Cretaceous Lime Stones

Gondwana Research, V. 7, No. 2, pp. 527-537. © 2004 International Association for Gondwana Research, Japan. ISSN: 1342-937X

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Gondwana Research

Carbon and Oxygen Isotopic Signatures in Albian-Danian Limestones of Cauvery Basin, Southeastern India J. Madhavaraju1*, I. Kolosov2, D. Buhlak2, J.S. Armstrong-Altrin3, S. Ramasamy1 and S.P. Mohan1 1

2 3

Department of Geology, School of Earth and Atmospheric Sciences, University of Madras, Guindy Campus, Chennai 600 025, India. *E-mail: [email protected] Institute of Geological Sciences, National Academy of Sciences of Belarus, Kuprevich Street 7, 220141 Minsk, Republic of Belarus Centro de Investigación en Energía, Universidad Nacional Autónoma de México (UNAM), Priv.Xochicalco s/n, Col.Centro, Apartado Postal 34, Temixco, Morelos 62580, México

(Manuscript received October 25, 2002; accepted September 17, 2003)

Abstract The Albian-Danian limestones of Cauvery Basin show a wide range of d13C and d18O values (–13.2 to +1.1‰ and – 9.0 to –2.5‰, respectively). The cement samples show negative carbon and oxygen isotope values (-18.9 to -3.9‰ and -9.0 to -4.3‰, respectively). The petrographic study reveals the presence of algae, molluscs, bryozoans, foraminifers and ostracods as major framework constituents. The limestones have microspar and equant sparry calcite cements. The pore spaces and vugs are filled with sparry calcite cement. The bivariate plot of δ13C and δ18O suggests that most of the samples fall in the freshwater limestone and meteoric field, while few samples fall in the marine limestone and soil calcite fields. The presence of sparry calcite cement, together with negative carbon and oxygen isotope values, indicates that these limestones have undergone meteoric diagenesis. Key words: C- and O- isotopes, limestone, diagenesis, Cauvery Basin, Southeastern India.

Introduction The carbon and oxygen isotopic composition of the skeletal carbonate sediments reflect the physico-chemical properties of the waters in which the organisms grow (Keith et al., 1964; Morrison and Brand, 1986) and also provide information regarding the diagenetic processes and environments, which initiate the conversion of skeletal carbonates into limestones (Jenkyns et al., 1994). It has been proved by many studies, that the carbon and oxygen isotopic composition of carbonate rocks provide useful information regarding physico-chemical conditions of precipitation, paleoclimate, paleo-oceanographic conditions, paleoecology and diagenetic conditions (James and Choquette, 1984; Wright, 1990). Because, the Carbon isotopic composition in carbonate minerals are mainly identified by the δ13C values of bi-carbonate/carbonate ions in the water, whereas the δ18O values are largely influenced by the isotopic composition of water and temperature of precipitation. Further more, carbon isotopic studies constitute an important tool to reconstruct the paleoenvironmental conditions. The carbonate rocks deposited in marine

environments tend to record the carbon isotopic composition of the ocean water (Scholle and Arthur, 1980). Similarly, the oxygen isotope studies from foraminifers and the paleobotanical record provide strong evidence that the Cretaceous period was substantially warmer than today (Crowley and North, 1991; Spicer and Corfield, 1992). Paleoclimatic conditions for a given region can be determined by studying temporal changes of meteoric diagenesis within a single lithology, particularly limestone, and the geochemical signature of the associated diagenetic products (James and Choquette, 1984). Stable isotope data from shallow-burial meteoric calcite cement will be more useful to distinguish sequence-specific “meteoric calcite line” (invariant δ18O and variable δ13C values; Lohmann, 1988). The calcite cements, which are useful for determining a meteoric calcite line, are mainly precipitated in isotopic equilibrium with local meteoric waters and these data can be used to estimate the oxygen isotopic composition of local meteoric waters during diagenesis (Smith and Dorobek, 1993). It appears that the carbon and oxygen isotope composition of carbonate rocks is useful for recognizing the diagenetic and sea level

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history of a sedimentary basin. A systematic study has been undertaken in this work to describe the possible reasons for the variations in carbon and oxygen isotopes and to interpret the diagenetic history by using the petrography of whole rock and calcite cement samples of Albian-Danian age limestones of Cauvery Basin.

