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Gondwana Research xxx (2010) xxx–xxx

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Barremian–Danian chemostratigraphic sequences of the Cauvery Basin, India:Implications on scales of stratigraphic correlation

Mu. Ramkumar a,⁎, D. Stüben b, Z. Berner b

a Department of Geology, Periyar University, Salem, 636011, Indiab Institüt der Mineralogie und Geochemie, Universität Karlsruhe, D-76128 Karlsruhe, Germany

⁎ Corresponding author. Present address: GeoZentruErlangen, Fachgruppe Paläoumwelt, Loewenichstrasse 28

E-mail address: [email protected] (Mu. Ram

1342-937X/$ – see front matter © 2010 International Adoi:10.1016/j.gr.2010.05.014

Please cite this article as: Ramkumar, Mu.,on scales of stratigraphic correlation, Gond

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Article history:Received 11 May 2009Received in revised form 26 May 2010Accepted 26 May 2010Available online xxxx

Keywords:ChemostratigraphyBarremian–DanianCauvery BasinIndia

Chemostratigraphic concepts postulate that distinct chemozones could be recognized from rock records tohelp correlate geographically separated strata. Many studies have upheld this view, but were limited toclassic boundaries such as Cenomanian–Turonian or Cretaceous–Tertiary. Only a few elements and isotopiccompositions were made use in such studies. An attempt to recognize chemozones of Barremian–Danianstrata of the Cauvery Basin, South India through whole-rock chemistry had revealed many complex signalsand depositional events as expressed by bulk geochemical profiles. The signals indicate major shift insedimentation pattern from dynamic depocenter to stable depositional conditions during Santonian. Varyingsensitivities of different elements towards prevalent depositional conditions are also inferred. Trends ofgeochemical composition indicated domination of single depositional control all through the depositionalhistory, basinal configuration that limited terrestrial sediment source and episodic influx of detritalmaterials. Presence of distinct change in whole-rock geochemistry across geochronological and lithostrati-graphic boundaries is deciphered through a statistical tool ANOVA. These results demonstrate that visualdelineation of chemozones in a long ranging rock record would be tenuous and has to be exercised withcaution.

© 2010 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.

1. Introduction

During the last two decades, chemostratigraphy has emerged to be apotential tool for correlation of widely separated strata where otherconventional stratigraphic methods fail or have limitations (Ramkumarand Sathish, 2006). Many recent studies have addressed the chemos-tratigraphic application (e.g., Sawaki et al., 2008; Ishikawaet al., 2008; LeGuerroue and Cozzi, 2010) and its potential as a vital tool forstratigraphic correlation, fixation of geological boundaries and petro-leum exploration. These studies attempted solving selective boundaryquestions (for example: Saltzman, 2002; Schroeder et al., 2004;Bergstoerm et al., 2006; Kouchinsky et al., 2007; Marquillas et al.,2007; Schroeder andGrotzinger, 2007; Handley et al., 2008; Robinson etal., 2009; Elrick et al., 2009; Ruhl et al., 2009). Many of the publicationshave either documented geochemical and or isotopic compositionaltrends across a chronological boundary or geochemical trends of limitedchronological span (for example: Brasier and Shields, 2000; Mutti andBernoulli, 2003; Saylor et al., 2005; Mutti et al., 2006; Nedelec et al.,2007; Kakizaki and Kano, 2009) thus concentrating on presence/

m Nordbayern der Universität, D-91054 Erlangen, Germany.kumar).

ssociation for Gondwana Research.

et al., Barremian–Danian chewana Res. (2010), doi:10.10

absence of similarity in geochemical profiles for a limited geologictime. Only selective elemental or isotopic data were employed in thesestudies.

As stratigraphic record is the outcome of an exogenic systemconsisting of geologic setting, changes in sea level, changes ingeochemical reactions between the sea and earth, climate and processesof sediment formation (Srinivasan, 1989), the ensuing sedimentaryrecord should show differences in bulk chemistry. As these changesproduce different combinations of minerals, primary differences in thechemistry of their constituent minerals or in the proportions ofaccessory phases such as heavy minerals and clays, there might bedistinctive major and trace elemental compositions of the sediments aswell which in turn forms the basis of chemostratigraphic application insub-dividing strata (Das, 1997). If this underlying concept of chemos-tratigraphyholds good, an ability to judge these differences, distinctnessof chemical composition and its causative factor (local, regional orglobal), should enable recognize and correlate chemozones at appro-priate scale (Ramkumar, 1999; Ramkumar and Sathish, 2006). Thus,applicability of chemostratigraphic technique as a potential tool forstratigraphic correlation could be tested only when its perceivedcapabilities are examined critically through analysis of whole-rockgeochemical trends of strata representing considerable time span andalso through demonstrating the ability of whole-rock chemistry todistinguish depositional units produced under varying conditions ofsedimentation.

Published by Elsevier B.V. All rights reserved.

mostratigraphic sequences of the Cauvery Basin, India: Implications16/j.gr.2010.05.014

http://dx.doi.org/10.1016/j.gr.2010.05.014

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Fig. 1. Sedimentary basins of India and location of the Cauvery Basin.

2 Mu. Ramkumar et al. / Gondwana Research xxx (2010) xxx–xxx

The exposed part of the Cauvery Basin, south India (Fig. 1) containsa more or less complete sedimentary record of the Upper Cretaceous–Lower Tertiary periods (Yadagiri and Govindan, 2000). The basincomprises many commercially exploitable oil reserves accumulatedunder “stratigraphic traps”, and warrants a high-resolution strati-graphic model (Raju and Misra, 1996). On the other hand, presence ofbarren rock sequences, patchy occurrence of fossils and occurrence ofexotic blocks (older rocks in younger sequences— Ramkumar, 2008a)have thwarted successful stratigraphic correlation and explorationthrough conventional methods in this basin (Ramkumar et al., 2004a).

To meet these two requirements namely, testing the potential ofchemostratigraphic modeling in long ranging strata and the need forhigh-resolution stratigraphic setup, we have examined geochemicalproperties of the Cauvery Basin, South India (Fig. 1).

2. Geology of the area

The Cauvery Basin formed as a result of fragmentation of Gondwana-land during Lower Cretaceous. It continued evolving till the end ofTertiary through rift, pull-apart, shelf sag and tilt phases (Prabhakar andZutshi, 1993), duringwhichmany episodes of transgression, regression,erosion and deposition took place to fill the basin. Comprehensivelithostratigraphyof onland part this basinwas presented by Tewari et al.(1996) and revised (Table 1; Fig. 2) by Ramkumar et al. (2004a). Fig. 3

Please cite this article as: Ramkumar, Mu., et al., Barremian–Danian cheon scales of stratigraphic correlation, Gondwana Res. (2010), doi:10.10

depicts the lateral and vertical variation of these members in theexposed area as observed in ten traverses shown in Fig. 2.

2.1. Facies characteristics of the Barremian–Danian strata

Sedimentation in the Cauvery Basin commenced with fluviatileand coastal marine deposition during Barremian. Deposition contin-ued until Danian. The depositional environments varied fromfluviatile, lacustrine, coastal marine to deep marine and continentalregimes during this period. These environmental changes haveimpacted the lithofacies association and succession. These are brieflypresented in the Table 2 (Figs. 4, 5, and 6).

