GENERAL INTRODUCTION OF MALEKHU AREA
1.1. Scope and Objective of the Present Study
The word Geology has been derived from the Greek words “geo” meaning the earth and “logos” meaning discourse. Geology is therefore the science of the earth. The science of geology includes the study of the earth as a whole, its origin, structure, composition, history and nature of processes which have given rise to its present state.
More recently, Geology has been defined approximately synonymous with solid sciences. In course of geological study, field works are indispensable to carry out many investigations and give the student a practical knowledge. The scope and objective of the present study in partial fulfillment of the requirement for the degree of Bachelor of Science in Geology are as follows:
v To use the clinometer compass & measure the attitude of rocks.
v To take the bearing of the distant object and identify the location in map.
v To prepare the route map.
v To study the different rock units / formations of the study area.
v To prepare the lithostratigraphy of the area.
v To prepare the geological map and cross-section.
v To identify the geological structure of the area.
v To prepare the columnar section.
v To know the geological history of the study area with respect to paleogeography, paleotectonics.
The study area lies in Bagmati Zone, 75 km northwest from Kathmandu valley along Prithvi Highway. The study area, a part of Lesser Himalaya lies in Central Nepal and covers the Malekhu Bazaar & the areas lying in the vicinity of Malekhu. Geographically it extends between 270 45′ 00” to 270 52′ 30” north of latitude and 840 47′ 30” to 840 52′ 30” east of longitude. It extends from Chalise in the east to Majhuwatar in the west & Kalidaha in the north to Chhepan in the south (Fig. 1).
The study area is linked with the Prithvi Highway, which is considered as a channel linking Kathmandu with Pokhara & other major cities of the country. Malekhu Bazaar is therefore accessible by motorable and graveled roads. The northern part of the study area is accessible through Malekhu – Dhading road. The remaining parts can be reached by narrow foot trails around the Malekhu and Thopal Khola.
1.4. Topography and Drainage
The topography of the area is very rugged. It includes hills, river valleys, escarpments, spurs, saddles, river plain and terrace etc. The lowest altitude of this area is about 340m at the Trishuli river valley and the highest altitude of this area is about 1525 m at the north of the Dharapani village.
The Trishuli River, a snow fed river of the Central Nepal, is the main drainage of Malekhu and its surrounding areas. It flows through the east to the west, more or less parallel to the structure strike. The main tributaries of the river in that area are Thopal and Malekhu Khola. Thopal Khola drains water from northern part with many sub-tributaries, where as Malekhu Khola runs from the south towards the north and joins with Trishuli River near Prithvi Highway (Fig. 2).
The area as a whole exhibits high drainage density. All the streams are in youth stage. Because of the high stream gradient, water flows with high current. The high flooding on the streams occur during rainy seasons from middle June to middle September.
Because of the variation in altitude, ranging from 350 m from sea-level (at the bank of Trishuli Nadi) to 1000m (neighboring hills), the local climate varies throughout the year. There is tropical to sub-tropical climate in valleys and temperate climate at higher areas. Physiographic condition, monsoon winds, altitude & vegetation controls the climate of the area.
Temperature ranges from 250c to 380c in summer and 100c to 200c during winter season. Valley area is relatively warmer than higher areas. Rainy season brings difficulties in habitation. Various disasters like floods, landslides etc. cause the damage to the highway bridges, agriculture lands, houses etc.
1.6. Land Use
This area represents a poor development of the usable land. Most of the areas are covered by steep hilly ridges, forests, grasslands, river valleys and gullies. The usable lands are the older river terraces and gentle to plain area irrigated by Trishuli River, Malekhu Khola & Thopal Khola of the area. Major crops of the area are sugarcane, maize, millet, wheat, rice etc. Many fruits such as papaya, lemon, banana etc. are found. Many lands are suffered by landslide due to heavy rainfall and deforestation. Small local mining also found in some places.
1.7. Flora and Fauna
This area is rich in flora and fauna. The type of vegetation changes with altitudes. At low altitude deciduous forest is found, which contains Sal (Shorea robusta), Chilaune (Schima walchina), Uttis (Alnus nepalansis), Sissao (Dalbergia sissao), Pine (Pinus roxburghi) and many other trees shrubs and herbs etc.
The faunas like mongoose, monkey, jackal, wildcat, squirrel etc. are found in the forest. Various types of reptiles and birds like kalij, maina, parrot, dove, sparrow, owls, eagles, crow etc. are also found. Besides, invertebrates such as insects, mollusks, annelidas and so on are found. Different types of fishes and other aquatic animals are inhabited in Trishuli and other rivers. Cow, buffalo, ox, sheep, goat poultry are most common domestic animals.
People of different classes and traditions live in this region. Higher parts of the area are inhabited by Magars, Gurungs, Tamangs, Kamis, Sharkis whereas majority of Bhramins, Chettries, Newars are inhabited in low lying hills. Malekhu, Gajuri, Benighat, Dhading Besi are highly populated in comparison of other parts because of good sanitation, school, power supply and motorable road. The people of that area are laborious, polite and friendly behaved. Agriculture and hotel business along the Highway are their economic backbone.
GEOLOGY OF NEPAL HIMALAYA
2.1. Himalaya in General
The world’s highest and largest mountain range the “Himalayas” extend for about 2400 km length where width varies from 160 km – 400 km and lies in the central part of the Eurasia and northwards of the Indian shield. The Himalayas are characterized by outstanding heights, a number of large scale thrusts, variable rock compositions and their isolated positions. The spectacular Himalayan mountain ranges are thrust against the Indian shield or rather under thrust by the later.
In the span of 2400 km from the east to the west, the Himalayas display diverse tectonic patterns. Full scale Himalayan features are developed in the central part of the Himalayas, which gradually disappear in western & eastern syntaxial bends.
2.1.1 Origin of the Himalaya
The Himalaya is the result of continental collision of Indian and Eurasian plate and represents a classic example of continent – continent collision giving rise to a mountain range.
Before the collision of these two continents, a sea, Tethys Sea, was present in between. The first movement took place during the Late Cretaceous – Early Eocene time. It resulted in emplacement of Dras Volcanics along the northern borders of the mountains. As the two continents Eurasia and India approach each other the intervening oceanic crust is subducted and a trench developed. A flysch wedge formed at the continental slope of the Eurasia and deltaic sediments develop over the continental slope of India. Further subduction of the oceanic crust resulted in the development of thrust wedge and folding of overlying sediments and as well these took place regional metamorphism and emplacement of granitic gneisses in the deeper parts of the orogeny. During the Late Eocene time, the collision between the two continental masses lid to the abduction of oceanic material in the form of ophiolites which then mixed with flysch type of deposits, which is also known as the upliftment of Tethys Himalaya. Then after the emplacement of tourmaline granites in the metamorphosed and granitic gneisses, that comprise the Higher Himalayan Zone, took place. During the Middle Miocene time, the rocks of the Lesser Himalayan Zone were deformed into broad folds trending parallel to the Himalayan Chain of mountains. Thrust sheets originating from the northern parts were piled one over the other in a southward transitional movement, due to the northward subduction of the Indian plate. The nappes were further folded and thrust faulted due to continued the south directed couple movement of the rising Himalaya. During the Middle Miocene time, a foredeep was formed between the rising Himalaya and the northern edge of the Peninsula. During the Pliocene – Pleistocene Epoch, the fourth phase of the Himalayan upheaval occurred resulting in the rise of the Himalayan upheaval took place after the Pleistocene glaciers had preceded into the Higher Himalayan regions.
In a plate tectonic model, the rise of the Himalaya has been attributed with the northward drift of the Indian plate and its subsequent collision with the Eurasian plate. The northward drift of the Indian plate began with the fragmentation of the Gondwana land and opening of Proto Indian Ocean during Jurassic Period. The paleomagnetic data from the floor of the Indian Ocean reveals that the northward drift of the Indian plate was most rapid during the Paleocene Epoch. This was also the time of extrusive volcanism over the major part of the Indian Peninsula. The collision of the Indian plate with the Eurasian plate occurred during the Early Eocene. It has been estimated that the collision of these two plates initially started from the western region and then proceeded eastwards. The collision retarded the pace of the northerly drift of the Indian plate. The drift resumed at the beginning of the Oligocene with a slightly changed direction of translation. It is considered that two horns of plate pushed the Tethys sediments in such a way that two hairpins (syntaxial bends) are formed on both sides of the Himalayas.
2.1.2 Transverse Division of Himalaya
Following Gansser (1964), the Himalayan chain is divided geologically and geographically into five major groups.
v The Punjab Himalaya
v The Kumaon Himalaya
v The Nepal Himalaya
v The Sikkim-Bhutan Himalaya
v The NEFA Himalaya
The Punjab Himalaya
The Punjab Himalaya lies between the Indus River in the west and the Sutlaj River in the east. This region is geographically very important. Any river does not cross across the Punjab Himalaya. It includes Kashmir and Spiti region. Its extension is about 550 km.
The Kumaon Himalaya
This Himalayan range runs from the Sutlaj River in the west to the Kali River in the east. It includes the Garhwal Himalayas and parts of the southern Tibet. The best known peaks are Nandadevi (7100 m), Badrinath (7096 m), Kedarnath (6940 m), Trishul (7120 m) etc. The extension of this range is about 320 km.
The Nepal Himalaya
This is the longest division of the Himalaya. It starts from the west at the Mahakali River and ends at the east by the Tista River. It is crowned by eight highest lofty peaks – Mt. Everest, Kanchanjunga, Lhotse, Makalu, Chou, Dhaulagiri, Manaslu and Annapurna I, among 14 peaks of the world exceeding 8000m. The extension of Nepal Himalaya is about 800 km.
The Sikkim – Bhutan Himalaya
The Sikkim – Bhutan Himalaya is confined between the Tista River in the west to the Eastern boundary of Bhutan in the east within which Sikkim and Bhutan lies. Its length is about 400 km.
The NEFA Himalaya
The term NEFA is an abbreviator form of “North East Frontier Agency”. This eastern part of the Himalayan range is about 440 km long. It extends from eastern boundary of Bhutan in the west to the cross – gorges of the Tsangpo – Bramhaputra in the east.
2.2. Geology of the Nepal Himalaya
Nepal occupies the central position of the southwardly convex Himalayan mountain arc and represents a typical unit showing all the regional structural units described by Gansser (1964). Infact, the Nepal Himalaya extends from the Mahakali River in the west to the Tista River in the east. It is interesting to note that 800 km of the 2400 km long Himalayan range lies within Nepal and therefore Nepal Himalaya is the longest division and of major segment of the Himalayan Range, which covers one third of it. Geographically it is bounded by the north latitudes of 260 22′ and 300 27′ and the east longitudes of 800 11′ and 880 27′.
