Proterozoic intracontinental basin: The Vindhyan example
Chandan Chakraborty
Geological Studies Unit, Indian Statistical Institute, 203 B.T. Road, Kolkata 700 108, India.
e-mail: chandan@isical.ac.in

Abstract: The Vindhyan basin is a classic example of Proterozoic intracontinental basin that developed in the central part of the Indian shield along with several other basins such as Cuddapah,Chattisgarh,etc.The strata are exposed in three major sectors:Son valley,Bundelkhand and Rajasthan. Substantially thick Vindhyan rocks have also been recognized under the Gangetic alluvium.The constituent stratigraphic units of the Vindhyan Supergroup are laterally correlatable and vertically stacked in similar fashion in individual sectors,but their stratigraphic and sedimentologic attributes vary from one sector to the other.The basin fill in the Son valley (around 3 km thick) exhibits an asymmetric cross-sectional geometry normal to the regional strike (ENE –WSW),with the thickest part occurring to the south,and is divisible into four groups:Semri,Kaimur,Rewa and Bhander,from bottom to top.The boundary between the Semri and Kaimur Groups is marked by a pronounced unconformity.The dominant lithologies include conglomerate,sandstone,shale, limestone as well as volcaniclastics.The Vindhyan strata de fine a broad,regional syncline trending ENE –WSW.The axis of the syncline is slightly curved (convex towards north)and plunges gently towards west.The average dip of the southern limb is greater than that of the northern limb rendering the axial plane to dip southerly. The Vindhyan succession can be divided into five major sequences separated by unconformities.
The unconformities are manifested in the field by angular discordances and/or long-term subaerial exposure features.Unconformity surfaces are disposed in the succession de fining what is known as progressive unconformity,indicating contemporaneous tectonic activity during sedimentation. Individual sequences comprise of a spectrum of depositional systems that include alluvial fan, fan delta,braidplain,braidplain delta,eolian sandsheet,siliciclastic/carbonate tidal flat,siliciclas- tic/carbonate shoreface,siliciclastic/carbonate shelf.The paleocurrents revealed by the Vindhyan strata are typically northerly suggesting that the evolving Satpura orogen served as the source for the Vindhyan sediments.However,the source for the clastics occurring within the Semri and the lower parts of the Kaimur and Rewa Groups in the Bundelkhand sector was perhaps the Bundelkhand Granite Gneiss,Bijawar and Gwalior Group of rocks as manifested by the southerly paleocurrent.
The sequences of the Vindhyan succession are composed of several systems tracts.The different paleogeographic settings characterizing the systems tracts within individual sequences are as follows:
Sequence 1:
• TST –alluvial fan –fan delta –shelf (Deoland and Arangi Formations)
• HST –carbonate ramp (Kajrahat Formation)
Keywords. Intracratonic basin;Proterozoic;depositional environments;sedimentation;sequence stratigraphy.


Outcrop signatures of relative sea level fall on a siliciclastic shelf: Examples from the Rewa Group of Proterozoic Vindhyan basin
Partha Pratim Chakraborty
Department of Applied Geology, Indian School of Mines, Dhanbad 826 004, India.
e-mail: partha − geology@yahoo.co.in

Abstract: The Rewa Group of the Vindhyan Supergroup in the Son valley begins with a thick ( ∼200 m) 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 de fine 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 di fferent 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:
•invariable presence of deeper water,shelfal shale below (Panna or Jhiri);
•the underlying shale at places shows signatures of emergence at the top;
•laterally impersistent,wedge-like geometry;and
•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.

  Stratigraphy, sedimentology and bulk organic geochemistry of black shales from the Proterozoic Vindhyan Supergroup (central India)
S Banerjee 1, ∗,S Dutta 2 ,S Paikaray 1 and U Mann 2
1 Department of Earth Sciences, Indian Institute of Technology Bombay, Powai, Mumbai 400 076, India.
2 Forschungszentrum J¨ ulich, Institut f¨ ur Chemie und Dynamik der Geosph¨ are, Sediment¨ are Systeme,
D-52425 J¨ ulich, Germany.
∗e-mail: santanu@iitb.ac.in

Abstract: Four organic-rich shale units of the Proterozoic Vindhyan sedimentary succession have been scanned to reveal their origin and hydrocarbon potential.The wavy-crinkly nature of the carbonaceous laminae is suggestive of a microbial mat origin of the shales.These shales are thus di fferent from Phanerozoic black shales which typically exhibit planar laminae.The hydrocarbon potential of the black shale units has been evaluated by Rock-Eval pyrolysis.Total organic carbon content of many of the shales exceeds 1%.The mean Tmax for the black shales translate to a vitrinite re flectance range of 2.05 –2.40%Rm based on standard conversion techniques.These shales have reached the catagenetic stage near the beginning of anthracite formation.


