Transition from marine to fluvial-dominated sediment supply at Shoalhaven prograded barrier, southeastern Australia
Prograded barriers are depositional coastal landforms which have the potential to reveal changes in the primary drivers of coastal evolution within their varied morphology. Beach ridges and intervening swales preserve paleoenvironmental records of coastal processes, relative sea level and storm events. Optically stimulated luminescence (OSL) dating of quartz grains, airborne LiDAR-derived morphology, and sediment texture and mineralogy reveal four different periods of morpho-sedimentary progradation history in the Shoalhaven barrier system in southeastern Australia. The barrier is composed of approximately 40 ridges that occupy an area of 15.2 km2, comprising an estimated sand volume of approximately 88,000,000 m3 above mean sea level. OSL dating of ten samples taken from a 940-m long transect across the Holocene system indicated that the barrier prograded at a slow rate of approximately 0.12 m/yr from 6130 ± 330 to 2400 ± 130 years ago and subsequently at a higher rate of 0.22 m/yr until 600 ± 130 years ago. More recently, an increase in historical accumulation and progradation rates has favoured development of an anomalously high foredune fronting the system with the formation of lower ridges in the past two centuries. Increasing angularity and feldspar content observed via Scanning Electron Microscope (SEM) and determined using X-ray Diffraction (XRD) analysis, respectively, imply a transition in sediment supply. Progradation has been sustained through delivery and reworking of marine sediments from offshore following the marine transgression, subsequently augmented by fluvial sands discharged to the coast by the Shoalhaven River. The adjustment in progradational rates and sediment provenance influenced the morphology and spacing of individual ridges and the regressive system as a whole. Average progradation rates for the Shoalhaven barrier, revised from those previously reported using radiocarbon dating, are considered lower than most of barriers studied in similar coastal environments around Australia, indicating the different ways that similar progradation systems have evolved.