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CARBONATE DIAGENESIS IN THE KALLANKURICHCHI FORMATION, ARIYALUR GROUP, SOUTH INDIA: A CONTINUUM OF PRIMARY EOGENETIC

SECONDARY TELOGENETIC HISTORY AND IMPLICATIONS

ON PETROLEUM PROSPECTS

Mu.RAMKUMAR Department of Earth Sciences, IIT-Bombay, Powai, Mumbai-400 076, India.

E-mail: muramkumar@yahoo.co.in

ABSTRACT Petrographic observations on diagenetic characteristics of the Kallankurichchi

Formation carbonates were made. Petrographic interpretations were supplemented by

geochemical, mineralogical and stable isotopic data to delineate prevalent five distinct

diagenetic zones viz., sediment-water interface, marine phreatic, meteoric vadose, meteoric

phreatic and meteoric closed/semiclosed system, that have acted through eo, meso and

telodiagenetic stages. Despite continuous action of diagenetic transformations regardless of

change in environmental conditions and zones were interpreted, predomination of one or

more processes during different stages of diagenesis was observed. Though the depositional

conditions could have supported extensive production of organic matter during deposition

of this formation, the role played by biological organisms such as borers during deposition

and diagenetic events such as occurrence of extensive chemical compaction and absence of

physical compaction during marine burial stage have contributed towards destruction of

organic matter continuously since deposition. Repeated occurrence of dissolution-

precipitation mode of diagenesis under oxygenated conditions had also made the rocks not

to serve as good source rocks. Similarly, owing to the availability of abundant cementing

material immediately and occurrence of prolific cementation, the carbonates are all well

cemented without significant unfilled pore spaces, rendering the formation not to be a

hydrocarbon reservoir. However, it is interpreted that, owing to its well cemented nature of

the Kallankurichchi Formation along with the occurrence of poorly cemented, loosely

packed and moderately well sorted siliciclastic deposits over and below, the

Kallankurichchi Formation can serve as stratigraphic trap and/or reservoir seal if those

siliciclastic deposits are found to be hydrocarbon reservoirs. It is also revealed that the

viscous hydrocarbon residues in dolomitic limestones of this formation are a localized

phenomena restricted within shell cavities that escaped open system of meteoric diagenesis.

Key Words: Carbonate diagenesis, stages and zones of diagenesis, timing of diagenetic events, hydrocarbon potential.

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INTRODUCTION

The Kallankurichchi Formation of Ariyalur Group, South India was under study for

its biostratigraphy (Rao, 1956; Sastry et al. 1968, 1972; Bhatia, 1984; Ayyasamy, 1990),

paleoecology (Guha, 1987; Petrovic & Ramamoorthy, 1993; Chandrasekaran & Ramkumar,

1994) and depositional environments (Nair, 1974, 1978; Ramkumar & Chandrasekaran,

1994; Madhavaraju, 1996; Madhavaraju & Ramasamy, 1999a, b; Ramkumar, 1995, 1999a,

2001; Fürsich & Pandey, 1999). Recently, this formation attracted the attention of

petroleum geologists owing to the occurrence of bitumen residues in crevasses and vugs

exposed in mine sections of this formation (Ramkumar, 1995, 2004a; Yadagiri & Govindan,

2000) and the oil find in Ariyalur-Pondicherry sub-basin of the Cauvery basin (Govindan &

Ramesh, 1995) wherein the Kallankurichchi Formation is located. Hydrocarbon potential

of this formation is substantiated by the fact that it is mostly made up of very thick

populations of large gryphea and inoceramus shells besides highly diversified microfaunal

composition that could have generated abundant organic matter to be converted into

significant quantities of hydrocarbon during diagenetic history. However, published data

are limited on its post depositional characteristics and that too based on geochemistry and

or stable isotopes (for example:- Madhavaraju et al. 2004; Ramkumar, 1996a, 1997, 1999b,

2004a; Ramkumar et al. 1996a). This paper examines diagenetic characteristics of the

carbonates of Kallankurichchi Formation based on petrographic data while geochemical,

mineralogical and stable isotopic data were utilized in supportive role.

LOCATION AND GEOLOGIC SETTING

The Kallankurichchi Formation is a prominent carbonate horizon of the Ariyalur

Group and is exposed as isolated outcrops. The general stratigraphic setup of the study area

(Fig.1) is as follows (after Sastry et al. 1968; Chandrasekaran & Ramkumar, 1995).

