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Lithostratigraphy, depositional history and sea levelchanges of the Cauvery Basin, southern India

MUTHUVAIRVASAMY RAMKUMAR1, DORIS STÜBEN

2 & ZSOLT BERNER2

Abstract. The sedimentary sequence exposed in the erstwhile Tiruchirapalli district hosts a more or less com-plete geological record of the Upper Cretaceous–Tertiary period. Systematic field mapping, collation of data onthe micro–meso scale lithology, sedimentary structures, petrography, faunal assemblage and facies relationshipsof these rocks, in the light of modern stratigraphic concepts, helped to enumerate the lithostratigraphic setupand depositional history of the basin. Spatial and temporal variations of the lithologies and revised stratigraph-ic units are presented in this paper. Many high frequency sea level cycles (presumably fourth or higher order)which stack up to form third order sea level cycles (six in number), which in turn form part of second ordercycles (two in number), including seven eustatic sea level peaks, have been recorded in this basin. Trend analy-sis of sea level curves indicates a gradual increase of the sea level from Barremian to Coniacian and a grad-ual decrease from Coniacian to Danian. Such lasting sea level trends had their influence on the sedimentationpattern and facies association. It is inferred that depositional bathymetry was maintained at a shallow–moder-ate level, primarily influenced by a lack of major subsidence during the depositional history of this basin. Thestudy also revealed a prevalent simple basin filling process and dominant control by sea level changes, ratherthan tectonic movements over the depositional regime.

Keywords: stratigraphic revision, sedimentary environments, sea level cycles, sequence development, CauveryBasin, southern India.

Apstrakt. Sedimentni niz otkriven u nekada{wem Tiru~irapali rejonu sadrì mawe ili vi{e celo-vit geolo{ki zapis gorwokredno–tercijarne periode. Sistematskim geolo{kim kartirawem, upore|i-vawem podataka mikro i mezo sleda litologije, sedimentnih struktura, petrografije, skupa faune istenskih facijalnih odnosa, u svetlu modernih stratigrafskih shvatawa, pospe{eno je stvarawe lito-stratigrafskih postavki i depozicione istorije basena. U ~lanku su prikazane prostorne i vremenskevarijacije litolo{kih i revidovanih stratigrafskih jedinica. Visoka u~estalost ciklusa kolebawaniova mora (po svoj prilici ~etvrtog ili vi{eg reda) stvorila je oblike tre}eg ciklusa morskog nivoa(ukupno {est), a u obrnutom delu ciklusa (dva), ukqu~uju}i i sedam eustati~nih pikova morskog nivoazabeleènih u ovom basenu. Trend analiza krivih nivoa mora ukazuje na postepeno pove}awe nivoa odbarema do konijaka i postepeno smawewe od konijaka do danskog kata. Takvo trajawe trendova morskognivoa je imalo uticaja na sedimentacioni model i asocijaciju facija. Zakqu~eno je da je depoziciona ba-timetrija odraàvala umeren plitkovodni nivo, prvobitno uzorkovan nedostatkom glavnog spu{tawa zavreme depozicione istorije basena. Studija je tako|e ukazala na jednostavne procese puwewa basena i natektonske pokrete kao dominantnu kontrolu promene nivoa mora tokom depozicionog reìma.

Kqu~ne re~i: stratigrafska revizija, sedimentne sredine, ciklusi kolebawa nivoa mora, sekventnirazvoj, Kaveri basen, ju`na Indija.

GEOLO[KI ANALI BALKANSKOGA POLUOSTRVA

ANNALES GÉOLOGIQUES DE LA PÉNINSULE BALKANIQUE65 (2002–2003) 1–27 BEOGRAD, decembar 2004

BELGRADE, December 2004

1 Department of Geology, Periyar University, 636011 Salem, Tamil Nadu, India. E–mail: [email protected] Institut für Mineralogie und Geochemie, Universität Karlsruhe, D–76128 Karlsruhe, Deutschland. E–mail: doris.stueben@

img.uni–karlsruhe.de; [email protected]–karlsruhe.de

Introduction

The Cauvery Basin, the southernmost basin (Fig. 1)in the Indian peninsular shield of the NE–SW trendingLate Jurassic–Early Cretaceous rift basins (POWELL etal., 1988) was created during the fragmentation of thesupercontinent (JAFER, 1996). A more or less completeUpper Cretaceous–Paleocene succession is exposed inthe Ariyalur–Pondicherry depression of this basin (SA-STRY & RAO, 1964). This succession, in view of itslithologic and faunal diversities, has attracted the atten-tion of geologists since the early studies of KAY (1840),BLANFORD (1862) and STOLICZKA (1861–1873). Many

hundreds of papers have been published since then,among which the works on microinvertebrates (CHIP-LONKAR, 1987), foraminifers (GOVINDAN et al., 1996), os-tracods (BHATIA, 1984), ammonites (AYYASAMY, 1990),nannofossils (JAFER & RAY, 1989; KALE & PHANSAL-KHAR, 1992; KALE et al., 2000), bryozoans (GUHA,

1987; GUHA & SENTHILNATHAN, 1990, 1996), lithostra-tigraphy (RAMANADHAN, 1968; BANERJI, 1972; SUNDA-RAM & RAO, 1986; RAMASAMY & BANERJI, 1991; TE-WARI et al. 1996) and tectonics, (KUMAR, 1983; PRABA-KAR & ZUTCHI, 1993) present a comprehensive account.In general, many of these publications were, to a majorpart, devoted to a single species or genera, group orformation/member/ lithology and, hence, a generaliseddepositional model for the whole of the basin has yet tobe documented. The economic discoveries of hydrocar-bons in this basin demand a more organised classificationof these strata which could help in designing improvedexploration strategies (RAJU & MISRA, 1996). As far as

the existing stratigraphic classifi-cations of the Cauvery Basinare concerned, as noted by TE-WARI et al. (1996), there is stilla reluctance to the recognitionof the association of faciesbased on stratigraphic classifi-cations. Furthermore, there hasbeen no synergic study eluci-dating the sedimentologic me-chanisms which acted duringthe course of the evolution ofthis basin. Thus, the goals ofthis study were:• to present a workable litho-

stratigraphy with detailed li-thological profiles, includingrecognisable units so as toproduce a realistic meso-micro scale lithological co-lumn. This was necessary,primarily because recognitionand fixation of any biostrati-graphic or chronostratigaphicboundaries rely mostly onthe correct recognition of la-teral and vertical spatial rela-tionships of the strata, and

• to enumerate the deposition-al history of the basin witha view of singling out thecontrols of the depositionaland stratigraphic continuumand breaks in order to de-duce the differential roles ofgeological agents and pro-cesses.

Methods and materials

Systematic field mapping on the scale of 1:50 000(the first ever in this part of basin) was conducted inand around the Ariyalur, Perambalur and Tiruchirapallidistricts, (erstwhile Tiruchirapalli district) in which the

MUTHUVAIRAVASAMY RAMKUMAR, DORIS STÜBEN & ZSOLT BERNER2

Fig. 1. Sedimentary basins of India.

largest exposure (a more complete one than the otherexposures located north of it – YADAGIRI & GOVINDAN,2000) is located. Data on the lithology, sedimentarystructures, tectonic structures, facies and faunal associ-ation (mega and ichno) were collected from naturalexposures, as well as well and mine sections. A totalof 308 locations were logged and sampled. The guide-lines and norms provided in EYESINGA (1970) and TheNorth American Stratigraphic Code (1983) were fol-lowed for the presentation of the systematic strati-graphy. The following procedures for major revision oflithostratigraphy, as suggested by HART et al. (2000),were adopted in this study: • primary field work in the exposed area,• detailed analysis of the relationships of the intrafor-

mational facies, • hierarchical use of groups, formations and members

in accordance with the stratigraphic codes,• formal definitions of type sections and historical pre-

cedent.The exposed beds were traced laterally and vertical-

ly. Gross lithology (meso scale), general faunal andichnofaunal information, petrography and sedimentarystructures were considered when defining the stratigra-phic units. The proposed lithostratigraphic setup is pre-sented in Table 1. The lateral and vertical variations ofthe lithostratigraphic units in the study area are present-ed in Fig. 2. The geographic distribution of the mem-bers is presented in Fig. 5. The stratigraphic informa-

tion, interpretations of depositional environments, majorgeological events (such as faulting, sea level changes,depositional breaks, etc.) generated with ground truthdata and laboratory analyses and published paleontolog-ic data enabled the elucidaton of the depositional histo-ry and prevalent geologic processes.

Systematic stratigraphy

TEWARI et al. (1996) presented a comprehensive ac-count on the lithostratigraphy of this basin. Following thecurrent practices in stratigraphic terminology, the classi-fication was modified and updated in the present work.

Sivaganga Formation

BANERJI (1982) recorded the extensive occurrence ofa conglomerate–sandstone unit in regions far south of thepresent study area and named it the Sivaganga Forma-tion. BANERJI & RAMASAMY (1991) and BANERJI et al.(1996) reported a similar unit in the Tiruchirapalli ex-posure. These beds mark the first Cretaceous sedimen-tation in this basin. Three discrete members constitutethis formation in the study area.

Basal Conglomerate Member

Nomenclature. This member corresponds to the Ko-vandankurichchi Conglomerate Member of TEWARI etal. (1996). As they named the overlying sandstonemember also with the prefix “Kovandankurichchi”, thenomenclature is amended herein to avoid repetition ofthe same geographic name for two stratigraphic units.Since this member forms the bottommost sedimentaryrecord of this basin, the prefix “Basal” has been added.

Stratotype: It is located at 78°55’48” E; 10°52’36” Nin the Kovandankurichchi Mine I, NE wall, Bench IV(mine owned by Dalmia Cements Ltd.).

Lithology, contacts and environment. These beds arelithoclastic boulder conglomerates that contain abundantclasts of granitic gneiss (basement rocks). The clasts aresub-angular to sub-rounded and range in size from 10 mto 60 cm. This lithological unit is clast supported andtowards the top shows a reduction in the size of the clastand an increase in the quantity of finer grains. Manysuch upward fining sequences have been observed to restover the highly faulted Archaen basement. These sedi-ments are essentially brought by rivers and deposited un-der high-energy conditions as coastal conglomerates inview of the widespread erosion of the fault scarps (BA-NERJI, 1983). They may be termed as type II conglome-rates according to the classification of PETTIJOHN (1957).

Geographic extension. This member is exposed onlyin the mine section in this part of the basin. Mine sec-tions expose only a limited portion of this unit, as all

Lithostratigraphy, depositional history and sea level changes of the Cauvery Basin, southern India 3

Table 1. Proposed lithostratigraphy of the Cretaceous–Paleo-cene strata, Cauvery Basin.

the limestone deposits are found only above this unit.All other sub-basins of Cauvery basin, where deep borewells were drilled, show existence of this member indirect contact with Archaen basement rocks.

Fossils and age. Subsurface counterparts contain Bar-remian–Aptian fossils of Globigerina boteriveca (GOVIN-DAN et al., 1996; RAJANIKANTH et al., 2000).

Thickness. 18 m.Equivalents from other areas. BANERJI (1982) report-

ed extensive exposures of this member in the southernpart of this basin.

Kovandankurichchi Sandstone Member

Nomenclature. It has been amended from the nomen-clature of TEWARI et al. (1996) to reflect its lithology.

Stratotype. It is located at 78°55’51” E; 10°52’34” Nin the Kovandankurichchi Mine I, NW wall, Bench III(mine owned by Dalmia Cements Ltd.).

Lithology, contacts and environment. These are grainsupported upward coarsening cyclic beds that show pa-rallel, even and thin to thick bedding. Individual bedset ranges in thickness from 20–100 cm. This memberrests over the Basal Conglomerate Member having a dis-tinct, non-depositional and erosional unconformity. It dif-fers from the underlying member in terms of the chan-ge in lithology and lack of granitic clasts. The sandsrange in size from granule to coarse silt and are carbo-nate cemented. The grains are well sorted within eachlamina and show rounded-well rounded shape. These re-present recurrent sheet flow deposits probably in a sub-aqueous fan deltaic environment. Towards the north,they grade to argillaceous siltstones and claystones, re-presenting deposition in a basinal setup.

