Late Quaternary channel and floodplain formation in a partly confined subtropical river, eastern Australia

Abstract

ABSTRACTAlong the eastern margin of Australia, hydrological variability reaches a peak in the subtropics of south‐east Queensland and many rivers have entrenched characteristics. To address the nature of entrenchment and the relationship with adjacent alluvium, this paper presents the results of detailed chrono‐stratigraphic analysis of alluvial units in the partly confined mid‐reaches of Lockyer Creek, Australia. Four sites were investigated using topographic, sedimentological and chronological data. Radiocarbon and single grain optically stimulated luminescence dating indicate a large proportion of the valley fill reflects a major phase of aggradation of fine‐grained alluvium from ca. 35 ka throughout the Last Glacial Cold Period. Synchronous incision of Pleistocene alluvial fills between 11.5 and 9.3 ka suggests the current entrenched Lockyer Creek formed in response to changes in late Quaternary climate. Holocene floodplains set within the entrenched Pleistocene valley floor have basal ages of ca. 7.5 ka, but whose proximal margins are still actively accreting. This Holocene fill has accreted over the mid‐ to late Holocene but overlaps with the contemporary hydrological regime. The sedimentary nature of the Holocene fill appears to be related to persistent antecedent controls in the form of bedrock and terrace constriction.