Age and origin of alluvial sediments within and flanking the Mt Lofty Ranges, southern South Australia: a Late Quaternary archive of climate and environmental change

Quaternary alluvial sediments occur within and on the flanks of the Mt Lofty Ranges of southern South Australia. Within the ranges they occupy colluvium-filled bedrock depressions, alluvial-fan sequences at hill/plain junctions and river terraces that flank major streamlines in both locations. Sediments ranging in age throughout the Quaternary have been identified, but this paper focuses on those deposits of Late Quaternary age. Luminescence dating has verified a Last Interglacial age (132-118 ka) for the most widespread of the alluvial units, the Pooraka Formation. A younger, Marine Isotope Stage 3, alluvial unit, in places containing bones of the extinct marsupial Diprotodon, has also been identified. Deposition of the alluvial sediments is associated with relatively warmer and wetter conditions, whereas the valleys that they occupy were eroded under drier climatic conditions. A more widespread occurrence of Stage 3 units is expected to be present but has not yet been verified. Cold, arid environments are inferred from the presence of dunes (18 ka) deposited during the Last Glacial Maximum when stream valleys were incised. Grey/black mid-Holocene alluvium (Waldeila Formation), forming present-day floodplains and low river terraces, equates with the Holocene Hypsithermal. The sequence of climatic changes revealed by these sediments is correlated with evidence of Late Quaternary climatic change from other Australian locations. The identification of equivalent units in different tectonic settings reveals that sedimentation is largely climatically driven although active tectonism may accelerate the supply of sediments available for transport.