Organic-rich shale in the Palaeoproterozoic Kajrahat Formation, Vindhyan Supergroup,Son Valley: implications and genesis

摘要

The paper attempts to interpret the textural attributes of the shelf-originated, organic-rich shale unit of theKajrahat Formation in the Proterozoic Vindhyan Supergroup. The black shales, alternating with thinly laminated dolomitebeds exhibit many textural features, indirectly indicating microbial mat growth on muddy siliciclastics. The microbial matfeatures are subdivided broadly into three different categories related to growth, destruction and diagenesis of microbialmat. Wavy and crinkly laminated fabric is very characteristic of the shales, thin mud drapes imparting the wavy laminaeconspicuous. Pyrite, formed because of anaerobic decay of microbial mat follows the carbonaceous laminae, rather thanbeing disseminated. Cohesive behaviour of the shales is manifested by the folded and contorted nature of the carbonaceouslaminae at places. Sticky nature of the microbial mats possibly generated the quartz-silt aggregates and abundant scatteredquartz silts within the carbonaceous laminae. The black shale unit of the Kajrahat Formation, thus, appears to havemicrobial mat origin like many other contemporaneous black shales reported from Precambrian black shales. Theoccurrence of microbially originated, organic-rich shale suggests curtailment of siliciclastic supply.