Carbonate Geochemistry

International Geology Review, Vol. 45, 2003, p. 16–26. Copyright © 2003 by V. H. Winston & Son, Inc. All rights reserved.

Geochemistry of Upper Miocene Kudankulam Limestones, Southern India J. S. ARMSTRONG-ALTRIN,1 SURENDRA P. VERMA, Centro de Investigación en Energía, Universidad Nacional Autónoma de México (UNAM), Priv. Xochicalco s/n, Col. Centro, Apartado Postal 34, Temixco, Mor. 62580, México

J. MADHAVARAJU, Department of Geology, University of Madras, Guindy Campus, Chennai-600025, India

YONG IL LEE, School of Earth and Environmental Sciences, Seoul National University, Seoul 151-742, South Korea AND

S. RAMASAMY

Department of Geology, University of Madras, Guindy Campus, Chennai-600025, India

Abstract Concentrations of major, trace, and rare-earth elements (REE) were measured in shallow marine limestone samples of the upper Miocene Kudankulam Formation, southern India, in order to investigate the geochemical variations among various litho-units. The CaCO 3 content is higher in algal limestone (AL; 92 ± 1, n = 3) and clastic limestone (CL; 90 ± 2, n = 3) than sandy shell limestone (SSL; 81 ± 1, n = 3). All trace elements exhibit lower concentrations than post-Archean Australian Shale (PAAS) values, except one SSL sample. Large variations in SREE content are observed among CL, SSL, and AL (~14–142, ~68–124, and ~38–98, respectively). Almost all limestone samples analyzed from the Kudankulam Formation show a small negative cerium anomaly (Ce/Ce* ~0.8– 0.9), except one AL sample, which lacks this cerium anomaly (Ce/Ce* ~1.04). Variations in Ce anomalies and SREE contents in Kudankulam limestone samples are mainly controlled by the amount of terrigenous sediments and diagenetic behavior. Shale-normalized REE patterns and (La/ Yb)s, La/Sc, La/Th, and Th/Sc ratios suggest that the detrital sediments present in the limestones were probably derived from felsic source rocks. The observed low content of U (0.9 ± 0.5, n = 9) and U/Th (0.2 ± 0.1, n = 9) ratio in these limestones are probably related to an oxygen-rich environment.

Introduction

tional environments such as widespread marine anoxia (Liu et al., 1988; German and Elderfield, 1990; Murray et al., 1991b), oceanic palaeo-redox conditions (Liu et al., 1988), proximity to source area (Murray et al., 1991b), surface productivity variations (Toyoda et al., 1990), lithology, and diagenesis (Nath et al., 1992; Madhavaraju and Ramasamy, 1999). It is important to study whether or not the redox changes observed in the water column are transmitted to the underlying sediments. These types of studies should be carried out on modern sediments with known environmental settings, which will help in resolving uncertainties regarding depositional environments of ancient sediments (Macfarlane et al., 1994). From a detailed study of carbonate sediments deposited from the oxygen minimum zone (OMZ) in the Arabian Sea, Nath et al. (1997) sug-

T HE BEHAVIOR OF rare-earth elements (REE) in marine waters, sediments, and carbonate rocks has been studied in detail by many workers (Ronov et al., 1967; Piper, 1974; Klinkhammer et al., 1983; De Baar et al., 1988; Elderfield et al., 1990; Sholkovitz, 1990). Dominant factors influencing the REE contents of carbonate rocks are: (1) the amount of terrigenous input (Piper, 1974; McLennan, 1989; Murray et al., 1991b); (2) variations in the oxygen level in the water column (Liu et al., 1988); and (3) biogenic sedimentation (Murphy and Raymond, 1984). The distribution of REE, particularly the Ce anomaly, in marine sediments and carbonate rocks has proved to be an excellent indicator of deposi1Corresponding

author; email: [email protected]

0020-6814/03/643/16-11 $10.00

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UPPER MIOCENE KUDANKULAM LIMESTONES

