Diagenesis of quartz in the Upper Proterozoic Kaimur Sandstones, Son Valley, central India

Sedimentary Geology, 73 (1991) 209-225 209

Elsevier Science Publishers B.V., Amsterdam

Diagenesis of quartz in the Upper Proterozoic Kaimur Sandstones, Son Valley, central India

S. Morad a, Ajit Bhattacharyya b I.S. A1-Aasm c and K. Ramseyer d

Department of Mineralogy and Petrology, Institute of Geology, Uppsala University, Box 555, S-751 22 Uppsala, Sweden b Department of Geological Sciences, Jadavpur University, Calcutta-700 032, India c Department of Geology, University of Windsor, Windsor, Ont. N9B 3P4, Canada

a Geologisches Institut, Universitiit Bern, CH-3012 Bern, Switzerland

(Received December 10, 1990; revised version accepted June 12, 1991)

ABSTRACT

Morad, S., Bhattacharyya, A., A1-Aasm, I.S. and Ramseyer, K., 1991. Diagenesis of quartz in the Upper Proterozoic Kaimur Sandstones, Son Valley, central India. Sediment. Geol., 73: 209-225.

The Upper Proterozoic Kaimur Sandstones in central India are quartz-, sublithic- and lithic-arenites cemented by quartz, illite and hematite. Diagenetic quartz occurs in five modes: syntaxial overgrowths, fracture healings, aggregates of small euhedral crystals, quartz resulting from the alteration of detrital silicates and from the recrystallization of quartz. Intergranu- lar pressure dissolution is suggested as the main source of silica with smaller contribution from other sources, such as silica dissolved in meteoric waters, stylolitization, clay-mineral diagenesis, and the alteration of detrital silicates. Studies on the fluid inclusions and oxygen isotopes of diagenetic quartz suggest that meteoric water modified by diagenetic reactions has mediated the quartz cementation.

Introduction

Diagenetic quartz is very widespread in sedimentary rocks and occurs in different styles and origins. Despite the extensive investigations on quartz cementation by numerous authors, aspects on the origin and mechanism of silica transport, the timing of cementation, and the controls on cement distribution remain debated.

Quartz overgrowths on detrital quartz are by far the most frequently described type of diagenetic quartz in sandstones. The less commonly reported types include fracture-healing quartz (Sippel, 1968; Burley et al., 1989; Glasmann et al., 1989), discrete or aggregates of interstitial quartz crystals which are often associated with clay minerals (Dapples, 1979; Morad, 1986), and crystals resulting from the alteration of detrital feldspars (Heald and Larese, 1973; Wallace, 1976; Morad and A1Dahan, 1986), micas (Morad, 1990) and other silicates such as sphene (Morad, 1989).

In the present study, five modes of diagenetic quartz are described and discussed from the Up- per Proterozoic Kaimur Sandstones belonging to the Son Valley Vindhyan Supergroup (central In- dia; Fig. 1). Out of the five modes, fracture-healing and recrystallization types of diagenetic quartz are not commonly described in the literature and probably overlooked. Detailed petrographic and geochemical studies were carried out on this diagenetic quartz in order to understand the geochemistry of the pore fluids as well as the temperature and mechanism of formation.

Methods of investigation

The inferred diagenetic sequence of the Kaimur Sandstones (43 samples were used) was based on detailed thin section and hot-cathode luminescence (hot-CL; Ramseyer et al., 1989) petrography, complemented with scanning electron microscopy (SEM) using an instrument equipped

0037-0738/91/$03.50 © 1991 - Elsevier Science Publishers B.V. All rights reserved

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DIAGENESIS OF QUARTZ IN THE UPPER PROTEROZOIC KAIMUR SANDSTONES, SON VALLEY 211

with a backscatter electron (BSE) and a cathodoluminescence (CL) detector (Grant and White,

1978; Warwick, 1987). Electron microprobe (EMP)

analyses were obtained using a Cameca Camebax SX50 and natural element-oxide standards. The CL, BSE and EMP investigations were performed on polished thin sections coated with a thin layer of carbon. For hot-CL studies, an electron voltage

of 30 keV and a beam current density of 0.4 k t A / m m 2 were used. Colour slides of the lumines-

cence features were taken with an Ektachrome 400 (27 DIN) colour transparency film and developed

at 800 ASA (30 DIN). The exposure times varied between 100 and 150 s depending on the objective

used (2.5 × or 6.3 × ). Modal analyses were carried out on B S E / C L images and luminescence

colour transparencies projected onto a 24 x 24 cm matt screen.

