Geomorphic Evolution and its Significance in Exploration Planning of the East Coast Bauxite Deposits of India

The East Coast Bauxite deposits top either the Khondalite or Charnockite Group of Archaean rocks that constitute the bulk of the Eastern Ghat physiographic province. Subsequent to the Eastern Ghat orogeny (1400-1600 m.y.), faulting in Mesozoic was the major tectonic event recorded in the Indian Peninsula.

The Eastern Ghat terrain was subjected to uninterrupted planation in the intervening period. The attainment of a clear macro-relief instead of a featureless plain (peneplain) is visualised as a consequence. This macro-relief, possibly modified to some extent due to Mesozoic faulting, after attaining stability and with the onset of monsoonal climate in upper Cretaceous promoted the process of residual chemical weathering. Chemical leaching was accentuated and optimised during Upper Tertiary-Quaternary period when there was copious rainfall with less of evapo-transpiration, resulting in thick bauxitic duricrusts. This view is at variance with the concept of an once extensive residuum formed at lower levels and what are now seen at high altitudes are their erosional relics, owing their elevation and discontinuous nature to post-residuum uplift followed by denudation. As the gibbsitic duricrusts formed insitu have not been disturbed later, an in-depth study of their geomorphic characteristics, especially the slope morphology, assumes added relevance as it aids geological prognosis of the behaviour of the pay zone in terms of grade and thickness fluctuations. Once the geomorphic evolution of the duricrusts has been logically reconstructed, a conceptual approach instead of a regular grid for exploration planning is considered more pragmatic not only for acquiring a geologic grip but also in minimising exploration costs.