Richard George

Modern and palaeogeographic trends in the salinisation of the Western Australian wheatbelt: a review

The Western Australian wheatbelt contains vast areas of agricultural land underlayed by saline and deeply weathered regolith derived from Archaean rocks and recent sediments. The region has been geologically stable since the late Permian, although the Archaean basement sustained some movement during the break-up of Gondwanaland and the northward drift of Australia from Antarctica. During the Early Cretaceous, Eocene and more recently, the wheatbelt region’s weathered mantle has been successively eroded by rivers. The palaeovalleys have been infilled with terrestrial and marine sediments, and subjected to ongoing deep weathering. During the Pliocene and Quaternary the region experienced alternating arid and wetter climates. These cyclic episodes influenced regolith development, affected vegetation species and catchment water balances, and also promoted the accumulation of massive volumes of salt. In more recent times, these salt stores have interacted with vegetation, soils, surface water bodies, and groundwater systems and left a distinctive and pervasive legacy in the landscape. Salinisation was manifest in the wheatbelt from as long ago as 2.8 Ma, concentrating in valley floors as arid and wetter cycles prevailed and while the continent migrated northwards. Today, agricultural development has altered the water balance on 20 Mha of cleared farmland. As a result, salinity is spreading, further degrading 300 000 ha of variably saline landscape that existed before the arrival of Europeans, and affecting an additional 1.1 Mha of formerly arable land. Unchecked by reduced rainfall or human-induced changes to the water balance, salinity may expand even further, potentially affecting 1.7–3.4 Mha of the wheatbelt’s agricultural land and its unique natural resources. This paper reviews the palaeogeography and palaeoclimates of the region and its hydrogeology and examines the nature of its susceptibility to salinisation. It poses questions about the relationship between palaeo-salinity and contemporary salinity, seeking geomorphic evidence to indicate whether salinity is likely to expand beyond extant palaeo-salinity markers. Finally, it considers the likely timeframes involved in salinisation and whether clearing-induced salinity will follow patterns similar to those observed from past saline episodes in the region.

Airborne Geophysics Provides Improved Spatial Information for the Management of Dryland Salinity

Biophysical information forms the technical base for farm planning and landscape management. Such information is required at spatial scales which are suited to the application of the user. Within the past five years, two catchments in the South West of Western Australia were studied to assess the contribution that airborne geophysics data (specifically electromagnetics, magnetics and radiometrics) makes, in conjunction with existing datasets, to the understanding and management of land degradation, principally dryland salinity. The Towerrinning (30 000 ha) and Toolibin surveys (70 000 ha) are located in areas of intensely weathered Archaean granitic terrain in a low gradient landscape. At both sites over 80 bores were drilled and extensive ground geophysical surveys were undertaken as a part of a formal process of calibration and landscape investigation.

The application of AEM to mapping sea-water intrusion at La Grange, WA

We describe interpretation of an AEM survey around the La Grange allocation area, WA. This survey was designed to map aquifer bounds and the sea water intrusion, and then to assess groundwater in the region, and to facilitate planning water use. The simple, stratified nature of sediments of the western onshore Canning Basin allowed us to use blocky layered earth models and we found that five-layer models were the most parsimonious. After deriving surfaces representing the top of the Jarlemai siltstone and the top of the sea water ingress, we were able to effectively characterise the spatial characteristics of the sea water intrusion. We found that in places, sea water intruded 40 km inland, and could be found at a depth of over 250 m.