Paleoshorelines and lowstand sedimentation on subtropical shelves: a case study from the Fraser Shelf, Australia

Abstract

Evidence of submerged paleoshorelines on tectonically stable continental shelves is represented by relict coastal depositional features that formed during periods of lower sea level. This study investigates two paleoshoreline features that extend 70 km from the Barwon Bank along the southeast Fraser Shelf, off eastern Australia. They are defined by well-lithified, mixed carbonate-siliciclastic deposits now at water depths of 60 m and between 80 and 100 m, located approximately 40 km seaward of the modern shoreline. High-resolution multibeam bathymetry data show geomorphic features that we interpret as paleodunes in the case of the 60 m feature, and paleobarriers in the case of the 80–100 m feature. Sub-bottom profiles also show evidence of prograded beach foresets and paleochannels as components of the Barwon Bank, which is a low and elongate bank that rises about 30 m above the surrounding shelf to a depth of 60 m. Carbonate cements in the rocks forming the paleoshoreline features reveal diagenetic influence of meteoric waters, suggesting that subaerial processes contributed to the formation and preservation of both features. Radiocarbon dating of bioclastic grains (coralline algae, benthic forams) yields an age range between 22.8 and 20.4 ka for the paleobarrier, and an age of 17.3 ka for the paleodune. The morphology of the submerged paleodunes is analogous to modern parabolic dunes on nearby Fraser and Moreton islands. The position and morphology of the paleodunes provide proxy information about the climate history of Australia during the late Quaternary, as these dunes are a likely consequence of the general continent-wide aridity during the Last Glacial Maximum. The preservation of the eolian dunes during marine transgression, despite their direct exposure to coastal hydrodynamic processes, suggests that the dunes were armoured, stabilised and lithified during the glacial lowstand, prior to drowning.