2.2. Depositional history and sea level changes

The depositional history in terms of depositional units, depositionalbreaks andmajor geological events is presented in the Table 3. This tableshows that depositional history of this basin was controlled predom-inantly by sea level changes. Raju and Ravindran (1990) and Raju et al.(1993) presented sea level curve for this basin based on foraminiferaldata that documented the presence of six 3rd order cycles caused byglacio-eustatic fluctuations. Ramkumar et al. (2004a) constructed sealevel curve for this basin based on lithofacies data,which is similar to thecurves presented by Raju et al. (1993) except that it additionally



Table 1Lithostratigraphy of the Cauvery Basin (after Ramkumar et al., 2004a).

3Mu. Ramkumar et al. / Gondwana Research xxx (2010) xxx–xxx

recorded fourth and higher order sea level cycles (Fig. 7) of glacio-eustatic origin. The global sea level peaks during 104 MY (Early–LateAlbian), 93.7 MY (±0.9; Middle to Late Cenomanian), 92.5 MY (±1;Early to Middle Turonian), 86.9 MY (±0.5; Early to Late Coniacian),85.5 MY (±1; Early to Late Santonian), 73 MY (±1; Late Campanian),69.4 MY (Early to LateMaastrichtian) and 63 MY (±0.5; Early toMiddleDanian)were observed to occur in this basin (Raju andRavindran, 1990;Raju et al., 1993; Ramkumar et al., 2004a). The 3rd order cycles areseparated by type I sequence boundaries (recognized through shift ofshoreline crossing shelf break as explicit in lithologic information,contact relationship between strata; evidences of subaerial exposureand erosion, advancement of fluvial channels over former offshoreregions, etc.), confirming the interpretation of sea level controlleddepositional pattern in this basin (Ramkumar et al., 2004a). Sedimen-tation in this basin took place in an epicontinental sea (Ramkumar et al.,2004a). Although sea level variations ranged from supratidal to basinallevels, shallow nature of the basin was maintained all through thehistory of the basin.

3. Materials and methods

Systematic field mapping in the scale of 1:50,000 was conductedthrough ten traverses (Fig. 2) in which 308 locations were logged andsampled. Construction of lateral and vertical facies variations asrecorded in these ten traverses allowed compiling a compositestratigraphic profile of Barremian–Danian strata that allowed selectionof 157 rock samples for analyzing trace elemental composition.Published information of these rocks in terms of facies and petrography(Ramkumar et al., 2004a), whole-rock mineralogy and clay mineralogy


and statistical analyses of whole-rock geochemistry (Ramkumar et al.,2006) suggested little or insignificant diagenetic alteration of the strataunder study. Ramkumar et al. (2006) also inferred predomination ofdepositional geochemical signatures rather than diagenetic imprints.From these 157 samples, 70 sampleswere further selected and analyzedfor major elemental composition. Trace and major elemental analyseswere performedwithXRF following the procedures discussed inKramar(1997) and Stüben et al. (2002). Analyses of 157 samples forpetrographic and bulk mineralogical data and 70 samples for claymineralogical compositions were also performed. This paper discussesonly the geochemical data, while the mineralogical and petrographicobservations are utilized only in a supportive role. Keeping in line withthe scope of this paper, listing out important depositional events,stratigraphic breaks, etc., and their influence on geochemistry areexamined. Initially, the geochemical profiles of absolute concentrationswere examined for recognizing distinct zones/cycles, covariance oftrends of profileswith reference to depositional units, presence/absenceof shift of trends across strata in/along which major geological eventsandprocesses thatmade imprints. Itwas followedbyexaminationof theprofiles towards presence of secular and or cyclic trends, determinationof their range and determination of their relationship with depositional,tectonic and sea level variations.

4. Trends of absolute geochemistry in response todepositional conditions

Broad scale alternations of siliciclastics and carbonates in thegeological record as a result of change in depositional processes andsea levels (Warzeski et al., 1996) is inferred from comparing the



Fig. 2. Geology of the study area (after Ramkumar et al., 2004a). Ten traverselines along which 308 locations were logged (shown with Roman numbers). From these 308 locations, acomposite stratigraphic column of Barremian–Danian was constructed and 157 representative samples were selected for geochemical analyses.


profiles of Si (Fig. 8a) and Al (Fig. 8b) with Ca (Fig. 8c). This relationshipindicates that as could be observed elsewhere, whenever siliciclasticdeposition ceased, carbonate deposition was initiated immediately inthe basin. Such occurrence of coeval episodes of Si, Ti, Al and K mayindicate transport of terrestrial quartz, feldspars and clay together(Pearce et al., 1999). The profiles of Zr (Fig. 8d), Rb (Fig. 9a) and Y(Fig. 9b), follow closely the pattern of Si. These elementswere primarilyassociated with detrital influx in terms of quartz, feldspars, clays andlithoclasts drawn from granitic gneiss, charnockite and pegmatitesource rocks that have accessory minerals composed of these elements(Singh and Rajamani, 2001). Guha and Mukhopadhyay (1996) alsoobserved sympathetic relationship of K with acid insolubles (non-carbonate clastic residues) in the rocks of this basin.

Prolific accumulation of Mn (Fig. 9c) during Albian may reflectanoxic/anaerobic environment of deposition as it is associatedwith Corgrich grey shale deposits that have no bioturbation (Peryt andWyrwicka,1993). Enrichment of Mn observed during latter part of Cenomanian–Turonian could have been due to depositional (as detrital influx—GuhaandMukhopadhyay, 1996) aswell as diagenetic imprints. Iron (Fig. 9d),like Mn, is closely associated with grey shale deposition indicative ofanoxic/anaerobic environment of deposition during Albian, followed bya broad peak during Cenomanian–Santonian as a result of highercontinental sediment influx (Brand, 1989) and diagenetic incorporationinto the Maastrichtian carbonates (as a result of diagenesis during late


stage in meteoric phreatic zone as evidenced by the presence of ferroancalcitic cements — Fig. 6b and c — Ramkumar, 2008b).

Profile of Ba (Fig. 10a) shows episodic detrital influx to thedepocenter. From the X-ray diffractograms, presence of Ba-Orthoclasehas been recognized and may be the reason for association of Ba withdetrital influxand siliciclastic deposits. AlthoughBa could associate itselfwith carbonates, its primary role as detrital influx (Guha andMukhopadhyay, 1996) has been indicated by the coincidences of Bapeaks with lithoclastic conglomerates, wherein large boulder-cobblesized feldspars are present (Fig. 5c). Ba adheres onto clay mineralsurfaces (Singh, 1978) in which case, influx of clay during detrital influxcould also be inferred. In either case, clastic source for Ba is indicated.Computation of excess Ba content in the sediments according to theformulae of Murray and Leinen (1996) with reference to PAAS (PostArchaen Average Shale) has not indicated overwhelming availability ofBa in these rocks except during late Maastrichtian to which Ramkumaret al. (2004b, 2005) have ascribed destabilization of gas hydrates due tosea level fall and resultant Ba enrichments in sediments. Observation ofinflux of unaltered feldspars together with other silicate minerals suchas quartz gain credence with the profile of Na (Fig. 10b), which showsnearly the same trend as Si. Influx of unaltered lithoclasts together withfeldspar clasts (Fig. 5c) suggests sudden major erosional eventsgenerated by tectonic movements. Petrographic and geochemicalfeatures suggest short transport distance for the eroded materials and



Fig. 3. Lateral and vertical distribution of stratigraphic members as measured along the ten traverselines shown in Fig. 2. The Arabic numbers given against each box in the legendindicate lithostratigraphic members as drawn and referred in the legend of Fig. 2.

