Diagenesis,facies and palaeocurrent analysis of Upper Rewa Sandstone around Sagar, Central India

Gurv K.Singh , Ashish K.Ri , Arvind K.Singh

a Department of Applied Geology,Dr.Harisingh Gour Vishwavidyalaya,Sagar 470 003,Madhya Pradesh,India

b Geological Survey of India, Shillong, India

Abstract The stratigraphic surface represented by a major contact between the Archean Bundelkhand Granite and the extensive Proterozoic Vindhyan sediments is a regional basement cover unconformity.This crystalline－sedimentary interface reflects an intense weathering of continental crust during marine transgression.Three time-transgressive sand deposition events viz.Kaimur,Rewa and Bhander are mainly witnessed during the entire Vindhyan sedimentation.Stratigraphically,the Upper Rewa Sandstone comprises one of these events in the Vindhyan Basin.Considerable progress has been made in interpreting these sandstones as a function of entirely marine process to a combination of tidal－fluvio－eolian activities.All the results have so far been attained on the basis of sedimentary facies, provenance, palaeocurrent analysis, and some of petrography.A distinct differentiation between marine and fluvial components of the Upper Rewa Sandstone still remains uncertain.Here, we use diagenesis as a parameter for the first time along with facies and palaeocurrent analysis to acquire a clear comprehension of marine- and fluvial-dominated processes.The present study spans 27 square kilometer area covering 15 stratigraphic sections with a collection of 571 directional data from the facies specific sedimentary structures,and 28 samples obtained for the thin section analysis.

Keywords Palaeocurrent, Diagenesis, Facies, Upper Rewa Sandstone, Vindhyan Basin

1.Introduction

Vindhyan Basin is the largest ENE－WSW-trending Proterozoic sedimentary succession of the Indian subcontinent fringing Bundelkhand Granite.It is bounded by Son Narmada North Fault(SNNF)and Great Boundary Fault(GBF)towards south and northwest respectively.A considerable northern portion of this basin however, is covered under the Indo-Gangetic alluvium.Some part of the Vindhyan sediments is also concealed under Deccan Traps in southwestern direction.Vindhyan sediments represent a several hundred meters thick succession primarily made up of the mature sandstone－-shale－carbonate association.Vindhyan system has a well-established lithostratigraphy by virtue of systematic mapping by Auden (1933) whereas other classical works include that of Oldham(1856)and Mallet(1869).Some significant contributions in terms of microbially induced sedimentary structures,sedimentary processes and sea level fluctuations can be found in Sarkar et al.(1996, 2002, 2004, 2005, and 2006).Recently, the hydrocarbon source rock potential of Vindhyan shales has also been attempted (Singh and Chakraborty, 2021).Auden (1933)divided the entire succession into Semri,Kaimur, Rewa and Bhander Series which were later modified as Groups.Commonly,Semri Group is referred as the Lower Vindhyan whereas other Groups are included under the Upper Vindhyan succession.

There are three major sand deposition events in Vindhyan Basin viz.Kaimur, Rewa and Bhander that accommodate a huge thickness.The present work is confined only to one of these sand deposition event namely Upper Rewa Sandstone which has been interpreted as shoal beach complex by Singh (1980).Fission－Track(F－T)dating gives a Neoproterozoic age(710±120 Ma) for the Upper Rewa Sandstone(Srivastava and Rajgopalan, 1988).Such a thick succession represents extensive Neoproterozoic continental denudation followed by marine transgression similar to what proposed by Peters and Gaines(2012).An interpretation of Upper Rewa Sandstone as Highstand Systems Tract (HST) given by Bose et al.(2001)confirms this contention.They have regarded these sandstones as sandwiched between two Transgressive Systems Tracts (TSTs) viz.Rewa Shale and Ganurgarh Shale respectively.This succession lacks the typical beach zonation,i.e.,shoreface－foreshore－backshore sequence.They were traditionally interpreted as entirely marine in nature (Banerjee, 1974; Singh,1976, 1980; Chanda and Bhattacharya, 1982; Prasad and Verma, 1991) based on sedimentary facies and palaeocurrent pattern (Verma and Shukla, 2015).It was also found that the succession can be divided into lower marine and upper fluvial transition(Chakraborty and Chaudhuri, 1990).The interplay of multiple depositional agents for Upper Rewa Sandstone was discussed by Bose and Chakraborty(1994)using facies association and palaeocurrent.