Geographic extension. This member is exposed inthe Kovandankurichchi mine and is observed to occurin stream sections, well cuttings and trenches up to theregion south of Perali. From south to north, there seemsto be a gradual reduction in the thickness and grain size


Fig. 2. Spatio–temporal variations of lithostratigraphic units along the traverses. Arabic number against each box refer to theindex of different members as given in Fig. 5. Similarly, the roman numbers refer to the transverse line, as drawn in Fig. 5.

with a simultaneous increase in argillaceous matrix. Inthe southern regions, these are essentially grain-support-ed fabrics. Towards the north, the rocks are matrix sup-ported, loosely packed and well cemented.

Fossils and age. Early Cretaceous palynoflora werereported from subsurface sections (bore well samples)of this member (VENKATACHALA, 1974; RAJANIKANTH etal., 2000).

Thickness. 24 m.Equivalents from other areas. BANERJI (1982) report-

ed this member and a Basal Conglomerate Member asconglomerate–sandstone succession of the Sivaganga For-mation. The availability of new exposures in the Kovan-dankurichchi mine and the tracing of them up to thesouth of Perali, with a distinct stratigraphic break in be-tween them, allowed us also to precisely define the stra-tigraphic relationships and position. BANERJI (1982) re-ported isolated outcrops of this member also in Virud-hachalam (north of the study area).

Terani Claystone Member

Nomenclature. Amended herein from the nomencla-ture of TEWARI et al. (1996) to reflect its lithology.

Stratotype: It is located at 78°54’01” E; 11°07’42” Nin the Terani clay mine (mine owned by Nataraj Ce-ramics Ltd.).

Lithology, contacts and environment. Traditionallythese have been termed as clay deposits resting overthe Archaen basement (GOVINDAN et al., 1996). How-ever, transition of Kovandankurichchi siltstone andsandstone into basinal clay and argillaceous siltstone,which have contact with Archaen rocks, were observedin new mine sections. This transition might have result-ed from a rise in the sea level and the cessation ofcoarse clastic influx. As these are primarily clays,severe continental weathering was also indicated. Theclays are white to pale brown in colour and show thefrequent occurrence of alternate bands of red and red-dish brown laminae. Less frequent interlayers of thin tothick ferruginous siltstone are also present. These ob-servations indicate the influence/existence of seasonalfloods that brought in a significant detrital influx to thedepo-center of this member, although this member wasdeposited in deeper regions of the basin (regions farbelow the base of a storm weather wave). The preva-lence of a faulted basin topography which might havehosted a narrow shelf and a sudden steep slope couldexplain the bypassing of silts and sands to the basinalsetting. Deposition took place principally through sus-pension settling, except for the siltstone/fine sandstonelaminae.

Geographic extension. These beds are generally foundto have been deposited in isolated patches in andaround Karai. They occur along the westernmost bor-ders of the sedimentary basin. The extensive distribu-tion of their counterparts has been reported from all

over the subsurface part of the basin (VENKATACHALA,1977; MAHESHWARI, 1986).

Fossils and age. The clay beds contain frequent im-pressions of Ptilophyllum sp. MAMGAIN et al. (1973)reported two Barremian ammonite species and one ino-ceramid species, as well as fossils of this plant. SASTRY

et al. (1977) and RAMASAMY & BANERJI (1991) assign-ed Aptian age to this member.

Thickness. 30 m.

Dalmiapuram Formation

Grey Shale Member

Nomenclature. The original nomenclature of RAMA-SAMY & BANERJI (1991) has been restored with anamendment herein. On account of water logging andthe non-accessibility during their visit to the mine, TE-WARI et al. (1996) termed this unit as Grey Siltstonebased on only one sample from this mine and anothersample from Perali. They mentioned the validity oftheir term in their paper and reported the non-exami-nation of the more “calcareous nature of the shale” asdescribed by earlier workers. The sampling conductedfor this study occurred during a period when dry minefloor was exposed, which allowed this unit to bedefined more precisely.

Stratotype. It is located at 78°55’51” E; 10°52’34” Nin the Kovandankurichchi mine I, NW wall, Bench III(mine owned by Dalmia Cements Ltd.).

Lithology, contacts and environment. This is princi-pally grey shale with frequent interbeds of fossiliferousgrey limestone (the thickness of which increases up-wards) and to a minor extent it is composed of a sig-nificant admixture of silt-sized siliciclastics. The ubiq-uitous grey colour is more pronounced where shalewithout any intercalation or admixture was encoun-tered. Deposition in a periodically restricted shallowmarine setting was indicated by lithological and faunalinformation. The lower contact is an unconformity sur-face associated with marine flooding and the uppercontact is non-depositional and erosional with an over-lying biostromal member.

Geographic extension. This member is well exposedin the Kovandankurichchi mine. Diagenetically alteredcounterparts are exposed in well sections located north-east of Kallagam and south of Perali. Many authors ha-ve recorded subsurface extensions of this member (e.g.SUBBARAMAN, 1968; GOVINDAN et al. 2000). Depositionof this member in isolated patches restricted the avail-ability of extensive exposures.

Fossils and age. KALE et al. (2000) reported a richassemblage of palyno fossils in this member. Thismember exposes the H. planispira planktonic formini-feral Zone (GOVINDAN et al., 2000). Ostracods, bry-ozoans, gastropod fragments have also been observedin this member (TEWARI et al., 1996). These faunal and


floral assemblages indicate Barremian to Albian age forthis member.

Thickness. 7 m.

Varagupadi Biostromal Limestone Member

Nomenclature. (Proposed herein). RAMASAMY & BA-NERJI (1991) and TEWARI et al. (1996) termed rocks ofthis member as Dalmiapuram Limestone Formation andMember respectively. As a more complete section ofthis member occurs in the Varagupadi mine and assign-ing two lithostratigraphic units with the same name isdiscouraged, this unit has been termed as VaragupadiBiostromal Limestone Member.

Stratotype. It is located at 78°54’57” E; 11°09’35” Nin the Varagupadi mine, Bench I and II (mine ownedby Vijay Cements Ltd.).

Lithology, contacts and environment. These are lime-stone beds containing various proportions of bioclasticand siliciclastic materials. They show wackstone to rud-stone fabric and have clasts of redeposited boundstones.Thin to thick bedded, even to parallel, bioclastic lime-stone beds which have drawn their detritus from reefspredominate. Typical reef cores are absent in this mem-ber. These are typical reef flank biostromal deposits de-posited under high-energy conditions and show signifi-cant lateral variation of facies and texture. These bedsare found to be directly overlying the Grey ShaleMember, while their reef core counterparts might havebeen located further west directly over the Archaenrocks, which in turn were eroded during a major mid-Cretaceous faulting (TEWARI et al., 1996; YADAGIRI &GOVINDAN, 2000; STEINHOFF & BANDEL, 2000). The in-terpretation of BANERJI et al. (1996) based on fieldcharacteristics that “these biostromal deposits were de-rived from some older biohermal bodies which are nolonger seen outcropping”, adds further support to thisassertion. The information that the Ariyalur–Pondicher-ry sub-basin had experienced erosion of a ca. 1.5 kmthick sediment pile (YADAGIRI & GOVINDAN, 2000) andthe occurrence of Olaipadi Member (detailed latter) at10 km directly west of the Dalmia mine confirm theprevalent but eroded reef counterpart of this biostromalmember. Many authors, however wrongly recognised amuch younger Dalmia Biohermal Member to be thebiohermal counterpart of this member. It is to be notedthat the biohermal member was subaerially exposed andunderwent karstification. Had it been the counterpart ofthis member, then how could this member have escap-ed similar karstification? Based on the common reefmineralogy and their easy susceptibility to subaerial ex-posure, it is assumed that as the Dalmia BiohermalMember contained highly unstable carbonates and a pre-valence of a high host to fluid bulk chemistry ratio, thediagenetic fluid (that moved downwards) attained satu-ration before reaching the lower member producingless/no recrystallisation of beds of this member. The

presence of disconnected vugs and voids in the bioher-mal member, indicates that the absence of interconnect-ed pores might have thwarted access of meteoric watersto the deeper regions, because of which, the biostromalmember, located there, escaped karstification. If so, thebiostromal member might be older than the Dalmia Bio-hermal Member.

The depositional relationship between this memberwith an eroded biohermal counterpart and the GreyShale Member could be such that deposition along faultscarps and differential growth of reef and reef derivedbioclastic limestone deposits introduced a temporary tolong term closure of parts of the depositional setting,whereby more anoxic, fine grained shale and grey lime-stone deposits were alternatively deposited. Throughthe passage of time, with the rise in the sea level, moreopen conditions of sea were introduced which permit-ted the deposition of bioclastic reef derived limestoneonly. Faulting, as evident from the Varagupadi mine,where the biostromal limestone beds rest directly overArchaen rocks, may also have influenced this change.However, the occurrence of alternating shale-bioclasticlimestone beds indicates that fluctuations in the sea le-vel must have taken place which were probably accom-panied by faulting and submergence with the net resultthat the sea level had an overwhelming influence onthe depositional pattern. The occurrence of granitic cob-bles at the bottom beds of this member suggests a reju-venated fresh erosion of basement rocks, implying thecessation of the deposition of the shale member by faultmovement, followed by a rise in the sea level and therejuvenation of bioclastic limestone deposition. Frombottom to top, the increase of the thickness of the greyfossiliferous limestone within the shale beds, the con-current reduction of the thickness of the grey shale bedand the increase of silt admixture in the shale, and thecomplete cessation of shale deposition and the presenceof only limestone at the top and, finally, the occurrenceof such an observed biostromal sequence make way forthe interpretation of facies variation over time under themajor influence of sea level changes.

Geographic extension. In the southern part of the ex-posed region, these are pure biostromal, bedded lime-stone deposits, with frequent erosional surfaces. Towardsthe north, they grade to arenaceous limestone beds withreef derived bioclastics intermixed with siliciclastics ofvarying proportions and at places also have argillaceousadmixture. Good exposures occur all along the westernmargin of this basin in the mine sections. Basementlithoclasts in the lower portion and argillaceous admix-tures towards the top are observed in this member,meaning a gradual increase in the sea level and anassociated reduction in the depositional energy condi-tions. The occurrence of high variation of thicknesswithin short distances and the contact of this memberwith both older sedimentary rocks and Archaen rocksalso confirms the superposition of an icrease in the sealevel and the movement of the reef core in a westerly


direction, which might also have resulted in a lateralshift of this member.

Fossils and age. The faunal composition of this mem-ber constitutes bioclasts of bivalves, rudists, corals, al-gae, foraminifers, ostracods, and bryozoans and echi-noid spines. GOVINDAN et al. (1996) assigned MiddleAlbian age to these biostromal beds.

Thickness. 23 m.

Dalmia Biohermal Limestone Member

Nomenclature. It is proposed herein.Stratotype. It is located at 78°57’23” E; 11°59’19” N

in the Kallakkudi mine II, Northern wall, Bench III andIV (mine owned by Dalmia Cements Ltd.).

Lithology, contacts and environment: These are purealgal and coral facies limestone beds that form a reefcore. The occurrence of the reef core stratigraphicallyabove the biostromal limestone deposits, the absence ofa reef core counterpart of the underlying biostromallimestone beds and the younger Olaipadi Member overthe biohermal member, as well as the direct overlap-ping of the much younger Kallakkudi Sandstone Mem-ber in the Kallakkudi mine are all suggestive of a pre-valent progradation of reef colonies over the former de-eper regions of the sea concomitant with a fall in thesea level (STEINHOFF & BANDEL, 2000). This 15 mthick, biohermal member contains massive pink togreyish white limestone deposits with abundant vugsand cavity fillings. The growth and movement ofcoral–algal colonies were suddenly truncated as a resultof mid-Cretaceous faulting. The upper contact of thismember is a forced regression and associated subaerialexposure surface which has even resulted in meteoricdiagenesis of these limestone deposits. The bottom con-tact is conformable with biostromal limestone and rep-resents a non-depositional surface. The occurrence oflithified carbonates clearly indicates a fall in the sea le-vel (associated with faulting) after the deposition of thebiohermal member as the sea level crossed the shelfedge (Type I sequence boundary). The absence of anOlaipadi Member over this member in the Kallakkudimine and the adjoining regions, as well as the super-position of the much younger Kallakkudi SandstoneMember directly over this limestone member in thisregion suggests that, in view of faulting, this areabecame topographically raised and served as a sedimentsource for the Olaipadi Member. On the contrary, theoccurrence of slope failure could also be interpreted aslarge-scale mass wasting, a phenomenon commonlyassociated with a fall in the sea level (SARG, 1988).The interpretation whereby slowly prograding reefsmoved towards former offshore regions with a fallingof the sea level adds support to this view as prograda-tion of reefs could have surpassed the angle of reposeand become unstable and might have contributedtowards the generation of mass wasting. However, the

presence of granitic clasts (more than 10 m diameter)in the younger Olaipadi Member (lithoclastic conglom-erate deposits – detailed latter) indicate fresh erosion ofcontinental materials, which paves the way for theinterpretation of a fall in the sea level concomitant withtectonic movement, whereby the change in the sea levelhad an overwhelming influence.