Geology and Stratigraphy The Cauvery Basin has been classified as a pericratonic rift basin formed along the eastern continental margin of Peninsular India (Sastri et al., 1981; Biswas et al., 1993). The block faulted architecture exhibited brittle accommodation of the stress regime, particularly during the initial phase of the Early Cretaceous period (Sastri et al., 1981; Prabhakar and Zutshi, 1993). The exposures of sedimentary rocks has been identified in five areas of Cauvery Basin viz. i) Pondicherry, ii) Vridhachalam, iii) Ariyalur, iv) Tanjore and v) Sivaganga. Among these, the sedimentary rocks are well developed in the Ariyalur area. Detailed and systematic work on these sedimentary rocks were carried out by Blanford (1862), who divided the various rock types into three distinct group i.e. i) Uttattur, ii) Trichinopoly and iii) Ariyalur. Numbers of lithostratigraphic classifications were proposed by many workers (e.g., Srivastava and Tewari, 1967; Banerji, 1972; Sastry et al. 1972; Sundaram and Rao, 1986; Ramasamy and Banerji, 1991; Sundaram et al., 2001). In this study, we have followed the lithostratigraphic classifications proposed by Sastry et al. (1972) and Sundaram et al. (2001). The distribution of rocks from different litho units are shown in Fig.1. Detailed lithostratigraphic classification and facies mosaic of the sedimentary rocks are given in Fig. 2. Uttattur Group Uttatur Group has been divided into four distinct formations viz. i) Terani Formation, ii) Arogyapuram Formation, iii) Dalmiapuram Formation and iv) Karai Formation (Sundaram et al., 2001). The Dalmiapuram and Karai Formations comprise limestone deposits. Based on the lithology, the Dalmiapuram Formation is divided into two distinct members i.e. lower grey shale member and upper limestone member. The limestones are biohermal and biostromal in nature. The biohermal limestone is around 20-25 m thick, massive, hard, pink to buff red colour and has a stromatactis structure (Yadagiri and Govindan, 2000). The biostromal limestone is soft and friable and off-white to brownish yellow in colour. The presence of calcareous algae, bryozoan and coral fragments in the biohermal limestone suggests a shallow marine environment (Banerji et al., 1996). The biostromal limestone contains abundant benthic foraminifers clearly

indicating that the bedded limestone was deposited under deeper shelf conditions with water column more than 50 m (Ramasamy and Banerji, 1991; Ramasamy et al., 1995). Based on foraminifers and ammonites, an Early to Middle Albian age has been assigned for the Dalmiapuram Formation (Ramasamy and Banerji, 1991). Karai Formation mainly consists of clastic-carbonate units, which are exposed as a linear, north-south belt. It is divided into two members i.e. Odium member and Kunnam member (Sundaram and Rao, 1986). The Kunnam member is characterised by grey sandy clay beds, siltstone and fine grained sandstone, which is alternate with thin band of argillaceous limestone (Sundaram and Rao, 1986). The limestone bands are less than 1 m thick. The Karai Formation is considered to represent an offshore, highstand depositional environment continuing the transgressive trend apparent for the underlying Dalmiapuram Formation (Sundaram et al., 2001). Based on the index fossils (foraminifers) a Late Albian-Early Turonian age has been assigned (Ramasamy and Banerji, 1991). Trichinopoly Group This group is divided into Kulakkalnattam and Anaipadi Formations (Sundaram and Rao, 1986). Kulakkalnattam Formation mainly comprises of basal sandstone, pebbly sandstone, coarse grained calcareous sandstone, shale and shell limestone. The limestone is 1 to 2 m thick, compact, grey, shell rich and is characterised by the presence of molluscan shells. Based on the diagnostic ammonite fossils, middle Turonian to Santonian age has been assigned for the Trichinopoly Group. Ariyalur Group The Ariyalur Group has a conformable relationship with the Trichinopoly Group but oversteps the basement along its southern part, between Kilapaluvur and Vettriyur villages. It is well exposed in the vicinity of the Ariyalur area. This group has been divided into four formations i.e. i) Sillakkudi, ii) Kallankurichchi, iii) Ottakkovil and iv) Kallamedu (Sastry et al., 1972). The Kallankurichchi Formation exhibits the considerable amount of limestone deposits. Kallankurichchi Formation is lithologically divided into two units i.e. i) clastic unit and ii) carbonate unit (Sastry et al., 1972). The fine to medium grained, calcareous, fossiliferous and pale green to yellowish brown sandstone was deposited in a near shore environment (Sundaram and Rao, 1986; Madhavaraju, 1996; Madhavaraju and Ramasamy, 1999a, b; Sundaram et al., 2001). The limestones mainly consist of sandy fossiliferous limestone, fossiliferous limestone and marl. The limestones are offwhite to orange yellow in colour, massive to thick bedded and interbedded with marl. The occurrence of Gondwana Research, V. 7, No. 2, 2004

