SIWALIK OF BAGMATI- GOSAIKUND REGION.pptx

This is the Formation of Bagmati
-gosaikund region of Nepal

1. SIWALIK OF BAGMATI- GOSAIKUND REGION PRESENTED BY 1- Purushottam Sharma 2-Karan raj Giri 3- Mahesh Nyaupane 4- Shreya Rana Magar KATHMANDU UNIVERSITY DEPARTMENT OF CIVIL ENGINEERING. ( B.E. in Mining Engineering) 1

2. N 2 Tibetan Tethys Himalaya ( Cambrian - cretaceous ) ………………… South Tibetan Detachment system( STDS) Higher Himalaya ( pre- cambrian ) ………………… Main Central Thrust(MCT) (-25-30 MY) Lesser Himalaya ( Late Precambrian to early Miocene ) ………………… Main Boundary Thrust (MBT) (-10-12 MY) Siwalik group ( Churia group) (CCT OR ICT to Dun Valley) ( Mid-Miocene to Pleistocene ) ………………… Himalayan Frontal Thrust (HFT) (-1 MY) Terai (indo- Gigantic plain) (Pleistocene to present)

3. Geography of Bagmati – Gosaikund Region This region is located in upper, middle and lower siwalik . The The siwalik sequence of Bagmati Gosaikund Region is the foothill bed as it is intensely folded and dislocated many times by imbricate thrust. Larger portion of this region (tract) has been covered by the upper siwalik and late Pleistocene – Holocene alluvial deposits of the Narayani and Budi Rapti river. This region consist of mainly Sedimentary, Meta Sediments and basic rocks. Fig.1 geological map of Bagmati-Gosaikund region 3

4. Fig.2 kokhajor Thrust and kokhajor khola Formation 4

5. Lithostratigraphy (classification of rocks on the basis of lithology) Sah et al.(1994) has mentioned that the siwalik of this region has been divided into Rapti Formation (lower siwalik ), Amlekhganj Formation (middle siwalik ), Churia khola Formation (upper siwalik conglomerates) and Churia mai Formation (upper siwalik boulder beds) Lower Region ( Rapti Formation) The lower region consists of cyclic interbedding of variegated (having same color) mud stone, green-gray to brown sand stone and also includes many calcareous clay stone or marl beds. The inner belt of the lower siwalik making up the hanging wall of Kokhajor Thrust and the Bastipur Thrust are comprised of relatively soft, medium to fine grained, red purple mud stone and sediments. 5

6. Fig3. Lower Siwalik beds, exhibiting mottled colours and biturbation structure. West of hetaunda . 6

7. Middle Region ( Amlekhganj Formation) It exposed North to Hetauda and Amlekhganj consist of 20-30 m thick cyclic alteration of very coarse – to-coarse grained pepper and salt colored sand stone passing into gray to dark gray mudstone and Samari Khola towards South consist of gray sand stone and Dark gray Mud stone. The succession towards the North of Samarai khola are composed of gray sand stone, dark-gray mud stone and red purple mud stone bed. Near the upper part the appearance of pebbly sandstones are found which grade the middle siwalik into the overlying upper siwalik . 7

8. Fig.4 Large through cross stratification in the pepper and salt sand stone at the north of Amlekhganj . 8

9. Fig.5 Planar cross-Lamination in the pepper-and-salt sandstone of the Middle Siwaliks . North of Amlekhganj Fig.6 Sandballs (Calcareous Concretions) in the sandstone of middle Siwaliks . North of Amlekhganj . 9

10. Upper Region ( Churia khola Formation- Conglomerates) In the upper region of Siwalik, The conglomerates are sometimes cemented by Calcite and they make the hard ground. In the inner belt of the Kokhajor Khola , the upper region comprise mainly soft but sometimes well cemented and hard (near Bhokteni ), pebble, cobble to boulder Conglomerates interbedded mainly with pebbly sand stones. On the right bank of Kokhajor Khola there is Conglomerates cliff on the right bank about 25M high and extends for 100M along the river coarse. 10

11. Upper Region ( Churia mai Formation - Boulderbeds ) This region of Formation exposed at Devitar contains a 1-7 m thick conglomerate bed, composed exclusively of sub angular to sub-rounded pebbles derived from red-purple and brown as well as Dark green, fine- to very fine-grained, quartzes sandstone or quartz arenite. Holocene alluvial deposits are wide spread in this region. Most of the Formation are concentrated along the main river courses, where they range in thickness from 10 to 30 m and more. 11

12. LITHOFACIES Lithology reflects compositional mineral content and facies are expressions of depositional environments . In Bagmati – Gosaikund region there are main seven Lithologicalfacies . Facies-1: This facies association was deposited by a meandering river, carrying fine sediments, and with floodplains exposed to weathering for a long time . Facies-2: A second facies association includes meduium - to coarse-grained , light gray sandstones interstratified with mottled mudstones laid down by flood flow dominateing meandering river system. Facies-3: A third facies association contains medium- to coarse-grained sandstones , fine-grained muddy sandstones, and gray mudstones deposited by a sandy meandering river suffering from recurrent floods. 12

13. CONT … Facies-4: This Facies marked by the appearance of coarse-to vary coarse- grained, Very thick bedded sandstone(up to 25M) this association is present in middle and upper part of the middle siwaliks . It was deposited in deep sandy braided rivers. Facies-5: A fifth facies association is represented by planar lamination, ripple lamination, and less-pronounced fining-upwards cycles they were deposited in a shallow sandy braided river. Facies-6: A sixth facies association comprises well-sorted pebble- to cobble conglomerates interbedded with yellow-brown sandstone lenses and gray mudstone beds it was formed by gravelly braided rivers. Facies-7: A seventh facies association is made up of poorly sorted boulder-conglomerates and well sorted pebble-conglomerates with a subordinate amount of gray sandstone and mudstone. I t was deposited by braided river system.( Nayakama and Ulak 1999) 13

14. STRUCTURE The Kokhajor Thrust has brought the lower Siwalik, over the upper Siwaliks . The Basan Thrust passes north of Chauda khola , Crosses the Bagmati river, and follows the right bank of the marin khola . The marin Thrust enters this region from the west of phaparbari , runs through the chauda khola , goes over the Bagmati river and follows the lower reach of the marin khola . In Siwalik range and also in Terai range blind Thrust are also detected. The eroded core of the frontal anticline is interrupted by a horizontal alluvial terrace of the Bagmati River. 14

15. Fig.7 Granular to pebbly sandstone exposed towards the upper part of the Middle Siwaliks . North of Amlekhganj . 15 Fig. 8 Scoured and cross laminated gray sandstone bed followed by channel deposits (conglomerate) in the Middle Siwaliks . North of the Samari Khola .

16. Fig.9 Emergent and blind thrusts in the Siwaliks and Terai of the Hetaunda area. 16

17. DISCONTINUOUS LESSER HIMALAYAN BELT In the upper reaches/ head of the kokajor khola , the lesser Himalayan rocks form a few 100m wide strip where the mainly the Benighat slate of the Nuwakot Complex crop out. In the Devitar neighborhood, the Kathmandu complex is Thrust over the lower Siwalik. The absence of the lesser Himalayan Range along the foothills of the Mahabharat range(Stocklin and Bhattrai 1977) indicates that quite recent movement of the main Central Thrust of this region. 17

18. 18 Fig.10

19. ANY QUERIES 19