Quartz-dominated Cenozoic gravels in southeastern Australia: where did all the quartz come from?

Upper Cenozoic fluvial gravels in the Canberra region are quartz-dominated (>95 vol% quartz), whereas modern bedload sediments contain less than 50 vol% quartz and a range of lithologies that broadly reflect the catchment geology. Potential reasons for the compositional difference include changes in catchment geology, sediment transport distance, post-depositional weathering and regolith history. Breakage of unsound clasts during transport does not explain the difference because sediment transport distances in the Cenozoic were similar to today. Post-depositional weathering of less resistant clasts is ruled out because the Cenozoic gravels are clast-supported and preserve original bedding. During much of the Cenozoic, rainfall was significantly higher than the present, and rainforest species were present in the local and regional vegetation. Weathering regimes were likely very different from today, and deep weathering of catchment lithologies may have favoured the supply of quartz-rich gravelly sediment to the river systems.