Genesis and environmental significance of Upper Cretaceous shell concentrations from the Cauvery Basin, southern India

Upper Cretaceous shallow marine calcarenites of the Upper Campanian-Maastri chtian Kallankurichchi Formation from the Cauvery Basin of southeastern Ind ia are rich in shell concentrations dominated by large bivalves with minor admixtures of brachiopods, bryozoans, and larger foraminifera. Rarely, thes e concentrations occur as lenses or pavements, but usually as beds 10 to 20 0 cm thick. Depending on the dominant skeletal elements, five types of shel l concentrations can be distinguished: Isognomon, Pycnodonte, Ceratostreon, inoceramid (?Platyceramus, Cataceramus), and polyspecific concentrations. These shell concentrations occur in various preservational states which ref lect the gradual mixing and mechanical destruction of former communities. A nalysis of the taphonomic signatures of their components and of the biofabr ic suggests that storm-induced waves and currents, reduced sediment input, the settling behaviour of taxa, and a high rate of production of biogenic h ardparts were responsible, in varying degrees, for the formation of the var ious shell concentrations. Accordingly they can be classified as storm wave , proximal tempestite, and primary biogenic concentrations. The shell conce ntrations formed below fair weather wave base in shallow, subtropical, low- stress environments. (C) 1999 Elsevier Science B.V. All rights reserved.