Group Age Formation Gross lithology Thickness

Kallamedu Formation Sandstone 100 m Maastrichtian Ottakoil Formation Sandstone 60 m

Ariyalur Kallankurichchi Formation Limestone 40 m Group ----------Unconformity-----------

Campanian Sillakkudi Formation Sandstone 400 m ----------Unconformity-----------

Trichinopoly Group

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K

allankurichchi Form

ation

AR

IYA

LU

R G

RO

UP

The formation has N-S extension of 35 km with a width of 500-3500m in the study

area (Fig.1). Sastry et al. (1972) assigned Maastrichtian age to this formation and it was

refined to Lower Maastrichtian by Ramamoorthy (1991) and Radulovic & Ramamoorthy

(1992). Hart et al. (2000) stated that deposition of this formation commenced during late

Campanian-Earliest Maastrichtian. Generalized lithological succession of the formation

(after Ramkumar, 1999a, 2001) is as follows.

Kallamedu Formation Ottakoil Formation

-------------Unconformity---------------

Gryphean L.St. Mbr. (18m) Gryphean fragmental shell l.st. Gryphean l.st.

Gryphean fragmental shell l.st. Bedded fragmental shell l.st.

Fragmental shell L.St. Mbr. (8m) HCS shell hash Cross bedded fragmental shell l.st. Bedded fragmental shell l.st.

Inoceramus L.St. Mbr. (8m) Inoceramus l.st.

Arenaceous gryphean l.st. Arenaceous Mbr. (6m) Silicious and limy conglomerate

-------------Unconformity--------------- Sillakkudi Formation

This formation consists predominantly of skeletal limestones and fragmental limestones

analogues to bank and bank derived materials of Nelson et al. (1962). According to

Dunham s textural classification (Dunham, 1962) modified by Embry & Klovan (1971), the

rocks of this formation are of wackstone to rudstone categories. On the basis of energy

index classification (Plumley et al. 1962), these rocks were deposited under quite to

moderate energy conditions with occasional short-lived high energy conditions

(Ramkumar, 1995, 2004b). Shallow marine, normal saline, well oxygenated water

conditions were prevalent during deposition as indicated by sedimentologic (Ramkumar,

1999a, 2001; Ramkumar & Chandrasekaran, 1994) and faunal (Ramkumar &

Chandrasekaran, 1996; Guha, 1987) studies. Deposition took place under photic region and

the rate of deposition was low for major part of the formation (Ramkumar, 1996b) except

the storm deposits (Ramkumar, 2001, 2004b). Six standard microfacies types of Wilson

(1975) are recognized from this formation (Ramkumar, 1995) and interpreted to have been

deposited in a distally steepened carbonate ramp setting (Ramkumar, 1995, 1999a).

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Deposition of this formation commenced with coastal conglomerates following

transgression during the Latest Campanian-Early Maastrichtian (Hart et al. 2000). Towards

top, the conglomeratic deposits show reduction in proportion and size of siliciclastics that

were increasingly replaced by gryphean colonies. In due course of sea level rise, the

gryphean bank shifted towards shallower regions and the locations previously occupied by

coastal conglomerates became middle shelf wherein typical inoceramus limestone started

developing. Break in sedimentation of inoceramus limestone was associated with

regression of sea level resulting in erosion of shell banks and middle shelf deposits and

resedimentation of them into biostromal deposits (Fürsich & Pandey, 1999; Ramkumar,

2004b). Again the sea level rose to create marine flooding surface and as a result of which,

gryphean shell banks started developing more widely than before. Towards top of these

gryphean shell banks, fragmented shells and minor amounts of siliciclastics are observed

indicating onset of regression and higher energy conditions. Occurrence of non-

depositional surface at the top of this formation and deposition of shallow marine

siliciclastics (Ottakoil Formation) immediately over the carbonates and conformable offlap

of much younger fluvial sand deposits (Kallamedu Formation) are all suggestive of gradual

regression associated with establishment of fluvial system during end Cretaceous

(Ramkumar, 1999a).

METHODS AND MATERIALS

The scope of this paper is to document the changes that took place to transform

carbonate sediments in to limestone

and the processes and agents involved in the course

of transformation. Systematic field survey coupled with collection of rock samples

representing complete petrographic profile (sensu Bathurst, 1987) of this formation from

natural exposures and mine sections were made. 380 thin sections were stained following

the procedures of Adams et al. (1988) and studied under polarized light. Supporting

inferences were collated from mineralogic (Ramkumar, 1995), geochemical (Ramkumar,

1996a, 1997, 1999b), isotopic (Ramkumar et al. 1996a) and structural (Ramkumar, 1995,

1996c, 2004a) data. Compilation of inferences from all these studies had given insights on

diagenetic processes, imprints and agents acted to produce limestone and enabled

construction of diagenetic model of the Kallankurichchi Formation. Analyzing the model

characteristics in the light of organic carbon preservation and maturation had provided

insights on hydrocarbon prospects of the Kallankurichchi Formation.

