GEOLOGICAL STUDY OF THE BUTWAL-TANSEN AREA WEST CENTRAL NEPAL #TEchTOT

GEOLOGICAL STUDY OF THE BUTWAL-TANSEN AREA

WEST CENTRAL NEPAL

Introduction

A field trip or excursion is a journey by a group of students to a place away from their normal environment. Excursions are arranged to allow students to observe some important aspect of geology. The purpose of the trip is usually to provide students with experience, develop co-operative feeling to work in group and to motivate towards the subject. Likewise, learning to use the topographical map in field and use for geological purpose is another benefit for the student. Besides, learning to use compass for various geological purpose is another benefit for the students. Similarly, identifying rock exposed in excursion sites and studying their mineralogical composition also adds an additional confidence to the students for gaining insight, how rock were the part of human civilization throughout the history. In the field trips students are allowed to explore investigate and collect samples which are beneficial in many ways for clear understanding as it is completely different method of learning than being a bookworm without any experience and motivation in real time. The purpose of the trip is usually observation for education, non-experimental research and to provide students with experiences. It gives good working knowledge of rocks minerals and fossils with their characteristics. It develop co-operative feelings among teacher and students and most importantly, it develops motivation towards the subject .If students are able to learn all the skills, the main purpose of having excursion is fulfilled .So students must understand this program is developed to provide specialist turning for students with an interest in earth science who are to pursue professional career in minerals and all explanation, extractive industries, environmental sciences of hydrology. Excursion in geological studying helps producing graduates capable of confronting and solving problem in earth science. Learning all these facts in the field and understanding theory future consequences is in an integral part of the geological excursion, is not written in a systematic order, the approach of enhancing geological knowledge through excursion would be incomplete. 

Butwal-Tansen area was selected as our excursion sites as the place is very rich in many geological structures, exposed rocks and soil horizon. For the partial fulfillment of bachelor’s degree 4th year in geology, field excursion to Butwal-Tansen area started from 2072/08/15 and ended 2072/9/13. This 28 days field work was focused to achieve knowledge and learn process of documentation of different geological activities, geological boundaries, technique to make, Geological map and columnar section. It also helped gaining practical knowledge about the field and in overall career development. Identifying different rock type minerals and geomorphic structure, collecting necessary data which is very useful to interpret the geological history of area is very important so, this scientific method is also focused.

Location of the study area

The study area is divided into Butwal and Tansen area, lies in Lumbini Zone of Western Development Region. Butwal lies in Rupandehi District.It is located 265 km west of Kathmandu ,161 km south of Pokhara and 22 km north of Siddharthanagar Bhairawaha, at the northern edge of the Terai plain below the Siwalik Hills. Similarly, Tansen lies in Palpa district. It is located on the highway between Butwal and Pokhara, on the crest of the Mahabharat Range or Lesser Himalaya overlooking the valley of the Kaligandaki River to the north. Geographically it extends between 83° 29’ 30” to 83° 35’ 30” East of longitude and 27° 45’ 30” and 27° 51’ 30” North of latitude.(fig: Location map)

 Accessibility

Geographically, Butwal is the midsection (Cross Roads) of the Nepal's National Highway, Mahendra Highway and Siddhartha Highway. It connects western Nepal to the capital Kathmandu through highway and air links and Tansen lies on highway between Butwal and Pokhara. Hence, the study area is accessible road ways, air ways and by narrow foot trails in remaining parts.

Geomorphology

The topography of the area is very rugged, steep escarpment towards the south. It includes hills, river, valley, spur, river plain, terrace and cliff etc. In the southern part lies flat and plain area and its attitude increased towards north. Topographically, the Siwaliks represents very young and imaturetopography. The maximum attitude of the area is 1035 m at northeast of BoteGau and minimum attitude of about 200 m in the river valleys. The Lesser Himalayan range has comparitively mild and mature topography .The highest attitude of this area is about 1491.9 m at northeast of Basantapur and Northwest of Sitdada  and  the lowest attitude of this area is about 350 m at the river valley of the Tinau. The Lesser Himalayas of Nepal includes both physiographic divisions of Midlands and Mahabharat Range. It has a comparatively mild and mature topography with gentle slopes and deeply dissected valleys, which suggest that the rivers are still furiously at work.

The main drainage of Tansen and its surrounding isTinaukhola.Bhainskati river and Hulandi River also drains the area as the tributaries of the Tinaukhola. Drainage system is dendritic as the system forms in planer surface and tributaries meets at low angle in branch like pattern.Some rivers like Harchal flows along the strike.

Land Use

Most of the area is covered by steep hilly ridges, forest, grassland, river valley, gullies etc. Useable land is the older river terrace irrigated by HulandiRiver, BhainskatiRiver, Tinaukholaetc. Due to altitudinal and climatic variation, from south to north, vegetation types changes very quickly with in short distance- even in few kilometers. The type of vegetation vary from the tropical to subtropical in the south in Terai to alpine and tundra in high altitude area in the north. Very small portion of pasture land is used for animal grazing. Many lands are suffered by landslide due to heavy rainfall, deforestation and poor structure. Some area is used for industrial purpose. Small local mining and quarrying are found in some places. 