A distinct differentiation between marine and fluvial processes in otherwise monotonously similar appearing rocks of Upper Rewa Sandstone still remains to be established.An account on the application of diagenesis to comprehend the sedimentary processes pertaining to Upper Rewa Sandstone is lacking in the works carried out so far (refer to Table 4 in Discussion).In the present work therefore, marine and fluvial processes are described with the help of three evidences including diagenesis, facies type, and palaeocurrent analysis.

The marine succession described by Bose and Chakraborty (1994) is referred as the “l(fā)ower unit”whereas those revealing fluvial process is mentioned as the “upper unit” in the present work.The prefixes“l(fā)ower”and“upper”reflect the stratigraphic position where the lower unit occurs below and is older than the upper unit (Fig.1).The terms syntaxial, passive pore fill and grain replacive cement frequently used in this work are taken from Worden and Burley (2009).

2.Methodology

This work is carried out around Sagar town(Fig.2)which is a part of Sagar District of Madhya Pradesh,Central India.A total of 27 km2geographical area is covered pertaining to the Survey of India toposheet No.55I/9.It comprises monotonous outcrops of reddish sandstone exposed along sections bounded by escarpments almost everywhere.Fifteen stratigraphic sections are studied, and their litholog is prepared using CorelDraw software.From all the sections, palaeocurrent azimuths are taken which amounts to about 571 directional data plotted on GeoRose software.The directional structures used for measurements include the current crescent, planar and trough crosslaminations, lee face of asymmetric current ripples,and less frequent herringbone cross-laminations.

All the procedural equations used in this work for the mathematical treatment of palaeocurrent data are taken from Rao and Sengupta (1972).

If θ is considered as the palaeocurrent azimuth and n is the total number of observations,then the sine and cosine components, viz.W and V respectively, are given by:

The percentage (L) and probability (P) for total number (n) of observations are respectively found from:

A simple operation of data uniformity test is performed as evidence in favor of preferred directions.The basis of this simple uniformity test (Rao and Sengupta, 1972) is the sample arc length or difference between the successive observations on the circumference.If the observations are numbered as α1, α2,… …, αnnumerically arranged such that α1is the smallest and αn, the largest, the arc lengths are given by:

The expected length of arc in this test is (360/n)°and the test statistic is half the sum of absolute derivations given by:

The smaller values of Unindicate a uniform data distribution whereas the larger values support the preferred direction.Additionally, 28 samples are thin sectioned for petrographical study of both lower unit as well as upper unit.

3.General description of the studied sections

In all the well preserved sections noticed, the lower unit consists of fine- to medium-grained sandstones with an alternating plane- to cross-laminated succession(Figs.3,4A－B).The planar cross-laminated unit shows a bidirectional palaeocurrent pattern,whereas the trough cross-laminated unit gives a dominant palaeocurrent pattern towards NW.Under microscope,the sandstones show a well-sorted nature with insignificant matrix and predominance of authigenic siliceous cement around framework grains.

Fig.2 Geographic location map of Sagar and adjoining areas.The studied localities are marked with numbers which correspond to the sections referred in Tables 1 and 2

Fig.3 A general litholog showing the lower unit as facies A and B and the upper unit as facies C,D and E(see Section 4 for description).Scale bar is 2.5 m.

The upper unit in each section consists of granulerich planar cross-laminated layers that laterally grade to trough cross-bedded sandstones.On the bedding plane of latter, several current crescents with parting lineation are found.All of them show a preferably unimodal trend towards NW.A predominance of more than 15%matrix with ferruginous and siliceous content is observed along with subtle variation in size and shape of framework grains in thin section.It is observed that the rocks are poorly sorted, and therefore correspond to the different process.The occurrence of liquefaction driven recumbent foresets is frequent in almost every unit reflecting basin instability.

4.Facies type description and interpretation

Based on the relative grain size and bedform of different units observed during field work, two facies groups were found.The lower unit shows two facies indicating an influence of marine process whereas the upper unit with three facies gives a dominant fluvial signature.The facies pattern observed in the present work pertaining to marine and fluvial processes are somewhat similar to that described by Bose and Chakraborty (1994), however, with some differences.

