| Abstract |
The Kali Gandaki Supergroup is extensively distributed in the Mahabharat Range and the Midland of the Lesser Himalayas in western Central Nepal. It is a thick sequence attaining more than 10 km in thi...ckness and ranging in age from Late Precambrian to early Palaeozoic, and is unconformably overlain by the Tansen Group of Gondwana rocks. There is no stratigraphic break throughout the whole sequence. The Kali Gandaki Supergroup is divided into three groups. The Low.er Group consists of the thick, monotonous, predominantly gritty phyllite and phyllite Andhi Formation and the overlying, strongly rippled and cross-bedded Naudanda Quartzite. Metabasites are sporadically interbedded in the Naudanda Quartzite. The Middle Group is composed of five conformable formations. The lowest Heklang Formation comprises monotonous, light-green phyllite with subordinate limestone and marl beds. The Virkot Formation represents cyclic sedimentation of sand dune and ephemeral lake and fluvial deposits that are characterized by reddish-purple shale and quartzite with sun cracks and some stromatolite beds. The Lower and Middle Members of the Chappani Formation comprise black, light-green and pink, calcareous laminated slate with many thin interbeds of column-shaped Kussiella and Collenia-type stromatolites. They are the deposits in a lagoonal environment. The Upper Member is made up of quartzite and reddish-purple and light-green shale with some beds of shale-pebble conglomerate. Sun cracks are common in shale beds. The Khoraidi Formation is unique in having a large number of stromatolite biostromes which totally attain 300 m in thickness. The stromatolites are domed to subspherical, and are dolomitic. Oolite, sandstone and shale rhythmite, and quartzose sandstone with abundant gypsum pseudomorphs are inter. bedded. This formation is referred to as the deposits in the intertidal zone under arid climate conditions. The Saidi Khola Formation consists of coarsely to thinly interlayered sandstone and shale with many burrows by bioturbation, and is interpreted to have been deposited on shallow subtidal to intertidal flats. The Upper Group consists of two formations; the Ramdighat of predominantly calcareous, varicoloured slate and the Kerabari largely of thick, monotonous, platy bedded dolomite and limestone. The Ramdighat indicates the deposition in the lagoonal environment and the Kerabari was deposited in the shallow shelf to intertidal environments. The Lesser Himalayas in the study area is structurally divided into two belts, a complexly folded outer belt and block-faulted inner belt by the active right-lateral Bari Gad Fault. The outer belt forms the Mahabharat Range to the north of the Main Boundary Thrust and the inner belt is expressed by the depressed topography of the Midland. The outer belt is subdivided into three structural units disposed from south to north; the Tansen Synclinorium, Angha Khola Recumbent Fold Belt and Khoraidi Fold Belt. The Tansen Synclinorium is largely occupied by the Tansen Group of Gondwanas and Tertiary rocks, and is overlain by the Palpa Klippe with intensely deformed melanges along the sole thrust. The Kali Gandaki Supergroup of the Angha Khola Belt wholly exhibits a reversed stratigraphy. On the basis of consistent Z-shaped asymmetric folds of various sizes and the bedding-cleavage relationship, it was con6rmed to form an inverted limb of a huge recumbent fold. In the Khoraidi Belt the supergroup shows northerly verging isoclinal folds. The southern part of the inner belt is segmented by longitudinal high-angle faults with the component of great vertical displacements more than 1000 m. Each block is usually occupied by a single or two formations and forms a broad open fold. The Lower Group thrusts upon the upper groups along the Phalebas Thrust in the southern front of the northern part. The juxtaposition of the strongly folded and thrust outer belt and the block-faulted inner belt in the Lesser Himalayas call be explained by differences of stress fields, compressional in the frontal part and tensional on the rear side of a large scale thrust-sheet associated with the Main Boundary Thrust. These structures are considered to have been formed after main activity of the Main Central Thrust.show more
|
Export Mendeley
25_03_p337