Microbial mat-induced sedimentary structures in siliciclastic sediments: Examples from the 1.6 Ga Chorhat Sandstone, Vindhyan Supergroup, M.P., India
Subir Sarkar 1, ∗Santanu Banerjee 2 ,Pradip Samanta 1 and
Silambuchelvan Jeevankumar 2
1 Department of Geological Sciences, Jadavpur University, Kolkata 700 032, India.
2 Department of Earth Sciences, Indian Institute of Technology Bombay, Powai, Mumbai 400 076, India.
∗e-mail: ssarkar@geology.jdvu.ac.in

Abstract: This paper addresses macroscopic signatures of microbial mat-related structures within the 1.6 Ga-old Chorhat Sandstone of the Semri Group –the basal stratigraphic unit of the Vindhyan succession in Son valley.The Chorhat Sandstone broadly represents a prograding succession of three depositional facies ranging from shallow shelf to coastal margin with aeolian sandsheet.The mat-mediated structures were generated because of plastic or brittle deformation of sand,turned cohesive and even thixotropic because of microbial mat growth.Mat growth also favoured abundant preservation of structures that usually have low preservation potential. Proli fic growth of microbial mat in the subtidal to intertidal zone of the Chorhat sea was facili-
tated due to lack of grazing and burrowing activities of organisms in the Precambrian.It further indicates low rate of sedimentation between the storms,as also attested by frequent superposition of storm-beds,even near the storm wave base.It also reduces erosion and that,in turn,would imply low sediment concentration in flows leading to development of bedforms that are likely to be smaller in size and isolated from each other in a single train in contrast to those that form in mat-free sands.

Petrographic observations suggestive of microbial mats from Rampur Shale and Bijaigarh Shale, Vindhyan basin, India
S Sur 1, ∗,J Schieber 2 and S Banerjee 3
1 Department of Geology and Geophysics, The University of Oklahoma, 100 East Boyd St.,
SEC Suite 810, Norman OK 73019, U.S.A.
2 Department of Geological Sciences, Indiana University, 1001 E 10th St., Bloomington, IN 47405, U.S.A.
3 Department of Earth Sciences, Indian Institute of Technology Bombay, Mumbai 400 076, India.
∗e-mail: ssur@ou.edu

Abstract: Petrographic observations of two Vindhyan black shales (Rampur Shale of the Semri Group and Bijaigarh Shale of the Kaimur Group)revealed the following features:
•general wavy lamination,
•contorted and folded thin shreds of organic matter,
•wavy pyritic laminae,
•‘teeth and socket ’structure.
These features are indirect evidence of microbial mat colonization during the Proterozoic.The microbial mats probably fixed carbon at the sediment surface,stabilized sediment and recycled organic matter and were the primary producers,unlike during the Phanerozoic time.

Palaeobiology of Mesoproterozoic Salkhan Limestone, Semri Group, Rohtas, Bihar, India: Systematics and significance
Mukund Sharma
Birbal Sahni Institute of Palaeobotany,53 University Road,Lucknow 226 007,India.
e-mail:sharmamukund1@redi ffmail.com

Abstract: Mesoproterozoic ( ∼1600 Ma old)Salkhan Limestone (Semri Group)of the Vindhyan Supergroup, exposed in Rohtas district of Bihar,India,preserves an abundant and varied ancient microbial assemblage.These microfossils are recorded in three distinctly occurring cherts viz.,bedded chert, stromatolitic chert and cherty stromatolites.27 morphoforms belonging to 14 genera and 21 species have been recognized.Six unnamed forms are also described. The microbial assemblage,almost exclusively composed of the remnants of cyanobacteria,is dominated by entophysalidacean members and short trichomes and can be termed as ‘typical Meso-proterozoic microbiotas ’.The assemblage includes characteristic mat-forming scytonematacean
and entophysalidacean cyanobacteria.Eoentophysalis is the dominant organism in the assemblage. Ellipsoidal akinetes of nostocalean cyanobacteria (Archaeollipsoides )and spherical unicells also occur;both are distinct from mat forming assemblage,allochthonous and possibly planktic.Co-occurrence of the microbiotas and precipitates is related to the depositional environment of the Mesoproterozoic tidal flats with high carbonate saturation.


