Petrology of the Vindhyan Rocks Around Rohtasgarh, India

Abstract

ABSTRACT The Vindhyans constitute a sequence of about 7000 feet of unfossiliferous Precambrian (Algonkian) sedimentary rocks which form the Vindhyan ranges of central India. The Kheinjua, Kaimur and Rohtas formations are exposed in the northeast portion of the area. The orthoquartzite-limestone association indicates a stable tectonic phase, as do uniform grain-size distributions, low 1 values, stable mineralogical composition, monomineralic stable heavy mineral suites, and high roundness of the grains constituting the sandstones. The sandstone bodies of the Amarkha, Akbarpur and Amjhora beds are wedge shaped, thickening to the south-southwest. In the Amarkha Formation a southwesterly paleocurrent is indicated by ripple marks and a regular variation of the frequency of shale fragments. Although the size distribution and mineral composition of the Akbarpur quartzites do not give any clue regarding the paleocurrent direction, they show cyclic variation of the skewness values as was observed by Folk and Ward. In the Murli Sandstone for which no thickness data is available, a regular southward change in the type of quartz is noted, indicating an approach towards a point of mixing of two sediments, one borne by a other probably flowing current, the other probably flowing in the general direction of south-southwest. In the absence of any evidence of bevelling due to erosion, the zero isopach of the beds is believed to roughly indicate the position of the shoreline. Examination of the position of the shoreline at different times reveals a migration of shoreline roughly towards the northeast.