Geology of Nepal

Ranjan Kumar Dahal, PhD

Department of Geology, Tribhuvan University, Ghantaghar, Kathmandu, Nepal

Please use this information extensively, but I request to acknowledge the source.

For Citation, use this line:

Dahal R.K., 2006, Geology of Nepal, published in personal home page www.ranjan.net.np.

Himalaya in brief

The Himalayan Range is a young mountain system of world. It is a broad continuous arc along the northern fringes of the Indian subcontinent, from the bend of the Indus River in the northwest to the Brahmaputra River in the east. The Himalayan mountain chain extends in an east-west direction between the wide plains of the Indus and Bramhaputra in the south and the vast expanse of the high Tibetan Plateau in the north. The limit of the Himalayas in the east and west is marked by the eastern and western arc of Himalayan bends. Between these bends the Himalayan range is approximately 2400 km long and 200 km to 300 km wide. The Himalayas cover an area of approximately 600,000 sq. km in south Asia.

Himalaya was formed by the collision of the Indian Plate with Tibetan (Eurasian) Plate around 55 millions years ago (Fig 1.1). Many scientists believe that at that time the northward moving Indian plate first touched the southern edge of Tibetan (Eurasian) plate.

Fig 1.1, Collision of Indian plate with Tibetan plate and formation of Himalaya (modified after USGS, 1999)

The mountain building (orogenic) process continues from the collision and the mountain is still on making process. This is noticeable by present day northward movement of India at the rate of 5 cm per year and the occurrences of frequent seismic shakes all along the Himalaya and its surroundings (Jackson and Bilhan, 1994, Pandey et al., 1995, Bilham et al., 1998). Most part of the drift is accommodated within the Himalaya by various thrusts as well as rising peaks. Prominent research of Himalaya was begun after seventies only when the plate tectonics theory was became popular among scientists and engineers. Undoubtedly, Himalaya became an ingenious natural laboratory to the researcher for testing many thoughts of plate tectonics and earth surface dynamics. Himalaya is one of the rare mountain ranges on the Earth where, in a single traverse, more than 50 km thick vertical section of mountain can be assessed from root to top. Good exposures of deep seated metamorphic rock sequences to the fossiliferous sedimentary rock on top are easily accessible for research. The Himalayan mountain system developed in a series of stages 30 to 50 million years ago and they are still active and continue to rise today. Himalaya is considered as a tectonically very active and vulnerable mountain system of world. In 1964, Augusto Gansser provided the first comprehensive picture of the Himalayas (Fig 1.2) and he had transversely divided the whole Himalayan range into following five major groups.

The Punjab Himalaya

It has the Himalayan range that is in between the Shatluj (east) and the Indus Rivers in the west. Its extension is about 550 Km.

The Kumaon Himalaya

Its extension is about 820 Km. This is the Himalayan range bordered easterly by the Mahakali River and westerly by the Satluj River.

The Nepal Himalaya

Nepal has the longest division of the Himalaya. Its extension is about 800 Km and starts from west at the Mahakali River and ends at the east by the Tista River.

Sikkim-Bhutan Himalaya

Its length is about 400 Km and extends between Sikkim and Bhutan.

The NEFA Himalaya

It stretches about 440 Km from the eastern boarder of Bhutan to the Tsangpo River in the east.

Longitudinally, Himalayan Range is also divided into five tectonic zones (Gansser, 1964).

Gangetic Plain

Sub-Himalayan Zone

Lesser Himalayan Zone

Higher Himalayan Zone

Tibetan-Tethys Himalayan Zone

These east-west extending zones run almost parallel to each other (Fig 1.2). They have different lithology, structure, and geological history.

Fig. 1.2 Longitudinal Subdivision of the Himalaya (modified after Gansser 1964)

Geological Framework of Nepal Himalaya

Nepal occupies the central sector of Himalayan arc. Nearly one third of the 2400 km long Himalayan range lies within Nepal. Similar to other parts of the Himalaya, from south to north, Nepal can be also subdivided into the following five major tectonic zones.

Gangetic Plain

Sub-Himalayan (Siwalik) Zone

Lesser Himalayan Zone

Higher Himalayan Zone

Tibetan-Tethys Himalayan Zone

Each of these zones is characterized by their own lithology, tectonics, structures and geological history. The generalized geological map is given in Fig 1.3.

