The end stage of barrier estuary infill: Insights from Jack Smith Lake, Southeastern Australia

Abstract Jack Smith Lake in southeastern Australia is an infilled saline coastal lake, disconnected from the ocean by a foredune barrier. This study explores the Holocene infill history of this estuary in order to understand the end‐stage evolution of barrier estuarine systems with negligible fluvial input. Geomorphological mapping reveals the lake is bounded by a double beach barrier sequence, with a central mud basin at mean sea level elevation. Sediment cores show the basin infilled vertically, with a basal shelly gravel unit (6,220–5,460 cal BP) overlain by a dense shell hash (4,240–4,050 cal BP) and capped by muddy sands (2,810–2,330 cal BP). Entrance closure occurred around 2,500–2,000 cal BP, corresponding to a regional phase of barrier progradation which was likely driven by falling sea levels and increased sediment supply. The infill sequence follows the classical model of marine‐dominated sedimentation, but without transitioning to a fluvial floodplain. Entrance closure terminated an abundant estuarine shellfish habitat, necessitating a shift in subsistence practices by Indigenous Gunai/Kurnai peoples from estuarine molluscs to surf beach bivalves. Jack Smith Lake exemplifies the end‐stage ‘saline coastal lake’ classification, highlighting the importance of defining such systems by their sedimentology and geomorphic evolution rather than salinity.