Chuaria circularis Chuaria circularis Chuaria circularis from the early Mesoproterozoic Suket Shale, Vindhyan Supergroup, India: Insights from light and electron microscopy and pyrolysis–gas chromatography
Suryendu Dutta 1,2,3, ∗,Michael Steiner 2 ,Santanu Banerjee 1 ,Bernd-Dietrich
Erdtmann 2 ,Silambuchelvan Jeevankumar 1 and Ulrich Mann 3
1 Department of Earth Sciences, Indian Institute of Technology Bombay, Powai, Mumbai 400 076, India.
2 Technische Universit¨at Berlin, Sekr. Ack 14, Ackerstrasse 71-76, 13355 Berlin, Germany.
3 Forschungszentrum J¨ ulich, Institut f¨ ur Chemie und Dynamik der Geosph¨ are, Sediment¨ are Systeme,
D-52425 J¨ ulich, Germany.
∗e-mail: s.dutta@fz-juelich.de

Abstract: Chuaria circularis (Walcott 1899)from the Suket Shale of the Vindhyan Supergroup (central India) has been reinvestigated for its morphology and chemical composition using biostatistics,electron microscopy and pyrolysis –gas chromatography.Morphology and microscopic investigations provide little clues on the speci fic biological a ffinity of Chuaria as numerous preservational artifacts seem to be incorporated.On the contrary,the predominance of n aliphatic pyrolysates of presently studied Chuaria from India rather supports an algal a ffinity.Moreover,the re flectance of C. circularis can be used to obtain a comparative maturity parameter of the Precambrian sediments.The review of the age and geographical distribution of C. circularis constrains that this species cannot be considered as an index fossil for the Proterozoic time.


Carbon and oxygen isotope compositions of the carbonatefacies in the Vindhyan Supergroup, central India
S Banerjee 1 ,S K Bhattacharya 2 and S Sarkar 3
1 Department of Earth Sciences, Indian Institute of Technology Bombay, Powai, Mumbai 400 076, India.
2 Physical Research Laboratory, Navrangpura, Ahmedabad 380 009, India.
3 Department of Geological Sciences, Jadavpur University, Kolkata 700 032, India.

Abstract: The Vindhyan sedimentary succession in central India spans a wide time bracket from the Paleopro- terozoic to the Neoproterozoic period.Chronostratigraphic signi ficance of stable carbon and oxygen isotope ratios of the carbonate phase in Vindhyan sediments has been discussed in some recent studies.However,the subtle controls of facies variation,depositional setting and post-depositional diagenesis on stable isotope compositions are not yet clearly understood.The Vindhyan Super-
group hosts four carbonate units,exhibiting a wide variability in depositional processes and paleogeography.A detailed facies-speci fic carbon and oxygen isotope study of the carbonate units was undertaken by us to investigate the e ffect of these processes and to identify the least altered isotope values.It is seen that both carbon and oxygen isotope compositions have been a ffected by early meteoric water diagenesis.The e ffect of diagenetic alteration is,however,more pronounced in case
of oxygen isotopes than carbon isotopes.Stable isotope compositions remained insensitive to facies only when sediments accumulated in a shallow shelf setting without being exposed.Major alteration of original isotope ratios was observed in case of shallow marine carbonates,which became exposed to meteoric fluids during early diagenetic stage.Duration of exposure possibly determined the magnitude of alteration and shift from the original values.Moreover,dolomitization is found to be accompanied by appreciable alteration of isotope compositions in some of the carbonates.The present study suggests that variations in sediment depositional settings,in particular the possibility of subaerial exposure,need to be considered while extracting chronostratigraphic signi ficance from δ 13 C data.


