Holocene sea-level change on the southeast coast of Australia: a review

A revised Holocene sea-level curve for the southeast coast of New South Wales, Australia, is presented based on a review of previously published geochronological results for fossil molluscs, organic-rich mud, mangrove roots and fixed biological indicators. It is supplemented by new radiocarbon and amino acid racemization-derived ages on fossil molluscs from transgressive sandsheet facies in back-barrier settings within shallow incised valleys along the southern coast of New South Wales. This data base has been limited to fossils with accurate descriptions of their facies associations and stratigraphic relationships to present mean sea level. Results show that sea level during the Holocene marine transgression rose to between −15 and −11 m at 9400—9000 cal. yr BP. Sea level then rose to approximately −5 m by 8500 cal. yr BP and to approximately −3.5 m between 8300 and 8000 cal. yr BP inundating shallow incised valleys resulting in the deposition of shell-rich transgressive sandsheets within shallow incised bedrock valleys. Present sea level was attained between 7900 and 7700 cal. yr BP, approximately 700—900 years earlier than previously proposed. Sea level continued to rise to between +1 and +1.5 m between 7700 and 7400 cal. yr BP, followed by a sea-level highstand that lasted until about 2000 cal. yr BP followed by a gradual fall to present. A series of minor negative and positive oscillations in relative sea level during the late-Holocene sea-level highstand appear to be superimposed over the general sea-level trend. However, the precise nature of the oscillations are difficult to quantify because of problems associated with accurately determining palaeotidal and wave regimes, climatic conditions and the antecedent morphology of the shallow marine environments during the mid Holocene.