Abstract
AbstractThe Vindhyan Supergroup represents the largest Proterozoic sedimentary basin fill in the Indian shield. In addition to some significant palaeobiological discoveries, the sedimentary sequence of the Vindhyan, particularly its argillaceous intervals, holds crucial information for our understanding of sedimentation dynamics in Proterozoic clastic shelves. Here we attempt an extensive, although not exhaustive, review of the physical characteristics of six argillaceous (shale) intervals (Arangi, Koldaha, Rampur, Bijaygarh, Rewa and Sirbu shale) from the Son valley sector, Vindhyan Basin, augmented with new observations to unravel the status of current understanding in terms of palaeo-flow dynamics, shelf sedimentation processes and dispersal pattern, depositional cyclicity and basinal tectonics. The sedimentary attributes of Vindhyan shales reveal their deposition largely in relative bathymetry fluctuating from distal shoreface or inner shelf (near to fair-weather wave base) to distal shelf below storm wave base. More often than not, the Vindhyan shelf was storm-infested and the operation of both storm-generated return flow and Coriolis-force-guided geostrophic currents are documented from different stratigraphic intervals of argillaceous successions. The thick arenaceous intervals interrupting the deposits of the Koldaha, Rewa and Sirbu shales at multiple stratigraphic levels indicate the presence of a fan delta and braided fluvial system during intermittent regressive stands of sea level or event deposition during a sea-level highstand, respectively. Based on facies pattern and flow vectors, a rift-related half-graben model is inferred for Arangi and Koldaha shale and a low-gradient stable-shelf model with well-defined energy gradient is proposed for successions from Rampur shale onwards.
Highlights
Among the different varieties of sedimentary rocks, namely clastic, chemical and organo-chemical, shales are the most abundant worldwide (> 70% of sedimentary cover) and are traditionally considered suitable for understanding the nature, scale, heterogeneity and physical sedimentation parameters in low-energy depositional environments
In combining data from the existing literature with the addition of new process-based sedimentary data, this paper critically examines six different shale members (Arangi, Koldaha, Rampur, Bijaygarh, Rewa and Sirbu) of the Vindhyan Supergroup exposed in the Son Valley, central India and attempts to understand the depositional dynamics operative on the Vindhyan shelf
It is inferred that Vindhyan shales were deposited largely in a marine-shelf environment with relative bathymetry ranging from inner to distal shelf conditions
Summary
Among the different varieties of sedimentary rocks, namely clastic, chemical and organo-chemical, shales are the most abundant worldwide (> 70% of sedimentary cover) and are traditionally considered suitable for understanding the nature, scale, heterogeneity and physical sedimentation parameters in low-energy depositional environments. It is generally believed that low settling velocity and flocculation allow even weak currents to disperse clay-sized particles over large areas; it has been argued from recent flume studies (Schieber, 2016) that flocculated mud can be transported by currents able to move bedload sand This new understanding has brought shale sedimentology to the forefront; what was previously interpreted as a deep stagnant environment from laminated shale successions may possibly be interpreted as belonging to an energetic and dynamic depositional environment. Careful studies of marine shale successions have revealed evidence of the operation of close-spaced multidirectional events such as distal tidal currents, stormwave impingement and unidirectional currents (bottom flow, turbidites) on the muddy sea floor In this backdrop, it is important to emphasize that the study of argillaceous formations present in the rock record can provide geological information to improve knowledge of issues that is are either very poorly understood or so far unaddressed. From the collation of data generated in this study along with available data in literature, we have attempted to summarize the current understanding in terms of palaeo-flow dynamics, sediment dispersal pattern, depositional setting and cyclicity, and basin tectonics
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