Shale mobility: From salt-like shale flow to fluid mobilization in gravity-driven deformation, the late Albian–Turonian White Pointer Delta (Ceduna Subbasin, Great Bight, Australia)

Large offshore depocenters above a weak detachment level (either salt or shale) can undergo gravity spreading and/or gliding. The gravitational systems (e.g., gliding deltas) are classically composed of an updip domain affected by extensional listric normal faults and a downdip domain affected by toe thrusts. While the role of salt in such systems is a classic tectonic process, the role and mechanical behavior of mobile shale levels in shale-prone gravity-driven systems are increasingly questioned. A three-dimensional seismic data set in the Ceduna Subbasin (Australia) displays the late Albian–Turonian White Pointer Delta (WPD) as having an unusual diversity of shale-cored structures. The early flow of shale resulted in depocenters showing wedges, internal unconformities, and shale diapirs and ridges, while fluidization of shales underneath a significant burial resulted in mud volcanism, secondary radial fault sets, and collapse features beneath the Campanian–Maastrichtian Hammerhead Delta, which lies above the WPD. Massive shale mobilization, together with downdip shortening and distal margin uplift, localized a major thrust in the core of the basin, ending the downward-propagating failure of the WPD. Mobilization of thick shale intervals, either as salt-like flow or mud volcanism, appears to have been a key process in the deformation, which should be considered at large scale for worldwide gravity-driven deformation systems.