Composition of river sediments from Kerala, southwest India: Inferences on lateritic weathering

The sediments at the lower reaches of 21 rivers from Kerala were investigated to understand the influence of lateritic weathering on their mineralogy and geochemistry. Kaolinite followed by gibbsite and quartz were present as major minerals and, illite, goethite and mixed layered clays as traces in the sediments. The sediments were enriched with Al, Fe, Ti, Mn and P and depleted with Ca, Na, Mg, K and Si relative to that of the Post-Archaean average Australian Shale (PAAS). Weathering indices indicated intense chemical weathering on source rocks. The SiO2/Al2O3 and Fe2O3/Al2O3 ratios and intensity of lateritisation revealed that the sediments represent 'kaolinitised to weakly lateritised' soils. The mean total-trace element (Sigma TE) content was lower in the clay fraction but higher in silt fraction than in PAAS. PAAS-normalized trace element distribution showed depletion of Rb, Sr and Cs and enrichment of other trace elements. The mean enrichment factor (mEF) of Sc, Cr, Co, Cu, Ga and Pb was more than 1 in both size fractions and, Zr and Hf were more than 2 only in silt fraction. The binary plots of major and trace elements revealed dominant felsic composition for the sediments in south Kerala and increasing mafic or intermediate composition between mafic and felsic for sediments towards north Kerala. The ternary plots of Zr-Cr-Ga and La-Th-Sc suggest that the sediments were essentially a mixture derived from basalts, granites, acidic and metamorphic rocks, and sediment composition close to that of granodiorite and granite. Influence of lateritic weathering is evident both on minerals, and major and transition trace element composition of sediments. Heavy minerals in silt fraction enhanced the trace element content in sediments of south Kerala.