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DIAGENETIC TRANSFORMATIONS

Micritization

In the study area, three types of micritic products occur, namely, walls of micro-

bores in shells (Plate 1.1), bioclasts with micritic coating (Plate 1.2) and bioclasts and tests

of microfossils that have lost their original microstructure completely to become peloids

(Plate 1.3) due to complete micritization. The first category includes borings of size ~1.5

mm diameter and length of about 3.2 mm observed in gryphea and alectryonia shells.

These bores are interconnected by galleries (sensu Bathurst, 1971) and exhibit a layer of

micritic wall with uniform thickness between bore-fill and unaffected shell structure.

Borings with these characteristics are interpreted to be of clinoid sponges (Bathurst, 1971;

Akpan & Farrow, 1985). The second type of micritization is a dense array of micritic rods

arranged centripetally in bioclasts. These intersect skeletal microstructure. On account of

dense arrangement of bores, the size of individual rods could not be measured. This array

looks like a coating around bioclast and is brown in color. Varying thicknesses of this

coating make the micritic wall irregular in appearance (Plate 1.2). Origin of the micritic

wall with these features is interpreted to be by endolithic algae (Bromley, 1967). Bathurst

(1966) opined that prolonged such boring by algae combined with secondary precipitation

of microcrystalline calcite results in formation of micritic envelop around carbonate

bioclasts. The third category occurs in the event of progressive intense micritization of

bioclasts, which results in loss of their original microstructure. The resultant peloids (Plate

1.3) are homogenous and consist of very finely crystalline calcite and organic matter.

Micritization during primary depositional to early part of secondary eogenetic stage

at or near the sediment-water interface is interpreted owing to three reasons detailed herein.

First is, the clinoid sponge borings occur exclusively in top side of gryphea and alectryonia

shells. As these organisms had had sedentary lifestyle (Brown, 1988), resting over

substratum (Ramkumar & Chandrasekaran, 1996), the successive sediment layers might

have slowly buried the shell and thus allowing boring by clinoids only on the exposed

portion of the shell above bottom sediment surface. As the gryphean bank deposition was

associated with hardgrounds (Ramkumar, 1996b), the depositional rate might have been

very low, allowing prolonged exposure of the shells to boring and thus the boring density is

high on gryphea and alectryonia shells. Second is, as all the micritization processes were

either the activities of endolithic algae or sponges, dependence of these organisms on

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availability of light gives clue to timing of micritization process as depositional stage at

sediment-water interface (Akpan & Farrow, 1985). If the associated bacterial reaction had

also to be considered as a cause of micritization, it might have been effective upto the top

meter of the sediment column (Droser & Bottjer, 1988; Obrochta et al. 2003) which

translates into maximum of earliest part of secondary eogenetic stage. Experimental study

of micritization process by Caudwell et al. (1997) had also suggested occurrence of the

process within eogenetic stage of diagenesis. The third is, the micritic envelops found

around mouldic porosity wherein the pore space is filled with cement spars characteristic of

secondary mesogenetic stage at phreatic-burial zone (blocky low magnesian non ferroan

calcite spar - LMNFC). Hence, development of micritic coating might have predated

marine phreatic zone of dissolution, that commenced far below sediment-water interface

and the zone of active burrowing by benthic organisms and sediment feeders, usually

ranging upto a meter from sediment-water interface (Akpan & Farrow, 1985; Meadows,

1986; Droser & Bottjer, 1988). As the gryphean shell banks were associated with hard

grounds, the time span to deposit a meter thick sediment cover might have been higher (in

the order of many kilo years - Droser & Bottjer, 1988), thus supporting the origin of

micritic coating well before the bioclasts reached the zone of marine phreatic dissolution.

This extended period of exposure of sediments to micritic coating may also explain the high

incidence of micrite-coated bioclasts in the study area.

Compaction

The compactional features expressed in the rocks of the Kallankurichchi Formation

are classified into two viz., mechanical and chemical compaction. Mechanical or physical

compaction results in dewatering, grain reorientation and slight grain deformation in the

early stage of burial diagenesis (Kim et al. 1985). Whole rock mass or selective fracturing

and deformation are possible during and after lithification (Kumar et al. 1992) due to

mechanical compaction. The chemical compaction (pressure solution) produces reduced

porosity as well as reduction in carbonate content of rocks and sediments (Bathurst, 1971).

While post-cementation compaction could be easily recognized, pre-cementation

compaction is indicated by breakage of components, deformation of trace fossils and

peloids, occurrence of fitted fabrics, orientation of elongated allochems, and crushing of

emptied micrite envelopes (Meyers, 1980; Bathurst, 1986; Ricken, 1986, 1987). Based on

these criteria, three episodes of compaction are recognized in the study area. Salient

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features of the compactional processes, stages and relative intensity as inferred from the

study area are presented in table 1. Quantitative measurements on compaction using trace

fossils revealed that the Kallankurichchi Formation had lost 11.85% (Ramkumar, 1996b) of

its depositional thickness (sensu Akpan & Farrow, 1985).