Objectives

 The principal objectives of the field work are to introduce student to various geological rock successions of the Nepal Himalaya, and to familiarize them various criteria and techniques to study geological elements to produce geological map of 1:25,000 scale. The other specific objectives of the study in partial fulfillment of the requirement for the degree of Bachelor of Science in geology are as listed below:

• Identify bedding plane, foliation plane, joints and learn their measurement techniques.
• Recognition, field description and interpretation of rocks, sedimentary structure, mode of preservation of fossils and their types and environment.
• To study geology of Western Nepal (i.e. Indo Gangatic Plane, Siwalik and Lesser Himalaya) and understanding its lithostratigraphy, sedimentology, rock types and depositional environment.
• Generate concept on order of superposition and direction of younging and correlation on rock units.
• To know the geological history of the study area by analyses of paleocurrent (data of imbricated pebbles, cross-bedding) and paleotectonics .
• Study on geological mapping techniques.
• Techniques of preparation of columnar section and route map.

Methodology

The understanding of Earth processes and environments over geological time is highly dependent upon both the experience that can only be gained through doing fieldwork, and the collection of reliable data and appropriate samples in the field. Geological Field Techniques is designed for students, amature, enthusiasts and professionals who have a background in geology and wish to collect field data on rocks and geological features.  Hence, students were allowed to use different types of field gear (instruments) and methods which were used to explore and investigate the excursion site

Due to lack of laboratory management only field techniques is used in excursion site. Topographical map of 1:25,000 scales is used to identify location and different geomorphological features. Similarly, permanent markers are used to write required information on samples or on sample bags containing samples. Field note book is used to note name and index of topographical map sheet, data of field work, sketching outcrop features and information on samples and photograph, etc. Similarly, geological hammers is used to obtain fresh surface of rock to study its composition, nature, mineralogy, weathering condition and identify different rock types in different formation. It is also used to powder rock sample for acid and photograph, for evidence is also collected. Also, hand lens is used to identify minerals and grain size of minerals. Drafting materials like tracing papers, protectors, pencils and eraser are used for finding and locating the location of excursion site, drawing maps and cross-sections. Similarly, compass is used to record orientation of geological features (like bedding, joints, faults, fold etc.), forward bearing and backward bearing. Compass is also used to locate observation points on the base map.

Geology of Nepal Himalaya

Introduction

   The Himalayan Range is the most impressive example on earth of an active collisional orogen and is highest, very highly active and youngest mountain system of the world. It combines rapid crustal shortening and thickening, intense denudation driven by monsoon climate, and frequent very large earthquake along an incomparably long and high mountain arc.The Himalaya was formed due to collision of two tectonic plates, Indian Plate and Eurasian Plate on the north at about 40 Ma (Dewey et al. 1988). Many scientists believe that at that time the northward moving Indian plate first touched the southern edge of Tibetan (Eurasian) plate. Before the collision of the plates, the place was occupied by Tethys Sea. It extends in 2400 km east-west direction. Its width varies from 230 to 320 km and bounded between the Indo-Gangetic Plain in the south and Tibetan Plateau in the north. Geographically, major part of Nepal (83%) falls within the mountainous region and the remaining portion lies in the northern edge of the alluvial plains of the Gangetic basin. Nepal Himalaya occupies the central 800km part. Geographically it is bounded by the north latitudes of 260 22′ and 300 27′ and the east longitudes of 800 11′ and 880 27′ and is divided into five distinct morpho-geotectonic zones (Fig 2: Geological map of Nepal and its cross section), from south to the north as (1) Terai Plain (2) Sub Himalaya (Siwalik Range), (3) Lesser Himalaya (Mahabharat Range and mid valleys) (4) Higher Himalaya and (5) Inner Himalaya (Tibetan Tethys). Each of these zones is clearly identified by their morphological, geological and tectonic features. Main Frontal Thrust (MFT), Main Boundary Thrust (MBT), Main Central Thrust (MCT) and South Tibetan Detachment Fault system (STDFS), from south to the north respectively are the major linear geological structures that act as the boundary line between the two consecutive units. 

 The mountain building (orogenic) process continues from the collision and the mountain is still on making process. Today, the Indian plate continues to be driven horizontally below the Tibetan Plateau, which forces the plateau to continue to move upwards.  This is noticeable by present day northward movement of India at the rate of 5 cm per year and the occurrences of frequent seismic shakes all along the Himalaya and its surroundings (Jackson and Bilhan, 1994, Pandey et al., 1995, Bilham et al., 1998). Himalaya is considered as a tectonically very active and vulnerable mountain system of world .All these unique features of the Himalaya has made this mountain range an important research field for a large number of the geoscientists.

 General sub-division of the Himalaya

Like the entire 2400 km long Himalayan Range, the Nepal Himalaya is also divided in to the five major tectonic zones from south to north, respectively (Gansser 1964, Hagan 1969 ).The tectonics are extending east to west and almost parallel to sub-parallel to each other are distinctive in their lithology, tectonics, structures, metamorphism, and geological history. The zones are as follows. (fig : The major tectonic subdivision of the Himalaya)

1.      Terai(Indo-Gangatic Plane)

2.      Sub Himalayas (Siwaliks or Churia zone) 

3.      Lesser Himalayan zone  (Mahabharat Midland Zone)

4.      Higher Himalayan zone  (Central Crystalline Axis)

5.      Tibetan-Tethys-zone

1. Terai (Indo-Gangatic Plane)

This zone is the southernmost tectonic division of Nepal and represents the northern edge of Indo–Gangetic alluvial plane and range in elevation from 100 to 200 m above MSL. The width of Terai varies from 10 to 50 km. It extends from the indo-Nepal boarder in the south to the base of churia or siwaliks hills in the north separated by Main Frontal Thrust (MFT) which is very well exposed at many places at the foot of Siwalik hills. Along this thrust, the Siwalik rock thrust over the alluvial sediments of the Indo-Gangetic plain. The Terai region is generally flat with very minor relief caused by river channel shifting and down warping of the basin. All the rivers emerging from the Nepal Himalaya flow straight to the south until they meet the Gangas River and finally head eastward towards the Bay of Bangal.

