Chemical Geology (Isotope Geoscience Section), 52 (1985) 337-348 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

337

IN SEARCH OF THE ORIGIN OF CEMENT IN SlLlClCLASl-IC SANDSTONES: AN ISCTTOPIC APPROACH

PRODIP K. DUTTA

Department of Geography and Geology, Indiana State University, Terre Haute, IN 47809 (U.S.A.)

(Received January 20,1984; revised and accepted October 5, 1984)

Abstract

Dutta, P.R., 1985. In search of the origin of cement in siliciclastic sandstones: An isotopic approach. Chem. Geol. (Isot. Geosci. Sect.), 52: 337-348.

In siliciclastic sediments only a minor amount of cement is internally derived. This implies that an external source has to be sought in sandstone diagenesis. Controversy exists about the external source of cement and the nature of the aqueous solution, viz. meteoric, hydrothermal and marine, and/or de- watering of shale. Isotope geochemistry may prove useful in solving this controversy.

The oxygen isotopic compositions of neoformed clay minerals are related to the isotopic composi- tion of the coexisting aqueous solution. Aqueous solutions of diverse origin such as meteoric, hydro- thermal and marine, and dewatering of shale, in general, bear characteristic isotopic signatures. Thus, the 6 180-values of authigenic clay minerals may be used to identify the nature of the original aqueous solution, provided the temperature of the environment is known and the extent and the nature of isotopic exchange between the minerals and fluids following crystallixation can be evaluated.

Oxygen isotopic compositions of early authigenic clay minerals in Permo-Wiic Gondwana sand- stones of Peninsular India have been used to infer the cement source. Sandstones of Salm&an age have an average 6*‘0-value of +5.046,, whereas a relatively high average 6 180-value of +13.2’& is observed in sandstones of Rhaetic age. 6 180-values of sandstones ranging in age between Sakmarian and Rhaetic have values between +5.0 and +13.2%,. This gradual increase of 6 ‘80-values with decreasing age shows a strong correlation with the changing latitudinal location of the sample site from a position around 60’S during Sakmarian time to a posltion around 38% during Rhaetic tie. The changing pattern of oxygen Isotopic composition of authigenic clay has been interpreted as a result of corresponding change in isotopic com- position of meteoric water due to the northerly drift of the Indian plate during Gondwana sedimentation. This suggests that the aqueous solution, involved in the early dlagenesis of Gondwana sandstones, is of meteoric origin.

1. Introduction

Cathodohuninescence studies have demon- strated that pressure solution, as an internal source of authigenic cement, cannot ac- count for the bulk reduction of porosity in sandstone by cementation in siiiciclastic sediments (Sippel, 1968; Sibley and BIatt, 1976; Land and Dutton, 1978). However,

a certain amount of cement can be derived internally, especially during deep burial diagenesis, and can be identified by con- ventional petrographic study. A significant part of the cement, in most cases, is ex- ternally derived from sources outside the sediment system. In sandstone diagenesis, therefore, an external source of authigenic cement has been sought. There is no con-

0168-9622/85/$03.30 0 1985 Elsevier Science Publishers B.V.

338

sensus of opinion regarding the nature of this source (Bjdrlykee, 1979; Blatt, 1979; Boles and Franks, 1979; Land and Dutton, 1979).

Aqueous solution from dewatering of shale due to compaction and dehydration has been suggested as a possible source of cement, particularly in marine environments where the shale/sandstone ratio is high (Carrigy and Mellon,. 1964; Boles and Franks, 1979; Land and Dutton, 1978). Generally it is accepted that - lo5 cm3 of aqueous solution of average composition is required to cement each cubic centimeter of pore volume of sediments (Bjerlykee, 1979; Blatt, 1979; Land and Dutton, 1979; Dutta, 1983). Bjdrlykee (1979) argued that such an enormous volume of pore solution can- not be obtained from dewatering of shale and the water must necessarily be meteoric in origin, percolating downward through the sediments under a hydraulic head. A similar view was expressed by Blatt (1979), Dutta (1981) and Dutta and Suttner (1985). Another remotely possible, but volumetrical- ly less significant, source of water could be hydrothermal fluids flowing upwards driven by a convective flow system.

It is possible that all of these sources, individually or collectively, supply the aque- ous solution necessary for cementation. In different environments and tectonic settings the relative importance of each of these sources would differ. Conventional petrographic techniques have limitations in identifying the external source of cement. Oxygen isotope geochemistry may prove useful in answering the question about the origin of cement in siliciclastic sedi- ments.

