rare earth element geochemistry of limestones cretaceous research 2010 v.31

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Cretaceous Research 31 (2010) 400e414

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Cretaceous Research

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Geochemistry of the Mural Formation (Aptian-Albian) of the Bisbee Group,Northern Sonora, Mexico

J. Madhavaraju a,*, C.M. González-León a, Yong Il Lee b, J.S. Armstrong-Altrin c, L.M. Reyes-Campero d

a Estación Regional del Noroeste, Instituto de Geologia, Universidad Nacional Autónoma de México, Apartado Postal 1039, Hermosillo, Sonora 83000, Méxicob School of Earth and Environmental Sciences, Seoul National University, Seoul 151-747, Republic of Koreac Instituto de Ciencias del Mar y Limnología, Geología Marina y Ambiental, Universidad Nacional Autónoma de México, Circuito Exterior s/n, 04510 México D.F., Méxicod Servicio Geológico Mexicano, Gerencia de Hidrogeología y Geología Ambiental, Blvd. Felipe Ángeles Km. 93.50-4, Pachuca de Soto, Hidalgo 42080, México

a r t i c l e i n f o

Article history:Received 8 May 2008Accepted in revised form 14 May 2010Available online 20 May 2010

Keywords:GeochemistryRare Earth ElementProvenanceAptianeAlbian LimestoneMural FormationNorthern SonoraMéxico

* Corresponding author.E-mail address: [email protected] (J. Madhav

0195-6671/$ e see front matter � 2010 Elsevier Ltd.doi:10.1016/j.cretres.2010.05.006

a b s t r a c t

The elemental content (major, trace and rare earth elements) of 35 AptianeAlbian limestone samplesfrom the Mural Formation has been determined to provide information on depositional conditions andprovenance. The limestones of the Mural Formation show large variations in terrigenous and carbonatecontents (1.2 to 42.3% and 57.7 to 98.8% respectively). Small variations are observed in CaO concentra-tions in the Tuape Shale, Cerro La Puerta and Mesa Quemada members whereas large variations arefound in the Cerro La Ceja, Los Coyotes and Cerro La Espina members. The majority of the limestonesshow high values of Th, Sc and Zr. Large variations in SREE content are observed among differentmembers of the Mural Formation. Most limestones from the Mural Formation record non-seawater-likeREEþY signatures. The limestones show large variations in Ce anomalies which may be due to mixing ofsediment components (biogenic and authigenic phases) and detrital materials including Fe-colloids fromfluvial input. Most of the limestones show positive Eu anomalies, but some samples show negative Euanomalies (Eu/Eu*: 0.42 to 2.62).

The large variations in terrigenous percentage, high Al2O3 and SREE contents, high LaN/YbN ratios, lowY/Ho ratios and non-seawater-like REE patterns suggest that the observed variations in SREE contentsare mainly controlled by the amount of detrital sediments in the limestones of the Mural Formation. Thelimestones of the Mural Formation were deposited under both coastal and open shelf environments, andthey exhibit non-seawater-like REEþ Y patterns. The presence of terrigenous materials in thesecarbonates as contaminants effectively masks the seawater signature due to their high concentration ofthe REE. Thus, trying to decipher the palaeoceanographic conditions represented by ancient carbonaterocks should be done cautiously since limestones deposited under open marine environments may alsobe contaminated by some amount of terrigenous particles. The presence of small quantities of terrige-nous materials in the limestones can also reveal source rock information. The La/Sc, La/Co, Th/Sc, Th/Cr,Th/Co and Cr/Th ratios suggest that the terrigenous materials present in the limestones were mainlyderived from a nearby exposed basement of intermediate to felsic igneous rocks.

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1. Introduction

Rare Earth Element (REE) concentrations, REE patterns, and theEu and Ce anomalies in marine sediments provide useful infor-mation on marine depositional environments. Many workers haveundertaken detailed studies on the REE to understand the path-ways of biogenic and terrigenous fluxes from the source to themarine sediments (Piper, 1974; Murray and Leinen, 1993;Sholkovitz et al., 1994). The REE concentrations in seawater aremainly controlled by factors relating to different input sources (e.g.,

araju).

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terrestrial input from continental weathering, hydrothermal input)and scavenging processes related to depth, salinity and oxygenlevels (Elderfield, 1988; Piepgras and Jacobsen, 1992; Greaves et al.,1999). Rare Earth Elements generally reflect uniform trivalentbehaviour except for Ce and Eu which exhibit multiple oxidationstates. Shale-normalized seawater REE patterns are characterizedby i) LREE depletion, ii) negative Ce anomalies and iii) a slightpositive La anomaly (e.g. de Baar et al., 1991; Bau and Dulski, 1996).Yand Ho are chemically similar in charge and ionic radius, but Ho ismore readily removed from seawater than Y because of its surfacecomplexation behaviour (Nozaki et al., 1997), thus seawaterexhibits distinctly a high Y/Ho ratio than the terrigenous materials(e.g. Bau, 1996).

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The distribution of REEs and Ce anomalies in marine sedimentsmay be influenced by depositional environments such as proximityto source area (Murray et al., 1991a), widespreadmarine anoxia (Liuet al., 1988; German and Elderfield, 1990; Murray et al., 1991b),surface productivity variation (Toyoda et al., 1990), oceanic redoxconditions (Liu et al., 1988; German and Elderfield, 1990) andlithology and diagenesis (Nath et al., 1992; Madhavaraju andRamasamy, 1999; Armstrong-Altrin et al., 2003; Madhavaraju andLee, 2009). Ce anomalies in marine sediments are considered bysome as reliable indicators for understanding the paleoredoxconditions (Liu et al., 1988), although several workers have raiseddoubts about their effectiveness (German and Elderfield, 1990;Murray et al., 1991b; Nath et al., 1992, 1997).

The REE signatures in ancient marine environment provideinformation on secular changes in detrital influx and oxygenationconditions in the water column (e.g. Holser, 1997; Kamber andWebb, 2001). The seawater signatures are, however, completelymasked by the incorporation of terrigenous materials, which haverelatively high, non-seawater-like REE contents (Murray et al.,1992;Webb and Kamber, 2000; Nothdurft et al., 2004; Madhavaraju andLee, 2009). The identification of the terrigenous particles presentin the marine carbonate rocks as contaminants is an importantaspect to understand the geochemistry of carbonate rocks.

The Lower Cretaceous, shallow marine siliciclastic and calcar-eous strata of the Mural Formation are exposed in northern Sonora,northwest Mexico in a 300 km long transect that extends fromSierra El Chanate (westernmost part) to Cerro El Caloso Pitaycachi(northeastern most outcrop). Along this transect, González-Leónet al. (2008) reported the stratigraphy and biostratigraphy ofseveral sections, including the Cerro Pimas and Sierra San Josésections (Figs. 1 and 2) of which we discuss herein the major, traceand REE geochemistry of their limestone beds. The aims of ourstudy are to determine the influence of terrigenousmaterials on theREE characteristics of carbonate rocks, to document the variationsin Ce anomalies and to unravel the probable reason for significantpositive Eu anomalies in the limestones of the Mural Formation.

SONORA ARIZONA

HERMOSILLO

Santa AnaCaborca

Sonoita

San Luis R. C.

NogalesCananea

Agua PrietaNaco

CerroPimas

SierraSan Jose

Guaymas

Obregon

Navojoa

0 60 12020Km

Mexico

113 00 110 00

31 00

29 00

27 00

Fig. 1. Location map of the studied sections of the Mural Formation.

2. Geology and Stratigraphy

The Lower Cretaceous sedimentary succession assigned to theBisbee Group is well exposed in the north-central part of the stateof Sonora, Mexico. This succession has similar stratigraphic andlithologic characteristics to the younger formations of the BisbeeGroup (Ransome,1904) of southern Arizona and NewMexico in theUnited States of America, and is correlative with strata exposed innorthern Mexico (Cantu-Chapa, 1976; Bilodeau and Lindberg, 1983;Mack et al., 1986; Dickinson et al., 1989; Jacques-Ayala, 1995;Lawton et al., 2004). In Arizona the Bisbee Group consists of theGlance Conglomerate and the Morita, Mural and Cintura Forma-tions that were deposited in a rift basin, termed the Bisbee Basin.The older unit is the Glance Conglomerate composed of cobble- toboulder-conglomerate with local interbeds of volcanic flows andtuffs, which represent syntectonic rift deposits (Bilodeau et al.,1987; Lawton et al., 2004).

The Morita and Cintura Formations are composed of reddishbrown siltstone and lenticular beds of arkose and feldspathicarenite (Dickinson et al., 1986; Klute, 1991) that were depositedunder fluvial conditions. These two formations are difficult todistinguish based only on their lithological characteristics. Hence,the intervening marine Mural Formation is key to understandingLower Cretaceous stratigraphy and basin configuration in the area.