Geologically, the Terai plain is composed of Pleistocene to Recent alluvial sediments. Quaternary deposits mainly boulders, sand, mud’s with thickness of alluvium is 1500 m are present. Coarser Terai sediments lie close to the mountain front, which is known as the Northern Terai zone (Bhabhar zone), where they form large alluvial fans. Finer sediments are found farther to the south and the Recent Terai sediments represents the latest foreland basin deposits of the Himalaya.

2. Sub-Himalaya (Siwaliks or churia zone)

This zone represents the lower hill of the Churia range and is bounded to the north by the Main Boundary Thrust (MBT) and to the south by the Main Frontal Thrust (MFT). Lesser Himalayan metasedimentary rocks have been thrust southward over the churia rocks along the MBT and large part of the Churia zone rocks must be buried beneath  the cover of the over thrust Lesser Himalayan rocks to the north. The Churia Hills abruptly rise from the plains of Terai and forms the southernmost mountain range of the Himalaya.The Siwaliks constitutes the narrow belt of 20 Km to 30 Km in width 5 Km to 6 Km in thickness runs east west. The age range is indicated as Middle Miocene to Early Pleistocene.

The Siwaliks Group in Nepal is composed of three units that are known as lower, middle and upper members. These units can be correlated with the Sub Himalaya of Pakistan and of northern India (Burbank, Beck & Mulder 1996).Sub-Himalaya is composed of fluvial sedimentary rocks like mudstone, sandstone, and conglomerate, shows the coarsening upward succession. Since middle Miocene (16-1 Ma) to lower Pleistocene, the sediments are accumulated in the basin due to rapid denudation of the Himalaya.

3. Lesser Himalaya Zone

The lesser Himalayan Zone lies in south of the Higher Himalaya that is separated by the Main Central Thrust and north of the Siwalik Range seperated by Main Boundary Thrust (MBT).The total width ranges from 60-80 km. From east to west, the Lesser Himalayan geology of Nepal shows much variation in stratigraphy, structures, magmatish and displays relatively subdued and mature geomorphology. 

Tectonically, the entire Lesser Himalayas consists of two sequence of rock: allochthonous and autochthonous. The zone is made up of low-grade metamorphic rock units, with over riding crystalline nappes, klippen and tectonic windows. Geologically the Lesser Himalaya consists of low grade metamorphic rocks like slate, phyllite, schist, quartzite, marble and sedimentary rocks like limestone and dolomite, shale etc. in the south. In some region there is some minor volcanic and some granitic rocks of Proterozoic –Cambrian age (2000-500 Ma). The stratigraphic thickness of the rock sequence in this zone is 10-20 km. The sedimentary rocks in the Lesser Himalaya barely yield fossil partly because there are much older (dating back to the period when life form were not abundant or diverse) and partly because they have been metamorphosed.

Hagen (1969) and Stocklin&Bhattarai (1977) have divided the Lesser Himalayan zone into two main geological and tectonic units, which are outlined as follows:

• Kathmandu Complex 
• Nawakot Complex

4. Higher Himalaya Zone

 Geologically, the Higher Himalayan Zone includes the rocks lying north of the Main Central Thrust (MCT) and south of normal fault system called as South Tibetan Detachment System (STDS) below the highly fossiliferous Tibetan-Tethys Zone and runs throughout the country. This zone is made up of 10-20 km thick high grade metamorphosed coarse grained rocks (schist and gneiss) and granites situated at altitude of 3000 m to over 8000m.These rocks are proterozoic-Cambrian age (2,000-500 Ma) and belong to the continental crust of the Indianplate. In addition, white granites (leucogranites) of Miocene-are (24-17 Ma) in upper part of the Higher Himalaya is found. It extends continuously along the entire length of the country as in whole Himalaya, and its width varies from place to place. Vast area of the Higher Himalaya is occupied by the Precambrian rocks. This sequence can be divided into four main units, as KyaniteSillimanitegneiss, pyroxenic,marbleand gneiss, banded gneiss, and augengneiss in the ascending order (Bordet, Colchen&LeFort 1972). However, later Le Fort (1975) revised this classification and divided into three formations as Formation I, Formation II and Formation III in the ascending order. This zone is characterized by extremely high relief, steep topography, rocky cliff and outcrops with little soil covered terrain.

5. Tibetan- Tethys Himalaya Zone

The Tibetan-Tethys Himalayas generally begins from the top of the Higher Himalayan Zone separated by the South Tibetan Detachment System (STDS) and extends to the north in Tibet, represents the deformed remnants of the northern edge of the Indian subcontinent. It has undergone very little metamorphosed except at its base where it is close the Higher Himalayan Crystalline rocks of the Higher Himalayan Zone. This zone is about 40 km wide and consists of Cambrian through Eocene sediments (sandstone, shale and limestone) which were deposited on the continental shelf of the Tethys Ocean. These sedimentary rocks contain many kinds of fossils including the Cretaceous-age ammonite (‘Shaligramshilla’ in Nepal) fossils.In Nepal these fossiliferous rocks are well developed in ThakKhola (Mustang)Manangand Dolpa area. In eastern part, amount of exposure of the Tibetan Tehys Zone is almost negligible and found only in top of the Mount Everest.

