Carbon and oxygen isotope compositions of the carbonate...

Carbon and oxygen isotope compositions of the carbonatefacies in the Vindhyan Supergroup, central India

S Banerjee1, S K Bhattacharya2 and S Sarkar3

1Department of Earth Sciences, Indian Institute of Technology Bombay, Powai, Mumbai 400 076, India.2Physical Research Laboratory, Navrangpura, Ahmedabad 380 009, India.

3Department of Geological Sciences, Jadavpur University, Kolkata 700 032, India.

The Vindhyan sedimentary succession in central India spans a wide time bracket from the Paleopro-terozoic to the Neoproterozoic period. Chronostratigraphic significance of stable carbon and oxygenisotope ratios of the carbonate phase in Vindhyan sediments has been discussed in some recentstudies. However, the subtle controls of facies variation, depositional setting and post-depositionaldiagenesis 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 pale-ogeography. A detailed facies-specific carbon and oxygen isotope study of the carbonate units wasundertaken by us to investigate the effect of these processes and to identify the least altered isotopevalues. It is seen that both carbon and oxygen isotope compositions have been affected by earlymeteoric water diagenesis. The effect of diagenetic alteration is, however, more pronounced in caseof oxygen isotopes than carbon isotopes. Stable isotope compositions remained insensitive to faciesonly when sediments accumulated in a shallow shelf setting without being exposed. Major alter-ation of original isotope ratios was observed in case of shallow marine carbonates, which becameexposed to meteoric fluids during early diagenetic stage. Duration of exposure possibly determinedthe magnitude of alteration and shift from the original values. Moreover, dolomitization is found tobe accompanied by appreciable alteration of isotope compositions in some of the carbonates. Thepresent study suggests that variations in sediment depositional settings, in particular the possibil-ity of subaerial exposure, need to be considered while extracting chronostratigraphic significancefrom δ13C data.

1. Introduction

Carbon and oxygen isotope data of Precambriancarbonates harbour a wealth of information on pasthydrospheres, atmospheres and biospheres (Veizeret al 1980, 1999; Beeunas and Knauth 1985).Carbon isotopes have provided a useful tool forcorrelation of end-Precambrian sedimentary suc-cessions of the world (Veizer et al 1992; Des Marais1994; Knoll et al 1995; Hall and Veizer 1996; Franket al 1997; Lindsay and Brasier 2000; Shields andVeizer 2002; Ray et al 2003). A common assump-tion of all the studies mentioned above is thatboth carbon and oxygen isotopes are insensitive

to sedimentary facies (see also Knoll et al 1986;Kaufman and Knoll 1995). Carbon and oxygenisotope analyses have been performed on the Pro-terozoic Vindhyan carbonates and their chronos-tratigraphic significance have been discussed inmany recent studies (Friedman et al 1996, 2004;Friedman and Chakraborty 1997; Kumar et al2002; Ray et al 2003). Ray et al (2003) provideddetailed carbon, oxygen and strontium isotope sys-tematics of the Vindhyan carbonates, discussedsecular variation of these isotopes and presentedprimary carbon and oxygen isotope values. Most ofthese studies recorded average Paleoproterozoic toNeoproterozoic carbon and oxygen isotope values

Keywords. Facies; meteoric water; diagenesis; Vindhyan; Proterozoic.

J. Earth Syst. Sci. 115, No. 1, February 2006, pp. 113–134© Printed in India. 113

114 S Banerjee et al

Figure 1. Map of India showing outcrop distribution of the Vindhyan basin in the Son valley area (a) and location mapof the study area (b). Location of Maihar is common in both the figures.

from the Vindhyan Supergroup. None of the above-mentioned studies, however, discussed isotope com-positions of the Vindhyan carbonates in relationto the carbonate facies. Samples for the studieswere selected ignoring facies and paleogeography ofthe constituent carbonate units, which could haveinfluenced isotope compositions. A few studies,however, considered carbonate facies while inter-preting the stable carbon and oxygen isotope data(Banerjee 1997; Sarkar et al 1998; Chakrabortyet al 2002; Chakraborty 2004; Banerjee et al 2005).Sarkar et al (1998) and Chakraborty et al (2002)recorded facies sensitivity of Neoproterozoic Bhan-der Limestone. In this context, it is appropriate toinvestigate the depositional and diagenetic controlson stable isotope compositions of the Vindhyancarbonates in detail.

The present paper discusses stable isotope com-positions of the Vindhyan carbonates in rela-tion to the sedimentary facies and paleogeographyto understand depositional and diagenetic influ-ences on the stable isotope compositions of allthe carbonate units of the Vindhyan Supergroup.The study attempts to compare chronostrati-graphic versus depositional/diagenetic controlson stable isotope compositions of the Vindhyancarbonates.

2. Geological background andage constraints

The Vindhyan succession of central India is two-tiered, about 4500 m thick and consists of mildlydeformed and un-metamorphosed siliciclastics andcarbonates exposed over wide areas in centralIndia (figure 1). The Semri Group comprises of

siliciclastics, carbonates and volcaniclastics (fig-ure 2). The Kajrahat Limestone and the RohtasLimestone are the two important carbonate unitsof the Semri Group (figure 2). A thin, discon-tinuous dolomite unit occurs at the mid-level ofthe predominantly siliciclastic Kheinjua Formationand is known as the Fawn Dolomite. The UpperVindhyan Group is dominated by clastics andthe sole carbonate unit, the Bhander Limestoneoccurs near the top of the succession (figure 2).Preservation of delicate sedimentary features is adistinct characteristic of the entire Vindhyan suc-cession. Detailed facies analysis is thus possible onall the carbonate units of the Vindhyan Super-group. Excellent preservation of sedimentary fea-tures, dominantly micritic nature of the carbonatesediments and lack of compaction features ruleout significant burial diagenesis of the carbon-ates (Bose et al 2001; Chakraborty 2004). Vind-hyan sedimentation took place largely in a shallowmarine setting (Chanda and Bhattacharyya 1982;Bose et al 2001), although minor paleogeographicshifts have been reported in a few recent stud-ies (Chakraborty 2004; Banerjee et al 2005). It isbelieved that the initial sedimentation took placein an intracratonic rift basin and then a transitionto intracratonic sag basin took place during theupper Vindhyan.

On the basis of 87Sr/86Sr ratios the age of theoldest carbonate unit, namely the Kajrahat Lime-stone, is assumed to be 1700 Ma old (Ray et al2003). Rasmussen et al (2002) and Ray et al (2002)determined the age of the overlying PorcellaniteFormation to be 1630 Ma by U/Pb SHRIMP datingtechnique. Sarangi et al (2004) determined an ageof 1600 Ma age for the Rohtas Limestone. The ageof the Fawn Dolomite is, therefore, between 1630

Carbon and oxygen isotope compositions 115

Figure 2. Vertical section showing details of the Vindhyan stratigraphy up to the formation level highlighting the fourcarbonate units (modified after Bose et al 2001). Ages of the carbonate units (after Ray et al 2003) are provided on theright of the vertical section.

and 1600 Ma. These data are consistent with thoseobtained by Rasmussen et al (2002) for the ash tuffunit at the base of the Rohtas Formation. The ageof the Bhander Limestone is not precisely deter-mined, but most likely it brackets an age rangebetween 650 and 750 Ma (Ray et al 2003).