The fossiliferous clastic and carbonate strata of the MuralFormation were deposited during a major marine transgressionduring AptianeAlbian time (Scott,1987) in the region of Sonora andArizona where it overlies the Morita Formation on a sharp ravine-ment surface and grades upward into the Cintura Formation.Lawton et al. (2004) defined six members in theMural Formation innorth-central Sonora (Fig. 2), which from the base upwards are theCerro La Ceja, Tuape Shale, Los Coyotes, Cerro La Puerta, Cerro LaEspina andMesa Quemadamembers. The lithostratigraphic studiesof different members of the Mural Formation show minor facieschanges from west to east. The facies characteristics and regionalcorrelation of different members of the Mural Formation indicatethat the depositional environments of this formation varied fromrestricted shelf with deltaic and fluvial influence to open shelf withcoral rudist buildups, to offshore shelf. For the present study, wehave collected limestone samples from the western part (CerroPimas e CP) and the eastern part (Sierra San José e SSJ) of theBisbee Basin in northern Sonora. Here the limestones of the MuralFormationwere deposited in a nearshore environment with deltaicand fluvial influence to open marine environments (González-Leónet al., 2008). Most of the limestone samples contain varied amountsof terrigenous materials.

The Cerro La Ceja (CLC) Member consists of interbedded bio-clastic limestone, siltstone and calcareous sandstone. The limestonebeds are grey, brown and dark yellowish brown, bioturbated andlocally sandy. Siltstone beds are grey, green and reddish brownwithcalcareous nodules. The Tuape Shale (TS) Member is mainlycomposed of grey to blackmudstone and shale, shaly limestone andsubordinate amount of siltstone and fine grained sandstone.Limestone occurs as thin beds which contain oysters and ammo-nites. The Los Coyotes (LC) Member consists of thin beds of brownmudstone, calcareous siltstone, shaly limestone, massive brownsiltstone, fine-grained sandstone and bioclastic limestone. Thismember contains abundant fossils such as oysters, trigoniids,gastropods, bivalves and echinoderms. The Cerro La Puerta (CLP)Member is composed of mostly black shale and thin beds of fine-grained sandstone and fossiliferous limestone. The limestoneexhibits distinct bedding-parallel burrows on the upper bedsurfaces, and it contains fossils including oysters and Orbitolina; theblack shale contains calcareous nodules. The Cerro La Espina (CLE)Member consists mainly of massive limestone with thin beds of

Fig. 2. Lithostratigraphic sections of the Mural Formation in Cerro Pimas and Sierra San José areas (modified after González-León et al., 2008).

J. Madhavaraju et al. / Cretaceous Research 31 (2010) 400e414402

Table 1Comparison of data of major oxides, trace and rare-earth elements for USGS refer-ence sample MAG-1 (marine sediment) with the literature USGS certificate ofanalysis (Govindaraju 1994; see also USGS website).

Oxide/Elements This Study* Govindaraju 1994 Limits of detection**

Mean �SiO2 50.72 50.4 0.96 e

Al2O3 16.48 16.4 0.30 5Fe2O3 7.11 6.8 0.60 20CaO 1.42 1.37 0.10 5MgO 3.11 3.0 0.10 5K2O 3.59 3.55 0.17 5Na2O 3.60 3.83 0.11 50MnO 0.10 0.098 0.009 5TiO2 0.70 0.75 0.07 15P2O5 0.17 0.16 0.021 15LOI 14.09 e e e

Co 20.2 20 1.6 0.0287Cr 77.1 97 8 e

Sc e 17 1 e

Y 24.91 28 3 0.0075Sr 133.7 150 15 e

Zr 125.6 130 13 e

Pb 28.44 24 3 0.0598Th 11.96 12 1 0.0473La 41.46 43 4 0.0044Ce 87.32 88 9 0.0030Pr 10.06 e e 0.0031Nd 38.10 38 5 0.0034Sm 7.61 7.5 0.6 0.0034Eu 1.66 1.6 0.14 0.0031Gd 5.89 5.8 0.7 0.0036Tb 1.28 0.960 0.090 0.0024Dy 5.92 5.2 0.3 0.0024Ho 1.16 1.0 0.1 0.0027Er 3.09 3.0 e 0.0024Tm 0.56 0.43 0.43 0.0023Yb 2.70 2.6 0.3 0.0029Lu 0.53 0.4 0.04 0.0018

* Major oxides in wt% are analysed by XRF (average of 43 measurements); traceelements in ppm by ICP-AES and ICP-MS (average of 6 measurements). The obtaineddata were not tested statistically to find out the discordant outliers and it will beundertaken in our future work (Barnett and Lewis, 1994; Verma, 2005; Verma andQuiroz-Ruiz, 2006a, 2006b, 2008; Verma et al., 2008).** Limit of Detection: Three times the standard deviation of seven blank measure-ments; For major elements in mg/L and for trace elements in ng/L.e not determined or not reported.


siltstone, mudstone, fine grained sandstone and shaly limestone.The limestone beds are lenticular and fossiliferous with Orbitolina,gastropods, corals, rudists and other bivalves and the shaly lime-stones contain abundant oysters. The Mesa Quemada (MQ)Member includes green mudstone, light grey or red siltstone,sandstone and bioclastic limestone. Sandstone beds are fine- tovery fine-grained with local parallel laminations. The bioclasticlimestone contains numerous oysters.

3. Material and methods

The stratigraphy of several sections of the Mural Formationexposed in the northern part of Sonora has recently been studied byLawton et al. (2004) and González-León et al. (2008). Among them,carbonate rocks are well exposed at the Cerro Pimas and Sierra SanJosé sections (Fig. 2). In our present study, thirty-five limestonesamples from these two sections were analyzed e twenty from theCerro Pimas section and fifteen from the Sierra San José section. Weconsider these to be representative limestone samples from thewestern and northeastern part of the Bisbee Basin in order toestablish the geochemical variations in the two sections that areseparated by about 150 km within the Bisbee Basin.

Care was taken to remove the weathered materials from thesurface of the limestone samples. The selected samples werewashed with distilled water several times, air dried and powderedin an agatemortar. Then, fused glass beads were prepared for majorelement analysis using a Phillip PW 1480 X-ray fluorescencespectrometerwith a rhodiumX-ray source (see Norrish and Hutton,1969; Giles et al., 1995). The accuracy of SiO2, Al2O3 and K2O arebetter than�1%, MnO is better than�2% and that of Fe2O3, CaO andMgO are better than �4%. Na2O, P2O5 and TiO2 are better than �6%,Trace elements (Cr, Sc, Sr and Zr) were measured using a Jobin Yvon138 Ultrace Inductively Coupled Plasma Atomic Emission Spec-trometer (ICP-AES). Rare Earth Elements and certain other traceelements (Co, Y, Th and Pb) were analyzed using a VG elemental PQII plus Inductively Coupled Plasma Mass Spectrometer (ICP-MS)(see Jarvis, 1988). The sedimentary geochemical standard rock,MAG-1, obtained from the USGS, was used for calibration. Theresult from the analyses of MAG-1 are compared with the pub-lished values compiled by Govindaraju (1994) which shows betterprecision of our data and also compatible with the published values(Table 1). The precision for trace elements like Co, Zr and Th arebetter than �3% whereas Cr, Sr, Y and Pb are better than �10%. Theanalytical accuracy of all REEs is better than�4% (except Tb, Dy, Ho,Tm and Lu). The precision of Tb, Dy, Ho, Tm and Lu are more than�10%. The limits of detection for the analytical procedure are alsolisted in Table 1. They mainly agree with the findings mentioned byearlier workers (Verma et al., 2002; Santoyo and Verma, 2003;Verma and Santoyo, 2005). Major, trace and Rare Earth elementswere analyzed at the Korea Basic Science Institute. Three analyseswere made for each sample and then averaged. Yttrium is insertedbetween Dy and Ho in the REE pattern according to its identicalcharge and similar radius (REEþ Y pattern, Bau, 1996). Rare EarthElements were normalized to the Post Archaean Australian Shale(PAAS) values of Taylor and McLennan (1985) for preparing REE-normalized diagrams. The Ce/Ce* (Ce anomaly) is calculated usingthe value of Ce (Cesample/CePAAS) and the predicted value of Ce* isobtained from the interpolation from the PAAS-normalised valuesof La and Pr. The Eu/Eu* (Eu anomaly) is also calculated in similarway using the values of Sm, Eu and Gd.