Geographic extension. This member is typically ex-posed in the Kallakkudi mine. Isolated and highlyweathered counter parts are observed in the northwest-ern region of this basin. Their very limited occurrence,also above the Biostromal Limestone Member, is inter-preted as being the result of reef growth along the faultscarp, a large scale fault later associated with erosion,which allowed the preservation of only relatively deep-er water biostromal and biohermal limestone deposits.

Fossils and age. The limestone consists of red algae,corals and bryozoans, bioclasts of gastropods, bivalves,echinoid plates, ostracods, foraminifers and sponges.Based on the faunal assemblage, Middle Albian age isassigned to this member (GOVINDAN et al., 1996).

Thickness. 15 m.

Olaipadi Conglomerate Member

Nomenclature. After TEWARI et al. (1996). Stratotype. It is located at 79°05’47” E; 11°19’12” N

in the Olaipadi mine, Bench I, II, III and IV (mineowned by Ramco Cements Ltd.).

Lithology, contacts and environment. These are es-sentially basinal silty clays and clays in which chaoticblocks (Fig. 3A) are embedded. The beds contain largeblocks of basement rocks (angular and sub-roundedboulders which are often >10 m in diameter), lithifiedcoralline limestone blocks (similar to the underlyingbiohermal limestones), claystones (Fig. 3B; similar tothat of the Terani Clay Member) and lithoclasts of ol-der conglomerates, etc. The occurrence of lithoclasticconglomeratic boulders (1–2 m diameter, extraforma-tional), limestone (intraformational) and basement boul-ders (extraformational) in an otherwise deep marine siltyclaystone is indicative of sudden fault movement asso-ciated with widespread erosion of coastal/nearshore se-dimentary rocks and their transportation by gravity andinstability of a newly created slope, bypassing the nar-row shelf, to the new depocenters. TEWARI et al. (1996)interpreted this member as having been deposited fromrapid flows of submarine debris/talus deposits at thefoot of eroding fault scarps. The fault movement imme-diately after the deposition/lithification of the underlyinglimestone member associated with subaerial erosion isclearly exhibited in the Kovandankurichchi mine andthe Kallakkudi mine, wherein offsetting of fault blocksis exposed in terms of displaced bedded limestonedeposits (Varagupadi Member).

Geographic extension. This is the most extensivemember of Dalmiapuram Formation. The beds extend


continuously from Tirupattur in the south to Govinda-rajapalaiyam in the north. The granitic gneissic boul-ders are abundant in the Tirupattur mine. Their size andfrequency is reduced towards the north, to be progres-sively replaced by lithified carbonate boulders drawnfrom the underlying biohermal member. It should benoted that the lithified claystone conglomerate boulderclasts, which occur in association with other clasts ofthis member, were not present elsewhere in the exposedarea as a distinct lithology/bed. This observation, in ad-dition to the complete absence of biohermal counter-parts of Varagupadi Biostromal Member and presenceof >10 m large basement clasts, supports the interpre-tation of the existence of such lithologies further westbefore faulting and complete erosion of them in view offaulting. Towards the top, deep marine clays grade intocalcareous siltstone and include granitic cobbles and mi-nor amounts of siliciclastic sands, which indicate a grad-

ual reduction of the sea level followed by the commenc-ment of the influx of fluvial sediment. The thickness ofthis member varies within short distances, which isindicative of a fault controlled depositional topography.

Fossils and age. Reworked fauna, namely: belem-nites, rudists, corals and serpulids characterize this mem-ber. The occurrence of this member at the dawn of theCenomanian, together with a sudden change in the envi-ronment and faulting could be interpreted as beeing theresult of a major basin development process.

Thickness: 65 m.

Kallakkudi Sandstone Member

Nomenclature. It is proposed herein. It correspondsto the Kallakkudi Sandstone Member and the Kallakku-di Siltstone Member of TEWARI et al. (1996) and the


Fig. 3. A. Large sized limestone chaotic block (a) embedded in deep marine silty clay (b) of the Olaipadi Member. Note theconformable relationship of silty clay bands with the boundary of lithoclast. Tirupattur quarry. B. Close up view of clasts ofolder sedimentaries in the Olaipadi Conglomerate Member. Note the presence of lithoclasts of every conceivable size and shape.a - Hammer (~ 30 cm in length) is placed for scale; b - Subangular–subrounded claystone lithoclasts; c - Subangular–angularlithoclasts of limestone. Also note the different orientation of the long axes of these clasts. Olaipadi quarry. C. Minor scaleload casts, slightly deformed by post depositional compaction. Tirupattur quarry. D. Close up view of the woody sandstone.Note the presence of petrified wood fragments (arrows) with different orientation and size. Near Garudamangalam village.

biostromal and reefal deposits of RAMASAMY & BANERJI

(1991). Although TEWARI et al. (1996) eradicated manydiscrepancies existing in previous studies with their re-vision, they overlooked the gradation of faces betweensandstone and siltstone members, the repetitive occur-rence of alternate sandstone and siltstone beds whichgrade to each other (may be the result of varying ener-gy and sediment influx and also of minor sea levelfluctuations) and lack any stratigraphic break.

Stratotype. It is located at 78°57’23” E; 11°59’19” Nin the Kallakkudi Mine II, Bench I and II (mine ownedby Dalmia Cements Ltd.).

Lithology, contacts and environment. This is a fine--coarse sandstone member with alternate medium tothick beds of silty clay, calcareous siltstones, bioclasticarenaceous limestone and gypsiferous clay. The inter-calations are recurrent and show typical Bouma sequen-ces, normal grading, load casts (Fig. 3C), channel andscour structures. The sandstones are thin to mediumbedded and always start over a gypsiferous band and/orarenaceous limestone bed with an erosional surface andgrade to finer clastics. Towards the northern regions,this member grades to more silty and clayey, but thegradation and gypsiferous bands are persistent with theaddition of ferruginous silty clay bands.

The depositional setting of this member ranges froma shallow coastal to a basinal setting which were underthe influence of a major fluvial source near coast and aturbidity current in the basinal part, respectively. The co-astal region must have been periodically closed/partiallyclosed to produce evaporitic layers. In the basinal part,deposition took place principally from a high-density tur-bidity current in a ramp developed over the OlaipadiMember. The talus like platform created a steep slopedturbiditic apron, fed by siliciclastics derived from a flu-vial source and eroded carbonate sands, bypassing thenarrow slope (newly created by mid-Cretaceous faulting)and reaching to the basinal region. The continued depo-sition of this member must have been sustained by a pro-grading apron/talus. The channel orientation, large-scalescour structures and the nature of the bedding, suggestflow from south to north (TEWARI et al., 1996).

Geographic extension. Next to the Olaipadi Member,this is a well-developed member of the DalmiapuramFormation. Its thickness was controlled by depositionaltopography. In the southern region, these beds showrecurrent bands of fining upward sequences of only sili-ciclastics with calcareous cements. From Varagupadi to-wards the north, they show a thickening and thinningnature and a cyclic occurrence of bioclastic arenaceouslimestone, coarse sandstone–silty clay and evaporitic lay-ers with frequent minor–significant erosional surfaces.

Fossils and age. TEWARI et al. (1996) recorded fora-miniferal population of the Rotalipora appenninica in-terval Zone to Rotalipora cushmani total range Zone,the belemnite species Tetrabelus seclusus, Parahebolitesblanfordi and Neohibolites sp., and mid Cretaceous phy-loceratid ammonite along with shell debris of Exogyra,

Alectryonia, echinoids and bryozoans. According to theseobservations, this member ranges in age from Late Al-bian to Late Cenomanian.

Thickness. 60 m.

Karai Formation

This formation has two members, namely: the Gypsi-ferous Clay Member and the Odiyam Sandy Clay Mem-ber, in stratigraphic order.

Gypsiferous Clay Member

Nomenclature. The nomenclature of RAMASAMY &BANERJI (1991) was restored with an amendment toreflect its lithology.

Stratotype. It is located between 78°54’33”–78°54’58”E; 11°07’22”–11°08’09” N in stream section exposures ofa traverse from east of Karai to west of Kulakkanattam.

Lithology, contacts and environment. This memberconsists of unconsolidated deep marine clays and siltyclays. Very thin to thick gypsiferous (synsedimentary)clay beds characterize this formation. These beds con-tain a thick population of belemnite rostrum and phos-phate nodules. The extensive occurrence of gypsumalong with celestite makes this member readily recog-nizable in the field. The deposition of this member tookplace under increasing sea level conditions. After reach-ing its maximum, the sea level started declining concur-rently with the deposition of relatively coarser siliciclas-tics and the development of syngenetic gypsum ceasedto continue, marking the end of the deposition of thismember. While a non-depositional unconformity surfaceseparates this member from the underlying member, theupper contact is non-depositional and erosional.

Geographic extension. This member is best exposedalong the Karai–Kulakkanattam road section, where anextensive sprawl of badland topography occurs. Fromsouth to north, gradual reductions of depth, number ofbelemnite and phosphate nodules and frequency of gyp-sum layers are observed.

Fossils and age. Gryphaea, Tubulostrum, belemnitesand foraminiferal assemblages belonging to Rotaliporasubticinensis interval Zone to the Praeglobotrancanahelvetica interval Zone are reported and demonstrate Ce-nomanian age for this member (GOVINDAN et al., 1996).

Thickness. 275 m.

Odiyam Sandy Clay Member

Nomenclature. Proposed herein, amending the no-menclatures of RAMASAMY & BANERJI (1991; UpperSandy Clay Member) and TEWARI et al. (1996; OdiyamSandstone Member) to maintain uniformity in the strati-graphic nomenclature and to reflect the lithology.


Stratotype. It is located between 78°54’20”–78°55’58”E; 11°07’18”–11°07’22” N in the exposures of streamsections along the Karai–Kulakkanattam road.

Lithology, contacts and environment. These are es-sentially silty clays and sandy clays deposited in a pro-gressively shallowing basin. This member differs fromthe lower member in terms of the conspicuous absenceof belemnites and the occurrence of ammonites. Loadstructures and syndepositional slump folds are also ob-served. There seem to have been a sudden change of thesedimentary source, as well as of the quantity and natureof the sediment influx between the lower member and up-per member of the Karai Formation. The upper portion ofthis member has localised pockets of fine sandstone alongwith ammonites. While the lower contact with the under-lying member is conformable, the upper contact is ero-sional and signifies a major regression and unconformity.

Geographic extension. This member is absent in thesouthern part of the study area and has maximum occur-rence in the northern regions. The beds progressivelythicken from south (Karai) to north (west of Kunnam) andsuddenly thin out to become absent further east and north.The beds have regionally varying proportions of a sandand silt admixture and also contain petrified wood logs,the patchy occurrence of bivalve shells and shell frag-ments. Lenses of calcareous silty clay and siltstone arealso observed in the middle portion of this member.

Fossils and age. The fossil population of this mem-ber includes Thalassinoides, Pecten, Exogyra, Alectryo-nia, ammonites, foraminifers and serpulids. GOVINDAN

et al. (1996) recorded a distinct faunal assemblage ofspecies of Rotalipora, Preaglobotruncana, Whiteinellaand Hedbergella and regard this lithological unit tohave been deposited during Cenomanian–Early Turoni-an. TEWARI et al. (1996) also determined the same agefor this member based on the occurrence of Whiteinellaarahaeocretacea (PASSAGNO).

Thickness. 175 m.

Garudamangalam Formation

Following BLANFORD (1862), SUNDARAM & RAO (1986)recognised this formation as Trichinopoly Group. Thisformation, exposed in and around Garudamangalam andthe town Trichinopoly located about 40 km away, restover Archaen rocks and thus, KOSSMAT (1897) namedthis unit as Garudamangalam. Yet, many authors con-tinued using the old terminology, which made BANERJI

(1983) and RAMASAMY & BANERJI (1991) to restore thenomenclature of KOSSMAT (1897). This formation com-prises three members.