Geology of Western Nepal of Lesser Himalaya

The western Nepal includes the area between the Marsyangdi in the east and the Bheririver in the west was. The main tectonic zones of this region include the following units from north to south:

• The high Himalayas composed of the Tibetan fossiliferous sedimentary zone and underlying central crystalline zone.
• The lesser Himalayas composed of low-grade metasedimentary, autochthonous to allochthonous rock units. This zone includes the Midlands and Mahabharat ranges.
• The Sub Himalayas (Siwaliks), composed of foredeepmolassic sediments.

The stratigraphy of the western Nepal Lesser Himalaya may be described separately as follow: 

1. Nawakot Unit 
2. Tansen Unit, and 
3. JajarkotNappe .

Except the crystalline rocks of the jajarkotNappe, the entire terrian of the western Nepal between the MBT and MCT is covered by the rocks of the Nawakot Units and the Tansen Units. The Section south and Southwest of Pokhara around Tansen has been investigated in detail by Sakai (1983,1985), whereas most of the western part up to Bheri River has been investigated by Fuchs and Frank(1970), Sharma et al, (1984) and Dhital and kizaki ( 1987 a, b).

Our study area of western Nepal Lesser Himalaya contains Tansen Group and Kali GandakiSupergroup.

Tansen Group

Tansen group is separated from the underlying Kali GandakiSupergroup by an unconformity of which trace is obscured by later faulting. Tansen Group (2400 m thick) starts from upper Carboniferous or lower Permian and reaches to the early Miocene. . The Sakai (1983; 1985) carries out the detail geology of this group. According to the Sakai, the Tansen Group consists of five formation, the late Carboniferous to Permian Sisne (250m-100m thick), late Jurassic to early Cretaceous Taltung(250 m), probably Cretaceous Amile (280m), middle to late Eocene Bhainskati (160 m), and probably early Miocene Dumri  (725 m), in increasing orer, the former three represents the Gondwanas. 

Kali Gandaki Supergroup

The Kali GandakiSupergroup is extensively distributed in the Mahabharat Range and the Midland of the Lesser Himalayas in the western Central Nepal and has no stratigraphic break throughout the whole sequence. It is a thick sequence attaining more than 10 Km in thickness and ranging in age from Late Precambrian to early Palaeozoic, and is unconformably overlain by the Tansen Group of Gondwana rocks. 

Sakai, 1983, 1984 has divided this  Supergroup in to three Group and nine formations.

(Fig: Lithostratigraphy)

Geology of Siwalik (Churia ) Group

Introduction

The excursion route extends from Jitgadhi (at the contact of HFT and Siwalik) to the meeting point of Tinaukhola and Jhumsakhola,NE of Dumai.The total length of excursion site is approximately 11 km.

Siwalik Zone abruptly rises from the Indo Gangetic Plain (called the Terai in Nepal) and forms the southernmost mountain range of the Himalaya. The Siwalik Range was formed by the uplift of the sedimentary rocks deposited in earlier foreland basin of the Himalaya similar to the modern foreland basin occupied by the Indo-GangeticPlain. In future, much of the present day northern Indo-Gangetic Plain will be converted into a mountain and join the Siwalik Range. The Himalaya is thus gradually propagating southward by slowly uplifting the adjacent plain towards south.

The Siwalik Zone consist of Neogene to Quaternary fluvial sedimentary rocks .This zone is bounded to the north by the Main Boundary Thrust (MBT) and to the south by the Main Frontal Thrust (MFT).The Lesser Himalayan rocks thrusted southward over the Siwalik rocks along the MBT, and a large part of the Siwalik Group has been buried beneath the overthrusted Lesser Himalayan rocks. The Siwalik Zone itself has thrusted over the young alluvium of the Indo-G agetic plain along the MFT.

The Siwalik group is divided into three units:Lower, Middle and upper. The rocks of this group are made up of fluvial sediments such as mudstone, sandstone and conglomerate, and are generally coarser-grained in the upper horizon.

Lithostratigraphy

Geological map of the study area is shown in fig.5, and lithostratigraphy in Fig1.

Three-fold classification of Siwaliks into lower, middle and upper units is well-established and is freely applied to the Churia Group from the beginning of the geological studies in Nepal (Auden,1935; Bordet,1961; Hagen,1969; Itihara et al, 1971; Sharma, 1973; Herail and Mascle, 1980; Mascle and Herail,1982; Yoshida and Arita, 1982; Herail eta al, 1986) that had been developed in the Potwar region of Pakistan and in western Indian Himalaya.Lithostratigraphic units proposed by different studies are summarized in Table 3.

Lower Siwalik

The Lower Siwalik Group is essentially composed of the alteration of fine-grained sediments such as variegated mudstone; siltstone and shale with subordinate amount of fine- grained sandstone. Wedge shaped beds of sandstone is present deposited by lateral accretion (plate 1) This formation can be further sub divided in to three members i.e. lower, middle and upper. Typically fining upward sequence is frequently observed suggesting fluvial cycle. Also, occurrence of paleosols is locally common (DeCelles et al 1998). A few plants remains as fossils are sometimes found.

Attitude : N60E/19NW

Middle Siwalik

Middle Siwalik consists of multistoried thick sandstone bed measuring several meters to tens of meter in thickness and alternating subordinate beds of mudstone. The typical feature of this formation is the presence of light colored sandstone of salt and pepper appearance due to abundance of biotite and presence of light colored quartz and feldspar. Sandstoneis less indurated and medium to coarse grained. A cycle of fining-upward sequence are commonly observed in which beds begins with coarse sandstone, gradually fines upward and ends up in thin clay layer or paleosols. Also, the clay layer serves good plant fossils and locally freshwater molluscs. This formation is also sub-divided in to lower and upper member.

Attitude : N61W/23NE.