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Fig. 1. Geological map of the Ariyalur area showing the distribution of Cretaceous and Paleocene rocks (modified after Sundaram et al. 2001).

Goupillaudina daguini, Siderolites calcitrapoides and Lepidorbitoides sp. cf. L.socialis in the Kallankurichchi Formation suggests Late Campanian and Early Maastrichtian age (Hart et al., 2001). The presence of Orbitoids with other larger foraminifers suggests that these limestones were deposited under shallow marine depositional environment (Sastry et al., 1972; Sundaram and Rao, 1986; Madhavaraju and Ramasamy, 1999a). Gondwana Research, V. 7, No. 2, 2004

Niniyur Formation Sundaram and Rao (1986) placed the Niniyur Formation (Paleocene) under the Ariyalur Group whereas most of the authors (Sastry et al. 1972; Madhavaraju, 1996; Madhavaraju and Ramasamy, 1999a, b) proposed separate status for Niniyur Formation and they placed the Niniyur Formation above the Ariyalur Group. It is mainly composed of sandstone, thick limestone beds

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interbedded with marl layers and sandy clay. The limestones are hard, light grey to light brown in colour and contain numerous fossils of molluscs, echinoderms, algae and foraminifers.

Methodology Over one hundred thin sections were stained in Alizarine-red S solution (Friedman, 1959), and also in combined organic and inorganic stains for iron-rich calcite (Katz and Friedman, 1965) to find out the mineralogical variations. For this study, the samples were selected from different formations with respect to their lithology and they are listed in Table 1. Forty representative samples were analyzed for carbon and oxygen isotopes, which include 29 whole rock samples and 11 calcite cement samples. The calcite cement was collected by using the dentist drill bit. The carbon and oxygen isotope study was carried out at the Lithohydrogeochemistry Laboratory, University of Belarus, Belarus. The limestone samples were treated with H3PO4 in vacuum at 25°C. The resulted CO2 was analysed using Mass Spectrometer. Normal corrections were applied and the results are reported in the standard per mil (‰) d-notation relative to the Pee Dee Belemnite (V-PDB) marine carbonate standard. Sample reproducibilities in the laboratory were better than 0.1‰ for both carbon and oxygen isotope values.

Petrography The petrographic study of carbonate rocks has been carried out to support the isotopic study. The distribution of major petrographic types is described in detail. Totally five major petrographic types have been identified i.e. i) mudstone, ii) wackestone, iii) packstone, iv) grainstone and v) boundstone. Mudstone is a mud supported rock type which contains numerous foraminiferal grains and glauconite pellets are observed to float in the micritic matrix. Small amount of clay material is present within this mud. It also contain a considerable amount of fine grained quartz and feldspar grains. This petrographic type is commonly observed in the Karai Formation. Wackestone dominates the carbonate rich sequence in the study area. All wackestones contain a very small amount of angular quartz and feldspar grains. Quartz grains are mostly monocrystalline exhibiting uniform extinction. The feldspars are of orthoclase, plagioclase and perthite. The wackestone has the bioclastic framework of algae, corals, molluscs, bryozoans, foraminifers, ostracodal carapaces and echinoid skeletal materials which are found in the micritic matrix. The sparry calcite mosaic has developed in

Table 1. Carbon and oxygen isotopes of Albian-Danian limestones of Ariyalur area of Cauvery Basin. S. No. 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40.

