Geology of the Tansen Group of the Lesser Himalaya in Nepal

This paper is concerned mainly with the stratigraphy of late Palaeozoic, Mesozoic and Cenozoic formations in the southern part of the Lesser Himalayas in the Tansen area, western Central Nepal. The rocks of this area are grouped into two major divisions, the Kali Gandaki Group and the overlying Tansen Group, which are separated by a distinct disconformity. The Tansen Group consists of five formations, the late Carboniferous to Permian Sisne (250-1000m thick), late Jurassic to early Cretaceous Taltung (250m), probably Cretaceous Amile (230m), middle to late Eocene Bhainskati (160m), and probably early Miocene Dumri (725m), in ascending order, the former three represent the Gondwanas. / The Sisne Formation comprises diamictite and black shale of glacial influence, and is correlated with the glaciofluvial Lower Gondwanas, the Talchir Boulder Beds in the Indian Peninsula and the Blaini Boulder Beds in the Kumaon Lesser Himalaya. It is disconformably overlain by the Taltung Formation with the basal conglomerate, the Charchare Conglomerate. The Taltung represents an upward-fining fluvial facies characterized by cyclic sedimentation in the lower part and rhythmic one in the upper part, and yields plant fossils of the Upper Gondwanas indicating late Jurassic to early Cretaceous age. In the basal part, basaltic lavas attaining altogether 200m in thickness are intercalated and have supplied enormous amounts of pebbles to the Charchare Conglomerate. These lavas are compared with the Rajmahal Traps of the eastern Indian Peninsula. The Amile Formation, which rests disconformably on the Taltung, is composed of quartzose sandstone and lesser amounts of shale accompanying bioturbated siltstone and limestone which yield poorly preserved corals and molluscs. The sandstones of the lower part are mottled with ferruginous matter. This formation is interpreted as the coastal plain to intertidal sediments. The Amile is conformably followed by the Bhainskati Formation which is made up of black shale and is accompanied in the lower part with many, thin, calcareous interbeds yielding foraminifers of middle Eocene age along with abundant marine molluscs and some vertebrates. In the uppermost part are oolitic hematite containing molluscs, red-purple and green mottled shale. This formation is correlated with the Subathu of the Simla and Garhwal Himalaya. The Dumri Formation is a thick series of fluvial deposits consisting interbeds of sandstone and lesser amounts of purple and green shale, and overlies the Bhainskati with a disconformity. The sandstones become much predominant and massive upwards. No fossils available for age determination were obtained, but the lithologic sequence of this formation shows close resemblance to the Dagshai of the Simla-Garhwal Himalaya. / The Tansen Group is disconformably underlain by the Kali Gandaki Group which consists of the Angha Khola Formation (3000m +) below and the Kerabari Formation (2000m) above. The Angha Khola is alternating beds of purple and green shale with dolomite bands, pinkish quartzose sandstone, arenaceous oolitic dolomite and dolomitic stromatolites. It shows sedimentary facies of a fluvial-sabkha-intertidal flat association. The Kerabari is composed of monotonous thick beds of dolomite with interbeds of stromatolite and oolite, and some thin bands of chert. It is referable to the deposits of a supratidal sabkha to a very shallow sea, mostly intertidal flat environments. As the upper part of the Kerabari includes many carbonaceous seams occasionally containing small plant remains, the Kerabari possibly includes Palaeozoic rocks. The Angha Khola may range down into the Precambrian. / The Tansen Group is embraced in a doubly plunging synclinorium separated from the Kali Gandaki Group by a boundary fault, and is covered by a Klippe of the Kali Gandaki accompanied with a pronounced rupture zone in the central part. It is deformed by folds and faults. The former is of open, southerly verging, asymmetric chevronshape, and the latter is longitudinal or bedding faults of an upthrust sense. In the southern flank of the central area, the Tansen Group forms an imbricate structure. These deformation were taken place after the deposition of the early Miocene Dumri Formation, and no intense diastrophism occurred during the deposition of the Kali Gandaki and the Tansen Groups, except epeirogenic movements that resulted in disconformities beneath the Sisne, Taltung, Amile and the Dumri formations.