3. Samples and methods

The constituent facies of the four carbonateunits of the Vindhyan Supergroup have been

documented in the Son valley area of MadhyaPradesh through detailed field and petrographicobservations carried out over a decade, the resultsof which have been presented partially elsewhere(Sarkar et al 1996, 1998; Banerjee 1997; Boseet al 2001; Chakraborty 2004; Banerjee et al2005). In this paper we have presented relativelydetailed facies description for the Semri Groupas only a few studies provide such information.Fresh samples were collected for isotope analysisfrom all four of the carbonate units of the Vin-dhyan Supergroup while noting their respective


Table

1.

Faci

esand

paleog

eogr

aphy

ofth

eK

ajrahatLim

esto

ne.

Faci

esD

escr

ipti

on

Inte

rpre

tation

Rep

rese

nta

tive

featu

res

Upper

div

isio

nm

icro

bia

lla

min

ite

Const

itute

dby

calc

imic

rite

s.C

onsist

sof

crin

kly

lam

inate

dlim

esto

nes

.B

ears

V-

shaped

des

icca

tion

crack

s.U

sually

occ

ur

at

the

top

ofth

esm

all

stro

mato

lite

faci

es.

Tow

ard

sth

eto

pofth

est

rom

ato

lite

succ

es-

sion

the

faci

esbec

om

esabnorm

ally

thic

ker

.O

ther

wise

thic

knes

sof

the

faci

esva

ries

from

5cm

to40

cm.

Cri

nkly

natu

reof

the

lam

i-nae

rese

mble

sm

icro

bia

lla

mi-

nite.

Des

icca

tion

crack

sin

dic

ate

emer

gen

ce.

(matc

h-s

tick

length

=2.5

cm)

Sm

all

stro

mato

lite

Consist

ing

of

smaller

stro

mato

lite

sw

ith

aver

age

hei

ght

3–5

cmand

hea

ddia

m-

eter

2cm

).T

hes

eusu

ally

occ

ur

abov

eth

ela

rger

stro

mato

lite

sand

under

the

faci

esG

and

hav

eaxia

ldes

icca

tion

crack

s.G

ener

ally

bra

nch

ing

innatu

reand

are

imbri

cate

d.

Str

om

ato

lite

axes

imbrica

-tion

indic

ate

snort

hw

este

rly

pale

ocu

rren

tdirec

tion.A

ver

age

thic

knes

softh

esm

aller

stro

mato

lite

faci

esis

16

cm.C

onst

itute

dby

calc

imic

rite

s.

Ver

ysh

allow

agit

ati

ng

sett

ing.

Des

icca

tion

crack

sin

dic

ate

sem

ergen

ce.

(matc

h-s

tick

length

=2.5

cm)

Larg

est

rom

ato

lite

Consist

ing

of

larg

erst

rom

ato

lite

s,w

ith

aver

age

hei

ght

and

hea

ddia

met

er20

cmand

7cm

.G

ener

ally

conic

al

innatu

re(C

onophyt

on),

wit

hout

bra

nch

ing

and

imbri

cati

on.

May

hav

eaxia

ldes

icca

tion

crack

s.In

terc

olu

mnar

are

as

are

narr

owand

filled

by

small

peb

ble

tom

ud

size

stro

mato

lite

fragm

ents

that

are

sele

ctiv

ely

dolo

mit

ized

.A

ver

age

thic

knes

sfo

rth

ela

rge

stro

mato

lite

faci

esis

38

cm.

Const

i-tu

ted

by

calc

imic

rite

s.

Gre

ate

rhei

ght

and

Conophyt

on

affi

nity

indic

ate

sdee

per

pale

o-

bath

ym

etry

.D

esic

cation

crack

sin

dic

ate

sem

ergen

ce.

(pen

length

=14

cm)

Carbon and oxygen isotope compositions 117M

iddle

div

isio

nsh

eet

bed

ded

dolo

mit

e

Consist

sofbuff

colo

ure

ddolo

stone.

Alt

er-

nate

sw

ith

faci

esC

.T

hey

usu

ally

hav

esh

eet

like

geo

met

ryand

sharp

base

.A

ver

age

thic

knes

sis

4cm

.C

om

pri

ses

of

subhed

ral

toeu

hed

ral

non-fer

roan

dolo

spars

.P

seudom

orp

hs

of

gypsu

mro

sett

esare

abundantl

ypre

sent.

Typic

al

swallow

tale

stru

cture

sare

loca

lly

reco

g-

niz

able

.D

espit

eth

egen

eral

mass

iven

ess,

thin

lam

inae

can

be

seen

under

mic

ro-

scope

and

rose

tte

stru

cture

sare

com

monly

upri

ght

on

thes

epla

nes

.

Poss

ible

alloch

thonous

dep

osits

.

(pen

length

=14.5

cm)

Gre

ylim

esto

ne

Com

prise

sof

dark

gre

yco

loure

dfa

intl

yla

min

ate

dlim

esto

ne.

Thic

knes

sof

the

faci

esdec

rease

supw

ard

from

1.3

mto

25

cm.

Com

pri

ses

of

dolo

mic

rite

s.C

om

-pri

ses

ofm

icriti

clim

esto

ne,

loca

lly

recr

ys-

tallis

ed.

Extr

emel

ysh

allow

rest

rict

edm

ari

ne

sett

ing.

Low

erdiv

isio

ncr

oss

-str

ati

fied

dolo

mit

e

Consist

sof

isola

ted

buff-c

olo

ure

ddolo

stone

bodie

s.T

hes

eare

usu

ally

cross

-str

ati

fied

.A

ver

age

cross

-set

thic

k-

nes

s=

35

cm.C

ross

-str

ati

fica

tions

indic

ate

nort

hw

este

rly

pale

ocu

rren

tdir

ecti

on.C

on-

sist

sofsu

bhed

ralnon-fer

roan

dolo

spars

.

Indic

ate

ssh

allow

mari

ne

ori

gin

ofth

edolo

mite

body.

(pen

length

=14

cm)

Mass

ive

dolo

stone

Consist

sof

buff

colo

ure

dm

ass

ive

dolo

-st

one.

Sed

imen

tary

stru

cture

sare

poorl

ypre

serv

ed.

Com

pri

ses

of

subhed

ral

non-

ferr

oan

dolo

spars

.

Diffi

cult

toin

terp

ret

the

dep

osi

-ti

onalse

ttin

g.