2. Previous work

The isotopic composition of silicate miner- als in sediments, precipitating from an aque- ous solution, is determined by: (1) the isotopic composition of the aqueous solu- tion with which the minerals are in equi-

librium; (2) the isotopic fractionation factors between the water and the coexisting miner- als; and (3) the temperature of the environ- ment (Dutta, 1983). Clay minerals appear to be in isotopic equilibrium with the co- existing water in sedimentary and diagenetic environments (Savin and Epstein, 1970). Once formed the clay minerals retain their isotopic composition, unless they are sub- jected to an appreciable change in tempera- ture and/or chemical and mineralogical changes (0’Nei.l and Khan&a, 1976; O’Neil, 1979). In laboratory experiments, it has been demonstrated that isotopic exchange of oxygen between water and structural sites in kaolin&e and illite is not significant at sedimentary and diagenetic tempera- tures (O’Neil and Kharaka, 1976). Signifi- cant oxygen isotope exchange is observed in kaolinite at 360°C involving mineralogical changes. Observed isotopic exchange in montmorillonite is, presumably, due to the presence of interlayer water (O’Neil and Kharaka, 1976). Isotopic reequilibra- tion of clay minerals at elevated tempera- ture during deep burial diagenesis has been demonstrated particularly in argillaceous sediments (Eslinger and Savin, 1973; Yeh and Savin, 1977; Eslinger et al., 1979). However, below 150°C the isotopic exchange between clay and water is negligibly slow (Eslinger et al., 1979).

Lawrence and Taylor (1971) demon- strated that 6D and 6 I80 of clay minerals and hydroxides in equilibrium with meteoric water in Quatemary soils of the U.S.A. show a strong correlation with 6D and S ‘so of corresponding meteoric water. This would suggest that if the isotopic composition of any clay mineral is known as well as the ambient temperature in which it was formed, then the source of water can be identified from its isotopic composition, provided the fractionation factor between the clay mineral and the water at that temperature is also known.

Since the isotopic composition of marine water is very distinct and can easily be identi-

fied in most cases, minerals precipitated from such a source will have distinct isotopic fingerprints. Hydrothermal waters have diverse origins and wide ranges of oxygen isotopic compositions. Nevertheless, it has been possible to identify the source of such water by their isotopic signatures (Hall et al., 1974; Ohmoto and Rye, 1974; O’Neil and Silberman, 1974; Sheppard and Taylor, 1974; Taylor, 1974). Thus, the isotopic composition of authigenic cement could be used to trace back the origin of the water involved in cementation.

The objective of this paper is to confirm the hypothesis described above to identify the source of cement in Gondwana sand- stones of the Raniganj Basin, India. The source of cement was identified by con- structing a. detailed diagenetic history and by determining the oxygen isotopic com-

80”SO’E 87°00+E

29oSO’N

2S040’N

339

position of early authigenic clay minerals in Gondwana sandstones. The oxygen isotope data were then related to the most probable source.

3. Diagenetic history of Gondwana sandstones

The Gondwana succession in the Raniganj Basin in India was selected for this study (Fig. 1). This sedimentary sequence is charac- terized by 3200 m of Permo-Triassic sedi- ments derived essentially from Precambrian crystalline sources. The sediments were deposited within a fluvial system in a block- faulted intracratonic basin. During the Gond- wana sedimentation, the axis of the basin shifted southward with time, which resulted in an onlap of a part of the younger sedi- ments on the crystalline basement (Fig. 1). As a result, the entire thickness of the Gond-

87’10’E

m MAHADEVA FM.

fJJlJ PANCHET FM.

a RANIGANJ FM.

IZZ BARREN MEASURES m BARAI<AR FM.

m TALCHIR FM.

cJ;1 ARCHAEAN FM.

- - FAULT

88O50’E 87’00’E 87’1 O’E

Fig. 1. Geological map of the Raniganj Btin showing the different formational units (simplified after Kriinen, 1960). The bet map ehowe the location of the Raniganj Basin in India.