Kulakkanattam Sandstone Member

Nomenclature. TEWARI et al. (1996) proposed thisnomenclature and defined this member. In the present re-

vision, the top beds, described by them as carbonate con-cretions, have been excluded. This member correspondsto the Kottarai Member of RAMASAMY & BANERJI (1991).

Stratotype. It is located between 78°56’22”–78°57’06”E; 11°07’22”–11°22’23” N in exposures along the Ka-rai–Kulakkanattam road.

Lithology, contacts and environment. The rocks of thismember are massive, yellowish brown ferruginous sand-stones and contain abundant admixtures of silt and clay.Localised concentrations of shell fragments, bivalves andgastropods (resembling a shell bank typical of subtidal tointertidal coastal lagoons) and ammonites are common.BANERJI et al. (1996) interpreted this unit as having beendeposited under a high-energy wave regime. A significantfeature of this member is that the sandstone containsabundant wood fragments (in which a high concentrationof bivalve boring and oyster encrustation are observed),which vary in size between 5 cm length and 2 cm diam-eter to 18 m length and 3.5 m diameter, at places foundto be fit enough to term them as woody sandstone (Fig.3D). Cross bedding, channel courses (1 m deep and sev-eral meters across), planar bedding and feeding traces arealso common in this member. The depositional surfacewas strongly bioturbated and riddled roots (BANDEL,2000). The rocks are well cemented. In the Karai–Kulak-kanattam traverse, an angular erosional unconformity wasfound to separate this member from the underlying Karaiformation. BANERJI (1972) and BANERJI et al. (1996)recognised this contact as a transgressive onlap.

Geographic extension. This member is totally absentin the northern and southern ends of the study area.From the central south part, it thickens towards the northand from the central north part, it thins down to becomeabsent in the northern extreme. The population of gas-tropod and bivalve shell fragments is very high in themiddle portion. The concentration of large – small woodtrunks and fragments is higher in the southern region.While the northern and southern regions show abundantargillaceous admixtures, the middle region always showsa coarse grain supported fabric and a bedded nature withfrequent bioclastic arenaceous limestone lenses.

Fossils and age. Abundant wood fragments encrust-ed with oysters, Pinna, Thalassinoides, Diplocraterion,Ophiomorpha and others molluscs. Age ranges fromMiddle–Late Turonian.

Thickness. 123 m.

Grey Sandstone Member

Nomenclature: Proposed herein.Stratotype. It is located between 79°00’02”–79°00’56”

E; 11°00’42”–11°01’24” N in the exposures of the Kal-padi–Ariyalur traverse (Fig. 3; traverse No. IV).

Lithology, contacts and environment. Rocks of thismember are highly compact, well cemented, massive andhave vertical burrow structures. The member has cyclic,parallel and even bedded alternative layers of barren and


highly fossiliferous (small molluscs and whole bivalve andother shells and fragments) sandy layers (Fig. 4A) thethickness of which varies regionally. The sands are wellsorted and well rounded. These sediments represent sub-tidal to supratidal sand bars deposited under wave and tidedominated environments. This member rests conformablyover the Kulakkanattam Member and has a distinct ero-sional and non-depositional surface. The upper contact isa non-depositional surface associated with marine flood-ing. The deposition of this member immediately over sub-tidal flat sediments may indicate an advancing deltaicplain due to increased flow and or a stable/regressing sea.The presence of normal grading, cyclic sedimentation,repetitive occurrence of fossiliferous and barren layers ofdifferent thickness and planar horizontal bedding are allindicative of a foreshore environment, wherein the sedi-ment influx was controlled by the relative intensity of afluvial source, often reworked during periods of dry sea-sons. The difference in the cemented nature of this mem-ber compared to the overlying and underlying membersconfirms the assertion that the sediments of these rockswere exposed and cemented periodically under subaerialconditions, to form beach rocks. At the base, in the north-ern and southern regions, the deposition of this memberstarted with calcareous cemented pebbly sandstone. TE-WARI et al. (1996) interpreted this unit as a sign of depo-sition during transgression in a tidal inlet or a delta plain,or within a barrier succession. BANDEL (2000) interpretedthis pebbly sandstone as a shift of the principal depocen-ter connected with regression and a structural disturbance.In either case, the deposition of this member represents alowstand system tract above the Karai sequence boundary.

Geographic extension. The shelly layers show regio-nal variation of faunal and bioclastic composition. In thesouth, the layers constitute essentially gastropods and inthe north bivalves. Discrete layers of well-sorted sandswith shell fragments in the form of shell lag depositsare also observed. The increase of argillaceous admix-ture towards the upper part of this member indicates adeepening of the depocenter, may be because of marineflooding. In the southern part, the beds are thin and sili-ciclastic. Towards the north, they become more calcare-ous, thick bedded and at places have a very high con-tent of bioclastic materials, which made earlier workersregard this unit as shell limestone (e.g. RAO, 1970).

Fossils and age. Teredolites burrows, feeding tracesof Thalassinoides, oysters, gastropods and molluscs.YADAGIRI & GOVINDAN (2000) assigned Late Turonianas the age of these beds.

Thickness. 35 m.

Anaipadi Sandstone Member

Nomenclature. Authors: TEWARI et al. (1996).Stratotype. It is located between 78°57’40”–78°59’35”

E; 11°07’06”–11°07’21” N in the exposures north ofAnaipadi–east of Kulakkanattam and west of Kulattur.

Lithology, contacts and environment. The Grey Sand-stone Member is conformably overlain by dirty yellowsilty sandstone which contains abundant ammonites(many of which are more than 1 m in diameter), Nau-tilus, brachiopods (rhynchonellids), molluscs and boredwood fragments. The massive nature of the rocks, fos-sil association and lithological characteristics indicate,probably deeper marine conditions. The occurrence oflithoclastic cobbles in the middle portion of this mem-ber suggests the prevalence of periodic high-energyconditions, continental erosion and increased terrestrialsediment influx. The presence of intercalated beds withrich fauna dominated by oysters or rhynchonellids indi-cates the exchange of shallow water environment foropen marine condition (BANDEL, 2000). The depositionof this member was truncated by a major regression.

Geographic extension. These are massive and thin-bedded claystones, silty claystones and clayey sandsto-nes in the south that gradually grade to silty clay inthe southern center and thin down. Again, from there,the thickness of these beds and sediment grain size in-crease and contain abundant large ammonites. Furthernorth, these were observed to be clayey siltstones withabundant shell fragments and ammonites.

Fossils and age. A rich assemblage of Coniacian am-monites represented by Kosmaticeras gr. theobaldianum,Kossmaticeras theobaldianum crassicostata, Puzosia sp.,Damesites aff. sugata and other fossils viz., Nautilus, mol-luscs, bored woods, encrusting oysters and rhynchonellidsis present in this member, all typical of Coniacian age. Thismember corresponds to the Kossmaticeras theobaldinum am-monite Zone of AYYASAMY (1990) and the Marginotruncanaplanktonic foraminiferal Zone of GOVINDAN et al. (1996).

Thickness. 215 m.

Ariyalur Group

The Ariyalur group is distinct from rest of the strati-graphic units of the Cauvery basin in terms of litholog-ical and faunal diversity. Although BLANFORD (1862)described this stratigraphic unit into a group, it wasSASTRY et al. (1964, 1968, 1972) who assigned a system-atic stratigraphic nomenclature. Yet, different workerscontinued utilizing varied terminologies and classificationsystems that posed difficulties in recognising them in thefield. Later, CHANDRASEKARAN & RAMKUMAR (1995) re-viewed all the existing classification systems of this groupand supplemented additional field, petrographic, faunaland sedimentologic data. Their study confirmed the appli-cability of the classification reported by SASTRY et al.(1972) through biostratigraphic, chronostratigraphic andlithostratigraphic boundaries. These are detailed herein.

Sillakkudi Formation

This formation consists of three members.


Varakuppai Lithoclastic Conglomerate Member

Nomenclature. It is proposed herein. This member in-cludes the lower beds of the Upper Sandstone Member ofSUNDARAM & RAO (1986) and an unnamed fluvial siltysandstone of TEWARI et al. (1996). These publicationsmention only a single exposure and there was not muchinformation on the extent, lateral variations and other char-acteristics. The detailed mapping conducted in this studyallowed a precise description of the rocks and enabledthem to be classified into a formal stratigraphic unit.

Stratotype. It is located at 78°55’06” E; 11°01’23” Nin exposures of a stream section west of Varakuppai.

Lithology, contacts and environment. Following thedeposition of the Anaipadi Member, there was a majorregression, possibly associated with the reactivation ofbasement faults resulting in the movement of the shore-line towards former offshore regions. The period of reju-venated significant sedimentation after this regressionwas primarily under a fluvial regime and the fluvialagent was turbulent enough to transport well-roundedbasement rocks, quartzite and older sedimentary rockboulders (Fig. 4B; lithoclastic boulders) in addition tounsorted coarse sand-pebble sized siliciclastics into thenew depocenter. These deposits are typically reverse

graded and show cyclic bedding, large scale cross bed-ding and lack any body fossils. They rest over older sed-imentary rocks with typical erosional surfaces. The ero-sional intensity was so high that these beds have directcontact with the Karai Formation, thus giving testimonyto the prevalent large scale undercutting, recycling andtransporting nature of the fluvial channels. Three suchmajor paleochannel courses were recognised with thehelp of remotely sensed data and field mapping (Fig. 5).

The occurrence of large scale cross bedding, muddrapes, fresh feldspar and sandstone clasts made TEWARI

et al. (1996) interpret this unit as a fluvial channel fillwith some marine tidal influence during periods of sea-sonal change in discharge. The cross bedded sequence haslarge lithoclasts of basement boulders along with claystoneboulders and this unit is underlain by large scale cross-bedded, coarse lithoclastic sandstone which contains insitu burrow filling (Thalassinoid - Fig. 4C). These obser-vations confirm marine influence over a drowning rivermouth. The lower portion of this member was devoid ofmarine influence. The inference of a progressively increas-ing marine influence towards the top (inferred from theoccurrence of sedimentary structures and Thalassinoid,typical of shallow coastal regions) of the member indi-cates the presence of a sequence boundary at its base.


Fig. 4. A. Alternate barren and shell rich thin beds of the Grey Sandstone Member. Southwest of Anaipadi Village. B. Sub-rounded lithoclastic boulder in the Varakuppai Member. West of Varakuppai village in a stream section. C. Thalassinoid bur-row in the upper portion of the Varakuppai Member. West of Varakuppai village in a stream section. D. Gravel sized feldsparclast in the Kallar Member. West of Kallankurichchi village in a stream section.

Geographic extension. These beds are typically exposedin the regions south and northwest of Alundalipur, northof Melarasur and south and west of Sadurbagam in streamand road sections. Further east, north and south, they rap-idly die out. In a stream section across a bridge whichconnects Alundalipur–Kallakkudi, these are excellentlypreserved and rest directly over the Karai Formation.

Fossils and age. No in situ fossils were observed,except a few Thalassinoides in the upper portion of themember. Subsurface sections indicate a Marginotrunca-na coronata – Dicarinella asymmetrica zone (GOVIN-DAN et al., 1996) of Santonian age.

Thickness. 45 m.

Sadurbagam Pebbly Sandstone Member


in a stream section east of Sadurbagam.