Sample No.

δ13C

δ18O

Z

D1 D2 D3 D4 U1 U2 U3 U4 G1 G2 G3 G4 G5 G6 G7 K1 K2 K3 K4 K5 K6 K7 K8 K9 N1 N2 N3 N4 N5 D1 D2 D3 G1 G4 K4 K5 K6 K8 N4 N5

+0.8 +0.3 -6.2 -1.7 -13.1 -5.5 -3.2 -4.3 -10.8 -11.5 -11.1 -8.2 +0.6 -0.3 +1.1 -1.9 -3.1 -3.7 -1.3 -0.5 -13.2 -10.6 -2.9 -2.2 -8.2 -5.1 -5.1 -4.8 -6.2 -7.6 -6.0 -8.2 -12.1 -6.4 -18.9 -10.5 -4.6 -3.9 -5.2 -7.4

-2.48 -3.16 -6.56 -8.98 -8.30 -8.11 -7.14 -7.50 -8.59 -8.30 -5.59 -8.69 -2.58 -4.62 -3.01 -4.62 6.46 -7.24 -6.27 -3.74 -5.98 -7.53 -8.86 -4.91 -5.49 -6.17 -4.91 -5.88 -7.82 -4.33 -7.04 -8.58 -9.04 -8.40 -8.30 -8.89 -8.96 -6.95 -7.43 -8.62

128 126 111 120 96 112 117 115 102 100 108 106 127 124 128 121 118 116 122 124 97 102 117 120 108 114 114 115 111 110 112 106 98 110 84 101 113 116 113 108

Lithology: 1-4 Algal limestone, 5-8 Argillaceous limestone, 9-15 Shell limestone, 16-20 Sandy fossilferous limestone, 21-24 Fossilferous limestone, 25 Nodular limestone, 26-29 Fossilferous limestone Sample Type: 1-29 Whole rock, 30-40 Cement, Formation: 1-4 Dalmiapuram, 5-8 Karai, 9-15 Kulakkalnattam, 16-24 Kallankurichchi, 25-29 Niniyur, 30-32 Dalmiapuram, 33-34 Kulakkalnattam, 35-38 Kallankurichchi, 39-40 Niniyur.

certain leached out cavities. This petrographic type is recorded in Karai, Kulakkalnattam, Kallankurichchi and Niniyur Formations. Packstone has framework elements of molluscans, algae, bryozoans, crinoids and foraminifera. The foraminifera includes uniserial and biserial forms, orbitoids and siderolites. Some organic framework is highly fragmented and broken into discrete particles. The cement is microsparite and sparry calcite. A number of hematite filled aggregate fragments are seen in the sparry Gondwana Research, V. 7, No. 2, 2004

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calcite cement. Some molluscan fragments with original shell structure are also found in the sparry calcite mosaic. The reworked organics are filled with hematite as well as zooecia of bryozoans. Thin film of micritic coating is found on the molluscan grains. At places, the algal induced micritization is prevailed and this process has resulted in cloudy appearances. Because of micritic envelop over the bioclasts, the internal structure of shell fragments is not completely dissolved in the diagenetic environment. Some algal lumps enclosing certain organic fragments are also seen. The inoceramus shell walls were bored by some borers. The cavities formed by such organic borings have rich accumulation of detritus, juvenile brachiopods, foraminiferal materials, pellets, opaque, bryozoans in micritic matrix whereas some cavities are completely filled with sparry calcite cement. This petrographic type is found in the Dalmiapuram, Kulakkalnattam, Kallankurichchi and Niniyur Formations. Grainstone has rich fragmented and micritic coated molluscan bioclasts, algal lumps and crusts represented by Girvanella, Lithothamnium, etc. The other organic fragments in this rock type include foraminifers represented by orbitoids, benthic rotaliid and uniserial and biserial smaller foraminifera. The foraminiferal chambers are filled with ferruginous carbonate mud. Echinoderm plates showing pore structures. It also encloses number of intraclasts and ferruginous clasts which are cemented by sparrite. This petrographic type is identified in the Kallankurichchi Formation. Boundstone exhibits growth structure of bryozoa on substratum and encrusted by Lithothamnium algae. Algal

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micritisation is prevalent and micritisation has also been observed even in the zooecial openings of the bryozoa. Other bioclasts in the framework include miliolid and other benthic rotaliid foraminifera, Girvanella alga, inoceramus and echinoid spines. Sparry calcite mosaic is seen in the cavities of leached organisms. Hematite fillings and iron rich aggregates enclosing ferruginous grains are also observed in the thin sections. This petrographic type is found in the Kallankurichchi Formation.