Thirty-five samples were analyzed using standard XRD proce-dures (Biscaye, 1965; Muller, 1967; Grim, 1968; Hardy & Tucker,1988) for whole rock mineralogy. The powder samples werescanned from 2e70� (2q) per minute. X-ray diffraction was used ina computer controlled Shimadzu Diffractometer system model

6000 with Cu ka radiation to estimate semi-quantitatively theminerals present. The dominant minerals identified in these lime-stone samples are quartz, feldspar and calcite. The clastic andcarbonate percentages are given in Table 2.

4. Results

The concentrations of major elements in the studied limestonessamples of the Mural Formation are given in Table 2. Large varia-tions are observed in SiO2 and Al2O3 contents (Table 2) amongdifferent members of the Mural Formation in both sections. In theCerro Pimas section, the CaO content in the CLC, TS, LC, CLE and MQvaries significantly (Table 2). Small variations are observed in CaOconcentrations in the CLC, TS, CLP, CLE and MQ members whereaslarge variations are found only in the LC Member of Sierra San Josésection (Table 2). The limestones from both sections show lowcontents of Fe2O3 (Table 2). Those major and trace elements whichare enriched in silicate minerals (eg. SiO2 and Al2O3) are higher inthe CLC, TS, CLE and MQ members than the LC member from theCerro Pimas sections. In contrast, major and trace elements that arehoused in the carbonate minerals (eg. CaO and Sr) are higher in theLCmember than the other members of the Cerro Pimas section. TheCLC, TS, LC and MQ members from the Sierra San José section are

Table 2Major oxides (wt%), trace and rare earth elements (ppm) concentrations for limestones of the Mural Formationa.

Member/Sample no Clastic % Carbonate % SiO2 Al2O3 Fe2O3 CaO MgO K2O Na2O MnO TiO2 P2O5 LOI Total Co Cr Sc Y Zr Sr

Cerro Pimas sectionMesa QuemadaCP47 10.8 89.2 9.1 0.50 0.80 49.9 0.20 0.03 0.13 0.08 0.03 0.05 39.0 99.82 4.2 11.3 1.00 5.1 1.0 425CP45 5.1 94.9 3.5 0.86 0.41 53.0 0.31 0.16 0.09 0.05 0.04 0.02 41.3 99.74 2.9 10.4 1.00 4.7 1.0 631Cerro La EspinaCP43 29.5 70.5 23.7 2.99 1.46 38.7 0.67 0.82 n.d. 0.15 0.12 0.03 31.4 100.00 4.3 9.1 2.21 6.1 24.1 554CP41 4.8 95.2 3.0 0.27 0.34 53.3 0.35 0.05 n.d. 0.06 0.02 0.02 42.0 99.41 2.8 1.0 0.41 5.2 0.6 405CP38 11.0 89.0 7.8 1.10 1.27 48.7 1.26 0.07 n.d. 0.12 0.05 0.02 39.1 99.49 6.9 4.9 1.40 10.7 9.4 544CP36 5.2 94.8 3.3 0.58 0.56 53.0 0.68 0.08 n.d. 0.09 0.02 0.02 41.4 99.73 4.6 3.5 1.20 14.1 2.6 529CP33 7.7 92.3 5.6 0.46 0.66 51.2 0.73 0.01 n.d. 0.08 0.03 0.02 40.8 99.59 3.2 4.6 0.60 9.5 2.8 746Los CoyotesCP28 2.8 97.2 1.8 0.14 0.28 54.1 0.35 0.01 n.d. 0.06 0.01 n.d. 42.7 99.45 2.9 1.4 0.20 1.7 1.0 540CP26 5.5 94.5 3.9 0.65 0.65 52.0 0.41 0.14 n.d. 0.08 0.04 0.02 41.3 99.19 3.7 11.0 0.60 4.1 4.4 428CP24 3.9 96.1 2.9 0.48 0.65 53.2 0.26 0.10 n.d. 0.06 0.03 0.02 41.9 99.60 4.3 6.0 0.60 4.4 4.2 440CP22 1.9 98.1 1.5 0.10 0.25 54.9 0.09 0.01 n.d. 0.07 0.01 0.01 42.4 99.34 3.0 3.5 0.20 10.3 1.0 218CP18 6.1 93.9 3.4 0.48 1.81 51.9 0.51 0.08 n.d. 0.25 0.02 0.02 41.1 99.57 10.1 7.7 0.60 6.0 2.6 368CP15 6.6 93.4 5.4 0.47 0.78 51.5 0.18 0.12 n.d. 0.25 0.02 0.01 40.5 99.23 4.5 6.3 0.60 6.5 4.4 471Tuape ShaleCP12 19.6 80.4 14.2 2.75 1.66 44.2 0.69 0.44 n.d. 0.15 0.12 0.05 35.5 99.76 4.6 10.8 1.79 7.1 14.4 866CP10 14.9 85.1 9.9 1.73 1.33 46.8 1.20 0.20 0.02 0.12 0.08 0.04 37.7 99.12 3.6 13.6 1.39 18.2 42.1 1116CP9 17.0 83.0 13.1 1.33 1.37 46.1 0.32 0.14 0.26 0.16 0.07 0.05 36.6 99.50 3.0 9.0 0.81 8.4 5.9 503CP6 20.8 79.2 16.9 2.04 1.36 43.5 0.45 0.45 0.03 0.08 0.10 0.02 34.6 99.53 3.2 15.3 2.19 13.2 24.1 705Cerro La CejaCP4 3.9 96.1 2.0 0.49 0.26 53.3 0.38 0.10 n.d. 0.01 0.03 0.02 42.5 99.09 2.2 7.7 0.61 2.5 1.6 510CP3 5.8 94.2 3.1 0.77 0.41 53.3 0.23 0.17 0.02 0.07 0.04 0.04 41.4 99.55 5.3 6.8 0.87 6.6 1.9 391CP1 29.3 70.7 23.3 2.84 1.59 38.0 0.52 0.29 0.66 0.69 0.12 0.06 31.1 99.17 3.4 9.1 1.78 9.5 24.8 298

Sierra San José sectionMesa QuemadaSSJ27 11.3 88.7 8.2 1.56 0.59 49.1 0.50 0.29 0.05 0.44 0.06 0.03 38.8 99.62 3.2 2.0 1.39 5.4 6.6 983Cerro La EspinaSSJ25 6.7 93.3 4.3 0.51 0.60 51.7 0.88 n.d. n.d. 0.07 0.02 0.01 41.3 99.39 2.8 4.0 0.40 4.3 3.8 313SSJ23 7.9 92.1 6.5 0.46 0.12 50.7 0.93 0.07 n.d. 0.02 0.02 0.01 40.7 99.53 2.2 3.6 0.20 1.5 10.1 390SSJ21 5.9 94.1 4.2 0.44 0.10 52.6 0.73 0.04 n.d. 0.01 0.01 0.01 41.7 99.84 2.1 2.8 0.20 1.2 6.2 439SSJ18 1.2 98.8 0.3 0.08 0.06 54.6 0.68 n.d. n.d. 0.01 0.01 0.01 43.4 99.15 2.1 2.7 0.20 0.8 1.8 407SSJ16 5.9 94.1 4.6 0.21 0.16 52.5 0.85 n.d. n.d. 0.01 0.02 0.01 41.4 99.76 1.98 3.0 0.20 1.5 5.0 434Cerro La PuertaSSJ11 7.3 92.7 4.7 0.90 0.50 51.7 0.96 0.06 n.d. 0.07 0.03 0.01 40.6 99.53 2.2 6.3 0.60 3.3 6.0 827SSJ10 6.1 93.7 3.9 0.84 0.44 52.6 0.90 0.08 n.d. 0.02 0.03 0.02 40.3 99.13 2.1 5.9 0.60 4.4 6.2 849SSJ9 7.8 93.2 4.8 0.78 0.37 52.3 0.78 0.10 n.d. 0.02 0.03 0.01 40.2 99.39 1.8 7.4 0.60 3.6 5.4 673Los CoyotesSSJ7 8.4 91.6 5.7 0.54 0.96 51.2 0.84 0.03 0.01 0.08 0.02 0.06 39.7 99.11 1.6 8.6 0.61 5.7 6.5 1183SSJ6 38.1 61.9 31.5 3.00 1.20 36.2 0.73 0.24 0.86 0.09 0.23 0.06 25.6 99.71 2.3 11.9 1.59 18.0 70.3 753Tuape ShaleSSJ5 17.0 83.0 12.5 1.78 0.86 46.1 0.72 0.20 0.34 0.08 0.10 0.04 36.5 99.22 2.2 15.2 1.42 8.0 52.2 699SSJ4 17.9 82.1 13.5 1.92 0.92 49.7 0.78 0.21 0.37 0.09 0.11 0.04 31.6 99.24 1.9 19.3 2.00 13.8 43.8 520Cerro La CejaSSJ3 42.3 57.7 31.7 6.26 0.94 37.3 0.80 0.59 2.03 0.29 0.21 0.08 19.9 100.10 2.7 10.5 2.42 11.3 57.4 528SSJ2 39.2 60.8 29.6 6.00 1.48 32.8 1.03 1.21 0.07 0.07 0.30 0.08 27.3 99.94 2.8 21.4 4.20 12.1 50.6 481

