Carbonate Fan Fabric Structures (FFS) in time and space: A case study from the Palaeoproterozoic Kajrahat Limestone, Vindhyan Supergroup, India

This paper describes and discusses the origin of the carbonate Fan Fabric Structures (FFS), a rare and typical feature of the Precambrian Eon, observed in the Kajrahat Limestone near the Kota area, Sonbhadra District, Uttar Pradesh, India. In the Kajrahat Limestone, FFS escaped later recrystallization and subsequent dolomitization which otherwise obliterates the depositional texture as noted in most of the Proterozoic carbonate deposits. Characteristic FFS is noted exclusively in the upper part of the Kajrahat Limestone, Semri Group, Vindhyan Supergroup. The FFS varies from microscopic to mesoscopic in size. The depositional environment of the hosting carbonate units is inferred with the help of FFS morphology and configuration. The absence of actual microfossils, however, is conspicuous suggesting very rapid lithification. The origin of aragonite crystals and the possible role of organisms in the formation of FFS are discussed. The study reveals that these carbonate fans were formed below the sediment-water interface by the interplay of sedimentation and vertically upward nucleation of the crystal. Low diversity of stromatolites is also present in the Kajrahat Limestone and the overlying Salkhan Limestone. The Rohtasgarh Limestone, the top-most unit of the Lower Vindhyan is, however, completely devoid of FFS and stromatolites therefore, it is inferred that the FFS are restricted in time and space.