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

Abstract

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