87Sr/86Sr anomalies in Late Cretaceous-Early Tertiary strata of the Cauvery basin, south India: Constraints on nature and rate of environmental changes across K–T boundary

878787Sr/868686Sr anomalies in Late Cretaceous-Early Tertiarystrata of the Cauvery basin, south India: Constraints

on nature and rate of environmental changesacross K–T boundary

Mu Ramkumar1,∗, Doris Stuben2, Zsolt Berner2 and Jens Schneider3

1Department of Geology, Periyar University, Salem 636 011, India.2Institut fur Mineralogie und Geochemie, Universitat Karlsruhe, D-76128 Karlsruhe, Germany.

3Institut fur Geowissenschaften und Lithospharenforschung, Justus-Liebig-Universitat,D-35032 Marburg, Germany.

∗e-mail: [email protected]

The Ariyalur–Pondicherry sub-basin of the Cauvery basin comprises a near complete stratigraphicrecord of Upper Cretaceous–Lower Tertiary periods. Earlier studies have documented variationsof clay mineral assemblages, change in microtexture of siliciclasts and many geochemical andstable isotopic anomalies far below the Cretaceous–Tertiary boundary (KTB) in these strata. Thispaper documents the occurrences of two positive 87Sr/86Sr anomalies preceding K–T boundary inthis basin and discusses plausible causes. Analysis of trace elemental and stable isotopic profiles,sedimentation history, petrography and mineralogy of the rocks reveal that while both the anom-alies may be due to increased detrital influx caused by sea level and climatic changes, the secondanomaly might have been influenced by Deccan volcanism which in turn predated KTB. Record ofsuch anomalies preceding K–T boundary supports the view of multi-causal step-wise extinction ofbiota across KTB.

1. Introduction

Biotic information of earth’s history is repletewith proliferation, depletion and/or disappear-ances of taxa from species to family levels. Accor-ding to Hallam (1981) there have been six majorextinctions during the Phanerozoic namely, LateCambrian, Late Ordovician, Late Devoinan, LatePermian, Late Triassic and Late Cretaceous, thathave shown 52, 24, 30, 50, 35 and 26% ofextinctions of families (after Newell 1967). Amongthese, the Late Cretaceous extinction event isconsidered to be very significant (Raup 1986) asa wide variety of organisms as big and diverseas fishes and dinosaurs to as small as marineplanktons either became extinct or dwindled innumber of species and population across the

Cretaceous–Tertiary boundary (KTB). Accordingto the decision of the KTB Working Group, theboundary is characterized by major planktonicforaminiferal extinction event and/or the firstappearance datum of Cenozoic species and a majorlithologic change, including the presence of a thinred layer at the base of the boundary clay markedby various geochemical anomalies (Canudo et al1991).

Documentation of widespread iridium anom-aly at the KTB by Alvarez (1986) and Alvarezet al (1980) and report of similar anomalies fromgeographically separated regions (for example,Schmitz 1992; Evans et al 1993; Sutherland 1994;Evans et al 1995; Albertao and Martins 1996;Zhao et al 2002; Shrivastava and Ahmad 2005)led to the interpretation of major bolide collision

Keywords. Cretaceous–Tertiary boundary; strontium isotope; biotic turnover; sea level changes.

J. Earth Syst. Sci. 119, No. 1, February 2010, pp. 1–17© Indian Academy of Sciences 1

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(Kring 2007), asteroid or comet shower dur-ing end Cretaceous as the cause of extinctionacross KTB. Despite considerable discussion (forexample, Tschudy et al 1984; Jiang and Gartner1986; Saito et al 1986; Sloan et al 1986; Hallam1987; Keller 1988a, 1988b; Frank and Arthur 1999;Keller et al 2003; Alegret and Thomas 2004;Ramkumar et al 2004a, 2005; Arinobu et al 2005;Hart et al 2005; Lamolda et al 2005; Paul 2005;Shrivastava and Ahmad 2005; Stuben et al 2005;Kawaragi et al 2009), consensus on the cause(s)of extinction across KTB remains elusive and thedebate still continues (for example, Keller et al1995, 1998, 2002a, 2003, 2007).

Initially, bolide impact was offered as an expla-nation for mass extinction of planktic foraminiferaat the KTB (Smit 1982; Arz et al 2001) based onoccurrences of Ir anomaly in few KTB sections.However, many studies (for example, Keller 1993;Keller et al 1995; Keller and Stinnesbeck 1996;Lopez-Oliva and Keller 1996; Keller et al 2003;Karoui-Yaakoub et al 2002) disapproved such even-tuality as there is no evidence to show that extinc-tion occurred exactly at KTB. While reviewing theIr and platinum group element abundance in sedi-ments, Sawlowicz (1993) suggested involvement ofmany processes other than bolide sources for Irenrichment. Stable isotope patterns in the marinestratigraphic record documented the climatic andbiologic effects (Hsu et al 1982; Hsu 1984; Hsuand McKenzie 1985; Frank and Arthur 1999) thatwere interpreted to have contributed towards bioticturnover across KTB. Few other authors haveoffered an alternative hypothesis of gradual dis-appearance of taxa due to relative sea-level changes(Hallam and Wignall 1999), or regional tectonicsor many other causes. They have reported theoccurrences of geochemical and other anomaliesnot across KTB but far away from the boundary.According to these studies, an increasing body ofevidence points to the presence of abrupt δ18O andδ13C changes associated with the KTB (Romeinand Smit 1981; Hsu et al 1982; Shackleton andHall 1984; Mount et al 1986; Zachos and Arthur1986). At number of localities, however, δ18O andδ13C changes have been found to precede the K–Tboundary (Keller et al 1998, 2003; Keller et al2008a, 2008b; 2009a, 2009b) as defined by theworldwide iridium anomaly (Renard et al 1984;Williams et al 1983; Mount et al 1986; Zachos andArthur 1986; Zhao et al 2002). Similarly, Kramaret al (2001); Stuben et al (2002); Keller et al(2002a) and Adatte et al (2002) recorded anom-alies of Platinum group elements far below K–Tboundary. Luciani (2002) reported extinction ofspecies far below the KTB in Tunisian KT record.Decline and extinction of nannofossils prior to