Lithology, contacts and environment: The contact be-tween the Varakuppai and the Sadurbagam Members isa marine flooding surface. The resting of the Sadur-bagam Member over Archaen rocks and the VarakuppaiMember clearly affirms that, on flooding, areas that hadnot been previously transgressed were brought under ma-rine conditions. SUNDARAM & RAO (1986) also stated that

the regions had remained structurally and topographical-ly high since the inception of this basin were broughtunder marine influence during Late Santonian–EarlyCampanian as a result of a fault movement. At the top,a distinct non-depositional surface separates this memberfrom the overlying member, which could be interpretedas the maximum flooding surface created at the dawn ofthe Campanian. Coarse siliciclastics with abundant ma-rine fauna, shell fragments, and varying proportions of acalcareous matrix were deposited concomitantly with therising of the sea level along the western margin of thenewly established coasts which forms this member. Atthe base of this member, an erosional surface followed


Fig. 5. Geographic distribution ofthe members of the Cauvery Basin.Legend:F1 = Initial block faulting, F2 = Faulting during Cenomanian, F3 = Faulting during Santonian, F4 = Faulting during post Danian,1 = Unclassified gneiss,2 = Charnockite & Gneiss,3 = Basal Conglomerate Member,4 = Kovandankurichchi Member,5 = Terani Member,6 = Grey Shale Member,7 = Varagupadi Member,8 = Dalmia Member,9 = Olaipadi Member,

10 = Kallakkudi Member,11 = Gypsiferous clay Member, 12 = Odiyam Member,13 = Kulakkanattam Member,14 = Grey Sandstone Member,15 = Anaipadi Member,16 = Varakuppai Member,17 = Sadurbagam Member,18 = Varanavasi Member,19 = Kallar Member,20 = Kattupiringiyam Member,21 = Tancem Member,22 = Srinivasapuram Member,23 = Ottakoil Formation,24 = Kallamedu Formation,25 = Anandavadi Member,26 = Periyakurichchi Member,27 = Cuddalore Sandstone,28 = Alluvium.

by a distinct cobble-pebble quartzite conglomerate isobserved, which marks the beginning of the sedimenta-tion of this member. The rocks show massive, thick tomedium, even and parallel bedding and rest over theVarakuppai Member. At places, pockets of shell rich car-bonate lenses with an abundant siliciclastic admixture arefound to occur, which made earlier workers, (SUNDARAM

& RAO, 1986; TEWARI et al. 1996) interpret this mem-ber as Lower Limestone Member and Kilpaluvur Grain-stone Member, respectively. CHANDRASEKARAN et al.(1996) recorded load casts, slump folds, pillow structuresand synerasis cracks in this member. The sandstone bedsfrequently show normal grading and low angle crossbedding. The occasional development of algal mounds,intimately associated with fault scarps of older sedimen-tary rocks, is also found to signify this member. Thesedimentary structural and lithological information sug-gest a subtidal to intertidal clastic depositional environ-ment for this member.

Geographic extension. The thickness of this membervaries drastically in the south, may be reflecting thedepositional topography as created by the fault scarp.The rocks are monotonously pebbly sandstones withmassive bedding in the north and are continuously bor-dered by the Paleo Sea except in the region northwestof this member, where the younger Varanavasi Memberrests directly over the Anaipadi Member.

Fossils and age. The fossil composition of this memberincludes inoceramids, rhynchonellids, terebratulids, Nau-tilus, echinoids, crinoids, fragments of corals and bivalvesand algal mounds, indicative of Late Santonian age.

Thickness. 80 m.

Varanavasi Sandstone Member


in a stream section southwest of Varanavasi.Lithology, contacts and environment. These are planar

sheet sands, primarily consisting of featureless, massive,thick bedded, coarse to medium grained sandstones.They rest over the pebbly sandstone member with a non-depositional surface, may be due to flooding by marinewaters. AYYASAMY (1990) recognised a hiatus betweenSantonian (Sadurbagam Member) and this member basedon the ammonite zonation. The beds contain intermittentfine pebble-coarse sand laminae and show normal grad-ing to reestablish the deposition of thick, massive, medi-um sand deposits, probably representing periods of high-er energy and sediment influx. The sands are looselypacked and cemented by a calcareous medium. Towardsthe top, serpulid colonies are frequently observed whichsurvived in a slowly waning sea (RAMKUMAR, 1997).The upper surface of this member represents an erosion-al surface associated with regression.

Geographic extension. These are the most extensivelithologies in this part of the basin, developed mainly due

to a constant supply of sediment and stable shelf condi-tions, the duration of which was longer than before orafter. They show the development of monotonous, thickto very thick beds with coarse-medium sands in all theregions, except an argillaceous admixture and less grain-to-grain contacts in the north. Periodic higher energy con-ditions, marked by the deposition of very coarse sand-stone lenses, pockets of shell hash, intraformational litho-clastic bounders in association with vertical cylindricalburrows and resedimented petrified wood logs are observ-ed in the middle portion. In the type section of this mem-ber, thin to medium, cross-bedded siliciclastics are record-ed. This member has a more or less uniform thickness.

Fossils and age. Important fauna of this member in-clude inoceramids, serpulids, Turritella, Globotruncanaarca, Globigerinelloides, Marginotruncana marginata,Whiteinella baltica and Archaeoglobigerina and indi-cate Campanian age. GOVINDAN et al. (1996) recordedtypical Campanian benthonic foraminifer Bolivinoidesculverensis and B. decoratus in this member. Thismember also belongs to the Karapadites karapadenseammonite Zone (SASTRY et al., 1968; 1972) and theGlobotruncana elevata - G. ventricosa planktonic fora-miniferal Zone (GOVINDAN et al. 1996).

Thickness. 270 m.

Kallankurichchi Formation

Recently, RAMKUMAR (1999) reported the lithostrati-graphic setup of this formation with four members,which are detailed herein.

Kallar Arenaceous Member

Nomenclature. Amended with a prefix from the no-menclature of RAMKUMAR (1999).

Stratotype. It is located at 79°07’24” E; 11°08’32” Nin a Kallar river section.

Lithology, contacts and environment. This memberconsists of conglomerates in which clasts of basementrocks, fresh feldspar (Fig. 4D), resedimented colonies ofserpulids and other older sedimentary rocks, which rangein size from coarse sand to boulders, are observed. Theclasts are well rounded and show characteristics of rede-position. The beds show typical normal grading. The sed-iments were brought to the depocenter during high veloc-ity flows (sensu CANYBARE & CROOK, 1968) and are pri-marily channel deposits and coastal conglomerates (HART

& PLINT, 1995). The occurrence of redeposited lithoclastsof serpulid sandstone (which occurs in situ on top of theSillakkudi Formation) along with fresh feldspar cobblesindicates severe erosion of both older sedimentaries anddistally located basement rocks. The upper portion of thismember shows colonisation of gryphean shells and areduction of the proportion of siliciclastic sediment. Thelower contact of this member is an erosional surface. The


upper contact is a non-depositional surface resulting frommarine flooding. The occurrence of larger foraminifera inthis unit, as described by HART et al. (2000), and theircounterparts in ther neritic regime of modern seas indi-cates the initiation of the deposition of this member invery shallow bathymetry with a slow increase in the sealevel (as evidenced by the colonisation of gryphea overthe conglomerate bed).

Geographic extension. This member rests directly overthe Sillakkudi Formation and could be readily recognizedin the field by distinct changes in lithology, colour, fauna,sedimentary structures, etc. Laterally, the proportion ofsiliciclastics varies to produce localized lenses of silici-clastic conglomerates and limy conglomerates.

Fossils and age. Orbitoids and sideralolites constitutethe dominant microfaunal composition. Gryphaea andAlectryonia are the major fauna of this member. The fos-sil assemblage indicates Lower Maastrichtian age (RADU-LOVI] & RAMAMOORTHY, 1992). Based on the largeoccurrence of foraminiferal, HART et al. (2000) assignedvery uppermost Campanian to very Early Maastrichtianage and equated the initiation of the deposition of thismember with a global eustatic sea level rise.

Thickness. 6 m.Equivalents from other areas. Based on the study of

subsurface borehole samples, NAIR (1974, 1978) report-ed this unit 50 km south from the present study area.

Kattupiringiyam Inoceramus Limestone Member


Stratotype. It is located at 79°09’30” E; 11°07’31” Nin the Kattupiringiyam mine NE Wall, Bench I (mineowned by Fixit Co. Ltd.).

Lithology, contacts and environment. This memberconsists of dusty brown friable carbonate sands (Fig.6A) with parallel, even and thick to very thick bedding,which contain only inoceramids and bryozoans. Thismember has a pronounced diagenetic bedding andabundant geopetal structures filled with mm to cm sizeddog tooth spars of low magnesian, non-ferroan calcite.Based on standard microfacies analysis, RAMKUMAR

(1995) interpreted this member as having been deposit-ed in a middle to outer shelf. This member has a non-depositional surface at the bottom and a distinct ero-sional surface at the top.

Geographic extension. This member is well develop-ed in the south and dies out in northern regions. Itshows monotonous, massive dusty brown limestonebeds with inoceramids all over the exposed regionswithout any lateral or vertical variation and also with-out any break in sedimentation.

Fossils and age. This members hosts thick popula-tions of inoceramids and bryozoans and was depositedduring Early Maastrichtian.

Thickness. 8 m.

Tancem Biostromal Limestone Member


Stratotype. It is located at 79°07’32” E; 11°09’26” Nin the Kallankurichchi Mine I, Bench I and II (mineowned by Tancem Ltd.).

Lithology, contacts and environment. The beds havelocal concentrations of fragments of Inoceramus, Gry-phaea, Exogyra, Stigmatophygus, Alectryonia, terabra-tulids and bryozoans, in the order of decreasing propor-tion, and are interpreted as having received their sourcematerials from adjacently located shell banks. Sporadicadmixture of siliciclastics and intraclasts are observed.The sorting is generally poor and the roundness variesfrom angular to well rounded. The size differs betweenfine sand to cobble and shows lateral and vertical gra-dation. These are the products of periodic and longlasting high-energy conditions, related with temporal re-gression, storm events and associated erosion and rede-position. They show a thin to thick, parallel, even bed-ded nature, cross bedding, normal grading, hummockycross stratification, feeding traces, escape structures andtidal channel structures and range in depositional envi-ronment from tidal channel to middle shelf. This mem-ber rests over the Inoceramids Limestone Member andthe gryphaean limestone beds of the Arenaceous Mem-ber with a distinct erosional surface. The upper surfaceis non-depositional and at places erosional.

Geographic extension. These beds occur all over theexposed area. They show lateral variation in composi-tion and concentration of shell fragments.

Fossils and age. Except Stigmatophygus and Planoli-tes, no whole fossils are present in this member. MI-TROVI]-PETROVI] & RAMAMOORTHY (1993) interpretedthis unit as having been deposited during Early Maas-trichtian.

Thickness. 8 m.Equivalents from other areas. These are the frag-

mental limestones of NAIR (1974, 1978) who reportedthem 50 km south of this location and also from off-shore bore wells.

Srinivasapuram Gryphaean Limestone Member


Stratotype. It is located at 79°07’38” E; 11°09’44” Nin the Kallankurichchi mine II, northern wall, Bench Iand II (mine owned by Tancem Ltd.).

Lithology, contacts and environment. This memberconsists of uniform, parallel, thick-very thick-beddedgryphaean shell banks which have a very high popula-tion of Gryphea (Fig. 6B), Exogyra, terebratulids, bry-ozoans and sponges. Non-depositional surfaces are ob-served occasionally in this member in an otherwisecontinuous, massive unit. A shoreward movement of


the shell banks is observed which may be interpretedas adjustments of shell colonies in response to a preva-lent sea level rise. The upper contact of the membershows a distinct erosional surface. Based on the occur-rence of extensive boring in the gryphaean shells,synsedimentary cementation, colonies of encrusting bry-ozoa over gryphaean shells and micritization of bio-clasts, as well as the lifestyle of gryphaeans, RAMKU-MAR (1996, 2000) interpreted this member as havingbeen deposited within 50 meters bathymetry under opensea conditions in a distally steepened ramp setting.

Geographic extension. These are the most extensivebeds of this formation and have a distinct very highpopulation of Gryphaea. Towards the top, admixture ofsiliciclastics and an increased proportion of shell frag-ments were observed, which indicate a fall in the sealevel and an associated increase in energy as a resultof which shell banks were eroded and redeposited.

Fossils and age. Gryphaea, Alectryonia, terebratu-lids, bryozoans, ostracods, sponges and ammonites. RA-DULOVI] & RAMAMOORTHY (1992) assigned Lower Ma-astrichtian age to this member. This member belongsto the Hauriceras rembda ammonite Zone (SASTRY etal., 1972; AYYASAMY, 1990).

Thickness. 18 m.Equivalents from other areas. NAIR (1974, 1978)

reported these beds from 50 km south of this locationand also from offshore regions.

Ottakoil Formation

Nomenclature. After SASTRY et al. (1972) and CHAN-DRASEKARAN & RAMKUMAR (1995).

Stratotype. It is located at 79°07’12” E; 11°01’26” Nin a stream section east of Ottakoil.

Lithology, contacts and environment. These are coar-se to medium sized, well-sorted, fossiliferous, low an-gle cross-bedded and planar to massive bedded sand-stones. They also show recurrent upward fining sequen-ces. The rocks contain abundant echinoids and a fewtrace fossils. The rocks are loosely cemented with acalcareous medium. These lithological and faunal char-acters indicate deposition in marginal marine settings ina shallowing basin. This formation rests over the Kal-lankurichchi Formation with disconformity and overlap-ping by the Kallamedu Formation. This formationmarks the end of marine Cretaceous deposition in thispart of the basin.

