Influence of provenance, weathering, and sedimentary processes on the elemental ratios of the fine-grained fraction of the bedload sediments from theVembanad Lake and the adjoining continental shelf, southwest coast of India

Geochemical studies, comprising twelve major elements and twenty-four trace elements, including the rare earth elements (REE), have been carried out o n the < 4 mu m clay size fractions of modern sediments from fluvial and bra ckish regimes of Vembanad Lake and also the inner continental shelf represe nting nearshore marine environments. The sediments in the basin are supplie d by many small rivers so that large areas of the upper continental crust o f the southwestern India can be sampled. Concentrations normalized to the average upper continental crust (UCC) show that the sediments studied are depleted in Rb, K, Ba, Ta, Hf,Na, and Ca an d enriched in Cs, U, Th, REEs, Sc, Fe, Co, and Cr. UCC-normalized patterns are strikingly similar for all the three sediment types, probably because o f efficient mixing of sedimentary material during weathering, transport, an d deposition. The depletion of Na, K, Ca, and Ba in sediments relative to U CC could be attributed to their mobility during weathering. The depletion o f Ta and Hf and enrichment of Th, REE, and Sc are probably related to prove nance characteristics, and enrichment of U is attributed to oxidation-reduc tion processes. Shale-normalized REE values reveal steep HREE patterns [(Tb-sample/Tb-shale )/(Yb-sample/Yb-shale)] >> [(La-sample/La-shale)/(Sm-sample/Sm-shale)] and flat LREE patterns. The shale normalized patterns for all the three types o f sediments are remarkably similar, with variations in the magnitude of LRE E enrichment (La-n/Yb-n varies between 1.72 to 2.57 times that of PAAS). LR EE enrichment and steep HREE patterns are attributed to felsic source-rock characteristics. Other geochemical characteristics such as high La/Th and l ow La/Sc and Th/Sc ratios compared to UCC also support a felsic provenance. Eu/Eu* values for all the three types are little more than 1, which is als o a characteristic feature of Archean crust. Furthermore, data plotted on d iscrimination diagrams of La/Th vs. Hf and La-Th-Sc cluster closer to grani te and away from basalt and komatiite and are consistent with source rock g eology. Geochemical data have also helped in ascertaining the weathering trends. Th e chemical index of alteration (CIA) has been used to quantify the degree o f weathering. CIA values range between 81 and 94 on a scale of 40-100, indi cating a high degree of alteration (fluvial sediments seem to be more alter ed). On an A-CN-K diagram, the data fall closer to the compositional fields of highly weathered minerals kaolinite, gibbsite, and chlorite. A high deg ree of weathering is also indicated by other plots such as K2O-Fe2O3-Al2O3 and log K vs. log Rb. The geochemical data are consistent with the studies on clay mineralogy (higher kaolinite) carried out by other authors on the s ediments of the same area. Secondary processes seem to affect other geochemical discriminants such as Co, Cr, Ni, and U, as revealed by plots of (1) La/Sc vs. Co/Th and (2) Th-H f-Co and Co/Th, Cr/Th ratios and makes them unsuitable for determining prov enance and tectonic setting.