KTB was recorded in Caravaka, Spain by Lamoldaet al (2005). While examining the palynofossilsacross KTB in the Scollard Formation of Alberta,Canada, wherein Ir anomaly is documented,Srivastava (1994) had unequivocally establishedthat there is no catastrophic extinction of theflora and the change in palynofossil composition isgradual across the KTB. Thus, the anomalies thatpredate K–T boundary raise questions on the rela-tive timing of biotic and isotopic events near theK–T boundary and their relevance to the impactscenario (Kaminski and Malmgren 1989).

Based on the suggestion of Hallam (1987),Glasby and Kunzendorf (1996) noted that exten-sive Deccan volcanism itself might have causedsea level fall during latest Maastrichtian to thetune of ∼100m, which in turn could have createdenvironmental stress (Keller et al 2002b), causingmany biota to disappear, the effect of which mighthave been profound across KTB (Abramovich andKeller 2002). As it is probably the most voluminouscontinental flood basaltic volcanic activity thatspanned less than 1 million year duration (Dessertet al 2001), its impact on environmental conditionsmight have been sudden and rapid. Sutherland(1994) is of the opinion that either the Chicxulubimpact or the Deccan volcanism could haveinduced environmental changes serious enough tosignificantly disturb ecosystems that in turn led tobiotic turnover across KTB. Acidification of oceanwater due to Deccan volcanism and resultant envi-ronmental deterioration, preceding KTB was inter-preted through Os isotopic profiles by Robinsonet al (2009) that imply not only the deterioration ofatmosphere by addition of CO2, but also the directinfluence of volcanism on marine environment byacidification that made it inhospitable for selectivespecies. The studies of Keller (2008b, 2009b) thatfollowed multidisciplinary approach to documentbio, litho, magnetostratigraphic, mineralogic, andsedimentological data of KTB of Krishna–Godavaribasin and stratigraphic records of Jhilmili areaof Madhya Pradesh, India respectively, suggestedthat significant part of Deccan volcanism itselfmight have ended prior to perceived KTB. Preda-ting of Deccan volcanism has been suggested byfew other authors too (Venkatesan et al 1993;Bhandari et al 1993a; Khadkikar et al 1999).In an earlier study, Keller et al (2003) inter-preted multiple events, namely, an impact prior toKTB that created environmental stress, second andmajor impact across KTB coeval with significantDeccan volcanism that created mass extinctionand another impact during Danian that delayedbiotic recovery. Report of impacts other thanChicxulub (for example, Negi et al 1993) addsupport to this interpretation of either multiple

Constraints on nature and rate of environmental changes across K–T boundary 3

impacts or multiple causes and resultant environ-mental deterioration. Morgan et al (2004) are ofthe opinion that there might be yet another causenamely deep-lithospheric blasts that vented outenormous amounts of CO2, CO and SO2 that mighthave disturbed seriously the ecosystem causingextinction across KTB.

In India, Ir anomalies are linked to the Dec-can volcanism (Pandey 1990; Bhandari et al 1993a,1993b, 1994, 1995) especially for those recordedin intertrappean clayey sediments (Anjar region ofCentral India – Khadkikar et al 1999; Shrivastavaand Ahamad 2005) and for the suspected occur-rence of Ir anomaly in the Kallamedu Formationof the Cauvery basin. The clay layers in intertrap-pean deposits and in the Kallamdeu Formationare presumed to have been formed by weatheringof volcanic ash and glass (Hansen and Mohabay2000). Recently, Rao et al (2002) recorded basicvolcanism near KTB in Godavari basin. As theGodavari basin is more proximal to the Cauverybasin than Deccan region, either Deccan volcanismor volcanic activity in Godavari basin could haveinfluenced depositional episodes across KTB of theCauvery basin. However, Madhavaraju et al (2002)after examining clay mineral occurrences of Maas-trichtian and Danian rocks of the Cauvery basin,concluded that Deccan volcanism had no signa-tures in these deposits. These observations sig-nify relatively contemporaneous meteoritic impactat Gulf of Mexico and Deccan and Godavari vol-canism in the Indian subcontinent. However, asthe Ir anomalies occur in intertrappean beds, it isinferred that the Deccan volcanism predates mete-oritic impact (Venkatesan et al 1993; Bhandari et al1993a, 1993b, 1994; Khadkikar et al 1999). In viewof the enormity of Deccan volcanism (Glasby andKunzendorf 1996) and the proximity of Godavaribasic volcanism to the Cauvery basin, it couldbe assumed that influence of either (Deccan orGodavari) volcanic activity could have been sig-nificant in the Cauvery basin than the meteoriticimpact in the Gulf of Mexico region.

