Vindhyan Supergroup - opengeology.in

The Vindhyan Supergroup is named after the scarped plateau mountains in the north of the Narmada Valley. Vindhyan basin is canceled under the Deccan Traps of the Malwa Plateau and the Quaternary sediments. The basin evolved as a vast intra-cratonic basin formed in response to intraplate stresses. The Vindhyan Basin was formed as a result of a large scale down warp in the northern part of the Indian Peninsula. Sedimentation started from Paleoproterozoic Era to the end of Neoproterozoic Era.

The Vindhyan Supergroup is comprised of mostly low dipping formations of sandstone, shale and carbonate, with a few beds of conglomerate and volcanoclastic rocks.

The Vindhyan sequences show characters of tectonic driven depositional cycles of various orders. The initiation of marine transgression over the basement seems to have taken in the eastern part of the basin. The sedimentation seems to have begun in an extensional regime that soon changed into plate-margin compression. The shift is represented by unconformity that divides Lower Vindhyan and Upper Vindhyan. Upper part of the Upper Vindhyan formation shows facies characters indicating a decline in the rate of tectonic subsidence.

Lower Vindhyan (Semri Group)

The rocks of this group are best exposed in the Son River Valley, Dhar Forest, along the Kaimur Escarpment and in the neighborhood of Chittorgarh in Rajasthan.

Deoland formation rests unconformably over the steeply dipping beds of the Bijawar Group in the Son river Valley sections. The diamictite in the lower part of the Deoland formation is regarded to be of glacial origin.
The deoland formation is conformably overlain by Kajrahat Formation that consists of a thick sequence of dolomitised and stromatolitic limestone with basal unit of beds of shale.

The Kajrahat formation is ovelain by Porcellanite Formation that consists of a sequence of volcanic ash, pyroclastic flows and surge deposits. Presence of quenching cracks, agglutinated particles, pumice fragmentats and volcanic bombs indicates intra-basinal volcanism.
The overlaying Kheinjua Formation begins with beds of dark shale, Fawn limestone and reworked pyroclastic sediments. The major part of formation is made up of sandstone shale intercalation. The fawn limestone has preserved stromatolitic forms, such as Conophyton cylindricus, Collenia columnaris, and Collenia sp. These forms have been correlated those occuring in Lower Ripheans of the Russian Platform.
The sandstone of Khenjus formation is sharply terminated by shale beds that form the basal part of the predominantly carbonate sequence comprising Rohtas Formation.

Upper Vindhyans (Kaimur, Rewa and Bhander Group)

In the Son-Narmada Valley and Dhar forest areas, the Upper Vindhyan rests over Semri Group with a well marked unconformity. In other areas, the Upper Vindhyan rests directly over the Bundelkhand Gneiss, Gwalior group and Bijawar group. Presence of coarser sediments in the south indicates that the Vindhyan paeo-sea was open to the north and perhaps covered the region now occupied by the Lesser Himalaya. Grain texture and red colors of some of the Upper Vindhyan sandstone suggest their aeolian origin and deposition under arid climatic conditions.

The lowermost unit of the Upper Vindhyan known as Kaimur Group begins with a thin horizon of conglomerate that contains pebbles of red colored jasper. The jespers were presumbly derived from the underlying succession of Bijawar and Gwalior Groups.
The conglomerate is overlain by Lower Kaimur Sandstone, Bijagarh shale and Upper kaimur Sandstone.
The sandstones form escarpments and contains beds of flagstone, siltstone, orthoquartzite, while shales shows the gentle topography.

The Kaimur group is overlain by Rewah Group which are separated by a diamondiferous horizon of conglomerate. Small diamonds occurs as placer deposits in pebbles of vein quartz, jasper and green quartzite in a matrix of fine clay. Another diamond bearing horizon occurs on the top of the Rewah succession. Rewah Group shows a greater lateral variation in facies characters than underlying and overlying groups of the Upper Vindhyan. This group is also have alternating shale and sandstone as Panna Shale, Lower Rewah Sandstone, Jhiri Shale, Upper Rewah Sandstone.

Bhander Group that forms the uppermost unit of Vindhyan comprises of five important members : Ganurgarh Shale, Bhander Limestone, Lower Bhander Sandstone, Sirbu Shale and Upper Bhander Sandstone. Red colored sandstone of the Upper Bhander sandstone have a uniform fine grained character making it a good building stone. Their character is also similar to that of the purple sandstone of the Salt Range that is assigned as Early Cambrian age.

Age of Vindhyan Super group

One of the earliest geochronologic dating was made of Rb-Sr isochron age of 1140 +/- 24 Ma for lampriote pipe in the overlying and younger Kaimur Group.
The U-Pb dating of zircons in the porcellanite Formation of the Semri Group has given age of about 1630 Ma for the volacnic event.
Rb-Sr dating of sedimentary glauconites from the sandstone formation of the semri group near chitrakut has given age of 1531-1409 Ma.
The unconformity between the two primary Vindhyan divisions covered the entire basin and a hiatus in deposition lasting a few hundred million years.
Pb-Pb dating of the Bhander limestone has given age of 650~ Ma.
Paleomagnetic studies on samples of Rewah and Bhander group sandstones showed similarities of the pole-positions to that of the Purple sandstone of the Salt Range of Lower Cambrian.
The mineral phlogopite from kimberlite near panna has given 40Ar/39Ar age of ~1075 Ma. Since the kimberlite intruded Upper Vindhyans the initiation of sedimentation of Upp. Vindhyans began not later than the begining of Neoproterozoic.

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