Erosional modification of a lateritized landscape over sedimentary rocks. Its effect on soil distribution

Abstract

Erosional modification of a landscape, originally mantled by laterite, has been studied in the Perth Sedimentary Basin of Western Australia. A basis for understanding the soil pattern has thus been provided. This pattern depends on the variation in parent material resulting from the differential stripping of the lateritic mantle which in turn is related to a sequence of changes from a gently undulating upland to broad valleys. Yellow sands, derived by colluviation from the surficial horizons of the lateritic mantle, dominate the upland surface. Lower down yellowish brown and reddish brown sands, having as their source the deeper horizons of the laterite, occupy areas of increased relief. Along the sides of broad valleys, brown and dark red sands dominate areas downslope from ferruginous sandstone exposed as a consequence of even more effective erosion. The laterite mantle appears to have been developed in a landscape already showing relief including a system of valleys. Evidence suggests subsequent uplift. However, rejuvenation of the drainage system consequent on this has had only very limited influence on the area. Rather, it is to climatic fluctuation that erosional processes have responded. The magnitude of these processes is related to landscape features, notably the sequential changes up along this ancient valley system.