Abstract

Geochemical studies, comprising twelve major elements and twenty-four trace elements, including the rare earth elements (REE), have been carried out on the < 4 µm clay size fractions of modern sediments from fluvial and brackish regimes of Vembanad Lake and also the inner continental shelf representing nearshore marine environments. The sediments in the basin are supplied by many small rivers so that large areas of the upper continental crust of 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 and 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 of efficient mixing of sedimentary material during weathering, transport, and deposition. The depletion of Na, K, Ca, and Ba in sediments relative to UCC could be attributed to their mobility during weathering. The depletion of Ta and Hf and enrichment of Th, REE, and Sc are probably related to provenance characteristics, and enrichment of U is attributed to oxidation-reduction processes.

Shale-normalized REE values reveal steep HREE patterns [(Tbsample/Tbshale)/(Ybsample/Ybshale)] >> [(Lasample/Lashale)/(Smsample /Smshale)] and flat LREE patterns. The shale-normalized patterns for all the three types of sediments are remarkably similar, with variations in the magnitude of LREE enrichment (Lan/Ybn varies between 1.72 to 2.57 times that of PAAS). LREE enrichment and steep HREE patterns are attributed to felsic source-rock characteristics. Other geochemical characteristics such as high La/Th and low 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 also a characteristic feature of Archean crust. Furthermore, data plotted on discrimination diagrams of La/Th vs. Hf and La-Th-Sc cluster closer to granite and away from basalt and komatiite and are consistent with source-rock geology.

Geochemical data have also helped in ascertaining the weathering trends. The chemical index of alteration (CIA) has been used to quantify the degree of weathering. CIA values range between 81 and 94 on a scale of 40-100, indicating a high degree of alteration (fluvial sediments seem to be more altered). On an A-CN-K diagram, the data fall closer to the compositional fields of highly weathered minerals kaolinite, gibbsite, and chlorite. A high degree 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 sediments 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-Hf-Co and Co/Th, Cr/Th ratios and makes them unsuitable for determining provenance and tectonic setting.

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