Member/Sample no Th Pb La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu SREE

Cerro Pimas sectionMesa QuemadaCP47 0.25 8 5.57 5.63 0.98 4.68 0.86 0.23 1.07 0.13 0.74 0.14 0.39 0.04 0.28 0.04 20.78CP45 0.39 29 5.91 10.27 1.52 7.46 1.15 0.23 1.24 0.15 0.83 0.16 0.45 0.05 0.35 0.04 29.81Cerro La EspinaCP43 1.55 254 6.16 12.44 1.38 7.28 1.31 0.39 1.62 0.21 1.03 0.22 0.62 0.09 0.59 0.09 33.43CP41 0.23 47 3.25 6.21 0.69 3.73 0.70 0.18 1.00 0.14 0.75 0.16 0.45 0.06 0.39 0.06 17.77CP38 1.01 22 7.75 17.38 1.85 10.06 1.67 0.43 2.38 0.32 1.62 0.33 0.91 0.12 0.77 0.11 45.70CP36 0.81 58 8.62 19.69 2.07 11.45 1.98 0.43 2.91 0.40 2.06 0.42 1.16 0.16 1.02 0.15 52.52CP33 0.64 43 6.78 13.62 1.49 8.07 1.39 0.36 2.07 0.28 1.40 0.28 0.76 0.10 0.61 0.09 37.27Los CoyotesCP28 e 93 1.07 1.70 0.19 1.03 0.20 0.06 0.30 0.04 0.22 0.05 0.14 0.02 0.12 0.02 5.22CP26 0.33 49 3.66 6.45 0.73 3.87 0.68 0.20 0.89 0.12 0.56 0.12 0.32 0.04 0.26 0.04 17.94CP24 0.39 135 3.19 5.65 0.66 3.52 0.67 0.18 0.90 0.13 0.66 0.14 0.39 0.05 0.34 0.05 16.53CP22 0.09 18 2.23 2.69 0.38 1.92 0.44 0.14 0.57 0.08 0.44 0.10 0.27 0.04 0.21 0.03 9.54CP18 0.57 25 4.74 9.17 1.04 5.51 1.10 0.37 1.34 0.18 0.92 0.18 0.55 0.07 0.43 0.06 25.66CP15 0.50 46 5.31 9.18 1.04 5.60 1.12 0.36 1.40 0.19 0.93 0.19 0.50 0.06 0.39 0.06 26.33Tuape ShaleCP12 1.27 40 9.18 18.68 2.04 10.97 1.78 0.69 2.12 0.26 1.19 0.23 0.65 0.08 0.53 0.08 48.48CP10 1.68 117 11.98 27.18 3.26 17.25 2.42 0.78 2.94 0.35 1.42 0.26 0.70 0.08 0.54 0.08 69.24C P9 1.44 79 20.88 50.49 4.53 26.72 2.87 1.03 3.69 0.40 1.52 0.28 0.82 0.09 0.56 0.08 113.96CP6 2.34 14 7.72 14.32 2.01 10.33 1.94 0.47 2.43 0.35 1.98 0.41 1.21 0.16 1.09 0.17 44.59Cerro La CejaCP4 1.51 19 4.63 7.40 1.03 5.05 0.75 0.17 0.89 0.12 0.45 0.09 0.24 0.04 0.19 0.03 21.08


Table 2 (continued )

Member/Sample no Th Pb La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu SREE

CP3 1.13 67 6.49 11.50 1.59 8.04 1.32 0.36 1.57 0.20 1.07 0.21 0.58 0.07 0.48 0.07 33.55CP1 1.34 41 13.51 28.94 3.05 16.52 2.17 0.49 2.77 0.33 1.45 0.28 0.85 0.12 0.77 0.12 71.37

Sierra San José sectionMesa QuemadaSSJ27 0.72 25 6.69 13.64 1.39 7.54 1.21 0.75 1.48 0.18 0.83 0.17 0.48 0.06 0.40 0.06 34.88Cerro La EspinaSSJ25 0.35 13 3.77 7.77 0.81 4.44 0.70 0.19 6.50 0.12 0.56 0.12 0.31 0.04 0.36 0.04 25.73SSJ23 0.12 7 1.06 1.87 0.21 1.10 0.18 0.05 0.24 0.03 0.17 0.04 0.115 0.018 0.11 0.017 5.21SSJ21 0.08 17 0.91 1.59 0.19 0.98 0.16 0.04 0.23 0.03 0.17 0.037 0.11 0.017 0.114 0.018 4.60SSJ18 0.01 9 0.53 0.73 0.09 0.45 0.09 0.028 0.14 0.014 0.08 0.017 0.05 0.007 0.043 0.006 2.28SSJ16 0.07 26 0.92 1.26 0.16 0.82 0.14 0.04 0.21 0.03 0.15 0.03 0.10 0.01 0.09 0.012 3.97Cerro La PuertaSSJ11 0.44 14 2.95 4.64 0.55 2.83 0.46 0.13 0.61 0.08 0.41 0.09 0.26 0.04 0.22 0.03 13.30SSJ10 0.31 34 2.37 3.55 0.46 2.34 0.41 0.11 0.55 0.08 0.40 0.09 0.24 0.03 0.21 0.03 10.87SSJ9 0.36 9 2.51 3.71 0.48 2.44 0.42 0.12 0.63 0.08 0.43 0.09 0.27 0.04 0.22 0.03 11.47Los CoyotesSSJ7 0.77 11 9.36 21.51 2.56 13.45 1.78 0.37 2.10 0.25 1.06 0.19 0.54 0.07 0.42 0.07 53.73SSJ6 3.11 15 19.48 43.03 4.57 24.40 3.38 0.68 4.45 0.62 3.10 0.65 1.91 0.27 1.77 0.27 108.58Tuape ShaleSSJ5 1.19 21 6.66 13.26 1.68 8.62 1.27 0.39 1.50 0.20 0.94 0.19 0.56 0.08 0.50 0.08 35.93SSJ4 1.97 34 12.19 19.72 2.58 13.31 2.06 0.44 2.65 0.36 1.81 0.39 1.14 0.15 0.98 0.15 57.93Cerro LaCejaSSJ3 2.80 15 10.55 21.80 2.41 12.68 1.93 0.52 2.37 0.34 1.77 0.38 1.11 0.16 1.06 0.16 57.24SSJ2 3.23 6 12.68 27.70 3.35 17.51 2.80 0.81 3.18 0.44 2.18 0.44 1.29 0.18 1.19 0.18 73.93

a Before data presentation, an attempt was made to round the data to the number of significant digits as suggested by Verma (2005).


more enriched in silicate minerals than the CLP and CLE members.In contrast, the CLP and CLE members are more enriched incarbonate minerals than the CLC, TS, LC and MQ members. Theobserved variations in the major oxides concentrations within thesamemember from different localities may be due to the amount ofclastic materials included in them.

The bivariate plots of major oxides including certain traceelements vs the percentage of clastic materials present in thecarbonate rocks provide useful information regarding the source ofthese materials (Parekh et al., 1977; Cullers, 2002). In addition,statistical approach will also useful to understand the statisticallyvalid or invalid correlation (Verma et al., 2006). A Plot of theS(SiO2þAl2O3þ Fe2O3þMgOþNa2Oþ K2Oþ TiO2) vs the per-centage of clastic materials show excellent linear correlation andalso the linear plot extending from the origin (Fig. 3a). This suggestsas expected that these oxides are mainly incorporated into theclastic materials rather than the calcite. In contrast, CaO and LOIhave a perfect negative correlation with the percentage of clasticmaterials, suggesting that they are incorporated into the carbonatephase (Fig. 3b and c). These results are consistent with the otherpublished studies (Parekh et al., 1977; Cullers, 2002).

Trace elements concentrations and their ratios are given inTables 2 and 3. The high field strength elements (HFSE), namely Zr,Yand Th, are resistant toweathering and alteration processes whencompared with other trace elements (Taylor and McLennan, 1985).The limestones from the LC and MQ members contain a lowconcentration of Zr when compared with other members at theCerro Pimas section (Table 2). The CLC, TS and LC members havehigher concentrations of Zr than the CLP, CLE and MQ members atSierra San José section (Table 2). Maximum concentrations of Y arefound in the CLC, TS, LC and CLE members whereas low concen-trations are found in the MQ Member at Cerro Pimas section(Table 2). In the Sierra San José section, the higher concentrations ofYare observed in the CLC, TS, LC andMQmembers than the CLP andCLE members. Overall, the limestones of the Mural Formationcontain high Sr content (Table 2).

