ELECTRICAL RESISTIVITY AND GROUND PENETRATING RADAR OBSERVATIONS OF PALAEOCHANNEL COMPLEXES, MURRAY-DARLING BASIN, AUSTRALIA

The Murray - Darling River system captures a drainage basin that spans > 1,000,000 km²(14% of the Australian continent) and is located in the south central portion of Australia. Its major rivers begin in the Australian Alps and flow west and south from sub-humid highlands onto semi-arid plains, discharging into the Southern Ocean near Adelaide, S.A. The present-day tributaries occupy small channels within a much larger meandering channel belt that has presented a classic palaeohydrological problem since Schumm first studied the system in the 1960s. Published and preliminary OSL ages suggest the largest channels date to late MIS3 (35-25 kya) while channels dating to the Last Glacial Maximum (18-22 kya) have proved difficult to identify and describe.

This project applied geoohysical techniques, including electrical resitivity/conductivity and ground penetrating radar to elucidate the stratigraphic architecture and context of palaeochannel belts assoicated with the Lachlan and Murrumbidgee Rivers. Geophysical data was acquired parallel to existing core transects and undated scroll plains. A total of 20 km of EM and 3 km of GPR were collected during the 2014 Austral winter. The resultant geophysical sections were correlated to lithology, interpreted and constrained by OSL chronologies.

GPR proved ineffective on the Lachlan River due to clay-rich sediments at the surface but was very useful at examining the upper several meters of the Murrumbidgee scrolled meander plains. Conversely, the EM proved effective in both systems at elucidating the larger scale lithological units associated with flood plains, channels, point bars and channel fill. Additional work is required to fully constrain the interpretation of the EM due to deeply weathered and dry sediments.

Overall, the integrated sections reveal a complex subsurface architecture buried beneath a variable thickness of modern overbank fine sediment with the EM capable of mapping paleaochannel systems at depths of at least 6 m. These types of data will assist in defining the characteristics of LGM channels and discerning phases of channel development. Ultimately, these results will support palaeohydrological modeling of runoff and snow melt in the Australian Alps associated with distinct channel phases associated with Last Glacial Maximum and later environments.