Geology and Geochemistry of Tertiary Clay Deposits in South Kerala

The sedimentary clay deposits in south Kerala are the source of excellent quality kaolinite, formed due to weathering of arkosic sediments of Tertiary and crystalline Precambrian rocks. It has been possible to differentiate two such kaolinite beds in south Kerala based on mineralogy, chemistry and field relationship. At places, the two kaolinite beds are separated by sandstone-clay intercalations. The upper clay bed contains higher amount of impurities of quartz, goethite, hematite and gibbsite. The concentration of these impurities decrease at depth and vary horizontally having no consistent pattern. Study of sedimentary structures within the kaolinite beds, presence of kaolinite pseudomorph of feldspar still maintaining crystal faces, and lignite bed, support the hypothesis that the beds were deposited after a short and rapid transportation in a fluctuating flow regime. The lower elay bed, underlying the sandstone, is impoverished in both quartz and iron minerals. The source material for this clay deposit is the pre-Miocene weathered regolith extensively developed on Precambrian chalnockite- khondalite suites of rocks. After burial, ferric iron oxide from these beds was leached away in a reducing environment. The variation of mineral content and its chemistry in the kaolinite beds, have been related to spatial difference in micro-environment in the stratigraphic horizons in the geologic past and at present.