Fluid inclusion microthermometric measure-

ments were performed on a petrographic microscope equipped with a Linkam TH600 heat ing-

freezing stage calibrated from - 1 0 0 to 400°C,

and using rock slices about 100 # m thick, polished

on both sides. The accuracy is estimated to be _ 2 °C during heating runs and + 0.5 ° C for cool-

ing runs. The heating rate for homogenization runs was 0 . 5 ° C / r a i n and for cooling runs was

2 ° C / m i n . The size of the inclusions on which measurements were made ranged between 7 and 15 #m. Due to the lack of data on burial depth, no

pressure correction of the homogenization temperature was made. Thus, homogenization tempera-

ture represents the minimum trapping temperature

of the inclusion (Roedder, 1979; Pagel et al., 1986). Estimates of salinity (wt.% NaC1 equivalents) were made from measurements of the final ice melting

temperature (Tm,~) of individual inclusions, in

terms of a N a C 1 - H 2 0 system (Potter et al., 1978) because of the uncertainty regarding the actual brine composition. The values obtained represent the average of minimum and maximum salinities for a given Tm~¢e in the binary system.

Oxygen isotope analyses were performed on three quartz overgrowth samples, one framework

quartz sample, and one sample made of a few quartz pebbles. The sandstones selected were quartz arenites. Separation of quartz overgrowths from framework quartz was performed according

to the method of Lee and Savin (1985), while

removal of free iron oxides from the quartz sep-

arates was made according to the method of Jack-

son (1979). The purity of each quartz separate was checked by the SEM and X-ray diffraction. Duplicate analyses gave results which were better than + 0.2 per mil. The oxygen isotope values are related to Standard Mean Ocean Water (SMOW).

Geological setting

Regional geology

An unmetamorphosed sequence of Upper Pro- terozoic sedimentary rocks about 4.5 km in thick-

ness occupies the northern fringe of peninsular

India (Fig. 1). These sedimentary rocks surround the batholitic Bundelkhand Granite and are known in Indian stratigraphy as the Vindhyan Super-

group which is subdivided into the Semri, Kaimur, Rewa and Bhander Groups (Table 1). Amongst

these, the Semri Group (lower Vindhyan) is domi-

nantly comprised of limestone with subordinate amounts of shale and sandstone, whereas the other

TABLE 1

Stratigraphy of the Vindhyan Supergroup

Group Formation

Bhander (580-1630 m)

Rewa (360-3000 m)

Kaimur (48-400 m)

Semri (760-3055 m)

Upper Bhander Sandstone Sirbu Shale Lower Bhander Sandstone Ganurgarh Shale

Upper Rewa Sandstone Jhiri Shale Lower Rewa Sandstone Parma Shale

Dhandraul Sandstone Mangesar Formation Bijaigarh Shale Lower Kaimur Sandstone

Rohtas Limestone Kheinjua Sandstone Fawn Limestone Porcellanite and Silicified Shale Kajrahat Limestone Basal Sandstone and Conglomerate

212 S. M O R A D ET AL.

(upper Vindhyan) groups are dominantly argillaceous and arenaceous with minor limestone.

The Kaimur Group is mainly arenaceous with a restricted occurrence of argillaceous intercalations. Its total thickness varies between 48 and 400 m and is lithostratigraphically subdivided into four formations. These are from base upward, the Lower Kaimur Sandstone, the Bijaigarh Shale, the Mangesar, and the Dhandraul Sandstone Forma- tions (Fig. 2). While all the three lower formations remain confined to the southern part of the basin, the Dhandraul Sandstone covers the whole of it.

The Lower Kaimur Sandstone Formation, in the eastem part of the basin, is a three-part sequence with silicified shale sandwiched between two sandstone sequences. Further west, the two sandstone sequences merge into a single sandstone

body. Texturally it is a fining-upward sequence with mean grain size varying between medium- coarse sand at the base, fining up to a very fine sand. The sandstone 'shows profuse ripple and trough cross-bedding often with signs of subaerial exposure such as developing suncracks.

The overlying Bijaigarh Shale Formation is a 60 m thick dark black carbonaceous facies with up to a metre-thick pyritiferous intercalations. Near its

lower and upper boundary, the shales are interbe- dded with thin siltstones. These siltstone beds are profusely rippled and have surface features such as rill marks, spring spits, wrinkle marks, and mudcracks.