Table 2Facies characteristics of the Barremian–Danian strata.

Formation Member Facies characteristics

Niniyur Periyakurichchi

Thick arenaceous bioclastic limestone bed followed by medium to thick, parallel, even bedded recurrent biostromal limestone and marltypify this member. Regionally varying concentrations of shell fragments and whole shells of bivalve, gastropod and remains of amphibia,pisces, algae, foraminifera and ostracoda are also observed.

Anandavadi Isolated coral mounds, impure arenaceous limestone and lenses of sandstone and clay, deposited in a restricted marine regime undersubtidal to intertidal regions. Occurrence of localized concentrations of shell fragments, coralline limestone and reef derived talus depositsare characteristics of this member. This member rests over the Kallamedu Formation with distinct disconformity. At top, an erosionalsurface is recognizable.

Kallamedu Unconsolidated, well rounded and poorly sorted barren sands with rare-scarce dinosaurian bone fragments (Fig. 6a). Towards top, thesegrade to medium to thin bedded, relatively highly argillaceous sandstones. Local occurrence of clays and silts with dispersed detritalquartz grains and sandy streaks, non-bedded nature, rare lamination and mud cracks in them indicate sedimentation as over bankdeposits. Towards top, this formation shows development of soil and return of continental conditions.

Ottakoil The rocks are coarse to medium sized, well-sorted, fossiliferous, low angle cross-bedded and planar to massive bedded sandstones withregionally varying sparse calcareous cement. They also show recurrent fining upward sequences. Abundant Stigmatophygus elatus(Fig. 5h) and few trace fossils indicative of shallow marine environment of deposition. This formation rests over KallankurichchiFormation with disconformity and overlapped by Kallamedu Formation.

Kallankurichchi Srinivasapuram

Uniform, parallel, thick–very thick bedded gryphean shell banks (Fig. 5f) with Terebratula, Exogyra, bryozoa and sponge. Extensive boringin gryphean shells, synsedimentary cementation, colonies of encrusting bryozoa over gryphean shells and micritization of bioclasts(Fig. 5g) deposition of this member in inner shelf.

Tancem Biostromal limestone beds (Fig. 5d) with thin to thick, parallel, even bedded nature, cross bedding, normal grading, hummocky crossstratification, feeding traces, escape structures and tidal channel structures. Local concentrations of various fossils, sporadic admixture ofsiliciclastics and intraclasts are observed.

Kattupiringiyam

Dusty brown friable carbonate sands with parallel, even and thick to very thick bedding that contains only Inoceramus and bryozoa. Thismember has diagenetic bedding and abundant geopetal structures filled with mm to cm sized dog tooth spars of low magnesian non-ferroan calcite (Fig. 5e). This member has non-depositional surface at bottom and has erosional surface at top.

Kallar Normal graded conglomerates in which well rounded clasts of basement rocks, fresh feldspar (Fig. 5c), resedimented colonies of serpulids

(continued on next page)


Please cite this article as: Ramkumar, Mu., et al., Barremian–Danian chemostratigraphic sequences of the Cauvery Basin, India: Implicationson scales of stratigraphic correlation, Gondwana Res. (2010), doi:10.1016/j.gr.2010.05.014


Table 2 (continued)

Formation Member Facies characteristics

and other older sedimentary rocks that range in size from coarse sand to boulder are observed. Lower contact of this member is anerosional surface. The upper contact is non-depositional surface resulted from marine flooding.

Sillakkudi Varanavasi Featureless, massive, thick to very thick bedded, coarse to medium grained sandstones. Occasional very coarse sandstone lenses, pocketsof shell hash, intraformational lithoclastic bounders in association with vertical cylindrical burrows and resedimented petrified wood logsare observed. The rocks rest over the pebbly sandstone member with non-depositional surface Upper surface of this member representserosional surface associated with regression.

Sadurbagam Coarse siliciclastics with abundant marine fauna, shell fragments and varying proportions of calcareous matrix At the base, an erosionalsurface followed by distinct cobble-pebble quartzite conglomerate is observed. The rocks show normal grading, low angle cross bedding,massive, thick tomedium, even and parallel bedding. At places, pockets of shell rich carbonate lenses with abundant siliciclastic admixtureare found to occur. Load casts, slump folds, pillow structures and synerasis cracks, occasional development of algal mounds are also found.

Varakuppai It rests over older sedimentary rocks with typical erosional surface. The erosional intensity was such high that, the beds have directcontact with much older Karai Formation. Fluviatile sandstones with well rounded basement rocks, quartzite and older sedimentary rockboulders (Fig. 5a) in addition to unsorted coarse sand-pebble sized siliciclastics constitute thismember. These are typically reverse gradedand show cyclic bedding, large scale cross bedding and lack any body fossils. Large scale cross bedding, mud drapes, fresh feldspar andsandstone clasts are also recorded. Towards top, thalassinoid burrows (Fig. 5b) are reported, that indicate gradual submergence of thedepocentre by rising sealevel.

Garudamangalam Anaipadi Massive and thin bedded claystones, silty claystones and clayey sandstones in south that gradually grade to silty clay in south centre andthin down. Again, from there, thickness of these beds and sediment grain size increase and contain abundant large ammonites. Furthernorth, these were observed to be clayey siltstones with abundant shell fragments and ammonites.

Greysandstone

Highly well cemented, sorted and rounded grains giving massive appearance. The beds are cyclic, parallel, even bedded alternative layersof barren and highly fossiliferous and sandy layers (Fig. 4f) with regionally varying thicknesses. This member rests conformably over theKulakkanattam member and has distinct erosional and non-depositional surface. Upper contact is non-depositional surface associatedwith marine flooding.

Kulakkanattam

Massive, yellowish brown ferruginous sandstones with abundant admixture of silt and clay. Localized concentrations of shell fragments,bivalves and gastropods and ammonites are common. It also contains abundant wood fragments (Fig. 4e) with extensive oyster boring.Cross bedding, channel courses, planar bedding and feeding traces are also common. The depositional surface was strongly bioturbatedand riddled by roots. An angular erosional unconformity separates this member from underlying Karai formation.

Karai Odiyam sandyclay

Siltyclays and sandy clays with abundant ammonites. Load structures and syndepositional slump folds are frequently observed. Upperportion of this member has localized pockets of fine sandstone along with ammonites. While the lower contact is conformable withunderlying member, upper contact is erosional.

