Geology and Structure of the Kota-Baidi Area, Tanahu District, Central Nepal, Lesser Himalaya

Abstract

The study area is a part of the Lesser Himalaya, central Nepal along the Kota-Baidi area of the Tanahu District. The aim of the study was to prepare a detailed geological map of the area in the scale of 1:25,000. The study area consists of both the rocks of Upper and Lower Groups of the Nawakot Complex, their age ranging from Late Precambrian to Early Paleozoic. The rocks of the Lower Nawakot Group consist of the Nourpul Formation and the Dhading Dolomite whereas the Upper Nawakot Group consists of the Benighat Slate and the Robang Formation. The Malekhu Limestone is missing in the area and the Benighat Slate is not found in normal stratigraphic position which is thrust over the younger sequence, the Robang Formation. The Nourpul Formation consists of mixed type of metasedimentary rocks like shale, mudstone, phyllite, metasandstone, dolomite and quartzite. Occasionally these rocks consist of sedimentary structures like mudcracks and ripple marks pointing right-way-up of the beds. Stratigraphically, the Nourpul Formation is followed upwards by the Dhading Dolomite. It consists of ridge forming stromatolitic dolomite with subordinates of phyllites and quartzites. The well developed columnar stromatolites of this formation show the beds to be in the normal position. The Benighat Slates is characterized by the monotonous sequence of grey to dark grey slate and phyllite with or without calcareous deposits. Based on the calcareous nature, a separate member has been mapped within the Benighat Slate as the Jhiku Carbonates. The youngest sequence of the area is mapped as the Robang Formation, consisting of intercalation of phyllite, metasandstone and quartzite with several bands of metabasites. The whole study area lies in the core of megasyncline which is named as Kota-Baidi Syncline. It is a tightly strike-out syncline whose trend and plunge is about 277°/17°. Another significant local fold is mapped as Makre Anticline whose trend and plunge is found to be 284°/8°. Two major thrust structures of the area are the Main Boundary Thrust (MBT) in the south and the Chherenga Khola Thrust, nearly parallel to the MBT, in the north. The MBT can be easily demarcated in the field based on lithology and topography whereas the Chherenga Khola Thrust is recognized only on the basis of abrupt change in lithological formations. Numerous mesoscopic folds and faults in different orientations, sedimentary structures, quartz veins, boudinages and pockets of several minerals are also recorded in the field.