Figure 3. Vertical section showing three divisions of the Kajrahat Limestone in the Son valley area (a). Logs showingconstituent facies of the lower division (b), middle division (c) and upper division (d). The given height in b, c and dcorresponds to the stratigraphic height from the base of the Kajrahat Limestone. Note cyclicity defined by large stromatolite,small stromatolite and algal laminite in the upper division (d).

facies and stratigraphic position. Samples wereselected covering the entire stratigraphy of eachof the carbonate units. The samples were col-lected from different locations in the Son valleyarea (figure 1). 40 samples were collected fromthe 255 m thick Kajrahat Limestone section nearKuteshwar area. 26 samples were collected fromthree sections of Fawn Dolomite around Chorhat,Markundi and Dhanjoria. 31 samples were col-lected from the Rohtas Limestone section of theRampur area. 52 samples were collected from sev-eral short sections of Bhander Limestone spreadover Maihar, Rewa, Aber and Satna and Sajjanpur(figure 1).

Carbon and oxygen isotope analysis of thesamples was performed at the Physical ResearchLaboratory, Ahmedabad. Due care was given toselect visibly unaltered micritic carbonate samples

identified by prior petrographic studies involvinglight microscope and SEM. Thin sections of car-bonate samples were stained by Dickson’s (1966)method. SEM studies were carried out with thehelp of JEOL JSM 840A Scanning Electron Micro-scope at IIT Bombay. Samples containing recrys-tallisation veins, crack-fills and neomorphic calciteswere discarded. Chosen samples were cleaned,disaggregated and then powdered. Carbon andoxygen isotopes ratios were measured by treatingthe powdered samples with H3PO4 at 50◦C for10 minutes, cleaning the evolved CO2 from watervapour and other condensable gases and analysingit in a VG Micromass 903 triple collector massspectrometer. The isotopic ratios δ13C and δ18Oare expressed with respect to the internationalstandard PDB (Craig 1957) and are reproducibleto ±0.1� at 1σ level. Reproducibility measured


Figure 4. Photographs showing representative features of the Fawn Dolomite: crinkle laminated dolomite (a), tepeestructures (b) fenestrae (white patches), (c) and peloids (d). Lens cap diameter = 5 cm, pen length = 14 cm, bar = 0.6 cm.

as 1σ for five samples of NBS-19 (δ13C = 1.95�,δ18O = −2.20�) was approximately 0.1� for bothδ13C and δ18O.

4. Carbonate facies

Summary of facies analysis of the three major car-bonate units are presented in tables 1, 2 and 3.A brief discussion on facies and paleogeography ofall four of the carbonate units in the study area(sampling sites) is presented below.

4.1 Kajrahat Limestone

The Kajrahat Limestone is up to 255 m thick inthe area around Kuteswar and comprises of threesuperposed divisions characterized by distinctivefacies assemblages (Banerjee 1997; table 1, fig-ure 3). The 60 m-thick basal division is predomi-nantly dolomitic and comprises of massive dolomitefacies and cross-stratified dolomite facies. The70 m-thick middle division is composed of verti-cal alternations of faintly laminated grey limestonefacies and buff coloured sheet-bedded dolomitefacies. Under the microscope, the sheet-beddeddolomite reveals pseudomorphs of gypsum rosettes.The 125 m-thick upper division predominantlycomprises of large stromatolite facies, small stro-matolite facies and microbial laminite facies. Thelarger stromatolite columns often show the pres-ence of axial vertical cracks caused by desiccation

(Bose et al 2001). Microscopic observation showsthat the cracks are filled up by non-ferroanlimpid calcispars. The smaller stromatolites, usu-ally imbricated, occur above the larger stromato-lites. Microbial laminites usually occur above thesmall stromatolites and bear V-shaped cracks filledby clear blocky non-ferroan calcite. Regular ver-tical stacking of large stromatolites, followed bysmall stromatolites and microbial laminites indi-cates the presence of a conspicuous sedimentarycyclicity within the Kajrahat Limestone (figure 3).Banerjee (1997) recorded several such cyclicitiesfrom the upper division of the Kajrahat Limestone.Despite their variations in form, all these microbialfacies are made of non-ferroan calcimicrite, recrys-tallized locally into non-ferroan calcite microsparto a variable extent.

Extensive dolomitization has erased many of theprimary sedimentary features in the lower divi-sion of the Kajrahat Limestone creating difficultyin paleogeographic reconstruction. The large-scalecross-stratified bodies possibly represent channel-fills. Their distribution in the lower and upperdivisions suggests a shallow marine environmentof deposition. Presence of oriented stromatoliteswithin the upper division corroborates the hypoth-esis of shallow marine origin (Hoffman 1976). Occa-sional extreme shallowing is indicated by gypsumcrystals in the sheet bedded dolomite facies, pre-sumably formed at the sediment–water interface.V-shaped cracks within the microbial laminite


Figure 5. Vertical section showing constituent facies of theRohtas Limestone in the Rampur area.

facies represent desiccation cracks and indicateemergence above the water level. The microbiallaminite facies possibly formed in very shallow con-ditions and represents a supratidal setting. Largestromatolite facies, on the other hand, possiblydeveloped in a deep subtidal setting, beneath fairweather wave base (Grotzinger 1986; Southgate1989). Smaller stromatolites exhibit consistent ori-entation indicating strong agitation in a shallowsubtidal setting (Hoffman 1976; Grotzinger 1986;Glumac and Walker 1997, 2000). Reduction of stro-matolite size is related to shallowing of the basin.Therefore, the periodic alternation of large stroma-tolite, small stromatolites and microbial laminiteswithin the Kajrahat Limestone indicates cyclicfluctuation in sea level leading to eventual exposure

in each cycle. The scale of marine regression wassuch that it caused repeated exposure of deep sub-tidal facies (large stromatolite). Obviously, dura-tion of exposure will be more in case of microbiallaminites compared to the larger stromatolites.

4.2 Fawn Dolomite

Carbonates occur as discontinuous bodies withinthe predominantly siliciclastic Kheinjua Forma-tion. In the study area, the carbonate unit isrestricted in thin (less than 10 m) local patcheswithout any formal stratigraphic status (Bose et al2001). The predominantly dolomitic body knownas Fawn Dolomite/Salkhan Limestone in the east-ern Son valley (Auden 1933; Prakash and Dalela1982) comprises almost exclusively of microbiallaminite (figure 4; Banerjee 1997) and occurs abovethe progradational fluvio-deltaic succession of theKoldaha Shale (Bose et al 2001). Tepee struc-tures can be seen at places. Fenestrae and bird’seye features can be seen under the microscope(figure 4). Although it consists of laminated non-ferroan dolomicrite, at places (Markundi) it com-prises of pelsparite and oosparite (figure 4).

