Modern and palaeogeographic trends in the salinisation of the Western Australian Wheatbelt

Abstract

The Western Australian Wheatbelt has been essentially 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 eroded by rivers. The palaeo-valleys have been infilled with terrestrial and marine sediments, and been subject to ongoing deep weathering. During the Pliocene and Quaternary the region experienced alternating arid and pluvial climates. These cyclic episodes shaped the regolith and impacted on vegetation species and catchment water balances and also promoted the accumulation of massive volumes of salt. In more recent times, this stored salt has interacted with vegetation, soils, surface waterbodies and groundwater systems and left a distinctive and pervasive legacy in the landscape. Salinisation was manifest in the Wheatbelt of WA from as long ago as 2.8 Ma, concentrating in valley floors as arid and pluvial cycles prevailed and whilst the continent migrated northwards. Today, as agricultural development has altered the water balance on 20 M ha of cleared farmland, salinity is again on the move, further degrading the 300,000 ha of variably saline land that existed before the arrival of Europeans, and spreading across an additional 1.1 M ha of formerly non-saline land. Unchecked by reduced rainfall or human induced changes to the water balance, salinity may expand even further, potentially affecting 1.7-3.4 M ha of the Wheatbelt?s agricultural land and its unique and indigenous 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 determine whether salinity is likely to expand beyond extant palaeo-salinity markers. Finally, it considers the likely timeframes involved in the recovery of systems from primary salinity and whether clearing-induced salinity will follow patterns similar to those observed from past saline episodes in the region.