2.2.1 Geology of the Nepal Himalaya In General
Following Upreti and Le Fort (1999), the Nepal Himalaya like other parts of the Himalaya can be divided into five latitudinal morphotectonic zones (Fig. 4) which are as follows:
v Gangetic Plain (Terai)
v Sub Himalaya (Siwalik)
v Lesser Himalaya (Mahabharat Midland Zone)
v Higher Himalaya (Central Crystalline Axis)
v Tibetan Tethys Himalaya
The discussion of the Gangetic plain (Terai) which forms the southernmost tectonic division of Nepal is beyond scope of this report. A brief interpretation of other morphotectonic zones is given here below:
184.108.40.206 Sub Himalaya
The Sub Himalaya (siwalik) lies in the southern part of the country and is represented by low hills, churia range. This zone is bounded to the north by Main Boundary Thrust (MBT) and to the south by the Himalayan Frontal Thrust (HFT) or Main Frontal Thrust (MFT).
This zone is 30 – 40 km wide and even extends up to 52 km. The main lithology of this zone is sandstone, shale & conglomerate. These sediments are derived from the Higher & Lesser Himalayan Rocks. The age of this zone is considered as Middle Miocene to Early Pleistocene & altitude ranges from 200 – 1300 m. This zone is divided into three lithological units namely: Lower, Middle & Upper Siwaliks.
220.127.116.11 Lesser Himalaya
The lesser Himalayas lie in between the Sub Himalayas and Higher Himalayas. Both the southern and northern limits of this zone are represented by thrust, the Main Boundary Thrust (MBT) and Main Central Thrust (MCT) respectively. Lesser Himalayas of Nepal includes the both physiographic division of Midlands and Mahabharat range. The altitude of this zone is about 2200 – 3300m. The total width ranges from 60 – 80 km. The Lesser Himalaya occupies about one third of the total area of Nepal, broad especially in western Nepal. It has a comparatively mild & mature topography with gentle slopes & deeply dissected valleys, which suggest that the rivers are still furiously working.
The Lesser Himalayas is made up mostly of the unfossiliferous sedimentary and meta-sedimentary rocks like shale, sandstone, conglomerate, slate, phyllite, schist, quartzite, limestone, dolomite etc. ranging in age from Precambrian to Eocene. The geology is complicated due to folding, faulting and thrusting, and are further complicated by metamorphism. Higher Himalayan rocks are slided over the meta-sedimentary rocks of Lesser Himalayas, while the MBT brought the older Lesser Himalayan rocks over the much younger Siwalik rocks. Tectonically, the entire Lesser Himalayas consists of two sequences of rocks: allochthonous and autochthonous-para autochthonous units; with various nappes, klippes and tectonic windows.
Hagen (1969) & Stocklin & Bhattarai (1977) have divided the Lesser Himalayan zone into two main geological and tectonic units, which are outlined as follows:
v Kathmandu Complex
v Nawakot Complex
The Nawakot Complex is further subdivided into two groups as
v Upper Nawakot Group
v Lower Nawakot Group
The Kathmandu Complex is also divided into two groups namely
1 Phulchouki Group
2 Bhimphedi Group
18.104.22.168 Higher Himalaya
The Higher Himalayan zone forms a distinctive zone comprising of high grade metamorphic and migmatitic rocks. This zone is bordered by Main Central Thrust (MCT) to the south and Tibetan Tethyan sediments of the Paleozoic Era to the north. The Higher Himalayan zone consists of about 10 km thick succession of crystalline rock of Himalaya extending continuously along the entire length of the country. The width is about 20 km. fourteen snow covered peaks of Himalaya lie in this zone. In Nepal, the Higher Himalayan zone consists essentially of high grade crystalline rocks including various kinds of gneisses, schists and migmatites. The age of rocks found in Higher Himalaya zone is Precambrian – Mesozoic and the granite was intruded in Tertiary Period as determined by radiometric dating. Gansser (1964) has divided the Higher Himalayan zone into two sections as follows:
v Everest Section
v Kaligandaki Section
This section is in the eastern part of Nepal Himalaya. According to Bordet (1961), it is further sub-divided into three major units viz.
v Everest Sediments
v Makalu Granite
v Barun Gneiss
Everest Sediments: Above the Makalu Granite the base of the Everest section shows black to dark green fine grained gneiss. Everest sediments are classified into following three types:
- Everest Limestone
- Everest Pelites
- Lower Calcareous Layer
Makalu Granite: The Makalu Granite is the part of orogenic intrusion, which represents the zone of possible differential movement between the lower Barun Gneiss to higher Everest Pelitic Formation as an intercalatic network of dykes. The granite is generally fine grained with biotite, muscovite & tourmaline.
Barun Gneiss: The Barun Gneiss forms the crystalline base of the normal unit leading upward to the sedimentary section of the Tibetan Tethys Himalaya.
This section is in the western part of the Nepal Himalaya. The rocks of this zone show a homoclinal structures. The thickness of rocks in the Kaligandaki section is 1400m & continuously increases upward reaching up to 5000m in Marsyangdi section. The main rock types of this section are garnetiferous schist, kyanite to sillimanite, garnet to mica banded gneisses of pellitic to arenaceous composition.
22.214.171.124 Tibetan – Tethys Himalaya
The Tibetan – Tethys zone is the northern most tectonic zone of the Himalaya occupying a wide belt of 40 – 75 km. Consisting of sedimentary rocks known as Tethyan sedimentary series. It lies between the South Tibetan Detachment Fault System (STDFS) and the Indus Tsangpo Suture Zone (ITS).The Tibetan – Tethys Zone generally begins from the top of the Higher Himalayan Zone and extends to Tibet in the north. This zone is characterized by fossiliferous rocks such as shale, limestone and sandstone ranging in age from Lower Paleozoic to Paleogene with granitic intrusion in many parts. The rock consists of large amount of fossils of protozoa, brachiopods, mollusca etc.
GEOLOGY OF CENTRAL NEPAL
The central Nepal includes the area between Dudhkoshi River in the east and Marsyangdi River in the west. The main tectonic zones of this region include the following units from north to south.
v Tibetan – Tethys Zone composed of fossiliferous sedimentary rocks.
v The higher Himalayas composed of crystalline rocks.
v The lesser Himalayas composed of low-grade meta-sedimentary, autochthonous to allochthonous rock units. This zone includes the midlands and Mahabharat ranges.
v The Sub-Himalaya (Siwaliks) composed of fore deep molassic sediments.
The geology of the Central Nepal around Kathmandu was first studied by Medlicott (1875). The other geologist who studied the geology of the area profoundly is Auden (1935), Hagen (1951, 1969), Hashimoto (1959, 1973), Stocklin & Bhattarai (1977), Stocklin (1980) etc.
3.2 Geology of Central Nepal (Between Koshi and Trisuli Rivers):
The tectonic succession and the various formations comprising different tectonic units of Central Nepal (Between Koshi and Trishuli Rivers) is based on the stratigraphy given by Stocklin & Bhattarai (1977) which is further revised by Stocklin (1980) and Rai (1998). Apart from the Tertiary Siwaliks, the rocks of the Central Nepal Lesser Himalaya can be grouped into the “Nawakot Complex” & the “Kathmandu Complex” the essential difference between the two complexes lies in their differing lithostratigraphic sequences. Equally important are the differences of metamorphic grade. The Kathmandu Complex, occurring in its highest part includes relatively high-grade metamorphic rocks such as biotitic quartzites, two mica schists, garnet schists, marble, gneisses, migmatites and also granites. The Nawakot Complex on the contrary contains no crystalline rocks but only meta-sediments of low metamorphic grade. Hagen (1969) based on lithological comparisons with the European Alps placed the Nawakot Complex in the Paleozoic to Mesozoic and attributed to the superposition of the Precambrian Kathmandu Complex to nappe structure. (Fig. 5)
The table of subdivision of the above complexes is given in table no. 1 with rock type, thickness & age according to Stocklin & Bhattarai (1977).
The Central Nepal excluding the Higher Himalayan crystalline rocks and Tibetan Tethys fossiliferous sedimentary rocks are described below:
3.2.1 Siwalik Group:
The name of this zone was introduced by Medlicott (1875) from the Siwalik Hills of Haridwar, India. The Siwalik, sub Himalaya or Churia is occupied by Himalaya on foreland basin deposits. The zone consists of Neogene to Quaternary fluvial sediments that form the southernmost hill range in Nepal. It is bounded to the north by the Main Boundary Thrust (MBT) and to the south by the Main Frontal Thrust (MFT). In Nepal the Siwaliks form a 30 – 40km wide foothill belt with average thickness of 5 – 6km and extend beneath the gangetic alluvium in the south. The Siwalik comprises Middle Miocene to Early Pleistocene deposits which are rich in vertebrate, invertebrate as well as plant fossils. The lithological succession of the Siwalik Group forms a coarsening upward sequence.
Lithologically, the Siwaliks of Nepal have been sub-divided from top to bottom into three stratigraphic units namely.
v Lower Siwalik
v Middle Siwalik
v Upper Siwalik
126.96.36.199 Lower Siwalik:
The Lower Siwalik basically consists of fine grained, hard, red, ash grey & reddish brown sandstone with pseudo conglomerates containing pebbles of clay and shale. The Lower Siwalik has a thickness of about 3,000m.
188.8.131.52 Middle Siwalik:
The Middle Siwalik comprises relatively of coarse, gray arkosic sandstones with small proportion of green and gray colored shales and clays. The thickness of Middle Siwalik is about 2500 – 3000m.
184.108.40.206 Upper Siwalik:
The Upper Siwalik is represented by conglomerates, sandstone, siltstone & some clay. Besides conglomerate with pebbles, Upper Siwalik is constituted of boulders of phyllite, formative siltstone, schist and purple quartzite. The size of the clast is increasing upward & sub-rounded. At the top part, consolidated lacking the calcareous matrix. The thickness of Upper Siwalik is about 1000m.
3.2.2 Nawakot Complex:
The name of this complex is derived from an old town and fortress of Nawakot in the Trishuli River valley. It was first used by Hagen (1969) in the form of Nawakot series and Nawakot Nappes. It constitutes large part of the midland of Nepal. The Nawakot Complex consists of pelitic and calcareous metasediments rarely exceeding the sericite – chlorite grade. The age of the complex ranges from Late Precambrian to Late Paleozoic. The rock of Nawakot Complex has been subdivided into two groups by an erosional unconformity (?) as:
v Lower Nawakot Group
v Upper Nawakot Group
220.127.116.11 Lower Nawakot Group:
The Main Boundary Thrust (MBT) separates the Lower Nawakot Group from the Siwalik Group. It consists of nonclastic sediments such as phyllite, slate, dolomite, limestone, metaclastic sediments, quartzite and metasandstone. This group is at least 6km thick but the base is not being exposed. The age of Lower Nawakot Group is Late Precambrian.
