A STUDY OF SEDIMENTARY AND TECTONIC STRUCTURES IN KROL 'A' FORMATION, RISHIKESH, GARHWAL LESSER HIMALAYA

Abstract

Garhwal Syncline is one of the six synclines identified within the Lesser Himalayan Fold and Thrust Belt. It folds the Neoproterozoic successions of Blaini-Krol-Tal Formations. The Krol Formation, which has been divided into three sub units viz., the Lower. Middle. and Upper Krols overlie the Infra-Krols and underlie Chakrata Formation. Krol A Formation, exposed in the stud area, comprises of argillaceous limestone interbedded with greenish grey shale which is calcareous at places. These rocks have undergone buckle folding with development of axial planar cleavage and intersection lineation on the cleavage plane. They show abundant mm-cm * scale sedimentary structures and the soft-sedimentary deformation structures in some bands. These primary structures have been distorted during the subsequent tectonic folding that resulted into the development of the Garhwal syncline. Within the study area, the mesoscopic tectonic folds plunge at steeper angles than the shallow plunge of the regional fold, the Garhwal Synclinc. The steepening of the plunge angle may he due to presence of an unexposed fault in the vicinity of the study area. The geometrical details of the soft-sedimentary deformation structures are studied under the microscope and in the polished sections. Study of these structures in Field reveals that they are co-axial with the fold hinge line of the tectonic folds. The co-axial nature of the two types of folds is probably due to the fact that the mild curviplanarity of the primary structures acted as the _ buckling instability that were amplified during the tectonic IhIding.