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Geology
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Geological Problems of Nepal, Himalaya and its solution
K. D. Bhattarai
Introduction
This paper deals mainly with the three aspects of geology of Nepal Himalaya, which are also the main objectives of the presentation, and they are:
- To provide out line information on geology of
Nepal Himalaya.
- To draw attention of the concerned community
about the existing geological problems of Nepal Himalaya
- To provide certain suggestion on the types of
works needed to solve the existing problems.
Location of Nepal Himalaya
Nepal is divided in to various morph tectonic zones as other parts of Himalaya. (fig. 2)
Fig.2 Generalised morphotectonic map
Out line information on geology of Nepal Himalaya.
Considerable knowledge on geology of Nepal Himalaya is now available due to the effort of foreign as well as local geologist. However due to various complication, lack of definite proof, personal jingoism of the workers, lack of cooperation among geologist working on Nepal Himalaya and also due to absence of regular communication and circulation of latest publication among active workers on Nepal Himalaya, a valid and acceptable to all model is still not ready.
Due to presence of fossils and fairly detail work especially on the neighboring part India, The Sivalik formation that is more or less continuous all along he entire foothill of the Himalaya is will understood to some extent.
Similarly due to considerable amount of work especially by foreign geologist on the Higher Himalaya and also due to presence of fossiliferous sequence ranging from comb rain to Miocene Higher Himalaya of Nepal are also to some extent better understood than the complex lesser Himalaya.
Due to presence of the Nappe, Klippe, over thrusting, Tectonic windows and many other problems along with the unfosailiferous nature of the sequence with few exception (5/28{ very little is understood about the geology of lesser Himalaya of Nepal. Systematic study detail investigation and logical explanation of the available geological data is absolutely necessary in this part of the country for better understanding. However it is not only the lesser Himalaya that requires the systematic study and logical explanation hut the sub Himalaya as well as Higher Himalaya also as mentioned earlier require in depth study of available information along with the support of the geological data that are available now.
Sivalik
It is commonly used for molasses type Neogene's sediments of Himalayan foot hill zone. The age range as indicated by mammalians fossils from middle Miocene to early Pliocene.
In Nepal the Siwalik system form 20-30 km foothill belt sand extend south into the subsurface of the genetics plain these terrestrial largely fluviatile deposits reach over 5 km in thickness. Not much detail study of Siwalik stratigraphy of Nepal has been done yet and is know n only in gross outline. However based on well studied area of neighboring India Krishnan, 1943, Wadia, Pilgrim 11910, 1913, Lewis 1937, Fatmi 1943, etc.) the three fold divisions of Siwalk which is well established in India is also widely accepted in principle for Nepal also. (22, 30) however Glennie and Ziegler (1964) divided the Siwalik group into lower Sandstone faces and upper conglomerate facies.
The three fold divisions of Siwalik into clayey sandy siwalik (Nahan by Lehner) the predominantly sandy middle Siwalik and he predominantly conglomeratic upper Siwalik is used in Nepal, but they are seen as a patchy outcrops North of MBT on the Surkhet area.
The top of the Upper Siwalik is always truncated by the MBT. The pebbles in he conglomerate appears to be strikingly different from the recent pebbles materials of the rivers. Specially in the section of central Nepal (27).
The different members of Siwalik are repeated several time due to folding and faulting (6) and the belt is characterized by broad gentle folds dying out towards the Genetic plain with minor sign thrust faulting nearer to MBT (Tony Hegen fig 83-9 pp 101-111).
Lesser Himalaya
The lesser Himalaya of Nepal is the complex zone both from point of view of stratigraphy and structure. Klippe, of metasedimentaries, with granitic elements physically overlie less metamorphosed or un metamorphosed un fossili ferrous sequence over great distance. The unfossiliferos sequence are seen through the over thrust metamorphic in 'window" and ' semi window". The paucity of fossili ferrous sediments and complex structure relationship of various proterozoic- paleo zole sequences is common feature of this belt.
These belt mainly consists of classic sediments with locally important carbonates zones. Some of these rocks are practically un metamorphosed but most arte converted low grade metamorphism to slate, Phyllites, quartzite's, and finely crystalline lime stones. It can be distinguished into a narrow outer tectonically more intense and a broad inner or midland tectonically less or undisturbed sedimentary belt, the two being separated by a discontinuous zones of schist's, gneisses, migmatites, and also minor granite known as the lesser Himalayan crystalline which seem to overlie the sediments in large synclinal cores. The Mahabharata range south of midlands comprises the outer sedimentary belts and parts of crystalline. However in the eastern lesser Himalaya the crystalline cover most of the midlands and merge with the central crystalline zone of high range.
The less metamorphosed piles f sedimentary rocks reaches 20 km or more. Weather this thickness resulted from mere sedimentation or tectonic repetition is a controversial matter and is one of foremost problem of Nepal Himalaya. Due o almost lack of fossils except in pulchouki (5) and Chandragiri (28) and reverse metamorphism the problem of lesser Himalaya becomes more complicated.
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