Table 1. Stages, types and features of compaction

Stages

Mechanical compaction Chemical compaction

Early

Low intensity. Grain deformation>Reoirientation > Dewater-ing > Production of Grain to Grain contacts.

Very low to negligible intensity. Pressure solution (?).

Burial

Low to medium intensity and acted scarcely. Dissolution of bioclasts and early cement>Reorientation of large elongated bioclasts in fissile, mud rich lithologies.

Highly intense. Mud and unstable mineralogic conversion>Production of Grain-Grain contacts> Intense dissolution along contacts>Fracturing and deformation of bioclasts>fusing of bioclasts.

Late

Very high to negligible intensity. Production of Grain-Grain contacts> Shattering of bioclasts> fracturing, folding and faulting of rock mass

Accompanied mechanical compaction in terms of production of sutured pressure solution seams.

First compaction episode is physical compaction represented by synsedimentary

dewatering, reorientation of grains, conversion of mud supported texture into grain

supported (Kenter, 1990) and slight deformation of weaker bioclasts. This episode is

comparatively insignificant and is evidenced only by the presence of elongated

foraminiferal tests aligned parallel to each other on a mud matrix. Second episode is,

chemical compaction produced during progressive burial stage in the form of dissolution of

mud and earlier formed cements (fibrous cement grown over grains at sediment-water

interface), increased percentage of grain-grain contacts, dissolution along grain contacts,

fracturing of bioclasts (evidenced primarily by crystal growth tectonics, namely, fracturing

of bioclasts by the pressure exerted by growing crystals

Plate 1.4), bending of grains,

fusing of grains and development of non-wavy microstylolitic seams. Prevalence of higher

bioturbation and boring in these rocks might have helped rapid cementation and sediment

consolidation during depositional stage itself, as could be observed elsewhere (eg.

Richardson et al. 2002). In view of high depositional packing and synsedimentary

cementation, these rocks underwent lesser degree of mechanical compaction during burial

stage. Also, various rock types have experienced differential amount of compaction and

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dissolution at this stage. While the gryphean bank deposits have escaped from significant

compaction during burial stage owing to their lithification in sediment-water interface itself,

the middle shelf deposits (inoceramus limestone) that remained less cemented during

deposition underwent significant compaction, producing typical diagenetic bedding

(Ramkumar, 2001). Such differential diagenetic processes have been reported to be

common (Bathurst, 1987; Westphal & Munnecke, 1997; Böhm et al. 2003). The third

compactional episode accompanied tectonic structural features acted during End-

Cretaceous. The tectonic movements in terms of faulting and folding have produced

fractures (Plate 1.5) that paved way for the development of grain-grain contacts, shattering

of grains, fusing of bioclasts, matrix and cement into single crystals and finally production

of complex stylolitic seams, extensive dissolution along fractures and solution movement

and precipitation along fractures-turned-channels (Plate 1.6). The final phase of

compactional episode was associated with higher degree of mechanical and chemical

compaction.

Porosity

The rocks under study were analyzed according to the carbonate pore systems

classification of Choquette and Pray (1970). Under fabric selective porosity category,

interparticle, intraparticle, intercrystal and mouldic pore types are recorded in these rocks.

Despite being defined by depositional packing, the interparticle porosity experienced little

changes during later stages of compaction (Plate 2.1) as indicated by its preservation in

most of the lithologies except inoceramus limestone deposits. The intraparticle porosity

belongs to predepositional in origin and shows interactions during secondary mesogenetic

stage as evidenced by burial stage dissolution and spar infilling in gryphea and inoceramus

shells. This pore type shows predepositional-depositional-burial and finally secondary

telogenetic stage alterations (Plate 2.2) signifying continuous diagenetic transformations of

these carbonates since deposition regardless of changes in diagenetic stages, zones and

processes. Intercrystal porosity belongs to secondary telogenetic stage. Mouldic porosity

shows clear affinity to marine phreatic-burial stage characterized by clear non ferroan

calcitic (NFC), blocky spars of marine phreatic zone cementation. The fracture, channel

and vug pore types (Plate 1.6) fall under the category of not fabric selective porosity. These

belong to secondary telogenetic stage of diagenesis. Fabric selective or not porosity

category is represented by boring (Plate 1.1 and 1.2) and burrow types. Their pore fills

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suggest depositional and secondary eogenetic origin respectively. As explained earlier,

occurrence of these porosity types indicates prolonged exposure of sediments at sediment-

water interface in well-oxygenated water under photic zone and low rate of deposition.

Compound and gradational pore types comprising basic porosity types are also observed.

