Environmental magnetic studies on some Quaternary sediments of varied depositional settings in the Indian sub-continent

Abstract

The efficacy and limitations of routine environmental magnetic approaches to characterize and compare the Quaternary sediments of varied depositional settings (fluvial, fluvio-lacustrine and marine) from the Indian sub-continent are discussed. The various studied profiles fall under tropical wet and dry, subtropical humid, cold arid and alpine climatic zones at altitudes varying from approx. 2000m below sea level to approx. 4500 msl. The magnetic mineral assemblages are characterized by magnetic susceptibility, high field hysteresis parameters and their ratios and bivariate plots. Besides applying the conventional models, new and multi-parametric environmental magnetic models for finer discrimination of the mineral magnetic assemblages (MMA) amongst these deposits is produced. In similar hinterland setup and climate zones, independent MMA characteristics are shown by the Late Quaternary frontal Himalayan basins: the Ganga basin, the intermontane Pinjor Dun and the folded Siwalik sequence. Such variations are attributed to diverse energy conditions due to varied basin morphology, climate and the source to sink proximity. Contrasting channel to overbank MMA relations are discernible during the glacial and interglacial periods within the Ganga Basin. The Siwalik sediments are notably restricted to antiferromagnetic region of the bivariate plots indicating formation and stability of iron oxides under warm humid to arid conditions. The post-Siwalik intermontane sediments show steep gradients in MMA domain within a short distance from proximal to distal fan regime. In the tropical Peninsular region, the source to sink relation is marked by the Deccan Trap province (source) and Bengal submarine fan (sink) at least since the Last Glacial Maxima (LGM) in the Middle fan region. The MMAs for the lacustrine and fluviolacustrine deposits in the Himalayan foothills as well as the Higher Himalaya indicate detrital variations governed by the fluvial dynamics in the region. This demands a more detail catchment to sink studies in each basin and to explore the climatic tele-connections amongst the varied altitudinal regions. Overall, this study indicates a strong detrital control on MMA governed by the energy conditions of the transporting media that in turn is controlled by catchment morphology, regional tectonics and climatic fluctuations. In the absence of suitable unmixing models, the new bivariate and diamond plots attempted here are helpful in describing the detrital modes within the basin and its comparison to other basins.