Geology of Nepal Himalaya
The kingdom of Nepal occupies the north-central position in
south Asia and is geographically sandwiched between Tibet (china) in the north
and India in the south. It is located in the central part of the 2400 km long
Himalayan arc and covers about one third of ites length (Figure 1) main
structural features of the Himalaya, after A Gansser, 1964. The Mechi and the
Mahakali rivers mark the eastern and western political boundaries with India.
Nepal covers an area of about 147,181 sq km and is bounded by north latitudes
26o 22’ and 30o 27’ and east longitude
80o 04’ and 88o 12’. Geometrically
it forms and elongate rectangle with the average E-W axial length of 885 km and
varying width of 130 to 255 km. geographically, major part of Nepal (83%) falls
within the mountainous region and the remaining portion covers the northern
edge of the alluvial plains of the Gangetic basin
Physiographic feature of Nepal
Hagen (1969) was among the earliest workers to propose the
physiographic subdivision of Nepal. The subdivisions discussed below are after
him with some modification (Figure 2 and Table 1)
1.
Terai
2.
Churia Range (Siwaliks) or Sub-Himalaya
3.
Dun Valleys
4.
Mahabharat range
5.
Midlands
6.
Fore Himalaya
7.
Higher Himalaya
8.
Inner Himalayan Valleys
9.
Tibetan Marginal Range
Table 1: Physiographic
subdivision of the Nepal Himalaya and their main geological unit
|
S.N.
|
Physiographic unit
|
Width, km
|
Main geologic unit
|
|
1
|
Terai (Indo-Gangetic Plain)
|
10-50
|
Northern part of the Indo-Gangetic
basin made up of Recent Alluvium.
|
|
2
|
Churia (Siwalik) Hills and Dun Valleys
|
10-50
|
Sub-Himalaya (Foreland basin) made up of Siwalik or Churia Group of
sedimentary rocks. Recent sediments cover the Dun Valleys.
|
|
3
|
Mahabharat Range
|
20-30
|
Lesser Himalayan unit made up of sedimentary, metamorphic and igneous
rocks
|
|
4
|
Midlands
|
40-60
|
Lesser Himalayan unit made up of
sedimentary, metamorphic and igneous rocks with thrust sheets
|
|
5
|
Fore Himalaya
|
10-50
|
Lesser and Higher Himalayan units
|
|
6
|
Great Himalaya
|
10-50
|
Higher Himalayan unit made up of high grade metamorphic rocks
|
|
7
|
Trans Himalaya
|
10-60
|
Tibetan or Tethys Himalayan unit
made up of fossiliferous sedimentary rocks
|
Terai
This area forming the southernmost part of the northern end
of the Gangetic plain and range in elevation from 100 to 200 m above msl (mean
sea level). The terai region is generally flat with minor relief caused by
river channel shifting downwarping of the basin. It extends from the
Nepal-India border in south to the base of the Churia hills in the north. The width
of Terai varies between 10-50 km and forms nearly continuous belt from east to
west except two areas. South of Chitwan and Rapti valley ot continuity of the
Terai plain is interrupted for about 70 km and 80 km respectively and the
Churia hills mark the political boundary between Nepal and India.
The study area of surface geology as well as the recent
subsurface geological data from large number of drillings made for the
groundwater investigation has led to the further subdivisions of Terai into
Northern (Bhabhar zone), Middle and Southern zones.
Northern Terrai zone (Bhabar zone): The northern
Terai or Bhabar zone lies adjacent to the foothills of Churia and extends
southward to a maximum width of about 12 km. This zone is mainly covered by
thick tropical forest and is composed of a thick sediment zone characterized by
boulders, pebbles, cobbles and coarse grained sand derieved mainly from the
rocks of the Himalayan beyond to the north.
Middle Terai zone: This is an intermediate zone
between Bhabar and the southern Terai Zone. It is about 10-12 km wide. This
zone has been resulted due to the inter-fingering to two distinctly different
facies of sediments. Differences in porosity and permeability between the
sediments of Bhabar zone and southern Terai and the marked change in elevation
has caused the development of spring line, natural ponds and marshy land in
this zone. The zone is characterized by pebbly and sandy sediments with with
few clay layers. Lithologically the pebbles are similar to the northern Terai
zone. This zone is particularly well developed in the Lumbini area of central,
west and eastern Nepal.