340

cz L 1 1 I

IPFTRO-+PFTROFA( IFS II +TRo-+ PTTROFA( IIS IV -----JcpFTR0 +-PFTRO-4 , FACKS , , tA( irs ( ‘1 I I Ill I

I I I ‘TALCHIRi

::: BARAKAR FM -$BARRd+RAK,GAN, FM ’ FM ; I MFASIJRI-S I

-‘N&-PAN< HFl FM df~q I I I

, I MAHADFVA I

Fig. 2. Gondwana succession in the Raniganj Basin, India, showing the different formations and petrofacies. Dif- ferent petrofacies show a gradual change in 6’sO-values of early authigenic clay minerals and the corresponding change in latitudinal locations of the Raniganj Basin during Gondwana sedimentation. The large symbols indicate the mean 6’*0-values of each petrofacies. The lower and upper hexagonal symbols indicate the locations of the Raniganj Basin during Salsmarian time (petrofacies I) and R&tic time (pekrofacies VI), respectively.

wana succession in the Raniganj Basin shown in Fig. 2 is never present at a particular section, and the maximum burial depth of the basal part of the section was - 3960 m. In a craton the corresponding temperature at this depth will be in the vicinity of - 100°C. The sandstones within this succession show wide variations in composition, ranging from arkose to quarts arenite. On the basis of lithological and paleontological attributes the Gondwana succession in the Raniganj Basin has been formally divided into six units which are termed formations (Krishnan, 1960). Subsequently, based on petrographic criteria alone, Dutta (1983) divided the same succession into six petrofacies (Fig. 2). Petrofacies III, an argillaceous unit, is not included in this study.

Authigenic cement in the Gondwana sand- stones can broadly be divided into two groups with respect to their time of forma- tion. The first group is termed “early dia-

genetic cement”. The minerals in this group were first to form and occur as overgrowth, pore-lining and pore-filling cement, and in- clude quartz, kaolinite, smectite and chlorite. None of these minerals appears to have any genetic relation&p with the detrital minerals, nor do they show any alteration- replacement relationship with the detrital minerals or among themselves. These miner- als seem to have been formed by precipita- tion from pore solution derived exter&ly outside the sediment system. Thermodynamic considerations and hydrologic constraints imply that the formation of &se early authlgenic minerals was accomplished dur- ing shallow burial, soon after sand deposi- tion (D&a, 1983).

The second group of minerals, referred to as “late diagenetic cement”, was formed - 42 Ma after the sediments were deposited. This dirtgenetic age was determined from K-Ar dating of late &genetic ill&e (D&a,

341

1983). The late diagenetic minerals, in addi- tion to illite, are carbonates of Ca, Mg and Fe, plagioclase, Fe-oxide, and a second generation of quartz. These minerals do show replacement-alteration relationships with the detrital and early authigenic miner- als. Mass-balance calculations show that the bulk of the chemical constituents neces- sary to form these minerals was derived internally from the dissolution of detritsl minerals like quarts, feldspars, muscovite and biotite, as well as from the early au- thigenic quartz and kaolinite. However, a certain amount of Fe, Mg, Ca and Mn was derived externally during this late diagenetic stage (Dutta, 1983).

The early diagenesis in the Gondwana sandstones appears to have a simple history during which the formation of cement is related only to the externally derived aque- ous solution. The depth of burial was rela- tively shallow and the temperature of the diagenetic environment was relatively close to the earth’s surface temperature (Dutta, 1983). The minerals precipitated from such a solution would also be expected to have a simple chemical history. Based on this assumption., the early authigenic clay miner- als were selected for oxygen isotopic analyses.

4. Analytical technique

Interstitial authigenic clay minerals in the <2-pm fraction in the Gondwana sand- stones were separated from coarse detritus and the rest of the authigenic minerals. Less than 2cm long chips of sandstones were tumbled in a. glass jar filled with dis- tilled water and four solid cylindrical tubes made of epoxy, for - 45 min. The resultant slurry was centrifuged repeatedly to separate the desired <2+m fraction.

A part of the extracted clay was used for X-ray identification of the clay’minerals and to determine the relative amount of each mineral species in each sample. About 10 mg of the remaining sample were used for isotopic analysis. Clay samples were

dried in a vacuum chamber heated to 110°C for 24 hr. After drying, samples were loaded immediately into the sample retort and evacuated. After evacuation, the samples were heated at 2OO*C for - 45 min. ,Any gas liberated was evacuated again. Next bromine pentafluoride was introduced into the sample retort and the sample was heated for - 15 hr. at 600°C. The extracted oxygen was then reacted with a carbon disc at a high temperature (550-6OO”C) to produce carbon dioxide. The oxygen isotope com- position of the sample relative to SMOW (standard mean ocean water) was deter- mined by analyzing the carbon dioxide in a Nuclide@ 6-60 triple isotope ratio mass spectrometer. The oxygen extraction tech- nique used in this work is very similar to that of Taylor and Epstein (1962) and Clay- ton and Mayeda (1963). The analytical un- certainty is better than 0.1%~~.

