Geochemistry of Rewalsar Lake sediment, Lesser Himalaya, India: implications for source-area weathering, provenance and tectonic setting

Abstract

Major, trace and rare earth element (REE) chemistry of fresh-water lake sediments, stituated in the middle Siwalik Formation (Pliocene), have been investigated. The log (Fe2O3/K2O) vs log (SiO2/Al2O3) plot of sediments discriminate the rock types of the catchment which are remarkably similar to those observed in thinsections. The Chemical Index of Alteration (CIA) between 75.2 to 78.1 and Chemical Index of Weathering (CIW) between 79.5 to 89.8 are higher than Post-Archean Australian Shale (PAAS), indicating moderate to high weathering at source area. The PAAS normalized REE patterns are similar and differentiate the major rock types on the basis of abundance of REEs in accordance to clay minerals, grain size, with the highest concentration in the finest grain sediments. It also indicates poor mixing of sediments. The consistency of REEs in sediments with those of rock types illustrate almost unaltered, unmodified transport of these from the provenance to the detritus deposited in the basin and also shows that the REEs are enriched in phyllosilicates. The major elements, Fe2O3, TiO2, MgO, K2O show strong positive correlation with Al2O3 and follow the trend of the latter, and REEs also indicate same characteristics, hence phyllosilicates are likely to be the promising host for REEs. The negative correlation of La, Yb and Y with Zr and positive correlation with REEs and Al2O3 do not indicate Zr control on REE distribution in the present case. The petrographic character of minerals in thinsection of rocks as well as of lake sediments and low Na2O/K2O ratio in the latter show short distance transport, textural immaturity and sorting of grains. The higher Zr/Th and Zr/Yb ratios in coarser sediments than in fine grain detritus are supporting evidence of sedimentary sorting. Chondrite normalized REE patterns and negative Eu anomaly of the lake sediments are similar to that of PAAS indicating that they may have been originally derived from differentiated silicic (felsic) and/or recycled sedimentary source and deposited in foreland basin. However, petrographic character, REEs and high field strength elements estimated on sediments, tends to support metamorphic source area for middle Siwalik detritus and is consistent to the observations made on rocks exposed in the catchment area, attesting to the rise of Himalaya in phases, exposing fresh rocks for erosion/weathering, lying northward in Lesser Himalayan domain. Plot of sediment chemistry of lakes on tectonic delineation diagrams show that Rewalsar detritus derived from middle Siwalik rocks are from passive margin setting. The discriminant function plots of sediment samples also indicate that they are derived from craton interior or a quartzose sedimentary orogenic terrain and deposited in a passive margin setting. The chemistry of Rewalsar sediments is thus a proxy for the middle Siwalik rocks deposited in the foreland basin and throw light on provenance, source area weathering, and tectonic setting of the source rock.