Fig 1.3, Geological map of Nepal (modified from Dahal, 2006)

These all tectonic zones are separated from each other by the thrust faults. The southernmost fault, the Main Frontal Thrust (MFT) separates the Sub-Himalayan (Siwalik) Zone from Gangetic Plains. The Main Boundary Thrust (MBT) separates the Lesser Himalayan Zone from Siwalik. The Main Central Thrust (MCT) separates the Higher Himalayan Zone from the Lesser Himalayan Zone. The South Tibetan Detachment System (STDS) marks the boundary between the Higher Himalayan Zone and the overlying fossiliferous sequence of the Tibetan-Tethys Himalayan Zone. The Indo-Tsangpo Suture Zone is the contact knot between Indian plate and Tibetan (Eurasian) Plate in terms of plate tectonics.

Fig 1.4, Generalized cross section of Himalaya (modified after Dahal 2006)

Geological Zones and Types of Materials

The main geological zones of the Nepal Himalaya are described below.

Gangetic Plain

The Gangetic Plain is also called as Terai Zone and it is the Nepalese portion of the Gangetic Plain that extends from the Indian Shield in the South to the Sub-Himalayan (Siwalik) Zone to the North. The plain is in less than 200 meters above sea level and usually has thick (nearly 1500 m) alluvial sediments. The alluvial sediments contain mainly boulder, gravel, silt and clay. The width of Terai Zone varies from 10 to 50 km and forms a nearly continuous belt from east to west. Exceptionally at two place, Chitwan and Rapti valleys, the Terai Zone is interrupted by Siwalik for 70 km and 80 km respectively. Terai Zone is a foreland basin and has sediment originated from peaks of Northern part. To the north, this zone is separated by an active thrust system called as the Main Frontal Thrust (MFT) with Siwalik. At some places along MFT, the Siwalik rocks are observed to rest over the recent sediments of the Terai (Dahal 2006).

A large number of borehole logs and geophysical investigation made during the groundwater investigation and petroleum exploration in Terai play a lead role to study the surface and subsurface geology of the Terai. It further helps to classify the Terai into Northern Terai or Bhabhar Zone, Middle Terai and Southern Terai.

Northern Terai (Bhabar Zone)

The northern Terai is adjoining to the foothills of Siwalik and continues southward to a maximum width of 12 km. This part of Terai is also known as Bhabar Zone. This zone is mainly composed of boulders, pebbles, cobbles and coarse sand derived from the rocks of Siwalik and Lesser Himalaya.  These boulders, pebbles, and cobbles are mostly made up of sandstones (Fig 1.5) and the rocks from the immediate northern vicinity. Bhabhar Zone acts as a recharge zone for the groundwater of Terai.  Most of the rivers loose their water while passing through this zone. In this Zone, water tables in wells show very sharp fluctuations between the summer and rainy seasons.  At some places the wells become completely dry in summer. Due to the very course nature of the sediments, low water table and quick percolation of rainwater, this zone is particularly not productive for agriculture and therefore ideal for the development of forest resources.

Middle Terai (Marshy) Zone

This is a narrow zone of about 10-12 km wide and lying between the Northern Terai Zone and the Southern Terai Zone. This zone is characterized by pebbly and brown to grey colored unconsolidated sandy sediments with few clay partings. Clay is mostly dark grey colored and intercalated with brown colored sand layers. The medium to coarse grained sandy layers possesses good groundwater reservoir. Because of marked change in elevation from Bhabar Zone, this zone comprises marked development of spring line, natural ponds, marshland and lakes (Dahal 2006). Immediate south of spring lines, there are many artesian layers are found in depth of 25 m to 200 m. The permeability of Middle Terai Zone diminishes towards south and finally non permeable layers are encountered in boundary of the Southern Terai Zone (Fig 1.5).

