The eustatic and tectonic origin of Neogene unconformities from the Great Australian Bight

The Neogene succession recovered during ODP Leg 182 from the Great Australian Bight comprises unconformity-bounded, cool-water carbonates. A total of 15 hiatuses, each lasting ∼0.5 Myr or more, are identified or inferred primarily on the basis of planktonic foraminifer biostratigraphy from the basal Miocene to early Pleistocene. They are interpreted as local manifestations of major third-order boundaries at about 23.8 (H1), 22.3 (H2), 20.5 (H3), 18.7 (H4), 16.4 (H5), 14.8 (H6), 13.5 (H7), 11.5 (H8), 9.3 (H9), 7.0 (H10), 6.0 (H11), 4.5 (H12), 3.5 (H13), 2.5 (H14), and 1. 5 Ma (H15). The coincidence of these hiatuses with third-order global sequence boundaries suggests a eustatic control complicated by local tectonics. Three mega-hiatuses at about 15–16 Ma, 8–9 Ma and 1.5–2.5 Ma, each lasting >5 Myr at selected sites, are interpreted as being caused by large-scale slope failure during times of differential uplift/subsidence and sudden changes in relative sea level. These results provide evidence that the evolution of the southern Australian margin during the Neogene occurred in steps, controlled by a W–E stress field in the course of Australia’s northward drift and by changes in relative sea level that triggered sedimentation as well as sediment packaging by unconformities.