Lithostratigraphy of the Neoproterozoic Chattisgarh Sequence, its Bearing on the Tectonics and Palaeogeography

The Neoproterozoic Purana succession in the eastern part of Chattisgarh basin around Sarangarh has been classified into a conglomerate-sandstone-shale dominated proximal assemblage, and a lithographic limestone-shale dominated distal assemblage. The proximal assemblage constitutes the Chandarpur Group, and unconformably overlies the Archean crystalline basement complex. The Chandarpur succession has been classified into three formations that were deposited in fan-fan delta, deep water prodelta and storm- tide dominated prograding shelf environments. The distal assemblage, the Raipur Group, conformably overlies the Chandarpur Group, and may be subdivided into two shale-dominated formations separated by a limestone-dominated formation. The limestone sequence, the Sarangarh Limestone, comprises a lower member of mixed carbonate-siliciclastic succession deposited in a storm dominated shallow water platform, and an upper member of pelagic limestone that grades upward into a deep water shale, the Gunderdehi Shale. The rapid transition from shallow water platformal succession to deep-water pelagic limestone and shale points to abrupt deepening of the basin and drowning of the craton. The peak of transgression is represented by a persistent horizon of black limestone, a product of basin wide anoxia. Disposition of facies belts in proximal and distal assemblages and palaeocurrent directions measured from different facies belts point to a north-northwesterly palaeoslope of the basin. Signatures of intense storm and tidal currents in different litho-units collectively point to an open marine circulation condition. It has been inferred that the basin was connected to a major seaway that skirted the northern and north-western margin of the craton. Development of thick fan-delta sequence at the base of the succession, occurrence of felsic welded tuff within the Gunderdehi Shale, thick sandstone-mudstone cyclothems in the Chandarpur Group, and abrupt drowning of the carbonate platform leading to pelagic sedimentation collectively point to major tectonic control on basin evolution. The basin developed as a cratonic rift and evolved into a deeply subsiding one, without any major stratigraphic hiatus, through episodic tectonic pulses.