Micromorphology and clay mineralogy of the palaeosols in the Lower Siwalik Sub-Group, Kangra sub-basin, NW Himalaya: a response to Himalayan orogeny and climate change
摘要
Paleopedology plays a crucial role in reconstructing paleoenvironments by providing direct evidence of interaction between the Earth’s surface and its various spheres. This study presents detailed paleopedological records from the ~ 400 m Lower Siwalik (LS) succession in the Kotla-Brail section of the Kangra sub-basin, Himalayan Foreland Basin (HFB), spanning 10.85 to 9.83 Ma. The field characteristics are marked by 2.5YR and 5YR hue colours, with Bw, Bt, Bk, and Bss horizons ranging in thickness from 1 to 2 m. The micromorphological analysis confirms varying degrees of pedogenic development and weathering intensities, blocky structure, biogenic activity, b-fabrics, rhizocretions, Fe/Mn mottles, pedogenic CaCO3, and clay coatings, indicating the dominance of strong paleopedogenic development in the lower ~ 200 m of the studied Lower Siwalik (LS) and moderately to weakly developed palaeosols in the upper ~ 200 m, transitioning to the Middle Siwalik (MS) succession. X-ray diffraction (XRD) analysis of the clay (< 2 μm) fractions in Lower Siwalik (LS) succession palaeosols indicates the dominance of kaolinite and a lesser proportion of smectite, with > 1% clay pedo-features and fewer pedogenic carbonates in the lower ~ 200 m, suggesting sub-humid to humid conditions with prolonged weathering and paleopedogenesis. In contrast, the upper ~ 200 m of the Lower Siwalik (LS) succession show a high smectite but low kaolinite content, along with a large amount of pedogenic carbonate, suggesting arid to semiarid conditions (10.3 to 9.9 Ma). This contrast between the lower and upper parts of the Lower Siwalik (LS) succession suggests a weakening of the southwestern monsoon towards the transitions to the Middle Siwalik at ~ 10 Ma.