Deep seismic reflection study over the Vindhyans of Rajasthan: Implications for geophysical setting of the basin
B RajendraPrasad摯瑬敳獩 and V Vijaya Rao摯瑬敳獩 ∗
National Geophysical Research Institute, Hyderabad 500 007, India.
∗e-mail: rajbitragunta@yahoo.com
∗∗e-mail: vijayraov@yahoo.co.in

Abstract: This paper presents results of high-resolution deep seismic re flection pro filing of the Proterozoic Vindhyan basin of the Rajasthan area along the Chandli –Bundi –Kota –Kunjer pro file.Seismic images have been used to estimate the thickness of Vindhyan strata as well as to understand the tectonic framework of the basin.The results are constrained by gravity,magnetic and magne-totelluric data.The study reveals gentle SE-dipping re flection bands representing the Vindhyan strata.The seismic sections depict gradual thickening of the Vindhyan succession towards south-east from Bundi.The velocities of the upper and lower Vindhyans are identi fied as 4.6 –4.8 km/s and 5.1 –5.3 km/s.The NW limit of the Vindhyan basin is demarcated by the Great Boundary Fault (GBF)that manifests as a 30 km wide NW dipping thrust fault extending to a depth of 30 km.

Age of the Vindhyan Supergroup: A review of recent findings
Jyotiranjan S Ray
Physical Research Laboratory, Navrangpura, Ahmedabad 380 009, India.
e-mail: jsray@prl.res.in

Abstract: The Vindhyan Supergroup of India is one of the largest and thickest sedimentary successions of the world.Deposited in an intra-cratonic basin,it is composed mostly of shallow marine deposits. It is believed to have recorded a substantial portion of Proterozoic time and therefore,likely to contain valuable information on the evolution of the atmosphere,climate,and life on our planet. It also contains some of the most disputed fossils of earliest animal life.Despite their importance, the absolute age of these rocks had remained unknown until recently.In this work I evaluate all the recent chronological information and discuss their implications.From the present findings it appears that the issues surrounding the age of the Lower Vindhyans in the Son valley are now resolved,whereas problems with the age of the Upper Vindhyans and that with the stratigraphic correlations remain to be answered.


Mesoproterozoic diamondiferous ultramafic pipes at Majhgawan and Hinota, Panna area, central India: Key to the nature of sub-continental lithospheric mantle beneath the Vindhyan basin
N V Chalapathi Rao
EPMA Laboratory, Mineralogy Section, Ore Dressing Division, Indian Bureau of Mines, Nagpur 440 016, India.
e-mail: nvcr100@gmail.com

Abstract: Amongst all the perceptible igneous manifestations (volcanic tu ffs and agglomerates,minor rhy olitic flows and andesites,dolerite dykes and sills near the basin margins,etc.)in the Vindhyan basin,the two Mesoproterozoic diamondiferous ultrama fic pipes intruding the Kaimur Group of sediments at Majhgawan and Hinota in the Panna area are not only the most conspicuous but also well-known and have relatively deeper mantle origin.Hence,these pipes constitute the only yet available ‘direct ’mantle samples from this region and their petrology,geochemistry and iso-tope systematics are of profound signi ficance in understanding the nature of the sub-continental lithospheric mantle beneath the Vindhyan basin.Their emplacement age ( ∼1100 Ma)also constitutes the only reliable minimum age constrain on the Lower Vindhyan Group of rocks.The Majhgawan and Hinota pipes share the petrological,geochemical and isotope characteristics of kimberlite,orangeite (Group II kimberlite)and lamproite and hence are recognised as belonging to a ‘transitional kimberlite –orangeite –lamproite ’rock type.The name majhagwanite has been proposed by this author to distinguish them from other primary diamond source rocks.The parent magma of the Majhgawan and Hinota pipes is envisaged to have been derived by very small (<1%)degrees of partial melting of a phlogopite-garnet lherzolite source (rich in titanium and barium)that has been previously subjected to an episode of initial depletion (extensive melting during continent formation)and subsequent metasomatism (enrichment).There is absence of any
subduction-related characteristics,such as large negative anomalies at Ta and Nb,and therefore, the source enrichment (metasomatism)of both these pipes is attributed to the volatile-and K-rich, extremely low-viscosity melts that leak continuously to semi-continuously from the asthenosphere and accumulate in the overlying lithosphere.Lithospheric/crustal extension,rather than decompression melting induced by a mantle plume,is favoured as the cause of melting of the source regions of Majhgawan and Hinota pipes.This paper is a review of the critical evaluation of the published work on these pipes based on contemporary knowledge derived from similar occurrences elsewhere.