The sands of time: A long term record of coastal dune formation and evolution at the Cooloola Sand Mass.

Abstract

The coastal dune archives of SE Queensland represent one of the largest, oldest and most complex dune field systems on Earth. They are also a unique archive of change in the sub-tropics, an area under-represented in the paleoenvironmental literature. Of these dune fields, the Cooloola Sand Mass (240 km2) is known to contain an exceptionally complete archive of dune activation and is world renowned for its podzol soil sequence and primary succession sequence. These made it a prime target for investigation. This thesis presents a new optically stimulated luminescence chronology for the Cooloola Sand Mass. This is supported by morphological mapping of the dune field, limited ground penetrating radar surveys and stratigraphic logging of cliff sections.In total, 52 OSL dates were obtained that span the middle Pleistocene to the latest Holocene. An extensive auger transect was undertaken across the dune field and the cliff exposure along Rainbow Beach was logged in detail.Younger dune activity was investigated at Carlo Sand Blow. Repeated Late-Holocene reactivations were recognised. Dune activity was shown to be largely driven by fire events and in this landscape the fires are likely to relate to aboriginal occupation. The importance of fire for the reactivation of a single blow-out contrasts with the primary role of sea-level change in driving wider dune field activations and emphasises the importance of scale in dune studies.Morphological mapping of the dune sequences was conducted to identify and delineate the major periods of dune building in the dune field. This high-resolution LiDAR based mapping involved classification of dunes using morphometric characteristics and allowed for a better resolved dune field map than was possible with older technologies. Nine phases of dune building that has occurred periodically since the middle Pleistocene. Five Holocene and four Pleistocene phases were recognised. The OSL dating identified distinct periods of activity at ca. 800 ka, 140 – 150 ka, 110 – 120 ka, 10-6 ka, 5-3.5 ka, 2.4-1.9 ka and 0.4-0.2 ka. Late Pleistocene (Marine Isotope Stage 5) and Holocene dune activity coincides with post-glacial marine transgressions which is consistent with dune formation during rising sea levels. This evidence strongly supports the Cooper-Thom hypothesis of coastal dune formation, which focuses on sediment release due to shoreface erosion during rising sea-levels. A stratigraphical and geochronological investigation was also conducted along the Rainbow Beach cliffs. The results of this investigation produced the oldest OSL ages from the Cooloola Sand Mass ever recorded. The earliest period of dune activity at Cooloola was dated to >900 ka, with magnetostratigraphy used on novel target materials to confirm the OSL chronology. The earliest phases of dune activity at the Cooloola Sand Mass were very different to the late Pleistocene dune fields. The latter were dominated by parabolic dunes whereas the former represent a set of transgressive sheets. This earlier style is attributed to a massive influx of sand to the modern dune field when enhanced sea-level change at the Middle Pleistocene Transition (MPT), released sediment stored offshore and drowned the coastal lowlands in sand. The MPT is widely recorded in marine records but the formation of the Cooloola Sand Mass is a unique and world class terrestrial archive of the geomorphic impact of the MPT.This thesis has substantially advanced the understanding of the formation of coastal dune fields in eastern Australia and the findings are likely to be widely applicable in passive margin settings worldwide. It has also provided new information about the Cooloola Sand Mass, which should assist in the nomination of the dune field as a new World Heritage Area. It is hoped that the mapping and chronology can re-invigorate coastal dune research in Australia.