Though such presumptions have been made, sys-tematic studies on environmental changes acrossthe KTB of the Cauvery basin are very few(for example, Sastry and Rao 1964; Madhavarajuet al 2002; Ramkumar et al 2004a, 2005). Sastryand Rao (1964) documented biotic changes acrossKTB. Madhavaraju et al (2002) studied claymineral composition to constrain on prevalent envi-ronmental changes across KTB. Ramkumar et al(2004a, 2005) recorded many geochemical and sta-ble isotopic anomalies in the Cauvery basin. Recordof Ba anomaly by these authors in the Cauverybasin far below the K–T boundary and similaroccurrences of Ba-anomalies in comparable strati-graphic sections located at NE-Mexico, Guatemala

and Israel supported the theory of multi-causalslowly deteriorating environmental condition asthe cause of extinction. In addition, these studieshave suggested yet another event namely release ofvolatile gases from buried gas hydrates owing tosea level lowering at the end of Cretaceous, whichmight have caused a chain of events, leading to dis-appearance of many taxa. Wignall (2001), whileexamining the relative role of volcanisms on per-ceived mass extinctions during geologic history alsostated that the Deccan volcanism itself would havetriggered dissociation of gas hydrates and initiateda chain of biotic responses.

From this review, it is apparent that thoughenvironmental conditions across KTB varied, thetiming of initiation of environmental changes hasbeen the topic of discussion and remains debated.As the Cauvery basin contains near completestratigraphic record of Upper Cretaceous–LowerTertiary, it is proposed to examine its environmen-tal characteristics. Thus, strontium isotopic com-position of the Maastrichtian–Danian strata of theCauvery basin was analyzed and the results arediscussed in this paper.

2. Geologic setting and depositionalhistory

A near complete, 236 m thick, Upper Cretaceous–Paleocene carbonate–siliciclastic succession isexposed in the Ariyalur–Pondicherry depression(Sastry and Rao 1964) of the Cauvery basin(figure 1). Maastrichtian–Danian boundary in thisbasin is represented by an unconformity surfaceacross which faunal and lithological characteristicsare highly different making the boundary readilyrecognized (Sastry and Rao 1964; Sastry et al1972; Nair 1978; Chandrasekaran and Ramkumar1995; Madhavaraju et al 2002; Ayyasamy 2006;Ramkumar et al 2004b). Depositional history of thestudy area is inferred from the field data, labora-tory study of facies characteristics and collation ofinformation from previous studies listed above andis enumerated herein.

Sedimentation of the lowermost deposits ofthe studied section, the Kallankurichchi Forma-tion, commenced with transgression during theLatest Campanian–Early Maastrichtian (Tewariet al 1996; Ramkumar 1999, 2004; Ramkumaret al 2004a, 2004b; Ayyasamy 2006). Towardstop, the deposits show reduction in proportionand size of siliciclastics that were increasinglyreplaced by gryphea colonies. In due course con-comitant with sea level rise, the gryphean bank hadshifted towards former shallower regions and thelocations previously occupied by coastal conglo-merate have become middle shelf wherein typical

4 Mu Ramkumar et al

Figure 1. Location of Cauvery basin and study area.

inoceramus limestone started developing. Breakin the sedimentation of inoceramus limestone wasassociated with the regression of sea level result-ing in erosion of shell banks and middle shelfdeposits and redeposition of them into biostromaldeposits. A significant sea level rise followingthis erosional event is recorded on a basin scale(Raju et al 1993; Fursich and Pandey 1999).Due to this rise, gryphean shell banks starteddeveloping more widely than before. At top,shell fragments and minor amounts of siliciclas-tics are observed in the Kallankurichchi Formation

indicating onset of regression and higher energyconditions. The occurrence of non-depositional sur-face at the top of this formation and deposition ofshallow marine siliciclastics (Ottakoil Formation)immediately over the carbonates and conformableofflap (Sastry and Rao 1964; Sastry et al 1972;Chandrasekaran and Ramkumar 1995; Tewari et al1996; Ramkumar et al 2004b) of much youngerfluvial sand deposits (Kallamedu Formation)are all suggestive of gradual regression associ-ated with establishment of fluvial system duringend Cretaceous. Towards top of the Kallamedu


Formation, paleosols are recorded implying firmestablishment of continental conditions in areasthat were submerged under Cretaceous sea sinceSantonian (Ayyasamy 2006). Based on strati-graphic variation of microtextures of quartz grainsof Kallamedu Formation, Madhavaraju et al (2006)interpreted a change from coastal marine to flu-vial deposition which is in conformity with theseinferences. At the beginning of Danian, transgres-sion took place that covered only the eastern partof the Kallamedu Formation (Ramkumar et al2004b). Presence of conformable contact betweenAnandavadi member and Kallamedu Formationand initiation of carbonate deposition from thebeginning of Danian are indicative of absenceof fluvial sediment supply and tectonic activity.Increase in sea level and establishment of shallow,wide shelf with open circulation have led to thedeposition of Periyakurichchi member of NiniyurFormation. At top, this member has distinct ero-sional unconformity, which in turn, when inter-preted along with the presence of huge thickness ofcontinental sandstone (>4000m thick CuddaloreSandstone Formation), indicates restoration of con-tinental conditions in this basin. The global sealevel peaks during 73 MY (±1; Late Campanian),69.4 MY (Early to Late Maastrichtian) and 63 MY(±0.5; Early to middle Danian) are observed (Rajuet al 1993) to occur in this basin.