Cementation

Micrite and sparry calcite are the cementing materials of the rocks of

Kallankurichchi Formation. Between these two major categories, micrite is found to be

limited and shows many degrees of late stage dissolution and neomrophic alteration, while

the sparite is extensive and shows comparatively lesser alteration except larger, low

magnesian non-ferroan calcite (LMNFC) spars. Sparite cement consists of four major types

of morphology namely, fibrous, bladed, syntaxial rim and equant. Cementation in this

formation took place in all the diagenetic zones but with variable form, mineralogy and

abundance that are detailed herein.

Cementation at the earliest stage of diagenesis in Kallankurichchi Formation is

represented by perpendicularly arranged fibrous and isopacheous cement. These fibrous

cement spars have regular but sharp boundaries. Width of the spars varies at about 20

microns and length varies upto 100 microns. These spars are transparent and distinctly non

ferroan calcitic (NFC) in mineralogy. The morphology and mineralogy (as revealed by

staining) of pristine spars suggest that these were originally high magnesian calcites (HMC).

As the rocks of Kallankurichchi Formation were deposited in shallow marine conditions,

prolific aragonite cementation could be expected. All the while, unstable nature of

aragonite cements gave way to dissolution and or transformation to calcite (HMC) to LMC

as could be observed in modern carbonate counterparts (Jan, 1998). Later stage alterations

of these spars into coarse neomorphic mosaics with rich inclusions are common (Plate 2.3).

These cement spars are found to occur in interparticle porosity and never fill the pore space

completely. The fibrous cement spars show eroded, dissolved and or corroded nature and it

is followed by a layer of bladed cement spars (Plate 2.3) indicating continued cementation

despite the change in environment through time. The fibrous cement spars are typically

associated with hard ground surfaces and from this association, it can be inferred that

prolonged exposure of sediments by prevalent slow/non deposition in conjunction with

other favorable physico-chemical conditions influenced precipitation of this type of cement

spars. The prevalent extensive boring and micritization associated with hardground would

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have supplied abundant mud to get dissolved in surrounding waters to get enrich with

dissolved CaCO3 resulting in precipitation of cement spars of sediment-water interface

(fibrous cement). The heightened metabolic activity (as evident from very thick population

of gryphea and other fauna) could also have increased the solubility of less stable calcite as

faunal metabolism increases dissolved CO2 in the water (Sanders, 2003). The bladed

cement spars are arranged perpendicular to the substrate. They have 30-80 micron wide and

3-4 orders in length. These spars are of NFC in original mineralogy. The fibrous and bladed

spars with these characteristics were regarded as marine cements (Emery et al. 1987)

representing cementation at sediment-water interface and just below the interface

respectively.

The next generation of cementation is represented by phreatic/burial zone in terms

of pore-filling blocky LMNFC cement spars (It is to be noted that in view of absence of any

distinct criterion distinguishing marine phreatic and burial stages of cementation in the

study area, combined with emergence of land immediately after deposition of

Kallankurichchi Formation and absence of any significant overburden for this formation,

the two zones marine phreatic and burial are utilized interchangeably). Cementation in this

zone occurred primarily in interparticle and intraparticle pores (Plate 2.2 and 2.4). The

LMNFC nature of blocky cement spars (Plate 2.3) is typical of marine phreatic zone. These

equant spars generally exhibit increase of size towards centre of the pores (Plate 2.2 and

2.5), compromise structure, competitive growth boundaries and frequent enfacial junctions

typifying cementing origin. In addition to cementation in interparticle and intraparticle pore

spaces, this zone supported precipitation of cement spars in two different mouldic pore

spaces. First type was created by selective dissolution of less stable mineralogic layers in

gryphea and alectryonia shells and refilling the mouldic pore spaces with fine-coarse

equants of LMNFC. These spars are present now as SFC-FC in view of late stage

neomorphism. The second type is more extensive in the form of dogtooth spar mosaic in

cavities of inoceramus shell moulds. These spars are uniform in composition (LMNFC) and

size and are preserved without any neomorphic alteration, may be because of their size and

or mineralogy (LMC). In view of comparative stability of LMC nature of the spars and

complete filling up of pores by these spars, entry of meteoric water during latter stages of

diagenesis into these pores was thwarted and thus, at many instances pristine calcite

spars are encountered.

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The NFC syntaxial rim cements were produced at burial zone and found around the

fragments of echinoderms that do not have micritic coating and in whose micropores were

not filled by micrite. Thus it is inferred that presence/absence of micrite and micritic

coating acted as a strong control over the nucleation of syntaxial rim cements. Optical

continuity between echinoderm fragment and rim cement and extension of cleavage planes

across both are also observed. Size of these spars ranges up to orders few millimeters

depending on available pore spaces. These are NFC in original mineralogy and show

alteration to SFC and FC along their peripheries due to alteration in meteoric phreatic zone.