Gypsiferousclay

Unconsolidated deep marine clays and silty clays. These beds contain thick population of belemnite rostrum and phosphate nodules.While a non-depositional unconformity surface separates this member from the underlying member, upper contact is non-depositionaland erosional. From south to north, gradual reduction of thickness, population of belemnite and phosphate nodules and frequency ofgypsum layers are observed.

Dalmiapuram Kallakkudi Fine-coarse sandstones with alternate medium to thick beds of silty clay, calcareous siltstones, bioclastic arenaceous limestone andgypsiferous clay. In the southern region, these beds show recurrent bands of fining upward sequences of siliciclastics with calcareouscements. The intercalations are recurrent and show typical Bouma sequences, normal grading, load casts and channel and scourstructures. Towards northern regions, this member grades to more silty and clayey, but gradation and gypsiferous bands are persistentwith an addition of ferruginous silty clay bands.

Olaipadi Basinal silty clays and clays in which chaotic blocks are embedded. The beds contain large blocks (Fig. 4a–d) of angular and subroundedbasement rocks, coralline limestone blocks, claystones and lithoclasts of older conglomerates, etc. Towards the top, deep marine claysgrade into calcareous siltstone and include granitic cobbles and minor amounts of siliciclastic sands.

Dalmia Pure algal and coral facies limestone beds that form reef core. Upper contact of this member is a forced regression surface.Varagupadi Limestone beds typical of reef flank biostromal beds deposited under high-energy conditions. Thin to thick bedded, even to parallel,

bioclastic limestone beds that have drawn their detritus from reefs predominate. These beds are found to be directly overlying the Greyshale member. The rocks show wackstone to rudstone fabric and have clasts of redeposited boundstones.

Grey shale Grey shale beds with frequent thickening upward interbeds of fossiliferous grey limestone and minor to significant admixture of silt sizedsiliciclastics. Lower contact of this member is an unconformity surface associated with marine flooding and the upper contact is non-depositional and erosional.

Sivaganga Terani clay White to brownish colored clay and argillaceous siltstone that show transition from Kovandankurichchi member. Beds aremassive to verythick in nature. Lower contact of this member is non-depositional surface.

Kovandankurichchi

Grain supported coarsening upward cyclic beds (20–100 cm thick each) of very coarse sandstones that show parallel, even and thin tothick bedding. Grains are well-sorted within each lamina and show rounded–well rounded shape. These represent recurrent sheet flowdeposits probably in a sub-aqueous fan deltaic environment.

Basalconglomerate

Recurrent fining upward sequences of lithoclastic conglomerates of fluviatile and coastal marine environments. Lithoclasts are of gneissicbasement rocks. Rests over basement rocks with distinct erosional surface. Upper contact is a non-depositional surface.


higher rate of sedimentation. These conditions might have buried thesediments and due to which prevented alteration of these sedimentsunder marine conditions. Recurrence of such events episodically, butwith reduced intensity overpassageof timemaybe interpretedbasedonthe prevalence of steep sloped fault blocks along basin margin duringinitial times, close proximity of source and depositional areas, limitedextent of source area as well as lack of major drainage to promotechemical weathering (Ramkumar et al., 2009). Presence of lowerconcentrations of Na during periods of carbonate deposition indicateseither insignificant detrital input or leaching of carbonates duringwhichspecific expulsion of Na took place or precipitation of low magnesiancalcite (LMC) that in turn allowed only little quantity of Na to getincorporated. However, the element Sr that tend to get expelled duringdiagenesis of carbonates (Guha and Mukhopadhyay, 1996; Delaney etal., 1996; Veizer et al., 1997; Calver, 2000), do not show any such trend(Fig. 10c) in the carbonate deposits of this basin. Had there been


diagenetic expulsion of Sr and Na, it must have been under meteoricphreatic zone of diagenesis, in which case, the elements such as Fe andMn should show inverse trends wherein the elements Na and Sr showproliferation (Kumar et al., 2002; Veizer et al., 1999). Absence of suchtrends on the profiles of those elements indicates the concentration ofNa as related only to insignificant detrital influx.

Mg shows multiple episodes of varying time spans among whichCenomanian–Coniacian episode is prominent (Fig. 10d). While thepositive excursions of Mg associated with siliciclastic deposition arecorroborated with the presence of feldspars and clay as revealed in XRDpatterns, the periods of prolific carbonate deposition shows less thanaverage values of Mg concentration meaning that the bioproducers (asbulk of the limestone deposits are shell banks and reefs) have precipitatedlow Mg calcites (Al Aasm and Veizer, 1982). It implies prevalence ofcomparatively cooler bottom waters and slow precipitation and or lowrate of precipitation of shell calcite (Bathurst, 1975; Schifano and Censi,



Fig. 4. Field photographs showing the facies characteristics of Olaipadi, Kulakanattam and Anaipadi members. a. Basal pebbly s.st-claystone facies exposed in a mine section locatedin Tirupattur village. Arrows indicate individual bouma sequences. Photograph measures 4.5 m×2.5 m. b. A large coralline limestone boulder embedded in fine grained cyclicdeposits. Note the conformable bedding that encircles the clast. c. Basement rock boulders (a), coralline limestone boulders (b) embedded in fine grained cyclic deposits. d. Close-upview of typical algal bindstone with fenestral porosities indicated by arrows. Note that this particular lithology has not been recorded anywhere in the older deposits in and aroundthe study area. One rupee coin indicated by bold arrow is placed for scale. This boulder is found embedded in fine grained deposits. e. Close-up view of theWoody sandstone. Note thepresence of petrified wood fragments (arrows) with differential orientation and size. Location of the photograph: Near Garudamangalam village. f. Alternate barren and shell richthin beds of Grey sandstone member. Location of the photograph: Southwest of Anaipadi Village.


1986) or post depositional leaching of Mg (Al Aasm and Veizer, 1982;Mucci, 1987). Absence of ghost structures and stylolitic seams except inregions wherein faulting and folding occurred, occasional and sporadicoccurrence of dogtooth spars (equant spars of diagenetic origin— Fig. 6c),preservation of prismatic and fibrous shell structures in mollusca andbrachiopod shells without much alteration and extensive occurrence ofunfilled voids of shell cavities and vugs (Fig. 6d)in limestone deposits areall indicativeof absenceof large scalediagenetic alterationgivingcredenceto the interpretation of deposition of limestones under highstands, thatmight have supported cooler bottom waters and thus promotedprecipitation of primary low magnesian calcite, thus explaining lowerMg during periods of carbonate deposition as primary signal.