4.1.Lower unit

The lower unit in basal part of nearly all the studied sections consists of two facies viz.facies A (rhythmic cross-stratified facies) and facies B (plane-laminated facies).

4.1.1.Facies A－Rhythmic cross-stratified sandstone facies

A set of unidirectional small scale trough cross stratification is interlaminated with a rhythmic alternation of planar or tangential cross laminations dipping in opposite directions (Fig.4A).These troughs give a consistent NW-directed palaeocurrent trend whereas the bipolar planar cross laminations indicate a bidirectional NE－SW pattern together with lee face of asymmetric ripples giving SW-directed palaeoflow.

Fig.4 A) Small scale trough cross stratification (central arrow) with rhythmic alteration of planar (left arrow) and tangential cross laminations(right arrow)in facies A.Scale bar is 20 cm;B)Planar-laminated facies,facies B,occurring in alteration with facies A.The hammer is 33 cm for scale.

4.1.2.Facies B－Plane-laminated sandstone facies

A succession of fine-grained,thinly-laminated subhorizontal to gently-inclined quartz arenites alternates with facies A (Fig.4B).In all the studied sections, facies B occurs without any other conspicuous feature,except that it denotes high energy beach sand of upper flow regime.

4.2.Upper unit

The upper unit comprises three facies viz.facies C(granule-rich planar cross-laminated sandstone facies), facies D (trough cross-stratified sandstone facies with current crescent structure), and facies E(fine- to medium-grained planar cross-laminated sandstone facies).They are differentiated from each other and with the facies of other associations on the basis of grain size, and type of bedform.

4.2.1.Facies C－Granule-rich planar crosslaminated sandstone facies

This facies is cross-laminated and with grains which are often coarser than 2 mm, therefore they are termed as granule-rich sandstone(Fig.5A).It occurs in lateral continuity of facies D and planar crosslaminations are abruptly truncated.

4.2.2.Facies D－Trough cross-stratified sandstone facies

Unlike facies C, the succession of this facies is characterized by lensoidal form and granule-free nature.The most prominent structure of facies D is largescale trough cross-stratification (Fig.5B) along with primary current lineation and current crescent structures (Fig.5C).All these bedforms consistently indicate the conditions of lower flow regime and a unidirectional trend towards NW.

Fig.5 A)Granule-rich planar cross-laminated fluvial sandstone of facies C exposed near Gharonda Ashram(refer to location 1 of Fig.2);B)Trough cross-stratified fluvial sandstone of facies D exposed near Gharonda Ashram(refer to location 1 of Fig.2);C)Fluvial sandstone of facies D with current crescent structure giving palaeoflow direction towards NW; D) Fine- to medium-grained planar cross-laminated sandstone of facies E giving lensoidal forms.

Fig.6 QmFLt ternary diagram based on Dickinson (1985) indicates a continental block provenance for the lower unit sandstones (marine origin samples), whereas shows the continental block and merger field provenance for the upper unit sandstones (fluvial origin samples).

Fig.7 QtFL ternary diagram based on Dickinson (1985) indicates the recycled orogen provenance for sandstones of both the units.

Taaz ibml eu t1h ;SRt a=tVi setci ctaolr pr easr aum ltae nte t r msaogf n aizt uim deu;t hLa=l rVeeacdtionrg mf raogmn i ttuhdeedpi reercctei onntaa gl ese;dPi m=ePnr ota braybsi ltirtuyc;tσu r=e sV aorfi at hnec el.o w See cr.u=n iStescatni odns.t ones.Where, n=Number of observations;?= Vector resultant

Table 2 Statistical parameters of azimuthal reading from the directional sedimentary structures of the upper unit sandstones.Where,n=Number of observations;?=Vector resultant azimuth; R=Vector resultant magnitude;L= Vector magnitude percentage; P=Probability;σ =Variance.Sec.=Section.

4.2.3.Facies E－Fine- to medium-grained planar cross-laminated sandstone facies

In almost all the studied sections,upper part of the fluvial association is demarcated by planar crosslaminated sandstones sandwiched between subhorizontal strata (Fig.5D).These low-angle bottomsets and topsets occur as master erosion surfaces on which high-angle foresets truncate either abruptly or tangentially giving an overall lensoidal form to the sandbody.