The Lower Nawakot Group consists of five formations from bottom to top, namely:
v Dhading Dolomite
v Nourpul Formation
v Dandagaon Phyllite
v Fagfog Quartzite
v Kuncha Formation
18.104.22.168.1 Dhading Dolomite (dh):
The name Dhading Dolomite was first derived for this unit by Arita et. al (1973) from the name of village Dhading.
Dhading Dolomite overlies to Nourpul Formation with transitional contact in between the ridge forming Dhading Dolomite consists of massive to thick bedded sequence of finely crystalline to dense dolomite. It is characterized by bluish – grey color and splintery fracture. Thin black slates are rarely intercalated. Stromatolites are profoundly developed in this formation, which are of collenia and conoplyton types (Upreti et. al. 1980). The convex side of the dome – shaped stromatolite facing opposite to the dip – direction, exhibits overturned strata.
The thickness of Dhading Dolomite ranges from 500 – 1000m. The age is considered as Late Precambrian as indicated by stromatolites. This formation was developed under shallow water environment. It is the youngest formation of Lower Nawakot Group.
22.214.171.124.2 Nourpul Formation (np):
The name was derived from the village Nourpul, located to the south of Dhading.
This formation overlies to Dandagaon Phyllite with sharp contact but conformably. The beginning basal member, the Purebesi Quartzite consists of pure white to greenish – white with yellow and pinkish varieties of fine to coarse grained quartzite. It is a good marker horizon, and shows cross bedding and ripple marks.
Above the Purebesi Quartzite, this formation consists of mostly phyllites with intercalations of quartzites and calcareous rocks. At the upper part dolomitic slate and metasandstones became major lithology. This part contains numerous mudcracks. The V – notch of the mudcracks point opposite to the dip-direction indicating the strata are overturned. The cross bedding, ripple marks and mudcracks manifest the shallow water depositional environment.
The thickness of Nourpul Formation is about 800m and age is Late Precambrian.
126.96.36.199.3 Dandagaon Phyllite (da):
This formation is named after the village of Dandagaon in the east of Dhading and is synonymous with “Daram Suite” (Talalov 1972).
The contact of Dandagaon Phyllite with underlying Fagfog Quartzite is a rapid pass through a transitional zone. This unit consists of uniform argillaceous to finely quartzite phyllites of dark blue – green color. Phyllites are darker than the Kuncha type phyllites, and can be distinguished by lacking stretching lineation and intersecting crenulations cleavage. This formation is characterized by reddish tints in slightly weathered condition. The phyllites are frequently folded in small – scale. Within this unit intercalation of thin quartzitic beds are found carbonaceous material makes a first sporadic appearance in the form of laminated calc phyllites and occasional thin bands of dolomite.
The thickness of Dandagaon Phyllite is about 1000m and its age is assumed to be Late Precambrian.
188.8.131.52.4 Fagfog Quartzite (fg):
This formation has been named after the village of Phakphuk in the lower Thopal Khola and was introduced by Arita et. al. (1973)
Fagfog Quartzite rests over Kuncha Formation with a sharp contact. It is a fine to coarse grained white orthoquartzite with several phyllitic intercalations. The rocks are massive, highly fractured and jointed. Well developed current and oscillation ripple marks are the salient features of this formation. The sharp crests of the oscillation ripple marks pointed opposite to the dip-direction of the bedding, indicates the overturned strata. This formation extends persistently for a long distance making prominent ridges and therefore is a good marker horizon in such a terrain of unfossiliferous sequences.
The thickness of Fagfog Quartzite is in the order of 400m. The formation was sedimented under shallow water environment and the age is assumed to be Late Precambrian.
184.108.40.206.5 Kuncha Formation (kn):
The formation derived its name from the village Kuncha in Lamjung district and was first introduced by Bordet (1961).
Kuncha Formation is the oldest formation of the Nawakot Complex, and perhaps of the entire Lesser Himalaya of Nepal. It is composed of a rather monotonous flysch – like entirely non calcareous sequence of alteration of phyllites, phyllitic quartzites and phyllitic gritstones. The upper section has more phyllite and phyllitic gritstones. The phyllites have oily luster and yellowish blue – grey and green – grey in color. The phyllitic gritstones contains opal like milky – grey or bluish quartz, giving a diagnostic feature to this formation (Arita et. al. 1973).
These rocks exhibit polyphase of deformation as indicated by differently oriented crenulation cleavages and stretching lineation. The rocks are folded in small – scale, giving an undulatory surface. The Kuncha Formation has sericite & chlorite as the metamorphic mineral but a deeper section shows the increase of metamorphism with the appearance of chloritized biotite and garnet. The metamorphic grade also increases in general nearer to the MCT.
The thickness of Kuncha Formation is about 5,000m, and the base being no where exposed. The age of this formation is supposed to be Precambrian. The contact with the overlying Fagfog Quartzite is seemed to be conformable and gradational.
Table No. 1 Stratigraphic Subdivisions of Central Nepal
|Unit||Formation||Main Lithology||Apparent Thickness (m)||Age|
|Godavari Limestone||Limestone, dolomite||300||Devonian|
|Chandagiri Limestone||Limestone||2,000||Cambro – Ordovician|
|Sopyang Formation||Calc-phyllite, slate||200||Cambrian|
|Tistung Formation||Metasandstone, phyllite||3,000||Late Precambrian|
|———————————- Transitional Contact ———————————-|
|Markhu Formation||Marble, schist||1,000||Late Precambrian|
|Kulikhani Formation||Quartzite, schist||2,000||Precambrian|
|Chisapani Quartzite||White quartzite||400||Precambrian|
|Kalitar Formation||Schist, quartzite||2,000||Precambrian|
|Raduwa Formation||Garnetiferous schist||1,000||Precambrian|
|————————- Mahabharat Thrust (MT) ————————-|
|Robang Formation||Phyllite, quartzite||200 – 1,000||Paleozoic|
|Malekhu Limestone||Limestone, dDolomite||800||Paleozoic|
|Benighat Slate||Slate, argillaceous dolomite||500 – 3,000||Paleozoic|
|————————- Unconformity (?)————————-|
|Dhading Dolomite||Stromatolitic dolomite||500 – 1,000||Late Precambrian|
|Nourpul Formation||Phyllite, quartzite, dolomite||800||Late Precambrian|
|Dandagaon Phyllite||Phyllite||1,000||Late Precambrian|
|Fagfog Quartzite||White quartzite||400||Late Precambrian|
|Kuncha Formation||Phyllite, quartzite, Conglomerate, gritstone||5,000||Late Precambrian|
|————————- Main Boundary Thrust (MBT) ————————-|
|Siwalik Group||Sandstone, mudstone, conglomerate||Several Kilometers||Neogene|
After Stocklin and Bhattarai, 1977
220.127.116.11 Upper Nawakot Group:
The Upper Nawakot Group comprises the upper part of the Nawakot Complex and overlies to Dhading Dolomite with an erosional unconformity marked by an abrupt lithological change. The age of this group is considered as Paleozoic. The thickness greatly varies from 2000 – 5000m. This group consists of slate, dolomite, limestone, phyllite & quartzite.
The Upper Nawakot Group is divided into three formations, which from bottom to top are as follows:
v Robang Formation
v Malekhu Limestone
v Benighat Slate
18.104.22.168.1 Robang Formation (rb):
This formation has been named as Robang Formation after the village Robang. Robang Formation lies over Malekhu Limestone with transitional contact in between. It consist basically phyllites & associated quartzites in a strongly tectonized condition immediately below the Mahabharat Thrust (MT). The lower portion is mainly thin bedded quartzite with intercalations of sericitic – chloritic green colored phyllites. The middle part of this formation, which are associated with intrusion of basic rocks, amphibolite, consisting mainly of hornblende and plagioclase, showing the discordant contact. Towards the upper section, the quartzite becomes major rock type. This quartzite is called Dunga Quartzite, member of the Robang Formation. It consists of medium grained, clean, white quartzite with minor green chloritic phyllite intercalations. The Robang Formation is overlain by highly ganetiferous schist.
It is the youngest formation of the Nawakot Complex. Mahabharat Thrust (MT), is the tectonic contact between Robang Formation and overlying Bhimphedi Group of Kathmandu Complex. The thickness of Robang Formation varies from 200 – 1000m and its age is assumed to be Paleozoic.
22.214.171.124.2 Malekhu Limestone (ml):
This formation has been named after the village of Malekhu lying on the Kathmandu Pokhara road at the confluence of the Trisuli River and the Malekhu Khola.
Malekhu Limestone overlies to Benighat Slate with transitional contact in between this formation consists of grayish white colored limestone. Which are fine grained to dense, hard with partings. The lowermost part of this formation is composed of dense siliceous dolomitic limestone with pale green sericite partings. The middle part is thinly to thickly bedded darker gray dolomites with frequent appearance of chert and carbonaceous slate. The uppermost part shows interbedded sequence of dolomite and phyllite with pale green sericite partings.
The thickness of Malekhu Limestone is about 800m and is of Paleozoic age.
126.96.36.199.3 Benighat Slate (bg):
The name of this formation is derived from the village Benighat at the confluence of Burhi Gandaki and Trishuli River.
This formation lies over Dhading Dolomite with angular unconformity in between this is the lowermost unit of upper Nawakot Group. It consists of slates, phyllite slates with intercalation of calcareous beds. Slates are dark bluish – gray to black in color. The black color is due to presence of carbon contents or graphite material and called as graphitic slates. These carbonaceous slates sometimes show the rust color, limestone weathering crust and also yellow and orange powdery effervescence of sulphur, sericite and chorite are present. The phyllitic slates are light greenish – grey in color due to the presence of chlorite. The thin calcareous beds present in the formation are known as Jhikhu Calcareous Beds.
The thickness of Benighat Slate ranges from 500 – 3000m. The age of the rocks of this formation is Paleozoic.
3.2.3 Kathmandu Complex:
The Kathmandu Complex entirely belongs to the Kathmandu Nappe, which is considered as the backbone of the tectonic framework of Central Nepal. It was first introduced by Hagen (1969), and later detailed mapping of the nappe was carried out by a team of geologists from the Department of Mines and Geology, Nepal.