The Mangesar Formation conformably overlies the Bijaigarh Shale and is a thin-bedded (even or parallel), flaggy, very fine-grained sandstone and

HANDRAUL SANDSTONE 54 - 228 m)

= - = m = =,~.,~ MANGESAR FORMATION h 0 0 - 200 rn)

BIJAIGARH SHALE (0- 60 m)

~ LOWER KAIMUR SANDSTONE (15- 20 m)

I ~ Trough cross-stratified sets

I-~'~'-~ Small scale cross-laminated set

F : : : ~ Even, parallel-laminated sets

~ Silicified shale

m Dark black carbonaceous shale

~ Mudcracks

I '-~'--I Erosional surface lined with shale intraclasts

Kaimur Group. Fig. 2. A generalized stratigraphic section of the

DIAGENESIS OF QUARTZ IN THE UPPER PROTEROZOIC KAIMUR SANDSTONES. SON VALLEY 213

siltstone with a high proportion of matrix material. Wave-ripple marks and mudcracks are very common.

The Dhandraul Sandstone is a coarsening-upward sequence with grain size varying between coarse silt and fine sand at the base, coarsening up to a medium sand towards the top. Large-scale, low-angle cross-bedded sets and cosets alternate with parallel laminated cosets. Ripple marks are, however, found toward the base and top of the sequence. Planar erosional surfaces within the Dhandraul Sandstone are also common.

The association of sedimentary structures and their spatial variation, textural properties of various lithologies and total facies association and variation suggest that the Kaimur Group represents ephemeral braided fiver deposits.

Petrology and diagenesis

Texturally, the sandstones are fine-, medium- or coarse-grained and moderately to poorly sorted quartz-, sublithic- and lithic-arenites (Folk, 1965). Monocrystalline quartz (45-94 vol.%) dominates the framework grains and displays features of pressure dissolution such as sutured, concavo-convex, and straight contacts. Metamorphic rock fragments (mostly phyllite and quartz-mica schist) occur at concentrations varying between 1 and 25%; some samples contain up to 30% shale intraclasts. The argillaceous rock fragments are partially to pervasively dissolved. K-feldspar (0.0-7%) dominates over plagioclase (0.0-0.2%) with microfractures in the detrital K-feldspar healed by diagenetic K-feldspar and quartz. Although mica and chert mostly form less than 1% of the sandstones, in some samples the amount of chert may be up to 8%. Cementing material in the Kaimur Sandstones includes quartz (trace to 12%), clay minerals (illite and chlorite; trace to 7%) and hematite (trace to 15%). One sample contains 3% fine sparite cement. Although porosity (determined by optical petrology) varies between 4 and 11% and is mainly primary intergranular porosity, evidence suggests that most of the primary porosity has been destroyed by mechani- cal compaction of argillaceous rock fragments and intergranular pressure dissolution of quartz.

The overall diagenetic history of the Kaimur Sandstones is relatively simple. At the eodia- genetic stage, detrital feldspars, muscovite and biotite were illitized and resulted in the precipitation of fine-crystalline quartz, hematite (as patchy coarse-crystalline pore fillings and as fine pigment coating framework grains) and Ti-oxides. Illite and, less commonly, chlorite precipitated around framework minerals and partially filled the interstitial space. Small amounts of authigenic K- feldspar formed as overgrowths, fracture-healings and replacement of detrital microcline in a manner similar to that in the Triassic Buntsandstein of Spain (Morad et al., 1989). During mesodiagene- sis, increased overburden pressure caused ductile deformation of argillaceous lithic fragments resulting in formation of pseudomatrix (Dickinson, 1970), as well as pressure dissolution and fracturing of detrital quartz. Precipitation of quartz overgrowths, fracture healing, and recrystallization of detrital quartz occurred.

Petrography of diagenetic quartz

Quartz is the predominant diagenetic mineral in the sandstones and occurs in five modes: (1) as syntaxial overgrowths around detrital quartz grains; (2) as fracture-healings; (3) as fine euhedral crystals embedded within interstitial clay minerals; (4) as the alteration product of detrital silicates; and (5) following the recrystallization of detrital quartz.

Mode 1. Authigenic quartz as syntaxial overgrowths occurs in varying amounts in the sandstones and reduces the primary porosity (Fig. 3A). It is least abundant in lithic and sublithic sandstones and in sandstones in which the framework grains are coated by authigenic clay minerals (Fig. 3B). Inhibition of overgrowth formation by clay coats supports the findings of Pittman and Lured- sen (1968), Heald and Larese (1974) and Pittman (1988). The apparent thickness of quartz overgrowths varies between 30 and 80/~m. Small ( ~< 15 /~m) fluid inclusions, rounded or irregular in shape, occur at the boundary between the overgrowth and the framework grains and sometimes within the overgrowths. In some instances, small amounts

214 S. MORAD ET AL.

of clay minerals are trapped between the overgrowth and the grain.