The marine phreatic zone is characterized by dissolution of earlier formed cement

spars (Böhm et al. 2003). It is evidenced by corroded nature of fibrous and bladed cement

spars (for example, in Lincolnshire limestone as reported by Emery et al. 1987) which are

superposed by blocky NFC spars (Plate 2.3). Transitions from bladed morphologic spars

through pore filling fine equants to large blocky spars of > 2mm are observed to occur in

these rocks and this phenomenon suggests continuation of cementation process from

depositional stage to marine burial stage. Extensive cementation in this zone could be

attributed to abundant supply of source materials through widespread selective dissolution

of less/unstable mineralogic grains (Dravis, 1996; Friedman, 1998; Booler & Tucker, 2002)

that was supported also by oxidation of organic matter in upper regions of this zone (Droser

& Bottjer, 1988). Simultaneous dissolution of less stable grains and precipitation of LMC

cement spars in this zone is explained by the predomination of larger cement spars

(enforced by slow precipitation of crystal and low amounts of Mg2+ incorporated into Ca2+

lattice positions in growing calcite crystal Pingtore, 1978) compared to other zones.

The next generation of cementation took place under meteoric conditions after

emergence of land from Cretaceous Sea. Meteoric cements are observed in fracture,

channel, vug (Plate 1.6), interparticle and intercrystalline pore spaces and are rarely

observed in intraparticle porosity except those affected by and adjoining fracture surfaces.

The meteoric phreatic zone of cementation is expressed by the presence of pore filling

coarse

fine equants of SFC, FC and strongly FC spars. Discontinuity surfaces are

observed in few of these cement spars as a result of water table variations that always

brought alternating oxygenating and reducing conditions. While the vadose cement could

have survived with spar enlargement and incorporation of Fe2+ into Ca2+ lattice positions of

growing calcite spar, the meteoric cement spar would not be stable and thus shows

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corrosion surfaces. Occurrence of alternate layers of NFC and FC (zoning fabric) in single

crystals without any corrosion surfaces in between (Plate 2.6) is interpreted as reflection of

changes in water table and bulk fluid chemical composition (Grover & Reed, 1983) and

stable isotopic composition (Dickson & Coleman, 1980) within meteoric phreatic zone. As

the uppermost part of the meteoric phreatic zone is normally oxygenated and might have

inhibited incorporation of iron into the ongoing precipitation of calcite spar, NFC layer was

precipitated. Change in water table (rising) could have inverted the oxygenating conditions

and promoted incorporation of iron (thus producing FC layer) in view of newly introduced

reducing conditions. Repeated such process as a result of changes in water table but

continued precipitation of spar is suggested for the origin of zoned cement spars. Presence

of non-corroded nature between differently stained bands of pink and blue (that indicate

non ferroan and ferroan nature respectively) also confirms this assertion. There are coarse

cement spars showing transition from SFC-FC and strongly FC nature and are interpreted

to be the result of variation in pore water chemistry and oscillation of pH from slightly

reducing to strongly reducing nature. The meteoric vadose cementation is negligible and is

represented by meniscus cement (Plate 3.1) and dripstone cement (Plate 3.2) of fine equants

with non-ferroan nature. Occurrence of extensive dissolution features in topmost beds of

Kallankurichchi Formation suggests that the dissolved constituents acted as source for

cementation in meteoric phreatic and vadose zones.

On the whole, it is inferred that cementation was extensive in marine burial stage

and it was followed by marine sediment-water interface, meteoric phreatic zone and finally

meteoric vadose zone in relative abundance. Source of cementing medium in all the zones

was available nearby and generally it was sourced from dissolution of unstable carbonate

grains and cements of earlier generation. Cementation took place continuously from

primary depositional stage to secondary telogenetic stage without any significant hiatus.

Though eo and mesogenetic cement spars undergone later stage alteration, pristine crystals

are also present, signifying dependence of late stage diagenesis on depositional packing and

early stage lithification.

Neomorphism

The term neomorphism was first utilized by Folk (1965) to denote all

transformations between a mineral and the same mineral or another of the same general

composition (Adams et al. 1988). It includes polymorphism (aragonite to calcite),

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recrystallization (mineralogy remains unchanged during transformation excluding the

transformation HMC-LMC), aggradation (a process whereby finely crystalline carbonate is

replaced by a coarser mosaic) and destruction (wherein coarser spars are replaced by fine,

inclusion rich spars). Based on these criteria, neomorphic alterations of the rocks studied

are discussed here under three different categories namely, neomorphism of bioclasts,

mictiric matrix and cement spars.

