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

Abstract

The geomorphological evolution of the Holocene wave-dominated barrier estuary at Burrill Lake on the New South Wales coast, Australia, has been investigated using a combination of seismic stratigraphy and 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 the Last Interglacial barrier provides the chronological framework for this investigation. Results from this paper show that the barrier estuary occupies a relatively narrow (< 1.5 km wide) and shallow (< 40 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 at the mouth of the incised valley. These sediments, deposited during the Last Interglacial sea-level highstand, were 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 emerging Holocene barrier resulted in the development of a low-energy 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 back-barrier central basin mud facies. The 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.