Southern Terai Zone

Southern Terai Zone is southern most part of Terai up to Nepal-India border and also continues into India. This zone consists of main sediments of Gangetic Plain. Basically, sand, silt and clay (Fig 1.5) are the main sediments of this zone. This zone is composed of finer sediments than the Middle Terai Zone. To the extreme south bordering the Indian Plains, the sediments become finer and also show change of facies. The water table is about 3 m below the surface and aquifers are poor (Fig 1.5). Only in old river channels area north-south extending better aquifers are found. Therefore, except at the northern part and along old river channels, there are particularly no good aquifers in the lower horizons (Dahal 2006).  For this reason, in the southern Terai of Nepal, the development of the groundwater also appears to be difficult by deep tube wells.

Fig 1.5, Subsurface condition of Terai Zone of Nepal (modified after, GRDP, 1994)

Sub-Himalayan (Siwalik) Zone

The Sub-Himalaya Zone is also called as Siwalik Zone and is delimited on the south by the Main Frontal Thrust (MFT) and on the north by the Main Boundary Thrust (MBT). It consists basically of fluvial deposits of the Neogene age (23 millions years to 1.6 millions years old). This Zone extends all along the Himalaya forming the southernmost hill range with width of 8 to 50 km. The Lesser Himalayan rocks thrust southward over the rocks of Siwalik along the MBT (Dahal, 2006). The general dip of beds of Siwalik has northward trend with varying angles and the overall strike is east-west. The Siwalik Zone has number of east-west running thrusts. Siwalik Zone is also rich with fossils. Fossils of plants, pisces, reptiles and mammals (Carnivora, Proboscidea, Artiodactyla, Rodentia and Primates) have been reported from Siwalik. The three-fold classification of Siwalik in Potwar region of Pakistan and western Indian Himalaya was freely applied to the equivalent Siwalik of Nepal (Burbank et al., 1996) from the beginning of the geological studies in Nepal. According to three fold classification, Siwalik can be classified as follow.

Lower Siwalik

Middle Siwalik

Upper Siwalik

The example of geological map of Siwalik around Hetauda area is given in Fig 1.6. The map illustrates the Upper, Middle and Lower Siwalik in the Hetauda and Amalekhgunj area. The other geological names (formations) of Upper, Middle and Lower Siwalik are also provided in map as formation names.

Fig 1.6, Geological Map of Hetauda-Bakiya Khola area (Adopted from Ulak and Nakayama, 1999)

Lower Siwalik

The Lower Siwalik consist of irregularly laminated beds of fine grained greenish sandstone and siltstone with mudstone. The alternating mudstone beds are thickly bedded and are variegated, red, purple, and brown coloured. The best exposures of Lower Siwalik are found in Surainaka, Amlekhgunj, Arun Khola, Barahchhetra and Rato Khola area of Nepal.

Middle Siwalik

The Middle Siwalik are comprised of medium to coarse grained salt-and-pepper (looks like mixture of salt and black pepper) sandstones interbedded with mudstone (Fig 1.7). This is differentiated from the Lower Siwalik in lacking variegated mudstone and sandstone. In upper part of the Middle Siwalik, pebbly sandstone beds are also found. In Middle Siwalik the sandstone beds have thickness mostly ranges from 1 m to 45 m. The exposures of Middle Siwalik are found mainly in Surkhet, Surai Khola, Hetauda, and Butwal.

Fig 1.7, Interbedding sandstone and mudstone in Middle Siwalik, Butwal-Tansen section of Siddhartha Highway

Upper Siwalik

The Upper Siwalik is comprised of conglomerate and boulder beds and subordinately sand and silt beds. The mudstone beds of the Upper Siwalik are massive and irregularly bedded and contain many invertebrate fossils including Brachiopods and Gastropods. The upper part of this sequence contains conglomerate beds, which have mostly boulder and cobble size rounded to subangular fragments of Lesser Himalayan rocks. In Bardibas, Hetauda, Bhalubang, and Chitwan the good exposure of Upper Siwalik can be seen.

Lesser Himalayan Zone

The Lesser Himalayan Zone is bounded to the north by the Main Central Thrust (MCT) and to the south by Main Boundary Thrust (MBT). MBT can be traced out in whole Nepal Himalaya and it can be also well observed in aerial photographs also (Fig 1.8 and Fig 1.9). The rocks of Lesser Himalayan Zone have been transported southwards in several thrust slices. Generally two types of sequences namely autochthonous and allochthonous can be distinguished in this Zone throughout the Himalayas. The both sequences of the Lesser Himalaya mainly have unfossiliferous, sedimentary, and metasedimentary rocks such as slate, phyllite, schist, quartzite, limestone, dolomite, etc, ranging in age from Precambrian to Eocene. There are also some granitic intrusions in this zone.