The crystalline rocks of the Kathmandu Nappe or Kathmandu Complex are separated by Mahabharat Thrust (MT) from underlying metasediments of the Lesser Himalayas or Nawakot Complex. The Kathmandu Nappe is believed to be connected to the root – zone in the Higher Himalayas of Langtang – Ganesh Himal area by a tectonic bridge through Gosainkunda Lekh (Hagen, 1969). And the MT is considered as continuation of MCT (Stocklin, 1980); however the provenances of the Kathmandu Nappe rocks have been in question. Based on stratigraphy, lithology and metamorphism Upreti and Le Fort (1999) recognized two thrust package in the Kathmandu transect, namely; the Gosaikunda crystalline Nappe (GSN), and the Kathmandu crystalline Nappe (KCN) (equivalent to Kathmandu Nappe), separated by MCT. The GCN corresponds to the continuation of the Higher Himalayan crystalline series (HHCS) of the Langtang Himal, exposed at the north of Kathmandu Valley, but not the KCN expose to the south. According to Upreti and Le Fort (1997), the MT, that separates the KCN from the Nawakot Complex, is not the direct continuation of MCT; but an independent thrust. The KCN is squeezed mass of rocks lying between MCT and MT, which is thrusted over the Nawakot Complex along MT during the crustal shortening.
The rocks of Kathmandu Nappe called Kathmandu Complex (Stocklin & Bhattarai, 1977; Stocklin 1980), is divided into two groups viz. the Precambrian Bhimphedi Group consisting of relatively high grade metamorphic rocks, and the Phulchouki Group of unmetamorphosed to weakly metamorphosed sediments containing fossils of Lower – Middle Paleozoic age. These two groups are separated by a transitional zone.
188.8.131.52 Bhimphedi Group:
This name was initially introduced by the Indian geologists in the form of “Bhimphedi Formation” or “Bhimphedis” to designate the relatively high – grade rocks that are widely developed around the town of Bhimphedi.
The Precambrian Bhimphedi Group consisting of relatively high grade metasediments shows steady decrease in the metamorphic grade from garnet schist at the bottom to the sericite – chlorite grade on the top. Metamorphic grade is relatively high along the MT. The total thickness of Bhimphedi Group is about 10km. The group consists of six formations, which are as follows from bottom to top.
v Markhu Formation
v Kulikhani Formation
v Chisapani Quartzite
v Kalitar Formation
v Bhainsedobhan Marble
v Raduwa Formation
184.108.40.206.1 Markhu Formation (mr):
The formation has derived its name from the village “Markhu” in Kulikhani valley and was introduced by Joshi (1973).
This formation overlies the Kulikhani Formation with a transitional contact. It is the youngest formation of the Bhimphedi Group of Kathmandu Complex. It has mixed lithology, consisting of schist, quartzite and marble in varying proportions. The marble is medium to coarse grained, white to pink in color, which makes up about 50% of the total rock volume in the area. The schist and quartzite between the marble bands are dark, biotitic, fine grained and also include phyllitic and calc – phyllitic varieties.
The thickness of Markhu Formation is about 1,000m. The age of the formation is assumed to be Late Precambrian.
220.127.116.11.2 Kulikhani Formation (ku):
The Kulikhani Formation has been named after the village and river of Kulikhani.
The Kulikhani Formation overlies the Chisapani Quartzite with a transitional contact. It is a well bedded alternation of fine grained biotite schist and impure strongly micaceous quartzite of dark and light green grey color. Isoclinal folds, microfolds and drag folds are distinct in this formation.
The thickness of Kulikhani Formation is about 2,000m and the age of the formation is Precambrian.
18.104.22.168.3 Chisapani Quartzite (cp):
The name is derived from the village and forest of Chisapani on the Bhimphedi – Kathmandu foot track and was first used by Joshi (1973).
The Chisapani Quartzite overlies the Kalitar Formation with transitional contact. It is conspicuous white or pale green clean orthoquartzite. It is fine grained thin to thick bedded quartzite showing strong cross bedding and ripple marks. Augen gneiss is intruded in this formation with xenolith in gneisses. The gneiss is represented by thin layer of biotite and lenticular eye shaped feldspars. The presence of cross bedding in Chisapani Quartzite indicates that the sediments were deposited under shallow water environment.
The thickness of Chisapani Quartzite is about 400m and is of the Precambrian age.
22.214.171.124.4 Kalitar Formation (ka):
The name Kalitar has been derived from the village Kalitar on the Tribhuvan Highway.
Kalitar Formation overlies to Bhainsedobhan Marble with a sharp contact. It is composed of dark green – gray biotite – muscovite schist with intercalations of impure micaceous quartzites. Garnet and amphibolite minerals are common in the lower part but disappear in the upper part. The micaceous schists are well bedded and fine grained.
The thickness of Kalitar Formation is about 2,000m and is of the Precambrian age.
126.96.36.199.5 Bhaisedobhan Marble (bd):
This formation was named after the village Bhainsedobhan on the Tribhuvan Highway.
The Bhainsedobhan Marble overlies to Raduwa Formation with a transitional contact. It is coarsely crystalline, well bedded to massive white marble containing mica in fine dispersion and in the basal & top parts, as partings and thin intercalations of biotite – garnet schist. Pyrite crystals are abundant in coarsely crystalline marble. This marble is a good marker horizon.
The thickness of Bhainsedobhan Marble is 800m. The age of this formation is Precambrian.
188.8.131.52.6 Raduwa Formation (ra):
The name Raduwa owes its origin to a village in the Gorangdi Valley. This formation is synonymous with “Garnetiferous Biotite Schist and Quartzite.”
Raduwa Formation is separated by Mahabharat Thrust (MT) from underlying Nawakot Complex. It is the oldest & lowermost formation of the Bhimphedi Group and consists of coarsely crystalline garnetiferous mica schist with quartzitic intercalations.
The mica schist is dark green grey to light grey color, containing characteristic real garnet which occurs in great abundance, sometimes crystal up to 1cm in diameter.
The thickness of Raduwa Formation is about 1,000m. According to Stocklin and Bhattarai (1977) the age of this formation is Precambrian.
184.108.40.206 Phulchauki Group:
The name of this group is derived from the Phulchauki Hill located to the south of Kathmandu Valley, and it is famous for containing fossils. The Phulchauki Group represented by argillo-arenaceous and argillo calcareous rocks can be composed with the “Chandragiri Series”. Phulchauki Group is correlated with the Tibetan sedimentary zones. The total thickness of Phulchauki Group is about 6 – 7km. The Bhimphedi Group passes into the Phulchauki Group with a significant break in sedimentation. The Phulchauki Group is divided into the following Formation from bottom to top as:
v Godavari Limestone
v Chitlang Formation
v Chandragiri Limestone
v Sopyang Formation
v Tistung Formation
220.127.116.11.1 Godavari Limestone:
The name of this formation was introduced from the Godavari Village at the north foot of the Phulchauki Hill.
The Godavari Limestone is the youngest rock formation in the Kathmandu Complex and forms the summit of the Phulchauki Hills. It occupies the core of the Phulchauki Syncline and which in turn is the very core of the entire Mahabharat Synclinorium. This formation is comprised of well – bedded green and purple argillaceous limestone with crinoidal fragments. The main part of the formation above the crinoidal limestone is the medium to coarse grained, massive, crystalline limestone and dolomite of grayish blue, pale brown to purple color.
The thickness of Godavari Limestone is about 300m and the age is assumed to be Devonian.
18.104.22.168.2 Chitlang Formation:
The name of this formation has been derived from the village Chitlang in the upper Chitlang Valley first used by Hagen (1969).
Chitlang Formation overlies to Chandragiri Limestone with transitional contact. It consists of dark purplish soft weathered slates with white quartzite interbedded in the lower part and argillaceous limestone in its upper part. It forms the true core of the Chandragiri Syncline. This unit contains 2 or 3 haematite layers 10 – 15m thick in the upper part and is rich in Silurian Fauna of trilobites, brachiopods, echinoderms etc.
The thickness of Chitlang Formation is about 1,000m and the age is Silurian.
22.214.171.124.3 Chandragiri Limestone:
The name is derived from the Chandragiri range southwest of Kathmandu, and first used by Auden (1935).
Chandragiri Limestone is the most prominent unit of the Phulchauki Group due to its thick limestone sequence and wider distribution forming the Phulchauki and Chandragiri Hill range. This formation is of massive appearance with frequent argillaceous parting especially in the more thinly bedded lowermost & uppermost parts. A band of white laminated quartzite is interbedded in the upper part of yellow to brown colored limestone of this formation.
The thickness of this formation is about 2,000m. The fossils of crinoids & echinoderms found in Chandragiri Limestone suggest that the rocks belong to Cambro – Ordovician age.
126.96.36.199.4 Sopyang Formation:
The name of this formation was derived from the village Sopyang on the Trivhuvan Highway.
The Sopyang Formation overlying the Tistung Formation is a transitional phase between the fine grained clastic Tistung Formation and the thick younger Chandragiri Limestone. The presence of dark argillaceaus slates, soft weathered phyllite, thinly bedded layers of argillaceous limestone of dark gray to black color are the salient features of this formation.
The thickness of Sopyang Formation is about 200m and the age is believed to be Cambrian age.
188.8.131.52.5 Tistung Formation:
The formation has been named after the village Tistung on the Tribhuvan Highway.
Tistung Formation overlies the Markhu Formation of Bhimphedi Group with transitional contact. This is the oldest formation of Phulchauki Group which consists of fine clastic sequences of metasandstones, siltstones, phyllites, slates and calc – phyllites. Very fine grained biotite is seen in the lower part but sericite and chlorite are the metamorphic minerals in the main part and in the overlying units. Quartzite gives way in places to distinct pink, buff & purple tints, particularly in the sandstone, which often shows red / green color banding with the interbedded green phyllite. Distinct color banding & intense purple and pink color in weathered condition is the typical characteristic of this formation.
Granitic pluton called Agra granite is intruded into the Tistung Formation. Agra granite is a leucocratic, medium to coarse grained, porphyritic with distinct phenocrysts of feldspars. The rock mostly consists of feldspar, quartz, biotite, with tourmaline and muscovite as accessories. This granite is S – type i.e. derived from the partial melting of sedimentary rocks. This granite is characterized by the presence of xenoliths (unmelted fragments of the country rocks). Around this intrusion contact metamorphism is pronounced and forms aureole of hornfels, within this hornfels pegmatite veins are found, which consist of large crystals of feldspar, quartz, tourmaline, biotite and muscovite. Age of Agra granite is about 165Ma (Talalov, 1972).
The total thickness of Tistung Formation is about 3,000m. The age of this formation is believed to be late Precambrian.