Results Whole rock samples The analysed samples show large variations in carbon isotope values (Table 1). The algal limestone of Dalmiapuram Formation show negative to slight positive values (-1.7 to +0.8‰). The argillaceous limestones (Karai Formation) show large negative values, which vary from -13.1 to -3.2‰. Likewise, the shell limestones of Kulakkalnattam Formation exhibits more negative values (-11.5 to -0.3‰), except two samples (G5 and G7), which shows slight positive value (+0.6 and +1.1‰). In Kallankurichchi Formation, least variations are observed in the sandy fossiliferous limestones (-3.7 to -0.5‰), whereas large variations are observed in the fossiliferous limestone (-13.2 to -2.2‰). In nodular and fossiliferous limestones (Niniyur Formation), the δ13C values vary from -8.2 to -4.8‰. The δ18O values for algal limestones of Dalmiapuram Formation range from -9.0 to -2.5‰. The argillaceous limestones of Karai Formation show little variation in

Fig. 2. Facies mosaic of the AlbianDanian succession of Ariyalur area of Cauvery Basin (modified after Sundaram et al. 2001). Gondwana Research, V. 7, No. 2, 2004

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oxygen isotope values (-8.3 to -7.1‰). The shell limestones of Kulakkalnattam Formation show negative values from -8.7 to -2.6‰. The limestones of Kallankurichchi Formation mostly show lower values than other formations. It is less in sandy fossiliferous limestones (-7.2 to -3.7‰) when compared with fossiliferous limestones (-8.9 to -4.9‰). Likewise, the nodular and fossiliferous limestones of Niniyur Formation also exhibit negative values (-7.8 to -4.9‰). Calcite cement samples The calcite cements separated from algal limestones (Dalmiapuram Formation) typically display δ13C values which range from -8.2 to -6.0‰, similarly, the δ18O values of algal limestone vary from -8.6 to -4.3‰. The δ13C isotope compositions of calcite cements from shell limestone (Kulakkalnattam Formation) exhibit a broad range of values from -12.1 to -6.4‰, whereas the δ18O values show small variations (-9.0 to -8.4‰). The calcite cement from sandy fossiliferous and fossiliferous limestones (Kallankurichchi Formation) are highly depleted in both δ13C (-18.9 to -3.9‰) and δ18O (-9.0 to 7.0‰) values. The carbon and oxygen isotope values of fosiliferous limestones from the Niniyur Formation also show small variations among them. Accordingly, the δ13C values range from -7.4 to -5.2‰, and δ18O values range from -8.6 to -7.4‰.

Discussion The pelagic and hemipelagic carbonates from different periods and localities show significant variations in isotopic compositions, which have been interpreted to reflect primary variations in oceanic isotopic signals, as the carbonate rocks contain significant amount of calcareous planktic micro and nannofossils (Scholle and Arthur, 1980; Jenkyns et al., 1994; Price et al., 1998; Rosales et al., 2001). The geologically old carbonate rocks that have undergone post-depositional diagenesis play a significant role in modifying the pristine isotopic ratio (Mitchell et al., 1997). Diagenetically stabilised sedimentary carbonates preserve the carbon isotopic composition of original carbonate muds within ±1‰ of the original values but show more scatter in the oxygen isotope values (Tucker and Wright, 1990). The Albian-Danian limestones exhibit wide variations in the carbon isotope values for whole rock (-13.2‰ to +1.1‰) and cement samples (-18.9 to -3.9‰). Such large variations suggest re-equilibration between rock components with isotopically light waters (fresh waters), and also indicate the presence of original marine signatures (unaltered or less altered) (Bellenca et al., 1995). Few samples from Dalmiapuram (S. Nos. D1 and D2) and