5. Analytical results

The S 180-values of 24 samples analyzed for this study sre given in Table I. The data are represented in two bivariant graphs: Fig. 2 shows the gradual change in 6180- values and the corresponding change in latitudinal location of the Raniganj Basin, assuming a continuous movement of the Indian plate toward the equator during Gondwana sedimentation. Fig. 3 shows the variations of 6 180-values with the absolute age of the samples. Absolute ages of sam- ples have been calculated on the basis of the rate of sedimentation during Gondwana time.

It is apparent in both figures that there is an overall systematic change showing increasing 6 180-values with the gradual shifting of the Raniganj Basin towards the equator and increasing 6 ‘so-values with decreasing age of sediments (Figs. 2 and 3). Lowest 6 ‘*O-values are observed in sam- ples from petrofacies I (mean 6 ‘so = +5.0$&) and relatively high values in petrofacies VI (mean &I80 = +13.2%). The mean S 180-

342

TABLE I

Oxygen isotope data of different petrofacies

Petrofacies Sample No.

Age of Mineral assemblage 6 I80 Average 6 InO sample (%,I of each Ofa)

mineral relative amount petrofacies

(%) VM

VI

V

RSP-1 195 RSP-2 196 RSP-6 197 RSP-8 198 RSP-10 200

RP-19 207

m-14 209

RP-9 215

W-1 8 216

IV RR-3

RR-9

228

233

RR-l 6 237

RR-20 241

RBM-3 243

RBM-4 244

II RB-3

RB-10

RB-16

RB-21

RB-25

I RT-4

RT-3

257

258

262

266

267

272

273

kaolinlte 100 kaolinite 100 kaolinite 100 kaolinite 100 kaolinite 100

kaolinite 61 smectite 39 kaolinite 53 smectite 47 kaolinite 33 smectite 67 kaolinite 46 smectite 51 illite 3

kaolinite and chlorite

illite kaolmite

and chlorite illite kaolinite

and chlorite illite kaolinite

and chlorite illite kaolinlte illits kaolinite illite

98 2

98 2

96 4

94 6

42 58 37 63

kaolinite 17 illite 83 kaolinite 86 illite 14 kaolinite 10 illite 90 kaolinite 73 illite 27 illite 100

kaoliite and chlorite

illite kaolinite

and chlorite smectite illite

78 22

77 5

18

+14.9 +13.2 +14.9 +13.2 +11.5 +11.4

+6.5 +10.4

+10.7

+9.6

+14.9

+8.1 +6.7

+5.7

+5.1

+7.3

+6.1

+7.3

+7.5 +8.1

+8.6

+8.2

+9.9

+6.3

+4.3 u5.00

+3.5

343

TABLE I (continued)

280

270

? 250

5

g 250

E

z 240

z

z 230

“0

$ 220

c” 3

0

210

4 200

190

Petrofacies Sample Age of Mineral assemblage 6 I80 Average 6 ‘*O No. sample (%o) of each

(Ma) mineral relative

amount petrofacies

(%I uo,)

RT-1 214

RT-7 278

kaolinite and chlorite

illite kaolinite

and chlorite smectite

59 +5.8 41

76 +6.3 24

(WHITEHORSE)

PETROFACIES VI

PETROFACIES V

PETROFACIES IV

PETROFACIES II

PETROFACIES I

8 18

C VALUE OF MODERN METEORIC WATER

(IDAHO-MONTANA)

\

DAY

0 -- OO B

b 0

_---

0

A

\

A

A

A

A ?

-,60

- 58

- 56

- 54

- 52

48

42

(COLORADO)

-30 -20 -10 6 18

0 10 20 0 (per mill

Fig. 3. Oxygen isotope compositions of early authigenic clay in different petrofacies show gradual change with the absolute age of the sediments. Expkrnation of Zetter aymboh: A = best-fit line representing the oxygen isotope values in petrofacies IV-VI; B = best-fit line representing the oxygen isotope values in petrofacies I and II; C = presenbday oxygen isotope line for meteoric water (based on data from Dansgaard, 1964; Lawrence and Taylor, 1971); D = estimated oxygen isotope line for formation water during the early diagenesis of different petrofacies. For further explanation see text. Latitudinal locations of water samples from North America and that of the Rani- ganj Basin during.the formation of different petrofacies are also shown.