As the carbonates of this basin are essentially shell banks and reefs,the total carbon (Fig. 11a), inorganic carbon (Fig. 11b) and organiccarbon (Fig. 11c) are all associated with periods of prolific carbonatedeposition and absent during the period of prolonged exposure (forexample, Santonian). This observation, together with the occurrence ofreduced concentrations of organic and inorganic carbon during prolificsiliciclastic deposition suggests proliferation of carbonates duringperiods of higher sea levels in this basin. It indirectly indicates partiallystratified waters and is supported by the occurrence of low magnesiancalcite and higher availability of nutrients associated with sea level rise(McKirdy et al., 2001). The nature of carbonates and organic carbon toget accumulated and preserved during periods of sea level highstand




Please cite this article as: Ramkumar, Mu., et al., Barremian–Danian chemostratigraphic sequences of the Cauvery Basin, India: Implicationson scales of stratigraphic correlation, Gondwana Res. (2010), doi:10.1016/j.gr.2010.05.014


Fig. 6. Facies characteristics of Kallamedu Formation and carbonate rocks of the study area. a. Kallamedu Formation contains fragments of dinosaurian bone remains strewnrandomly in the rocks. b. Photomicrograph depicting fracture porosity in which late stage meteoric phereatic zone cements could be observed. The blue colored ferroan calciticcement spars indicate diagenetic incorporation of Mn and Fe into the calcite crystals. Scale bar 0.8 mm. c. Microphotograph showing a perfect intraparticle porosity manifested bybryozoan froands. This porosity is filled by mesodiagenetic marine phreatic zone cement spars that underwent neomorphism in meteoric phreatic zone. Note the gradual increase inspar size towards centre of the pore (that indicate originally cement spar nature of these crystals), zoned nature of crystals (that indicate varying zones of diagenetic cementprecipitation) and variable tones of blue coloration of spars (that indicate variable amounts of inclusion of Fe in Ca lattice positions of calcite crystal). Scale bar 0.2 mm. d. Sporadicoccurrences of vug-filling dogtooth spars as shown in this photograph indicate prevalence of marine conditions of early stage diagenesis and predominance of lowmagnesian calciteas the original mineralogy of the carbonate rocks. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)


(Kumar et al., 1995) explains the observed changes. Presence of perfectsympathetic behavior of carbonate and Corg contents in the carbonatedeposits implies preservation of primary signals (Meyers and Simoneit,1989). Rangel et al. (2000) also noted increase and decrease of organiccarbonwith sea level rise and fall, respectively. The inverse relationshipsof Si and Al with Corg indicate purely marine origin for Corg rather thancontinental influx (Lyle et al., 1992). During lowstands, the fluvialsystems that existed onland might have advanced over former offshoreregions bringing in higher terrestrial influx besides higher circulationand oxygenated environment of deposition. Such conditions thwartorganic carbon accumulation/preservation in ensuing siliciclastic sedi-ments and might have been the reason for reduced/non-accumulationof Corg during periods of siliciclastic deposition.

Fig. 5. Field and photomicrographs of rocks of the Sillakkudi, Kallankurichchi and Ottakoil fophotograph: West of Varakuppai village in a stream section. b. Thalassinoid burrow in uppevillage in a stream section. c. Fresh Ba-orthoclase clast embedded in the conglomerate bedsfragmental shell limestone bed (b) with a sharp erosional surface in between (pen placed forlived periodic high-energy conditions prevalent during the deposition of Tancem membeKallankurichchi village. e. Photomicrograph showing the non-ferreon calcitic spars filled inwhich thin layer of blade shaped non-ferreon calcite, followed by equant shaped dogtoothCyanide and gives shades of pink color due to its lowmagnesian and non-ferreon nature. ScalSrinivasapuram member of Kallankurichchi Formation. Location of the photograph: TANCEMand micritic wall development. Note the organic matter rich micrite filled in the mocroboremicritic coating towards centre of the bioclast. Scale bar 0.5 mm. h. Sandstone with Stigma


Phosphorus shows distinct peaks during Albian, Cenomanian andMaastrichtian and can be directly related to individual eustatic andtectonic events (Hoppie and Garrison, 2001). Several studies haveassumed that P weathering and input to the oceans are similar to theweathering and input of strontium (Raymo, 1994). Occurrence of peaksimmediately succeeding sea level changes in this basin affirm the view ofFilippelli and Delaney (1994) that fluctuations in sea level and resultantchanges in shelf area alter oceanic Pmass balance, primarily induced by Predistribution within a sedimentary basin. Rangel et al. (2000) docu-mented peaks of P during sea level changes inMagdalena Basin. Record ofrestricted water circulation, upwelling, higher primary productivity aswell asorganic carbonaccumulation andpreservation (Rangel et al., 2000;McKirdy et al., 2001)may explain the P peak in the Albian deposits of this

rmations. a. Subrounded lithoclastic boulder in the Varakuppai member. Location of ther portion of the Varakuppai member. Location of the photograph: West of Varakuppaiof Kallar member, Kallankurichchi Formation. d. Carbonate sand beds (a) overlain byscale rests on this surface, indicated by an arrow). These are the products of short-longr of Kallankurichchi Formation. Location of the photograph: TANCEM mines, west ofa geopetal structure. The lower most part of the photograph shows the shell wall overspars are found. The thin section was stained with Alizerine Red S and Potassium Ferrie bar is 0.5 mm. f. Thick population of insitu gryphean shells. These constitute the bulk ofmines south of Srinivasapuram village. g. Photomicrograph showing the microboring

s, irregular thickness of the micritic coating around bioclast and gradual movement oftophygus elatus characterize the Ottakoil Formation.



Table 3Depositional units, breaks and geological events in the Cauvery Basin.


basin. Occurrence of strong upwelling during sea level highstand is acommon phenomenon that induces increased phosphate accumulation(Warzeski et al., 1996; Hoppie and Garrison, 2001). Enrichment of Pduring Albian occurred within the oxygen minimum zone in an area ofpervasive coastal upwelling and reduced terrigenous influx (Hoppie andGarrison, 2001). Prevalence of oxygen minimum zone was indicated bythe absence of bioturbation (Spalletti et al., 2001) in the Albian deposits.

Fig. 7. Relative sea level curve of Barremian–Danian strata


The other twopeaksmight have been resulted by sediment redistribution(Compton et al., 1993; Ruttenberg, 1993) associated with sea level fall, afeature also recorded by Rangel et al. (2000) in Magdalena Basin.Ramkumar et al. (2009) discussed the trends of P in these strata understudy. According to them, the rocks where these two peaks are observedcontain cross bedding, graded bedding, abundant abraded faunal remainsand trace fossils, all indicative of storm events (Fig. 5d) and deposition in

of the Cauvery Basin (After Ramkumar et al., 2004a).



Fig. 8. Profiles of Si, Al, Ca and Zr across Barremian–Danian strata of the Cauvery Basin.


oxygenated environment (Ramkumar, 1996; Spalletti et al., 2001).Sulphur shows three distinct peaks (Fig. 11d), associated with shale(lowermostpeakcoeval to thedepositionofGreyshalemember indicativeof its association with higher organic matter preservation), gypsum(Varagupadi member-syngenetic gypsum) and resedimented olderlithoclasts during Santonian.