Mode 2. The detrital quartz show intricate hair-like healed fractures varying in thickness from less than 1/~m to 25 #m. The fractures are clouded by stringers of small (~< 1-10 /tm) rounded fluid inclusions which were trapped when the micro-

fracture was healed by local-scale diffusive mass transfer or sealed by precipitation of material transported from distances greater than grain scale (Smith and Evans, 1984).

Fracture-healing quartz is either optically continuous or discontinuous with the host detrital quartz. Using the BSE/CL imaging techniques,

Fig. 3. A. Authigenic quartz overgrowths occluding pore space in a quartz arenite. FD is a detrital K-feldspar. SEM micrograph. B.

Authigenic quartz-overgrowth formation was inhibited on sites where thick illite coatings on detrital quartz occur (arrow). Quartz

overgrowths occur at sites where there are no or only thin illite coatings. Optical micrograph, crossed nicols.


Fig. 4. A. BSE/CL image showing a detrital quartz (bright) pervasively fractured and healed by diagenetic quartz (dark; arrows). Note the presence of tiny inclusions in the domains of diagenetic quartz. B. Fracture planes delineated by stringers of fluid inclusion cut across adjacent grains and the quartz overgrowths (arrows). C. A quartz grain heavily clouded by fluid inclusions due to pervasive fracturing and healing; the quartz overgrowth (arrows) contains much smaller amounts of fluid inclusions. D. A diagenetic polycrystalline quartz clouded by fluid inclusions and formed by fracturing of detrital quartz. E. A detrital polycrystalline quartz which is almost free of fluid inclusions. F. A framework quartz grain that displays undulosity induced in source rocks (small arrows) cut by diagenetic fracture-healing quartz delineated by stringers of fluid inclusions (larger arrows). This feature produces a very

complex undulosity pattern in the quartz grains. B-F are optical micrographs, crossed nicols.

216 S. M O R A D ET AL.

the diagenetic quartz filling the fractures is non- luminescing, whereas the host detrital quartz has bright luminescence (Fig. 4A). The fractures strad- dle adjacent grains and their overgrowths, suggesting their in-situ origin. These are known as unsta- ble fractures and might outline a former through- going shear zone (Fig. 4B; cf. Roedder, 1984). However, fracturing and healing of detrital quartz

commonly occur due to burial compaction (Eichentopf, 1987).

Pervasively fractured and healed quartz grains are sometimes completely clouded by fluid inclusions. Quartz overgrowths on such grains are devoid of or contain smaller amounts of inclusions (Fig. 4C) which might indicate that fracturing has occurred in the source rocks or occurred in situ,

Fig. 5. A. Two quartz grains (1 and 2) that display a concavo-convex contact due to pressure dissolution and are intensively

deformed. Planes of deformation are marked by the occurrence of abundan t fluid inclusions. B. The same quartz grains as in A, but

taken after rotation of the microscope stage to show that considerable undulosi ty is induced to grain 1 at sites of max imum pressure

dissolution (arrows). Optical micrographs, X-nicols.


but has not cut across the overgrowths. Whether fractures propagate into the overgrowth or not is most likely controlled by the physical characteris- tics of the grain-overgrowth boundaries which might act as elastic discontinuities. Onasch (1990) suggested, however, that the failure of microfractures to propagate in the Lower Silurian Mas- sanutten Sandstone of the Appalachian Great Val- ley Province could be due to rapid closure. Rapid healing is supported by experiments of Smith and Evans (1984) who found that, in the presence of water, microfractures heal in only several hours at 400 ° C. They predicted that at 200 ° C, microfractures would have geologically short lifetimes. In- tragranular microfractures will tend to form where grain boundaries act as stress risers (Gallagher et al., 1974).

Some of the intensively fractured quartz grains are heavily clouded by fluid inclusions and display patchy extinction that is similar to polycrystalline quartz (cf. Sippel, 1968) (Fig. 4D). Such a diagenetic polycrystallinity is induced due to miso- rientation of regions in the quartz grains that resemble subgrains formed during metamorphism (Tullis and Yund, 1987).