3. Methods and materials

Systematic field mapping in the scale of 1:50,000covering an area of about 150 km2 in and aroundeast of Ariyalur (figure 2) was conducted forthe collection of rock samples and data on litho-logy, contact relationships, sedimentary and tec-tonic structures and faunal association (megaand ichno) from natural exposures, dug-well andmine sections (traverse lines are marked in thefigure 2). Details of facies association, field char-acteristics and depositional environments of thestudied section are presented in our other paper(Ramkumar et al 2004b). A composite strati-graphic profile representing continuous strati-graphic record of Maastrichtian–Danian stratawas constructed (figure 3) that allowed selectionof 47 representative whole-rock powder samples(sampling locations in stratigraphic order aremarked in the figure 3) for analyzing their traceelemental compositions and bulk mineralogy. Fromthese 47 samples, 24 alternate samples were ana-lyzed for strontium isotope, major elements, sta-ble isotopes and clay mineralogy. Trace and majorelemental analyses were performed with XRF fol-lowing the procedures discussed in Kramar (1997)and Ramkumar et al (2006). Stable isotopes were

analyzed against PDB standards and expressedin per mil unit, details of which are discussedin Keller et al (1998) and Stuben et al (2002).Strontium isotopic analyses were made followingthe method of Horwitz et al (1991a, 1991b) andBirck (1986). Laboratory methods for the stron-tium isotope analysis followed instrumental andoperating specifications as discussed in Degryseand Schneider (2008). Values of 87Sr/86Sr weredetermined with a VG-354 five collector mass spec-trometer using the multi-dynamic routines thatinclude corrections for isobaric interference from87Rb. Data have been normalized to a value of0.1194 for 86Sr/88Sr. Repeated measurements ofthe NBS 987 standard over the duration of analysesyielded an average 87Sr/86Sr ratio of 0.71025 ± 4(2s mean, n = 24). In this paper, we discuss the87Sr/86Sr record of the K–T section of the Cau-very basin and the data on stable isotopic, traceelemental, petrographic and mineralogical data areutilized to support the interpretations on strontiumisotopic data.

4. Results

Chronologically averaged 87Sr/86Sr values show ageneral reducing trend from Campanian (aver-age 0.7145) to Maastrichtian (average 0.7127)and finally to Danian (average 0.7095). How-ever, apparent fluctuations exist among lithos-tratigraphic formation-wise averaged values; viz.,Sillakkudi Fm. (0.7239), Kallankurichchi Fm.(0.7094), Ottakoil Fm. (0.7168), Kallamedu Fm.(0.7176) and Niniyur Fm. (0.7095). These fluctua-tions are due to the lithological control over stron-tium isotopic composition as siliciliclastics andcarbonates show higher and lesser values respec-tively. The Kallankurichchi and Niniyur formationsare predominantly carbonates and show Sr iso-topic composition close to seawater value, signify-ing their biogeochemical nature and also that theyare not altered from pristine nature.

Though the 87Sr/86Sr values remained arounda mean of 0.7080 since Cambrian (Ebneth et al2001), the Cauvery basin samples show an aver-age 87Sr/86Sr value of 0.7120, on a higher side.One potential error in the data may be due to theanalyses of whole rock powders. Recalculation ofmean by removing anomalous values (one samplefrom Campanian rocks and four samples from LateMaastrichtian rocks) had provided 87Sr/86Sr meanof 0.7096 which is close to the world average of0.7080, seawater value of 0.70916 (Davis et al2003) and K–T boundary value reported elsewhere(0.707845 – Sugarman et al 1995; McArthur et al1998). Nevertheless, this mean value is higher thanthe amount that could be supplied by river (0.7116)

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or benthic (0.7084) or hydrothermal (0.7037)sources (after Davis et al 2003). Facies andgeochemical characterization of the rocks understudy (Ramkumar 2004, 2007, 2008; Ramkumaret al 2004a, 2004b, 2005, 2006) have also indi-cated preservation of primary depositional char-acteristics in terms of geochemical composition inthese rocks. Hence, the data on whole rock powdersamples could be considered as compatible withseparates of carbonates, phosphates and barites asbeing analyzed elsewhere (for example, Veizer et al1999). Geochemical profiles (figure 4) of selectedmajor and trace elements and stable and stron-tium isotope compositions of the studied sectionreveal that there are many positive and nega-tive excursions of different elements, but most ofthose anomalies occur below the K–T boundary(65.4 my). Among all these profiles, two positive87Sr/86Sr anomalies located below the K–T boun-dary are predominant. Except the anomalies, thestrontium isotopic record of the study area followsthe general trend of 87Sr/86Sr curve reported else-where (for example, Martin and Macdougall 1991)and thus confirm the assumption of preservationof primary signal in the Cauvery basin. Thus, theanomalies are considered as primary signals andtheir causative factors are examined.

