The levels of intertidal environments of prograding Holocene coastal complexes of northeast Spencer Gulf have been precisely measured and their relation with the tidal range determined. Beach ridges and the top of subtidal Posidonia sea-grass deposits are the best indicators of relative sea level. Bases of active beach ridges range from + 1.1 m to + 1.8 m A.H.D. (Australian Height Datum) and the upper limit of Posidonia growth is <-2 m A.H.D. The elevation of old preserved examples of these facies has been established along transects across intertidal and supratidal plains. 14C dating has been carried out on unaltered mollusc shells recovered from beach ridges and the base of the regressive intertidal facies above the subtidal sea-grass facies. Sea-grass root fibres and shell hash from the top of the buried subtidal facies have also been dated. A Holocene sea-level history has been constructed for north-east Spencer Gulf, and shows the following events: 6000- 4000 years B.P. - probable construction of a shingle ridge at + 3-4 m at the peak of the Holocene transgression, followed by about 1 m fall of sea level, but little progradation, owing to low rates of carbonate production; 4000-3000 years B.P. - formation of regressive carbonate shorelines with fall of relative sea level from about +2 m to about + 1.5 m, and the construction of beach ridges; 3000-2000 years B.P. - no beach ridge construction along surveyed transects, but continued progradation of shorelines with a further 1.0 m fall in relative sea level; 2000 years B.P. to Present - construction of beach ridges accompanied shore-line progradation, and a fall in relative sea level of 0.5-1.0 m. A contributing cause of relative movement of sea level in the region may be continuing tectonic uplift, as evidenced by the high seismicity of the adjacent Flinders Ranges.
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