A Quaternary carbonate contourite drift system on the Eyre Terrace, Great Australian Bight: implications for the origin of prograding shelf-edge–upper slope clinoforms, southern Australian passive margin

Abstract

An appraisal of the Quaternary shelf-margin succession in the western Great Australian Bight has yielded new insights into the development of this prograding passive margin. Our analysis of seismic reflection and sedimentary data from the Eyre Terrace and adjacent outer shelf challenges the established view that this shelf-margin wedge represents a prograding carbonate ramp. Instead, we identify separate outer shelf and upper slope depositional systems: the outer shelf comprises an aggrading to prograding succession whereas the upper slope was constructed and shaped predominantly by alongslope processes driven by the major boundary currents that form the Southern Australia Current System. The latter resulted in the formation of a spectacular 500 m thick, basin-scale, elongate-mounded carbonate contourite drift (the ‘Eyre Terrace Drift’) and associated sediment waves. Sheeted drift and infill drift geometries are also observed. Key sedimentary attributes include fine-grained sediment, multi-scale gradational bed contacts and pervasive bioturbation. The upper flank of the drift provided a platform across which the prograding outermost-shelf facies progressively migrated. The resulting stratigraphic pattern of the Quaternary shelf-margin clinoforms was formed by a combination of ‘conventional’ clinoform progradation from the outer shelf and ‘contouritic clinoform’ progradation and drift accretion on the upper slope.