Geographic extension. It developed only in the cen-tral region of the exposed area and ubiquitously con-tains Stigmatophygus elatus in thick populations. Thecalcareous admixture varies regionally. The lower por-tion of this member shows a medium-bedded naturethat grade to a massive and very thick-bedded naturetowards the top.

Fossils and age. The faunal composition includesStigmatophygus elatus, Durania mutabilis, Thalassino-


Fig. 6. A. Carbonate sand beds (a) overlain by a fragment-ed shell limestone bed (b) with a sharp erosional surfacein between (pen placed for scale rests on this surface, indi-cated by an arrow). These are the products of short–longlived periodic high energy conditions prevalent during thedeposition of the Tancem Member of the KallankurichchiFormation. Tancem mines, west of Kallankurichchi village.B. Thick population of insitu gryphean shells. These con-stitute the bulk of the Srinivasapuram Member of theKallankurichchi Formation. Tancem mines, south of Srini-vasapuram village. C. Conformable, but erosional surfacecontact between the Kallankurichchi Formation (a) and theKallamedu Formation (b) as exposed in a mine locatedsoutheast of the Kallankurichchi Formation. Note the scourstructure, (indicated by an arrow) presumable representinga cross section of a fluvial channel, in which the poorlyconsolidated sandstones were filled.

ides, Ophiomorpha, Dactyloidites, Nautilus and frag-ments of Gryphaea and Alectryonia. RADULOVI] & RA-MAMOORTHY (1992) and MITROVI]-PETROVI] & RAMA-MOORTHY, (1993) interpreted this formation to havebeen deposited during middle Maastrichtian. This for-mation belongs to the Pachydiscus otacodensis ammo-nite Zone (AYYASAMY, 1990) and the G. gansseri fora-miniferal Zone (GOVINDAN et al., 1996)

Thickness. 40 m.Equivalents from other areas. NAIR (1978) reported

the occurrence of this formation in offshore regionsthrough a study of borehole samples.

Kallamedu Formation

Nomenclature. After Sastry et al. (1972) and CHAN-DRASEKARAN & RAMKUMAR (1995).

Stratotype. It is located at 79°08’39” E; 11°11’46” Nin stream sections north and northeast of Kallamedu.

Lithology, contacts and environment. This formationconsists of loosely packed sands drawn from older sedi-mentary rocks. The sediments are well rounded, poorlysorted and non-consolidated and are barren except for afew fragments of dinosaurian bones. The local occurrenceof clays and silts with dispersed detrital quartz grains andsandy streaks, its non-bedded nature, rare lamination andmud cracks are all indicative of prevalent sedimentationas over bank deposits associated with a channel. This for-mation primarily consists of white, coarse to mediumgrained immature, often pebbly sands with a clay matrixand localized clay lenses. These characteristics indicateprimary deposition in and around shallow channels (Fig.6C). Towards the top, this formation shows the develop-ment of soil and a return to continental conditions. Thus,the upper surface represents a non-depositional and ero-sional surface and also forms a sequence boundary.

Geographic extension. Next to the Sillakkudi Forma-tion, this is the most extensive formation of the AriyalurGroup. It covers the whole region between theKallankurichchi and Niniyur Formations. It shows de-velopment of monotonous, massive, loosely packed sandswith localised and isolated patches of clayey soil. In thebottom portion, along the central region of the study area,higher frequencies of texturally matured resedimented peb-bles are observed along with a few dinosaurian bone frag-ments. Towards the top, these rocks grade from mediumto thin bedded, relatively highly argillaceous sandstones.

Fossils and age. Relatively barren except for a fewdinosaurian bone fragments belonging to Carnosaur(YADAGIRI & AYYASAMY, 1987). MITROVI]-PETROVI] &RAMAMOORTHY (1993) and GOVINDAN et al. (1996)assigned Late Maastrichtian age to this formation. Thisformation represents the Abathomphalus mayoroensisplanktonic foraminiferal Zone (TRIPATHI & MAMGAIN,1987) and is consistent with Upper Maastrichtian paly-noassemblage (VENKATACHALA & SHARMA, 1974) whichincludes Aquilapollenites bengalensis, Cranwellia cau-

veriensis, Araucariacites australis, Tricolpites microre-ticulatus and Triporopollenites minimus (SAHNI et al.,1996).

Thickness. 100 m.Equivalents from other areas. MASTHAN (1978) re-

ported subsurface extension of this formation in off-shore regions.

Niniyur Formation

At the beginning of Danian, sedimentation startedagain to be associated with a rise in the sea level inthis basin. The presence of a conformable relationshipbetween this formation and the Kallamedu Formationsuggests the absence of major tectonic activity andindicates simple sea level fall and sea level rise. Thisformation comprises two members.

Anandavadi Arenaceous Member


in stream sections south and northeast of Anandavadi.Lithology, contacts and environment. It consists of

isolated coral mounds, impure arenaceous limestone,sandstone and clay lenses. The deposition of this mem-ber might have taken place in a restricted marineregime under subtidal to intertidal regions. The occur-rence of localised concentrations of shell fragments,well-indurated coralline limestone and reef-derived ta-lus deposits are characteristics of this member. Thismember rests over the Kallamedu Formation with a dis-tinct disconformity and is exposed in stream sections,road cuttings and well cuttings located in the southernpart of the study area. The upper contact is an erosion-al surface associated with marine flooding.

Geographic extension. While it is principally isolat-ed coral algal colonies, they are connected through thinveneers of bioclastic and arenaceous counterparts in thesouth. More arenaceous coral algal limestone depositssignify this member towards the north.

Fossils and age. SASTRY & RAO (1964) recorded adrastic decrease in fossil population from the underly-ing Cretaceous beds to this member. The fossil compo-sition consists of corals, algae, serpulids, bivalves andgastropods. This member was deposited during Danian(SAHNI et al., 1996).

Thickness. 30 m.Equivalents from other areas. NAIR & VIJAYAM

(1980) reported an extensive occurrence of this mem-ber in offshore subsurface regions.

Periyakurichchi Biostromal Member

Nomenclature. It is proposed herein.


Stratotype. It is located at 79°12’38” E; 11°17’54” Nin the southern wall Bench I, II and III of the Periy-akurichchi Mine (mine owned by India Cements Ltd.).

Lithology, contacts and environment. This is themost well-developed member of this formation andconsists primarily of medium to thick, even to parallelbedded alternate biostromal limestones and marls.Different concentrations of shell fragments and wholeshells of bivalve, gastropod and remains of amphibia,pisces, algae, foraminifera and ostracoda are alsoobserved. The deposition of this member occurred dur-ing a sea level highstand under open marine conditions.At the top, the sudden truncation of deposition fol-lowed a fall in the sea level and widespread erosion,the restoration of a continental environment and theprevalence of a tropical climate are evidenced by thepresence of the Cuddalore Sandstone Formation ofMiocene to Pliocene age with distinct unconformityover this member. The end of the deposition of thismember marks the end of marine sedimentation in thispart of the basin.

Geographic extension. Good exposures of this mem-ber are found in the Periyakurichchi and Ichangadumines. Beds of this member are traced to the east oflower members and they gradually thicken eastwardsand northwards. Contact between this member and theunderlying member is an erosional surface. A thick are-naceous bioclastic limestone bed rests over this surfaceand towards the top, alternate limestone and marl bedsof medium, parallel, even bedded nature typify thismember. Good exposures of this member are limitedonly to mine sections owing to the presence of verythick piles of overlying Cuddalore Sandstone Formationand Recent alluvium.

Fossils and age. Fossil assemblage of this membercomprises of algae, corals, bivalves, amphibiae, piscesand gastropods. Fossil assemblage with the nautiloidHercoglossa danica indicates Danian age (NAIR & VI-JAYAM, 1980; SAHNI et al., 1996).

Thickness. 26 m. Equivalents from other areas. NAIR & VIJAYAM (1980)

reported extensive occurrence of this member in offshoresubsurface regions.

Discussion on systematic stratigraphy

While the present study enables the exposed strata tobe defined into 22 members and 9 formations, it was notfelt possible to “accommodate” them into groups, as theexisting terminologies of groups are either non-relevantor rest over Archaen rocks. For example, the UttaturGroup as envisaged by TEWARI et al. (1996), is foundto be untenable. While TEWARI et al. (1996) eliminat-ed the use of the term Trichinopoly on the grounds thatit is far from the exposed area and rests over Archaenrocks, they themselves termed the Dalmiapuram andGarudamangalam Formations as the “Uttatur” Group

after the village name “Uttatur”, which in turn restsover Archaen rocks. In another case, a single formation(the Sivaganga Formation), which too is not typical ofits original characteristics of the type area (Sivagangaarea located south of the present study area) could notbe assigned with a Group as was done by TEWARI etal. (1996). The established guidelines of lithostratigra-phy specifically state that “while the members need tobe grouped under a formation, it is not necessary to putdifferent formations into a group” (EYESINGA, 1970;North American Stratigraphic Code, 1983). Thus, weexclude the terms Gondwana Group (assigned to theSivaganga Formation) and the Uttatur Group, (assignedto the Dalmiapuram and Garudamangalam formations)of TEWARI et al. (1996). While their study largely erad-icated the discrepancies existing in the literature, iterred in small counts. As reported by them in their pa-per, their study was limited only to “key locations” and“rarely individual beds are traced”, which in turn hasnecessitated the present study. Any meaningful litho-stratigraphic report should contain information on litho-logic types, their lateral and vertical variations, faunaand other criteria, such as sedimentary structure andstratigraphic breaks, which could be recognized andtraced in the field are necessary. The mapping of thestudy area on a 1:50 000 scale and the tracing of indi-vidual beds all over the exposed area (assisted by theavailability of newer mines, trenches and road sec-tions), allowed a more accurate documentation of thestratigraphic setup of the area. The total thickness ofthe exposed strata in the study area is 1696 m. How-ever, the subsurface counterparts of these strata accountfor about 5000 m and, hence, this onland exposurecould be considered as a condensed section. GOVINDAN

et al. (1996) state that in the deeper part of the basin,the strata are similar to the ones established in theTiruchirapalli–Ariyalur outcrops, except for the thick-nesses of the individual beds. Thus, the revised lithos-tratigraphic classification presented in this paper couldalso be put to use for offshore subsurface counterparts.

Depositional history

The rift between India–Australia–Antarctica duringLate Jurassic–Early Cretaceous resulted in block faultingof Precambrian terrain of India, creating a series of sed-imentary basins along the east coast, among which theCauvery basin is the southernmost sedimentary basin.The basement tectonics, as enumerated by PRABAKAR &ZUTSHI (1993), details that this basin continued to evolveuntil the end of Tertiary through rift, pull-apart, shelf sagand tilt phases, during which many episodes of trans-gression, regression, erosion and deposition took place tofill the basin. Seismic sounding and deep borehole logdata revealed that this basin consists of a number of sub-basins (Fig. 7), differentiated by many highs (SASTRY etal., 1977; KUMAR, 1983). The Pondicherry sub-basin is


the northernmost and contains three main exposuresmappable in outcrops. The structural grains mapped inthe field are presented in Fig. 5. It is a general phenom-enon in this basin that the folding and fracturing of therocks were always associated with faulting. Based onfield criteria and lithological association and displace-ment, the basin scale tectonic movements that occurredin the study area were interpreted viz., initial block fault-ing (Refer F1 in Fig. 5), movement of fault blocks dur-ing Cenomanian (F2 in Fig. 5), reactivation of older faultblocks and creation of new fault during Santonian (F3in Fig. 5) and reactivation of fault blocks during post-Danian–pre-Quaternary (F4 in Fig. 5). It should be stat-ed that, in addition to these major fault movements, therewere minor and local scale tectonic movements, all ofwhich were confined only to the adjustment of faultblocks along the pre-existing fault planes. The differencein trends of post-Danian fault movements affected theMiocene to Pliocene sandstones, Danian limestones andMaastrichtian deposits, as depicted in Fig. 3. The sedi-mentary response to the prevalent major tectonic andother geologic processes in the light of present observa-tions are detailed herein.

