Texture, mineralogy and geochemistry of late Quaternary sediments of the Mahi River basin, western India: Implications to climate and tectonics

Abstract We carried out textural, petrological, mineralogical and multi-element geochemical investigations of a ~25 m sediment core from the lower reaches of the Mahi River basin to understand the influence of climate and tectonics in the Gujarat Alluvial Plain (GAP) over the last >115 thousand years (ka) (Marine Isotopic Stage; MIS-5e). Ternary diagrams of texture analysis, A–CN–K, and A–CNK–FM including mafic index of alteration (MIA) and elemental ratios, suggest that most of the sediments are sandy-silt to silty-sand in nature and show incipient to moderate degree of chemical weathering. Additionally, the higher concentrations of lithic fragments (17%), feldspar (9%), and the presence of pyroxene and amphibole are also suggestive of increased physical weathering rather than chemical weathering. An increase in silt, clay and organic carbon contents register an increase in immobile elements (Al2O3, TiO2, FeOt, K2O, REEs, and Al2O3/SiO2), suggesting grain size control in the sediment geochemistry. The increase in sand and secondarily produced carbonates reduces the absolute concentration of all other elements due to the quartz and calcite dilution effect, respectively. Four major tectonoclimatic zones, namely 1 to 4 (oldest to youngest), have been identified based on the observed changes. The extrapolated Bayesian-base age model suggests that the bottom zone 1 (~25–17.64 m depth) was deposited between >115 and 60 ka (MIS-5a-e) during the enhanced phase of the Indian summer monsoon (ISM), as indicated by higher Rb/Sr ratio and dominance of kaolinite-illite over smectite. However, the deposition of gravel/coarser sediment horizons is also linked to base level change due to localized tectonic activities. The terminal part of zone 1 shows relatively drier conditions that continued through zone 2 (~60–54 ka; MIS-4), represented by lower values of immobile elements and Rb/Sr and kaolinite/smectite ratios. However, during Zone 3 (~54–38 ka; MIS-3), the sediment became coarser, implying enhanced rainfall or tectonic activity and a higher sedimentation rate. The top-most zone 4 (~38.0–6.7 ka MIS-3, 1) marks a fluctuating trend as indicated by the presence of gravely sand in the lower part of this zone (at ~5 m and ~3 m depth) due to intense rainfall or tectonic activity at ~37–38 ka. However, the upper part of this zone is dominated by sand and silt as evidenced by the decreasing chemical index of alteration (CIA) values and lower Rb/Sr ratio (