344

values for petrofacies II, IV and V are +8.1, +6.7 and +10.4%0, respectively. The in- creasing trend of S ‘*O-values in petrofacies IV and V fit well with the decreasing age of the sediments. The only exception to this pattern is shown by petrofacies II where the mean 6 “‘O-value is higher than the suc- ceeding petrofacies IV. In Fig. 3, the mean S 18O-values of petrofacies IV-VI fall along a straight line (line A). A second line (line B) drawn parallel to the line A1 roughly connects the mean values of petrofacies I and II, indicating a shift towards 6 “0 enrichment in these two petrofacies com- pared to the younger sediments.

Petrofacies I, II and two samples at the base of petrofacies IV contain considerable amounts of late diagenetic illite (Table I). Extensive dissolution of detrital and early authigenic minerals and formation of late diagenetic minerals have been observed in these sediments. It has been mentioned earlier that the maximum burial temperature at- tained in the oldest sediments in the Raniganj Basin was in the vicinity of u 100°C. On the other hand, the samples in petrofacies V and VI have no or very little late diagenetic minerals including illite. This part of the sequence does not show dissolution of any detrital or early authigenic minerals ex- cept for the alteration of biotite and forma- tion of Fe-oxide. The maximum burial temperature of petrofacies V and VI probably ranged between 75°C (basal part of petro- facies V) and 40°C (top part of petrofacies VI). Except for two samples at the base, there is very little illite in petrofacies IV. However, a certain amount of late diagenetic minerals other than illite and .dissolution of detrital and early authigenic minerals have been observed in this petrofacies. The maximum burial temperature in petrofacies IV was less than 100°C.

6. Discussion

The overall trend of increasing d “O-values with decreasing age and latitudinal shift

of the Raniganj Basin can be explained in two different ways: (a) equilibration of the early authigenic minerals in different petrofacies with the ever changing meteoric waters having different isotopic composi- tions as the Indian plate shifted towards the equator; and (b) modification of isotopic values due to isotopic exchange between the early authigenic minerals with the post- early diagenetic formation water. In the rest of this discussion these two aspects will be examined, and a probable explana- tion for the isotopic variations in different petrofacies will be offered.

It has been demonstrated that the forma- tion waters in many sedimentary basins are of meteoric origin (Clayton et al., 1966). However, the isotopic composition of this meteoric water is modified with increasing burial depth. Samples with highest salinity and burial temperature tend to possess water most enriched in ‘*O and vice versa (Clayton et al., 1966; Kharaka et al., 1973). Generally, the 8180-values of formation water increase at a rate of l-2%& per 10°C of burial temperature (Clayton et al., 1966).

The Raniganj Basin has been lying with- in the tropics for more than 100 Ma (Dietz and Holden, 1970). The formation water in Gondwana sediments during this time would be isotopically heavier compared to the formation water during early diagenesis due to both lower latitudinal location of the basin and increased burial depth. Any isotopic exchange during this time would not only increase the 6180-values of the clay minerals but also would have greatly reduced the wide variations that are observed in 6 ‘sOvalues in different petrofacies. Isotope data in this study do not show any such trend, indicating that the oxygen isotopic exchange was not a major phenomenon after the clay minerals were formed during early diagenesis. This is m conformity with the laboratory and field evidences that the isotopic exchange between clay and water is negligible in sedimentary and diagenetic environments (Eslinger and Savin, 1973;

O’Neil and Kharaka, 1976; Yeh and Savin, 1977; Eslinger et al., 1979). A relatively good fit of the data along line A (Fig. 3) also suggests #at the isotopic exchange had been negligible at least in petrofacies IV-VI.

Inferred 6 180-values, assuming only frac- tionation, of the formation water during early diagenesis of petrofacies V and VI indicate that the early authigenic clay miner- als (kaolinite and smectite, both of which have the same mineral-water fractiona- tion factor) were precipitated from aqueous solutions whose 6 ‘*O-values were -16.2 and -13.4&l, respectively. These 6 “‘O- values in petrofacies V and VI correspond to the average 6 180-values of presentday meteoric waters of Idaho-Montana (-16.0%) and Colorado (-13.05Go) (Table II). Water samples from Colorado came from an area lying between 37” and 41% latitudes (Lawrence and Taylor, 1971). Paleogeographic reconstructions of the Indian plate during the formation of petrofacies VI places Raniganj Basin around 38% (Diets and Holden, 1970), a comparable position to that of the location of Colorado with

TABLE II

345

respect to the latitudinal value. Since the early diagenetic episode took place within a few million years after deposition, it follows that the early authigenic clay minerals in petrofacies VI must have equilibrated with the then meteoric water. Similar arguments hold good for petrofacies V. Calculation of 6 ‘sOvalues of formation water in petro- facies IV, during early diagenesis, involves a little uncertainty because of the presence of chlorite in the sample. Taking the kao- linite-water fractionation factor (0~ = 1.027; Savin and Epstein, 1970) for the entire early authigenic mineral assemblage, a 6 ‘*O-value of -19.9% is obtained for the formation water during early diagenesis of petrofacies IV. Considering the relatively high propor- tion of kaolinite in petrofacies IV, the above value is probably a reasonable approxima- tion.

