CROSSING GREAT SANDY – TWO GPR TRANSECTS ON THE WORLD’S LARGEST SAND ISLAND, FRASER ISLAND, QUEENSLAND AUSTRALIA

Fraser Island is the world’s largest sand island and sits at the northern end of the world’s longest down drift sand system. The sand that supplies the aeolian system, for development of coastal dunes with relief in excess of 240 m, is sourced from erosion of the Hawkesbury Sandstone, some 1,100 km south in New South Wales. The oldest aeolian sands in Great Sandy are dated to the first half of the Quaternary with preliminary dates exceed 1 Mya. The Island is a series of layered and/or shingled aeolian morphosequences that have been mapped by Ward (2006) and reinterpreted by Patton and Shulmeister (2019) based on surficial expression and inferred soil relationships.

During 2016, a series of GPR surveys were conducted on Fraser Island including two complete transects orientated roughly toward the NW. Transects began at the beach on the Coral Sea and extended over the island to beaches or low bluffs on Great Sand Strait or Hervey Bay. A MALA GX 160 MHz system was hand drug over the 18+ km long transects. Numerous shallow (< 5 m) sand auger cores were collected along transects to verify field interpretation and assist in understanding the 28 m deep GPR record.

GPR transects were correlated with the morphosequences of Patton and others (2019) and GPR facies were identified based on geophysical and radar stratigraphy. The facies were interpreted for depositional environment, soil development, post-depositional modification and palaeo-wind indicators.

GPR facies reflecting the soil units indicative of podsols (A/O, E, B, C) were identified, and associated with the active soils and numerous palaeosols. In the eastern sections, where the youngest morphosequences are located, radar reflectors and facies were related to parabolic dune deposition and potential former blowouts. Toward the west, as the age increases, the reflectors associated with primary depositional structures were replaced with stacked palaeosol sequences consisting of E-B-C with the A/O absent and the C difficult to differentiate, deeper in the radar section.

Surface expressions of morphosequences and mapped boundaries generally align well with the interpreted subsurface, however, as the age of the morphosequence increases, offset subsurface and surface expressions are observed with the surface expression generally further west of the subsurface interpretation.