5. Systematic trends in geochemical profiles

Veizer et al. (1997) have demonstrated that the sedimentationsystem is dominated by cyclic processes that operate on a hierarchy of


temporal and spatial scales on which short-lived events are super-imposed. As a consequence, only a net result of cycles and eventscould be recognized in rock records. Observations on geochemicalprofiles of absolute concentrations of the Cauvery Basin also typify theobservations of Veizer et al. (1997). These profiles reveal multitudesof events (as detailed in previous section) due to which, variations ofresponses of different elements produce noise that deters recognitionof distinct chemozones from the geochemical profiles. To suppressnoise and to elucidate clear patterns, the data were analyzed forsystematic trends. Two types of trend lines namely, linear (to analyzethe data for secular change in geochemical composition with age) and



Fig. 9. Profiles of Rb, Y, Mn and Fe across Barremian–Danian strata of the Cauvery Basin.


polynomial (to analyze the data for cycles of variation over time)wereutilized in the present study.

5.1. Secular trends

Although absolute concentrations of Si, Ti, Ca,Mg, P, Fe, Ctot, andCinorgshow variations, they were found to bemore or less uniform all throughthe basinal history as indicated by the linear trend lines. That thedepositional basin remained essentially a shallow epicontinental seathroughout the depositional history, which however was inundatedand exposed frequently, is evident from the linear trend line drawn oversea level curve in Fig. 7. Al and Sr show noticeable reduction inconcentration from older to younger deposits. Reduction of Al (Fig. 8b)maybe reflectiveof either reductionofweathering in continental regions


and deprivation of availability of feldspars and clays for transport to thedepocenters and/or overall lithological variations of the sedimentaryrecord (Andrew et al., 1996).While gradual deprivation of feldspars andclays is supported by the linear trend line of K (Fig. 12a) and XRDpatterns, lithofacies variation on a basin scale (wherein dominantproportion of siliciclastics in older deposits and carbonates in youngerdeposits couldbeobserved) suggest the roles of bothof these causes. Sr isspatially conservative in the sea and any changes in Sr concentration in asedimentary record could be linked to prevalent contemporaneousprocesses (Andersen and Delaney, 2000. Reduction of strontium fromolder to younger strata (Fig. 10c) could be interpreted as decreasingterrestrial input or sea level or both (Martin andMacdougall, 1991). It isto be noted in respect of relative sea level (RSL) that, although in totalitythe RSL showed pulses of waning and waxing, a general pattern of



Fig. 10. Profiles Ba, Na, Sr and Mg across Barremian–Danian strata of the Cauvery Basin.


increase from Barremian to Santonian and decrease thenceforth up toDanian could be observed, meaning sea level controlled nature for theresponses of Sr. Many workers have documented covariation of Sr withsea level (e.g. Renard, 1986; Nandy et al., 1995) and it holds true to thisbasin also. The elementsMn, Zr, Y and Ba show increase in concentrationwith decreasing age. While enrichment of few of these elementsgenerally indicates terrestrial influx, presence of carbonates towardsyounger strata and the nature of Fe and Mn to get incorporated intocarbonates could not be ruled out.

5.2. Cyclic trends

The polynomial curves generated with sixth order polynomialfunctions are presented in the Figs. 8–12. From these figures, presence


of two distinct peaks could be observed. Non-identical nature of thesetwo peaks in terms of time and space, may be as a result of differentialresponse of those elements to prevalent environmental conditionscould be observed. Comparison of these peaks with the polynomialcurve of relative sea level indicates that, the peaks/cycles of elementalconcentrations are highly cross correlated, ascertaining prevalentinfluence of relative sea level changes over depositional pattern in thisbasin. Presence of alternation of carbonate and siliciclastic lithofaciesin this basin confirms the interpretation of deposition of siliciclasticsduring sea level lowstands and carbonates during sea level high-stands. It is also evidenced from the Fig. 12b which exhibits therelationships between sea level and concentrations of Ca and Si. Thisobservation is further substantiated by comparing the Figs. 8a, c and12b with Tables 1 and 3 wherein distinct change in lithological and



Fig. 11. Profiles of Ctot, Cinorg, Corg and S across Barremian–Danian strata of the Cauvery Basin.


associated geochemical trends could be inferred. Such lithofaciesalternations created by sea level variations are considered to betriggered by global scale climatic reversals (Srinivasan, 1989). Sarg(1988) observed that, sedimentary basins starve for detrital sedi-ments during high stands that lead to development of carbonates.Studies on recent marine sediments also indicate carbonate sedimen-tation only in regions starved for terrigenous sediments. Ruffel andRawson (1994) and Soreghan (1997) suggested that dry periodsmight cause a deficit in terrigenous supply and favor deposition ofcarbonates. This inference could also support interpretation ofoccurrences of sea level highstands during interglacial periods(warmer than glacial periods) and resultant general aridity and


deprivation of clastic sediment supply. The glacial periods bring inadded quantum of terrigenous supply to the depocentre in view ofshelf erosion (Kampschulte et al., 2001) and fluvial system advance-ment (Sarg, 1988; Carter et al., 1991). As the Cretaceous Period hadexperienced extended green house effect (Thiry, 2000), the sea levellowering may have influenced weathering and erosion (Hay, 1994).Erosion during periods of lower sea level, influenced also by theproximity to source rocks and adequate slope could be inferred fromthe configuration of the basin and paleochannels (Fig. 2) that wereassociated with sea level reduction. Presence of unaltered lithoclastsand feldspar clasts in rocks that immediately followed regressivesurfaces also suggests mechanical erosion, rapid and short duration of



Fig. 12. a. Profile of K across Barremian–Danian strata of the Cauvery Basin. b. Relationship between relative sealevel, absolute concentrations of Ca and Si in the stratigraphic context.Comparison of this figure with elemental absolute concentration cycles marked as 1 to 6 in Fig. 8a (Si) and c (Ca) suggests prevalence of this relationship at various temporal scales(cycles within cycles).


transport and quick burial. Such rapid mechanical erosion andtextural immaturity of sediments could produce covariation of Siand Al besides other elements associated with quartz, feldspar andother silicates as could be observed in sympathetic relationship ofpolynomial peaks of Al and coeval peaks of K, Mg, Ti and Fe, althoughin a different magnitude.

The element Y, which relatively gets not affected by diageneticalterations (Andrew et al., 1996; Das, 1997), shows a peculiarpolynomial peak across Aptian–Albian boundary, subdued natureduring most of the successive period till latter part of Middle Albianand gradual increase then afterwards reaching significant peak duringConiacian followed by gradual decrease. In magnitude and scale, itmimics Zr. Presence of its short and significantly prominent peaksexactly coinciding fault movements and associated change in sedimen-tation pattern indicate its influx immediately after major tectonicmovements and resultant change in nature, quantum and compositionof detrital influx into the basin. Sedimentation pattern and nature ofsediments of theperiodsbetweenBarremian–ConiacianandSantonian–Danianweredifferent andare reflected in thepatternsofY andZrduringthese two periods. The differences between these elements in terms oftemporal resolutionmay be a consequence of their differential response(Whitford et al., 1996) to prevalent depositional environmentalconditions. Higher accumulation of these elements up to Coniacianand their subdued nature after Coniacian could be attributed to thechanges brought in by major tectonic activity occurred during


Santonian, across which significant changes in proximity of sedimentsource and nature and quantum of detrital influx were witnessed(Sundaram and Rao, 1986), which in turn could have influenced thetrends of these metals across the said boundary. It is interesting to notethat when the positive excursions of these metals ceased to exist, Bashows a polynomial peak. However, during late Maastrichtian, peaks ofall these elements show covariation meaning dual role for Ba (detritalinflux, adsorption onto claymineral surfaces as well as association withcarbonates) as couldbeobservedelsewhere (Kumar et al., 1995). Bishop(1988) recorded covariationof Bawithmarineorganicmatter as a resultof barite precipitation in microenvironments during decomposition ofparticulate organic matter. On the contrary, when the organic carbonwas derived from terrestrial sources, it covaries with aluminosilicates(Dymond et al., 1992). Inferences on behavior of Ba in the light of aboveobservations indicate that in this basin, trends of Ba follows essentiallyof Zr (except during Danian), Ti and Al. The positive excursions of Ba areassociated with negative excursions of Ca and Corg, indicate primarydetrital signature and by implication, primary marine origin for theorganic matter. Prevalence of lesser circulation at bottom water duringcarbonate deposition would not have allowed significant Ba to formmicroenvironmental barite and hence, such a possibility for the Baabundance could be ruled out.