A range of neomorphic alterations in bioclasts from simple loss of microstructural

features (Plate 3.2) to complete dissolution-recrystallisation and later stage alteration to

ferroan calcitic microspars is observed depending on original mineralogy and size of the

clasts. The fragments of inoceramus shell originally had prismatic structure and

aragonitic/HMC mineralogy (Brown, 1988). It shows complete loss of prismatic structure

(Plate 1.4) to NFC and SFC microspar. Complete dissolution of inoceramus shell and

filling the cavity with fine-coarse equants of cement spars are also very common. Next to

inoceramus, shell fragments of gryphea, exogyra and alectryonia show neomorphic

alteration in terms of selective dissolution of their wall structure and refilling of those pores

by fine coarse NFC, SFC and FC neomorphic spars. Generally the molluscan grains show

transitional/blurred contact with the micritic matrix in view of neomorphic sparitization.

Cross cutting mosaics of neomorphic spar fabric are also common and thus it indicates

phreatic zone of neomorphic alteration through the force of crystallization controlled

mechanism (Folk, 1965). Complete fusing of framework grains and cement with matrix to

become single crystal is also recorded in few samples. The bioclasts of bryozoa, algae and

echinodermata and tests of ostracoda and foraminifera show loss of microstructure and

sparitization to lesser extent, indicating either early stage lithification into LMC and or

stable calcitic mineralogy (Brand, 1991). The brachiopod shell fragments are the least

altered bioclasts. Their shell structure is not showing any signs of loss in view of stable

calcite content (i.e., their original LMC mineralogy

Al Aasm & Veizer, 1982; Popp et al.

1988).

The neomorphic spars in matrix are patchy in occurrence, locally show size

variation, many degrees of alteration to NFC, SFC and FC and have irregular boundaries

confirming neomorphic origin. Generally, micrites contain abundant organic matter that

prevents neomorphic alteration (Bathurst, 1971). However, alteration of micrite to

microspars and pseudospars are also observed meaning the neomorphic conditions were

14

severe and or the organic matter content of micrite might have been low. The latter being

the cause of extensive neomorphic alteration of micrite could be gauged from the facts that

significant neomorphism took place at meteoric phreatic zone that followed extensive

dissolution and coeval destruction of organic matter in marine burial and meteoric vadose

zones. Neomorphic alteration of peloids to NFC microsparites indicate meteoric vadose

zone of alteration.

Neomorphism of cement spars is exhibited by reduction in size and conversion of

earlier formed spars to SFC and FC spars. The neomorphically altered cement spars are

translucent, inclusion rich, have irregular boundaries with relics of precursors. The large

NFC equant spars of marine phreatic zone cements show reduction size and conversion to

micro NFC, SFC and FC (Plate 1.1, 2.2, 2.5). The late stage meteoric cements have also

undergone neomorphic alteration to microspars (Plate 2.6, 3.2). The changes observed in

cements indicate that the transformations of metastable to stable phases are accompanied by

obliteration of prior fabrics and compositions (Wilkinson & Given, 1986), often leaving

ghost structures of precursors.

Dolomitization

Presence of dolomite is found in a limited region of the study area where the rocks

are faulted (Ramkumar, 1995, 2004a). X-Ray diffraction patterns of few selected samples

also confirm the presence of dolomites in these rocks (Fig.2). Dolomite crystals occur only

in micritic matrix as inclusion rich, cloudy euhedral rhombs (Plate 3.3). Individual rhomb

size ranges upto 5 micron. Their small sizes, association with micritic matrix, cloudy nature

and restricted occurrence to structurally complex area are all suggestive of replacement

origin (Ramkumar, 1995, 2004a). Selective dolomitization of lime mud and non-mimetic

nature are comparable to dolostones of Burlington

Keokuk Formation as reported by

Choquette et al. (1992).

After emergence of land from Cretaceous sea, a portion of the intensely folded

Kallankurichchi Formation limestone beds were faulted and a downthrown fault block had

plunged into impermeable, unconsolidated, clayey sandstone deposits of Kallamedu

Formation which in turn had restricted the plunged rock mass from open circulation of pore

fluids (Ramkumar, 1995, 1996c, 2004a). By this, a specific physico-chemical environment

different from other limestone beds of the Kallankurichchi Formation was created wherein

replacement of micron sized calcite crystals (micritic matrix) into dolomite took place. The

15

intense neomorphic alteration of associated bioclasts (HMC-LMC) might have supplied

Mg2+ required for dolomitization of micritic mass. Frank and Lohmann (1996) have also

recorded dolomitization of susceptible finer grains during HMC

LMC conversion of

larger bioclasts and other carbonate components in closed system of diagenesis. Such

immediate source for Mg has been reported to be common for selective diagenetic

dolomitization (Melim et al. 2002; Böhm et al. 2003; Pierre & Rouchy, 2004) in a zone of

contrasting permeabilities (in adjacently located Niniyur Formation of the Cauvery basin by

