Sedimentology of the Early Jurassic limestone beds of the Kota Formation: record of carbonate wetlands in a continental rift basin of India

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Research Article

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Journal of Paleolimnology


Pranhita–Godavari is one of the major rift basins of peninsular India that was actively filled when the Indian landmass was a part of Gondwanaland, the southern supercontinent. The basin-fill succession of this continental rift basin is characterized mainly by fluvial sandstones and mudstones. In the Early Jurassic this basin hosted a freshwater carbonate deposit characterized by a 20–30 m thick alternation of laminated and massive limestones. The remains and traces of land vertebrates, invertebrates and plants suggest that this limestone serves as one of the rare examples of freshwater carbonate depositing environment within a continental rift basin. The limestone interval is characterized by repeatedly stacked meter-thick laminated and massive carbonate-rich sediments. These sediments are classified into two facies associations representing two main depositional domains of a rift-bound, large, freshwater wetland system: a shallow lacustrine palaeoenvironment and a palustrine paleoenvironment. The former palaeoenvironment is characterized by laminated carbonate facies and the later one is by mottled, nodular and bioturbated limestone. Repeated influx of siliclastics by surface flow into ca-rich water bodies produced the laminated carbonates. Whereas, subaerial modifications and reworking of primary deposits resulted in palustrine carbonates. Vertebrate tracks, evidences of rooting, and the absence of deep basinal facies in this succession along with repetitive stacking of sediments formed in subaqueous and subaerial conditions indicates that the depositional environment was more akin to a modern wetland. This Jurassic carbonate wetland developed in the rift basin when the watershed on the adjacent rift shoulder exposed Proterozoic carbonate rocks that provided the ions necessary for carbonate accumulation. The repeated stacking of meter-scale lacustrine and palustrine carbonates in this limestone reflect long-periodicity fluctuations in water level possibly related to subsidence events of the rift valley.

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