All along the western margin of the basin, the base-ment rocks show typical northeast to southwest faultlines, which mark the initial block faulting. As a result

of this faulting and the associated down-throw of thePondicherry sub-basin, widespread transgression wasaffected and the sedimentation of the Sivaganga For-mation commenced. The Precambrian basement rocks,located farther west of the basin margin fault lines,were severely eroded and transported to the depocenterbefore inheriting alteration and maturity and, thus, freshsub-angular basement rock boulders and cobbles withfeldspathic pebbles are found to occur in this forma-tion. As the intensity of the energy conditions were re-duced, the sediment grain size was also reduced. Signi-ficant sedimentation commenced with the establishmentof a fluvial source onland and a submarine fan delta inthe basin (represented by the Kovandankurichchi Sand-stone Member). Gradation of this sandstone memberinto a deep marine claystone–siltstone member (the Te-rani Member) indicates stable environmental conditionsup to the end of the deposition of the Terani Member.The deposition of the Terani member was brought toan end due to renewed faulting introducing an angularunconformity associated with erosion and resedimenta-tion of older sedimentary rocks. The rejuvenated sedi-mentation was through the deposition of shale and sha-le–limestone alternate beds of the Grey Shale Memberof the Dalmiapuram Formation. The depocenter waspartially and periodically closed, during which timegrey shale was deposited. Whenever open conditions ofsea circulation were introduced, bioclastic limestonebands were deposited. The thickness of the limestoneinterbeds increases towards the top, indicating an incre-ase in the duration of the openness of the sea, whichculminated in the development of a biostromal memberover it. This biostromal member contains principallycoral clasts and algal fragments with varying propor-tions of clasts of bryozoa, bivalvia and gastropoda, inaddition to reef-dwelling microfauna. Siliciclastic ad-mixture is significant to minor in proportion and varyrandomly. The beds of this member are parallel, evento uneven, thin to thick bedded and have frequent ero-sional surfaces in between. All these features signifydeposition under a photic zone and the principal depo-sitional loci to be between subtidal and storm weatherwave base. Typical coral reef deposits developed overthis member that moved gradually towards offshoreregions owing to a fall in the sea level. At the top ofthis biohermal limestone member, a major erosional sur-face associated with faulting and regression is observ-ed. This faulting exposed the subtidal–storm weatherwave base deposits to subaerial conditions which led tokarstification. It also paved the way for the depositionof the Olaipadi Conglomerate Member which containslarge boulders (many of which are more than 10 m indiameter) of basement rocks, lithoclasts of similar size,drawn from underlying bioclastic and coral limestone,Terani claystone and lithoclasts of older sedimentaryconglomerates, all embedded in basinal clay sediments.The angular to sub-rounded nature of the boulders, thepossession of basement as well as lithified lithoclasts


Fig. 7. Structural configuration of the Cauvery Basin (afterCHANDRA, 1991).

of older sedimentary rocks in the basinal sedimentsclearly indicate a major faulting event, the creation ofa steep slope and a short transportation distance. Thepresence of argillaceous siltstone over these boulderswith lamination parallel to the boulder boundaries indi-cates the restoration of normal depositional conditionsand a gradual increase in the sea level. The depositionof argillaceous sediments was ended by rejuvenation ofthe fluvial source, resulting in a persistent influx ofcoarse to finer clastics and a suspended sediment load.This new set of environmental conditions led to thedeposition of the Kallakkudi Calcareous SandstoneMember. This member is sandy in the southern regionand clayey in the northern region. The occurrence ofrecurrent Bouma sequences, which always top with agypsiferous layer, followed by an erosional surface andagain by an another Bouma sequence in this memberindicates the dynamic nature of the depocentre, theepisodic closure of the sea, the exposure of sedimentsto subaerial conditions, rejuvenation of the depositionwith a rise in the sea level, deposition under the influ-ence of turbidity currents and the gradual facies changefrom near shore to deep sea. On the whole, the inter-pretation could be that the deposition of this membertook place in a slowly sinking basin and/or depositionwith episodic rises and falls coupled in the sea levelwith active fault block adjustment (to a minor degree)after major movement.

With the sinking of the coastal basin and/or sea levelrise, deep marine conditions were established and thickpiles of the Karai Formation clays were deposited. Thedeposition of about 450 m of thick clay alternated withferruginous silty clays and gypsiferous layers suggestsa well developed fluvial system onland which continu-ously supplied a suspended sediment load to deep ma-rine regions. The thick population of belemnites, siltyadmixture, alternate thin-thick laminae of ferruginoussilty clay and gypsiferous clay bands are frequent inthe southern region, which is indicative of the deposi-tion in shallower regions of palaeosea also. The shal-lower southern regions were periodically exposed sub-aerially due to minor sea level oscillations to produceevaporites. The top surface is marked by a pronouncederosional surface, which suggests major regression atthe end of the deposition of this formation. This ero-sional surface is overlain immediately by subtidal–su-pratidal ferruginous sandstones along with shell bankstypical of an estuary and shell hash typical of shallowwater shoals/distributary mouth bars that represent theKulakkanattam and Grey Sandstone Members of theGarudamangalam Formation. Together, their occurrenc-es indicate a retreat of the shoreline and an associatedadvancement of a fluvial system over the former off-shore areas. This inference is substantiated by the sud-den appearance of large tree trunks in these sandstones.Although the boundary between the Karai clays and theKulakkanattam sandstones is an erosional unconformi-ty, the presence of a conformable relationship and near

parallel bedding planes of rocks between them suggestsa simple variation of the sea level and the introductionof newer environmental conditions, rather than a fault-controlled environmental change across the boundary.Deep-water conditions were restored again in this partof the basin with the introduction of the deposition ofthe Anaipadi Sandstone Member which shows a grad-ual increase in the sea level. A break in the sedimen-tation, probably influenced by major regression waswitnessed at the end of the deposition of the AnaipadiMember.

Renewed transgression initiated during the MiddleSantonian covered the regions located north and souththat had not been transgressed previously. This wide-spread transgression, associated with downwarping offault blocks, submerged coastal tracts up to Pondicherryin the north, resulting in generation of Archaen–Cam-panian contact (faultline located north of Kilpalur – re-fer Fig. 5). This period was associated with widespreaderosion of basement rocks and older sedimentaries andresedimentation of them in the newly created depocen-ters. The Sillakkudi Formation of Ariyalur Group,which was the product of this widespread transgression,has three members. The lowermost member is a fluvialunit and shows gradual transition to deposition undermarine influence towards the top. Major channels thatcut older sedimentary rocks (Fig. 5), having a width ofmore than a kilometer and 30 m deep, were recognisedin the field. The strata of this member have reversegrading and are predominantly basement boulder andlithoclastic conglomerates. They also show large scaleadvancing cross beds, alternated with ferruginous s.st.foresets. The continuing increase in the sea level sub-merged the fluvial/estuarine mouth sediments and atypical coastal marine member started to develop, inwhich the prevalent deposition was in subtidal to inter-tidal environments. Towards the top, the VaranavasiMember shows the frequent occurrence of pebbly sand-stone layers (may be as a result of prevalent periodichigher energy conditions), erosional surfaces and re-worked fauna. The localised serpulid colonies found atthe top of this member indicate the cessation of sedi-ment supply, a reduction in the sea level, a reduced cir-culation and lower energy conditions. A major erosion-al unconformity separates this formation from the over-lying Kallankurichchi Formation.

The renewed transgression during the Latest Cam-panian–Early Maastrichtian was marked by widespreaderosion of basement rocks and older sedimentaries.However, the size of the basement boulders and litho-clasts of older sedimentaries in the basal conglomeratemember of this formation rarely exceed 30 cm and aremore rounded than their older counter parts. Further-more, these clasts seemed to be recycled from oldersedimentaries rather than sourced fresh from basementrocks. Thus, the Kallar Arenaceous Member has litho-clastic conglomerate deposits at its base and rests overthe underlying Sillakkudi Formation with distinct angu-


lar erosional unconformity. The Kallankurichchi For-mation is essentially a carbonate unit that denotes thecessation of a suply of fluvial sediment which existedduring Santonian–Campanian. As the initial marine flo-oding started to wane, the deposits show a reduction inthe proportion and size of siliciclastics, which wereincreasingly replaced by gryphean colonies marking thebeginning of a carbonate sedimentation. As the sea le-vel gradually increased, the gryphean bank shifted to-wards shallower regions and the locations previouslyoccupied by coastal conglomerates became middleshelves, on which typical inoceramid limestone starteddeveloping. The break in the sedimentation of thismember was associated with a regression in the sealevel, which transformed the middle to outer shelf re-gions into intertidal to fair weather wave base regions.These newer depositional conditions resulted in the ero-sion of shell banks and middle shelf deposits and theirredeposition into biostromal deposits (the Tancem Bio-stromal Member). As the energy conditions were highand the deposition took place in shallower regions, fre-quent non-depositional and erosional surfaces, punctu-ated with cross-bedded carbonate sand beds and tidalchannel grainstones and storm deposits with HCS(hummocky cross stratification) were deposited. Again,the sea level rose to create a marine flooding surfaceand as a result of which, gryphean shell banks starteddeveloping more widely than before, whereby the Srini-vasapuram Gryphean Limestone Member was formed.Towards the top of this member, shell fragments andminor amounts of siliciclastics are observed that mayindicate the onset of regression and associated introduc-tion of higher energy conditions. The occurrence of anon-depositional surface at the top of this formationand the deposition of shallow marine siliciclastics (theOttakoil Formation) in a restricted region immediatelyover the predominantly carbonate depocenter and con-formable offlap of much younger fluvial sand deposits(the Kallamedu Formation) are all suggestive of a grad-ual regression associated with the re-establishment of afluvial system at the end of Cretaceous Period. To-wards the top of the Kallamedu Formation, paleosolsare recorded, implying the abandonment of a river sys-tem and the restoration of continental conditions at theend of Cretaceous. At the beginning of Danian, trans-gression occurrerd which covered only the eastern partof the Kallamedu Formation. The presence of a con-formable contact of the Anandavadi Member with theKallamedu Formation and the initiation of carbonatedeposition from the beginning of Danian are indicativeof the absence of any fluvial sediment supply and theabsence of tectonic activity at this time. The increasein sea level and the establishment of a shallow, wideshelf with open circulation paved the way for the depo-sition of the Periyakurichchi Member. At the top, thismember has distinct erosional unconformity, which inturn, when interpreted along with the presence of ahuge thickness of continental sandstone (>4000 m the

thick Cuddalore Sandstone Formation), clearly indicatesthe restoration of continental conditions in this basin.The absence of any other marine strata over the Cudda-lore Sandstone Formation (Miocene to Pliocene in age)suggests that the sea regressed at the end of Danianand has never returned.

Sea level changes

The present day interest in sedimentary response tosea level changes owes its origin to the stratigraphicconcepts evolved from the work of SLOSS (1963) byVAIL et al. (1977). A sequence is interpreted to havebeen deposited during a cycle of eustatic sea levelchange, starting and ending in the vicinity of the inflec-tion points on the falling limbs of the sea level curve.The development of the theory of sequence analysisproceeded from higher to lower orders of sea levelchange. Thus, the first sea level chart of VAIL et al.(1977) dealt with 3rd order cycles, the first major revi-sion of it by HAQ et al. (1987) incorporated fourthorder cycles. Later, it was shown by CARTER et al.(1991) that the fifth order cycles correspond closely tothe Exxon SSM and there is abundant evidence fromthe studies of post-glacial sediments that the facies pat-terns of sea level cycles are characteristic of a fewthousand years, i.e., an infraseventh order sea levelcycle. Based on these principles and the methods devel-oped by earlier workers, attempts have been made toanalyse the prevalent sea level changes during the dep-osition of the Cretaceous–Paleocene strata under study,using information drawn from lithology, sedimentaryand tectonic structures, environments of deposition, pet-rography and fossil assemblage.

