Microfabric analysis of Proterozoic Vindhyan shales from central India and their paleogeographic implications

摘要

Three shale horizons from the 1.6-0.6 Ga old Vindhyan basin were identified for detailed microfabric analysis and their possible paleogeographic implications. All the shale are predominantly composed of illitic clays. Koldaha Shale and Sirbu Shale contain randomly oriented coarser quartz-silt grains within clay matrix and there is a marked absence of organic matter. Clay minerals are mostly arranged in face-to-edge and edge-to-edge fashion. In contrast, the Bijaigarh Shale is crinkle-laminated and contains pyrite crystals within intergranular spaces. Face-to-face orientation of clay matrix and organic filaments result parallel - to crinkle - laminated microfabric. Rapid sedimentation and little transportation of sediments were possibly responsible for the random orientation of clays in Koldaha Shale and Sirbu Shale. Presence of coarse-grained siliciclastics in these shales possibly indicate deposition in a high-energy shallow marine setting, while presence of pyrite crystals, lower abundance of siliciclastics and planar to crinkle-laminated microfabric within Bijaigarh Shale is indicative of slow sedimentation and deposition in relatively deeper offshore environment. Microbial mats might have caused wavy-crinkly laminated microfabric and pyrite crystallization.