Relative sea-level records preserved in Holocene beach-ridge strandplains - An example from tropical northeastern Australia

Northeastern Queensland is a far-field site from ice caps formed during the Last Glacial. However, the region has experienced glacio-hydroisostatically driven coastal elevation change during the Holocene that generated a distinctive relative sea-level record. We tested whether this subtle vertical movement of the coast is recorded in the morphology and stratigraphy of the beach-ridge strandplain at Cowley Beach, which comprises a series of 36 prominent and several subdued or truncated beach ridges. Previous studies have shown that the strandplain records the long-term progradation of the coast, with the ridges comprising coarse-medium sand that was deposited during both fairweather (foreshore and berm facies) and tropical cyclone events (erosional unconformities; storm-surge deposits) over the past approximately 7 ka. We utilised a fine-scale digital elevation model of the strandplain to examine beach ridge morphology and to compare the elevation pattern with published Holocene sea-level data for the region. The overall elevation pattern of the strandplain surface is consistent with the regional sea-level history, that includes a highstand at around 6 ka BP generated by isostatic uplift, and the elevation of the berm facies matches well robust coral sea-level indicator data. We also modelled the strandplain topographic patterns that would result under stable and continually rising Holocene sea-levels. The frequencies of elevation classes across the strandplain are normally distributed when the strandplain is deposited during a period of stable sea level, and negatively skewed with a continually rising sea level, consistent with the expected patterns. The results indicate the sensitivity of the elevation of the top of the berm facies preserved in beach ridges to subtle changes in sea level (tens of cm over a thousand years). Our results further underline the utility of beach-ridge strandplains, a globally distributed coastal landform, for building relatively detailed regional records of sea level during the Holocene at sites currently lacking this information.