17

FIG. 1. Simplified geological map of southern India showing the location of the study area (modified after Singh and Rajamani 2001). Abbreviations in inset: G = Gujarat; K = Kerala; Kr = Karaikal beds; S = Sri Lanka; J = Jaffna Formation.

gested that there is no variation in Ce anomaly deposited between these carbonates and those outside the OMZ. More recently, Madhavaraju and Ramasamy (1999) studied shallow-marine Maastrichtian carbonates of southern India to provide useful information regarding the geological environment such as terrigenous inputs and redox conditions at the time and place of deposition. They suggested that U and U/Th ratios are useful for paleoredox interpretations. No systematic geochemical work has yet been carried out on the limestones of the upper Miocene Kudankulam Formation. We report new major, trace-element, and REE data along with information on the depositional environment. Our main objective is to gauge the usefulness of uranium in predicting the paleoredox condit ions, and the probable reason for the variation in Ce concentration and Ce anomalies in the Kudankulam limestones.

Geology and Stratigraphy In the southeastern part of Peninsular India (Fig. 1), limestone deposits have been discovered that indicate marine influence during the formation of the southern part of Tamil Nadu (Bruckner, 1988). This deposit has been called Kudankulam Formation, which crops out 20 km NE of Kanniyakumari near the village of Kudankulam. Isolated exposures are present near villages such as Navalady, Manapaud, Tiruchendur, and Sattankulam (Fig. 2). The maximum altitude of this bed is ~51 m near Kudankulam and +20 m above msl at Tiruchendur. Sedimentary rocks of the Kudankulam Formation are dominated by clastic and carbonate rocks (Armstrong Altrin Sam, 1998). These strata are present in well cuttings, stream sections, and in a number of quarry sections. The clastic and carbonate rocks contain numerous fossils such as bryozoans, molluscs, echinoderms, algae, and foraminifera. The thickness of the sedimentary unit is only 4 to 5 m.

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ARMSTRONG-ALTRIN ET AL.

FIG. 2. Simplified geological map of the Kudankulam Formation, showing sample locations.

Hence, the area has not attracted many researchers. Bruckner (1988) has made an attempt to establish sea-level fluctuations based on the coastal carbonates of Tamil Nadu. He selected the Kudankulam limestones as the marker to reconstruct sea-level changes. The facies build-up and petrological data suggest that these limestones were deposited by a shallow sea, and formed in a near-shore environment (Bruckner, 1988). The Kudankulam Formation in southern India is most probably the equivalent of the Jaffna Formation of Sri Lanka. The occurrence of index microfossils like Austroltrillina howchini sp. and Taberina malabarica sp. places the age of the Jaffna Formation in the upper part of the late Miocene, specifically in the Burdigalian stage (Sahni, 1979; Cooray, 1984; Bruckner, 1988). Hence, the limestones of the Kudankulam Formation are either of Burdigalian or of late Miocene–Pliocene age. Detailed systematic work on the stratigraphy and petrography was reported by Ramasamy et al. (1994). Armstrong Altrin Sam and Ramasamy (1997), and Ramasamy and Armstrong Altrin Sam (1998) have carried out petrography and major-element geochemistry on this limestone formation. Finally, from a stable isotope study, it has been concluded that these limestones were subjected to meteoric or fresh water phreatic diagenesis (Armstrong Altrin Sam et al., 2001).

Analytical Methods Samples were collected from surface outcrops and quarry sections. Nine representative samples were analyzed—three from clastic limestone (CL), three from sandy shell limestone (SSL), and three from algal limestone (AL). These samples were washed with distilled water, air dried, and then ground in an agate mortar. The analytical techniques proposed by Shapiro (1975) were adopted for the preparation of solutions for major and trace elements. Si and Al were determined using a spectrophotometer. Fe was analyzed by atomic absorption spectrometer. CaCO3 was determined by a titration method using EDTA. Trace-element (Ba, Co, Cr, Cu, Ni, Sc, Sr, V, Zn, and Zr) concentrations were determined using a Jobin Yvon 138 Ultrace inductively coupled plasma atomic emission spectrometer (ICPAES). Rare-earth elements (REE) and additional trace elements (Cs, Hf, Nb, Pb, Rb, Th, U, and Y) were analyzed by a VG Elemental PQII Plus inductively coupled plasma mass spectrometer (ICP-MS) (Jarvis, 1988). United States Geological Survey Standard MAG-1 was used for calibration. All traceand rare-earth elements were analyzed at the Korea Basic Science Institute. Three analyses were made for each sample and averaged. Analytical precision for both trace elements and REE is better than 5%. For preparing REE-normalized diagrams, post-