Kulakkalnattam (S. Nos. G5 and G7) Formations exhibit slight positive carbon isotope values (+0.8 and +0.3, +0.6 and +1.1‰ respectively; Table 1). During transgression, more amount of organic matter is locked in the marginal areas, resulting in the enrichment of 13C, while at the regressive phase of the sea, the locked-up organic matter is eroded and oxidised, resulting in 12C enrichment in the deep ocean (Broecker, 1982). Positive carbon excursions have been encountered at several stratigraphic horizons during Mesozoic time, which reflect the times of increased organic productivity and/or enhanced organic deposition within the oceans (Jenkyns, 1980; Arthur et al., 1987). Most of the organic carbon in the marine realms are synthesised and observed that they are deposited in the shelf and marginal marine environments, where upwelling and riverine input of nutrients are locally important (Pelet, 1987). The limestones from different formations exhibit negative δ13C values with extreme variations (-13.2 to -0.3‰ for whole rock and -18.9 to -3.9‰ for cement samples; Table 1), except four samples. The observed large variations in carbon isotopic values probably reflect the contribution of variable amounts of soil-derived organic carbon to meteoric pore waters (Allan and Matthews, 1982). The negative values of δ13C are mainly due to biogenic production of CO2 in the soil (Cerling and Hay, 1986) and indicate subaerial exposure, because of incorporation of lighter carbon isotope from soil-borne carbon dioxide and decay of terrestrial matter (Hudson, 1977). Soil carbonates form in soils with a net water deficit, generally in soils where precipitation is less than about 100 cm per year (Jenny, 1980). Since most of the samples for the present work are collected from exposed Table 2. Stable isotope data for calcite cement samples for the AlbianDanian limestones of Cauvery Basin. Sample No.

Sample d13Ccalcite d18Ocalcite d18Ocalcite d18Owater (SMOW)b type (‰ PDB) (‰ PDB) (SMOW)a 20°C 25° C

D1 calcite D2 calcite D3 calcite G1 calcite G4 calcite K4 calcite K5 calcite K6 calcite K8 calcite N4 calcite N5 calcite average (n = 11)

-7.6 -4.3 26.4 -3.4 -2.3 -6.0 -7.0 23.6 -6.1 -5.0 -8.2 -8.6 22.0 -7.7 -6.6 -12.1 -9.0 21.5 -8.1 -7.1 -6.4 -8.4 22.2 -7.5 -6.4 -18.9 -8.3 22.3 -7.4 -6.3 -10.5 -8.9 21.7 -8.0 -6.9 -4.6 -9.0 21.6 -8.0 -7.0 -3.9 -7.0 23.7 -6.0 -5.0 -5.2 -7.4 23.2 -6.5 -5.4 -7.4 -8.6 22.0 -7.7 -6.6 -8.3± 4.3 -7.9± 1.4 22.8±1.4 -6.9±1.4 -5.9±1.4

n=number of samples, aδ18Ocalcite (SMOW) =1.03086 δ18Ocalcite (‰ PDB) + 30.86 (Friedman and O’Neil, 1977) b these values are calculated by assuming a paleotemperature of precipitation of 20° C and 25° C (e.g., Wright, 1987) and using the calcite paleothermometer of Friedman and O’Neil (1977). Gondwana Research, V. 7, No. 2, 2004