Fenestrae and bird’s eye features suggest rot-ting of microbial mat and subsequent expulsion ofmicrobially generated gases. Tepee structures alsosuggest collapse of mat cover due to expulsion ofgases. The carbonate body sandwiched betweensiliciclastic strata packages of the Koldaha Shaleand the Chorhat Sandstone is considered to be ofestuarine origin near Chorhat (Banerjee 1997). Thecarbonate bodies near Markundi and Dhanjoriapossibly have features of lagoonal and supratidaldeposits respectively (Banerjee 1997).

4.3 Rohtas Limestone

The Rohtas Limestone gradationally overlies theRampur Shale in the Son valley area. The latterpredominantly consists of black shale with thinlimestone interbeds (<3 cm thick). Rohtas Lime-stone in the Rampur area comprises of six facies(Banerjee et al 2005; figure 5, table 2) describedas: black shale facies, ribbon limestone facies, thin-bedded limestone facies, calcarenite facies, flakeconglomerate facies and non-edgewise conglomer-ate facies. Black shale facies alternates with rib-bon limestone facies and flake conglomerate faciesin the lower parts of the Rohtas Limestone. Middleand upper part of the Rohtas Limestone consistof thin-bedded limestone facies, calc-arenite facies,flake-conglomerate facies and non-edgewise con-glomerate facies. Wavy and crinkly laminae arecharacteristic of the black shale facies (Schieber1999). The ribbon limestone facies is character-ized by alternations of seamed- and non-seamed


bands; dissolution seams are abundant in theseamed band, which appears black while the non-seamed laminae appear white. The frequency ofoccurrence of dissolution seams decreases upwardthrough a seamed band towards the gradationalcontact of the immediately overlying non-seamedband; bases of the non-seamed bands are, however,always sharp. The thin-bedded limestone faciesis characterized by wave rippled bed tops. Thecalcarenite facies displays hummocky cross strat-ification (De Raaf et al 1977), climbing ripplelaminae and unidirectional cross strata. The flake-conglomerate facies displays conspicuous edgewisefabric exhibited by flake-like carbonate intraclasts.Non-edgewise conglomerate facies displays chaoticarrangement of irregular clasts.

The presence of wave ripples, hummocky crossstratification and edgewise conglomerates in theRohtas Limestone suggests deposition on the shelf.In the lower part, black shale alternates with dis-solution seams bearing ribbon limestone facies,both lacking current features suggesting deepwater sedimentation. Upward, the black shalefacies is replaced by edgewise conglomerate facies,hummocky cross-stratified and wave rippled lime-stone indicating overall progradation. Lack ofemergence features in these limestones suggeststhat the water level never became shallow. Ribbonlimestone facies represents decoupling of beds intoa lower compacted part and an upper uncompactedpart; pre-compaction differential cementation hasbeen inferred for the non-seamed bands (Banerjeeet al 2005).

4.4 Bhander Limestone

Sarkar et al (1996) and Chakraborty (2004) havepresented a detailed description of the faciesand paleogeography of the Bhander Limestone.The ramp-originated Bhander Limestone succes-sion is thought to have developed by forward–backward shifts of shore-parallel facies beltsbetween shoreface and distal offshore. The Bhan-der Limestone consists of five constituent facieswhose depositional regime varied widely rang-ing from distal offshore to shoreface (table 3,figure 6). Most of the facies are repetitive innature and appear throughout the successionrecording several transitions between offshore andnearshore domains. Amongst the constituent faciesthe stromatolite facies represents the widest rangein paleogeography. Lateral transition of the stro-matolite facies with the planar and cross-stratifiedones has also been recorded. Sarkar et al (1998)and Chakraborty (2004) have recorded basin wideregression leading to exposure at three differentlevels marked by unconformities and correlativeconformities.

4.5 Mineralogical variation acrossthe carbonate facies

Out of the four carbonate units of the Vind-hyan Supergroup the Fawn Dolostone is entirelydolomitic. Mineralogical data of the constituentfacies of the remaining carbonate units are givenin table 1, table 2 and table 3 along with thedescription of facies. In the study area, the RohtasLimestone consists predominantly of calcimicritesand calcisiltites (table 2). In Kajrahat Lime-stone small stromatolite facies, large stromatolitefacies, microbial laminite facies and grey limestonefacies consist solely of calcimicrites; whereas sheetbedded dolomite facies, cross-stratified dolomiteand massive dolomite comprise predominantly ofdolomicrites. In the case of Bhander Limestone thestromatolite facies and planar and cross-stratifiedfacies are predominantly made up of calcite.Plane laminated limestone is, however, dolomitisedfrequently.

5. Isotope compositions ofthe carbonate units

5.1 Primary carbon and oxygen isotope valuesfor Paleoproterozoic to Neoproterozoic

carbonates

Many workers believe that C-isotopic signaturesare usually well preserved in Proterozoic limestones(Knoll et al 1986). However, several examples ofpost-depositional isotopic shifts have been docu-mented in recent years (Kaufmann et al 1992;Sarkar et al 1998; Ray et al 2003). Unfortunately,the δ13C and δ18O values of late Paleoproterozoicand Mesoproterozoic carbonates are not well con-strained. Primary or near-original δ13C values forthe late Paleoproterozoic to early Mesoproterozoiccarbonates are found to be within 0 ± 2� (Veizeret al 1992; Lindsay and Brasier 2000; Shields andVeizer 2002; Ray et al 2003). Near-original δ18Ovalues of the calcitic carbonates for the entireProterozoic is estimated to be −6.5 ± 1.5� (Rayet al 2003); contemporary dolomites are usuallyenriched in 18O by 2–3� (Land 1980). The pri-mary δ13C values of the Neoproterozoic carbonatesare believed to be 3 ± 2� (Ray et al 2003). In thefollowing discussion the isotope ratios of the Vind-hyan carbonate samples obtained by us have beencompared with the primary range of values sug-gested by Ray et al (2003).

5.2 Kajrahat Limestone

δ13C values of 40 samples from different facies ofthe Kajrahat Limestone were measured and they


Table

2.

Faci

esand

paleog

eogr

aphy

ofth

eRohta

sLim

esto

ne.

Faci

esD

escr

ipti

on

Inte

rpre

tations

Rep

rese

nta

tive

vie

ws

Non-e

dgew

ise

conglo

mer

ate

Consist

sof

intr

acl

ast

sw

hic

hare

chaoti

-ca

lly

arr

anged

and

wit

hout

gra

din

g.

The

bed

shav

ela

tera

lly

variable

thic

knes

s(u

pto

60

cm).

Rev

erse

gra

din

gm

aybe

seen

at

the

base

ofth

ebed

s.

Dep

ositi

on

poss

ibly

took

pla

cefr

om

deb

ris

flow

sof

hig

hm

atr

ixst

rength

or

modi-

fied

gra

inflow

sw

ith

inte

rnalgra

infr

icti

on.

Str

ong

basa

lsh

ear

cause

din

ver

segra

din

g.

