Clay Mineralogy and Major and Trace Element Geochemistry of Recent Sediments in Rivers Along the West Coast of India: Implications for Provenance and Chemical Weathering

The clay mineralogy and major and trace element geochemistry of the sediments deposited at the lower reaches of 90 medium and minor rivers from five states along the west coast of India indicate distinct clay mineral assemblages in the Archean–Proterozoic (A-P) terrain and Deccan Trap (DT) terrain. The sediments from A-P terrain are dominated by kaolinite, with minor illite and gibbsite and traces of goethite, and those from DT terrain are dominated by smectite with minor illite, kaolinite and chlorite. The sediments are depleted of Si, Ca, Mg, Na and K relative to those of Post-Archean average Australian Shale. The SiO2/Al2O3 ratio of the sediments suggests lateritic soils in the A-P terrain and non-lateritic, chemically weathered soils in the DT terrain. Weathering indices indicate strong weathering in the clay fractions of all sediments. The silt fractions of sediments from Goa, Maharashtra and Gujarat exhibit intermediate to weak weathering and influence by hydraulic sorting processes and source rock characteristics. The total trace element content (∑TE) was higher in the silt fractions than in clay fractions of all sediments, and peaks of high ∑TE occur in the silt fractions of Kerala and Maharashtra. The silt fractions exhibit relatively high Th, U, La, Zr and Hf from A-P terrain, and high Sc, Cr, Co, Ni, V and Ga from DT terrain. The Th/U and Rb/Sr ratios are controlled by the intensity of weathering and lithology of source rocks. The standard plots using trace elements reveal that the clay fractions of sediments are more mafic from both the terrains, while silt fractions exhibit intermediate provenance between felsic and mafic sources. Since mafic component-dominated clays are transported to the adjacent seas and oceans, it would be a challenge to identify the provenance of clays from granitic terrain in the oceans using trace element chemistry.