Geology of the area between Bardibas and Sindhuli Madi, Sub-Himalaya, Central Nepal

The Siwalik Group (Sub-Himalaya) is the east-west extending youngest mountain belt formed by the sediments derived from the rising Himalaya on the north. The Siwalik Group is separated from the Lesser Himalaya to the north by the Main Boundary Thrust (MBT), and the Indo-Gangetic Plain to the south by the Main Frontal Thrust (MFT). The study was carried out mainly along the Ratu Nadi, Kamala Nadi river sections and was lithostratigraphically divided into the Lower Siwalik, Middle Siwalik, and Upper Siwalik, from the bottom to the top section, respectively. The Lower Siwalik is composed of fine-grained greenish grey calcareous sandstone (subarkose and lithic arenite), variegated mudstone, and calcareous siltstone. The Lower Siwalik is exposed due to the Marine Khola Thrust in the Gadyauli Khola, and sandstone there is more indurated and non-calcareous. The Middle Siwalik is mainly comprised of the medium- to coarse-grained salt-and-pepper sandstone (arkose and subarkose) with large cross lamination, calcareous sand lenses, convolute bedding, dark grey siltstone and mudstone. Plant fossils are also present in the finely laminated clay bed of the Middle Siwalik along the Ratu Nadi section. The Upper Siwalik comprises of mud- to sand-supported pebble to cobble conglomerates.

Geological Study of Chatara–Barahakshetra Section, Sunsari-Udayapur District, Eastern Nepal

Geological mapping was carried out in the Chatara–Barahakshetra area of east Nepal. The Siwalik of the Chatara–Barahakshetra area is divided into the Lower Siwalik and the Middle Siwalik. Each unit is further subdivided into lower and upper members. The Central Churia Thrust is overturned in the study area. Highly deformed Siwalik contain several outcrop-scale as well as a large-scale folds. Bulletin of Department of Geology, vol. 20-21, 2018, pp: 49-58

Leaf Impressions of Terminalia (Combretaceae) and Daphnogene (Lauraceae) from the Middle Siwalik of the Chatara-Barahakshetra Area, Eastern Nepal

Leaves of Terminalia (Combretaceae) and Daphnogene (Lauraceae) are described from the lower member of Lower Siwalik of Chatara- Barahakshetra area, eastern Nepal. The habit, habitat and present day distribution of the comparable extant taxa indicates the existence of tropical to sub-tropical evergreen to semi evergreen forest with humid climate in the area during the deposition of the Siwalik sediments. Bulletin of Department of Geology, vol. 20-21, 2018, pp: 21-28

Engineering geological study of the Mai Khola Hyroelectric Project, Ilam, eastern Nepal

The Mai Khola Hydroelectric Project, a run-of-river scheme, has a capacity 15.6 MW. It has design discharge of 16 m3/s, design net head of 112.71 m and includes 2192 m long inverted-D shaped headrace tunnel with 4.3 m diameter, concrete dam of 10.6 m height and semi-surface powerhouse. The project area consists of rocks of the Middle Siwalik Subgroup, comprising of sandstone, siltstone and mudstone, inter bedded frequently. Sandstone is predominant in head works area, headrace tunnel and is completely absent in a surge tank, and penstock alignment. Siltstone alternating with thin layer of mudstone is predominant in powerhouse area. The headrace tunnel outlet portal and surge shaft lie on the hill slope characterized by colluvial deposits. The penstock alignment passes through highly weathered siltstone and mudstone. The semi-surface powerhouse and the tailrace canal lie on the lower alluvial terrace. The Main Boundary Thrust (MBT) is the major structure observed at about 90 m upstream from the weir axis. The average Q-value of rock mass along the headrace tunnel surface mapping was 0.062–1.33 and after excavation the value was 0.004–0.23. An extremely poor to poor relation was observed between the rock mass class on surface mapping and exceptionally poor to very poor on excavation. Analysing the results of the surface and underground study of the rock mass, the excess support is required during construction.

Erosional unroofing of Himalaya in far western Nepal: a detrital zircon U-Pb geochronology and petrography study

Since the collision between the Indian and Asian plates, several peripheral foreland basin were formed, and started to accumulate the sediments from the hinterland Himalayan orogeny. The sediments deposited at the northern tip of the Greater India have been uplifted, exhumed after the activation of several south propagating thrusts and finally transported to the foreland basin by southward flowing fluvial system. We present petrography and detrital zircon dating for the interpretation of possible provenance of the Neogene Siwalik foreland basin sediments in far western Nepal. The QFL ternary plot for provenance analysis show a 'recycled orogeny' field for the studied sandstone samples, indicating Tethys Himalaya, Higher Himalaya and Lesser Himalaya as the source of the foreland basin sediments. The detrital zircon U-Pb ages of the studied samples have shown that during the time of deposition there was dominant numbers of detritus supplied from the Tethys and upper Lesser Himalaya. Subsequently the amount of the Higher and Lower Lesser Himalaya increased during the time of deposition of the Middle Siwalik.

Dipterocarpus Gaertn. (Dipterocarpaceae) leaf from the Middle Siwalik of eastern Nepal and its phytogeographic and climatic significance

A leaf of Dipterocarpus (Dipterocarpaceae) is described from the Lower member of Middle Siwalik of eastern Nepal. Its presence indicates that during the deposition of the sediments there was a warm and humid climate with dry season of not more than 3–4 months. The modern distribution of the genus and family reveals that nowadays they have disappeared from the modern flora of Nepal. The most plausible reason for their disappearance might be an increase in length of the dry season caused by the upliftment of the Himalaya.

Lithostratigraphy of the Siwalik Group, Khutia Khola section, Far Western Nepal Himalaya

The Neogene fluival sediments (Siwalik Group) forming the southernmost hills in the Himalaya is well exposed along the Khutia Khola in the Far Western Nepal. The newly established lithostratigraphy is subdivided into the Jagati Formation (2110 m, equivalent to the Lower Siwalik) and the Kala Formation (2050 m, equivalent to the Middle Siwalik) in ascending order. Each formation is further divided into three members; the lower, middle and upper, based on the ratio of mudstone vs. sandstone beds as well as color and grain size of sandstone. The Jagati Formation is characterized by reddish-brown mudstones interbedded with very fine- to coarse-grained sandstones. Paleosols characterized with burrows, rhizoliths, desiccation cracks, concretions and nodules are present in higher proportion than thinly laminated or massive mudstone beds. The Kala Formation comprises thin- to thick-bedded, very fine- to very coarse-grained sandstones and pebbly sandstones interbedded with reddishbrown, greenish-grey to dark-grey mudstones. Sandstone beds exhibit "salt and pepper" appearance. In the lower part of the upper member sub-rounded to rounded pebbles are scattered along with trough and planar cross-laminations in sandstone beds, whereas the gravel size tends to be larger and few cobble size gravels also appear in the upper part. Thinly laminated or massive mudstones are common and paleosols are less frequent. The age of the boundary between the Jagati and Kala Formations (Lower-Middle Siwaliks) is around 11.05 Ma. The boundaries age between the middle and upper members of the Jagati Formation is around 12.7 Ma and between the lower and middle members of the Kala Formation is around 9.0 Ma. The dominance of finer sediments and thinner sandstone units than that of neighboring Siwalik successions indicate the fluvial fancies of the Khutia Khola section is deposited by a small river system and may represent the inter fluve setting of major river systems.