UPPER MIOCENE KUDANKULAM LIMESTONES

Archean Australian Shale (PAAS) values listed by Taylor and McLennan (1985) were used. The ratio of [Ce/Ce*] (Ce anomaly) is defined using the calculated value of [Ce] (Cesample /CePAAS) and the predicted value of [Ce*] obtained by the interpolation from the PAAS-normalized values of La and Pr. The calculation of Eu anomaly [Eu/Eu*] was done in a similar way using the observed values of Sm, Eu, and Gd.

Results Petrography Clastic limestone (CL). The bioclasts are exceptionally large. Large foraminifers followed by bryozoan, algal, molluscan, and crinoidal fragments dominate the bioclasts. It also contains important amounts of subrounded medium-sized quartz grains and reworked bioclasts. A few peloidal grains also are present. Micritization of bioclasts is prevalent. The grains are cemented by sparry calcite. Sandy shell limestone (SSL). This litho-unit contains micritized molluscan grains along with bryozoan bioclasts. They are highly fragmented. Many of the bioclasts are subrounded, suggesting deposition in a high-energy environment. Scattered, subrounded linear quartz grains also are present. Micritization was accomplished by endolithic algae in an early diagenetic environment. The molluscan bioclasts show lamellar shell wall structures. Algal limestone (AL). The lithology consists of assorted bioclasts of bryozoa, molluscs, ostracods, foraminifers, including miliolids, echinoderms, and algal elements (Lithothamnium sp. and Amphiroa sp.). Many molluscan shell fragments are completely leached out, and the voids are filled with sparry calcite, reflecting vadose-fresh water diagenetic environments. Several larger foraminifers and miliolid smaller foraminifer also were identified. The association of bioclasts such as bryozoans, molluscs, echinoderms, algae, and larger foraminifers suggests that the depositional environment was shallow marginal marine. The fragmented and rounded nature of most bioclasts reveals a moderate to highly agitated environment in a bank-like setting. Considering the skeletal fabrics and the association of packstone with coated and worn bioclasts, standard microfacies (SMF) no. 10 is suggested for the dominant carbonate deposition of this area. Facies Zone-7 (Flugel, 1982) may be appropriate to demonstrate the occurrence of dominant particles from high-energy environments on shoals, which

19

have moved down local slopes to be deposited in quiet water (Armstrong Altrin Sam and Ramasamy, 2000). The occurrence of clotted micrite and other calcrete-related features reveal that these deposits were periodically subaerially exposed.

Major elements Conc entrations of major elements in the Kudankulam limestones are listed in Table 1. Small variations are found in Si and CaCO 3 contents (~0.7–2.6 and ~88–91, respectively) in CL. In SSL the content of Si ranges from ~5.1 to 5.9 (Table 1). Like Si, small variation is observed in CaCO3 content for SSL (~80–82; Table 1). A low concentration of Si is observed in AL, which varies from ~0.7 to 2.6, whereas their CaCO3 content is higher (~92– 93). The Al content in CL, SSL, and AL varies from ~0.1 to 0.8, ~0.6 to 0.9, and ~0.2 to 0.3, respectively (Table 1). The content of Fe is very low in the Kudankulam limestones. Geochemical compositions reveal the enrichment of terrigenous sediments in the CL and SSL relative to AL. Si vs. CaCO3 shows a clear negative correlation (correlation coefficient r = –0.86, number of samples n = 9), which probably suggests that these two elements exhibit different modes of origin. The silica is derived mainly from siliclastic sediments, whereas the CaCO3 was derived from carbonate cements.