GEOLOGY OF THE STUDY AREA
In course of the fieldwork, the geology of Malekhu area along Malekhu – Dhading road and Malekhu Khola have been studied. The Stratigraphy of the studied area can be discussed under two major heads, the Nawakot Complex and the Kathmandu Complex. Each complex has been divided into two groups, the Nawakot complex comprises the Lower Nawakot Group and the Upper Nawakot Group: the distinction has been made to indicate a possible major stratigraphic unconformity between the two groups. The Kathmandu Complex is divided into the (Lower) Bhimphedi Group and the (Upper) Phulchauki Group, the two being stratigraphically continuous but distinguished by different grades of metamorphism. Geological Route Map, Geological Cross-Section and Stratigraphic Column are shown in Fig. 6, Fig. 7 & Fig. 8 respectively.
4.1.1 Nawakot Complex:
The Nawakot Complex is divided into two groups:
v Upper Nawakot Group
v Lower Nawakot Group
184.108.40.206 Lower Nawakot Group:
The rocks of the Lower Nawakot Group are separated from upper Nawakot Group by an erosional unconformity (?). This group is further subdivided into five formations, which are:
v Dhading Dolomite
v Nourpul Formation
v Dandagaon Phyllite
v Fagfog Qurtzite
v Kuncha Formation
220.127.116.11.1 Dhading Dolomite (dh):
The Dhading Dolomite is observed on the left bank of Thopal Khola near Katle Danda. The exposed rock is gray colored, highly fractured massive Dolomite.
On the exposure along Malekhu-Dhading road, stromatolitic structures are observed (Fig. 12). They are convex downward, which reveals that the rock sequence is overturned. The fresh color of rocks is light gray and weathered color is yellow. The rocks are fine grained.
The thickness of Dhading Dolomite is about 500m with attitude of N 800 E / 790 N W.
18.104.22.168.2 Nourpul Formation (np):
The exposure of Nourpul Formation lies in between Mawi Khola and bridge over Thopal Khola along Malekhu-Dhading road, where the rocks are seen on a steep hill. The formation is consisted of dark gray phyllites, metasandstone and a few carbonaceous phyllite bands with quartz veins. The basal part contains quartzitic zones while the middle part consists of phyllite with some intercalations of quartzite and carbonate. The upper part is composed of dolomitic quartzite of green to blue green color. In the rocks, there is the presence of mica mineral along with metasandstone. In metasandstone, mudcracks (fig. 10), ripple marks and cross bedding (fig. 11) are seen.
The thickness of this formation is found to be about 1600m and the attitude of foliation plane is of N 750 E / 840 N W.
22.214.171.124.3 Dandagaon Phyllite (da):
The Dandagaon Phyllite is exposed about 150 – 200m north of the Thopal Khola Bridge along Malekhu-Dhading road near the village Bungchung. It consists of graphitic phyllite of bluish gray color with interbeds of green gray chloritic phyllites. Quartz veins across and parallel to the foliation plane is observed. Crenulation cleavage is distinct in rock exposures of this formation. The fresh color of the rock is dark gray.
The thickness of Dandagaon Phyllite is about 500m and the attitude of foliation plane is of N 750 E / 840 N W.
126.96.36.199.4 Fagfog Quartzite (fg):
The Fagfog Quartzite overlying the Kuncha Formation is found along the Malekhu – Dhading road on the right bank of Thopal Khola where the rocks are exposed on a cliff. The Fagfog Quartzite is milky white in color. The beds are blocky to massive and highly jointed. The rocks are medium to coarse grained. The formation essentially consists of crystalline transparent bimodal orthoquartzite. The major sedimentary structures present are the ripple marks which being symmetrical are called oscillation ripple marks and asymmetrical are called current ripple marks (Fig. 9).
The thickness of this formation is about 400m and the attitude of foliation plane is of N 820 E / 720 N W.
188.8.131.52.5 Kuncha Formation (kn):
The Kuncha Formation lies in the northern part of Malekhu and around the Thopal Khola Valley. The rocks of this formation also occur around the Trisuli River area to the north of Dhading and most of the hills between Thopal Khola and Lower Marsyangdi in the western part. The formation consists of dark green to dirty green and light green to dark green colored phyllite. Lineation and foliation are distinct in the phyllite. The fresh color of the rocks is greenish gray and weathered color is yellow & brown. Many minor folds with crenulations cleavage are also present.
The thickness of Kuncha Formation in the study area is more than 2,000m and the attitude of foliation plane is of N 750 E / 700 N W.
184.108.40.206 Upper Nawakot Group:
The upper Nawokot Group consists of the following three formations:
v Robang Formation
v Malekhu Limestone
v Benighat Slate
220.127.116.11.1 Robang Formation (rb):
The Robang Formation is well exposed on the both bank of the Malekhu Khola about 550m south from the Malekhu Bridge. The outcrop consists mainly of phyllite, which is associated with quartzite. The quartzite is massive, thick bedded and yellowish white in color. The phyllites are seen as dark green patches. One can also find greenish colored metamorphic rock, amphibolite. In this zone, a hill of quartzite (Dunga Quartzite) is observed. Foliation plane, joints and fracture are distinct in Robang Phyllites. There are numerous faults in this formation.
The thickness of Robang Formation is about 950m with attitude of N 800 E / 850 S E. This is the uppermost formation of the Nawakot Complex, above to which there is rocks of Kathmandu Complex and separated by a tectonic boundary called Mahabharat Thrust (MT).
18.104.22.168.2 Malekhu Limestone (ml):
This formation is observed at an outcrop of about 50m width and 5m height on the left bank of Trisuli River. The formation has crystalline calcareous limestone of light yellow to white color. They are blocky to massive with fine to coarse grained. The rocks are thinly laminated and the thickness of lamina is from 2 – 5mm. The rocks in this formation partly include gray dolomite and partly shown as a carbonate intercalation of carbonaceous phyllites. The middle part is the main part, which is more thickly bedded dolomite or dolomitic limestone of dark gray color. Quartz veins are intercalated parallel to the foliation. Boudinage structures are observed in this formation.
The thickness of this formation is about 800m and the limestone strikes along N 840 E with dip amount 840 due S E.
22.214.171.124.3 Benighat Slate (bg):
A good out crop of Benighat Slate is observed at about 300 N E of the suspension bridge over the Trishuli River along the Malekhu-Dhading road, this slate is exposed in so many places. This formation consists of gray colored slate and highly carbonaceous graphitic slate. Minor folds are observed in this formation. Intercalation of quartz veins 2 – 4cm thick is also observed.
The thickness of Benighat Slate is more than 1,700m and the attitude is of N 750 E / 820 S E.
4.1.2 Kathmandu Complex:
The Kathmandu Complex is composed of high – grade metamorphic rocks. In Malekhu area, rocks of Precambrian Bhimphedi Group are exposed. In the study period, the rocks of Phulchauki Group and also uppermost formation of Bhimphedi Group namely Markhu Formation was not observed.
126.96.36.199 Bhimphedi Group:
The Bhimphedi Group has six formations, but the uppermost, Markhu Formation was not covered in the study plan. So only five formations namely from older to younger below are studied:
v Kulikhani Formation
v Chisapani Quartzite
v Kalitar Formation
v Bhainsedobhan Marble
v Raduwa Formation
188.8.131.52.1 Kulikhani Formation (ku):
The Kulikhani Formation is exposed at the right bank of Malekhu Khola, just above the augen geniss of Chisapani Quartzite. The rocks of this formation consist mainly of schist, phyllite and metasandstone. Biotite is predominant in this formation giving overall black appearance to the rocks of this formation. The exposed rocks are dark quartzite with excess of mica generally biotite.
The attitude of foliation plane of this formation is of N 780 E / 640 S E.
184.108.40.206.2 Chisapani Quartzite (cp):
Fine exposure of rocks belonging to the Chisapani Quartzite is found at Chepan towards the east on the junction of Aap Khola and Malekhu Khola. The exposed rock mainly comprised of coarse grained quartzite containing micaceous minerals. The fresh color of the unit is white and weathered color is pale yellow. On moving about 30m upstream from the location, a huge exposure of augen gneiss containing augen shaped feldspar, tourmaline, biotite etc. is found (Fig. 18). The thickness of augen gneiss is about 50m. At the base of Chisapani Quartzite, plunging folds are seen, as well as isoclinal folds are also seen.
The thickness of this formation is found to be 350m and the attitude of foliation plane is of N 750 E / 650 S E.
220.127.116.11.3 Kalitar Formation (ka):
The Kalitar Formation overlying the Bhainsedobhan Marble is found to occur at the left bank of Dhobi Khola. The main constituent rock of this formation is mica schist. The exposed rock consists of strongly micaceous quartzite, which is on the whole subordinate to the schist. The general color of rocks in the exposure is dark green gray with brownish weathering color. The schist shows varying schistosities which obscure the bedding.
The thickness of Kalitar Formation is about 1700m and the attitude of foliation plane is of N 720 E / 840 S E.
18.104.22.168.4 Bhainsedobhan Marble (bd):
The Bhainsedobhan Marble is well exposed near Beltar. The rock has massive appearance but at close view, shows always distinct bedding and fine calcite crystals. The marble contains some mica and few pyrite crystals. The color of marble is greenish brown. The rocks are medium to coarse grained.
The thickness of Bhainsedobhan Marble is about 125m with attitude of N 740 E / 700 S E.
22.214.171.124.5 Raduwa Formation (ra):
A good exposure of Raduwa Formation is in the western part along the Malekhu Khola above the Mahabharat Thrust (MT). The dominant lithology of this formation is garnetiferous – biotite schist. The schist is generally coarse in texture in which garnet crystals are in great amount. The schist contains lenses and nodules of quartz. The rocks show minor folds also. At some place, the rocks are intercalated by strongly micaceous gray quartzite. Quartzite layers are developed along the schistosity plane, which consists of predominantly biotite, muscovite, quartz and few grains of garnet & feldspar.
The thickness of Raduwa Formation is about 350m and the attitude of foliation plane is of N 700 E / 740 S E.
4.2 Geological Structures:
The study of geological structure i.e. structural geology is the study of the architecture of the rocks in so far as it has resulted from deformation. As such, the aim of structural geology is to determine and explain the architecture of the rocks as observed in the field. Geological structure aid in study of sequence of events in an area, physical forces that produced the observed structure, nature of the causative forces and the depositional environment.
The geological structures observed in Malekhu area can be discussed under two major heads.
v Primary Structures
v Secondary Structures
4.2.1 Primary Structures:
Primary Structures are the structures produced during depositional time. Various types of Primary structures observed in Malekhu are as follow:
Ripple Marks of both types (oscillation and current) are well developed in Fagfog Quartzite and Nourpul Formation. In Fagfog Quartzite, oscillation ripple marks are well exposed along with few current ripple marks. The oscillation ripple marks are symmetrical and consist of broad troughs that are convex downward and of sharp crest that point upward. In Fagfog Quartzite the sharp crest of the oscillation ripple point in opposite direction to that of the dip direction of the beds, indicating the beds are overturned. The sharp crests points towards the younger beds whereas the rounded trough is convex towards the older beds. On the other hand, current ripple marks are asymmetrical and both trough and crest are rounded. Such ripples develop when a current, either of water or of air moves across sand or mud. (Fig. 9)
Fig. 9: Ripple Marks in the Fagfog Quartzite.