Upper Siwalik

Upper Siwalik consists of very coarse sediments such as boulders, conglomerate with minor proportion mudstone intercalation. Conglomerate is mainly composed rounded to well-rounded fragments of Lesser Himalayan rocks. Sorting is very poor. The mudstone beds of the Upper Siwalik are massive and irregularly bedded and contain many invertebrate fossils including brachiopods and Gastropods.

Geological study of western Nepal Lesser Himalaya

Introduction

The excursion route extends from Jitgadhi to Tansen. The geology of the western Nepal Lesser Himalaya between the Marsyangdi River in the east and the Bheri River in the west was  studies mainly by Bordet et al (1964 a, b), Boudenhausen et al (1964), Nadgir and Nanda (1966), Fuchs (1967), Hagen (1969), Fuch and frank (1970), Talov (1972), Nanda (1973), Hashimoto et al (1973) , Sharma (1977), Upereti and Merh (1978), Uperiti et al (1980, 1984), Sakai (1984, 1985), Hayashi et al (1984), Sharma et al (1984), Arita et al (1984), Bashyal (1986), Dhital and kizaki (1991, 1992), Works by Department of Mines and Geology( DMG).The stratigraphy discussed in this report is primarily based on field work and Sakai(1983, 1985 ).The excursion route was 

The Lesser Himalaya lies in between the Siwalik and Higher Himalaya. Both the southern and the northern limits of this zone are represented by thrust, the Main Boundary Thrust (MBT) and Main Central Thrust (MCT) respectively. Lesser Himalaya of Nepal includes the both physiographic division of midlands and Mahabharata range. The altitude of this zone ranges from 2200-3300 m. 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 comparatively mild and mature topography with gentle slopes and deeply dissected valleys, which suggests that the rivers are still furiously working. The Lesser Himalaya is made up of mostly of the unfossiliferous, sedimentary and meta-sedimentary rocks like shale, sandstone, conglomerate, slate, phyllite, schist, granite, limestone, dolomite etc. ranging in age from Precambrian to Eocene. The geology is complicated due to folding, faulting, and thrusting are further complicated by meta-sedimentary rocks of Lesser Himalaya, while the MBT brought the older Lesser Himalayan rocks over the much younger Siwalik rocks. Tectonically the entire Lesser Himalaya consists of two sequences of rocks, allocthonous  andautocthonous units with various nappes, klippes  and tectonic windows.

Sakai(1983, 1985) have  divided a west central lesser Himalayan zone into following two main geological and tectonic units, which are outlined as follows:

1. Tansen Group
2. Kali Gandaki Supergroup

The Tansen Group consists of Post Gondawanas and Gondwanas rock and has following formations namely

• Sisne Formation
• Taltung Formation
• Amile
• Bhainskati Formation
• Dumri Formation

The Kali GandakiSupergroup is further divided in to upper, middle and lower Group and consist of following Formation namely

• Andhi Formation
• Naudanda Quartzite
• Heklang Formation
• Virkot Formation
• Chappani Formation
• Khoraidi Formation
• SaidiKhola Formation
• Ramdighat Formation
• Kerabari Formation

The table of sub-division of above Groups is given in Table 2 according to Sakai (1983, 1985).

Stratigraphy and Lithology:

The stratigraphy of study area can be discussed under two major heads, the Tansen Group and Kali GandakiSupergroup which is further divided into five and nine formation respectively.

Tansen Group

Geological map of the study area is shown in fig.6, and lithostratigraphy in Table.2.

The geology of the Tansen Group is important from the view point that at a number of places fossiliferous sequence ranging age from Permo- Carboniferous to Lower Miocene(Gondwana and Post Gondwana) rocks have been mapped.

The Gondwanas

The Gondawanas sequence is sequence, unconformably overlying the older metasedimentary formation is largely a nonmarine sequence though some formation of marine origin have been identified.Gondwana rock in western Nepal may be divided into lower and Upper Gondwanas depending upen their fossil assemblages and other characters.

 Lower Gondwanas

The Lower Gondwanas are well developed in western Nepal in Tansen, Phalabang and Sallyan,  areas. In the Tansen area, this  formation has been described as Sisne Formation by (Sakai 1983,85). The Sisne Formation (1983) represtents the oldest rocks of Tansen Group and well exposed in TansenSynclinorium. This formation rests disconformably on the Kerabari Formation of the Kali Gandaki Super Group (Sakai 1985) with a basal conglomerate derived from the underlying Kerabari Formation.

Sisne Formation

It is the lowest unit of the Tansen Group and belongs to Gondawanas class. The name 'Sisne' was derived from the Sisnegaun(Sakai 1983), is distributed from the Sisnegaun in the west to Charchere village to the east, more or less along the Aulis Khola.

This formation is disconformably overlain by the Taltung formation and mainly consists of diamictite and bioturbated mudstone or mudstone converted into slates with some beds of sandstones and conglomerates. Diamictite and black shale influence and is correlated with the glaciofluvial lower gondwana. Diamictite is unstratified scattered clasts of various shapes and sizes. Conglomerate is the diamictite whose proportion of the clasts is large. The clasts are made up of gnesis, limestones, granites, dolomites, sandstones and less of slate and phyllites. The clasts vary in size from pebble to cobble to sand size but sometimes exceeds. Dark grey to black Claystone forms a major constituent of Sisne Formation.

The thickness of this formation is about 1000m and attitude of bedding plane is N85E/75SW

Taltung Formation

The name 'Taltung' was derived from the Taltung village (Sakai 1983). The Taltung Formation is extended from north of Sisnegaun in west to Gautul in the east.