Patchy extinction involves more continuous and irregular changes in extinction position as compared to the sharp boundaries of subgrains. More- over, the detrital polycrystalline quartz derived from igneous or metamorphic source rocks is either devoid of inclusions or has inclusions delineating boundaries of subgrains formed as a result of grain fracturing (Fig. 4E); similar inclusions do not occur in the host detrital quartz. Wilkins and Barkas (1978) pointed out that fluid inclusions are eliminated from quartz during metamorphic recrystallization but the exact mechanism by which this occurs is not well understood. Complex undulosity patterns of framework quartz was observed to result upon superimposition of undulosity that is detrital metamorphic in origin and undulosity induced due to burial fracturing (Fig. 4F). In many cases, it appears that quartz grain deformation is more intensive at the sites of pressure dissolution contacts with adjacent quartz grains (Fig. 5A, B). Some of the fractures in quartz are filled with diagenetic illite rather than healed by quartz.

Fracturing of quartz is a brittle deformation feature and is produced when the local stress exceeds the local strength due to tectonic deformation, temporal and spatial differences in temperature, stress release, or higher overburden pressure than pore pressure (Tuckwell, 1979; Kranz, 1983).

Mode 3. Aggregates of small ( < 30/tm) euhedral quartz crystals are distributed patchily in the interstitial spaces of many lithic and sublithic arenites. This type of quartz is associated with authigenic illite (Fig. 6A), and constitutes nearly 3% in some sandstones.

Mode 4. Diagenetic quartz has also resulted from the precipitation of silica released upon the dissolution and illitization of detrital biotite and muscovite. Such quartz, which is common in some lithic and sublithic arenites, occurs as several discrete crystals or aggregates of crystals that are embedded along the cleavage traces of illitized micas, or occur in their immediate vicinity (Fig. 6B). Few pseudomorphs composed of Ti-oxides and quartz were observed and believed to result from decomposition of detrital sphene (cf. Morad, 1989).

Mode 5. Distortion of quartz grains and formation of deformation lamellae during burial diagenesis of sandstones have been described by Taylor (1950) and Phipps (1969). In the Kaimur Sand- stones, detrital quartz has been recrystallized into diagenetic quartz (cf. Sippel, 1968; Roedder, 1984) that is clouded by tiny fluid inclusions. Recrys- tallization is either patchy or occurs along sub- planar optically distinct (relative to the host) deformation lamellae in the detrital quartz and, sometimes, the overgrowths (Fig. 7). In other words, this pattern of deformation has induced undulosity to the framework quartz. The deformation lamellae are subbasal or basal in quartz (Carter, 1971).

In contrast to fracture-healing quartz, framework quartz which shows deformation lamellae is most common in sandstones rich in interstitial clay minerals. In some cases, the microfractures cut across, and thus post-date, the deformation lamellae. Onasch (1990) observed in the Lower Silurian Massanutten Sandstone, central Appa- lachian, that deformation lamellae, undulous ex-

218 s. MORAD ET AL.

Fig. 6. A. Fine-crystalline quartz (Q) surrounded by illite flakes (arrows) in a lithic sandstone. B. Detrital biotite (Bi) altered into

illite ( IL) ; note the formation of quartz along traces of cleavage planes (arrows) and as patches (Q) in pores adjacent to the altered

biotites. SEM micrographs.


tinction and subgrains are the earliest microstruc- tures and are the product of dislocation flow.

A common feature in the Kaimur Sandstones is

that the fractures and deformation lamellae cut across the whole grain suggesting that deformation occurred mainly due to burial rather than

tectonic effects (Eichentopf, 1987). We are, however, uncertain whether some of the recrystallized

or, at least, deformed quartz grains are derived

from the source rocks or not. The deformation of the Kaimur Sandstone has

not changed the original grain boundaries of detri-

Fig. 7. A. A detrital quartz grain that has been subjected to deformation. The deformation iamellae that cut into the quartz overgrowths are heavily clouded by fluid inclusions due to recrystallization. Optical micrograph, crossed nicols. B. A B S E / C L image

showing that diagenetic quartz along the deformation lamellae is non-luminescing, whereas relict parent detrital quartz is bright luminescing.

220 S. MORAD ET AL.

tal quartz or the texture of the sandstones. These features are common in low-temperature deforma-

tion (Groshong, 1988).

Hot-CL and microprobe analyses of quartz

The luminescence colours displayed by quartz under the optical microscope are a poorly understood and debated subject and controlled by a number of factors, including the degree of crystal- lattice order, twinning patterns, the stress state, and the presence of various types of trace elements which act either as activators or quenchers (Matter and Ramseyer, 1985; Ramseyer et al., 1988). Under the hot-CL, the detrital quartz of the Kaimur Sandstones is characterized by blue, pink and violet colours, whereas diagenetic quartz is non-luminescing, or shows various shades of dull reddish-brown typical of relatively defect-free growth of quartz at low temperatures (Marshall, 1988).