In Fig. 3, the S 180-value of present-day meteoric water from Whitehorse (Dansgaard, 1964) and average 6180-values from Idaho- Montana and Colorado areas (Lawrence and Taylor, 1971) are plotted against their respec- tive latitudinal locations and a best-fit line has been obtained (line C). This line may

Latitudinal locations of three isotope sample sites in North America and of the Baniganj Basin during the sedi- mentation of different petrofacies and the respective oxygen isotope composition of modern-day meteoric water for North American locations and the formation water during early diagenesls of Gondwana sediments

Place name/ petrofacies

Latitudinal location of 6 ‘8a-values of Calculated 6 **O-values isotope sample site in present-day of formation water North America and of meteoric water during early dlagenesis the Baniganj Basin during (O&) of Gondwana sandstone the sedimentation of Rw) different petrofacies

Whitehorse, Yukon Terr., Canada 60’36’N -22.0 Idaho-Montana, U.S.A. 45% -16.5 Colorado, U.S.A. 39% -13.0

Petrofacies I 60’S -26.4 II 55”3O’S -23.4 IV 49”3O’S -19.9 v 43”s -16.2 VI 38”s -13.4

346

be taken as the modemday 6 l*O line for meteoric water. The Idaho-Montana and Colorado locations have been obtained by taking the mean latitudinal locations of the sample sites (Table II). In the same figure another line (line D) has been drawn, connecting the 81s0-values of estimated early diagenetic formation water for petro- facies IV-VI (Table II). Latitudinal loca- tions of the Raniganj Basin during the deposi- tion of petrofacies I and VI are known. Assuming a uniform movement of the Indian plate during this time, the latitudinal loca- tion of the Raniganj Basin at any particular time can also be reasonably calculated. The age of the sediment and the corresponding latitudinal location of the basin at that particular time is obtained -from Fig. 3. The same figure shows close correlation between line D, representing the S ‘so-values of formation water during the early diagenesis of petrofacies IV-VI and the 6l*O line of modem-day meteoric water (line C). This indicates that the early authigenic clay minerals in petrofacies IV-VI were formed from an aqueous solution of meteoric origin.

It has been mentioned earlier that the 6 ‘*O-values of petrofacies I and II show an enrichment compared to the rest of the suc- cession. This observation and the presence of appreciable amounts of late diagenetic illite suggest that there is probably some noise in the data, making interpretation difficult. The 6 ‘*O-values of petrofacies II are anomalously high and do not fit with- in the overall pattern of increasing 6’*0- values with decreasing age of sediments. This anomaly of higher 6 ‘*O-values in petro- facies II compared to petrofacies IV may be due to: (a) equilibration of early au- thigenic clays with isotopically heavier water compared to the formation water in petrofacies IV during early diagenesis, or (b) due to the presence of a significant amount of late ,diagenetic illite in the sam- ple, or (c) isotopic exchange of early au- thigenic clay with formation water at a greater burial depth during late diagenesis.

Since the early diagenetic history of the entire Gondwana succession is similar, the possibility of equilibration with isotopically heavier water in petrofacies II compared to petrofacies IV at shallow depth during early diagenesis may be ruled out. The pres- ence of late diagenetic illite as impurities would add noise to the early authigenic data. Interestingly, however, a pure illite sample (RB-25, Table I) has the lowest 6 ‘*O-value (+6.3%0) in this petrofacies. Samples having higher kaolin& content (RB-I 0 and RB-21) have higher 6 ‘*O-values (Table I). Therefore, it appears that the presence of late diagenetic illite would lower the 6 ‘*O-values of samples rather than in- crease their values. This is reasonable since illite is a late diagenetic mineral which formed at a higher burial temperature and has a lower fractionation factor (ar*-wakr = 1.023; Faure, 1977) compared to the kaolinite-water system (a = 1.027; Savin and Epstein, 1970). Thus it seems that there may be some iso- topic exchange in petrofacies II as well as in petrofacies I with postearly authigenic formation water at elevated temperature due to burial.