(coin

dia

met

er=

2.5

cm)

Fla

ke

conglo

mer

ate

Consist

sof

flake-

like

carb

onate

intr

acl

ast

s(u

pto

18

cmin

length

and

2.2

.cm

inth

ick-

nes

s).C

last

sare

poorly

sort

edand

gra

din

gis

abse

nt.

Edgew

ise

clast

fabri

cis

chara

c-te

rist

icfo

rth

efa

cies

.

Epis

odic

natu

reof

the

flow

isre

vea

led

by

the

occ

urr

ence

of

thes

ecl

ast

sin

muddy

sedim

ents

.Str

ongly

erosive

storm

-dri

ven

com

bin

edflow

isth

em

ost

likel

yagen

tto

pro

duce

the

edgew

ise

conglo

mer

ate

s.

(coin

dia

met

er=

2.5

cm)

Calc

are

nit

eC

onsi

sts

of

tabula

rand

sharp

-base

dbed

sof

calc

are

nit

ew

ith

occ

asional

basa

lcl

ast

conce

ntr

ati

on.

Show

sta

bula

rand

dow

nslope

wed

gin

gcr

oss

stra

tifica

tions.

Hum

mock

ycr

oss

stra

tifica

tions.

Over

lyin

gpla

nar

lam

inae

are

also

pre

sent.

Clim

bin

gri

pple

sm

ayocc

ur

abov

epla

nar

lam

inae

ina

few

case

s.

Pro

duct

sof

epis

odic

storm

-origin

ate

dw

ave-

cum

-curr

ent

com

bin

edflow

.W

anin

gnatu

reof

the

flow

isev

iden

tby

the

basa

lcl

ast

conce

ntr

ati

on

and

pla

nar

lam

inae

pass

ing

eith

erto

small

hum

mock

sor

wav

eri

pple

s.

Hum

mock

ycr

oss

stra

tifica

tion

(pen

length

=14

cm)

Carbon and oxygen isotope compositions 123T

hin

bed

ded

lim

esto

ne

Thin

bed

ded

,w

ave

ripple

d(a

v.w

avel

ength

7cm

and

am

plitu

de

1.5

cm)

and

consist

sof

calc

isilti

te.

Shallow

wate

rdep

ositi

on

abov

eth

efa

irw

eath

erw

ave

base

.

Wav

eri

pple

s(m

atc

h-s

tick

length

=4.5

cm)

Rib

bon

lim

esto

ne

Late

rally

per

sist

ent,

alt

ernate

dark

and

whit

eca

lcim

icri

tebands.

The

dark

erbands

conta

innum

erous

disso

luti

on

seam

sva

ry-

ing

inle

ngth

from

1m

mto

4.5

cm.

The

white

bands

are

dev

oid

of

seam

sex

cept

at

thei

rbase

.T

he

conta

ctbet

wee

nth

ese

am

edand

the

over

lyin

gnon-s

eam

edla

m-

inae

isgra

dati

onal.

Each

couple

tco

mpri

sing

seam

edla

min

ae

and

its

imm

edia

tely

over

lyin

gnon-s

eam

edla

min

ae

repre

sents

pro

duct

sof

asingle

sedim

enta

tion

phase

.A

bse

nce

of

curr

ent

stru

cture

ssu

gges

tsdep

osi

tion

ben

eath

the

wav

ebase

.

Alter

nati

on

ofse

am

ed(d

ark

,arr

owed

)and

non-s

eam

edla

min

ae

(whit

e)(c

oin

dia

me-

ter=

2.5

cm)

Bla

cksh

ale

Even

-bed

ded

,fiss

ile

and

carb

onace

ous

shale

s(T

OC

up

to1.8

%)

inte

rbed

ded

wit

hth

in(<

1cm

)m

icriti

clim

esto

ne

lam

inae.

Shale

show

sw

avy

lam

inae,

pyri

tes

occ

ur

alo

ng

the

carb

onace

ous

lam

inae.

Calm

,quie

tand

anox

icdep

osi

tional

set-

ting.

Oute

rsh

elf

dep

osi

ts.

Wav

yla

main

esu

gges

tsit

sm

icro

bia

lm

at

origin

.

Bla

cksh

ale

-lim

esto

ne

alt

ernati

on

124 S Banerjee et alTable

3.

Faci

esand

paleog

eogr

aphy

ofth

eBhander

Lim

esto

ne.

Faci

es/fa

cies

code

Des

crip

tions

Pale

ogeo

gra

phy

Str

om

ato

lite

(SM

)D

ark

-colo

ure

dst

rom

ato

lite

sof

both

bio

ther

mal

and

stra

tifo

rmty

pe.

Most

lydom

al

and

com

pos-

ite

type.

Som

eare

ofm

icro

-dig

itate

form

.D

ispla

ypre

ferr

edorien

tation

at

pla

ces.

Com

monly

ass

o-

ciate

dw

ith

PC

Sand

HL.

Rare

lyov

erlies

PLM

faci

es.C

onsi

sts

ofnon-fer

roan

calc

imic

rite

s.

Str

ati

form

type

indic

ate

sver

ysh

allow

dep

th.

Shallow

agit

ate

dsu

bti

dal

sett

ing

for

dig

itate

stro

-m

ato

lite

sand

inner

shel

fse

ttin

gfo

rco

mposite

dom

alfo

rms.

(ham

mer

length

=41

cm)

Pla

ne

lam

inate

dm

udst

one

(PLM

)G

rey-c

olo

ure

dlim

em

udst

one

alt

ernati

ng

wit

hth

insilici

clast

icm

uds.

Exhib

its

pla

nar

lam

ina-

tion.

Thic

k-t

hin

alter

nations

bet

wee

nca

lcare

ous

lam

inae,

whic

hat

pla

ces

impart

sin

tern

al

gra

d-

ing.

Gra

des

into

HL,

SH

and

rare

lySM

faci

es.

Consi

sts

of

non-fer

roan

calc

imic

rite

s,fr

equen

tly

dolo

mit

ized

.

Dis

tal

offsh

ore

,bel

owst

orm

wav

ebase

.Poss

ible

spri

ng

tide

pro

duct

s.

(pen

length

=14

cm)

Pla

nar

and

cross

stra

tified

(PC

S)

Lig

ht

colo

ure

d,

wel

lso

rted

calc

are

nit

e.Loca

lly

ooliti

cand

pel

oid

al.

Tabula

rin

bed

geo

met

ry.P

la-

narand

cross

-str

ati

fica

tion

com

mon.E

xhib

itsher

-ringbone

cross

-str

ati

fica

tion

loca

lly.

Poly

modal

curr

ent

patt

ern.

Gra

des

into

HL,

SH

and

rare

lySM

faci

es.C

onsi

sts

ofca

lc-a

renit

e.

Wav

e-and

tide-

affec

ted

shore

face

.

