Structural evolution of the Early Cretaceous depocentres, Otway Basin, Victoria

The dominant control on the (transition between) depositional settings of the Crayfish Group of the Otway Basin in Victoria, Australia has been determined. The study first involved seismic mapping of six stratigraphic units within the Early Cretaceous, continental Crayfish Group. The resulting 3D structural model was used to identify major Early Cretaceous depocentres, and to determine which Crayfish Group sediments are restricted to individual rift depocentres and which are more widespread as a result of inter-connectivity of the basin. Five structural cross-sections were then constructed across each major depocentre of the basin; these were balanced and restored, and missing section estimated, in order to test the validity of the structural interpretations. This also enabled analysis of differing extensional rates within each depocentre and the calculation of the cumulative displacement of each major bounding fault. Results show that displacement rate, growth and linkage of the faults, as well as the amount of subsidence within the depocentres, had a significant effect on the distribution and development of the major facies within the Crayfish Group. The Casterton Formation and Sawpit Shale equivalent/McEachern Sandstone were restricted to rapidly subsiding, structurally controlled depocentres in the west, while the succeeding Sawpit Sandstone equivalent was deposited within the same depocentres, across the intervening structural highs and in the eastern part of the basin where depocentres had just begun to form. The Pretty Hill Formation shows a similar distribution pattern, while the overlying fine-grained Laira Formation also drapes structural highs but is replaced in the east by coarser-grained sediments of the upper Pretty Hill Formation. Extension was locally up to 21% in the central Otway Basin but was much less in the eastern Otway Basin.