Kerrie M. Tomkins

Evidence of Late Cenozoic uplift and climate change in the stratigraphy of the Macquarie River valley, New South Wales

The stratigraphy of the alluvial sediments filling the Macquarie River valley, New South Wales, was investigated to test models of landscape evolution of the rifted margin Eastern Highlands of Australia. In the neighbouring Lachlan River valley, the stratigraphy of Neogene sediments has been incorporated into a model proposing denudation and episodic passive (denudational isostatic) uplift of the highlands throughout the Neogene. In this study, we aimed to test the regional extent of this model and the influence of sediment supply on the stratigraphic record and its interpretation. The sediments of the Macquarie River valley show three major changes in the depositional regime. First, erosion of the valley basement in the Late Miocene was followed by (and possibly synchronous with) deposition of a basal clay and sand unit in the Late Miocene. Second, the valley was filled with a wedge of sediment containing abundant gravel and sand in the Pliocene; and third, the older gravelly unit was eroded and reworked in the Quaternary, and a sandy clay unit, which forms the modern floodplain, was deposited. Deformation of the Middle Miocene basalt‐filled valley provides good evidence for substantial uplift of the highlands in the Middle‐Late Miocene, synchronous with or shortly after wide‐spread volcanism, but continuing no later than the Late Miocene. The Pleistocene incision is best accounted for by climate change to drier conditions, triggering a sediment‐starved response by the Macquarie River, rather than further uplift. While valley incision and subsequent infilling can be viewed as a response to tectonism, the internal sedimentology and stratigraphy of the sediments appears to respond to fluctuations in sediment supply brought about by extrinsic climate change and intrinsic catchment lithology. The first‐order tectonic events are not synchronous with uplift in the Lachlan valley and are restricted to relatively local spatial scales. However, the secondary climatic forcing has a broader regional expression.