A prominent polynomial Zr peak spanning from Late Albian tolatter part of Middle Campanian shows covariation with terrigenousinflux. The followed up period (Late Campanian–Middle




Maastrichtian), representing predominant carbonate deposition, alsohas intercalations of siliciclastics (Middle Maastrichtian), but withcomparatively no Zr content. General association of Zr with heavyminerals (Pearce and Jarvis, 1991) gives surmise to interpret recyclingof siliciclastics from an intrabasinal marine source during this timespan. Prevalence of lower energy environment capable of scavengingand recycling only light minerals could also be inferred. This period ofsubdued Zr concentration/accumulation was overlain by a suddenspurt of fluvial deposits (Kallamedu Formation) that shows significantconcentration of Zr. The quantum of Zr kept on increasing towardsDanian, meaning survival of fluvial system across K/T boundary. Thisobservation also supports the view espoused by Sastry and Rao(1964) that the K/T boundary in this basin primarily indicates shift ofprincipal loci of deposition rather than major hiatus and rejuvenatedsedimentation. While analyzing strontium isotopic trends across K/T,Martin and Macdougall (1991) stated that there were increasedcontinental weathering and higher detrital influx into the worldoceans during this time. Based on strontium isotopic trends of LateMaastrichtian–Danian stratigraphic record of Cauvery Basin, Ramku-mar et al. (2010) have also interpreted significant detrital influx inthis part of the basin.

Although many events that have been advocated to occur acrossK/T transition, a marked biotic turnover owing to nutrient depletion(resulting either from impact scenario or extensive volcanic erup-tion) has been found to be consistent world over (Tappan, 1967;Peryt et al., 1993). Similar observations could be made from thecurves of P and organic carbon in this basin. Phosphorus formsessential ingredient for primary production in life cycle and any delayin additional input from terrestrial sources would be devastating,resulting in higher biotic turnover (Tappan, 1967). Covariation of Corgalong with nutrients such as P could also indicate purely marinesource for the organic matter and non-influx of terrestrial organicmatter (Raymo et al., 1997). Present observation of nutrientdepletion in the Cauvery Basin across K/T boundary affirms theviews of works cited above and also the view of Saraswati et al.(1993) who have stated that the close of Mesozoic era marks thebeginning of climatic deterioration in the Indian subcontinent. As thisclimatic change is a global phenomenon (Meyers and Simoneit,1989), it is not surprising to find major lithological changesassociated with various geochemical anomalies across this boundary(Kaminski and Malmgren, 1989; Canudo et al., 1991; Peryt et al.,1993; Ramkumar et al., 2004b, 2005; 2006, 2009, 2010).

The trends of Cinorg, Corg and Ctot apparently mimic Ca, explainingtheir dependence on carbonate deposition controlled by globalclimatic and sea level changes (as discussed earlier). Tu et al. (1999)have shown intimate relationships between marine organic matter,atmospheric CO2 content, climate and sea level. Dependence of Corgavailability with carbonate deposition indicates its marine biologicalorigin (Peryt and Wyrwicka, 1993) and low bottom water oxygen-ation (Pratt, 1984). Oxic nature of the depositional waters during thesea level lowstands and associated deposition of siliciclastics thwartaccumulation and preservation of organic carbon in the sediments(Friedman and Chakraborty, 1997), thus restricting its higherconcentration associated only with carbonates. This interpretation,when combined with the interpretation of carbonate depositionduring sea level highstands, implies stratification and lesser mixingduring carbonate deposition. Under such conditions, interpretation ofprevalence of cooler bottom waters and slow precipitation of shellcalcite and LMC as original mineralogy could further be ascertained.Judging from higher preservation of Corg in carbonates and depositionof carbonates essentially during sea level highstands, non-existence/reduced significance of fluviatile sediment transport, lesser/absenceof continental water influx into depocenter, reduced verticalcirculation (Calver, 2000) and higher primary productivity duringcarbonate deposition could also be inferred. Enhanced preservation oforganic matter may have been caused by widespread conditions of


poorly oxygenated bottom waters and inefficient recycling of organicmatter by benthic scavengers (Pratt, 1985). Production of organicmatter by sedentary organisms and absence of abnormal increase ofCorg from background values indicate absence of strong upwellingduring the periods of carbonate deposition and prevalence of reducedoxygen levels (nevertheless, not low enough to discourage sedentaryorganisms). Such inference may explain absence of completedegradation of Corg (Peryt et al., 1993). Occurrence of alternation ofsiliciclastics and carbonates, among which higher Corg, in carbonatescoupled with record of global signals of sea level highs in this basinpave way for interpretation of falling pCO2 due to extensive organicmatter burial, resulting in climatic cooling (Jeans et al., 1991; Ulicny,1992; Raymo et al., 1997) and sea level decrease (Tu et al., 1999). Asstated earlier, due to lesser temperature gradient, climatic cooling andresultant sea level fall would have lengthened fluvial systems totransport detrital material that in turn could have balanced Corg burialthrough cessation of carbonate deposition. Influx of continentalwaters coupled with reduced bathymetry could have significantlyincreased available oxygen in the waters and increased mixing(Kampschulte et al., 2001), promoting destruction of Corg (McKirdyet al., 2001). This mechanism could explain the alternate occurrenceof siliciclastic and carbonate deposits as a consequence of global scaleclimatic fluctuations.

During Barremian to Danian, Indian plate was an Islandcontinent akin to present day Australia, wherein climatic conditionsare controlled by temperature of surrounding seawater. As theCretaceous Period is suggested to have experienced extendedgreenhouse effect, slightest change in seawater temperaturewould have caused glaciers to retreat or advance, which in turnmight have caused high-frequency sea level oscillations. Theseinferences explain the occurrences of bimodal peaks of polynomialcurves within which high-frequency variations in profiles ofabsolute concentrations.