5.Provenance and palaeocurrent analysis

The modal composition is made with the compositions of monocrystalline quartz (Qm), feldspar (F) and total lithic fragments (Lt) based on the ternary QmFLtplot (Fig.6) proposed by Dickinson (1985).It shows a continental block provenance for all the lower unit sandstones whereas a mix provenance of continental block and merger field of mature rocks for upper unit sandstones.Another ternary plot, QtFL plot, using Qt(total quartz composition), F (feldspar composition)and L (lithic fragments) indicates mostly recycled orogen provenance for both the units based on Dickinson(1985) (Fig.7).

Fig.8 The polymodal palaeocurrent pattern appears polymodal for the lower unit sandstones giving a dominant NW-trend of longshore current and sediment dispersal pattern through NE-directed flow and SW-directed ebb current.

The palaeocurrent data for the lower unit sandstones give a polymodal plot(Table 1;Fig.8)and give peculiar information about the palaeo-shoreline and sediment dispersal pattern.The trough crosslaminations show a persistently uniform trend directed towards NW in all the studied sections.It gives vector resultant azimuth of N311.1°, vector magnitude of 21.62, and probability values varying from 7.49×10-15to 2×10-5calculated from equations(1), (2), and (4) respectively.It accounts to about 83.71% data that conform to N311.1°direction from equation(3).This unidirectional consistency of trough cross-laminated unit of facies A implies an effect of longshore current for a NW－SE-directed palaeoshoreline which accords with that suggested by Verma and Shukla(2015).Another palaeocurrent trend towards NE－SW gives the flow and ebb directions represented by herringbone and tangential crosslaminated sandstone.It gives vector resultant azimuth of N38.66°, vector magnitude of 10.12, and about 90.3%data with probability values varying from 3×10-5to 1.5×10-2calculated from equations(1－4)respectively for the flow direction.For the ebb direction, the results based on the same parameters consecutively are N213.4°, 10.83, 73.77% data with probability values varying from 2 × 10-5to 1.3 × 10-3.Both the patterns are obtained from the sandstones of facies A which indicates a typical tidal setting for this succession.

The directional structures used for investigating palaeocurrent of upper unit include trough crossstratifications, current crescent structures, planar cross-laminations and lee face of asymmetric ripples.All of data and plots indicate a distinct unimodal pattern of palaeoflow towards NW (Table 2; Fig.9).The vector resultant azimuth points towards N307.56°,vector magnitude is 14.87, and 71.48% data with probability values vary from 9 × 10-21to 10-1calculated from equations(1),(2),(3)and(4)respectively.Unlike polymodal palaeocurrent pattern of plots obtained for the lower unit sandstones, this pattern is purely unidirectional.The values of probability(P)for both the units are close to zero, which means that it has a strictly non-uniform distribution and has a preferred orientation.

Fig.9 The unimodal palaeocurrent pattern appears for the upper unit sandstones showing a dominant direction towards NW.

6.Texture and diagenesis

The lower unit of Upper Rewa Sandstone around Sagar is predominantly composed of well-sorted to moderately-sorted, rounded to sub-rounded, finegrained to medium-grained quartz arenites (Fig.10A).These sandstones have average framework-grain compositions of >95% quartz and <10% rock fragments.Detrital quartz is the major framework constituent consisting mostly of monocrystalline grains, of which some of them display small,rounded overgrowths.

The upper unit sediments comprise poorly-sorted,sub-rounded to angular, very fine- to coarse-grained sandstones (Fig.11A).The predominance of detrital quartz in the framework grains is slightly less (85%－90%)compared to that of lower unit sandstones.Matrix forms a considerable part of this unit (>15%) and consists of quartz and iron oxide.Rock fragments including chert and metamorphic polycrystalline quartz grains are present but in very minor amounts.

It can therefore be seen that the framework-grain mineralogy of the Upper Rewa Sandstone reflects a contrast between the lower unit and the upper unit.The relative composition within each unit however remains relatively constant in almost every section with no significant variations among individual facies.A difference in the sediment source between the two units can be one of the most likely causes for the variations in sandstone composition observed in Upper Rewa Sandstone.These sediments were modified by a series of diagenetic changes including mechanical compaction, early quartz cementation, frameworkgrain and cement dissolution.There is no variation found in authigenic mineral type or abundance among individual units nor is there any significant trend vertically within each unit.Small scale variations that occur locally most likely reflect the lithological variation within the unit.Additionally, the diagenetic sequence found in both units of Upper Rewa Sandstone differs considerably despite a similarity in frameworkgrain mineralogy.