(pen

length

=14

cm)

Het

erolith

ic(H

L)

Exhib

its

repea

ted

alt

ernati

on

bet

wee

nca

lcare

-nit

eand

lim

em

udst

one.

Shee

t-like

bed

geo

met

ry.

Wav

eri

pple

and

hum

mock

ycr

oss

stra

tifica

tions

pre

sent

inte

rnally

and

gutt

erca

stocc

ur

at

the

sole

s.T

horo

ughly

dolo

mit

ized

.

Sto

rmdom

inate

din

ner

shel

f.

Shale

(SH

)G

reen

ish

gre

ysi

lty

shale

,pla

ne

lam

inate

d;

thin

lim

esto

ne/

silt

stone

layer

sin

terv

ene

at

pla

ces.

Upw

ard

gra

din

gin

tore

ddish

colo

ur

silty

shale

wit

hisola

ted

sand-fi

lled

scours

.

Dista

loffsh

ore

close

toth

est

orm

wav

ebase

.


Figure 6. Representative log showing constituent facies ofthe Bhander Limestone in the Sajjanpur area.

range from 1.1� to −2.5� (table 4, figure 7a).δ18O values for the same samples vary from −7.9�to −13.4� (figure 7a). δ13C versus δ18O cross-plot reveals conspicuous clusters of data points forindividual facies. The sheet-bedded dolomite faciesexhibits the highest δ13C and δ18O values whilethe microbial laminite facies exhibits the lowestones. The average δ13C value shifts from low pos-itive (1.0� in sheet bedded dolomite facies and0.3� in case of large stromatolite facies) to slightlynegative (−0.2� in massive dolomite facies andfaintly laminated limestone facies, −0.3� cross-stratified dolomite facies and −0.4� in small stro-matolite facies) to still more negative (−1.7�in microbial laminite facies). A similar trend is

also observed in the case of δ18O values; averageδ18O value gradually shifts towards more negativevalues in the same hierarchy from sheet-beddeddolomite facies (−9.3� in sheet-bedded dolomitefacies, −9.6� in larger stromatolite facies, −10.3�in smaller stromatolite facies, −10.6% in mas-sive dolomite facies and microbial laminite facies,−10.9� in faintly laminated limestone facies and−11.7� in cross-stratified dolomite facies). Cross-plot of δ13C and δ18O values shows moderate cor-relation with values from sheet-bedded dolomitefacies and microbial laminite facies occupying twoends of the correlation line while values fromother facies near the middle part of the correla-tion line (figure 7a). It also shows wide scatterof data points for the microbial laminite facies;amongst the microbially originated components ofthe upper division, the large stromatolite faciesexhibits the least scatter and small stromatolitefacies exhibits moderate scatter of data points.Amongst the seven carbonate facies sheet-beddeddolomite exhibits least scatter of data points andhighest δ13C and δ18O values. In the upper division,each of the stromatolite cycles shows this generaltrend; δ13C and δ18O values become more depletedgoing across from large stromatolite facies to smallstromatolite facies to microbial laminite facies.δ13C and δ18O values, thus, are unrelated to verti-cal stratigraphy and found to be dominantly facies-controlled.

Moderate correlation of δ13C and δ18O values(r2 = 0.3) and wide scatter of the data points ofthe Kajrahat Limestone possibly suggest alterationof most carbon and oxygen isotope values. Exceptthe microbial laminite facies the δ13C values ofthe remaining facies remain confined within a nar-row range (close to 0�). δ18O values of the sam-ples exhibit wide variation. It can be surmisedthat the alteration effect is less pronounced inthe case of carbon isotopes. It seems that thehigher δ-values, e.g., those for the sheet-beddeddolomite facies and the large stromatolite faciesare least altered and represent near-primary ratios(Veizer et al 1992). Higher δ18O values in sheetbedded dolomite facies may also be related todolomitic mineralogy of the samples (Land 1980).As mentioned before, most of the facies constitut-ing the Kajrahat Limestone bears evidence of expo-sure of the depositional substrate. The δ13C andδ18O values of small stromatolites and microbiallaminites and even cross-stratified dolomites andsheet bedded dolomites probably represent alter-ation by isotopically lighter fluids at an early stageof diagenesis. Such isotopically lighter fluids canbe generated either by organic maturation duringburial or organically charged meteoric water (seealso Booler and Tucker 2002). Lack of compactionwithin the Kajrahat Limestone, however, suggests


Table 4. Carbon and oxygen isotope compositions for the constituent facies of the Kajrahat Limestone.

Distance from theSample no. Facies base of the member δ13C(�) δ18O(�)

KJ-40 Microbial laminite 250.4 m −0.5 −12.5









KJ-31 Small stromatolite 250.4 m −0.9 −9.7









KJ-22 Large stromatolite 250 m 0.3 −9.6









KJ-13 Sheet bedded dolomite 129 m 1.0 −9.0

KJ-12 Sheet bedded dolomite 115.1 m 0.9 −9.7




KJ-8 Grey limestone 115 m −0.1 −10.9

KJ-7 Grey limestone 103 m −0.1 −11.0

KJ-6 Grey limestone 72m −0.1 −11.1

KJ-5 Grey limestone 61m −0.3 −10.8

KJ-4 Cross stratified dolomite 48m −0.2 −13.4



KJ-1 Massive dolomite 24m −0.2 −10.6

that diagenetic resetting of carbon and oxygen iso-tope compositions took place in near surface con-ditions. Amongst the microbially originated faciesof the upper division, microbial laminite facies, aswell as both large and small stromatolites, show

emergence features. However, the isotope ratiosof large stromatolites are least altered. Scatterin data points and moderate correlation of val-ues in the case of larger stromatolites suggestminor alteration from original values indicating a


Figure 7. δ13C versus δ18O plot of the samples of the constituent facies of the Kajrahat Limestone samples displaying widescatter of data points (a). δ13C versus δ18O plot of the Fawn Dolomite samples from three areas, namely Chorhat, Markundiand Dhanjoria (b). δ13C versus δ18O plot of the samples of the constituent facies of the Rohtas Limestone samples (c) andBhander Limestone (d). Primary box of Ray et al (2003) is indicated by the rectangle in each figure. Note that none ofthe samples of the Kajrahat Limestone plot within the primary box and all but one sample of the Rohtas Limestone plotsoutside the primary box.

short duration of exposure in this case (Joachimsky1994). Microbial laminites, on the contrary, showlarge depletion of δ13C and δ18O values suggestinglonger duration of exposure. Small stromatolitesshow moderate depletion of δ13C and δ18O valuesin comparison to the large stromatolites. It is pos-sible that the composition of the meteoric fluidsvaried over time causing the observed scatter in theδ-values. Similar fluctuations in response to shal-lowing leading to exposure have been documentedfrom many Phanerozoic sequences (Joachimsky1994). δ18O values of the Kajrahat Limestoneare highly depleted compared to contemporaneouscarbonates and possibly represent considerablediagenetic alteration. As a consequence, none ofthe values plot within the primary box of Rayet al (2003) although the range of δ13C valuesremains within the ‘primary’ range (figure 7a).Diagenetic fluids do not contain much CO2 toreset original carbon isotope signature unless the

alteration effect is extensive and the original car-bonate sediments are thoroughly recrystallised.Significant post depositional resetting leading tolowering of δ18O values took place possibly for mostof the samples of the Kajrahat Limestone.