Fig 1.8, Aerial photograph of Udaypur district (eastern Nepal), well marked Main Boundary Thrust (MBT) is passing through middle of photograph

Fig 1.9, MBT observed in Butwal-Tansen section of Siddhartha Highway

From east to west, the Lesser Himalayan Zone of Nepal varies in rock type, age, structures, and igneous rock intrusion. Eastern Nepal is characterized by the development of extensive thrust sheets (allochthonous) of high grade metamorphic rocks (gneiss and schist) which have moved southwards. Below this sequence, due to erosion, large exposure of the low-grade metamorphic rocks (autochthonous) can be seen. In Central Nepal, a large thrust sheet called the Kathmandu Nappe (allochthonous) covers a wide area around the Kathmandu region. Whereas west of Kathmandu, between the Budhi Gandaki and Bheri rivers, amount of transported high grade metamorphic rocks (allochthonous) is very low and the area is generally covered by autochthonous sequence. But in west of the Bheri River, up to the western border of Nepal (Dadeldhura-Baitadi) high-grade metamorphic rocks reappear and cover much of the terrain.

The Higher Himalayan Zone

The Higher Himalayan zone mainly consists of huge pile of strongly metamorphosed rocks. Geologically, the Higher Himalayan Zone includes the rocks lying north of the Main Central Thrust (MCT) and below the highly fossiliferous Tibetan-Tethys Zone. This zone is separated with Tibetan-Tethys Zone by normal fault system called as South Tibetan Detachment System (STDS). Higher Himalayan Zone consists of an approximately 10 km thick succession of strongly metamorphosed coarse grained rocks. It extends continuously along the entire length of the country as in whole Himalaya, and its width varies from place to place (see Fig 1.3). The kyanite - sillimanite minerals bearing gneisses, schists, and marbles of the zone form the basement of the Tibetan-Tethys Zones. Granites are found in the upper part of the unit.

The Tibetan-Tethys Zone

The Tibetan-Tethys Zone lies in northern part of the country. It begins from the top of the STDS (Fig 1.10) and extends to the north in Tibet. In Nepal, the fossiliferous rocks of the Tibetan-Tethys Zone are well-developed in Mustang (Fig 1.11), Manang and Dolpa area. In eastern part, amount of exposure of the Tibetan Tehys Zone is almost negligible and found only in top of the Mount Everest (Fig 1.3). Most of the other Great Himalayan peaks of Nepal such as Manaslu, Annapurna, and Dhaulagiri have rocks of Tibetan-Tethys Zone. This zone is composed of sedimentary rocks, such as shale, limestone, and sandstone, ranging in age from Cambrian to Eocene.  This zone in some area is found as continuous deposits of Higher Himalayan Zone without normal fault.

 

Fig 1.10, South Tibetan Detachment System (STDS) separating Higher Himalayan Zone from Tibetan-Tethys Zone, Chhaktan Khola, north west from Kokhethati, Mutang (Adopted from Dahal 2006)

 

Fig 1.11, Cliff of limestone belongs to Tibetan-Tethys Zone, Jomsom, Mustang

Physiography of the Nepal Himalaya

Physiographic division of Nepal has been in practice since 1950s. It was 1969, Tony Hagen successively divided Nepal into eight well defined physiographic provinces from south to north. These provinces are E-W running and can also be incorporated in Indian Himalayan belt. The Hagen classification is the most appropriate classification and represents all characteristic physiographic zones of Nepal. Some geographer and geomorphologists also used five fold classifications in the general sense namely Terai, Churai, Middle Mountain, High Mountain and High Himalaya. Nevertheless, detail physiographical provinces of Nepal are given in Table 1.1 and Fig 1.12. and Fig 1.13 illustrates the generalized physiographic profile of the Nepal Himalaya.

