Formation of Di- and trioctahedral smectite as evidence for paleoclimatic changes in Southern and Central Peninsular India

Abstract

Di- and trioctahedral smectites in saprolites, developed on Precambrian gneiss and Gondwana sediments in southern and central Peninsular India, indicate a climatic change from tropical humid to semi-arid during the Plio-Pleistocene transition period. Well crystallised dioctahedral smectite was the first weathering product of Peninsular gneiss, and was subsequently transformed to kaolinite in ferruginous soils formed in a pre-Pliocene tropical humid climate. When the humid climate terminated both these clay minerals were preserved to the present. Silt and clay size trioctahedral smectite (i.e. low-charge vermiculite) in these saprolites is regarded as the alteration product of biotite formed in arid climate since the Plio-Pleistocene transition period. We conclude that the ferruginous soils on Peninsular gneiss and Gondwana sediments are relict paleosols, but are polygenetic because of their subsequent alterations.