Plots of La, Ce, Sc and Th vs S(SiO2þAl2O3þ Fe2O3þMgOþNa2Oþ K2Oþ TiO2) yield significant positive correlation with the

linear plots emanating slightly above the intersects of the X and Yaxes (Fig. 4aed). This suggests that these elements are mainlyhoused in the clastic materials. Likewise, Sm, Eu, Tb, Yb, and Lushow a positive correlation with S(SiO2þAl2O3þ Fe2O3þMgOþNa2Oþ K2Oþ TiO2) which suggest that these elements are mainlyassociated with terrigenous particles (statistically significant ata strict significance level of 0.001; linear correlation coefficientr¼ 0.64; 0.63; 0.71; 0.78; 0.79; respectively, n¼ 35). These resultsare consistent with the other published studies (Parekh et al., 1977;Cullers, 2002).

The enrichment and depletion of REE in the sediment arecontrolled by the major processes such as the terrigenous inputfrom the continental area, authigenic removal of REE from thewater column and early diagenesis (Sholkovitz, 1988). Seawatercontributes lesser amount of REE to the sediments where as thesediments contain high REE concentration show non-seawater-likepattern (Nothdurft et al., 2004). The PAAS normalized Seawater REEpatterns (Fig. 5) are characterized by (1) uniform light REE deple-tion, (2) a negative Ce anomaly, and (3) a slight positive La anomaly(e.g., De Baar et al., 1991; Bau and Dulski, 1996) and higher Y/Horatios (e.g., Bau, 1996). The limestones from the CP and SSJsections show non-seawater-like REEþ Y patterns (CLC:NdN/YbN¼ 1.52� 0.42, n¼ 5; TS: 1.95�1.17, n¼ 6; LC: 1.18� 0.65,n¼ 8; CLP: 0.97� 0.08, n¼ 3, CLE: 0.92� 0.15, n¼ 10; MQ:1.57� 0.19, n¼ 3; Fig. 6aef). Most of the samples contain positive Laand negative Ce anomalies although some samples show slightlypositive La and negative Ce anomalies to no anomalies. Most of thelimestones from the CP and SSJ sections contain chondritic Y/Horatios (CLC: 30.1�2.7, n¼ 5; TS: 34.2� 5.0, n¼ 5; LC: 32.1�2.4,n¼ 7; CLE: 35.4� 6.1, n¼ 10; MQ: 32.5� 3.3, n¼ 3; respectively,Y/Ho Chondritic ratio:w28), but the limestones of the CLP memberfrom the SSJ section contain slightly higher Y/Ho ratios (CLP:41.8� 6.4, n¼ 3). Two samples contain high Y/Ho ratios (CP10:70.15 and CP22: 102.5) which have been statistically proved asoutliers using the method proposed by Verma and Quiroz-Ruiz(2006a,b). So, we have not included those samples with discor-dant outliers while calculating the mean and standard deviation toimprove the authenticity of the data set (Verma et al., 2008).

a

b

c

Fig. 3. a, plot of the S(SiO2þ Al2O3þ Fe2O3þMgOþNa2Oþ K2Oþ TiO2) vs clasticpercentage in the limestones of the Mural Formation provides a linear plot witha correlation coefficient of 0.99. b, plot of the clastic (%) vs CaO content gives significantnegative correlation with a correlation coefficient of �0.98. c, clastic (%) vs LOI plotexhibits a linear plot with a correlation coefficient of �0.98.


5. Discussion

5.1. Ce Anomaly

Ce/Ce* ratios in limestones of the Cerro Pimas section rangesfrom 0.56 to 1.20, with a mean value of 0.91 (n¼ 20); in limestonesof the Sierra San José section this ratio ranges from 0.73 to 1.05 witha mean value of 0.90 (n¼ 15). Noticeable variations are observedamong different members of the Mural Formation. Many lime-stones of the Mural Formation exhibit less negative Ce anomaliesthan the deep-sea Indian Ocean carbonates (Nath et al., 1992),Arabian sea sediments (Nath et al., 1997), Cretaceous carbonatesfrom Southern Alps (Bellanca et al., 1997), Maastrichtian carbonatesof Southern India (Madhavaraju and Ramasamy, 1999), Indian

Ocean waters (Bertram and Elderfield, 1993) and Pacific Oceanwaters (Zhang and Nozaki, 1996).

The depletion of Ce relative to the adjacent REE is one of thecharacteristic features of modern seawater. In seawater Ce/Ce*

values range from <0.1 to 0.4 (Elderfield and Greaves, 1982;Piepgras and Jacobsen, 1992), but it is 1 in average shale(Murray et al., 1991a). The deficiency of Ce in seawater resultfrom the oxidation of Ceþ3 to the less soluble Ceþ4 and subse-quently its removal from the seawater through scavenging bysuspended particles which settle through the water column(Sholkovitz et al., 1994).

The Ce/Ce* values are not correlated very well with Al, Th and Zr(r¼ 0.40; 0.35; 0.33; respectively), but Ce values are positivelycorrelated with Al, Th and Zr (statistically significant at a strictsignificance level of 0.001; linear correlation coefficient r¼ 0.55;0.53; 0.58; respectively). Such moderate correlation of Ce and Ce/Ce* with Al, Th and Zr suggests that other factors in addition todetrital input might have controlled the Ce distribution in thestudied limestones.

Seawater and marine carbonates exhibit a Ce deficient naturedue to the scavenging of Ceþ4 by FeeMn oxides in the deep seaenvironments (Elderfield, 1988). The deep sea regions having a lowsedimentation rate with a well developed oxic water column witha more active scavenging processes which initiate the coprecipi-tation of Ce(OH) on to FeeMn coatings on sedimentary particles. Inthe studied limestones the Ce/Ce* values, however, are not corre-lated with a scavenging-type particle reactive element (Ce/Ce* vsPb: r¼ 0.24), which indicates that the observed variations in Ceanomalies are probably unrelated to a scavenging process.Furthermore, the limestones of the Mural Formation weredeposited in shallow marine environments (Lawton et al., 2004;González-León et al., 2008) where scavenging processes arelimited when compared with deep sea regions.

Many limestone samples from the Mural Formation containpositive Ce anomalies. The positive Ce anomalies mainly occur dueto the paleoredox conditions (German and Elderfield, 1990),lithology and diagenesis (Nath et al., 1992; Madhavaraju andRamasamy, 1999; Armstrong-Altrin et al., 2003; Madhavaraju andLee, 2009) and Fe-organiceREE rich colloids from the riverineinput (Sholkovitz, 1992).

The variations in the bottom water oxygenation level in thecarbonate rocks have been estimated using Ce/Ce* ratios (Wanget al., 1986; Piper, 1991). In the present study, Ce anomalies areinversely correlated with CaO contents (r¼�0.48) suggestingthat the Ce/Ce* values, however, not related to bottom wateroxygenation. The inclusion of REE-rich river-borne colloids in thecoastal fringing reef resulted in the lack of a negative Ce anomaly(Nothdurft et al., 2004). The fluvial deltaic-influenced facies arepresent in the Cerro la Puerta and Cerro la Espina members atCerro Pimas locations (González-León et al., 2008). The absenceof a large Ce anomaly requires inclusion of material with a posi-tive Ce anomaly relative to PAAS, and Fe-colloids from riverineinput have such positive Ce anomaly (Sholkovitz, 1992).A comparison of Ce/Ce* values with the concentrations of Feindicates that there is a moderate correlation ((statisticallysignificant at a strict significance level of 0.001; linear correlationcoefficient r¼ 0.57) between them in these limestones. Hence,the observed variations in Ce content and Ce anomalies in thelimestones of the Mural Formation may be due to the mixing ofdifferent portions of sediment components (dominantly biogenicand authigenic phases) which inherited the seawater-like Ceanomaly and of detrital materials (mainly alumino-silicates) withcrust-like Ce anomaly. In addition, incorporation of Fe-colloidsfrom riverine input might be partially responsible for the varia-tions in Ce anomalies.

Table 3Elemental ratios for limestones of the Mural Formation.

