Freshwater Microbialites in Early Jurassic Fluvial Strata of the Pranhita-Godavari Gondwana Basin, India

Abstract

The role of microbes in the precipitation of carbonates in various modern freshwater environments like lakes, rivers, and springs is currently being actively researched. Extensive research over the last few decades has documented some of the distinctive features of the freshwater microbial carbonates. However, identification of the signatures of microbes in carbonate precipitation remains a challenging task in the case of geologically ancient deposits. It is especially difficult for deposits older than the Neogene, because of diagenetic imprints on the primary fabric. Due to this limitation, the habitats of microbes are poorly constrained for a significant part of the Phanerozoic. In this study, we report the field occurrences and micro-morphological features of thin carbonate bodies encased in a thick siliciclastic alluvial succession. The succession is Early Jurassic in age and formed in a continental rift basin, the Pranhita-Godavari Gondwana Basin of peninsular India. It has been demonstrated that in spite of diagenetic modifications, a number of microscopic and sub-microscopic features of these carbonates can be equated to those found in modern freshwater microbialites. Seven facies have been identified from these freshwater carbonates. Among them, the first three are mainly autochthonous deposits, and the others show incorporation of detrital components (partly or completely allochthonous). The geometry of the carbonate bodies and the mode of association of the various textural features suggest three main types of primary carbonate accumulation and a fourth type representing coeval reworking of the primary deposits. The stratigraphic relationship between the carbonate and siliciclastic components of this succession reveals the mode of mutual interactions between the siliciclastic and carbonate depositing processes.