Trace elements Trace-element concentrations of the Kudankulam limestones are also listed in Table 1. The largeion lithophile elements (LILE; Sr, Rb, Cs, and Ba) have low concentrations compared to PAAS (postArchean Australian Shale; Fig. 3). One sample from SSL (sample no. 5) shows high concentrations of Sr and Ba with respect to the average composition of PAAS. The ferromagnesian trace elements Co, Cr, and V are depleted, and Ni is somewhat enriched, particularly in the SSL sample No. 5 (Fig. 3). The enrichment of Ni, particularly in the SSL, is mainly due to the high content of feldspars present in this litho-unit. High-field-strength elements (HFSE) such as Zr, Y, Nb, Hf, Th, and U are resistant to weathering and alteration processes compared to other trace elements (Taylor and McLennan, 1985; Bhatia and Crook, 1986; Feng and Kerrich, 1990). These elements also have lower concentrations in comparison with PAAS. The variations in the elemental concentrations in the CL, SSL, and AL are mainly due to

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ARMSTRONG-ALTRIN ET AL.

TABLE 1. Major (wt%), Trace (ppm), and Rare-Earth Element (ppm) Data for the Kudankulam Limestones Rock type: Sample: Si Al Fe Ba Co Cr Cs Cu Hf Nb Ni Pb Rb Sc Sr Th U V Y Zn Zr La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu CaCO3

SREE Eu/Eu* Ce/Ce* (La/Yb)s U/Th Th/Sc La/Sc La/Th

Clastic limestone (CL) 1 2 3 2.63 0.75 0.08 72 11.4 11.2 0.42 6.10 0.88 7.62 85 11.3 15.8 2.48 47 15.6 0.78 66.6 7.90 16.0 13.2

0.67 0.14 0.08 62 1.68 6.39 0.04 6.39 0.54 0.95 63 6.57 10.1 0.67 59 1.45 0.43 11.3 2.59 11.15 6.14

2.36 0.71 0.26 60 7.27 5.38 0.26 7.60 0.76 5.09 66 3.99 9.90 1.40 40 9.70 0.92 17.4 5.83 14.4 12.2

34.3 63.3 7.47 26.4 3.81 0.34 2.96 0.37 1.79 0.22 0.77 0.09 0.56 0.07 91 142 0.47 0.90 4.49 0.05 6.31 13.84 2.20

3.45 5.53 0.69 2.43 0.43 0.09 0.47 0.07 0.41 0.04 0.22 0.03 0.20 0.02 88 14 0.89 0.82 1.29 0.30 2.15 5.12 2.38

25.2 47.9 5.64 19.9 2.82 0.21 2.13 0.26 1.25 0.14 0.54 0.06 0.39 0.05 91 106 0.40 0.92 4.81 0.09 6.95 18.07 2.60

Sandy shell limestone (SSL) 4 5 6 5.86 0.87 0.20 124 1.90 22.1 0.12 7.44 1.23 5.63 119 9.6 16.4 4.31 43 10.1 1.96 32.0 16.8 16.4 33.3

7

Algal limestone (AL) 8 9

5.12 0.56 0.40 822 1.73 30.4 0.09 8.61 0.69 2.92 801 5.88 23.1 3.11 584 3.87 1.24 29.4 7.29 8.37 11.72

5.17 0.57 0.02 59 1.84 12.0 0.09 2.65 0.55 2.62 64 2.89 9.9 1.99 122 5.20 0.83 11.8 8.40 4.09 7.70

1.61 0.27 0.14 41 1.61 11.4 0.07 6.67 0.44 1.51 37 2.78 6.16 1.84 139 2.61 0.63 9.05 5.08 5.96 5.24

2.62 0.21 0.62 96 2.21 11.3 0.25 10.7 0.55 2.61 113 5.92 14.6 2.61 84 5.14 1.40 18.6 15.7 14.3 6.81

0.74 0.27 0.53 91 7.53 2.68 0.22 4.14 0.56 3.7 96 4.29 11.9 0.97 45 4.90 0.32 11.5 4.18 7.31 6.09

