Aggradation and incision since the very late Pleistocene in the Naas River, south-eastern Australia

Abstract

Past phases of aggradation and incision have been studied along a 10 km reach in the Naas Valley in south-eastern Australia. Detailed mapping of the stratigraphy and an ambitious dating exercise, involving 23 optical dates, have been used to distinguish the different periods of aggradation and incision. The dated alluvial sequence shows that a period of aggradation occurred in the very late Pleistocene (ca. 14,000-12,000 years ago). Alluvial deposits are absent for the period 12,000-3300 years ago. Whether this truly reflects no deposition or a series of aggradation and erosion cycles remains unresolved. Aggradation dominated between 3300 and 900 years ago, punctuated by a short incision event around 1300 years ago. Gully erosion contemporaneous with this incision phase is also recorded. Incision has dominated during the last 900 years, cutting down to bedrock. This incision, which is still ongoing, has not been a continuous process, but involved short periods of aggradation. The exposed bedrock and large boulders display numerous scour holes indicating that bedrock has been exposed and abraided for lengthy periods in the past. The aggradation and incision cycles in the late Holocene gave rise to three groups of terraces, today visible along the study reach. Possible causes for the different periods of erosion and deposition are discussed in the context of late Pleistocene and Holocene climate change, land use impacts, and intrinsic factors.