This formation consist of olive green sandstone,siltstone and conglomerate with volcanic detritus.In the basal part basaltic lava are intercalated  and have supplied enormous amount of pebbles to the charchare Conglomerate. The conglomerate consists of densly packed well sorted pebbles and cobble. Compositionally, the conglomerates are mainly made up of volcanic rocks and quartzites. Basic volcanic rocks (Aulis volcans) make a distinct member of Taltung Formation. Wood fragments are common in the sandstones. 

Taltung represents an upward fining fluvial facied characterized by cyclic sedimentation in lowerpart and rhythemic one in the upper part and yields plant fossil of upper gondwana indicating the Jurassic to early cretaseous age.

The thickness of this formation is about 300m and attitude of the bedding plane is N65W/59SE.

Amile Formation

The name of this Formation was derived from the Amilekhola running west near Taltung (Sakai, 1983). This formation is extended in the both limbs of the TansenSynclinorium.

This formation lies disconformably on the Taltung, is composed of quartose sandstone and lesser amount of shale accompanying bioturbuted siltstone and lime stone which yields poorly preserved corals and molluscs.Also, carbonaceous shales containing coalified wood fragments is found.

Its total thickness of this formation is about 280m and is interpreted as the coastal plain to intertidal sediments of Paleocene to late Cretaceous age.

The attitude of the foliation plane is N88E/80SE.

Bhainskati Formation

The name of this formation was derived from the Bhainskati Khola (Sakai, 1983).This formation is well exposed along the Bhainskati Khola.

This formation consist largely of black shale with many thin calcareous and fissilliferousinter beds in the middle part and caricolored  mainly green and red purple shale and red iron oolitic hematite beds in upper part. It consist of Black shale and is accompanied in the lower part with many thin calcareous interbeds yielding foraminifers of Eocene age along with abundant marine molluscs and some vertebrate. Hematite beds occur at several horizons in the upper part and extensively traceable, although the thickness of the beds and the iron content are locally varied. 

The thickness is about 160m thick and is supposed to be of Eocene age. Because of its unique litho facies consisting largely of black shale containing abundant marine fossils, this formation is one of the most distinctive beds among the geologic sequence in the lesser Himalayas.

Attitude of the bedding plane is N88E/74SE.

Dumri Formation

The name Dumri was derived from the Dumre village (Sakai, 1983). This Formation represents the youngest unit, not only the Tansen Group  but also in whole Lesser Himalayan rock sequences. This formation is well exposed along the Bhainskati Khola, Tinau Khola around Dumre, Masyam, Jorpipal, Olesaetc.Sakai (1983).

This formation occupies the top of the Tansen Group, overlies  disconformily over the eroded surface of BhainskatiFormation.It is a thick series of fluvial deposits consisting inter bed of medium-grained , bluish grey to greenish grey sandstone and lesser amount of purple and green shale. Sandstone becomes much predominant and massive upwards. Strong bioturbation is common in shale and can be easily recognized by their mottled structures.The sandstone are quartzose wacke in composition. Several intraformtional shale-pebble conglomerate is also noted.Tree trunk is also found in is formation. (plate 2).

The thickness of this formation is about 725 m and the age assigned to these rocks is Oligocene to early Miocene.

Attitude of the bedding plane is N60W/77NE.

Kali Gandaki Supergroup

Geological map of the study area is shown in fig.6, and lithostratigraphy in Table

The Kali gandakiSupergroup consists of pre- Gondwana rocks, is divided into three groups; the Lower, Middle and Upper Groups(table.2) and in to nine Formations.The Lower Group is monotonous , thick sequence of predominantly argillite and argillaceous gritstone with lesser amount of quartzite and meta-basic rocks. The Middle is extremely varied sequence composed of quartzite, dolomite, limestone and phyllitic slate. The Upper is made up of thick dolomite , limestone and varicolourted calcareous slate.

The stratigraphic correlation of western Nepal Lesser Himalaya (Sakai, 1983) with that of central Nepal (Stocklin and Bhattarai, 1978) is shown in Table.

1.The Lower Kali Gandaki Group

This formation consists of monotonous, thick succession of alteration of phyllite, phylliticgritstoneand as lesser amount of quartzite. In Kali Gandaki area, they are more than 2 km thick and are composed of slate and phyllite. The rock are regarded as the lowest sequence of the Lesser Himalayan rocks in the lithological similarities to the Kuncha Formation, its great thickness incomparable to other formation and is having carbonate rocks which have enough thickness to be compared with the carbonate formation in the overlying two groups.

2.The Middle Kali Gandaki Group

The middle kali Gandaki Group is distributed in both the outer and inner belts. In the outer belt, this group crops out in three areas. The First is in the area between the Bari Gad Fault and the NBF of the TansenSynclonorium. The second is in a narrow belt between the MBT and the SBF, where the group forms a homoclinical structure of the southern limb of the TansenSynclonorium. The third is the palpaKlippe that tectonically overlies the Tansen Group in the central part of the Tansensynclonorium. The Middle Group is subdivided into five formation as compared with the monotonous sequence of the upper and Lower Grpup, and is characterized by arenaceous and calcareous rocks that are considered to have been deposited under arid shoreline environments.

3.The Upper Kali Gandaki Group

The Upper kali Gandaki Group is distributed around the Tansensynclonorium in the outer belt. It is normally distributed in the lower valley of the AndhiKhola in the inner belt. 

               This group consists of two formations; the Ramdhighat and the Kerabari in ascending order is a monotonous beds of dolomite attaining 2 km in thickness. The basal black limestone of the KerabariForaion is separated from the main part as the Ri-Rilimestine Member. The upper group ,as a whole characterized by thick carbonate rocks and its total thickness attains 2900m.

