Transformations of Soil Materials Under Semi-Arid Conditions In Valley Floors In S.W Australia.

The south west of Western Australia is characterised by an ancient deeply weathered landscape of low relief formed under humid conditions and which is now experiencing a semi-arid climate.  The region is largely underlain by Archaean granitic and gneissic rocks that have been altered to deep laterite profiles that are now highly dissected by erosion. The main weathering products are kaolinite and iron oxides.  However carbonates, gypsum and other evaporite minerals are now present in the regolith having accumulated from ground waters in valley floors under the current climate.

 

Valley floors contain diverse arrangements of alluvium, colluvium and residual lateritic regolith together with evaporite deposits.  Most regolith materials show evidence of alteration by solute-rich groundwater under the semi-arid conditions that have existed in the region for the past million years.  Although kaolinite is the dominant clay mineral; in some cases these sediments have been replaced or impregnated by calcite and dolomite with illite, palygorskite or smectite now being dominant clay minerals in a dolomite matrix.  Scanning electron microscopy with EDS of thin sections enables mineralogically distinct materials to be identified and delineated enabling recognition of the types and episodes of alteration.  Some valley floor regolith remains dominated by kaolinite with very little variation in matrix material.  For these materials, poorly sorted angular quartz and feldspar suggest limited transport of saprolite material with little subsequent alteration.  However goethite and hematite have precipitated at redox fronts or texture boundaries throughout the matrix.  Often matrix material is a mixture of clay minerals; commonly kaolinite, smectite and illite with relic peds of pure kaolinite persisting within the mixed clay mineral matrix, similarly clay peds persist in some calcareous regolith.  This paper considers possible sequences of conditions and events in this landscape as evidenced by soil morphology and mineralogy.