5. Discussion

Inferences on 87Sr/86Sr are drawn based on thenotion that past variations of strontium isotopiccomposition are due to the changes in relative con-tribution of mantle Sr input to the ocean water andsupply of Sr from continental weathering. Majororogenic movements also tend to influence Sr iso-topic values (Godderis and Francois 1995) in theensuing marine sedimentary record. Based on thePhanerozoic record of 87Sr/86Sr, Veizer et al (1999)concluded that variations in strontium isotopiccompositions of sedimentary rocks reflect princi-pally the waxing and waning of Sr input fromrivers (continental influx) versus the input fromthe submarine hydrothermal systems. According toGodderis and Francois (1995), the oceanic stron-tium isotopic ratio of Phanerozoic shows high val-ues in the Precambrian (0.709) followed by lowervalues during much of the Palaeozoic and Mesozoiceras (0.707) and finally a sharp increase during theCenozoic (after 40 Ma) to the present value (0.709).Superimposed on this long term trend there aremany oscillations of various amplitudes throughoutthe Phanerozoic. In order to find the reason of thesefluctuations, several authors have built up moreor less complex Sr cycles. The rivers deliver highradiogenic Sr (isotopic ratio = 0.7119; Holland1984) to the ocean by carrying the weathering

product of igneous and sedimentary rocks, whilethe Sr exchanges during hydrothermal processesat mid-ocean ridges supply low radiogenic Sr (iso-topic ratio = 0.703; Holland 1984) to the ocean.The removal of Sr out of the ocean by carbon-ate deposition and its recycling through diagene-sis of carbonate sediments play a secondary role.While reviewing the geochemical and isotopic char-acteristics of Permian–Triassic and Cretaceous–Tertiary, Holser and Magaritz (1992) stated thatthere could be four reasons due to which stron-tium isotopic compositional profile showed pertur-bations namely, influx of radiogenic Sr eroded fromcratons, an acid rain consequent to bolide impact,explosive volcanism related to the eruption of Dec-can basalts, erosion consequent to regression andany/all of these, though the relative contribution ofthem in an area has to be interpreted according tothe local criteria. The 87Sr/86Sr composition of thestudy area is examined in the light of these causes.

5.1 Deccan/Godavari volcanism

Recent studies have established that peak vol-canism of Deccan basalts took place during LateMaastrichtian. The double positive anomalies of87Sr/86Sr in rocks under study also occur dur-ing Late Maastrichtian, permitting surmise theircause to be related to volcanism. As the adjacentlylocated Godavari basin also has shown volcanicactivity during this time (Keller 2003, 2005; Kelleret al 2008b), it is possible that these anomalieswere created under the influence of either Dec-can volcanism or Godavari volcanism. India waslocated at around 40◦ southern latitudes duringMaastrichtian and passing through the Reunionhotspot and also might have been under the influ-ence of Ninetyeast midoceanic ridge (Keller 2005),the submarine basaltic flows could have suppliedradiogenic strontium (Davis et al 2003) resultingin the positive 87Sr/86Sr anomalies of Late Maas-trichtian deposits. As the studied rocks generallyshow higher values of 87Sr/86Sr composition, preva-lence of basaltic lava ejection probably from Nine-tyeast ridge and peaking of 87Sr/86Sr values duringinitiation of Deccan volcanism could be inferred.Jones et al (1994) have constructed strontium iso-topic curve for Jurassic and Cretaceous periods inwhich documented a sharp positive anomaly dur-ing Late Maastrichtian. They have explained it interms of a mid-oceanic ridge hydrothermal flux.Though these presumptions could be made merelyby the coeval nature of 87Sr/86Sr anomalies andvolcanism, it would not be prudent to comparewell-constrained age data available for those vol-canic events and non-available nature of the rocksunder study. In addition, the observations that theCauvery basin sediments generally contain higher


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Sr isotopic composition than that can be sourcedfrom hydrothermal or volcanic source discountssuch possibility. The strontium isotopic anom-alies occur far below the suspected clay rich layer(presumed to be the altered products of volcanicash/glass) and hence also, it would be prudent todiscount volcanism as the cause of these anom-alies. The clay mineral analysis of Madhavarajuet al (2002) does not record any influence of Dec-can volcanism in the rocks under study. How-ever, considering the quantum of Deccan volcanismand its global scale environmental ramifications, itwould be difficult to affirm or discount its influ-ence with the available data. Additional data andvery-high resolution sampling and establishment ofchronological datum may be required to documentthe influence of Deccan or Godavari volcanism inthe Cauvery basin.

5.2 Source area weathering and periodicinflux of siliciclastics

While simulating the weathering process andresultant 87Sr/86Sr value, Probst et al (2000)observed that there is an initial spike of Srisotopic composition ranging up to 0.742. In thelight of this observation, occurrences of generallyhigher 87Sr/86Sr values in the studied section andits gradual reducing trend from Campanian toDanian suggest initiation of significant weather-ing of granitic gneiss and granitic provenance afterthe tectonic movement during Santonian (F3 infigure 2) and progressive decrease of radiogenicSr contribution due to weakening of weathering,which is in conformity with the general absenceof chemical weathering in the source area andthe presence of unaltered basement rock clasts.Collection of available data on strontium iso-topic values of source rock types has given thefollowing information. Gneisses which form thebasement rock in much of the south Indian penin-sular shield contain 87Sr/86Sr value of 0.75 (Derryand France-Lanord 1996) which add support to theobservation of generally higher values of 87Sr/86Srcomposition of the studied rocks. However, thespikes of 87Sr/86Sr in terms of positive anomaliescould not be explained by source area weatheringalone. Additionally, episodic/periodic influx of sili-ciclastics from provenance areas could be assumedfor these positive anomalies. Incidentally, one ofthe Sr anomalies occurs together with Ba anomaly(figures 3 and 4) that was interpreted to be of theresult of abundant influx of siliciclastics followingsea level retreat (Ramkumar et al 2005). The silici-clastics contain fresh and unaltered Ba-orthoclaseclasts. As potash feldspars contain higher 87Sr/86Srvalue of 0.782738 ± 5 (Probst et al 2000), peri-odic influx siliciclastics with these feldspars could

