SHALE MICROFACIES AND ORIGIN OF THE MIDDLE PROTEROZOIC RAMPUR SHALE OF THE ROHTAS FORMATION, VINDHYAN BASIN, INDIA

The Rampur Shale of the Rohtas Formation of the Vindhyan Supergroup (central India) is a dolomitic black shale with limestone interbeds. It is 1.6 billion years old, about 60 m thick, laterally extensive, and has an average TOC content of 2%. With detailed petrographic observations three shale microfacies types can be distinguished on the basis of proportion of clay, dolomicrite, silt and carbonaceous material. The three microfacies types are: 1) clayey shale facies; 2) carbonaceous silty shale facies; 3) carbonaceous streak shale facies. Clayey shales are dominated by clay lenses, show an anastomosing fabric of clay lenses and thin carbonaceous films, and contain scattered quartz silt and dolosiltite in a clay- dolomite matrix. Carbonaceous silty shales differ from clayey shales by having fewer clay lenses, more carbonaceous material, and a higher silt content. Carbonaceous streak shales are characterized by silt and carbonaceous flakes that are randomly distributed in a clay- dolomite matrix. Presence of micro- scouring and sand size clasts in clayey shale facies indicates the action of strong bottom currents, and thus the clay lenses could have originated as current sorted (now flattened) clay rip-ups and/or as erosional remnants of clay drapes. In carbonaceous silty shales, fewer clay lenses and paucity of erosional features may indicate that they were deposited more distal than the clayey shales. Wavy- crinkly fabric and cohesive behavior of carbonaceous laminae in carbonaceous silty shales can be interpreted as microbial mat colonization of the seabed between clastic depositional events. Abundance of silt in carbonaceous laminae may indicate particle trapping by microbial mats. Massive and non-laminated nature of carbonaceous streak shales suggests rapid deposition from suspension or prolonged sedimentation pulses. Absence of erosional features and clay lenses suggests little or no bottom current reworking and most distal deposition of the three facies.