Mudcracks are well exposed at the left bank of Mawi Khola, which consists basically of calcareous sandstone. Mudcracks are developed when deposited sediments are exposed to the Sun. As a result water of the sediments will be evaporated and ‘V’ shaped empty structure or cracks will be developed. Then further deposition over that bed will fill up the empty ‘V’ shaped cracks, and due to various processes that will be developed into rocks. The pointed end of ‘V’ points towards older strata & open end points towards younger strata. In Nourpul Formation, the open end points towards opposite direction to that of dip direction. This indicates that the beds are overturned.
Fig. 10: Mudcracks in the Nourpul Formation
In Nourpul Formation, cross bedding with tangential lower contact are observed. The cross beds are sharply truncated above and are tangential to the true bedding below. The cross beds in planar cross bedding are inclined to the true bedding at a considerable angle at both their upper and lower extremities. (Fig. 11)
Fig. 11: Cross Bedding in the Nourpul Formation
Stromatolites are dome shaped structures produced by algae. The pointed end of the stromatolite indicates towards older strata and dome shaped rounded end points towards younger strata. But the stromatolite observed in Dhading Dolomite have pointed end directing up and rounded end directing below, which indicates that the beds are overturned. (Fig. 12)
Fig. 12: Stomatolite seen in the Dhading Dolomite
4.2.2 Secondary Structures:
The Secondary Structures are produced after the deposition of sediments and due to stress applied in it. The various types of secondary structures observed during geological excursion along Malekhu Khola and Malekhu-Dhading road are as follow:
An unconformity is a surface of erosion or non deposition, usually the former – that separates the younger strata from older rocks. An erosional unconformity is found separating the Upper and Lower Nawakot Groups, in between Dhading Dolomite & Benighat Slate. However this unconformity is still in controversy.
The rocks of the study area is comprised of numerous faults of which are thrust.
Thrust – Mahabharat Thrust (MT):
The low angled reverse fault is called thrust. In the study area, there was a thrust, the Mahabharat Thrust (MT) separating the Lesser Himalaya into two complexes, Nawakot Complex and Kathmandu Complex. It is observed 1km S-E from Malekhu Bazaar and is believed to be the continuation of MCT. A sharp reversal in the regional metamorphism from chlorite grade below to garnet amphibolite grade above testifies the presence of Mahabharat Thrust. The regional discordance along which both the underlying & overlying stratigraphic sequences are truncated to a variable degree also indicates the presence of Mahabharat Thrust (MT). Some Geologists believe that a marked change in structural pattern reveals the presence of MT. The MT is best expressed in the south west flank of the synclinorium, where the rocks below the discontinuity (Upper Nawakot Group) shows shear folding, slicing and imbrication, whereas the rock immediately above the discontinuity (lower part of Bhimphedi Group) appears as a competent and rigid.
In the study area several other faults were also observed. A reverse fault is clearly observed in Malekhu Limestone about 300m upstream along Malekhu Khola from the old bridge. Beside this fault in this area, there are so many normal & reverse faults in Robang Formation, which might have been produced due to Mahabharat Thrust in the upper part of Robang Formation. Minor faults were also observed along the left bank of Malekhu Khola at Robang Formation.
Fig. 13: Reverse Fault in the Robang Formation.
In many metamorphic rocks the schistosity may be crinkled into small folds and eventually the mica flakes are rotated into discrete zones parallel to the axial planes of the crinkles. Displacement may take place along these zones and the rocks tend to break parallel to these zones. This type of cleavage is termed as crenulation cleavage. It can be observed in the rock of Kuncha Formation and Raduwa Formation.
Boundinage or “Sausage Structure” is clearly the result of stretching at right angles to the line of junction of the individual units. The line of junction of the individual units may be called the boudin line and is a lineation. This structure is observed about 5m up from the suspension bridge on the right bank of Trishuli River. This boudinage structure of quartz vein is in the Malekhu Limestone.
Fig. 14: Boudinage Structure in the Malekhu Limestone.
The rocks of the study area are affected by folding of all scales. Some are major of regional scale and some are minor of local scale. There are few drag folds as well.
The Major fold is the large regional Mahabharat Synclinorium of which Malekhu is in the northwestern flank. It has the youngest rock in its core. The flanks of this area are steep. In some places it is almost vertical and in northern part, it is slightly overturned.
Many minor folds are observed in several formations. The beds of Dandagaon Phyllite are highly folded which may be due to local faulting. The rocks of Benighat Slate are also folded. Some folds are also seen in Malekhu Limestone and Chisapani Quartzite.
Fig. 15: Plunging Fold in the Malekhu Limestone.
Drag folds are observed in both Kathmandu and Nawakot Complexes. The drag fold in the Malekhu Limestone is ‘Z’ type, which indicates that it is the left limb of an anticline or right limb of a syncline. Drag folds also indicate large plunging folds. Plunging folds are observed in the Malekhu Limestone.
It is apparent that a change in physiochemical and tectonic conditions commonly upsets the stability of the pre-existing country rock and later on tends to reform themselves into new rocks stable under the new set of conditions. The change or reformation may either be in mineral composition or in texture or in both and under appropriate conditions in chemical composition as well. The process in which rock is derived form pre-existing rock by mineralogical, chemical or structural change due to change in temperature, pressure and chemical environment at depth in the earths crust is called metamorphism. The rock so produced is called metamorphic rocks. The rocks are not wholly melted but just partially melted and re-crystallized. Most of the metamorphic rocks found around Malekhu area are formed due to regional metamorphism.
Rai et. al. (1998) calculated the pressure and temperature of the rocks of Kalitar and Kulikhani Formations along the Malekhu Khola as follows:
Table No. 2 Pressure & Temperature of Kalitar & Kulikhani Formations:
|Formation||Temperature (oC)||Pressure (MPa)|
After Rai et. al. (1998)
The grade of metamorphism is increasing towards Main Central Thrust (MCT), whereas near Mahabharat Thrust (MT) grade of metamorphism is low i.e. there is low grade index minerals such as chlorite, biotite near MT but the high grade index minerals such as sillimanite, kyanite are towards MCT. So the study area exhibits an inverse metamorphism.
The rocks that have undergone the metamorphism in the study area are given as:
Slates are produced due to the metamorphism of argillaceous rock. Slates are predominant in Benighat Slate. They are gray to black colored and are not thicker than 5 – 8cm.
Phyllites are metamorphic rocks resulted from slate. Phyllite has foliated structure better than slate. Crenulated green colored phyllite is found in Kuncha Formation whereas dark green gray phyllite is found in Dandagaon Phyllite. Light colored phyllite with intercalation of quartzite has been found in Robang Formation.
Schist is a regionally metamorphosed rock characterized by parallel arrangement of the constituent minerals. Raduwa Formation consists of garnetiferous schist of greenish gray color. It is highly foliated. The size of garnet ranges from 5mm – 10mm. It is translucent & red in color.
Gneiss and Augen Gneiss:
Banded gneiss was formed during high grade regional metamorphism. They are coarse grained with well developed gneissic structure, when the gneiss has large eye shaped crystal of feldspar, they are called augen gneiss. Augen gneiss is formed due to metamorphism of plutonic rock granite. It is observed in uppermost part of Chisapani Quartzite near the confluence of the Tudi Khola and the Malekhu Khola, which is about 50m thick.
Fig. 16: Augen Gneiss in the Chisapani Quartzite.
Amphibolite is dark greenish gray colored, medium to coarse grained metamorphic rock, mainly composed of hornblende & feldspar, in which hornblende is in large percentage than feldspar. It is found in Robang Formation at about 500m towards the south of the Malekhu Bridge on the left bank of the Malekhu Khola.
Quartzites are fine grained, compact and hard rock, quartzites form the chief lithology of formations like Fagfog Quartzite, Dunga Quartzite of Robang Formation, Chisapani Quartzite etc. and are present in other formations too.
Coarse grained, crystalline & yellowish white colored marble is found in Bhainsedobhan Marble. Marble is metamorphosed form of dolomite and limestone.
Xenolith is the metamorphosed piece of foreign rock occurring near the margin of a batholith or other types of intrusion, within the solidified granite, which is derived from the invaded country rock. The xenoliths of metasandstone are observed in the granite boulders spreaded along the Malekhu Khola.
Fig. 17: Xenolith in Granite Boulders along the Malekhu Khola.
When magmatic intrusion takes place, then the heat is transferred from magma to the adjacent country rocks. Due to such heat, minerals & textures of country rock changes. In one word, “Metamorphism” takes place in the country rock. The resulting rock is spotted like tiger, which is known as hornfels. The process of such type of metamorphism is known as Contact Metamorphism. In the study area, hornfels boulders were seen along the bank of Malekhu Khola.
Fig. 18: Spotted Hornfels along the bank of Malekhu Khola
Magmatism is a process of consolidation of magma at or near the earth’s surface. In the study area, two magmatic intrusions were observed. The first one of them is amphibolite of Robang Formation. It is a basic igneous rock, which was formed due to – at first intrusion of basic magma in the country rock and followed by metamorphism. The minerals present in amphibolite are hornblende and plagioclase. The amphobolite is sync-sedimentary with the country rock.
The second intrusive rock was observed in the upper section of Chisapani Quartzite on the right bank of Malekhu Khola few meters upstream from the confluence of Tundi Khola & Malekhu Khola. The intrusive rock is augen gneiss. At first magma intruded into the country rock & slow cooling took place as a result the crystalline granite was formed. Then due to metamorphism the granite changed into gneiss in which feldspar took the shape of eye. So, thus formed gneiss is called augen gneiss. The main minerals are quartz, feldspar, tourmaline, muscovite & biotite.
GEOLOGY OF INDIVIDUAL AREA
The individual study area of Group ‘E’ towards from Malekhu (khola) bazar along Prithvi Highway to chalise (gauri) in west and Chepantari in North to Katahare in West. The area overall occupies about 6 sq.km Prithvi Highway passes through middle of the area. Geological cross-section of the individual study area is shown in Fig.22
Stratigraphically, the individual area of field study falls within the Trasitional contact between Upper Nawakot group of Nawakot Complex and Bhimphedi group of Kathmandu Complex . Stratigraphy of the individual study area is given below in table no. 3.