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surfaces, the protracted pedogenic process could be responsible for the shift towards low negative δ13C in some samples. Among various factors contributing δ 13C variation, the role of organic matter is well known (Craig, 1953). Since the organic matter production is related to sea-level variation, attempts are being made to relate the δ13C variation with sea-level change. The positive shifts are seen to occur during transgression and the negative shifts during regression. The rise in sea level leads to decrease in the production of organic matter and slows down the decay of organic matter, which returns the isotopically depleted carbon to the dissolved inorganic carbon reservoir. Thus, the amount of CO2 with light isotopic ratio reduces in the seawater and consequently the δ13C increases. The reverse is true when the sea level falls (Renard, 1986). Similarly, the negative δ13C shifts in the carbon-isotope signature for the Albian-Danian limestones of this study seems to be linked with lowering of eustatic sea level (Shackleton et al., 1983; Berger and Vincent, 1986; Weissert, 1989; Follmi et al., 1994). The role of plants is also indicated by the relatively negative δ13C values, because the isotopic composition of soil carbonates is controlled by CO2 in soil gases, whose carbon isotopic composition is mainly controlled by the proportion of C3 and C4 plants in the local ecosystem. Recent studies have demonstrated that the oxygen and carbon isotope compositions of carbonates can be used to infer palaeoclimate and palaeoecology (Anderson and Arthur, 1983; Quade et al., 1995; Rajagopalan et al., 1999). C3 plants, which include trees, most shrubs and herbs, and cool season grasses (montane) have δ13C values between -25‰ and -32‰; C4 plants, which include grasses favoured by warm growing seasons and a few shrubs in the families Euphorbiaceae and Chenopodiaceae, they have δ13C values between -10‰ and -14‰ (Quade et al., 1989). Some whole rock and carbonate cement samples of this study (Albian-Danian age) show very low negative values of δ13C (e.g. -13.2 to -10.6‰ and -18.9 to -10.5‰, respectively), mainly due to the effects of pedogenic alteration on these limestones as we discussed earlier. It is generally believed that the plants utilizing C4 or CAM photosynthetic path ways did not evolve until the Miocene. Recently, however, the possible presence of non C3 plants in the late Cretaceous was suggested by Bocherens et al. (1994). But there is no evidence for C4 plants in world wide during Cretaceous. Latorre et al. (1997) and Quade et al. (1989) suggested that the starting of C4 plants at 7.3 – 6.7 Ma. The reason for the expansion of C4 plants in the late Neogene is falling of pCO2 levels and climate change. Believing, instead of the glacial sea and upliftment of Himalayas. Thus the δ13C isotopic values for the limestones (soil derived carbonates) of Cauvery Gondwana Research, V. 7, No. 2, 2004

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Basin indicate an ecosystem dominated by C3 plants. The dominance of C3 plant in the Cretaceous system was discussed by many workers (Lee, 1999; Cerling et al., 1992; Latorre et al., 1997; Scott, 2002). Like δ13C values, we observed large variations also in 18 δ O values, which range from –9.0 to -2.6‰ for whole rock samples and -9.0 to -4.3‰ for cement samples. Wide range and negative δ18O (Table 1) values in carbonate rocks have been interpreted elsewhere as having resulted from the mixing of marine and fresh waters related to regressive sea-level cycles (Carpenter et al., 1988; Longstaffe et al., 1992; Ludvigson et al., 1994). Fluctuating relative sea levels controlled the position of the coastal aquifer, with sea-level rises resulting in more marine pore fluids, and lowering of relative sea level resulted in seaward shifts of the coastal mixing zone and consequent freshening of pore fluids. Carbonate cements precipitated during this active hydrological history recorded the isotopic and trace element characteristics of the parent fluids (Coniglio et al., 2000). Thus, the negative δ18O shifts in oxygen isotopic signature of the Albian-Danian limestones of Cauvery Basin also seems to be linked with lowering of eustatic sea levels. The δ13C and δ18O values were plotted in a bivariate plot which show most of the samples fall below the zero line whereas few samples fall just above the zero line (Fig. 3). Ancient marine carbonates reveal δ13C values close to zero (Faure, 1986) on the PDB scale, while slightly negative values (-4.9‰) are observed in freshwater carbonates (Hudson, 1977). Likewise, these data were also plotted in the bivariate plot which was initially proposed by Hudson (1977) and later modified by Nelson and Smith (1996), they distinguished various isotopic fields for carbonates of different origins, by using New Zealand carbonate δ18O and δ13C data from a variety of published and unpublished sources (Fig. 4). In this bivariate plot, most of the whole rock samples fall in the fresh water limestone and meteoric cement fields, whereas some samples fall in the marine limestone as well as in the soil calcite field. The Albian-Danian limestones of Cauvery Basin were initially deposited in a marine environment, later these limestones were subjected to meteoric diagenesis. These limestones represent mudstone, wackestone, packstone, grainstone and boundstone combination. Most of the limestones enclosing algae, coral, foraminifera, ostracod carapaces and bryozoans which indicate that these limestones were deposited in the shallow marine environment. The petrographic study reveals that the microsparite and sparry calcite cements are present in the pore spaces and bioclasts chambers which strongly support the meteoric diagenesis. The diagenetic pathway is not complete because some