The use of the BSE detector sometimes allows the distinction between fracture-healing (dark grey) and detrital (light grey) quartz types (Fig. 8). This feature indicates that the average atomic number is higher in the detrital quartz than in the fracture-healing (fine-crystalline) quartz (see, e.g., White et al., 1984). However, the EMP analyses (using the SXSPCT program for element detec-

tion) revealed a pure SiO 2 composition for both types of quartz (Fig. 8). The amounts of trace elements in these quartz types were below the detection limit of the EMP (less than about 100 ppm). Therefore, the difference in average atomic number between the two types of quartz is probably related to presence of an 0 2- vacancy (Jones and Embree, 1976) a n d / o r to different amounts of water (see, e.g., Kats, 1962; Elphick and Graham, 1988). Water in quartz occurs: (1) as discrete H 2 0 or H 2 molecules occupying channel- ways or defects in the quartz; (2) as O H - ions involved in a 4 O H - = SiO 4- hydrogrossular-type substitution; (3) as hydrolized S i -O-Si bonds of the form S i - O - H - H - O - S i ; or (4) as H ÷ ions compensating for the replacement of Si 4÷ by A13 +

(in other words H ÷ occurs as HALO2). The solubility of water in quartz is very limited and thus might not account for the atomic number differences among the diagenetic and detrital quartz types displayed by the BSE detector. On the other hand, water is abundant (as much as 20% but usually 3 to 9%; Bates and Jackson, 1980) in amorphous and cryptocrystalline types of S i O 2.

Ramseyer and Mullis (1990) have found that short-lived (i.e. change colour during electron bombardment) bluish and green luminescence colours show a relationship with the crystal structure of quartz and are probably stimulated by posi-

Fig. 8. A BSE image of fractured and healed quartz grain. An unknown considerable difference in chemical composition has given

rise to the darker grey shades of the diagenetic quartz than the detrital host.


tively charged trace elements (H + or Li +) stored in structural channels parallel to the c-axis of the quartz crystal. However, the solubility of H ÷ in quartz is accompanied with charge-compensating A1 (or Fe 3+) replacing Si in the lattice. In the quartz types displayed in Fig. 8, AI in detrital and diagenetic quartz was below the detection limit of the EMP except for one diagenetic quartz (formed by recrystallization of detrital quartz) which con- tained 900 ppm A1. No other element-composi- tional differences were detected among these two quartz phases. Although this "Al-rich" quartz is expected to have a darker grey colour than the Al-poor host (due to the lower atomic number of A1 than Si), the reverse was observed; the reason behind this behaviour is unclear. Matter and Ramseyer (1985) and Henry et al. (1986) found that low-intensity luminescent diagenetic quartz contains high concentrations of aluminium. More recently, Ramseyer et al. (1988) concluded that at high beam current densities, the aluminum-poor (< 500 ppm) quartz luminesces either blue, blue- green, bottle-green, yellow, pink or red. At low beam current densities, Al-poor quartz is non- luminescent (Ramseyer and Mullis, 1990).

Distinction between quartz overgrowth and a detrital quartz core by means of the BSE detector was not possible and thus reflects the absence of significant variations in the chemical composition.

Fluid inclusion studies on diagenetic quartz

In order to gain an idea about the nature of pore waters in terms of temperature, salinity and chemical composition, fluid inclusion studies were undertaken. All the fluid inclusions examined are two-phase, liquid-dominated liquid-vapour inclusions. Many inclusions in the undeformed quartz overgrowth are primary, whereas those in the recrystallized and fracture-healing quartz are con- sidered to be secondary because criteria for primary origin are not satisfied (Roedder, 1984). Fluid inclusions in the diagenetic quartz of different modes (overgrowths, fracture filling, and recrystallization) have homogenization temperatures of 102-156°C (Fig. 9) and contain saline solutions (8 to 16 wt.% NaC1 equivalents). The range of homogenization temperature in quartz over-

~10 c o

c 6 "6 L 4.

~2 Z

Quartz overgrowth

100 120 1#0 160 Homogenization temperature (°E)

I0 Fracture-heating and recrystatlizafion '~ 8

,E 6

oJ Jc l

• 2

1()0 120 140 160 Homogenization temperature (°C)

Fig. 9. Homogenization temperatures from secondary two-

phase inclusions in (A) quartz overgrowth and (B) fracture

healing.

growths is lower (102-134°C) than in quartz formed by recrystallization or fracture-healing (122-156C). Distinction of fluid inclusions among the latter two types of diagenetic quartz was dif- ficult. The relatively high ( - 4 2 to -33°C) eutectic temperatures (temperature of initial melting) suggest that the brines were simple in composition and dominated by NaC1-H20, the main type in most deep sedimentary basins.

