Provenance composition, paleo-weathering and tectonic setting of Himalayan foreland basin sediments, Kumaun Sub-Himalaya, India

Clastic rocks of foreland basin from Nandhaur Sanctuary area, Kumaun Sub-Himalaya were studied to interpret their lithological composition, provenance, source area weathering characteristics and tectonic setting. Detailed geochemical investigation on 33 mudstone and petrographic study of 25 sandstone samples have been carried out. Sandstones are classified as quartz-arenite to sublith-arenite; lithic to subarkosic arenite and lithic to arkosic greywacke using detrital mode analysis. Chemically, mudstone is identified as greywacke to lithic-arenite and also straddles between shale and wacke with a few in lithic-arenite field. CIA values vary from 55.8 to 78.7 for Lower Siwalik, 53–78.3 for Middle Siwalik and, 58.8–81.7 for Upper Siwalik sediments. These results suggest that differential weathering conditions acted upon the sediment provenance: under steady to non-steady-state, variable climatic conditions, and Cenozoic tectonic uplifting along Himalayan thrusts. Chemical data in Al2O3-CaO-Na2O-K2O (A-CN-K) plot depart from the granite-granodiorite field towards the illite-muscovite indicating weathering trend in provenance. Chemically, mudstone bears the signature of Passive Margin tectonic setting of deposition which is inherited from the source rock. High field strength element (HFSE) and Rare Earth Element (REE) content of mudstone support the felsic upper crustal material as source, whereas transition elements advocate minor contribution from mafic rocks. Chondrite normalized high Light REE (LREE)/ Heavy REE (HREE) ratio (11.15–15.11), sloping LREE and near-flat HREE, negative Eu anomaly (Eu/Eu* = 0.6–0.88) indicate the evolved upper crustal material as the prominent source. The trace element ratio in mudstone; La/Sc (0.71–5.86), Cr/Th (2.42–9.50), Th/Co (0.07–0.95), and La/Co (0.15–2.31), supports the sediment derivation from felsic sources however Th/Co values indicate the mixed source rocks. The geochemistry of mudstone shows affinity with the rocks of high-grade Higher Himalaya and low-grade Lesser Himalaya and is a useful proxy for understanding the various tectonic and paleo-climatic conditions that prevailed during the evolution of Himalayan Foreland Basin.