Southern Terai zone: This zone lies to the south of
the Middle Terai zone and extends southward the India-Nepal boarder and
continues further into India. This zone represents the typical Gangetic plain.
It is mainly composed of the finer sediments consisting of sand, silt and clay.
The land systems in the terai can be divided into the flood
plains of the river and recent and older river terraces. Generally gravelly and
bouldery soils prevail along the foothills and sandy and /or silty soils are
found in most of the parts. The region often suffers from divesting floods and droughts.
The terai has experienced a massive deforestation since the control on malaria.
Except for the protected forests almost the entire terai is cultivated.
Churia Range (Siwaliks) or Sub-Himalaya
The churia hills abruptly rise from the plains of terai and
forms the southernmost mountain range of the Himalaya. The churia hills rise to
altitude from 900 to 1500 m and show an arcuate face to the south. They form continuous
east – west ranges all along the length of the country with steep escarpment
towards south. Churia hills generally exhibits very rugged topography with deeply
dissected gullies and steep slopes. In spite of the thick forest cover, the
area is very dry and with poor soil development. Due to this fragile character,
a great amount of sediment is contributed to the rivers originating from the
churia hills. Most of the streams originating from the churia hills remain
nearly dry during most time of the year except in the monsoon period. There are
several dun valleys within the churia hills. Some of the important ones are
Trijuga, East Rapti, Nawalpur, Deukhuri, Dang and Surkhet Valleys.
The churia hills are characterized by low terraces, alluvial
fans, bad land, and thin sandy soil covers. The dun valleys are filled up by
the alluvial and fan deposits. Most of the dun valleys are cultivated where as
the siwaliks since last few decades are encroached by more number of people
resulting in the degraded forest, over gazed pasture lands and bad-lands.
Relatively less settlement are found on the Siwalik Hills where as the dun
valleys and larger river valleys are densely populated.
Mahabharat Range
The mahabharat range derives its name from the famous Hindu
epic the Mahabharat. It rises up to 3000 m and extends throughout the length of
the country. The range raises high among the surrounding of the churia hills
and the midlands and significantly controls the climate of the region. At a few
places the mahabhart range is intersected by the major rivers of the country
through which all the water of Nepal originating north of the mahabharat range
is drained off the south. In contrast to the churia hills and the Midlands, the
Mahabharat range is topographically distinct with its towering height, rugged
nature and steep southern slopes.
The Mahabharat range is characterized by the concentration
of the population along the ridge and gently dipping northern slopes. There are
degraded forest and pasture lands.
Midlands
The Midland zone is bounded by the towering snow-clad great
Himalayan range on the north and Mahabharat range on south. The midland zone
has a width of 40-60 km in ranges in elevation between 200 and 3000m. The
midlands consist of low hills, river valleys and tectonic basins form the most
important physiographic province of Nepal. This zone, in contrast to other
physiographic divisions, exhibits a mature and subdued landscape. Within the
midlands are the large valleys of Kathmandu, Banepa, Panchkhal in central
Nepal, Pokhara and Mariphant in western Nepal and Patan in far western Nepal.
The midland zone is drained by a large number of rivers and
streams with predominatly N-S and E-W trending valleys. The larger rivers with
their predominantly N-S course, when reach the northern slope of the Mahabharat
range, suddenly deflect making right angle bends and flow along E-W direction
for a long distances collecting waters of many other N-S flowing rivers and
streams on their way. The rivers breach the barrier of the Mahabharat range only
at a few places. The major rivers flowing through the midlands have very low
gradient and form extensive Quaternary terraces along their courses.
The midlands are marked by the diversity in the land use and
land systems. The soils range from the ancient river terrace to the deeply
weathered residual soil. The river valleys are densely populated and
cultivated. On the other hand, some of the valleys are filled up by the
lacustrine deposit. Cultivated wetlands are found either on the river terraces
or on the gently dipping slopes with colluvial and residual soils. The midland
zone is densely populated comprising nearly half of the country’s population.
Fore Himalaya
Hagen (1969) defined a separate physiographic unit
intermediate between the midlands and the great Himalayan ranges and named it as Fore Himalaya. The Fore Himalayan
zone is 10 to 50 km wide with altitude generally more than 3000 m. Solukhumbu
in eastern Nepal and Dhorpatan and Jumla in the western Nepal belongs to this
zone. The fore Himalaya is generally covered by forest with sparse population.