Considering a similar early diagenetic history for the entire Gondwana succession, it is reasonable to assume, as a first approxi- mation, that the average 6 ‘*O-values of petrofacies I and II would lie on line A, if there had been no post-crystallization isotopic enrichment. This approximation would give S’*O-values of +1.2 and +3.2srO, for petrofacies I and II, respectively. Using these values, the 6 ‘*O-values of formation water in petrofacies I and II during early diagenesis were found to be -26.4 and -23.4%, respectively (Fig. 3; Table II). In determining the isotopic composition of formation water, the fractionation factor for the kaolinite-water system has been taken since kaolinite is the only early au- thigenic clay mineral in petrofacies II and also constitutes a part of the early authigenic clays in petrofacies I. The same fractionation factor (“kaolinWater = 1.027) has been

347

used in estimating the isotopic composi- tion of formation water for all the petrofacies during early diagenesis. This causes the average 6 l*O-values of all the petrofacies to plot close to line D, a line parallel to line A (Fig. 3). Line D is reasonably close to the 6 I80 line of the present-day meteoric water (line C). The closeness of these two lines suggests that the formation water during early diagenesis of Gondwana sedi- ments was of meteoric origin.

7. Conclusions

Based on a detailed reconstruction of the diagenetic history and determination of oxygen isotopic composition of authigenic clay minerals the following conclusions can be made:

(1) The oxygen isotopic composition of authigenic cement can be used to identify the nature of water involved in diagenesis in certain geological settings, particularly in continental sediments, provided the ex- tent of post-crystallixation isotopic exchange can be evaluated independently.

(2) Isotopic exchange probably takes place in kaolinite even at a burial tempera- ture around 100°C for geologically old sedi- ments of Permian age.

Acknowledgements

Funds for partially supporting the field work in India were provided by the Depart ment of Geology, Indiana University, Bloo- mington. I am thankful to the Director General, Geological Survey of India, for permission to collect samples and for provid- ing logistics in the field. Encouragement for this research work by Dr. Lee J. Suttner is gratefully acknowledged. I am also thankful to A. Basu, E. Merino, D. Bhattacharya, R. Howe, N. Shaffer and A. Swenson for their help in this work. The manuscript has bene- fitted much from reviews by two unknown reviewers. I am grateful to both of them.

References

Bj&lykee, K., 1979. Discutwion of Cementation of samfstones, by J.R. Boles and S.G. Franks. J. Sediment. Petrol., 49: 1368-1359.

Blatt, H., 1979. Diagenetic processes in sandstones. In: P.A. Scholle and P.R. -luger (Editors), Aspect8 of Diagenesis. Sot. Econ. Paieontol. Mineral., Tulsa, Okla., Spec. Publ. No. 26, pp. 141-157.

Boles, J.R. and Franks, S.G., 1979. Cementation of sandstones - Reply to discussion of Cementa- tion of sandstones, by J.R. Boles and S.G. Franks. J. Sediment. Petrol., 49: 1362.

Carrigy, M.A. and Mellon, G.B., 1964. Authigenic clay mineral cements in Cretaceous and Tertiary sandstones of Alberta. J. Sediment. Petrol, 34: 461-472.

Clayton, R.N. and Mayeda, T.K., 1963. The use of bromine pentafluoride in the extraction of oxygen from oxide8 and siiicates for isotopic analysis. Geochim. Cosmochim. Acta, 27: 43-52.

Clayton, R.N., Friedman, I., Graff, D.L., Mayeda, T.K., Meents, W.F. and Shimp, N.F., 1966. The origin of saline formation waters, 1. Isotopic com.position. J. Geophys. Res., 71: 3869-3882.

Croweii, J.C. and Frakes, L.A., 1970. Ancient Gond- wana giaciations. In: S.H. Haughton (Editor), Proceedings and Paper8 of 2nd Gondwana Sym- posium, South Africa, CSIR (Count. Sci. Ind. Res.), Pretoria, pp. 469-476.

Dansgaard, W., 1964. Stable isotopes in precipita- tion. Tellus, 4: 436-460.

Diet+ R.S. and Holden, J.C., 1970. Reconstruction of Pangaea: Breakup and dispersion of continents - Permian to present. J. Geophys. Res., 76: 4939-4956.

Dutta, P.K., 1981. Early authigenic clay minerale in sandstones a8 paieoclimatic indicators. Geol. Sot. Am., Annu. Meet., Cincinnati, Ohio, p. 443 (abstract).

