Ganurgarh Shale of Southeastern Rajasthan, India: A Precambrian Regressive Sequence of Lagoon-Tidal Flat Origin

Abstract

ABSTRACT The Ganurgarh Shale is the basal formation of the Bhander Group, the uppermost stratigraphic sequence of the Vindhyan Supergroup (Precambrian). A 40-km long outcrop belt of this formation was examined in the Singoli-Mandalgarh area of southeastern Rajasthan with a view to reconstructing the depositional environments. In the study area, the Ganurgarh Shale is 22 m thick and shows a perfectly conformable relationship with the underlying Rewa Sandstone and the overlying Bhander Limestone. Fine clastics and parallel laminations characterize the entire formation. However, a lower, grey-blue unit A containing small-scale lenses of silty sandstones is distinguishable from an upper, red-coloured unit B which lacks lenses. The significant sedimentary features in the formation are micro-cross-lamination, very small ripple marks of symmetrical, flat-topped and interference types, rill marks, mud cracks, raindrop imprints and shale pebbles. Depositional environments of the formation were interpreted on the basis of lithology, geometry of rock-units, sedimentary structures, sediment dispersal patterns and vertical facies changes. The unit A was deposited in a shore lagoon-tidal flat complex. The lagoon was filled up and the shore prograded giving rise to supratidal flats on which the unit B was deposited. The formation, thus, represents a regressive sequence formed in a shore lagoon-tidal flat complex.