Separating tectonic, eustatic, and episodic depositional processes in the stratigraphic record of the basins of southeastern and northwestern Australia (Cretaceous Period)

Abstract

Four dynamic processes interact to create the stratigraphic record in a basin: climate, sediment supply, eustasy, and subsidence. To identify the contribution of each of these processes, techniques that can uniquely separate the effect of each component process on the stratigraphic record must be applied to produce a thorough understanding of the process, how it evolves through time and interacts with other processes. Recently published techniques (sequence stratigraphy, one- and three-dimensional backstripping), as well as techniques developed herein (Relational Tectonic Analysis), provide high resolution basin eustatic and tectonic histories. Integrating these new techniques with standard stratigraphic methods illuminates aspects of basin evolution heretofore hypothesized but unrecognized. Application of integrated analysis techniques indicates that plate tectonic reorganization events occurring during the Cretaceous Period had profound effects on the recognition of important sequence stratigraphic surfaces in southeastern and northwestern Australia. Seven reorganization events are recognized to have influenced the Cretaceous motion history of Australia. Eleven third-order eustatic cycles are recognized to have occurred during deposition of the Neocomian sediments of the Exmouth Plateau/North West Shelf. Sixteen third-order eustatic cycles are defined in Late Cretaceous sediments of the Otway Basin. The numeric ages determined from these twenty-seven biochronostratigraphically dated eustatic cycles are used to produce one- and three-dimensional subsidence histories. Each tectonic event creates a paired increased subsidence-decreased subsidence cycle. Each change in subsidence induces changes in the space available for sediment deposition. Changes in subsidence that create more space for sediments causes depositional systems to retrograde and transgressing the continent. Transgressive aspects of sediments deposited during each sea level rise associated with third-order eustatic cycle are amplified and therefore easier to recognize. Concurrently, the ability to recognize the regressive aspects of each eustatic cycle is diminished. If subsidence change reduces space available for sediments, the opposite effect occurs: depositional systems prograde, and regressive aspects of the third-order cycles are enhanced. Not all subsidence rate changes are due to tectonic reorganizations. Change in flexural subsidence induced by sudden large localized sediment influxes also cause enhancement of aspects of third-order eustatic depositional patterns.