Rare-earth elements 30.5 16.3 51.7 29.7 6.09 3.26 22.1 11.6 3.51 2.27 0.56 0.64 3.21 1.78 0.43 0.25 2.38 1.40 0.39 0.21 1.30 0.70 0.16 0.09 0.98 0.53 0.14 0.08 80 81 124 69 0.78 1.48 0.90 0.93 2.29 2.26 0.20 0.32 2.34 1.24 7.08 5.23 3.03 4.20

16.2 28.5 3.51 12.6 1.96 0.30 1.84 0.25 1.42 0.21 0.75 0.09 0.60 0.08 82 68 0.74 0.86 2.00 0.16 2.62 8.18 3.13

8.66 16.6 1.87 6.79 1.12 0.17 1.10 0.16 0.85 0.12 0.43 0.05 0.33 0.04 92 38 0.71 0.94 1.92 0.24 1.42 4.72 3.32

19.3 42.8 4.58 17.4 3.35 0.66 3.47 0.51 2.92 0.49 1.50 0.19 1.20 0.17 93 98 0.89 1.04 1.18 0.27 1.97 7.39 3.80

15.0 25.2 2.95 10.3 1.51 0.21 1.26 0.16 0.81 0.09 0.38 0.04 0.28 0.04 92 58 0.69 0.86 3.96 0.07 5.06 15.47 3.06

UPPER MIOCENE KUDANKULAM LIMESTONES

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FIG. 3. Multi-element diagram for the Kudankulam limestones, normalized against average post-Archean Australian Shale (PAAS; Taylor and McLennan, 1985). These PAAS values are (in ppm): Co = 23; Ni = 55; Cr = 110; V = 150; Sr = 200; Rb = 160; Cs = 15; Ba = 650; Pb = 20; Zr = 210; Y = 27; Nb = 19; Hf = 5; Th = 14.6; U = 3.1.

FIG. 4. Shale-normalized REE plots for the Kudankulam limestones with sample numbers. PAAS normalization values from Taylor and McLennan (1985) are (in ppm): La = 38; Ce = 80; Pr = 8.9; Nd = 32; Sm = 5.6; Eu = 1.1; Gd = 4.7; Tb = 0.77; Dy = 4.4; Ho = 1.0; Er = 2.9; Tm = 0.40; Yb = 2.8; Lu = 0.43.

differences in their lithologies, as described in the petrographic study.

Rare-earth elements The results of REE concentrations are also presented in Table 1. The shale-normalized REE concentrations are less than one for all Kudankulam limestone samples (Fig. 4). CL, SSL, and AL exhibit large variations in SREE content (~14–142, ~68– 124, and ~38–98, respectively). Generally, the REE contents are lower in limestone samples than clastic sediments. High contents of REE in clastic sedi-

ments are mainly due to the occurrence of silt and clay fractions, because REE are readily accommodated in the clay structure (McLennan, 1989). The presence of contrasting amounts of terrigenous sediments may cause the differences in the REE contents among Kudanku lam limestones. The low content of REE in some samples could be due to REE dilution by carbonate minerals. The shale-normalized REE patterns of these limestones (Fig. 4) show a slight enrichment in light REE relative to heavy REE. In this diagram, most of these samples show a small negative Ce anomaly

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ARMSTRONG-ALTRIN ET AL.

TABLE 2. Average Values of Kudankulam Limestones in This Study Compared to Values of Shallowand Deep-Marine Carbonate Sediments

Kudankulam carbonate1

Shallow-marine carbonate2

Arabian Sea carbonate sediments3

Indian Ocean carbonate sediments5

Ce/Ce*

0.90 ± 0.06

0.76 ± 0.16

0.84 ± 0.06

0.56

(La/Yb)s

2.7 ± 1.4

1.8 ± 0.5

0.8 ± 0.2

1.03

SREE

80 ± 40

73 ± 20

78 ± 40

–

CaCO 3

88 ± 5

75 ± 15

51 ± 22

65.3

Eu/Eu*

0.78 ± 0.31

0.58 ± 0.11

1.15 ± 0.08

–

U U/Th

0.9 ± 0.5

0.7 ± 0.5

0.19 ± 0.10

0.22 ± 0.29

6±

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2.1 ± 0.54

– –

1This

study; n = 9. and Ramasamy, 1999; Late Cretaceous, n = 8. 3Nath et al., 1997; n = 9. 4Values from oxygen minimum zone; n = 4. 5Nath et al., 1992. 2Madhavaraju

