Transcontinental Cainozoic paleovalleys of Western Australia

Abstract

800-2400 km long transcontinental paleovalleys straddle the modern landscape of Western Australia (WA). These valleys formed following emergence of the Canning Basin at the end of the Lower Cretaceous, and reached their greatest development during Eocene time. They owe their preservation to limited erosion/burial and to an overall drying climate since the Upper Cretaceous. They represent the largest network of inactive valleys visible at the Earth’s surface. These valleys debouch at their downstream ends into well-dated depocentres and paleo-shorelines, which provide age constraints and firm milestones of their temporal and spatial evolution. Drainage network evolution and valley long profiles constrain the timing of long (~1000 km) to intermediate wavelength (~200 km) variations in uplift and subsidence rates over the continental interior. We use these data to decipher the respective contributions of regional tectonics and dynamic topography to the evolution of the Northwest Shelf. Upon emergence uplift determined the shape of the initial drainage that started to drain the emerging landmass. Later, a changing field of surface uplift triggered drainage rearrangements in the early Cenozoic. Rearrangement resulted in piecemeal rerouting of water and sediments towards the North West Shelf. Increasing aridity during the Neogene contributed to the tectonic defeat of some of these rivers. We use the Badlands surface model developed by the Basin Genesis Hub to quantify sediment and water delivery to the North West Shelf during the lifespan of the drainage. Further, there is some debate regarding the chronology of aridification in the continental interior. We use the modelling to derive the first quantitative estimates of the water balance to see if numeric water balance assessments can reconcile discrepant sets of paleoclimatic proxies.