Fig.10 A) Marginal marine quartz arenite under thin section (plane-polarized light) showing rounded to sub-rounded grains with syntaxial authigenic silica cement.Scale bar is 150 μm; B) Passive pore-fill cement(cross-polarized light)shown by red arrow.Scale bar is 7.5 μm; C)Cross-polarized view showing linear grain contact (blue arrow) and syntaxial quartz overgrowth with detrital grain boundary (yellow arrow and red arrow respectively).Scale bar is 7.5 μm;D)Partial grain dissolution indicated by grain replacive cement(cross-polarized light)shown by red arrow.Scale bar is 7.5 μm.

6.1.Mechanical compaction

The effect of mechanical compaction in the lower unit sandstones is reflected by the presence of long and concavo－convex grain contacts.In some sections of the basin, sandstones indicate substantial mechanical compaction, which resulted in subsequent dissolution porosity.Such a compaction in these sandstones occurred by means of enhanced grain-packing density through grain rotation.Additionally, quartz grains display interpenetrative, presolved contacts as a function of pressure solution(Fig.11D).The upper unit sandstones, however were compacted by both grain rearrangement and plastic deformation of ductile components.Unlike that of lower unit sandstones,they show much less frequency of straight and concavo－convex contacts due to considerable matrix content.

6.2.Matrix and cement

Total authigenic cement in the lower unit sandstones is approximately 20% for which the most pronounced authigenic mineral cement consists of quartz as syntaxial overgrowth (Fig.10C).The quartz however, is also present as passive pore-fill cement in minor amounts (Fig.10B).On the basis of petrographic analysis, multiple generations of quartz cement formed during early and late stages of diagenesis.The variation of cement types and complexity of textural relations between individual cement phases suggest the possible presence of other kind of cements that are not recognized.Syntaxial quartz overgrowth predates passive pore-fill cements(Fig.10B) and is the most abundant in sandstones of the lower unit.The syntaxial nature of this quartz cement indicates that detrital grains served as nucleation sites for early cement growth.In syntaxially cemented sandstones, quartz comprises optically continuous crystals along with pore-fill cement.The relatively high intergranular volumes reflected by profuse cementation characterizing these rocks reveal only minor mechanical compaction which occurred prior to cementation.

Fig.11 A) Thin section photomicrograph (plane-polarized light) of fluvial sandstone with high matrix content (red arrow).Scale bar is 150 μm; B) Cross-polarized view of authigenic quartz overgrowth (yellow arrow) and post-dating iron-oxide-rimming detrital grain (red arrow).Scale bar is 7.5 μm;C)Extensive grain replacive cement(cross-polarized light)denoting dissolution shown by red arrow.Scale bar is 7.5 μm;D)Photomicrograph(cross-polarized light)showing sutured grain contact or stylolites(red arrows)due to pressure solution.Scale bar is 150 μm.

Table 3 A comparison chart of diagenetic events between the two units of Upper Rewa Sandstone.Thick bar shows stronger intensity of the event whereas dashed line bar indicates mild extent(modified after Pitman et al., 2000).

Ferruginous matrix is ubiquitous, somewhat irregular and is distributed in random patches in sandstones of the upper unit, reflecting a high depositional porosity and permeability.Such features also suggest that iron oxide formerly may have been more widespread in pore fluid.At many places, ferruginous matrix is in contact with early quartz overgrowths; and commonly does not replace the cement.A considerable fraction of ferruginous matrix in the basin coexists with quartz cement.The authigenic syntaxial quartz cement forms compositionally homogeneous euhedral crystals around detrital grains but some of them exhibit Fe-rich overgrowths (Fig.11B).Ferruginous matrix predates other authigenic syntaxial quartz overgrowth in the upper unit sandstones.Some of the detrital quartz grains have ferruginous cement with straight boundaries, but elsewhere, the iron oxide forms a kind of matrix that infills secondary pores.At a few places, ferruginous cement displays slightly curved crystal contacts.