(Ce/Ce* ~0.8–0.9), whereas one AL sample exhibits no Ce anomaly (Ce/Ce* ~1.04; AL sample No. 8; Table 1). Similarly, most of the Kudankulam limestone samples also show negative Eu anomalies (Eu/ Eu* ~0.4–0.9) except for a single SSL sample, which shows a positive Eu anomaly (SSL sample No. 5; Eu/ Eu* ~1.5; Table 1). The presence of a positive Eu anomaly, particularly in this sample, is consistent with feldspar enrichment.

Discussion Source of REE and provenance characteristics Average SREE contents of the Kudankula m limestone samples (80 ± 40, n = 9, Table 2) is similar to those of Upper Cretaceous shallow-marine carbonates (Madhavaraju and Ramasamy, 1999), as well as modern carbonates of the Arabian Sea (Nath et al., 1997; Table 2). For individual litho-units, there is a large variation in SREE content (CL 88 ± 66, n = 3; SSL 87 ± 32, n = 3; AL 65 ± 31, n = 3). Differences in SREE content among individual samples are mainly related to variations in the amount of terrigenous sediment included in these limestone samples. This seems to be supported by generally lower contents of Si and Al and higher content of CaCO3 in AL than SSL and CL, and further suggests that SREE contents are a function of non-carbonate impurities.

In the Kudankulam limestones, the average ratio of (La/Yb)s (2.4 ± 1.4, n = 9) is somewhat higher than the shallow-marine carbonates of southern India, Arabian Sea carbonate sediments, and Indian Ocean carbonates (Table 2). Also, the (La/Yb)s ratio is higher than the postulated average value for terrigenous sediments [(La/Yb) s = 1.3; Sholkovitz, 1990]. For the individual litho-units, the (La/Yb)s ratios for CL, SSL, and AL are 3.5 ± 2.0, 2.2 ± 0.2, and 2.4 ± 1.4, respectively (n = 3). Differences in (La/Yb)s ratios among various litho-units may be related to: (1) changes in REE input from the source terrain; and (2) diagenetic remobilization and exchange with interstitial water (Murray et al., 1991a), as well as to a decreasing trend of (La/Yb) s ratio with depth (Worash and Valera, 2002). Such diagenetic processes have been documented in recent, shallow, buried estuaries (Sholkovitz et al., 1989). Our study area reflects shallow-marine conditions where the fractionation of REE should have been low. The nature of the source rocks can be identified from REE patterns and (La/Yb) s ratios. Certain trace-element ratios such as Th/Sc, La/Sc, and La/ Th also are useful to infer the nature of the source rocks, because they are sensitive to average provenance compositions. Th is a highly incompatible element, whereas Sc is relatively compatible. Both of these elements are relatively uniformly trans-

UPPER MIOCENE KUDANKULAM LIMESTONES

FIG. 5. Bivariate plot of Sr – Eu/Eu* for the Kudankulam limestones. Note the enrichment of Sr in one sample (sample No. 5) from SSL (sandy shell limestone). The samples falling below the line (Eu/Eu* = 1) have, by definition, a negative Eu anomaly.

ferred into terrigenous sediments from the source through sedimentation (Taylor and McLennan, 1985). The Kudankulam limestone samples exhibit slightly LREE enriched and flat HREE patterns (Fig. 4) with somewhat high average ratios of (La/ Yb)s, Th/Sc, La/Sc, and La/Th (2.7 ± 1.4, 3.3 ± 2.2, 10 ± 5, and 3.1 ± 0.6 respectively, n = 9; Table 1), implying that the terrigenous sediments present in the shallow-marine Kudankulam limestones were derived mainly from felsic source rocks.