Quartz grains which have been subjected to deformation-recrystallization or fracturing-healing by diagenetic quartz have inclusions that are characterized by higher salinities (11-16 wt.% NaC1) than inclusions in the overgrowths (8-12 wt.% NaC1).

Oxygen isotope values

The oxygen isotope values (8]80) of the three quartz overgrowth samples are + 15.7, + 16.2 and + 16.8 per mil, while that of the detrital unaltered (by recrystallization or fracture-healing as revealed by petrographic and CL examinations) quartz pebbles is + 11.8 per mil. As the O-isotope value of the detrital quartz is known, an estimate of the composition of diagenetic quartz can be made by using the modal analyses of detrital and

2 2 2 s . M O R A D E T A L .

diagenetic quartz of the BSE/CL images and the luminescence colour transparencies. The uncer- tainties included in this method are errors in estimation of the diagenetic/detrital quartz ratio and in the assumption that the 8180 of the quartz pebbles represents average values for detrital quartz. Determination of 8180 for recrystallized and fracture-healing quartz types separately was not possible.

Sources of silica cement

As pointed out above, detrital quartz grains in many of the Kaimur Sandstones have been exten- sively affected by pressure dissolution. Pressure dissolution is less common in the lithic sandstones that contain abundant argillaceous fragments or abundant intergranular clay minerals. The amount of silica derived from the pressure dissolution of detrital quartz was determined by point counting overlap quartz (portion of a detrital grain that is inferred to have been dissolved by intergranular pressure dissolution; Houseknecht, 1984) and accounts for 60% of the total quartz cement. In many samples, pressure dissolution and the precipitation of overgrowth occur on thin-section scale, whereas in other samples, pressure dissolution is volumetrically more important than the amounts of quartz precipitated. The latter sandstones may be referred to as silica exporters (Houseknecht, 1988). Six out of the fifteen sandstones from the Dhandraul are silica exporters, whereas most sandstones of the Lower Kaimur (8 out of 9 samples) are silica importers (Fig. 10). Many of the data points plot close to the 1:1 line indicating that the sandstones closely approach mass balance with respect to silica. The significant positive correlation of data in Fig. 10 indicates a genetic relationship between the amounts of quartz cement and the volume loss due to pressure dissolution.

The remaining amounts of silica have been derived from the meteoric waters (groundwaters have an average of 31 ppm; White et al., 1963) and from pressure dissolution along stylolites, clay-mineral transformation (e.g., smectite to illite; Towe, 1962; Siever, 1962), the dissolution of chert, chalcedony and argillaceous rock fragments,

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O_ >,

0

0 Lower Kaimur Sandstone E] Mangesar Formation

/

12- A Dhandraul Sandstone /

/ I0

8

o A @ 0

2 L, & g 1'0 1'2 Quartz cement (%)

Fig. 10. Plot of quartz cement against volume of detrital grains lost by pressure dissolution. Straight line divides samples for which volume loss by pressure dissolution exceeds volume of

quartz cement (silica exporters) from samples for which volume of quartz cement exceeds volume lost by pressure dissolution (silica importers). Correlation coefficient for the data points is

+ 0.69.

and the alteration of detrital silicates. The SEM and standard petrographic examinations revealed that detrital quartz grains are severely etched due to dissolution, particularly in fine- to medium- grained sandstones rich in interstitial clay minerals. Non-pressure dissolution of quartz in sandstones has been described by several authors (e.g., Cassan et al., 1981; Burley and Kantorowicz, 1986; Leder and Park, 1986; Hurst and Bjorkum, 1986). How- ever, the distribution and amounts of these processes in the studied basin were not quantified.