Facies analysis, interpretation of palaeoenvironmentand depositional history, provided information on thesea level changes during the evolution of the basin,from which a sea level curve for the strata under studywas constructed and is presented in Fig. 8. This curve


Fig. 8. Relative sea level changes in the Cauvery Basin.

agrees on the whole with the sea level curve construct-ed by RAJU & RAVINDRAN (1990) and RAJU et al.(1993), except that the present study recognised theoccurrence of high frequency cycles in addition to sec-ond order cycles (detailed below). This figure showsthe occurrence of six sea level cycles of 3rd order with-in which many higher frequency cycles could be recog-nised. The period from Barremian to Coniacian showsthe frequent occurrence of sea level lows and highs thatmay be interpreted as prevalent high frequency higherorder cycles. The period from Coniacian to Danianshows a gradual sea level rise and fall, punctuated withlowerer frequency higher order cycles. The sea levelrise during Santonian–Early Campanian shows a steadi-ly increasing pattern, which coincides with the periodof the initiation of the reduction in tectonic activities inthe basin. While global sea level peaks during 104 Ma(Early–Late Albian), 93.7 Ma (±0.9; Middle to LateCenomanian), 92.5 Ma (±1; Early to Middle Turonian),86.9 Ma (±0.5; Early to Late Coniacian), 85.5 Ma (±1;Early to Late Santonian), 73 Ma (±1; Late Campanian),69.4 Ma (Early to Late Maastrichtian) and 63 Ma(±0.5; Early to Middle Danian) were shown to occurtypically in this basin (RAJU et al., 1993), the occur-rence of many other peaks within these cycles (exceptfor a few, which co-incide with basinal and local scaletectonic movements) indicate sea level cycles on a localand regional scale too.

The 3rd order cycles are separated by Type I se-quence boundaries (i.e., recognized through shifts ofthe shoreline crossing shelf breaks, explicit lithologicinformation, contact relationships between strata; evi-dence of subaerial exposure and erosion, advancementof fluvial channels over former offshore regions, etc.),giving ample evidence for the interpretation of thedepositional pattern in this basin being controlled bythe sea level. Such a pattern is further substantiated byFigs. 9 and 10, which portray the different sea levelconditions for each lithostratigraphic formations (Fig. 9)

and members (Fig. 10). The average relative sea levelsof different chronostratigraphic units are presented inTable 2. It shows the highest relative sea level (RSL)during Campanian and the lowest during Barremian.The standard deviation of the RSL for different agesindicates the prevalence of highly varying sea levelsduring Cenomanian and comparatively stable deposi-tional conditions during Aptian. The occurrence of peri-odic high energy conditions associated with sea levelchanges led to the deposition of storm beds duringCampanian (Varanavasi Member of the Sillakkudi For-mation) and lower Maastrichtian (Tancem Member ofthe Kallankurichchi Formation) as was interpreted onthe basis of independent lithological and sedimentarystructural analyses, which in turn are confirmed by thehigh standard deviation values of these members (Table2). When the grand average values of the RSL and thestandard deviation are considered, it could be said thatthe deposition occurred in a shallow marin environmentfor most of the history of the basin and that there were


Fig. 9. Relative sea level of different formations.

Fig. 10. Relative sea level of different members.

Table 2. Statistics of the relative sea level in the Cauvery Basin.

dynamic sea level fluctuations during the entire dura-tion of deposition.

While the absolute values of the relative sea levelchanges show six third order sea level cycles, the trendlines, drawn with the help of a statistical noise reduc-tion technique, indicate the prevalance of depositionalcontrols during the evolutionary history of the basin.Since the sedimentation took place on an epicontinen-tal sea, the bathymetry was kept at shallow to modest

levels throughout the history of the basin (<50 m – asindicated by the linear curve in Fig. 11). Although, vari-ations allowed the attainment of either supratidal (0–2 mbathymetry) or basinal (~200 m) levels, the shallownature of the basin was maintained (an interpretationsupported by the grand average value of the RSL – Ta-ble 2). This observation confirms the limited influenceof tectonic forces in the basinal history. The movingaverage trend line, which shows the values at fixed in-tervals and projects the normal course of future change,supports this interpretation. If any non-fit between themoving average curve and the actual relative sea levelcurve are observed, then, they may be considered to bethe result of a sudden change, due either to basinal sub-sidence or uplift or to a multifold increase of the sed-imentation rate and the resultant reduction of the depo-sitional bathymetry. This means that the gradual increa-se or decrease of the sea level was superseded or over-whelmed by causes other than eustatic sea level chang-es. Using this assumption, it is concluded that a markedshift between these two curves occurs whenever majortectonic activity or fault block adjustment occurred dur-ing the deposition (for example, during Aptian, Albianand Cenomanian). It is interesting to note that suchmismatches between the RSL curve and the movingaverage curve are either not large or absent during theperiod between Campanian–Danian, a period interpret-

ed as having had more stable (tectonically) deposition-al conditions than its older counterpart. These infer-ences support the interpretation of a sea level controlleddepositional history of this basin. The polynomial trendline, which tracks and follows the actual data trendsand assigns a most likely trend by way of a polynomi-al fit, indicates the presence of distinct peaks of the sealevel during Albian and Turonian and distinct lows dur-ing Cenomanian and Maastrichtian (Fig. 11).

Conclusions

Following the current practices in stratigraphic clas-sification and terminology, revision, amendment and re-organization of the existing lithostratigraphic setup wereattempted. In the present study an updated lithostra-tigraphic setup of the area with 22 members and 10 for-mations (Table 1) is presented.

Enumeration of the tectonic structures and history ofthe Cauvery Basin indicates that after the initial blockfaulting and the commencement of sedimentation duringLate Jurassic–Early Cretaceous, the intensity of tecton-ic control over the sedimentation was small, althoughthe basin continued evolving till the end of Tertiary.

Sedimentation in this basin commenced in a shallowwater environment and most of the deposits representdeposition in a shallow epicontinental sea, punctuatedwith slope and basinal deposits. An increase of the sed-iment-starved nature of the basin from older to youngerstrata has also been inferred.

The sedimentation history of the basin was dominat-ed by prevalent sea level changes, many of which wereeustatic in nature. Six 3rd order cycles were recognised,within which many higher order cycles are embedded.These higher order cycles correspond to the lithostrati-graphic members. Based on the average duration of themembers, it could be interpreted that the high frequen-


Fig. 11. Pattern of relative sea le-vel changes in the Cauvery Basin.a. Non-fit of actual the RSL curveand the moving average trend lineduring Aptian; b. Non-fit of theactual RSL curve and the movingaverage trend line during Albian;c. Non-fit of the actual RSL curveand the moving average trend lineduring Cenomanian.

cy cycles may belong to 4th order cycles. The polyno-mial trend line drawn over the sea level curve indicatesthat the third order cycles form part of presumably sec-ond order cycles. The stacked-up nature of these cyclesof different orders is in conformity with the observa-tions of GRAMMER et al. (1996).

Acknowledgements

MR acknowledges the financial assistance of the Alexandervon Humboldt Foundation, Germany and the Council of Scien-tific and Industrial Research, New Delhi, India. Prof. H. S.PANDALAI, Professor and Head, Prof. P. K. SARASWATI andProf. D. CHANDRASEKARAM, IIT–Bombay, are thanked for aca-demic support and laboratory facilities. Mr. NICKOLOSKI pre-pared thin sections for the petrographic study. Dr. V. SUBRA-MANIAN, Reader, Department of Geology, National College, Ti-ruchirapalli, India, is thanked for his help and logistic supportduring the initial stages of this study. Mr. T. SREEKUMAR,Department of Geology, National College is thanked for assis-tance during the field survey. Permission to collect sampleswas accorded by the managers and geologists of the mines:Dalmia Cements Pvt. Ltd, Tancem Mines, Ramco Cements,Nataraj Ceramics Ltd., Vijay Cements, Fixit Mines, ParveenMines and Minerals Ltd., Alagappa Cements, Rasi Cements,Tan–India Mines, Tamin Mines and Chettiyar Mines. The edi-tor of this journal is thanked for his keen interest and sugges-tions that have improved the style and content of this paper.

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Rezime

Litostratigrafija, evolucija depozi-cionih sredina i promena morskog nivoaKaveri basena (ju`na Indija)

Kaveri Basen, najju`niji rift platforme In-dijskoga poluostrva, pru`a se pravcem severois-tok–jugozapad. Nastao je fragmentacijom Gondvan-skoga nadkontinenta tokom gorwe jure i dowekrede. Ovom basenu pripada depresija Arijalur––Pondi~eri u kojoj je razvijena skoro kompletnasukcesija tvorevina gorwe krede i paleocena. Oovom basenu objavqene su brojne publikacije, kojese bave ili samo jednom vrstom ili rodom, jednomformacijom, ~lanom ili odre|enom litologijom,dok je u ovom radu predstavqen jedan sveobuhvatandepozicioni model, te sinteza sedimentolo{kihmehanizama koji su delovali tokom razvoja ovogbasena. Date su litostratigrafske karakteristike


i detaqni litolo{ki stubovi, ukqu~uju}i prepoz-natqive jedinice na tim profilima. Nazna~eni sufaktori va`ni za depozicionu evoluciju basena.Prikazani su stratigrafski kontinuitet i diskon-tinuitet, kao posledica delovawa brojnih agenasa.Ovim su olak{ane regionalne i globalne strati-grafske korelacije, a unapr|ena je i strategija ve-zana za istraìvawe ugqovodonika u ovom basenu.

Kori{}ewem savremenih stratigrafskih sazna-wa, sistematsko kartirawe, upore|ivawe lito-lo{kih podataka na raznim nivoima, sedimenta-cione strukture i sastav faune, kao i facijalnakorelacija, omogu}ili su sagledavawe litolo{kogsastava i depozicione istorije ovog basena. Ovakveanalize su, u okviru prou~avanih naslaga, omogu}i-le izdvajawe 10 formacija sa 22 ~lana. Revidiranastratigrafija, data u ovom radu, omogu}ila je usa-gla{avawe stavova iznetih u ranijim radovima.Opis litolo{kih promena (u pravcu i upravno napravac pruàwa), uz wihov stratigrafski kon-tekst, olak{ali su korelaciju slojeva zastupqenihu ovom basenu. Na osnovu terenskih podataka olitolo{kim asocijacijama i tektonskim pokreti-ma u okviru ovog basena, izvr{ena je interpretaci-ja procesa, po~ev od blokovskog rasedawa, reakti-vacije rasednih blokova tokom cenomana, reakti-vacije starijih rasednih blokova, formirawa ra-seda tokom santona i reaktivirawa rasednih blo-kova u intervalu posle danskog kata, a pre kvar-tara. Treba navesti da su uz glavne pokrete du`raseda, postojali i mawi, lokalni, tektonski po-kreti, vezani za prilago|avawe rasednih blokovauz ranije postoje}e rasedne ravni. Na osnovu tek-tonskih struktura se ~ini da je posle inicijalnograsedawa blokova i po~etka sedimentacije tokomkasne jure i dowe krede, uticaj tektonike u odnosuna sedimentaciju bio mawi u Kaveri basenu.

Sveukupna analiza facija, faunisti~kih asoci-jacija i ìvotne sredine, dala je podatke o rela-tivnoj batimetriji pojedinih depozicionih jedini-ca, koje, posmatrano u celini, govore o promenamanivoa mora tokom istorije ovog basena. Brojniciklusi morskog nivoa visoke u~estalosti (poseb-no ciklusi ~etvrtog ili vi{eg reda), zajedno for-miraju cikluse tre}eg reda (ukupno {est), kojipredstavqaju delove cilusa drugog reda (ukupno dva)i obuhvataju ukupno sedam eustatu~kih kolebawa(pikova) nivoa mora ovoga basena. Period od bare-ma do konijaka ukazuje na ~este pojave maksimuma iminimuma nivoa mora, koji se mogu objasniti do-minirawem ciklusa vi{eg ranga. Tokom periodakonijak–danski kat, postepena izdizawa i spu{ta-wa nivoa mora su u ravnoteì sa mawom u~esta-lo{}u ciklusa vi{eg reda. Nivo mora tokom peri-oda santon–dowi kampan je u stalnom porastu, {tose poklapa sa periodom redukcije tektonske aktiv-nosti u ovom basenu. Sastav basena i topografijapriobaqa uslovili su da maksimumi nivoa morarezultuju u pove}anoj depoziciji karbonata, dokminimumi nivoa mora omogu}avaju depoziciju si-liciklasti~nih sedimenata. Analiza trenda krivenivoa mora ukazuje na postepeni porast nivoa moraod barema do konijaka, a postepeni pad od konijakado danskog kata. Ovakva postojana kretawa nivoamora uticala su na na~in sedimentacije, kao i naodlike prisutnih facija. Pretpostavqa se da je du-bina depozicione sredine odràvana na umerenoplitkom nivou pre svega posledica nedostatkazna~ajnijeg taloèwa tokom evolucije basena. Is-traìvawa su pokazala da vi{e preovla|uju jedno-stavni procesi zapuwavawa basena, odnosno meha-nizmi koji su uslovqavali promene nivoa mora,nego tektonski pokreti.


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