5.3 Fawn Dolomite

Stable isotope compositions of time equivalentdolomite bodies of the Kheinjua Formation (FawnDolomite) vary considerably (table 5). The datapoints of Chorhat, Markundi and Dhanjoriasamples form three separate clusters (figure 7b).The data points of the Fawn Dolostone exhibitgood correlation between δ13C and δ18O values(r2 = 0.7). The Chorhat samples are character-ized by consistently negative δ13C values rangingfrom −0.9� to −2.8� with an average of −1.9�.In contrast, the Dhanjoria samples exhibit consis-tently positive δ13C values ranging from 0.9� to


Table 5. Carbon and oxygen isotope compositions of the Fawn Dolostone indifferent places.

Sample no. Facies/ lithology δ13C (�) δ18O (�)

Chorhat section

Kh-11 Microbial laminite −1.1 −8.2











Dhanjoria section

Kh-20 Microbial laminite 1.5 −6.1









Markundi section







1.8� the average being 1.4�. The Markundi sam-ples exhibit δ13C values ranging between 0.5� to−1.2� and are highly depleted in δ18O. δ18O val-ues of the samples steadily become more negativefrom Dhanjoria (average −6.6�) to Chorhat (aver-age −8.2�) to Markundi (average −9.1�).

The formation of distinct clusters of data pointsfor samples collected from different localities sug-gests dominant control of paleogeography and localpore fluid on the isotope compositions. As all thesamples are mainly dolomitic clustering of pointsin separate groups it cannot be attributed to min-eralogical variation. Diagenetic alteration by mete-oric water is equivocal in the inferred depositionalsetting for the Fawn Dolomite. Meteoric fluidsof different compositions might have shifted orig-inal carbon and oxygen isotope values towardsdepleted values. δ13C and δ18O values of most of

the Dhanjoria samples remain restricted withina narrow range and possibly represent the orig-inal carbon and oxygen isotope signature. How-ever, δ18O values of the Markundi samples aremuch depleted and δ13C values of the same exhibita wider range. Modest alteration by isotopicallylighter fluids is suggested in the latter case. Goodcorrelation between δ13C and δ18O values fur-ther suggests possible meteoric diagenetic alter-ation of the dolomites whose original compositionwas very much similar to the Dhanjoria samples.Petrographic studies show several generations ofpore-filling cements within the peloidal dolomites(figures 4c, 4d), corroborating pervasive diageneticalteration. Ray et al (2003) recorded very highMn/Sr and 87Sr/86Sr and considered that the sam-ples belonging to the Fawn Dolomite are heav-ily altered. Paleogeographic setting of the Fawn


Table 6. Carbon and oxygen isotope composition for the constituent facies of the Rohtas Limestone.

Distance fromSample the base of

no. Facies the member δ13C (�) δ18O (�)

RL-31 Thin bedded limestone 152 −1.3 −5.7

RL-30 Flake conglomerate, clasts 151 −1.4 −8.7

RL-29 Calcarenite 149 −1.5 −6.9



RL-26 Flake conglomerates, clasts 140 −1.3 −7.2

RL-25 Non-edgewise conglomerate, clasts 90 −1.4 −7.4



RL-22 Flake conglomerates, clasts 15.5 −1.0 −5.8

RL-21 Flake conglomerates, clasts 5.6 −0.9 −6.0

RL-20 Ribbon limestone, non-seamed 23 −0.6 −5.2

RL-19 Ribbon limestone, seamed 23 −1.3 −6.8











RL-8 Ribbon limestone, non-seamed 8.5 −1.5 −5.6

RL-7 Ribbon limestone, seamed 8.5 −1.3 −7.6



RL-4 Ribbon limestone, non-seamed 3.5 −0.5 −6.3

RL-3 Ribbon limestone, seamed 3.5 −1.4 −7.7



Dolomite is the main cause of such an extensivealteration of primary isotope values. As a conse-quence of such extensive alteration possibly mostof the samples of Markundi and Chorhat plot out-side the primary box of Ray et al (2003); onlythe samples of Dhanjoria plot inside the primarybox.

5.4 Rohtas Limestone

Isotope analyses of 31 samples from all the faciesconstituting the Rohtas Limestone are given intable 6 and shown in figure 7(c). 20 samples belongto the 10 couplets of the seamed- and non-seamedbands. δ13C values of the samples remain restrictedin a narrow range from −1.5� to −0.3� (average

−1�) whereas δ18O values range from −5.2� to−8.7� (average −6.5�). δ13C versus δ18O cross-plot reveals two broad clusters of data points,one dominated by ribbon limestone facies (seamedpart) and the other consists of data points belong-ing mostly to ribbon limestone facies (but being thenon-seamed part). There is a modest correlationbetween δ13C and δ18O values (r2 = 0.6). Samplesof the calc-arenite facies and thin-bedded lime-stone facies also plot in the latter cluster. δ13C andδ18O ratios of 10 couplets taken from the seamedand non-seamed bands in ribbon limestone faciesreveal a systematic pattern of variation. Everyupward transition from seamed lamina to its over-lying non-seamed lamina is associated with enrich-ment in heavy carbon and oxygen isotopes. Within


Table 7. Carbon and oxygen isotope composition of the constituent facies ofthe Bhander Limestone.

Sample no. Facies δ13C (�) δ18O (�)

Maihar section

1/PLM Plane laminated mudstone −6.6 2.9







Satna section







Maihar section

1/PCS Planar and cross-stratified −5.1 −1.7





6/PCS Planar and cross-stratified −5.2 0.3


Satna section








Aber section


16/PCS Planar and cross-stratified −4 −1.8

17/PCS Planar and cross-stratified −4 −1.4

Maihar section

1/SM Stromatolite −5.2 0.5









Table 7. (Continued).

Sample no. Facies δ13C (�) δ18O (�)

Sajjanpur section




Aber section


13/SM Stromatolite −11 3.7

14/SM Stromatolite −8 1







21/SM Stromatolite −6 0.9


each couplet, the non-seamed band is enriched in18O by 0.7� to 2.1� (average 1.2�). The sametrend is also recorded in 13C (non-seamed band isenriched in 13C, average enrichment being 0.6�).The shifts are minor in magnitude but significantin their recurrent unidirectional nature and highfrequency.