explain the positive anomalies or at least the anom-aly wherein Ba anomaly is also reported. Probstet al (2000) also stated that mineral-weatheringand soil formation in areas rich in potash feldsparcould also supply radiogenic strontium to thesediments. In which case, influx of either unal-tered potash feldspars and/or significant chemicalweathering in the source area and their influx tothe depositional environments could be inferred.Bullen et al (1997) have proposed that significantquantities of radiogenic Sr may be leached fromK-feldspar during weathering of granitoid prove-nances. As the provenance areas of the Cauverybasin consist of charnockites and granitic gneissesthat contain significant quantities of K-feldspars,the inference of radiogenic Sr could be ascertained.However, occurrences of unaltered clasts precludesignificant chemical weathering in the source area.In addition, only one positive anomaly is corrobo-rated with fresh feldspars a-la significant siliciclas-tic influx, reasons for the occurrence of anotherpositive anomaly have to be identified.

If the reports of Madhavaraju et al (2002, 2006)who have reported prevalent rapid climatologicalchanges in the Cauvery basin during Late Creta-ceous are considered, the spikes of Sr isotope inthe form of two positive anomalies might be theresult of rapid and short climatic changes thataccompanied siliciclastic influx events. It may alsobe due to the fact that, as the carbonates aredeposited during sea level highstands a conditionthat allows less of drainage area to get exposed forweathering and transport, eventually allowing onlylittle quantity of radiogenic Sr to reach depocen-ters (Denison et al 1997). In such a scenario, thenature of sea level oscillations and resultant changein depositional conditions as well as source areaenvironmental conditions has to be examined.

5.3 Sea level changes and resultant strontiumisotopic excursions

A pronounced sea level low occurred during LateMaastrichtian on a global scale (Hakansson andThomsen 1999) and the same is recorded in theCauvery basin too (Raju et al 1993; Ramkumaret al 2004b). The sea level curve of the Cauverybasin shows a long term decreasing trend that cul-minated at KTB lowstand similar to the observa-tions of Adatte et al (2002) on K–T section locatedin Tunisia as a result of middle and high latitudecooling. Comparison of sea level curve and geo-chemical profiles of the studied section shows thatthe positive anomalies of 87Sr/86Sr form part of sealevel retreats. As deposition in this basin repre-sent a major oscillating carbonate-siliciclastic sys-tem coinciding with sea level high-low respectively(Ramkumar 2004; Ramkumar et al 2004a, 2004b),


the periods of sea level low represent advancing ofcontinental river systems that might have broughtin higher terrigenous influx, the positive anom-alies could be interpreted as a direct result ofclastic input. Occurrence of positive excursions of87Sr/86Sr during sea level lows confirm this infer-ence and also the views of Wallmann (2001) andKampschulte et al (2001) that periods of glaciation(sea level lows) influence higher erosion in conti-nents (Ebneth et al 2001) and increased terrigenousinflux (Veizer et al 1999). These views are alsoascertained by the observed higher mean value of87Sr/86Sr of the Cauvery basin strata than theworld average. Dessert et al (2001) proved thatthe strontium isotopic values of end-Cretaceousseawater increased synchronous with Deccan vol-canism and pre-volcanic values were restored aftersignificant volcanic activity, thus forming a positiveanomaly across KTB. They have suggested strongcontinental weathering as the reason for this spike.Whitford et al (1996) stated that 87Sr/86Sr in rocksthat have abundant riverine source material havea mean of 0.7117 whereas a hydrothermal sourcewould record a mean value of 0.703. According tothese estimates also, enhanced continental weath-ering and riverine influx that have dominated depo-sition in the Cretaceous–Tertiary deposits of theCauvery basin which in turn influenced by sea levelretreats could be inferred.

Occurrences of inverse relationships betweenlinear and polynomial trendlines of relative sealevel and terrigenous elements Sr, Si, Al and Ti(figure 4), ascertain the influence of continentalinflux concomitant with sea level reduction. As theelemental concentration of Sr shows a broad peakcovering the two positive anomalies of 87Sr/86Srcurve, coeval with positive excursions of terrige-nous elements such as Si and Ti, these observa-tions gain importance. As observed by Martin andMacdougall (1991) in K–T sections that spawnworld over, the smoothed trendline (polynomialtrend) shows a broad positive anomaly across K–Tboundary in the Cauvery basin. They termed suchanomaly as a consequence of higher influx of radio-genic Sr to the ocean during deposition. They alsoopined that, as Deccan volcanism was coeval toK–T boundary event, weathering of volcanic rocksmay have influenced reduction of 87Sr/86Sr valuesin the early Cenozoic as is evident in figure 4.

