Petrography of Neogene Siwalik Sandstone of the Himalayan Foreland Basin, Garhwal Himalaya: Implications for Source-Area Tectonics and Climate

The 1.6 km thick Neogene fluvial sandstone (Middle Siwalik; 9-5.23 Ma) in the Dehradun sub-basin (west central sector of Himalayan Foreland Basin) is characterised by distinct grey colour, and salt and pepper texture. They can be divided into poorly indurated (friable), Matrix-Rich Sandstone (MRS) and indurated (hard), Matrix-Poor Sandstone (MPS). The sandstone is classified as lithic arenite (Q₅₅F₅R₄₀; Mx₂; 50%), lithic graywacke (Q₆₁F₃R₃₆Mx₂₄ 29%) and sublithic arenite (Q₇₅F₅R₂₀ Mx₃; 21%). The MRS is fine- to very fine-grained lithic graywacke, and the MPS is rnediurn to fine-grained lithic to sublilhic arenite.

The framework composition of the sandstone points to its derivation from a provenance comprising sedimentary, low to medium grade metamorphic and igneous source rocks. Significant increase of metamorphic element above 750ml level (7.65 Ma) and particularly from 900m level (7.50 Ma) suggests a widespread erosion of the uplifted Himalayan gneiss or the Higher Himalayan Crystallines (HHC). The uplift of Higher Himalayan Crystalline was due to tectonic adjustment along the Main Central Thrust (MCT) between 7.65-7.50 Ma. Results of this study also suggest fluctuating semi-arid to semi-humid climate till 8.5 Ma. However, a distinct deviation from semi-arid to humid climatic phase is observed between 8.5 and 7.6 Ma.