Paleo-drainage and structural deformation during Gondwana breakup: insights from the 3D Geometry of the Bunbury Basalt

Abstract

The Bunbury Basalt is a series of lava flows in the Perth Basin, Western Australia, and is interpreted to be related to the breakup of eastern Gondwana during the Early Cretaceous. We integrate new aeromagnetic images with available well intersections and outcrop constraints to establish the first 3D model of the Bunbury Basalt. The model reveals that the flows can be up to 100 m thick and are predominantly confined to two paleochannels and their tributaries situated in the Bunbury Trough in the southern Perth Basin. The Donnybrook paleochannel flows proximal to the Darling Fault and is cut by the Bunbury paleochannel, which is positioned centrally in the Bunbury Trough and contains two flow episodes. The model illustrates a dominantly north–south axial paleo-drainage pattern that existed in the southern Perth Basin prior to Gondwana breakup. The 3D model provides a present-day volume of 90 km3 with an aerial extent of 3300 km2. Offsets of the Bunbury Basalt have been used to identify new northeast and northwest trending faults in the southern Perth Basin, and broad folding is interpreted to a consequence of drag into the Darling and Busselton Faults. The source vents for the Bunbury Basalt were probably located at extensional jogs at intersections between the Darling and Busselton Faults with subordinate oblique faults.