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limestone samples still retain their original marine signature (S. Nos. D1, D2, G5, and G7; Table 1). The isotopic values of calcite cements also are plotted in the same bivariate plot. Most of the samples fall in the freshwater limestone and meteoric cement fields except two samples, which fall in the soil calcite field, this further support the possibility for the prevalence of diagenetic transformation in these limestones. It is further confirmed by following the equation proposed by Keith and Weber (1964), which is used here to discriminate marine and freshwater limestones; where Z = a(δ13C + 50) + b(δ18O + 50) in which ‘a’ and ‘b’ are 2.048 and 0.498 respectively. The limestones with Z values above 120 are considered as marine, whereas those with Z values below 120 would be classified as freshwater type. In the present study (whole rock samples), eight samples show Z values above 120, whereas nineteen samples exhibit Z values below 120; two samples have Z values close to 120 (Table 1). Furthermore, the Z values are less than 120 for all the cement samples and come under the fresh water type. The wide range of δ18O compositions is thus interpreted to reflect fluctuations, possibly related to climate, in local marine baseline compositions. Pore waters in shallow buried sediment communicated freely with overlying sea water and precipitated early diagenetic calcite characterized by a broad range of δ18O values. Assuming the calcite samples formed during shallow diagenesis, the δ18O values of pore waters that precipitated these calcites are estimated to range from -8.1 to -3.4‰ (average value of -6.9 ± 1.4‰, SMOW; Table 2) at 20°C and from -7.1 to 2.3‰ (average value of -5.9 ± 1.4‰, SMOW; Table 2). The observed values are low which indicates that the diagenetic fluids contain mainly of meteoric waters. Lee (1995) estimated the oxygen isotope compositions of the Early Cretaceous meteoric water was about -6‰ SMOW.

Fig. 4. Reference δ18O - δ13C diagram showing isotope fields for a selection of carbonate components, sediments, limestones, cements, dolomites and concretions (after Nelson and Smith, 1996)

So our pore water values are in good agreement with the Early Cretaceous meteoric water. The whole rock and cement isotopic composition suggest that the dominant diagenetic signatures present in these limestones are mainly due to meteoric diagenesis.

Conclusion

Fig. 3. Bivariate plot of δ18O - δ13C for limestones of Cauvery Basin.

The limestones of Cauvery Basin were deposited under shallow marine environment and during the initial period, the basin experienced calm environment which initiated the development of micritic mud, and the pore spaces and bioclasts were filled with micrite. Later, the deposited limestones were subjected to freshwater phreatic environment, which is favorable for the formation of equant sparry calcite cement. The limestones enclose algal remains, molluscs, bryozoans, foraminifers and ostracods. Gondwana Research, V. 7, No. 2, 2004

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Thus, the petrographic study reveals the presence of sparry calcite cement in the pore spaces and vugs, which strongly support the evidence of meteoric input. The isotopically negative δ13C and δ18O values may result when stabilisation of marine limestones are largely influenced by meteoric water as evidenced through the petrographic study. The bivariate plot of δ13C and δ18O suggests that most of the limestone samples are freshwater and meteoric cement types. Finally the petrographic and isotopic studies on Albian-Danian limestones of Cauvery Basin indicate that they were deposited in a marine environment and subsequently underwent diagenesis in a meteoric environment.

Acknowledgments The first author would like to thank the Department of Science and Technology, Government of India, for providing financial assistance under the Young Scientist Scheme (No. SR/FTP/ES-110/2001). We are thankful to Prof. M. Santhosh and Dr. M. Satish Kumar for their useful suggestions during this study. We would like to thank an anonymous reviewer who offered critical comments which helped us to improve our presentation. This work was partly supported by UGC SAP Phase II, UGC COSIST and DST-FIST programs of the Department of Geology, University of Madras, and PAPIIT grant IN-106199 to JSA(UNAM, Mexico).

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