The Cause of Breaks in Holocene Beach Ridge Progradation at Bengello Beach

Abstract

Based on the combination of sediment dates, changes in topography and stratigraphic changes evident in ground-penetrating radar (GPR), breaks in Holocene beach ridge progradation at Bengello Beach occurred at 6000, 5000, 3500, 3000, 2500 and 1500 cal BP. The use of GPR to visualise the subsurface stratigraphy identified changes in the elevation of a shelly deposit, located at approximately the present sea level, indicating changes in sea level of up to 1 m at those times. The timing of these sea level changes correlate closely to local and international dates for oscillations in sea level. The Bruun Rule was used to model the barrier evolution of Bengello Beach in response to previously proposed alternative sea level oscillations. The results indicate that the sea level oscillations are likely to have contributed to the formation of the breaks in progradation. However, the precision of the Bruun Rule is likely to have an error factor associated with it. Thus, the Bruun Rule should still be used with caution when predicting the quantitative effects of future sea level rise on the shoreface.

The GPR profiles also show that many of the break locations are associated with scarps up to 5 m high. The presence of these large scarps indicates the likelihood of storm wave erosion at those times. However, given that they are highly correlated, it is not possible to isolate the sea level changes from the wave climate changes across the barrier. It appears that the combined effects of sea level and wave climate anomalies contributed to the periods of erosion throughout the Late Holocene. These periods of erosion are comparable to the events observed in the 1970’s along the New South Wales coast, which were also attributed to sea level anomalies and wave events.