DEPOSITIONAL ENVIRONMENTS AND GEOTECTONIC FRAMEWORK: SOUTHERN AUSTRALIAN CONTINENTAL MARGIN

Abstract

Three principal phases occurred in the development of the basins of the southern Australian continental margin: epi-continental, marginal continental and oceanic. These correspond generally to the phases of margin development proposed by Falvey (1974): pre-rift, rift valley, and post-breakup; but tectonic and depositional transitions are not necessarily contemporaneous.Prior to the Upper Cretaceous, the region of the present day southern Australian margin lay well within the Eastern Gondwanaland continent, essentially barred from deep ocean basins. During the Upper Cretaceous the series of epicontinental basins was increasingly subjected to marine breakthroughs. Thus marine ingressive horizons were deposited along an incipient rift valley between the primitive Indian Ocean and Tasman Sea. Rift valley subsidence, possibly related to deep crustal metamorphism, was most significant on the flanks of the rift zone. Further marine influence during the Paleocene ('infra-breakup') and early Eocene corresponded to the onset of seafloor spreading between Australia and Antarctica. The neo-breakup phase is dominated by shelf and plateau subsidence and spreading ridge development, with topography influencing ocean current. The changing palaeogeography can be accurately illustrated by computer-derived reconstructions based on quantitative sea-floor spreading data. Quantitative thermal uplift/subsidence models can be used to estimate post-breakup water depth of the subsiding ocean basin and the continental margin. A complex pattern of transgressive continental deposition and submarine erosion diminished with the gradual widening of the Southern Ocean and the establishment of circumpolar ocean current paths. Oceanic basins dominated the margin through the Neogene.