Sequence evolution in the eastern Chhattisgarh Basin: constraints on correlation and stratigraphic analysis

Abstract

The Proterozoic succession in the eastern part of the Mesoproterozoic Chhattisgarh Basin comprises two unconformity-bounded sequences. Sequence I represents the Chhattisgarh Supergroup of earlier workers. It overlies rocks of the basement complex with a profound unconformity. Sequence II unconformably overlies Sequnce I, and represents the closing phase of basin evolution during the early Neoproterozoic time. It is unconformably overlain by rocks of the Gondwana Supergroup.
 The Lohardi and Gomarda formations at the lower part of the Chandarpur Group of Sequence I comprise an immature succession of conglomerate, sandstone and shale deposited in fan-delta - pro-delta environments, marked by rapid facies changes, variable rates of sediment influx, and uneven rates of subsidence and creation of accommodation space. The Kansapathar Sandstone in the upper part of the Chandarpur Group, by contrast, comprises a sheet of mature arenite deposited in a macrotidal shelf. The immature assemblage is best developed in the eastern part of the basin, and rapidly thins out towards west, where the Kansapathar arenite directly overlies the basement. The Raipur Group provides an excellent example of cyclic sedimentation of red shale and limestone. It comprises three shale-dominated intervals and two carbonate-dominated intervals, organized into multiple shallowing-up cycles. The lower carbonate succession, the Sarangarh Limestone, developed as a shallow water un-rimmed platform and evolved into a deep water ramp, with an extensive thin sheet of black limestone facies. Stromatolites are conspicuously absent in the Sarangarh Limestone. Small stromatolite bioherms appear in the Gunderdehi Shale which overlies the ramp succession, and abundant growth of stromatolite is noted in the upper carbonate succession which evolved as a rimmed platform. A thick ignimbrite horizon in the Churtela Shale attests to major felsic volcanism and termination of the Sequence at ~1000 Ma.
 The Kansapathar Sandstone, the black limestone facies of the Sarangarh Limestone, and the Gunderdehi Shale embedded with small stromatolite bioherms can be used as key marker horizons to overcome the problem of intrabasinal correlation. The marker horizons can be traced from the western part to the eastern part of the basin. The stromatolites in the Gunderdehi Shale and in the Saradih Limestone further provide a biostratigraphic frame, subject to detailed morphologic and microstructural analysis, for possible chronostratigraphic classification.