Andhi Formation

The name of this Formation is derived from the AndhiKhola (Sakai, 1985).

It is the oldest lithounit of Kali Gandaki Supergroup.This formation consist of thick monotonous sequence of greenis- grey to brownish-grey phyllitic slate with silver luster and with occasional interbeds of thin, calcareous siltstone up to 10 cm thick. The phyllite are thinly foliated. The attitude of the beds of phyllite and quartzite were more or less similar.

The thickness of this formation is about 2000 m belongs to Late Precambrian age is correlated with Kunchha Formation of Central Nepal (Stocklin and Bhattarai, 1978).

Attitude of the foliation plane is N40W/67NE.

Naudanda Quartzite

The name of this Formation is derived from the Naudada cliffs of Sangja district (Sakai, 1985). 

This formation comprises a fine- to coarse-grained , white quartzite arenite with several interbeds of phyllite, conglomerate and metabasite. The quartzite are strongly rippled and cross-bedded. Oscillation type ripple mark suggest shallow marine environment. This formation is not visible due to our limited time.

The total thickness of this formation is about 400 m belongs to Late Precambrian age and is correlated with Fagfog quartzite of Central Nepal ( Stocklin and Bhattarai, 1977 and 19  ).

Attitude of the foliation plane is N85W/68NE.

Heklang Formation

The name of this Formation is derived from the Heklang village (Sakai, 1985).

This formation is composed mainly of dark-green phyllitic slate or phyllite with brown and grey tints caused due to weathering intercaliated with fine marl beds. Thickness of individual beds of phyllite ranges from 1 cm to 4 cm.Thephyllite is intercalated at several horizons with up to 10 meters thick layer that frequently contain fine calcareous, laminae and bands.This layer are associated with calcareous sanstone, marl and dolomite. Foliation plane are developed in the phyllite. The carbonate rocks often weather a coffee brown color. This formation can be exposed in Madanpokhari and Telgha of Tansen area. 

The total thickness of this formation is about 800 m belongs to Late Precambrian age and is correlated with DandagaunPhyllite of Central Nepal ( Stocklin and Bhattarai, 1977 ).

At Bhalebas, attitude of the foliation plane is N60W/77NE.

Virkot Formation

The name of this Formation is derived from the Virkot village (Sakai, 1983, 1984, 1985).This Formation is repeatedly distributed by folding in the are between the TansenSynclinorium and the Badi Gad Fault.

This formation consists of white to pink quartzite and reddish-purple phyllitic slate interlayering with each other giving  characteristiccolourbandingThe lower part of the formation is predominantly reddish-purple phyllitic slate, and the upper part comprises predominantly quartzose sandstone..

The total thickness of this formation is about 510 m belongs to Late Precambrian age and is correlated withNourpulFormation. This formation exhibits an upward-coarsening and thickening sequence as a whole.

At the southern limb of the synclinorium, attitude of the foliation plane is N35E/15NE.

Chappani Formation

The name of this Formation is derived from the Chappani village (Sakai, 1883, 1984, 1985).This Formation is distributed in to two to four belts, having been repeated by folding , in the area between Badi Gad Fault and the TansenSynclinorium.

At this location red purple colored shale are found which contains mud cracks and ripple marks. In the beds of slate domed shaped stromatolites are present which indicates the beds are overturned..The lower and  middle part of the Chappani Formation comprise of black, light green and pink, calcareous laminated slate with many thin inter beds of stromatolites. The upper part contains quartzite and reddish purple and light green shale with same bed of shale and pebble conglomerate.

The total thickness of this formation is about 400 m belongs to Late Precambrian age and is correlated with Nourpul formation .

Attitude of the foliation plane is N75E/86NE.

Khoraidi Formation

The name of this Formation is derived from the Khoraidi village (Sakai, 1883, 1984, 1985).

This formation is unique in having a large number of stromatolites. The stromatolites are dome shaped, sub-spherical and dolomitic. The dolomite is grey white to ash color. The stromatolites in Khoraidi Formation are concave upward sequence but in normal sequence stromatolites are concave downward which indicates beds are overturned. (plate 3)

The total thickness of this formation is about 350 m is correlated with Dhading Dolomite of Central Nepal ( Stocklin and Bhattarai, ).

Attitude of the bedding Plane is N15W/36NE.

SaidiKhola Formation

The name of this Formation is derived from the SaidiKhola (Sakai, 1883, 1984, 1985). The Saidi Khola Formation repeatedly crops out by folding in the area between the Bari Gad Fault and NBF of the Tansen synclinonium .

The bed rocks contain chiefly of coarsely inter layered purple sandstone and grey shale. The shale are bioturbeted, which means they were acted by micro-organisms during their formation. This alternating thin beds of shale and sandstone is called rythmite. This rythmitic sequence of shale and sandstone indicates tidal deposits. They were deposited during high tide and low tide. The boulders present there are of dolomites which contains stromatolites with different structures of faulting and folding. Some of they even have mud-cracks and ripple marks also.

The total thickness of this formation is about 400 m.

Attitude of the bedding plane is N60E/49SE

Ramdighat Formation

The Ramdighat Formation is named from a village Ramdighat on the southern bank of the Kaligandaki (Sakai, 1983, 1984, 1985).This formation is located in the area between the Bari Gad fault and NBF of the TansenSynclonorium.

This formation consists of calcareous argillite and minor amount of thin limestone and coarse clastic rocks are extremely rare.