Table no. 3 Stratigraphy of the Individual Study Area
|Group||Formation||Main Lithology||Apparent Thickness (m)||Age *|
|Bhimphedi Group||Bhainse Dhobhan Marble Formation||Marble||800||Precambrian|
|Raduwa Formation||Granetic Schist||1000||Precambrian|
|————————- Mahabharat Thrust (MT) ————————-|
|Upper Nawakot Group||Robang Formation||Phyllite, Quartzite||200-1000||Paleozoic|
|Malekhu Limestone||Limestone, Dolomite||800||Paleozoic|
* After Stocklin And Bhattarai (1977)
5.1 Nawakot Complex:
Nawakot Complex is sub-divided into two groups. They are Lower Nawakot Group and Upper Nawakot Group. But in the individual study area only Upper Nawakot Group was observed.
5.1.1 Upper Nawakot Group:
The following formations of Upper Nawakot Group of Nawakot Complex were also observed in the study area.
It is the name after the place Malekhu .Our individual area covers large portion of this formation .An outcrops is obtained on the left bank of Trishuli River. The rock is light grey coloured, fine grained laminated dolomite, thin to thick bedded and jointed. Quartz veins and small drag folds are seen .Thin to thick dolomite bands interbeded with quartz. The attitude of beds is S880 E / 600 S W. Its Traditional contact with Robang formation is traced along Chepantari to Bhumedada. In the contact zone, intercalation of dolomite with Phyllite layer increases towards the lower side indicating commencement of Robang Formation. The thickness of this formation is found to be 600m.
Robang Formation (rb
This formation occupies a small area of our individual area . An exposure is observed at the left bank of Trisuli river .The rock is medium grained light green in color and gives soapy feeling. It is highly failted and quartz venis are also introduced in some places. The attitudes of beds is N 80o E / 70o SE. The quartzite is very hard and compact with Greyish White in colour, quartize thinly interbeded with grey schist is observed at the turning of Trishuli river. A few meter ahead towards the Bhumedada the highly sheared garnet with the dark red color embedded in foliated schist is observed. Thus, it is the extension of Mahabharat trust. So, it is highly disturbed zone. Then, the schist layer increases towards the Dadagaon indicating the commencement of Raduwa formation.
The rocks of Robang Formation is separated from Raduwa Formation by Mahabharat Thrust, which passes through Chalise .This formation is about 400m thick and its age is Paleozoic.
5.2 Kathmandu Complex:
The Kathmandu Complex is also sub-divided into two groups. One is Bhimphedi Group and other is Phulchauki Group. But in the individual study area only Bhimphedi Group was observed.
5.2.1 Bhimphedi Group:
The following formation of Bhimphedi Group of Kathmandu Complex lies in the Individual Study Area.
Raduwa Formation (ra):
This formation is named after Raduwa village from lower Gandaki valley and is youngest formation of Bhimphedi Group. The exposed rock of this formation are rarely found and are grey colored granetiferous schist with quartzitc intercalation .The garnet is high graded with size of approximately 0.1cm to0.5cm .the color of schist s dark green to greenish grey .They are rarely exposed from chailse to phurke khola .The attitude of foliation is S750 W / 700 SE. Near the chaise a left bank of Trisuli river schist with quartz veins are clearly exposed .then the calcite layer increase towards the lowerside towards Katahare indicating the commencement of Bhainsedhovan Marble. The thickness of this formation was found to be 600m.
This formation occupies small area of our individual area. An exposure is rarely observed as the area is covered with vegetation .The name Bhainsedhovan was derived from the village Bhainsedhovan. An exposure observed at katahare is dirty colored marble with shiny coarse crystal and well bedded. It readily reacts with dil. HCL. The attitude of beds are S 70oW / 80oSE the thickness of this formation is found to be 200m.
GEOLOGICAL HISTORY OF THE STUDY AREA
The study area lies entirely within the Central Nepal Lesser Himalaya. The rocks of the area are divided into two complexes: Nawakot Complex and Kathmandu Complex. The lack of fossils in this area creates a problem to geologists for the historical interpretation. The presence of abundant sedimentary structures i.e. depositional structure play a key role in revealing facts about the depositional environment of sediments. The type and grade of metamorphism along with the foliation & schistosity trend provide deformational history.
In the study area, stratigraphically, the Kuncha Formation is the oldest formation, which is shallow marine deposition as there is the presence of flute cast & graded bedding. The different types of crenulation cleavage and stretching lineation indicate multiphase deformation and polymetamorphism during its development courses.
The Fagfog Quartzite contains frequent current as well as oscillation ripple marks, indicating the shallow marine deposition. The Dandagaon Phyllite do not contain any primary features, but the rather dark colored fine grained and rapid transitional relation with Fagfog Quartzite, usually indicate a general subsidence of the depositional basin.
Conformable, following the Dandagaon Phyllite, the Nourpul Formation again exhibits the shallow water deposition, as there is frequent ripple marks & mudcracks. The mudcracks indicate the partial exposure of the deposited sediments above the water surface to be well dried by the Sun. Since the stratigraphically upward directions of these sedimentary structures are pointing opposite to the dip direction, indicates the strata are overturned.
Nourpul Formation is followed by Dhading Dolomite. This gradual lithological change indicates gradual change in depositional environment, by changing the climate and water composition, which favor the precipitation of calcareous sediments. The Dhading Dolomite contains abundant stromatolites, indicating its deposition in deep marine level. The presence of massive beds indicates a long lasted homogeneous condition. Ultimately the depositional basin uplifted and general erosion occurred. After some duration of time subsidence was followed forming an erosional unconformity. Above this erosional surface lies the Benighat Slate, containing graphitic slates with calcareous intercalations indicating deep water deposits of finer sediments. Malekhu Limestone comprises of Precambrian sediments and is a deep marine basin. Robang Formation is shallow water deposits containing ferrigeneous & clastic material.
The overall Nawakot Complex consists of low-grade metamorphic rocks such as slate, phyllite, metasandstone, quartzite, dolomite & limestone indication a regional metamorphism at low temperature & pressure condition.
Over the Nawakot Complex lies the Kathmandu Complex. The Kathmandu complex begins with a high-grade metamorphic garnet bearing crystalline rocks. The contact between the Nawakot Complex and Kathmandu Complex is a tectonic one, the Mahabharat Thrust (MT). But due to the metamorphism, stratigraphy and tectonic settings, the MT is now considered as an independent thrust and the Kathmandu Complex is the squeezed mass of rocks lying between the MT and MCT. (Upreti and Le Fort 1999)
The Kathmandu Complex, especially the Bhimphedi Group rocks are deposited in deep to shallow water condition. Raduwa Formation, Kalitar Formation, Chisapani Quartzite & Kulikhani Formation are shallow marine to continental deposit. Bhainsedobhan Marble is a deep marine deposit. The Kalitar Formation & Kulikhani Formation contains similar lithology indicating similar environment of deposition. Due to Mahabharat Thrust, reverse metamorphic rock-garnetiferous schist is present in the Raduwa formation. The snowball shaped garnet shows shadow structure & minor faults are present in Robang Formation. The whole Bhimphedi Goup is considered as Precambrian in age.
ECONOMIC MINERAL OCCURRENCES
Minerals which are economically valuable are called economic mineral. They can be used directly or valuable product can be extracted from them. The economic minerals are divided into 2 major groups, metallic & non-metallic minerals. In the Malekhu area some economic minerals are occurred and few of them are identified. Only the traces of these minerals can be seen along the traverse. The economically important mineral types are as follows.
v Metallic minerals
v Non Metallic minerals
7.1. Metallic Minerals
Metallic minerals are those, which contains some metals. In the study area, such metallic minerals are found in very small quantity, only in traces. Metallic minerals like pyrite, malachite, azurite are seen in the study area. Those minerals are low in quality & quantity.
Pyrite[FeS2]: A yellowish iron containing mineral, can be seen in the beds of Bhainsedobhan Marble. It is also known as fool’s gold.
Malachite[Cu2CO3(OH)2]: A green colored copper containing mineral, is found in highly jointed Malekhu Limestone, at the north-ward bank of Trishuli River near its contact with Benighat Slate.
Azurite[CuCO3Cu(OH)2]: A blue colored copper containing mineral is found in highly jointed Malekhu Limestone, at the north-ward bank of Trishuli River near its contact with Benighat Slate
7.2. Non Metallic Minerals:
Non metallic minerals are described under following subtitles:
Calcite: Mostly calcite are in micro-crystalline form in limestone, and relatively coarsely crystalline in marble. These are very common in Malekhu Limestone, Dhading Dolomite. Calcite is used in cement factory and as mortar in building purpose. Those mineral are used as fertilizer to reduce acidity of soil.
Quartz: It is one of the major rock forming mineral found in this area, but is mostly occurred in impure form containing many inclusion and impurities. Quartz veins are present in many formations and mainly found in Robang and Raduwa Formation. Here gem quality quartz is very rare.
Garnet: Garnet is relatively hard, reddish metamorphic index minerals, found in medium grade metamorphic rocks. These are found in garnetiferrous schist of Raduwa Formation and can also be seen in the lower part of Kalitar Formation. The size of the mineral varies from 1mm to 1cm or even more. They are not in pure form, which contain many inclusions and are not gem quality but useful for making sandpaper.
Tourmaline: The Tourmaline crystals are found in granite boulders along Malekhu Khola and in other intrusive rocks there. As they are small in size and poor in quality, they cannot be used as gemstone.
Feldspar: Feldspar found in Malekhu area are of various types. eg. Na-Feldspar, K-Feldspar etc. They are also found in augen gneiss, in Kulikhani Formation and on the granite boulders along Malekhu Khola. They are used for the manufacture of porcelain.
Hornblende: Dark green to gray black crystals of hornblende is found in massive amphibolite rock of Robang Formation.
7.3. Some Economically Useful Rocks
Besides these metallic and non-metallic minerals, some rocks are used as construction materials. They are as follows:
Slate: Slate is low grade regional metamorphic rock, exhibiting excellent slaty cleavage. These rocks are mainly seen in Benighat Slate. This rock can be split in planar surface along the cleavage plane. As a result this is locally used as roofing materials.
Marble: Marble is crystalline metamorphic, composed mostly of re-crystallized calcite. Chiefly limestone and sometime dolomite. It is dominant lithology of Bhainsedobhan Marble and is used in building construction in the form of block, slabs and chips.
Limestone: Limestone is found in Malekhu Limestone. It is light gray colored fine grained and massive. Limestone is important for cement production but these limestone have little Mg in its content and is siliceous. So, it is not used for manufacturing cement. It is used for producing lime.
Quartzite: Quartzite is metamorphosed sandstone, and found in Fagfog Quartzite, Dunga Quartzite, Chisapani Quartzite and some in other formations. Quartzite is chiefly used in building materials because of its hardness, strength and resistant to weathering.