Gomez-Alday et al (2004) are of the opinionthat negative excursion of stable isotopes, parti-cularly the carbon isotope is typical of lowstandsystems tract, which is in conformity with thepresent observations. The carbon isotopic pro-file shows a gradual negative excursion, may bedue to sea level fall (Marquillas et al 2007), expo-sure of previously submarine regions to subaerial

conditions, oxidation of carbon stored there andresultant depletion of C13 (Holser and Magaritz1992). As observed by Holser and Magaritz (1992),the return of C13 to the normal trend across Danianis so swift, in conformity with the interpretation ofreturn of sea level to normal levels during Danian.The oxygen isotopic trend, though could be statedto be indicative of these inferences, shows fewminor fluctuations, may be due to ‘local effectsincluding neomorphism or spar cement’. However,warming up before the KTB as indicated by deple-tion and rapid cooling and again gradual shift towarm trends are indicated.

Based on the amendment proposed by Donovanet al (1988) for the Late Cretaceous–Early Tertiarysea level curve of Haq et al (1988), Holser andMagaritz (1993) stated that there was a globalscale fall of sea level at the end of Maas-trichtian and a faster return of sea level duringDanian (also observed from many K–T sectionsof the World – Schulte et al 2006). It impliesthat the end Cretaceous was a period of rapidsea level changes as a result of climatic varia-tions. While analyzing palaeoclimatic variationsthrough clay mineral composition of the Cauverybasin under study, Madhavaraju et al (2002) haveobserved higher variability in the proportion of claymineral abundances in stratigraphic context duringLate Maastrichtian and interpreted alternationsbetween seasonal variations and physical weath-ering and transport. These observations, togetherwith rapid and global scale sea level variations,suggest sea level controlled variations in sedimen-tary pattern that might have reflected in trends ofstrontium isotopic trends.

In a weathering environment, the calcite getsdissolved initially followed by sheet silicate dissolu-tion (Taylor et al 2000), in which case, weatheringof newly exposed regions due to sea level fall andsupply of Sr initially from leaches of calcite and fol-lowed by from leaches of silicates could be inferred,explaining the double peaked positive 87Sr/86Sranomaly. This inference calls for additional expla-nation pertaining to the spurt in supply (sharppositive excursions). If periodic variation of sedi-ment influx (dry deposits and wet deposits) asinterpreted by clay mineral analysis (Madhavarajuet al 2002) is considered, such behaviour of Sr iso-topic composition could be justified. Dissolutionof calcite is suggested to be a significant contri-butor of radiogenic strontium (Taylor et al 2000).These authors have also noted that after initialspike, with time, the end-products reach stron-tium isotopic ratio of near equal or more thanthe precursor due to release of Sr, which mayexplain the spiked and higher values of Sr iso-topic composition in the studied K–T section.


As the rocks under study were deposited underlowstand systems tract during which large swathsof previously marine regimes were exposed tometeoric conditions that subjected predominantlymarine carbonates (Kallankurichchi Formation) toundergo meteoric conditions of leaching, spikesof calcite dissolution events and resultant enrich-ment of Sr isotopic composition of contemporane-ous sediments (Kallamedu Formation) could haveresulted. Jones et al (1994) have also suggestedthat diagenetic flux (Sr isotopic influx drawnfrom leaching of subaerially exposed carbonates)could also be a reason for spike in strontium iso-topic curve. Derry and France-Lanord (1996) haveobserved sudden spikes in strontium isotopic com-position and attributed them to be the resultof change in intensity of chemical weathering ofsource rock and the shift in the ratio betweensilicate and carbonate weathering.

Weathering under oxidizing conditions promotethe sheet silicates to degenerate rapidly (Tayloret al 2000), that may be responsible for rapidrelease of Sr, which in turn may have influencedthe formation of Sr isotopic positive excursions.Exposure of former marine deposits to the sub-aerial conditions, due to sea level fall might havecreated conducive milieu for oxidizing conditions.At this juncture, the interpretation of destabi-lization of gas hydrates due to sea level fall andpromotion of Ba anomaly by Ramkumar et al(2005) gains importance. As the Sr anomaly occurstogether with the Ba anomaly, influence of oxi-dation of organic matter and weathering of base-ment and marine rocks under oxidizing conditionsalso could not be ruled out. On analyzing thelong term trends of mantle and continental con-tributions of Sr isotope, Godderis and Francois(1995) recorded sudden spikes and attributed itdue to CO2 degassing. This is in resonance with thehigher greenhouse effect during end-Cretaceous,more particularly suits with our interpretationof destabilization of gas hydrate and release ofmethane gas into atmosphere due to sea levelfall and oxidation of organic carbon stored in theformer submarine regions.

Thus, the double peaked strontium isotopicexcursions could be interpreted as a result ofthe combination of various factors such as sealevel fall due to global scale climatic change,exposure of former marine regimes to subaerialconditions and diagenetic release together withincrease of siliciclastics influx to the new depocen-ters and the episodic nature of influx of siliciclasticsinflux. Important observation in this inference is allthese changes have occurred prior to Cretaceous–Tertiary boundary as validated by the extensivereview presented in the introductory section ofthe paper as well as the stratigraphic location

of the observed anomalies (figure 3). Implicationsof these observations are discussed in the followingsection.

