacustrine sequence from the Eromanga Basin. This sequence has been proposed as an analogue for the Cenomanian deltaic lobe in the Ceduna sub-basin, which has yet to be penetrated by drilling. 3. AFTA and VR data from outcropping rocks in the Eyre Peninsula, and subsurface rocks retrieved by drilling in the Polda sub-basin, to the NE of the Ceduna sub-basin. These data point to substantial exhumation of this region during the late Cretaceous. 4. Zircon U-Pb and fission track ages from the Turonian-Maastrichtian sequence penetrated by the offshore Gnarlyknots-1 well. Interpretation of these ages suggests that this sequence was largely sourced from recycled Permian-Early Cretaceous cover and underlying basement rocks eroded from the proximal, NE basin margin. The integration of these onshore and offshore datasets provides new, valuable insights into the Cretaceous palaeogeography of the Ceduna sub-basin, the tectonic processes controlling the input of clastic sediments, and the prospectivity of this frontier exploration region.

AAPG Datapages/Search and Discovery Article #90217 © 2015 International Conference & Exhibition, Melbourne, Australia, September 13-16, 2015