Member/Sample no La/Sc La/Co Th/Sc Th/Co Th/Cr Cr/Th Ce/Ce* Eu/Eu* LaN/YbN NdN/YbN Y/Ho

Cerro Pimas sectionMesa QuemadaCP47 5.57 1.33 0.25 0.06 0.02 45.00 0.56 1.13 1.47 1.39 36.07CP45 5.91 2.05 0.39 0.14 0.04 26.69 0.79 0.91 1.25 1.77 29.56Cerro La EspinaCP43 2.79 1.44 0.70 0.36 0.17 5.89 0.99 1.26 0.77 1.10 27.68CP41 7.93 1.17 0.56 0.08 0.23 4.43 0.95 1.01 0.62 0.80 32.75CP38 5.54 1.12 0.72 0.15 0.21 4.84 1.05 1.02 0.74 1.09 32.33CP36 7.18 1.89 0.68 0.18 0.23 4.32 1.07 0.84 0.62 0.93 33.45CP33 11.3 2.13 1.07 0.20 0.14 7.14 0.99 1.00 0.82 1.10 33.96Los CoyotesCP28 5.35 0.37 e e e e 0.84 1.17 0.44 0.70 33.60CP26 6.10 0.99 0.55 0.09 0.03 33.27 0.91 1.21 1.04 1.24 34.00CP24 5.32 0.73 0.65 0.09 0.07 15.26 0.89 1.09 0.69 0.86 31.57CP22 11.15 0.74 0.45 0.03 0.03 38.78 0.67 1.33 0.78 0.77 102.50CP18 7.90 0.47 0.95 0.06 0.07 13.44 0.95 1.44 0.82 1.07 33.28CP15 8.85 1.17 0.83 0.11 0.08 12.54 0.89 1.35 1.01 1.20 34.05Tuape ShaleCP12 5.13 2.02 0.71 0.28 0.12 8.47 1.00 1.67 1.28 1.72 31.04CP10 8.62 3.32 1.21 0.47 0.12 8.10 1.00 1.38 1.64 2.66 70.15C P9 25.78 7.01 1.78 0.48 0.16 6.23 1.20 1.49 2.75 3.96 29.89CP6 3.53 2.40 1.07 0.73 0.15 6.54 0.84 1.02 0.52 0.79 32.12Cerro La CejaCP4 7.59 2.10 2.48 0.68 0.20 5.09 0.78 0.98 1.81 2.22 28.00CP3 7.46 1.22 1.30 0.21 0.17 6.01 0.82 1.17 0.99 1.39 31.48CP1 7.59 4.01 0.75 0.40 0.15 6.81 1.04 0.94 1.30 1.78 34.00

Sierra San José sectionMesa QuemadaSSJ27 4.81 2.12 0.52 0.23 0.36 2.76 1.00 2.64 1.23 1.56 31.88Cerro La EspinaSSJ25 9.43 1.35 0.88 0.13 0.09 11.37 1.03 0.42 0.77 1.02 35.42SSJ23 5.30 0.49 0.60 0.06 0.03 29.58 0.88 1.12 0.72 0.83 40.00SSJ21 4.55 0.43 0.40 0.04 0.03 34.88 0.87 0.98 0.62 0.73 32.70SSJ18 2.65 0.25 0.05 e e e 0.75 1.22 1.00 0.87 48.24SSJ16 4.60 0.46 0.35 0.04 0.02 42.71 0.73 1.09 0.75 0.75 45.94Cerro La PuertaSSJ11 4.92 1.34 0.73 0.20 0.07 14.39 0.83 1.15 0.99 1.06 36.22SSJ10 3.95 1.11 0.52 0.15 0.05 19.10 0.79 1.10 0.85 0.93 51.65SSJ9 4.18 1.38 0.60 0.20 0.05 20.58 0.77 1.10 0.84 0.91 39.14Los CoyotesSSJ7 15.34 5.78 1.26 0.48 0.09 11.16 1.01 0.90 1.64 2.66 30.11SSJ6 12.25 8.66 1.96 1.38 0.26 3.83 1.05 0.83 0.81 1.15 27.71Tuape ShaleSSJ5 4.69 3.03 0.84 0.54 0.08 12.75 0.92 1.33 0.98 1.44 42.32SSJ4 6.10 6.45 0.99 1.04 0.10 9.80 0.82 0.89 0.92 1.13 35.46Cerro La CejaSSJ3 4.36 3.89 1.16 1.03 0.27 3.74 1.00 1.14 0.73 0.99 29.66SSJ2 3.02 4.59 0.77 1.17 0.15 6.63 1.00 1.28 0.79 1.23 27.34


5.2. Behaviour of Europium

The limestones from Cerro Pimas and Sierra San Jose sectionsshow large variations in Eu anomalies (Eu/Eu*: 0.84 to 1.67; 0.42 to2.64; respectively). Most of the limestone samples contain positiveEu anomalies, whereas few samples contain negative Eu anomalies.Positive Eu anomalies aremainly found either in sediments affectedby hydrothermal solutions (Michard et al., 1983; German et al.,1993; Siby et al., 2008); intense diagenesis (Murray et al., 1991b;MacRae et al., 1992) or variations in plagioclase content (Nathet al., 1992). Positive Eu anomalies are not common in seawater,which resulted due to hydrothermal discharges along mid-oceanridges (Klinkhammer et al., 1983, 1994). Positive Eu anomalies havebeen well documented for hydrothermal vent fluids and sedimentparticulates in active ridge system (Michard et al., 1983; Germanet al., 1990, 1999; Douville et al., 1999). Hydrothermal solutionsmainly originate in the deep marine environments but such anorigin is unlikely for the limestones of the Mural Formation whichwere deposited in shallow marine environments (Lawton et al.,2004; González-León et al., 2008).

Positive Eu anomalies have been reported from Amazon fanmuds in which Eu2þ is precipitated from pore waters duringdiagenesis (MacRae et al., 1992). Unlike Ce which can undergooxidation state changes in ambient seawater conditions, redoxtransformations from Eu3þ to Eu2þ require low oxidation-reductionpotentials (pH 2e4) and high temperatures (>200 �C) (Sverjensky,1984; Bau, 1991). These conditions are generally absent in shallowmarine environments. Petrographic and geochemical studies,suggest that the studied limestones were not subjected to intensediagenesis. The positive correlation between Eu and the immobileelements such as Y, Th and Zr (statistically significant at a strictsignificance level of 0.001; linear correlation coefficient r¼ 0.69;0.74; 0.53; respectively) supports the nondiagenetic influence onthis element. The inclusion of small amount of feldspars may leadto positive Eu anomalies in the bulk sediments (Murray et al.,1991b). In the present study, Eu contents show significant posi-tive correlation with Al2O3 which suggest the detrital origin of thiselement. Hence, the observed variations in Eu anomalies in thelimestones of the Mural Formation may be due to the presence offeldspar content rather than hydrothermal events and diagenesis.

Fig. 4. a, S(SiO2þ Al2O3þ Fe2O3þMgOþNa2Oþ K2Oþ TiO2) vs Sc show significantpositive correlation with correlation coefficient of 0.85. b, plot of the S(SiO2þ Al2O3

þ Fe2O3þMgOþNa2Oþ K2Oþ TiO2) vs Th content provides positive correlation withcoefficient of 0.72. c, S(SiO2þ Al2O3þ Fe2O3þMgOþNa2Oþ K2Oþ TiO2) vs La plotexhibits positive correlation (correlation coefficient of 0.70). d, plot of theS(SiO2þ Al2O3þ Fe2O3þMgOþNa2Oþ K2Oþ TiO2) vs Ce content gives positivecorrelation with a correlation coefficient of 0.67.

0.01

0.1

1

10

La Ce Pr Nd Sm Eu Gd Tb Dy Y Ho Er Tm Yb Lu

NPSWCoral SeaSouth Fiji basinBay of BengalAndaman Sea

Elements

01 x SA

AP/reta

waeS-6

Fig. 5. PAAS normalized REE patterns of modern seawaters (NPSW: North Pacificshallow water, Sagami trough (Alibio and Nozaki, 1999), Coral Sea: Coral sea (SouthPacific shallow seawater, Zhang and Nozaki, 1996), South Fiji Basin: South Fiji Basin e

Station SA12 (Zhang and Nozaki, 1996), Bay of Bengal: Bay of Bengal shallow water(Nozaki and Alibio, 2003) and Andaman Sea: Andaman Sea shallow water (Nozaki andAlibio, 2003).


This interpretation is further supported by the enrichment of Sr inthese limestone samples.

5.3. Source of REE

REE data have been used extensively to assess the pathways ofbiogenic and terrigenous fluxes from the source to the marinesediments (Piper, 1974; Murray and Leinen, 1993; Sholkovitz et al.,1994; Wray, 1995; Cullers, 1995). The concentration of REEs inseawater is mainly controlled by factors relating to input sourcesand scavenging processes related to depth, salinity, and oxygenlevels (Elderfield, 1988; Piepgras and Jacobsen, 1992; Bertram andElderfield, 1993; Greaves et al., 1999).

The limestones of the Mural Formation exhibit distinctly non-seawater-like patterns (Fig. 6aef) which resulted from the presenceof a variety of contaminants. The major source of contaminants arelikely: 1) terrigenous fine-grained sediments having high REEcontent with non-seawater-like pattern (Elderfield et al., 1990), 2)Fe andMn oxides (Bau et al., 1996) and 3) phosphates having a highaffinity for REEs in diagenetic fluids (Byrne et al., 1996).