Synthesis

The Kaimur Sandstones have undergone a complex history of quartz cementation, fracture- healing and recrystallization. The two latter changes resulted in clouding of framework quartz by tiny fluid inclusions and induced undulosity and polycrystallinity. Polycrystalline quartz of metamorphic and igneous origins is crystal-clear or contains small amounts of inclusions which are concentrated along subgrain boundaries. Hence, future studies on the use of undulosity of framework quartz in source-rock identification (Blatt


Fracture-healing a n d / ~ / / 1 4 0 , recrysfattizafion ~ J / ,~/~ /

100. / / / ~ / overgrowfh

/ / / / 8o E

%,//

-20 -15 -10 -5 0 5 8180water %0 (SMOW)

Fig. 11. Temperature against water oxygen isotopic composition plot for diagenetic quartz using the following fractionation equation (Friedman and O'Neil, 1977): 103 lna(quartz-H20) = 3.38(106)T -2 -2.90. Diagenetic quartz overgrowth has formed earlier from more depleted waters than fracture-healing and recrystallization quartz types. The dashed lines show the

possible trend of pore water evolution.

and Christie, 1963; Folk, 1965; Basu et al., 1975) should be made with great caution. Moreover, interpretation of ~180 data on framework quartz should be made using a careful microscopic (light, hot-CL, and BSE/CL) search for the presence of recrystallized and fracture healing quartz.

Some clues to the origin and diagenetic evolution of the pore waters involved can be derived from fluid inclusion and oxygen isotope data. Figure 11 presents the possible range of 8180 values of the pore water in equilibrium with the different types of diagenetic quartz. The low ~180 values ( - 5 to - 1 per rail) and the moderate salinities (8 to 12 wt.% NaC1 equivalents) of the pore waters which mediated the precipitation of quartz overgrowth suggest a meteoric water modified by diagenetic reactions and probably mixing with deep brines. The Kaimur Group is underlain by marine limestones, shales and sandstones that likely contain entrapped and modified connate marine waters that could move upward by compaction.

The range of homogenization temperature for fluid inclusions in the quartz overgrowths (102- 134°C) suggest cementation at significant burial depths (3-5 km assuming normal geothermal gradients). Although quartz cementation by meteoric waters has been concluded by several authors to be important at relatively shallow (<

several hundred metres) burial depths (Blatt, 1979; Kantorowicz, 1985; Molenaar, 1986; Dutta and Suttner, 1986), deep-burial (> 2 km) cementation by pore waters with a strong meteoric component also occurs (Ayalon and Longstaffe, 1988; Dutton and Land, 1988).

The meteoric water was further modified during recrystallization of detrital quartz grains and fracture healing by diagenetic quartz. This is evi- denced by the relatively heavier ~18Owate r values

(Fig. 11), the higher salinities (11-16 wt.% NaCI), and the higher homogenization temperatures (122-156 o C), compared to pore water responsible for the formation of quartz overgrowths.

Conclusions

(1) Fracture-healing and development of deformation lamellae in the Kaimur Sandstone have: (a) resulted in clouding of the diagenetic quartz by stringers of tiny fluid inclusions, (b) induced undulosity into the framework quartz, and (c) formed diagenetic polycrystalline quartz.

(2) Propagation of microfractures unstably across grains and grain-overgrowths boundaries is not necessary during sandstone deformation because these boundaries would likely act as elastic discontinuities. Another explanation for the failure of microfractures to propagate is their rapid healing. Further studies are, nevertheless, needed in order to establish reliable petrographic criteria on the basis of which distinction between in-situ and detrital metamorphic quartz deformation can be made.

(3) Formation of diagenetic quartz occurred at a temperature range greater than 102 to 156°C from meteoric water that has been modified by diagenetic reactions. Formation of quartz overgrowths occurred at somewhat lower temperatures (102-134°C) than quartz formed during recrystallization and fracture-healing (122-156 o C).

Acknowledgements

The study has been financed by the Swedis Natural Science Research Council, NFR (to SM), the Center of Advanced Study in Economic Geol- ogy, Jadavpur University, India (to AB), and the

224 s MORAD ET AL.

N S E R C , C a n a d a ( to ISA) . T h e B S E a n d B S E / C L

w o r k w e r e c o n d u c t e d a t S a g a P e t r o l e u m a.s.

( S a n d v i k a , N o r w a y ) u n d e r t he s u p e r v i s i o n o f T o r

M e l l e m . T h e m i c r o p r o b e a n a l y s i s w a s a i d e d b y

H a n s H a r r y s o n , U p p s a l a . T h e ho t -C1 s t u d i e s w e r e

c o n d u c t e d a t G e o l o g i s c h e s I n s t i t u t , U n i v e r s i t a t

Be rn . T h e c r i t i ca l r e a d i n g o f t h e m a n u s c r i p t b y

Dr s . J. Hu l l , F. L o n g s t a f f e , P.J . M c K e e v e r , O.

W a l d e r h a u g a n d t w o j o u r n a l r e f e r ee s is g r a t e f u l l y

a c k n o w l e d g e d .

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Diagenesis of quartz in the Upper Proterozoic Kaimur Sandstones, Son Valley, central India