Table 1.1, Physiographical division of the Nepal Himalaya (modified after Upreti, 1999)

SN	Geomorphic Unit	Width (km)	Altitudes (m)	Main Rock Types	Main processes for landform development
1	Terai (Northern edge of the Gangetic Plain)	20-50	100-200	Alluvium: coarse gravels in the north near the foot of the mountains, gradually becoming finer southward	River deposition, erosion and tectonic upliftment
2	Churia Range (Siwaliks)	10-50	200-1300	Sandstone, mudstone, shale and conglomerate.	Tectonic upliftment, erosion, and slope failure
3	Dun Valleys	5-30	200-300	Valleys within the Churia Hills filled up by coarse to fine alluvial sediments	River deposition, erosion and tectonic upliftment
4	Mahabharat Range	10-35	1000-3000	Schist, phyllite, gneiss, quartzite, granite and limestone belonging to the Lesser Himalayan Zone	Tectonic upliftment, Weathering, erosion, and slope failure
5	Midlands	40-60	300-2000	Schist, phyllite, gneiss, quartzite, granite, limestone geologically belonging to the Lesser Himalayan Zone	Tectonic upliftment, Weathering, erosion, and slope failure
6	Fore Himalaya	20-70	2000-5000	Gneisses, schists, phyllites and marbles mostly belonging to the northern edge of the Lesser Himalayan Zone	Tectonic upliftment, Weathering, erosion, and slope failure
7	Higher Himalaya	10-60	>5000	Gneisses, schists, migmatites and marbles belonging to the Higher Himalayan Zone	Tectonic upliftment, Weathering, erosion (rivers and glaciers), and slope failure
8	Inner and Trans Himalaya	5-50	2500-4500	Gneisses, schists and marbles of the Higher Himalayan Zone and Tethyan sediments (limestones, shale, sandstone etc.) belonging to the Tibetan-Tethys Zone	Tectonic upliftment,  wind and glacial erosion, and slope degradation by rock disintegrations

Fig 1.12, Physiography of the Nepal Himalaya (after Dahal and Hasegawa, 2008)

Fig 1.13, Generalized geological cross section of the Nepal Himalaya (modified after Dahal 2006)

 

References

Dahal, R., K., 2006, Geology for Technical Students, Bhrikuti Academic Publications, Kathmandu, Nepal, 756p.

GRDP, 1994, Reassessment of the groundwater development strategy for irrigation in the Terai, Vol. 3 Groundwater, HMG, Department of Irrigation, Groundwater Resources Development Project

Hagen, T., 1969, Report on the geological survey of Nepal preliminary reconnaissance: Zürich, Mémoires de la soc. Helvétique des sci. naturelles, 185 p.

Jackson M. and Bilham roger (1994) Constraints on Himalayan deformation inferred from vertical velocity fields in Nepal and Tibet. Journal of Geophysical Research 99 (B7) 13897-13912.

Kizaki, K., 1994, An Outline of the Himalayan Upheaveal, A case study of the Nepal Himalayas, Kathmandu, Japan International Cooperation Agency (JICA), 127p.

Le Fort , P. 1994, French Earth Sciences research in Himalaya region. Alliance Francaise publication, Kathmandu, 174 pp.

Pandey M. R. Tandukar R. P. Avouac J. P. Lavé J. and Massot J. P. (1995) Interseismic strain accumulation on the Himalayan crustal ramp (Nepal): Geophysical Research Letters, 22, 751-754.

Ulak P., D., Nakayama, K., 1999, Lithostratigraphy and the evolution of the the fluvial style of the Siwalik Group in the Hetauda-Bakiya Khola area, Central Nepal. Bull.Dept. Geol. Tribhuvan Univesity,  6-14.

Upreti, 1999, An overview of the stratigraphy and tectonics of the Nepal Himalaya, Journal of Asian Earth Sciences 17 (1999) pp. 577- 606.

Upreti, B. N. and Yoshida, M, (Eds.) 2005, Guidebook for Himalayan Trekkers, Series No. 1, Geology and Natural Hazards along the Kaligandai Vallley, nepal, Department of Geology, Tri-Chandra Campus, Tribhuvan University, Kathmandu, Nepal, 165p.

USGS, 1999, Understanding Plate Motion, URL: http://pubs.usgs.gov/publications/text/understanding.html