Outcrop signatures of relative sea level fall on a siliciclastic shelf: Examples from the Rewa Group of Proterozoic Vindhyan basin |
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Authors: | Partha Pratim Chakraborty |
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Institution: | (1) Department of Applied Geology, Indian School of Mines, 826 004 Dhanbad, India |
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Abstract: | The Rewa Group of the Vindhyan Supergroup in the Son valley begins with a thick (∼200m) dominantly shaly, shelfal succession,
occurring between the Dhandraul Formation of the Kaimur Group (fluvial sandstone) below and Drammondganj Formation of the
Rewa Group (marginal marine sandstone) above. Such a stratigraphic disposition indicates a sharp rise in relative sea level
at the onset of Rewa sedimentation, inducing a shelfal depth to the Vindhyan basin. However, a number of wedge-shaped, sandstone/conglomerate
bodies (maximum thickness 23.5 m) occur at multiple stratigraphic levels within the aforesaid deeper water shale succession,
which appear to be of much shallow water origin representing regressive deposits. Though these bodies do not define a single
physically continuous unit, either vertically or laterally, they are still designated by a single term ‘Asan Sandstone’ in
the literature. On the other hand, the encasing shelfal shales are termed as Panna and Jhiri Shales, in accordance with their
occurrence below or above the so-called ‘Asan Sandstone’. The present study reveals that in different sections spread over
the Son valley, there are several discrete regressive wedges occurring vertically, and their depositional environment is also
variable, ranging between braided fluvial, shoreface fan and braid delta. The features common to most of the regressive coarser
clastic bodies are:
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•invariable presence of deeper water, shelfal shale below (Panna or Jhiri);
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•the underlying shale at places shows signatures of emergence at the top;
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•laterally impersistent, wedge-like geometry; and
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•presence of granular transgressive lags at the top. These coarser clastic wedges record several episodes of regressive deposition
during short-term falling stage or lowstand of relative sea level at the early phase of Rewa sedimentation. Each individual
phase of regressive deposition was, however, followed by flooding and resumption of shelf mud deposition.
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Keywords: | Neoproterozoic Rewa Group paleoenvironment sea level regression |
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