The geomorphological evolution of a wave-dominated barrier estuary: Burrill Lake, New South Wales, Australia

The geomorphological evolution of the Holocene wave-dominated barrier estuary at Burrill Lake on the New South Wales coast, Australia, has been delineated using a combination of seismic stratigraphy and the lithostratigraphic analysis of vibracores collected from the back-barrier estuarine environment. A combination of radiocarbon and aspartic acid racemisation-derived ages obtained on Holocene fossil molluscs, and the thermoluminescent signal in remnant Last Interglacial barrier sediments provides the chronological framework for this investigation. Results from this paper show that the barrier estuary occupies a relatively narrow (\u3c1.5 km wide) and shallow (\u3c40 m deep) incised bedrock valley formed during sea-level lowstands. Late Pleistocene sedimentary successions and remnants of the Last Interglacial barrier have been preserved within the incised valley axis and the mouth of the incised valley. These sediments, deposited during the Last Interglacial sea-level highstand, have subsequently been partially removed during the last glacial maximum. Overlying the antecedent late Pleistocene landsurface is a near basin-wide basal marine sand deposited in response to rising sea level associated with the most recent post-glacial marine transgression, which inundated the shallow incised valley ca.7800 years ago. More open marine conditions, with a diverse assemblage of estuarine and marine mollusc species, persisted until ca. 4500 years ago when the stabilizing Holocene barrier resulted in the development of a lowenergy back-barrier lagoonal environment. A late Holocene 1-2 m regression of sea level ca. 3000 years ago further restricted oceanic circulation, increased the rate of fluvial bay-head delta progradation and the extension of the backbarrier central basin mud facies. This evolutionary model of barrier estuary evolution developed for Burrill Lake is consistent with recent research conducted in Lake Illawarra and St Georges Basin and can be applied to other estuaries that have formed in relatively shallow and narrow incised bedrock valleys on tectonically stable, wave-dominated coastlines