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 in filled 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.