The depositional setting for the RohtasLimestone was confined to shelf zones. None ofthe constituent facies represent emergence of thedepositional substrate above water level rulingout diagenetic resetting of isotope compositionsby meteoric water. The data points of the RohtasLimestone are, therefore, most likely to retain theoriginal isotope composition. The tight cluster ofthe data points and the narrow range of δ13C valuesfrom −1.5� to −0.3� suggest possible retentionof primary isotope signature in most of the samplesof Rohtas Limestone. Differential cementation,however, caused marginal shifts in δ13C and δ18Ovalues within the alternate bands of ribbon lime-stone facies (Banerjee et al 2005). The non-seamedlaminae in ribbon limestone facies are alwaysenriched in heavier isotopes of carbon and oxygenthan the seamed laminae. The modest correlationbetween δ13C and δ18O values can be explainedby genetically linked δ13C and δ18O variations ofthe seamed- and non-seamed laminae. Banerjeeet al (2005) attributed such repeated shifts inthe ribbon limestone facies to pre-compaction dif-ferential cementation of the non-seamed bands.Marine cements are always isotopically heavierthan the marine sediments; abundance of such

cements within the non-seamed bands can accountfor the observed enrichment of heavier isotopes inthe non-seamed bands (Banerjee et al 2005). Mostof the data points of the Rohtas Limestones plotwithin the primary box of Ray et al (2003) andthereby corroborate the fact that neither δ13C norδ18O values are affected by meteoric diagenesis.

5.5 Bhander Limestone

In a cross-plot of δ13C versus δ18O the data pointsbelonging to the three constituent facies of Bhan-der Limestone, e.g., plane laminated mudstone,stromatolites and planar and cross-stratified lime-stone plot as three separate clusters with very littleoverlap (table 7, figure 7d). Overall the data pointsof the Bhander Limestone exhibit a wide scat-ter (compare figure 7a–d). Figure 10 shows weaknegative correlation between δ13C and δ18O val-ues (r2 = 0.3). The data points of the stromatolitefacies exhibit larger scatter in comparison to thosebelonging to the planar and cross-stratified faciesand plane laminated mudstone facies. The samplesfrom the stromatolite facies exhibit consistentlypositive δ13C values ranging from 0.5� to 4.9�(average 3�). The samples of plane laminatedmudstone facies also display consistently positiveδ13C values ranging between 2.9� and 4.5� (aver-age 3.6�), whereas the planar and cross-stratifiedlimestone exhibit low negative to low positivevalues ranging between −1.8� and 0.3�. Aver-age δ18O values for stromatolite facies and forplane laminated mudstone facies are −7.7� and


−4.3�, respectively. Planar and cross-stratifiedfacies exhibit intermediate δ18O values rangingbetween −4� and −7.4� (average −5.1�).

The occurrence of separate clusters of datapoints belonging to different facies suggests thatthe carbon and oxygen isotope compositions areto a large extent controlled by the nature of thefacies. Amongst all the carbonate units of the Vin-dhyan Supergroup the data points of the BhanderLimestone represent a wide scatter (compare fig-ure 7a to d) and correlation between δ13C and δ18Ovalues of the samples is very poor. Moderate alter-ation of isotope compositions is thereby suggestedfor most of the samples of the Bhander Lime-stone. Depositional regime for the Bhander Lime-stone varied widely extending from distal offshoreto shoreface, the planar and cross-stratified faciesrepresenting the shallowest bathymetry. Therefore,the observed depletion of δ13C in planar and cross-stratified limestone is possibly because of extensivediagenetic alteration due to prolonged exposure insub-aerial condition. Sarkar et al (1998) reportedextremely depleted δ13C values in meteoric dia-genetic cements close to the unconformity sur-faces where the δ18O values of the same cementswere similar to that of the carbonate sediments.It is intriguing that the δ13C values are mostlydepleted in planar and cross stratified facies with-out much depletion of δ18O values in the same sam-ples. Depleted δ18O values of the stromatolite faciesare possibly caused by diagenetic resetting by iso-topically lighter fluids. Paleogeography of the stro-matolite facies varies considerably and allows thecarbonates to interact with pore fluids of differentcompositions. The stromatolite facies thus exhibitsmaximum scatter of data points out of the threefacies. In plane laminated mudstone facies occur-rence of enriched δ18O values could be due to thedolomitization effect (Land 1980). All the sam-ples belonging to the planar and cross stratifiedfacies plot outside and a few samples from stroma-tolite facies and plane laminated mudstone faciesplot inside the primary box of Ray et al (2003). Itcan be mentioned that meteoric diagenetic alter-ation of isotope values is already anticipated for thesamples of planar and cross stratified facies basedon paleogeographic setting and their effect on iso-tope ratios. Plot of the data points of the planarand cross stratified facies outside the ‘primary box’further supports the concept of altered isotopesignature within the facies.

6. Conclusions

The carbonate facies of the Vindhyan Super-group represents a wide spectrum of depositional

processes. Supratidal, shoreface, intertidal, shal-low subtidal, lagoonal, estuarine, inner and outershelf carbonates characterise wide variations infacies and paleogeography. Carbon and oxygenisotope compositions of the constituent facies ofthe different carbonate units frequently form clus-ters of data points with moderate to negligibleoverlap. Original carbon and oxygen isotope com-positions of many of the carbonate samples arealtered because of diagenetic resetting by isotopi-cally lighter fluids. However, carbon and oxygenisotope compositions of the Rohtas Limestone pos-sibly remained unaltered because depositional sub-strate was not exposed to allow percolation ofmeteoric fluids through the carbonate sediments.On the contrary, most of the samples of Kajra-hat Limestone (intertidal to supratidal origin) rep-resent depleted δ18O values possibly because ofdiagenetic resetting by isotopically lighter fluids.Scatter of the data points in the latter possiblyrelates to duration of exposure of the depositionalsubstrata. Both δ13C and δ18O values of the othertwo carbonate units, namely Bhander Limestoneand Fawn Dolostone have been altered consider-ably because of meteoric diagenetic effect. Besidesthe depositional setting, mineralogy of the carbon-ates are also responsible for the observed scatterin the data points and formation of clusters; ele-vated δ18O values have been noticed in a few faciesconsisting essentially of dolomites. Stable isotopecompositions possibly remain close to primary val-ues when early diagenesis takes place in marinesetting, e.g., in the case of Rohtas Limestone.Chemostratigraphic significance of carbon and oxy-gen data is thus reduced in the case of very shallowmarine carbonates, which are likely to be flushedby meteoric water. Our interpretations regardingnear primary and altered isotope signatures matchclosely with the inferences drawn by Ray et al(2003).

Acknowledgements

Authors are thankful to P K Bose of Jadavpur Uni-versity for his constant encouragement. We thankMr. R A Jani for help in isotope analysis. Theauthors are thankful to Graham Shields and KaremAzmy for their critical comments and suggestionson an earlier version of the manuscript. SB isthankful to the Department of Science and Tech-nology, Govt. of India for financial support.

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