THE OTWAY BASIN: EARLY CRETACEOUS RIFTING TO NEOGENE INVERSION

Abstract

A regional seismic interpretation ot the on shore Otway Basin has been completed and used to determine the basin's structural history.

Sedimentation commenced in the Tithonian-Berriasian with the deposition of the volcanogenic Casterton Formation and continued into the Berriasian-Barremian with the deposition in elongate half graben, of thick fluviolacustrine sediments of the Crayfish Group, typically thickening dramatically towards the bounding faults. The NW to W trend of Crayfish Group depocentres and their major bounding faults suggest that the initial extension direction was N-S to NE-SW in the Late Jurassic-Early Cretaceous. Dextral transtensional movement occurred along the Trumpet Fault in the west of the basin and was complemented by sinistral transtension on the major NNE striking faults of the Torquay Sub-basin in the east during this period.

The dip direction of the pre-Barremian bounding faults changes a number of times along the northern margin of the basin. These changes occur across transfer/accommodation zones of complex faulting and folding, not over discrete transfer faults.

Faulting and related uplift resulted in partial erosion of the Crayfish Group from a number of structural highs, prior to the Aptian. The half graben faults are overlain by Eumeralla Formation indicating that active rifting had ceased by the Aptian in the onshore Otway Basin. Further erosion occurred following post-Albian faulting and uplift prior to the Paleocene, in particular within the eastern part of the basin.

During deposition of the Sherbrook Group in the Late Cretaceous, fault reactivation produced minor, shallow grabens within the older half graben systems. Major movement also continued along the Tartwaup Fault Zone, resulting in basin deepening toward the SW. This fault activity continued into the Paleocene-Early Eocene during deposition of the Wangerrip Group. In the Eocene, the Southern Ocean spreading rates changed from slow to fast, resulting in the late-Early Eocene deltaic sediment of the Upper Wangerrip Group covering some of the earlier extension faults. Compression, resulting in right-lateral wrenching and inversion of previous faults, occurred during the Miocene-Recent. Pliocene-Holocene volcanic activity occurred along zones of weakness related to these fault systems.