Tidal shelf sedimentation in the Neoproterozoic Chattisgarh succession of central India |
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Authors: | Sarbani Patranabis Deb |
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Institution: | (1) Geological Studies Unit, Indian Statistical Institute, 203, B. T. Road, 700 108 Kolkata, India |
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Abstract: | The Neoproterozoic Kansapathar Sandstone of the Chattisgarh basin, a shallow marine shelf bar sequence, consists of mineralogically
and texturally mature sandstones with subordinate siltstones, mudstones and conglomerates. The sediments were transported,
reworked and deposited in subtidal environments by strong tidal currents of macrotidal regime as well as storms, and accumulated
as discrete shoaling-upward features, separated from each other by muddy to low-energy sandy deposits. The sandbodies developed
into shoaling up linear bars, often more than a kilometre in length, through accretion of thick cross-stratified units in
transverse directions under the influence of ebb and flood tidal currents, as well as in longitudinal direction affected by
southeasterly flowing along-shore currents. The aggrading upper surfaces of the bars experienced protracted reworking by strong
oscillatory wave currents leading to extensive development of subaqueous 2D or 3D dunes mantled with lag pebble deposits at
different points. With continued shoaling and progradation, the bars amalgamated into large sandstone sheets with the development
of high energy beach deposits and coastal sand flats in the uppermost part of the sequence. The presence of rill marks, flat-topped
ripples, wrinkle marks, desiccation cracks and adhesion warts point to intertidal conditions with intermittent exposure. The
high energy sandstone bars overlie a thick mudstone-dominated shelf sequence across a sharp interface indicating rapid change
in the sea-level, provenance, rate of sediment generation and sediment input, and circulation condition in the shelf. A quiet
muddy shelf was replaced by a major sand-depositing environment with strong, open marine circulation. An interplay of tidal
currents, oscillatory wave currents and storm currents generated a complex flow pattern that varied in time and space from
bimodal-bipolar to strongly unimodal flows.
Close parallelism of wave ripple crests, trend of linear bars and unidirectional flows suggest that the elongate bars were
parallel to sub-parallel to the coastline, and were strongly influenced by along shore drift. The inferred coastline was broadly
N-S. The large-scale structures in the bar sandstones, emplacement of vast amount of sand and migration of large bedforms
under strong macrotidal currents collectively indicate that the Kansapathar shelf was intimately connected with an open ocean
basin towards north-northwest. |
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Keywords: | Neoproterozoic Kansapathar Sandstone shelf sedimentation macrotidal |
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