6.3.Dissolution

Dissolution can be regarded as a sort of chemical compaction (Worden and Burley, 2009 and references therein) that follows soon after the mechanical compaction and enhances porosity.Based on visually estimated content of matrix and several cement types,approximately all of the porosity in both units of the Upper Rewa Sandstone is intergranular.Although the lower unit sandstones have a dominance of primary porosity but a part of the overall porosity in the upper unit sandstone is of secondary origin,resulting from the predominance of early diagenetic secondary matrix.In thin section,evidence of dissolution includes embayed quartz overgrowths,well-developed authigenic cement adjacent to extensively leached areas (Figs.10D and 11C).The presence of relict ferruginous cement between detrital grain and authigenic overgrowth(Fig.11B) indicates that one or more generations of quartz and ferruginous cement are present in the upper unit sandstones.However,the lower unit sandstones are still tightly cemented by authigenic quartz (Fig.10C).Intragranular porosity resulting from partial dissolution of detrital grains is present at a few places throughout the Upper Rewa Sandstone in small amounts.Moreover,minor porosity due to grain dissolution seems insignificant compared to the total sandstone porosity.

Table 4 Representative contributions enlisted for the Upper Rewa Sandstone,Central India.

7.Paragenetic sequence

An overview of the diagenetic processes that may have influenced the genesis of the Upper Rewa Sandstone is useful to figure out the paragenetic sequence.We then use this understanding based on the combined effects of mechanical compaction and mineral cementation to differentiate the sandstones of both the units.

The petrography of the lower unit sandstones reveals that the quartz cement was precipitated both at early and late stages during the burial history.In contrast to early quartz,which is the dominant cement in these sandstones,later quartz,is a minor diagenetic phase as a passive pore fill and grain replacive cement.It seems quite likely that early quartz cementation was achieved through silica derived from both external and internal sources.Though noticed as interpenetrative grain contact, an effect of grain dissolution and pressure solution is less frequent in the lower unit sandstones.

A predominance of matrix (>15%) is a major characteristic of the upper unit that separates it petrographically from the lower unit.It has subsequently caused another difference in its response to compaction as matrix behaves plastically.Much of this matrix is ferruginous in nature surrounding detrital quartz and predates authigenic silica overgrowth (Fig.11B).The silica might have been derived from meteoric ground waters circulating through the sandstones as soon as they were deposited.In the later stage or deeper burial, pressure solution along with stylolites and framework-grain contacts might have added additional silica to pore fluids, which subsequently moved by diffusion to nearby sites of precipitation.Incipient stylolitization (Fig.11D) and grain-to-grain contact dissolution are observed in nearly all the sandstones of the upper unit.A comparison of diagenetic stages between the lower and upper unit sandstones is given in Table 3(modified after Pitman et al.,2000).In spite of a subtle difference in the way of compaction, the two units have a considerable similarity during early burial history in terms of early authigenic quartz cementation.However, there is a different behavior observed in the upper unit sandstones during the mesogenesis.Unlike that of the lower unit sandstones,they show much less intensity of pressure solution and have a profuse ferruginous intergranular fill.

Major diagenetic events observed in the lower unit sandstones include: 1) syntaxial quartz cementation;2) secondary cement and minor framework grain dissolution; 3) passive pore fill and grain replacive cement.The paragenetic sequence for the upper unit sandstones includes: 1) profuse ferruginous matrix; 2)quartz overgrowth and syntaxial cementation; 3) secondary ferruginous cement and extensive framework grain dissolution.

8.Discussion

The observations based on facies type and palaeocurrent pattern described in this work reflect a characteristic shallow marine tidal succession for the lower unit of Upper Rewa Sandstone.It is characterized by prominent bar cross-bedding, megaripple lamination and absence of typical beach zonation(Singh, 1980).On the basis of the same parameters,the upper unit differs distinctly by the absence of herringbone cross-stratification, and shows a consistent unimodal palaeocurrent pattern.This unit has been interpreted to represent the signature of a fluvial-dominated process by Bose and Chakraborty(1994).Further, this contrast between the two units becomes more pronounced with the aid of petrographical study.Such an approach combining diagenesis with facies and palaeocurrent parameters given in the present work have not been attempted so far(Table 4).