The total thickness of this formation is about 750 m is correlated with Benighat Slate of Central Nepal (Stocklin and Bhattarai). The Ramdhighat slate has possibly been deposited in a lagoon or large bay that was fringed with an intertidal flat with an algal marsh zone. The lithofacieschanges from the Saidikhola  Formation to RamdhighatFormatiob indicates a transgression or sbbsidence of the depositional basin.

Attitude of the foliation plane is N5E/40SE

Kerabari Formation

The name of this Formation is derived from the Kerabari village (Sakai, 1883, 1984, 1985).The Kerabari Formation, the uppermost unit of the Kali Gadali Super group, is most extremely distributed in Tansen area.

The kerabari Formation conformably rest on Ramdighat formation. The basal black limestone is designated as Riri Member.

The total thickness of this formation is about 2100 m is correlated with Malekhu limestone of Central Nepal (Stocklin and Bhattarai).

At Kerabari, the attitude of the bedding plane is N65W/51NE

Geological structure

The minor structure found in our survey are

• Bedding Plane:  It is the plane between two beds of a sequence.

• Foliation Plane: It is the plane that develops perpendicular to the direction of the application of the stress in metamorphic rocks.

• Lateral Accretion: Lateral accretion is the depositional structure formed by a meandering river system at the depositional bank as the river gradually changes its course towards cutting bank. It is found Dumri Formation. (plate 1)

• Flute Cast: When turbidity flow occurs on soft sediments, the coarser sediment make groove called flute on the soft sediment and some other sediments is deposited on the groove, called as cast. It is seen in Saidi Khola Formation.

Some other structures are Cross Bedding , Algal Structure, Pencil cleavage etc.

The Major geological structures includes the

• MBT : It is a major thrust that seperates siwalik from the lesser Himalaya.

• Palpa Klippe: the appearance of older Heklang and Virkot Formations on much younger Dumri Formation is due to a thrust called palpa klippe thrust. Since its root zone is not clear, it is termed as Palpa Klippe. 

• Tansen Synclinorium: The reappearance of Heklang Formation and then appearance of Amile Formation suggests a fold. The fold is non other than a syncline called Tansen Synclinorium. 
• In this study area Kerabari Formation of Kaligandaki Supergroup is followed by Sisne, Taltung, Amile, Bhainskati and Dumre Formation of Tansen Group. A number of syncline folds lying one over another had been observed in the observation point along the Siddhartha Highway heading towards Tansen. The general attitude is S70W/ 100NW and N60W/ 30SW, which suggests that there is presence of syncline fold. The value of dipping on opposite sides mentioned above proves the presence of syncline in large scale forming Tansensynclinorium.

• Main Boundary Thrust (MBT):The Main Boundary Thrust is the one of the major thrust system which extends almost all length of the entire Himalayan range. As it separates the Sub-Himalayas and Lesser Himalayas in the entire Himalayan range, in our study area it does the so. Here, on both side of it lies the Kerabari Dolomite (Lesser Himalayas) and Siwalik rocks in north and south part respectively. In the present study area, MBT is observed along the Siddhartha Highway, near 24 mile. This thrust system can be observed well along the Harichal Khola.

Conclusion

Excursion held from 2072/08/15 to  2072/9/13 was very much beneficial and success, to provide students with experience and knowledge. Students not only learned about geological features but also develop co-operative feelings to work in groups.  It also helped students to motivate towards the subject and explore, investigate on their own, to clear their curiosity.

  After long days of hard work, we realized complete dedication, early preparation, co-operative feeling and proper guidance of teacher are very much important. The geological site we visited is the outcomes of the never ending geological process that started millions of years ago. So, the process is continuing and will continue and will always shape the dynamic earth. The structure we studied gave us some general idea, how the structures were formed.  Identifying the fossils, minerals present in the exposed rock and using topographical map to locate saddle, peak and our own location was very helpful. Also, learning to use geological field equipment helped build our confidence.

  So, field excursion plays very important role in producing best future geologist. So, it is likely to organize such field trip with participation and support from student themselves also. We can learn the importance of excursion by using Charles Darwin as example who contributed lot to science through the use of field trips. 

From the study of western Nepal lesser Himalaya and Siwalik following major conclusion is concluded-

• Geologically,from south to north, two sequence of rock is studied in Western Nepal Lesser Himalaya. They are of Tansen Group and Kali GandakiSupergroup in which former is younger.
• Sub-Himalaya is composed of fluvial sedimentary rocks like mudstone, sandstone, and conglomerate, shows the coarsening upward succession.
• The Lower Himalayan Zone consists of low to medium grade metasediments.
• Tectonically, the entire Lesser Himalayas consists of two sequence of rock: allochthonous and autochthonous. 
• Both Group have different composition, metamorphic grade and difference in age.
• The Tansen Group consist of Gondawana and Post Gondawanas rocks.
• Recent Terai sediments represents the latest foreland basin deposits of the Himalaya

References:

Handbook of Field in geology with reference to siwalik. Central Development of Geology.

Sakai, H, 1982, Geology of the Tansen Group of the Lesser Himalaya, West Central Nepal. Tecnical Report of Department of Geology, Tribhuvan University, Kathmandu.

Sakai, H, 1983, Geology of Tansen Group of the Lesser Himalaya in Nepal

Sakai, H, 1985, Geology of Kali Gandaki Supergroup of the lesser himalaya in Nepal.

Tokuoka,T., Takayasu, k., Yoshida,-M., Hisatomi, k., 1986. The Churia (Siwali) group of the Arung khola area, west central Nepal.

Note: 

• This is excursion report prepared by students of geology.
• This post is intended to help student pursuing geology as reference material.

Feel free to comment and recommend new geology related post.

Thank you.

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