TECTONIC EFFECTS ON GEOMORPHOLOGY AND SEDIMENTATION IN PARTS OF WESTERN BASTAR CRATON

Abstract

The Pranhita-Godavari basin is basically a rift basin between the western Bastar and Eastern Dharwar cratonic nuclei. The study area falls in the NE part of the eastern margin of the Pranhita-Godavari basin near Chandrapur, Maharashtra. Western Bastar craton, here, exhibits the peripheral Proterozoic sedimentary cover. This fossil rift has witnessed several phases of opening and closing in response to the Precambrian tectonics. This rift basin has various synthetic and antithetic faults which have shaped the basin-margin geomorphology and sedimentology by intermittent activities. The established stratigraphy of the region explains the Pakhal, Penganga and the Sullavai Group sediments which are discriminated by unconformable sequences. Multiple episodes of subsidence caused the sediments to adjust in the crustal spaces and deposition of some sedimentary sequences. Previously deposited sequences were very thin as they were just deposited over the Archaean basement. The region has shown sub-dendritic to sub-parallel drainage patterns and it varies with rocktype and structure of the particular region. The central Neo-Proterozoic part of the region has also shown hydrothermal activity in the form of veins and has reduced the rock porosity and permeability. Therefore, exceptions in the drainage morphology has been recorded as compared to its adjacent regions. The previous westerly slope precluded the sedimentary sequences to move westerly which is supported by the imbricated clasts and palaeocurrent directions through cross bedding. However, the present drainage follows the southerly and easterly directions showing a complete inversion of the drainage in response to the major tectonic activities. Upper part of the study area along the basin margin, there were closed sub-basinal areas during the Neo-Proterozoic time which has recorded the presence of paleosol embedded within a clastic sedimentary sequence. The magnetic susceptibility and the paleo- weathering data help in the reconstruction of paleoclimate during the formation of the paleosol. Its analysis has revealed low temperatures and high physical weathering which coincides with other parameters. In the middle of the area, lies about a 400m thick limestone sequence (Penganga limestone). Its equivalent has been reported in the western margin of the PG basin. The dolomitic limestone deposited along the basin ii margin disintegrated due to various fault activities which caused mass flow on the westerly slope of the basin margin. There are many limestone facies documented in the three major mass-flow events which describe tectonic activities and flow parameters. However, in-situ stromatolitic formation in between the mass-flows indicates intermittent tectonically stable periods and origin of life in the basin. The Ground Penetrating Radar (GPR) was used to acquire subsurface data in relation to unconformity or any fault trace and various profiles have given ample evidences of offlap, onlap and some unconformable surfaces which helped in basin interpretation as well as tectonics associated with it. Some sedimentary sequences which are common along the basin margin are the fan-delta sequences. Dongargaon fan-delta within study area is one of them. Different sedimentary facies like fine grained sandstone, and intercalatory sandstone-conglomerate sequence and shale facies were mapped on 1:1000 scale to analyze the facies assembles in detail with respect to sea-level fluctuation and subsidence. Basic model for the evolution has been identified and explained on the basis of exhaustive field evidences and it has been correlated with the overall geology and geomorphology of the region to get some inference about the tectonic history and sedimentation of the area. Eventually, various processes along the basin margin including the formation of fan-delta sequence or mass flow events or some paleosol formation during the period of non-deposition in the fossilized rift basin may conclude about the local basin-margin processes due to tectonism and sedimentation. Also, present geomorphology of the terrain is another manifestation of the same.