The maximum depth of eogenesis is around 1 km－2 km which gives the temperature limit of about 30°－70°assuming 20°/km－30°/km as the geothermal gradient (Worden and Burley, 2009).Quartz cementation is a significant process only at temperatures greater than 70°－80°(Giles et al.,2000)and becomes more pronounced at over 80°(Bj?rlykke and Egeberg,1993).In the present study, it implies that quartz cementation ranges from late eogenetic to mesogenetic stage for both the units.Authigenic quartz cementation is more dominant in the lower unit sandstones which indicate a highly saline pore fluid(Sullivan et al., 1994).Such a widespread authigenic quartz cementation denotes normally pressured sand(Osborne and Swarbrick, 1999) where hydrostatic stress equals to the pore fluid pressure.

Though formed at the similar diagenetic stage,quartz cementation in the upper unit sandstones is far less and therefore gives no indication of high salinity as revealed by that of the lower unit.Additionally,authigenic quartz as cement is relatively much less together with prominent stylolites developed at grain boundaries.It reveals a state where fluid pressure exceeds hydrostatic stress (overpressure fluid),thereby reducing the effective stress (Worden and Burley, 2009).Mechanical compaction is a response of a vertically incident normal stress which leads to porosity reduction along with quartz cementation in sandstones.The porosity enhancement however,denotes dissolution of framework grains and cements,which is relatively minor in intensity for both the units.

Overall, the lower marine unit is characterized by nearly sub-rounded, well-sorted, normally-pressured sandstones with highly saline pore fluid.The features pertaining to facies and palaeocurrent pattern of the upper unit resemble a braided fluvial system(Bose and Chakraborty, 1994).All the upper unit sandstones are poorly-sorted, matrix-dominated, overpressured,having profuse stylolites without any evidence of a saline pore fluid.A subtle change in cementation pattern has been used as a proxy to delineate sequence boundary (Pitman et al., 2000).Similarly,results of this study provide an alternative method to recognize a facies based unconformity in intracratonic basins like Vindhyan.Furthermore, it indicates that the sandstone deposition event (Singh, 1980) documented for the Rewa Group involves both marine and fluvial processes, but their outcrop appearance is strikingly similar.The Neoproterozoic (Srivastava and Rajgopalan, 1988) siliciclastic sedimentation of Upper Rewa Sandstone can be attributed to the intense chemical weathering of continental crust.It seems somewhat similar to a widespread unconformity related Neoproterozoic continental denudation and subsequent sedimentation proposed by Peters and Gaines (2012).The cause for a profuse Neoproterozoic continental denudation as a function of sea level fluctuation still remains to be explored.

9.Conclusions

The major findings from this work are as follows.

1) The two units of Upper Rewa Sandstone differ considerably in terms of facies type with signatures of shallow marine tidal influence in the lower unit sandstones.The sandstones of the upper unit lack any bidirectional structures and are moderate to poorly sorted.

2) There is also a strong contrast in palaeocurrent patterns between the two units.The polymodal plot of the lower unit shows NW－SE-directed palaeoshoreline.A consistent unimodal pattern of the upper unit indicates the NW-directed palaeoflow suggesting a fluvial origin.

3) Lithification of the lower unit is attributed to the authigenic quartz overgrowth whereas that of the upper unit is strongly affected by the pressure solution.Such an approach can be successfully applied for marking at least a facies based unconformity between rocks with similar lithology having no angular discordance and even without any geochronological data or age marker fossils.

Availability of data and materials

Data supporting the findings of this work are available upon request from the corresponding author.

Funding

This work has not received any financial support.

Authors'contributions

Gaurav K.Singh: Conceptualization, Methodology,Investigation,Original draft preparation;Ashish K.Rai:Resources, Field work, Draft preparation; Arvind K.Singh: Data curation.

All authors read and approved the final proof.

Conflicts of interest

All authors hereby declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

We are thankful to Mr.Ratandeep Jauhari for his cooperation in the preparation of location map.Mr.Vinod Kanojiya is especially thanked for making several thin sections and every technical staff is hereby acknowledged for their help.One of us(Ashish K.Rai)is grateful to the University Grants Commission(UGC) for fellowship.