No abstract

The Siwalik Group, ranging from the Early Miocene to Pleistocene, is believed to be deposited in the fluvial environment and controlled by contemporary Himalayan tectonics and climate. In this study, we established the fluvial environment and its controlling factors responsible for the deposition of the Siwalik succession along the Muksar Khola section in the eastern Nepal Himalaya. Five sedimentary facies associations are identified; these are interpreted as the deposits of flood plain-dominated fine-grained meandering river (FA1), flood-dominated overbank environment (FA2), sandy meandering river (FA3), anastomosing river (FA4), and debris flow-dominated gravelly braided river (FA5). These changes in the fluvial system occurred around 10.5 Ma, 10.0 Ma, 5.9 Ma and 3.5 Ma, defined by existing magnetostratigraphy constraints, due to the effects of hinterland tectonics, climate and sea-level change and continuous drifting of the foreland basin towards the hinterland concerning depositional age. The thick succession of an intraformational conglomerate reveals intensification of the monsoon started around 10.5 Ma in the eastern Nepal Himalaya. The present study also shows asynchronous exhumation of the Himalaya from east to west brought a significant difference in the fluvial environment of the Neogene foreland basin.

The Siwalik Group extending east to west co-linear to the main Himalayan range is well exposed along the Muksar Khola section, Siraha-Udayapur district, eastern Nepal Himalaya. Siwalik Group in the present study area is divided into the Lower, Middle, and Upper Siwaliks based on grain size and sandstone-mudstone proportion. The Lower Siwaliks is characterized by very fine- to fine-grained, light grey sandstone interbedded with dark grey to olive black mudstone. The Middle Siwaliks, is characterized by the domination of fine- to coarse-grained sandstone, and based on the lithology and bed thickness it is divided into two members. The lower member is dominated by fine- to medium-grained “salt and pepper” sandstone with dark greenish to olive-grey mudstone while, the upper member is dominated by light grey to white medium- to coarse-grained sandstone with grey, dark grey to black mudstone. Increase in the grain size and thickness of sandstone beds, increase in the proportion of mudstone, decrease in induration of sandstone and decrease in the proportion of biotite grain in sandstone makes upper member different from lower member of the Middle Siwaliks. The Upper Siwaliks is characterized by very thick beds of clast supported conglomerate associated with coarse- to very coarse-grained, very thick bedded sandstone and dull yellowish-grey to grey mudstone. The boundary between the Lower Siwaliks and the Middle Siwaliks, lower member and upper member of the Middle Siwaliks, and the Upper Siwaliks are 10.0 Ma, 5.7 Ma, and 3.5 Ma, respectively. The present study records the presence of a large succession of intra-formational conglomerate succession in the Lower Siwaliks.

Bengal Fan Miocene sediments were collected during International Ocean Discovery Program Expedition 354 and investigated using petrographic and detrital garnet chemistry analyses. The Miocene Siwalik Group, which is composed of sediments deposited in the Himalayan foreland basin, was also analyzed for comparison with the Bengal Fan data for the provenance change during the Miocene. Our petrographic analyses revealed that the Miocene sediments of the Bengal Fan and Siwalik Group consist predominantly of Higher Himalayan Crystalline (HHC)-derived detritus such as chloritoid, staurolite, sillimanite, and/or kyanite, which appear among the accessory minerals. The chemistry of the detrital garnet varies across the stratigraphy; most of the garnet is rich in almandine and poor in spessartine and pyrope.However, pyrope-rich garnet, which is considered to originate from the HHC core (granulite facies), was found in the lower to upper Miocene deposits. The deposition of HHC-derived detrital garnet began before the Middle Miocene (15 Ma) and before the Late Miocene (10-9 Ma) in the Siwalik Group. The Bengal Fan data, by contrast, indicated that pyrope-rich garnet appeared in the Early Miocene (17.3 Ma) and Late Miocene (8.5-6.5 Ma). We conclude that the Bengal Fan sediments record the erosion of the HHC zone since the Early Miocene that appears in the Siwalik sediments. Furthermore, we found that the HHC-derived inputs decreased from the late Middle Miocene (12 Ma) to the early Middle Miocene (10 Ma) in both the Nepal Himalaya foreland basin and the Bengal Fan. The disappearance of the HHC-derived detritus is probably the result of dilution by Lesser Himalayan detritus, which suggests that the Lesser Himalayan zone, which is composed of metamorphosed and unmetamorphosed sedimentary rocks, was uplifted.

The Siwalik Group, ranging from the Early Miocene to Pleistocene, is believed to be controlled by contemporary Himalayan tectonics and climate. In this study, we established the fluvial system responsible for the deposition of the Siwalik succession along the Muksar Khola section and its controlling factors. Five sedimentary facies associations are identified which are interpreted as the deposits of flood plain dominated fine-grained meandering river (FA1), flood dominated overbank environment (FA2), sandy meandering river (FA3), anastomosing river (FA4), and debris flow dominated gravelly braided river (FA5). These change in fluvial style occurred around 10.5 Ma, 10.0 Ma, 5.9 Ma and 3.5 Ma due to the effects of hinterland tectonics, climate and sea-level change. The thick succession of intraformational conglomerate reveals the intensification of monsoon started around 10.5 Ma in the eastern Nepal Himalaya. The present study show asynchronous exhumation of the Himalaya east to west brought significant difference on the fluvial environment of the Neogene foreland basin. Moreover, this study also reveals continuous drifting of the foreland basin towards the hinterland concerning depositional age.