The concentration of Al2O3 is closely related to clay content. So,Al2O3 concentration is considered as a proxy for shale contamina-tion (Nothdurft et al., 2004). Most of the limestones from the MuralFormation show high values for Al2O3 (0.44 to 6.26%; except fivesamples which show low concentration) when compared to theaverage values of siliciclastic-contaminated carbonate rocks (0.42%;Veizer, 1983). The Al2O3 concentration shows positive correlationwith the

PREE content (statistically significant at a strict signifi-

cance level of 0.001; linear correlation coefficient r¼ 0.59, n¼ 35)which suggest a moderate contamination.

Trace elements such as Th and Sc have been used as indicators ofshale contamination because of their higher concentrations in thePAAS than in marine carbonates (Webb and Kamber, 2000). The Thand Sc concentrations correlatewellwithAl2O3 contents (statisticallysignificant at a strict significance level of 0.001; linear correlationcoefficient r¼ 0.81, r¼ 0.90; respectively) which suggest the pres-enceof shale contamination in the limestonesof theMuralFormation.

Many modern carbonates (particularly foraminifera) werecontaminated by ferromanganese crusts (Palmer, 1985), whichshow a high affinity for the REEs, and the crusts incorporate themdisproportionately (Bau et al., 1996). The limestones of the presentstudy show a positive correlation between the

PREE and Fe

contents (statistically significant at a strict significance level of

0.01

0.1

1


CP1 CP3 CP4SSJ2 SSJ3

Elements

SA

AP/elp

maS

0.1

1

10


CP6 CP9 CP10

CP12 SSJ4 SSJ5

Elements

SA

AP/elp

maS

b

0.01

0.1

1


CP15 CP18 CP22 CP24

CP26 CP28 SSJ6 SSJ7

Elements

SA

AP/elp

maS

c

a

0.01

0.1

1


SSJ9 SSJ10 SSJ11

Elements

SA

AP/elp

maS

d

0.001

0.01

0.1

1

10


CP33 CP36 CP38 CP41 CP43

SSJ16 SSJ18 SSJ21 SSJ23 SSJ25

Elements

SA

AP/elp

maS

e

0.01

0.1

1


CP45 CP47 SSJ27

Elements

SA

AP/elp

maSf

Fig. 6. a, REE patterns of limestones of the CLC Member. b, PAAS normalised REE patterns of the TS Member. c, REE patterns of the LC Member. d, REE patterns of limestones of theCLP Member. e, REE patterns of limestones of the CLE Member. f, PAAS normalized REE patterns of limestones of the MQ Member.


0.001; linear correlation coefficient r¼ 0.70). The average Fe2O3concentrations (0.27%, n¼ 35) of the present study is within therange of average carbonate (0.38%, Veizer, 1983) which suggest thatthe diagenetic Fe2O3 does not play a significant role in controllingthe REE patterns in these limestones.

Phosphates mainly incorporate REE disproportionally and theyare altered easily by diagenesis (Reynard et al., 1999). The lime-stones of the present study show a positive correlation betweenP

REE and P2O5 (statistically significant at a strict significance levelof 0.001; linear correlation coefficient r¼ 0.74). A poor correlationhas been observed between P and NdN/YbN ratios (statisticallysignificant at a strict significance level of 0.01; linear correlationcoefficient r¼ 0.42) in the limestones of the Mural Formation.Hence it is unlikely that the presence of minor quantity of P2O5could affect the REE patterns of limestones of the Mural Formation.Thus, our data suggest that contamination by phosphate mineralsor ferromanganese coatings is not likely.

Yttrium is not removed from the seawater effectively whencompared with its geological twin Ho, due to differing surfacecomplex stabilities, thereby leading to a significant superchondriticmarine Y/Ho ratio (Hogdahl et al., 1968; Zhang et al., 1994; Bau et al.,1995; Bau, 1996; Nozaki et al., 1997). The chemical sediments freefromcontamination generally display Y/Ho ratios between44 and74.But contaminations due to terrestrial detritus and volcanic ash havefairly constant chondritic Y/Ho values ofw28. The limestones of theMural Formation contain large variations in Y/Ho ratio (27.34 to102.50). Like the Y/Ho ratio, Th and Sc also showsignificant variationsamong differentmembers of theMural Formation. Such variations in

these limestones may be due to contamination by terrestrial detritus(Webb and Kamber, 2000). In the present study, most of the lime-stones of the Mural Formation show high values of Th, Sc and Zr andchondritic Y/Ho ratios, which suggests that these limestones appearto have been contaminated by terrigenous materials.

The different members of the Mural Formation show slightvariations in LaN/YbN ratios (Table 4). The LaN/YbN ratios of the CLC,TS andMQmembers aremore or less similar to the values proposedby Condie (1991; about 1.0) and Sholkovitz (1990; about 1.3) forterrigenous materials whereas the LC, CLP and CLE members showlower LaN/YbN ratios. The observed variations in the LaN/YbN ratiosin the limestones of the Mural Formation suggest that the REEsignals were mainly influenced by the incorporation of terrigenousmaterials into them. The LaN/YbN ratios of the Mural Formation aremore or less similar to the Arabian Sea carbonate sediments (Nathet al., 1997) and Indian Ocean carbonate sediments (Nath et al.,1992) and lower than the shallow marine Albian and Maas-trichtian limestones (Madhavaraju and Lee, 2009; Madhavaraju andRamasamy, 1999; Table 4) of the Cauvery Basin and KudankulamFormation (Armstrong-Altrin et al., 2003) of South India (Table 4).

The limestones from the Mural Formation show non-seawater-like patterns. The representative samples of the present study werecompared with the limestones having non-seawater-like patterns(Fig. 7; Late Devonian coastal fringing reef, Nothdurft et al., 2004;Albian limestone, Madhavaraju and Lee, 2009; Maastrichtianlimestone, Madhavaraju and Ramasamy, 1999; Miocene limestone,Armstrong-Altrin et al., 2003) which suggests that the inclusions ofterrigenous materials in the carbonates as contaminants will mask

Table

4Ave

rage

geoc

hem

ical

values

oftheMuralFo

rmationco

mpared

tosh

allow

anddee

pmarinesedim

ents.

MuralF

ormationae

fMural

Form

ationg

(ave

rage

)

Albian

limestoneh

Maa

strich

tian

limestonei

Kudan

kulam

Form

ationj

Arabian

Sea

carbon

ate

sedim

ents

k

IndianOcean

carbon

ate

sedim

ents

lCLC

aTS

bLC

cCLP

dCLE

eMQf

Ce/Ce*

0.93

�0.12

0.96

�0.14

0.90

�0.12

0.80

�0.03

0.93

�0.12

0.78

�0.22

0.91

�0.13

0.97

�0.13

0.76

�0.2

0.9�0.1

0.84

�0.1

0.56

LaN/Yb N

1.12

�0.40

1.35

�0.80

0.90

�0.35

0.89

�0.08

0.74

�0.12

1.32

�0.13

1.0�0.45

1.91

�0.22

1.8�0.5

2.7�1.4

0.8�0.2

1.03

SREE

51�23

62�28

33�34

12�1

23�19

28�7

35�28

39�25

73�20

80�40

78�40

e

CaO

43�10

46�2

51�6

52�0.5

51�5

51�2

49�6

48�6

42�8

49�3

29�12

36.6

Eu/Eu*

1.1�0.14

1.3�0.29

1.17

�0.22

1.12

�0.03

1.0�0.24

1.6�0.9

1.16

�0.34

1.2�0.08

0.58

�0.1

0.78

�0.3

1.15

�0.1

e

aPresen

tstudy,

n¼5.

bPresen

tstudy,

n¼6.

cPresen

tstudy,

n¼8.

dPresen

tstudy,

n¼3.

ePresen

tstudy,

n¼10

.fPresen

tstudy,

n¼3.

gPresen

tstudy,

n¼35

.hMad

hav

arajuan

dLe

e,20

09,n

¼8.

iMad

hav

arajuan

dRam

asam

y,19

99,n

¼8.

jArm

strong-Altrinet

al.,20

03,n

¼9.

kNathet

al.,19

97,n

¼9.

lNathet

al.,19

92,n

¼4.