The population is concentrated on the river valleys.
Great Himalaya
The hills of the midland zone and fore Himalaya slowly rise
to the north and give way to snow-capped ranges of the great (or higher)
Himalaya. Nepal contains not only the highest peak of Mount Everest (Sagarmatha
8848m) but also the greatest number of high peaks of the world. Unlike the
other physiographic units, the great Himalaya does not comprise a single range
but there are several discontinuous and overlapping parallel ranges. The trend
of these ranges varies from E-W and NW-SE to NE-SW. the N-S flowing major
rivers originating beyond great Himalaya have dissected these ranges forming
some of the deepest gorge of the world (Hagen 1969).
The great Himalayan zone is mostly covered by glaciers, snow
peaks, conifer forests, rocky slopes and talus and colluvial deposits.
Topographically this mountain range shows extremely rugged terrain with very
steep slopes and deeply cut valleys. Most of the soil is made up of the
landslide or talus material and the gracial rill with a few fluvio-glacial
deposits along the river banks. The great Himalayan zone, the taller species of
plans are found to confine below 4000 m elevation, while above that zone below
permanent snow covered area, shrubs and algae are the only vegetations
developed. Lacks of good agriculture land and extremely cold climatic condition
have contributed to a very low density of population in this zone.
Trans Himalaya
The Trans Himalaya is situated to the north of the higher
Himalaya and south of the Tibetan marginal range. Beyond the marginal range
lies the Tibetan plateau. It includes several inner Himalaya valleys and hills.
From east to west important valleys are Rolwaling, Kutung, Manang, Mustang
(Thakkhola), Dolpa (Sanju), Mugu, and Humla. Some of these valleys are very
large in size. The length and width of these valleys are variable (Hagen 1969).
As the Trans Himalaya lies in the region of high altitude
and rain shadow, it is generally covered by talus and colluvial deposits, bare
rocky slopes, snow and ice. Though it lies within the Himalayan zone, the
climatic condition and geomorphic features are quite different from the
Himalaya and resemble more to the Tibetan plateau. However, compared to the
cold desert climateof the Tibetan plateau, the inner Himalayan valleys receive
some rain (below 250 mm in average). The rain bearing cloud enters these
valleys through the deep gorges between the higher Himalayan ranges causing the
rain that supports conifer forest and the agriculture in these valleys. In this
zone, human settlements have been found upto an altitude of 4300 m (Phopa Gaon
in Langu Valley).
Geological framework of Nepal
Major Tectonic Zones
As in other parts of the Himalayan sections, geologically
Nepal can be divided into the following five major tectonic zones from south to
north, each zone characterized by their own lithology, tectonics, structures
and history.
1.
Terai zone
2.
Churia Zone (Siwalik zone)
3.
Lesser Himalaya (Midland and Mahabharat zone)
4.
Higher Himalaya
5.
Tethys zone
Geologically Nepal can also be divided into four transverse
zones namely eastern Nepal, central Nepal, western Nepal, and far-western Nepal
each of them separated by a major river. These transverse zones too are quite
distinct in their stratigraphy, structures and tectonics.
Terai Tectonic Zone
This zone represents the northern edge of the Indo-Gangetic
alluvial plain and forms the southernmost tectonic division of Nepal. In the
north it is delineated by the main frontal thrust (MFT) which is exposed at
many places. The terai plain gradually raises from a altitude of 100 m in the
south to 200 m in the north. However, there seems to be slso a gradual regional
slope from west to east in further south to that the Ganges river system flows
towards east to the Bay of Bengal. All the rivers emerging from the neapal
Himalaya first flow the south and finally head eastward towards the bay of
Bengal.
Geologically, the terai plain in covered by recent alluvium
the average thickness of alluvium is 1500 m. The basement topography of the Ganges
basin is not uniform. There appears to have a number of transverse ridges and
valleys and therefore the depth of basement widely varies. The terai sediments
(Pleistocene to recent) were deposited over the churia group of rocks, which in
turn rest over the Gondwanas or younger rocks (Eocene-Oligocene?). The recent
alluvium in terai is brought in by the rivers coming from the hills in the
north. The rivers make fan shaped deposition when they leave the mountains.
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