A 1500 year record of coastal sediment accumulation preserved in beach deposits at Keppel Bay, Queensland, Australia

Abstract

The coastal plain formed by a series of relict beach ridges on the margin of Keppel Bay, central Queensland, Australia, preserves a detailed record of coastal sediment accumulation. Importantly, this record spans the historical period and extends back to early Holocene times, well before European settlement. The relict and modern beach ridges comprise fine sand that was originally deposited in Keppel Bay during flood discharge events of the Fitzroy River. This sediment was then reworked onto the shoreline by the prevailing tide, wave and wind-generated currents. Optically stimulated luminescence (OSL) ages of the ridges reveal a long-term positive coastal sediment budget in Keppel Bay and show that during approximately the last 1500 years, sets of beach ridges were emplaced in rapid episodes, up to a few decades long. The OSL ages of the ridges suggests there is a 500-200 yr periodicity to these phases of rapid sediment accumulation. Our preliminary interpretation of these data is that this pattern of formation relates to the recurrence interval of high magnitude flood discharge events of the Fitzroy River and there has been a general decline in the rate at which sediment has accumulated on the Keppel Bay coast during the last 1500 years. This reduction may reflect a long-term decline in major rainfall events in the Fitzroy River catchment. The trace element composition of ridges deposited during the last 100 yrs indicates there has been a greater contribution from areas of basaltic soils within the catchment. This trace-element record reveals the impact on river and coastal sediment composition brought on by changes in catchment land use.