5.4 Nature of change of environments duringLate Maastrichtian–Early Danian

Having established the links between sea levelretreats, enhanced continental weathering, clasticinput and positive excursions of 87Sr/86Sr, implica-tions of the inferences on the probable causes thatmight have produced higher biotic turnover acrossKTB are examined.

The positive anomalies are short-lived andrecord a spurt of 87Sr/86Sr from background values.These can be linked with major retreat of sealevel, which in turn indicate concomitant cool-ing of global atmosphere. Among global climaticreversals, cooling trends tend to be gradual whilewarming trends are abrupt (Crowley and North1988; Koch et al 1992) as evident from the pro-files of carbon and oxygen isotopes (figure 4) ofthe study area also. Paleo data imply a significantwarming from the Aptian to Albian, a period ofextreme warmth extending from the Albian to theSantonian, followed by a general cooling trend dur-ing the Campanian to the Maastrichtian (Veizeret al 1999; Wallmann 2001). The cooling during theLate Cretaceous was followed by a warming duringthe Early Tertiary with a sudden increase in tem-perature at the K–T boundary (Wolfe 1990), whichin turn is in conformity with the excursions of sta-ble isotopic data (figure 4), nevertheless with lesssignificant shifts.

The record of two positive 87Sr/86Sr anom-alies preceding K–T boundary, thus supports theview of the authors reviewed earlier that K–Tboundary event could not have been caused by asudden and single event, rather by many eventsand environmental factors. The marine environ-mental niches might have been under stress, whichin turn were aggravated by meteoritic impact (inand around Chicxulub in the Yucantan Peninsula)and the Deccan volcanism (on a global scale –sensu Glasby and Kunzendorf 1996; Martin andMacdougall 1991) and Godavari volcanism (at leastin a regional scale). While studying the REE ofclay minerals of Lameta Formation of Jabalpurand Deccan basalts, Salil et al (1997) opined thatthe REE patterns of them seem to be identicaland interpreted Deccan basalt weathered productsto be the provenance of those Maastrichtian clayminerals of Lamata Formation, thus suggestingprevalence of significant Deccan volcanism wellbefore KTB. As it may involve tens of thou-sands of years for emplaced basalt to weather andform clay layer to get transported to Late Maas-trichtian deposits, the volcanic event might be


predating far below KTB. In a sense, the occur-rences of Sr isotopic anomalies far below KTBcould, thus have been influenced either directlyby volcanism or by influx of weathered productsof volcanic ash. Occurrence of Caranosaur bonefragments followed by clayey sandstones (figure 3),presumably derived from weathering of volcanicglass in rocks above the second strontium anomalysignifies the influence of Deccan volcanism in thedeposition of Cauvery basin during terminal Cre-taceous period. Ramkumar et al (2005) who havestudied the Barium anomaly in the Maastrichtian–Danian strata of the Cauvery basin inferred dete-rioration of environment during end Cretaceous,besides suggesting yet another cause, viz., emis-sion of methane gas into the atmosphere, preced-ing K–T boundary. They have advocated that thesea level retreat after prolonged organic matteraccumulation (deposition of Kallankurichchi For-mation) might have introduced destabilization ofgas hydrates, which in turn might have producedBa anomaly. Occurrence of first strontium isotopicanomaly coeval with Ba anomaly interpreted tohave resulted due to sea level retreat and increaseddetrital influx are in tune with the published inter-pretations. Rapid variations of climate, sea level,clay mineral assemblages in sedimentary record ofLate Maastrichtian are all observed in the Cauverybasin similar to many other sedimentary basins andthese changes could have contributed towards step-wise extinction of species prior to KTB.

6. Conclusions

This paper has recorded the occurrence of 87Sr/86Srin Cretaceous–Tertiary deposits of the Cauverybasin preceding K–T boundary. The inferencesdrawn based on relative sea level and elementalconcentrations support the view that the 87Sr/86Srcomposition of the Cretaceous–Tertiary seawaterdepended on the quantum of continental influx.Record of positive 87Sr/86Sr anomalies before K–Tboundary is in conformity with the views of Kelleret al (1998) and Glasby and Kunzendorf (1996)that much of the environmental deterioration com-menced before the K–T boundary that led tothe culmination of higher faunal turnover acrossthe boundary. These authors stated that, one of thefactors that influenced higher faunal turnover couldhave been sea level change as a result of global cli-mate change, which in turn might have been influ-enced by Deccan volcanism. Present study broadlyfollows their inferences.

Acknowledgements

Research on the study area was commencedthrough the financial assistance of Alexander von

Humboldt Foundation, Germany and Council ofScientific and Industrial Research, New Delhi,India and is currently supported by research grantfrom University Grants Commission, New Delhi.Permission to collect samples was accorded bythe mines managers and geologists of Ms. DalmiaCements Pvt. Ltd., TANCEM Mines, RamcoCements, Nataraj Ceramics Ltd., Vijay Cements,Fixit Mines, Parveen Mines and Minerals Ltd.,Alagappa Cements, Rasi Cements, Tan-IndiaMines, TAMIN Mines and Chettiyar Mines.

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MS received 8 January 2009; revised 17 June 2009; accepted 6 December 2009

Documents

87Sr/86Sr anomalies in Late Cretaceous-Early Tertiary strata of the Cauvery basin, south India: Constraints on nature and rate of environmental changes across K–T boundary