Pebble, Cobble, Boulder and Sand: These are frequently found around this area and are used for the construction of road, building, and infrastructure development and so on.
GEOLOGICAL EXCURSION ALONG THE SOUTHERN PART OF THE KATHMANDU BASIN
Kathmandu Valley is located in the Lesser Himalaya, and was an ancient lake named as Paleo-Kathmandu Lake till 11 kyr B.P. (Sakai et al. 2001). This basin is filled with very thick (up to 550m thick) sequence of lacustrine and fluvial deposits of Plio-Pleistocene age (Yoshida and Igarashi 1984), (Moribayashi and Mauro 1980). First, significant works have been carried out by Yoshida and Igarashi (1984), Yoshida and Gautam (1988), Dongol (1985, 1987), Sah et al. (1995), and Sakai (2001). Yoshida and Igarashi (1984) first established lithostratigraphic unit; the Lukundol Formation, which was the older lake sediments covered by three fluvial terraces i.e. the Chapagaon, Boregaon and Pyangaon. According to them, lake became narrow and shifted toward the north due to upheaval of the Mahabharat range. They proposed the units such as the Gokarna, Thimi and Patan formations, which belonged to their younger stage deposits. T. Sakai et al. (2001) reported that these formations belonged to the fluvio-deltaic deposits Sah et al.(1995) proposed six stratigraphic units for the whole basin-fill sediments, i.e. Tarebhir, Lukundol, Sunakothi, Tokha, Kalimati and Thimi formations. Sakai (2001) proposed different stratigraphic units in the south, central and northern part of the basin. Paudel (2004), Paudel and Sakai (2004 and 2006) explored lithological variation found from south to north of the basin on the basis of geological mapping including texture and composition of the sediments. The redefined stratigraphic names are the Tarebhir, Lukundol, Itaiti, Kalimati, Sunakothi formations and Terrace gravel deposits in ascending order.
On the other hand, southernmost part of the basin, where oldest basin-fill sediments overlie the basement rock with an angular unconformity, is the important source of information regarding the history of the early stage of the Paleo-Kathmandu Lake. Moreover, in shallow and marginal lacustrine environments, relatively small change in lake level can influence on the depositional condition. In such areas, several lithological changes can take place (Bustillo et al. 2002). In this paper, we mainly focused on distribution of lithostratigraphic units in the southern marginal part (Fig.1), their correlation with the previously defined stratigraphic units, and their depositional environments.
Sedimentological and stratigraphic studies of the Kathmandu basin fill sediments.
- To see & observe the oldest basin filled sediments of the Kathmandu valley
- To see & observe the clear picture when & how the ancient Kathmandu Lake did was originated & disappeared.
- To observe the sedimentary strata & distinguished then into hard rock strata & sedimentary strata in field.
- To observe the fossils in the strata.
- To observe the sedimentary structure such as cross bedding pebble orientation & its significance for the geological interpretation
The basin-fill sediments are broadly divided into three sedimentary facies: the fluvio-deltaic facies from north to center, fluvio-lacustrine facies from south to center, and gravelly fan and fluvial facies from southern margin of the basin (Sakai 2001). Dongol (1985 and 1987) classified the valley-fill sediments into older Lukundol Formation and younger Kalimati Clay. Further, the Lukundol Formation was divided into four members: the Tarebhir basal gravel member, Kaseri Nayankhandi lignite, Nakhu Khola mudstone, and Champi-Itahari gravel in ascending order. Gravel of the Tarebhir Formation was interpreted to be originated from southern rim of the basin. This gravel sequence represents fan deposits, which are spreading from south to northand is covered by 6 m thick conglomerate with pebbly sandstone and cross-bedded sand. Clast consists of gneiss and mica schist, originated from the Shivapuri Gneiss to the north. The present field study indicates that the whole sequence of the Tarebhir Formation is not alluvial fan deposits from south.
Yoshida and Igarashi (1984) proposed three different terraces above the Lukundol Formation. Similarly, Shrestha et al. (1999) showed the Chapagaon Formation along the Nakhu Khola area. Sawamura (2001) showed the Kalimati Formation as the lateral facies change of the Itaiti and the Lukundol formations. Therefore, different authors have proposed differently the stratigraphic units of the southern part of the basin. With geological mapping, texture and composition of the sediments, we conclude that Lukundol Formation is the fluvial and marginal swamp facies, which is different from the lacustrine facies of the Kalimati Formation that extends upto the southern part of the basin, and changes into the fluvio-lacustrine facies of the Sunakothi Formation, and is also covered by the Terrace gravel deposits.
The Itaiti Formation is restricted only in the southern margin of the basin and shows alluvial fan. Northwards from the southern margin are thick gravel deposits covering the Sunakothi Formation and is the terrace gravel deposits in this study. The Kalimati Formation is not a lateral facies change of the Itaiti and Lukundol formations (Sawamura 2001). The Chapagaon Formation mentationed by Shrestha et al. (1999) does not cover the Lukundol Formation along the whole Nakhu Khola area. A detailed lithostratigraphy and their distribution from south to central part of the basin is given, and the following stratigraphic units; Tarebhir, Lukundol, Itaiti, Kalimati, Sunakothi formations and terrace gravel deposits, are described from bottom to top.
4. DEPOSITIONAL ENVIRONMENTS
Four facies in the southern part of the Kathmandu Basin are (a) fluvial facies of the Tarebhir and the lower part of the Lukundol formation, (b) fluvial-swamp facies of the middle and upper part of the Lukundol Formation, (c) lacustrine facies of the Kalimati Formation and lakeshore or fluvio-lacustrine facies of the Sunakothi Formation, and (d) alluvial fan facies of the Itaiti Formation.
Fluvial-swamp facies of the Lukundol Formation shows the repetation of the fluvial and swamp facies before the lacustrine facies was formed in the southern basin margin. The Kalimati Formation shows the open lacustrine facies whereas the Sunakothi Formation shows the fluvio-lacustrine facies, which is gradually changing into the fluvial facies. Alluvial fan facies of the Itaiti Formation itself is divisible into proximal alluvial fan facies lying in the south, and distal braided river facies lying towards the north.
Vertically they changed from fluvial facies to fluvial-swamp facies and to lacustrine facies, and from lacustrine to gravelly fluvial facies. The gravelly braided river changed onto the alluvial fan. To the north, the braided river changed onto the fluvial swamp (Lukundol Formation). The fluvial-swamp extended 3.5 km to the north from the basin margin to the open lacustrine Kalimati Formation, and vertically changed into the fluvio-lacustrine Sunakothi Formation. The fluvio-lacustrine facies covered the gravelly fluvial facies is pinching towards the south, while the open lacustrine facies of the Kalimati Formation is gradually thickening toward the north.
Extensive poorly drained swamp depositional environment of the Lukundol Formation in the southern part laterally changed into the well-drain swamp towards the center (Bunmati-Khupi). In this area no small lacustrine condition prevailed. Fluvial system of the Lukundol Formation directly changed into open lacustrine system of the Kalimati Formation. Composition of the detritus of the Lukundol Formation showed that the main source was from north. However, around the periphery of the basement rock, detritus was perhaps transported from east, west and south. Paleo current and compositional data of the Sunakothi sediments show southern sources.
The product of Himalayan Orogeny is at the time of Miocene, Pliocene and Pleistocene phase and subsequent strong erosion has given rise to a particular relief. A number of contributions have been made by different geologists for solving problems regarding geological and geo-morphological evolution, tectonics, stratigraphy but a lot still remains to be done.
The study area (Malekhu Area) is a part of Lesser Himalayas which lies in Central Nepal. Geologically it consists of two sequences of rocks: Nawakot Complex and Kathmandu Complex. Autochthonous to Paraoutochthonous low grade metamorphic rock of Nawakot complex is separated by Mahabharat Thrust (MT) from the high grade metamorphic rock of Bhimphedi Group of Kathmandu Complex.
The Nawakot Complex is further subdivided into Lower & Upper Nawakot Group, which is separated by an erosional unconformity. Nawakot Complex is composed of low grade metamorphic rocks such as slate, phyllite, metasandstone, quartzite, dolomite, limestone etc. belonging to the low green schist facies.
Similarly, Kathmandu Complex is subdivided into the Bhimphedi Group & Phulchauki Goup. Bhimphedi Group consists of Precambrian – Cambrian. Low grade to High grade metamorphic rocks such as: quartzites, micaceous schist, marble, garnetiferous schist etc. Whereas Phulchauki Group is composed of low grade metamorphic to un-metamorphosed fossiliferous sedimentary rocks of Cambrian to Devonian age.
Tectonically it can be said that Kathmandu Nappe is thrusted over the Nawakot Complex separated by MT, which is considered as the direct continuation of the MCT. However if MT were a inner continuation of MCT, the grade of metamorphism should have increased from base of MT to higher section of Kathmandu Nappe but there is the presence of medium to low grade rocks such as schist, phyllite, quartzite and there is no sign of high grade metamorphic minerals such as kyanite and sillimanite. In contrast to this, in other section of Nepal along the MCT, higher grade index mineral kyanite and sillimanite are found. So, on the basis of metamorphism, stratigraphy and tectonics, Upreti & Le Fort. (1999) considered MT as an independent thrust.
The grade of metamorphism decreases from garnet mineral to biotite showing progressive metamorphism above MT. below MT, the rocks are of biotitic group which shows an inverted metamorphism.
Small amount of some economic minerals are also found in the area such as azurite, haematite, magnetite, malachite, calcite, garnet, quartz etc. are present in low quality and quantity but are non-economic grade. Formations of both Complexes contain frequent primary sedimentary structures. From the study of these structures, it is noted that the rocks of this region were deposited in shallow water continental shelves, with some intermediate to deep water deposition and underwent different episode of tectonic activities giving rise to the present condition. Finally, the logical conclusion can be drawn that the Central Nepal comprises a very typical and complicated geological history.
On the basis of mapable distinct lithology, stratigraphy of the Kathmandu basin-fill sediments are redefined as the Tarebhir, Lukundol, Itaiti, Kalimati, Sunakothi Formations and Terrace gravel deposits. Among these units Itaiti Formation is restricted in the southern marginal part of the basin, and the rest on the Precambrian basement and the Lukundol Formation, except the terrace gravel deposits that erosionally covered the fluvio-lacustrine of the Sunakothi Formation from south to the central part of the basin. This terrace gravel was deposited during and after the ancient Paleo-Kathmandu Lake while gravel sequence of the Itaiti Formation was deposited before the origin of the proper Paleo-Kathmandu Lake. Redefined Sunakothi Formation and the Terrace gravel deposits were continuously extended from the south to the central part of the basin, and were deposited during the last stage of the Paleo-Kathmandu Lake. All the detritus of these two units were transported from the south of the Kathmandu Valley.
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