Origins of Holocene coastal strandplains in Southeast Australia: Shoreface sand supply driven by disequilibrium morphology

• Holocene strandplain growth in southeast Australia was primarily driven by shoreface sand supply in response to disequilibrium morphology
• Steady rates of volumetric Holocene strandplain growth would have required a secondary driver from around 3 kyr BP onwards, such as alongshore sand supply and/or relative sea-level fall
• Late-Holocene relative sea-level fall could only have contributed a minor portion of strandplain growth through enhanced shoreface sand supply driven by forced regression
• Rates of volumetric strandplain growth and shoreline progradation may be inconsistent through time due to evolving shoreface geometry
• Shoreface sand supply may persist at subtle rates on some southeast Australian beaches promoting shoreline stability

Coastal barriers store depositional records of past environmental conditions, such as sea level, wave climate and sedimentary regime. The embayed highstand coast of southeast Australia features a diverse range of coastal sand barriers, suggesting varying depositional responses to Holocene environmental conditions. In particular, the varying chronologies of prograded-barrier strandplains along a passive margin, with a predominantly autochthonous sedimentary regime, raises questions about relative sea-level change, and sediment sharing within and between compartments during the Holocene. Here we apply detailed geological data and geochronology from the Holocene prograded-barrier system at Tuncurry, within a morphodynamic modelling approach, to investigate the depositional response of the coastal system to possible drivers of strandplain growth, including: (1) forced regression driven by mid- to late-Holocene relative sea-level fall; (2) time-varying external sand supply via the alongshore transport system; and, (3) shoreface sand supply in response to disequilibrium morphology and stable sea level. Comparison between the simulated depositional response of the coastal system and the geological records suggests that progressively weakening and depth-decaying shoreface sand supply, in response to disequilibrium morphology, was the primary driver of Holocene strandplain growth. Alongshore sand transport into the Tuncurry compartment via a headland-attached shelf sand body may have provided a secondary sand source, although the simulated barrier-shoreface evolution precludes a dominant external sand supply. Mid- to late-Holocene sea level-fall from a highstand level within the range of uncertainty in available indicators (1.5 m above present), could only have contributed a minor portion of strandplain growth, by process of forced regression. The simulations demonstrate the potential sensitivity of strandplain chronologies to the sampling location (i.e. shoreface or dune facies), which emerges from coupled barrier-shoreface evolution that may support time-decaying volumetric growth and steady shoreline progradation simultaneously. Shoreface sand supply driven by the ongoing relaxation of disequilibrium morphology may persist at subtle rates (1–2 m3/m/yr) today on some southeast Australian beaches, promoting shoreline stability, and potentially moderating initial shoreline response to sea-level rise.