PALEOPROTEROZOIC CONDENSED ZONE SEDIMENTS IN THE KAJARAHAT FORMATION, VINDHYAN SUPERGROUP, CENTRAL INDIA

In the Proterozoic Semri Group (Vindhyan Basin, central India), a 1.7Ga.-old and ~7.5m thick marine black shale (TOC 2.2-4.6%) overlies the fining upward Deoland siliciclastic succession and is in turn overlain by monotonous stromatolite-algal laminated carbonates of the Kajarahat Limestone. Petrographic microscope and SEM study shows two compositional elements: (1) continuous to discontinuous clay-dolomicrite stringers that are intercalated with (2) carbonaceous laminae that contain a mixture of organic matter, clays, quartz silt, and dolosiltite grains. The silt component also includes metamorphic and volcanic grains. Wavy laminated dolomite layers (1-8cm thick) are interbedded with the black shale. Wavy texture, pyrite following carbonaceous laminae, and carbonaceous roll-ups suggest a microbial mat origin for the black shale laminae. Wavy lamination in dolomitic interbeds is suggestive of microbial mat colonization as well, but detrital cores of dolomite grains suggest that most of the dolomite was swept in from basin marginal carbonate platforms. Sporadic, but laterally persistent layers with biotite flakes or an abundance of feldspar are probably distal volcanic ash falls. Sharp-based graded clay-dolomicrite layers may be distal storm deposits or fine-grained turbidites. A host of lamina-confined deformation features like micro-faults, slump folds and micro-boudins, in association with disrupted wavy laminae at selected levels, indicate gravity driven mass-movement on a sloping surface, possibly triggered by tectonic disturbances. Slow mud accumulation was at times interrupted by bottom hugging currents that carried clays, quartz silt, organic detritus and microbial mat fragments across the surface. Situated on top of a transgressive systems tract, this black shale represents a condensed zone that records the first major rise of relative sea level in the Vindhyan basin. Whether this was due to eustasy or tectonic subsidence will require further study.