Depositional Environmental History of the Chhattisgarh Basin, M.P., Based On Stromatolites and Microbiota

The Proterozoic Chhattisgarh basin, comprising 2000m thick sediments mainly limestonel/dolomite, sandstone and shale, shows prolific growth of stromatolites and presence of micro biota dominantly cyanobacteria. Added to these there are many sedimentary features and structures found in the rocks throughoutthe sequence. Depositional environment is thus well doculnented by physicochemical factors in general and biological features in particular.

Based on all these biogeological features, it has been deduced that the deposition is largely marine under tidal flat conditions, sources of sediments being both precipitation as well as terrigenous material. Sedimentation shows a distinct cyclic pattern commencing with arenaceous facies culminating into argillaceous facies with intervening calcareous facies. During the waning stage of sedimentary history of the Chhattisgarh basin, the cycles repeated rapidly, interpreted from the intercalatory sedimentation of the upper horizon, namely Raipur Limestone. Disposition of stromatolites ancLtheir morphologies reveal low as well as high-energy conditions of deposition, levels of deposition being both intertidal and subtidal ones. The sedimentological, mineralogical and petrological evidences point towards an alternating oxidising and reducing environments throughout the sequence irrespective of cycles or lithologies.

Overwhelming dominance of procaryotic algae and absence (7 or near absence) of eucaryotic algae have possibly the following implications, namely the temperature during deposition was raised above 40°C at least for major part during deposition and also the pH of the environment. was 7 or more.