Rates of Shoreline Progradation During the Last 1,700 Years at Beachmere, Southeastern Queensland, Australia, Based on Optical Ages of Beach Ridges

Updated: 2018-04-22

The morphology and chronostratigraphy of seven beach ridges at Beachmere, southeastern Queensland, Australia, provide a record of changes in shoreline accretion and relative sea level over the last 1,700 years. Optical dating of beds of pebbly sand that form the inner 7th ridge reveals that 1,700 -140 yrs ago relative sea level at the site was approximately 1 m higher than at present. During the subsequent 600 yrs, the coast prograded at ~ 0.16 m yr-1 until ridge 6 was emplaced 1,140 ?80 yrs ago; this increased to a rate of ~0.40 m yr-1 as ridges 5 (optical age: 860 ?70 yrs) to 1 (140 - 50 yrs) were emplaced. A relatively wide (~200 m) intertidal to supratidal flat between ridges 4 (690 - 60 yrs) and 3 (190 ?40 yrs) marks a change from well-defined and regularly spaced ridges, 7 - 4 (~ 95 m apart), to lower amplitude closely spaced ridges, 3 ? 1 (< 33 m apart). Ridges 3 ? 1 have Optical ages that overlap within their 1 ? uncertainty and probably represent a rapid phase of shoreline accretion related to an increase in sediment supply associated with European settlement. The optical age of ridge 1 (140 ? 50 yrs), which sits immediately behind the modern beach, provides a maximum age for the onset of the modern erosional regime at the site.