SCARP SANDSTONE: AN EXAMPLE OF ESTUARINE SEDIMENTATION WITHIN THE MESOPROTEROZOIC KAIMUR GROUP OF THE VINDHYAN BASIN,(MIRZAPUR, U.P.), INDIA

摘要

Famous for its controversial origin, the Precambrian Scarp Sandstone Member in the studied hillock sections is 20-25 m thick. The rock successions are made up of 3-6 m thick channelized, vertically stacked and laterally shifting sand bodies. Based on primary sedimentary structures such as small to large scale trough- and- herringbone cross - beddings, wave, current and interference ripples, tidal bundles with mud drapes, four major lithofacies and at least two of them, divisible into subfacies are identified and ascribed to above the wave base shallow marine origin. However, channelized nature of the sand bodies and their hundreds of meter wide lateral extent made up of shoaling cycles, and north-west directed polymodal palaeocurrent pattern, imply deposition in sub-tidal to inter-tidal domains of estuarine setting. Nevertheless, the overall coarsening upward trend in the grain-size in the measured sections indicates tendency of deposition in a transgressive system. Sandstones show both textural and mineralogical maturity. Fine to medium-grained quartz arenites having sub- populations of both mono and polycrystalline quartz and heavy minerals such as hypersthene, olivine and zircon demand sediment supply from a provenance comprising rocks of metamorphic, sedimentary and igneous origin.