Behavior of europium The Eu/Eu * ratio of the Kudankulam limestone samples ranges from ~0.40 to 0.89, except for one SSL sample, which shows a remarkably high Eu/Eu* ratio (Eu/Eu * ~1.5, SSL sample No. 5, Table 1). Generally, the absence of a negative Eu anomaly and the prevalence of a positive Eu anomaly in shale-normalized REE patterns are due to either eolian input (Elderfield, 1988) or hydrothermal solutions (Michard et al., 1983; Worash and Valera, 2002). Hydrothermal solutions originate in the deep-sea environment, but our study area is located along a coastal belt. Because only one Kudankulam limestone sample (SSL sample No. 5) lacks a negative Eu anomaly (or has a positive Eu anomaly), it may be due to local enrichment of feldspar rather than a regional effect such as eolian input or hydrothermal solutions. This interpretation is further supported by the remarkable enrichment of Sr in this sample (Table 1; see the isolated sample in Fig. 5). Finally, the predominance of a negative Eu anomaly in the Kudankulam limestones reveals that the terrigenous part of these samples was probably derived from felsic source rocks.

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Significance of uranium in the marine environment Uranium (U) is thought to be conservative in oxygenated seawater (Ku et al., 1977), because of the formation of stable U+4 and soluble U+6. In oxic seawater, uranium is present in high concentrations as the species uranyl tricarbonate [UO 2 (CO 3 ) 3 4– ], whereas under reducing conditions the soluble U+6 is readily converted into insoluble U+4, which can be removed from the solution onto sediment surfaces (Barnes and Cochran, 1990). Uranium is mobile, whereas Th is relatively immobile in aqueous solutions (Anderson et al., 1983; Nozaki et al., 1981; Wright et al., 1984). In continental-margin environments, uranium is readily fractionated from Th, just like Ce, which is fractionated from other REE (Whittaker and Kyser, 1993). In the Kudankulam limestones, the U content is very low (range ~0.32–1.96, mean 0.9 ± 0.5, n = 9; Table 2) compared to sediments derived from the oxygen minimum zone (Nath et al., 1997). However, U concentrations of the Kudankulam limestones are similar to the shallow-marine carbonates deposited under oxic conditions (Madhavaraju and Ramasamy, 1999; Table 2). Therefore, we propose that the observed low content of U in the Kudankulam limestones is related to the oxygenation level in the water column. In an oxic environment, U is easily removed from the sediments and transferred into the water column. In a reducing environment, on the other hand, U is removed from sea water and precipitates onto the sediments. In some cases, lack of significant reduction of U+6 to U+4 has been observed in anoxic and suboxic waters (Anderson, 1987; Anderson et al., 1989). In this context, sedimentary geochemists have made an attempt to employ the U/ Th ratio, rather than the U concentration, as a redox indicator (Wright et al., 1984; Jones and Manning, 1994). U/Th ratios above 1.25 have been used to infer suboxic and anoxic conditions. The U/Th ratio is high (>1.25) in Arabian Sea sediments (Nath et al., 1997) collected from the oxygen minimum zone (OMZ; Table 2). The U/Th ratio (~0.05–0.32, 0.19 ± 0.10, n = 9; Table 2) is low in the Kudankulam limestone samples compared to samples deposited under anoxic and suboxic conditions. However, U/Th in these limestones is comparable with shallow-marine carbonates of southern India deposited under an oxic environment (Madhavaraju and Ramasamy, 1999; Table 2), which clearly suggests that the Kudankulam limestones were deposited under oxic conditions. Furthermore, there is almost no remarkable variation in U contents and U/Th ratios among

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ARMSTRONG-ALTRIN ET AL.

Kudankulam limestone samples, suggesting a lack of significant variations in oxygen level in the water column during deposition of these shallow-marine limestones. Thus, U and U/Th ratios could be considered as useful indicators for paleoredox conditions.