6. Geochemical signatures of the depositional units

Collectively, the geochemical profiles and trend lines indicate adisparity in the behavior of geochemical characteristics of the strataon either side of Santonian. A major tectonic movement duringSantonian that caused transgression in areas that have remainedtopographically highlands since inception of the basin (Barremian)had resulted in change of depositional pattern from dynamicdepocentre to more stable shelf conditions and is indicated by twomajor polynomial trend peaks of most of the elements acrossSantonian. However, only these two distinct signatures and a generalpattern of geochemical trends that follow sea level cycles could beobserved explicitly. It also follows from the profiles that these twomajor cycles contain multitudes of minor cycles, that may form high-resolution chemozones, but distinction of them manually would betenuous. Hence, alternative procedures capable of deducing geo-chemical signals, typical of separable depositional units at varyingscales were explored. The underlying postulate of chemostratigraphyis that there is always a distinct geochemical signature for differentdepositional units as a result of varying depositional conditions(Williams et al., 1988; Carter et al., 1991) at the scales between firstorder (∼100 ma.) and infra seventh order (∼20 ka.). Many studieshave documented diurnal and seasonal changes (Raymo et al., 1997;Ramkumar, 2001) in sedimentary records through geochemistry.Given cognizance to these, efforts weremade to examine geochemicaldistinctness of the depositional units in terms of samples of differentgeological ages and lithostratigraphic formations and members. Ifdepositional conditions were so dynamic to produce varieties oflithofacies associations at scales varying frommany tens of millions ofyears down to diurnal scales, the difference in dynamism might bereflected in geochemical composition too. In this context, absence ofclear and visible zonation in geochemical profiles might be




interpreted as the limitations associated with visual inspection ofabsolute concentrations. To test this assumption, mean values ofelemental concentrations pertaining to various depositional unitswere examined, which enabled to answer the question that if absoluteconcentrations were inconclusive in establishing distinct chemo-zones, could the statistical measures define distinct depositionalunits? In other words, if the geochemical profiles were found to be toocomplex and pose difficulties in delineation of distinct chemozones,could the depositional units established by lithostratigraphic criteriabe considered as distinct chemozones (provided they have distinctchemical signature)? Many previous studies have observed distinctgeochemical anomalies and shift of geochemical trends acrosslithostratigraphic boundaries (for example, Andrew et al., 1996;Pelechaty et al., 1996; Pearce and Jarvis, 1991; Pearce et al., 1999) andformed basis for this presumption. In addition, lithofacies associationsof this basin were the result of global sea level changes (Ramkumar etal., 2004b, 2006).

The Figs. 8–12, together with the Tables 1 and 3 reveal that there aredistinct compositional changes between the depositional units of thebasin, as defined by positive and negative excursions of elementalprofiles across geochronological and lithostratigraphic boundaries.However, these high-frequency cycles seem to have lesser visibilitywhen whole of the profile (from Barremian–Danian) is examinedvisually. In such a case, recognizing high-resolution chemozones basedonly on visual inspection of geochemical profiles of long ranging stratawould be misleading. These diagrams also indicate that, although theextent of positive or negative excursions of absolute concentrationsseem to pose difficulty in recognition of individual cycles, the quantumof excursion appears distinct, meaning, the mean values of eachdepositional unit may show noticeable differences from that ofjuxtaposed units. To test the existence of significant difference betweenmeans of two groups of samples, t-test is employed. If there are morethan two groups, a statistical technique namely “analysis of variance”(ANOVA) is utilized (Gupta and Kapoor, 1985). This method was foundto be useful in geological applications such as recognizing the texturaland chemical compositional disparity among recent sedimentaryenvironments (Ramkumar and Murty, 2000). ANOVA brings out thevariance/homogeneitybetween severalmeans by separationof varianceascribable to onegroupof causes fromthe varianceascribable to anothergroup. From the chemostratigraphic point of view, performing ANOVAcould answer the questions such as “whether there is distinctgeochemical signature for each depositional unit? and if present,whether an element or few elements with significant differencebetween depositional units could be categorized as chemostratigraphicindices and in turn, help distinguish chemozones in a long ranging stratasuch as that of the Cauvery Basin?”

While performing ANOVA on the geochemical data set, the samplesof different chronological, formational and member units were treatedasdifferentpopulations. Out of 30variables used, all the variables exceptRSL, Al, and Nb, showed significant variance between differentgeochronologic units. All the variables except Y, P and S showedsignificant difference between formations. Similarly, all the variablesexcept P showed variance between members. These results indicatepresence of significant differences among the depositional units at thescale of geochronological age, formation and member, but thedifferences are subtle due to which manual recognition of geochemicalshifts could not bemade easily. The lists of variables that havenodistinctvariation across the groups showed different combinations. Phosphorushas not shown any difference pertaining to lithostratigraphic units.Although relative sea level history of the basin was not significantlydifferent across geochronologic age units, it had significant role at thescales of formation and member, reinforcing the view held earlier thatthe depositional pattern, lithological association and history of the basinwere mainly influenced by sea level changes and also the fact thatsignificant environmental changes took place across lithostratigraphicboundaries in this basin rather than chronological boundaries.


7. Conclusions

• Strata of the Cauvery Basin preserve primary depositional signa-tures influenced by global climatic and sea level fluctuations. Owingto the dominant control of sea level changes all through thedepositional history of the basin and also due to relative stability ofsea level and limited and episodic availability of continental influx,the deposits have apparent similarity in whole-rock geochemicalsignatures leading to generation of subtle and complex signals inprofiles of absolute elemental concentrations.

• Conventional observation of geochemical profiles shed light onmajor geological events in terms of detrital influx, basin starvationand changes in depositional pattern rather than portraying distincthigh-resolution chemozones or distinct signatures for each of thedepositional units.

• Presence of almost all the elemental concentrations in the results ofANOVA that show significant difference in variance at 95%confidence level indicate the occurrence of distinct geochemicalsignatures for all the depositional units. It means that whole-rockchemical compositions of rocks could serve as chemostratigraphicindices.

Acknowledgements

MR acknowledges the financial assistance of Alexander vonHumboldt Foundation, Germany. Council of Scientific and IndustrialResearch, New Delhi, India provided financial support during initialfield survey. Currently, this work is being supported by theDepartment of Science and Technology, New Delhi. Prof.M.Santoshand Prof. Alan Collins read earlier versions of the manuscript andprovided several constructive suggestions. Authors thank the com-ments and suggestions of the anonymous reviewers that have helpedto present the data and interpretations lucidly. Special thanks are dueto the scientific personnel of Institute of Mineralogy and Geochem-istry, University of Karlsruhe, Germany, namely, Dr. Utz Kramar forXRF analytical facilities, Dr. Karotke and Mrs. Oetzel for XRD analyses,Mr. Predrag Zrinjsak for carbon and sulphur analyses and Dr. Ott forcomputing facilities. Shri. T. Sreekumar, Geologist, OFI, Mumbai, isthanked for assistance during the field survey. Permission to collectsamples was accorded by the mines managers and geologists ofMessers. Alagappa cements, Chettiyar mines Dalmia Cements, FixitMines, Pvt.Ltd, Nataraj Ceramics Ltd., Parveen mines and MineralsLtd., Ramco Cements, Rasi cements, TANCEM Mines, TAMIN mines,Tan-India Mines and Vijay Cements.

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