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, FeO^t, 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 (<0.5), shows a reduction in the monsoon precipitation and stable tectonic conditions, besides the initiation of an arid phase in western India.

我们对马希河流域下游约 25 m 的沉积岩心进行了结构、岩石学、矿物学和多元素地球化学调查，以了解过去古吉拉特邦冲积平原 (GAP) 气候和构造的影响 > 11.5 万年 (ka)（海洋同位素阶段；MIS-5e）。包括基性蚀变指数 (MIA) 和元素比的结构分析三元图 A-CN-K 和 A-CNK-FM 表明，大多数沉积物本质上是砂质粉砂到粉砂，并显示出初期中等程度的化学风化。此外，较高浓度的岩屑 (17%)、长石 (9%) 以及辉石和闪石的存在也表明物理风化而不是化学风化增加。淤泥增多，₂ O ₃、TiO ₂、FeO ^t、K ₂ O、稀土元素和Al ₂ O ₃ /SiO ₂)，表明沉积物地球化学中的粒度控制。由于石英和方解石的稀释作用，沙子和二次生成的碳酸盐的增加分别降低了所有其他元素的绝对浓度。根据观察到的变化，已经确定了四个主要的构造气候带，即 1 到 4（最老到最年轻）。外推的贝叶斯基础年龄模型表明，在印度夏季风 (ISM) 的增强阶段，底部区域 1（~25-17.64 m 深）沉积在 >115 至 60 ka (MIS-5a-e) 之间，如由更高的 Rb/Sr 比和高岭石-伊利石优于蒙脱石来表明。然而，由于局部构造活动，砾石/较粗沉积层的沉积也与基础水平变化有关。区域 1 的终端部分显示出相对干燥的条件，持续通过区域 2（~60-54 ka；MIS-4），表现为较低值的不动元素和 Rb/Sr 和高岭石/蒙脱石比率。然而，在第 3 区（~54-38 ka；MIS-3），沉积物变得更粗，这意味着降雨或构造活动增强，沉积速率更高。最顶部的第 4 区（~38.0-6.7 ka MIS-3, 1）标志着波动趋势，如该区下部（~5 m 和~3 m 深）存在砾石所表明的约 37-38 ka 的强降雨或构造活动。然而，该区域的上部主要是沙子和淤泥，化学蚀变指数 (CIA) 值下降和 Rb/Sr 比值降低 (<0.5) 证明了这一点，表明季风降水减少和稳定的构造条件，