0.01

0.1

1

10


CP1 CP4 CP9 CP10CP33 SSJ2 SSJ3 SSJ6AL DCFR MAL ML

Elements

SA

AP/elp

maS

Fig. 7. Representative samples of the Mural Formation with non-seawater-like signa-tures are compared with limestones exhibit LREE enriched REEþ Y pattern (DCFR: LateDevonian coastal fringing reef (Nothdurft et al., 2004), AL: Albian limestone(Madhavaraju and Lee, 2009), MAL: Maastrichtian limestone (Madhavaraju andRamasamy, 1999) and ML: Miocene limestone (Kudankulam Formation, Armstrong-Altrin et al., 2003).


the seawater signature due to their high concentration of the REE.The high content of Al2O3 and SREE, the elevated concentrations ofTh and Sc, chondritic Y/Ho ratios, high LaN/YbN ratios and non-seawater-like REE patterns indicate that the terrigenous contami-nations in the limestones of the Mural Formation are responsiblefor such variations in REE signals.

The preservation of seawater REEþY pattern in limestone willonly occur if detrital materials, marine or diagenetic authigenicphosphates (eg. Rasmussen et al., 1998) and ferromanganeseencrustations (eg. Reitner, 1993) are nearly absent from the lime-stones. The inclusions of terrigenous materials in the carbonates ascontaminants will mask the seawater signature due to their highconcentration of REE in them. So, the depositional environment oflimestone is more important to understand the REE geochemistry.In the present study, the limestone samples from LC Member (at CPsection) and CLP and CLE members (at SSJ section) were depositedin an open shelf environment with little terrigneous contaminationwhereas the limestones from CLE (at CP section) and LC (at SSJsection) members contain high concentrations of terrigenousmaterials, respectively due to fluvial deltaic influence and delta-front depositional environments (González-León et al., 2008). Thus,the limestones deposited under both coastal and open shelf envi-ronments exhibit non-seawater-like REEþ Y patterns.

The present study reveals that the limestones deposited in openshelf environments are also contaminated by some terrigenousparticles. Hence, care may be taken to study the REE geochemistryof ancient limestone. The limestones devoid of terrigenous particlesare suitable for understanding the REE patterns of ancient shallowseawater and also they serve as a valuable seawater proxy. Thepresence of a small quantity of terrigenous materials in the lime-stones will also reveal source rock information.

The concentrations of certain immobile elements like La and Thare higher in silicic than in basic igneous rocks (Cullers, 1995). Thefelsic andmafic rocks show significant variations in La/Sc, La/Co, Th/Sc, Th/Co and Th/Cr ratios which are most useful in understandingthe provenance composition (Wronkiewicz and Condie, 1990; Coxet al., 1995; Cullers, 1995). The extent to which these elementalratios that are useful in unraveling the provenance of terrigenousmaterials present in the carbonate rock is clearly addressed byCullers (2002). In the present study, La/Sc, La/Co, Th/Co, Th/Cr, Cr/Th, and Th/Sc are similar over a range of theS(SiO2þAl2O3þ Fe2O3þMgOþNa2OþK2Oþ TiO2) (Fig. 8). Ourresults are generally consistent with other studies (Cullers, 2002).

Table 5Range of elemental ratios of the Mural Formation compared to felsic rocks, mafic rocks, Upper Continental Crust (UCC) and Post-Archaean Australian Shale (PAAS).

Range of Mural Formationa Range of sedimentsb Upper Continental Crustc Post-Archaean Australianaverage shalec

Felsic rocks Mafic rocks

Eu/Eu* 0.42e2.64 0.40e0.94 0.71e0.95 0.63 0.66La/Sc 2.65e25.78 2.50e16.3 0.43e0.86 2.21 2.40La/Co 0.25e8.66 1.80e13.8 0.14e0.38 1.76 1.65Th/Sc 0.05e2.48 0.84e20.5 0.05e0.22 0.79 0.90Th/Co 0.03e1.38 0.67e19.4 0.04e1.40 0.63 0.63Th/Cr 0.02e0.36 0.13e2.7 0.018e0.046 0.13 0.13Cr/Th 2.76e45.0 4.00e15.0 25e500 7.76 7.53

a Present study, n¼ 35.b Cullers (1994, 2000); Cullers and Podkovyrov (2000); Cullers et al. (1988).c Taylor and McLennan (1985).

Fig. 8. Bivariate plots for the limestones of theMural Formation. a,S(SiO2þ Al2O3þ Fe2O3þMgOþNa2Oþ K2Oþ TiO2) vs La/Sc. b,S(SiO2þ Al2O3þ Fe2O3þMgOþNa2Oþ K2Oþ TiO2)vs La/Co. c, S(SiO2þ Al2O3þ Fe2O3þMgOþNa2Oþ K2Oþ TiO2) vs Th/Sc. d, S(SiO2þ Al2O3þ Fe2O3þMgOþNa2Oþ K2Oþ TiO2) vs Th/Co. e, S(SiO2þ Al2O3þ Fe2O3þMgOþNa2Oþ K2Oþ TiO2) vs Th/Cr. f, S(SiO2þ Al2O3þ Fe2O3þMgOþNa2Oþ K2Oþ TiO2) vs Eu/Eu*.


The La/Sc, La/Co, Th/Co, Th/Cr, Cr/Th, and Th/Sc ratios of the lime-stones of the Mural Formation have been compared with felsic andmafic rocks (fine fraction) as well as to upper continental crust(UCC) and PAAS values (Table 5) which suggest that these ratios arewithin the range of intermediate to felsic rocks.

6. Conclusions

The high content of Al2O3, SREE, Th and Sc, low Y/Ho ratios, highLaN/YbN ratios and non-seawater-like REE patterns in the

limestones of the Mural Formation indicate that the terrigenouscontaminations is responsible for the variations in REE signals. Thelimestones of the Mural Formation were compared with the lime-stones having non-seawater-like patterns that indicate the inclu-sion of terrigenous materials in the carbonates, as contaminantswill mask the seawater signature due to their high concentration ofthe REE in them. The limestone samples from CLP member shownegative Ce anomalies (Ce/Ce*: 0.77 to 0.83, ave. 0.80� 0.03, n¼ 3)whereas CLC, TS, LC, CLE and MQmembers show both negative andpositive Ce anomalies (Ce/Ce*: 0.78 to 1.04, ave. 0.93� 0.12, n¼ 5;


0.82 to 1.20, ave. 0.96� 0.14, n¼ 6; 0.67 to 1.05, ave. 0.90� 0.12,n¼ 8; 0.73 to 1.07, ave. 0.93� 0.12, n¼ 10; 0.56 to 1.00, ave.0.78� 0.22, n¼ 3; respectively). The observed variations in Ceanomalies resulted from the inclusion of terrigenous materials aswell as Fe-rich colloids from rivers. The limestones of the MuralFormation contain both negative and positive Eu anomalies relativeto the PAAS. The CLP member shows least variations in Eu anom-alies (Eu/Eu*: 1.10 to 1.15, ave. 1.12� 0.03, n¼ 3) whereas CLC, TS,LC, CLE and MQ members exhibit large variations in Eu anomalies(Eu/Eu*: 0.94 to 1.28, ave. 1.10� 0.14, n¼ 5; 0.89 to 1.67, ave.1.30� 0.29, n¼ 6; 0.83 to 1.44, ave. 1.17� 0.22, n¼ 8; 0.42 to 1.26,ave. 1.0� 0.24, n¼ 10; 0.91 to 2.64, ave. 1.6� 0.9, n¼ 3; respec-tively). The observed positive Eu anomalies in the limestones arelikely controlled by the feldspar content.

The elemental ratios (La/Sc, La/Co, Th/Co, Th/Cr, Cr/Th, and Th/Sc)which are characteristics of provenance of terrigenous materialsshow the minimal variations with the changing percentage of theS(SiO2þAl2O3þ Fe2O3þMgOþNa2Oþ K2Oþ TiO2). The La/Sc, La/Co, Th/Co, Th/Cr, Cr/Th, andTh/Sc ratios of the limestones of theMuralFormation have been compared with felsic and mafic rocks, uppercontinental crust (UCC) and PAAS values which indicate that theterrigenous materials included in the limestones of the MuralFormationsweremainlyderived fromthe intermediate to felsic rocks.

Acknowledgements

The first author would like to thank Dr. Thierry Calmus, ERNO,Instituto de Geología, Universidad Nacional Autónoma de Mexicofor his support and encouragement during this work. We wouldlike to thank Prof. S.P. Verma and Prof. R.L. Cullers for their criticalreviews and constructive comments. We would like to thank Dr.Hannes Löser for his help during the field work. We acknowledgethe support rendered by Universidad Nacional Autónoma deMexico through PAPIIT Project No.IN121506-3. The field study ofthis work is partly supported by PAPIIT Project No. IN107803-3. Wethank Mr. Pablo Peñaflor for powdering of limestone samples forgeochemical studies. We also thank Dr. Teresa Pi I Puig, Instituto deGeología, Universidad Nacional Autónoma de México, México forher help in XRD analysis. This research was partly supported byKorea Science and Engineering Foundation (KOSEF) grants (R01-2000-000-00056-0 to YIL).

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