Dutta, P.K., 1983. The role of climate in the evolu- tion of detritai and authigenic mineralogy in sandstone8 from the Gondwana Supergroup, India. Ph.D. Thesis, Indiana University, Blooming- ton, Ind., 169 pp.

Dutta, P.K. and Suttner, L.J., 1986. Aiiuviai sand- stone composition and paleociimate, Part II. Authigenic mineralogy. J. Sediment. Petrol. (in re- view).

Esiinger, E.V. and Savin, S., 1973. Oxygen isotope geochemistry of the buriai metamorphic rock8 of Belt Supergroup, Glacier National Park, Mon- tana. Geol. Sot. Am. Bull., 84: 2649-2560.

Eslinger, E.V., Savin, 6. and Yeh, H., 1979. Oxygen isotope geothermometry of diageneticaily altered shales. In: P.A. Scholie and P.H. S&luger (Edi-

348

tors), Aspects of Dlagenesis. Sot. Econ. Paleontol. Mineral., Tulsa, Okla., Spec. Publ. No. 26, pp. 113-124.

Faure, G., 1977. Principles of Isotope Geology. Wiley, New York, N.Y., 464 pp.

Hall, W.E., Friedman, I. and Nash, J.T., 1974. Fluid inclusion and light stable isotope study of the Climax molybdenum deposits, Colorado. Econ. Geol., 69: 884-901.

Kharaka, Y.K., Berry, A.F. and Friedman, I., 1973. Isotopic composition of oil-field brines from Kettleman North Dome, California and their geological implications. Geochim. Cosmochim. Acta, 37: 1899-1908.

Krishnan, MS., 1960. Geology of India and Burma. Higginbothams, Madras, 604 pp.

Land, L.S. and Dutton, S.P., 1978. Cementation of a Pennsylvanian deltaic sandstone: Isotopic data. J. Sediment. Petrol., 48: 1167-1176.

Land, L.S. and Dutton, S.P., 1979. Reply to discus- sion of Cementation of sandstones, by J.R. Boles and S.G. Frank. J. Sediment. Petrol., 49: 1369- 1361.

Lawrence, J.R. and Taylor, H.P., 1971. Deuterlum and oxygen-18 correlation: clay minerals and hydroxides in Quaternary soils compared to meteoric waters. Geochim. Cosmochim. Acts, 35: 993-1003.

Ohmoto, H. and Rye, R.O., 1974. Hydrogen and oxygen compositions of fluid inclusions in the Kuroko deposits, Japan. Econ. Geol., 69: 947- 963.

O’Neil, J.R., 1979. Stable isotope geochemistry in rocks and minerals. In: E. Jiiger and J.C. Hun- ziker (Editors), Lectures in Isotope Geology. Springer, Berlin, pp. 235-263.

O’Neil, J.R. and Kharaka, Y.K., 1976. Hydrogen and oxygen isotope exchange reactions between clay minerals and water. Geochim. Cosmochim. Acta, 46: 241-246.

O’Neil, J.R. and Silberman, M.L., 1974. Stable isotope relations in epithermal Au-Ag deposits. Econ. Geol., 69: 902-909.

Savin, S. and Epstein, S., 1970. The oxygen and hydrogen isotope geochemistry of clay minerals. Geochim. Cosmochim. Acta, 34 : 25-42.

Sheppard, S.M.F. and Taylor, Jr., H.P., 1974. Hy- drogen and oxygen isotope evidence for the origins of waters in the Boulder batholith and the Butte ore deposits. Econ. Geol., 69: 926- 946.

Sibley, D.F. and Blatt, H., 1976. Intergranular pres- sure solution and cementation of Tuscarora orthoquartzite. J. Sediment. Petrol., 46: 881- 896.

Sippel, R.F., 1968. Sandstone petrology, evidence from luminescence petrography. J. Sediment. Petrol., 38: 530-554.

Taylor, Jr., H.P., 1974. The application of oxygen and hydrogen isotope studies to problems of hydrothermal alteration and ore deposition. Econ. Geol., 69: 848-883.

Taylor, Jr., H.P. and Epstein, S., 1962. Relationship between 1sO/160 ratios in coexisting minerals of igneous and metamorphic rocks. Geol. Sot. Am. Bull., 73: 461-480.

Yeh, H. and Savin, S., 1977. The mechanism of burial diagenetic reactions in argillaceous sedi- ments, 3. Oxygen isotopic evidence. Geol. Sot. Am. Bull., 88: 1321-1330.