Variations in cerium contents and cerium anomalies Numerous studies has been carried out on the application of Ce in the marine phases for inferring paleoceanographic conditions (Grandjean et al., 1987, 1988; Hu et al., 1988; Liu et al., 1988; Grandjean and Albarede, 1989; German and Elderfield, 1990; Nath et al., 1997). The depletion of Ce in oceanic water results from redox changes of cerium relative to the rest of the REE (Elderfield, 1988; Piepgras and Jacobsen, 1992; Nath et al., 1994). Ce/Ce* ratios in CL range from ~0.82–0.92, with a mean value of 0.88 ± 0.06 (n = 3); in SSL this ratio ranges from ~0.86 to 0.93, with a mean value of 0.89 ± 0.04 (n = 3). Thus, there is no remarkable difference in Ce anomalies between CL and SSL, indicating that there was not much fluctuation in bottomwater oxygen level. Somewhat larger variations in Ce/Ce* ratios are present in AL (~0.86–1.04, 0.95 ± 0.09, n = 3), although the number of samples are very limited. The observed negative Ce anomalies in the Kudankulam limestones (Table 2) are smaller than those of deep-sea carbonates of the Indian Ocean (Nath et al., 1992), Arabian Sea sediments (Nath et al., 1997), and shallow-marine Maastrichtian carbonates of the Cauvery Basin, southern India (Madhavaraju and Ramasamy, 1999). Both Ce concentrations and Ce anomalies can probably be explained by variations in terrigenous sediments in the Kudankulam limestones as well as some other processes such as diagenesis. The absence of a negative Ce anomaly in AL (1.04; sample No. 8) implies that, apart from lithological input, diagenesis may play a significant role in incorporation of REE (particularly Ce). Porewater nutrient studies (Nath and Mudholkar, 1989) document Ce uptake and positive Ce anomalies. Mollusk shell fragments exhibit either a positive Ce anomaly or no anomaly at all (Elderfield and Sholkovitz, 1987; Sholkovitz and Elderfield, 1988; German and Elderfield, 1989; Sholkovitz et al., 1989). Therefore, the absence of a negative Ce anomaly in AL is probably unrelated to paleoredox conditions, because limestones from different litho-units of the Kudankulam Formation deposited in the near-shore shallow-marine environment show oxic conditions

where scavenging processes are negligible. The AL exhibits numerous fossils such as mollusks and foraminifera. Apart from this, these limestones seem to have undergone some sort of diagenetic process (Armstrong Altrin Sam et al., 2001), which might have played a major role in eliminating a negative Ce anomaly in this sample (AL sample No. 8). Because the Kudankulam limestones were deposited in a shallow-marine, oxic environment, terrigenous sediments from nearby crystalline source rocks could also have been deposited. The Ce behavior recorded in the limestones suggests that REE fractionation in such sediments is not useful for paleoredox reconstructions. Hence, observed Ce contents and Ce anomalies in the shallow-marine Kudankulam limestones resulted from variations in terrigenous sediment inp ut, as well as diagenetic processes.

Conclusions REE patterns and La/Sc, La/Th, Th/Sc, and (La/ Yb) s ratios together with negative Eu anomalies demonstrate that terrigenous sediments present in the Kudankulam limestones were mainly derived from felsic source rocks. In these limestones, the (La/Yb)s ratio is higher than the average values of terrigenous sediments. All but one Kudankulam limestone sample exhibits negative Ce anomalies. Variations in (La/Yb)s ratios and Ce anomalies may have resulted from differences in detrital sediments and diagenetic effects. Furthermore, the behavior of Ce in the Kudankulam limestones suggests that REE fractionation in shallow-marine sediments is not very useful for paleoredox studies. Low but constant values of both U content and the U/Th ratio suggest the prevalence of oxic environments in the sediment/water interface during deposition of the Kudankulam limestones. Finally, our study reveals that U can be considered as a useful indicator for paleoredox conditions.

Acknowledgments We are grateful to Prof. S. P. Mohan, Head, Department of Geology, University of Madras for his help and for providing laboratory facilities through the UGC SAP II and UGC COSIST programs. We thank Prof. G. Mongelli, Italy and Prof. P. K. Banerjee, Emeritus Scientist, Jadavpur University, Calcutta for their help during the study. This work was partly supported by PAPIIT grant IN-100596.

UPPER MIOCENE KUDANKULAM LIMESTONES

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