Manifestation of Intra - Foreland Thrusting in the Neogene Himalayan Foreland Basin Fill

Neogene basin fills of the Middle and Upper Siwalik Subgroup in the central sector of Indian foreland basin record changes in fluvial architecture, dispersal pattern and provenance in response to deformation and uplift of the frontal Himalaya. The 2.4 km thick Neogene Siwalik succession in the Subathu sub-basin exposed in the vicinity of Nahan Thyst (intra-foreland thrust or IFT) shows fine-to medium-grained grey sandstones of multi-storey, sheet type deposited by transverse trunk rivers. These sandstones are gradually replaced by fine- grained buff sandstone of ribbon type deposited by relatively small piedmont rivers. A significant change in sediment dispersal from SE to SW is observed at 5.5 Ma for grey sandstone and to SE around 4.8 Ma for buff sandstone. Both the sandstones are lithic arenites with dominance of sedimentary rock fragments in the buff sandstone. The filling pattern and occurrence of softsediment deformation structures, particularly between 4.8 and 2.6 Ma, indicate tectonically controlled re-organisation of drainage pattern and gradual change in source area from Lesser Himalayan to Sub-Himalayan regions. These changes after 4.8 Ma are mainly governed by intra-foreland thrust activity, which resulted in partitioning of foreland basin. Further, a significant overlap of Tertiary clast-bearing conglomerate with that of pre-Tertiary clast-bearing conglomerate at around 1.77 Ma shows a major reactivation along IFT.