Nair & Vijayam, 1980 and in Gorden Group carbonates of Tasmania, Australia, by Rao,

1990). Owing to the restricted flow of pore fluids, the additional Mg2+ released from

conversion of larger HMC to LMC was not able to escape and contributed towards

enrichment of porewaters with Mg2+ and that in turn replaced the most vulnerable micrite

matrix into dolomite. This selective replacement of micritic matrix suggests poor

dolomitization conditions and limited availability of Mg2+ and higher surface to volume

ratio of micrite that made it susceptible to dolomitizaion (Carbonate sedimentology seminar,

Indiana University, 1987; Moss & Tucker, 1996). The dolomite crystals truncate when

they have contact with larger bioclasts. The contact planes of bioclasts adjacent to

dolomitized mud do not show any dissolution phenomena (Plate 3.3). These features

indicate that the porewaters that dolomitized micritic matrix were not supersaturated with

respect to the bulk shell calcite to effect wide spread shell calcite dissolution (Brand, 1991;

Emery et al. 1987). Dolomitization of mud might have depended on and simultaneous Ca2+

expulsion and incorporation of Mg2+ via fluid transport (Maliva, 1998).

Geochemical, stable isotopic and mineralogic indications on diagenetic environments

Depositional and diagenetic environments of carbonates of the Kallankurichchi

Formation based on geochemical and stable isotopic composition of whole rocks and

separates of rock components were reported earlier (Ramkumar, 1995, 1996a, b;

Ramkumar et al. 1996a, 2004a, b). These papers presented a detailed account on sampling

and analytical methods and data on geochemical and isotopic characteristics of the rocks

under study. Hence, only a brief note on chemical, isotopic and mineralogic signatures is

presented here with reference to diagenetic interpretations based on petrography.

The geochemical analytical results show that concentration of Ca varies from 1.23

to 61.32 wt.%; Mg ranges between 0.1875 and 21.255 wt.%; Fe ranges from 0.2727 to 6.62

wt.%; Al ranges from 0.0304 to12.5 wt.%; Na varies from 7 to 202 ppm; K varies from 1 to

16

7.5 ppm; Sr varies from 79 to 424 ppm; Mn varies from 112 to 1020 ppm and Ba varies

from 20.5 to 1040 ppm. These values, in isolation and in conjunction with petrographic

observations indicate that, the rocks have experienced marine as well as fresh water

diagenesis. Similar conclusions were drawn by Madhavaraju et al (2004) based on stable

isotopic composition of these rocks.

The intensively depleted nature of Sr, Na and Mg suggest fresh water, open system

of diagenesis wherein entry of diagenetic fluid with same initial composition to the rock

mass took place (Banner & Hanson, 1990). Their very low concentrations associated with

higher concentrations of Fe and Mn support of the interpretation that the imprints of

meteoric diagenesis dominated other zones of diagenesis. Their higher concentrations also

suggest very low rate of precipitation of diagenetic calcite spars in meteoric phreatic zone.

Based on these observations and the statement of Pingtore (1978), it is interpreted that the

predominant diagenetic transformation in these rocks was through calcite

calcite rather

than aragonite

calcite. The ghost structures of fibrous and bladed spars (characteristic of

HMC and aragonite), prevalent alternate layers of stable and unstable mineralogy in shells

and casts of shell walls are all suggestive of prevalence of aragonite during depositional

stage. Hence, it could be interpreted that before the introduction of meteoric diagenesis,

there might have been transformations involving dissolution and conversion of unstable

mineralogic components (HMC and aragonite) of the rocks preferably in marine regime

itself. The X-ray diffractograms of few selected samples have also established the absence

of aragonite and the presence of either calcite or dolomite in the samples studied (Fig.2).

The stable isotopic analyses of selected whole rock samples, separates of shell,

bioclast and cement spars suggest that the studied rocks underwent transformations at least

in three zones of diagenesis (Fig.3) with dominant transformations under isotopically

lighter water (meteoric water). Though the broad range exhibited by the stable isotopic

ratios ( 18O -7.23 to -2.0; 13C -15.2 to 0.14) indicates extensive reequilibration of

carbonates in isotopically light waters (Shieh et al. 2002), presence of marine signatures in

few of the samples (Fig.3) suggests preservation of pristine carbonate phases,

comparatively less altered nature of shell materials (Gryphea shell: 18O -5.6; 13C 0.14;

Inoceramus shell: 18O -3.02; 13C 0.43) and depleted nature of marine calcite spar ( 18O -

6.71; 13C -4.64) and thus differential response of diagenetic components and variable

accessibility of diagenetic fluids to solid components (Morad et al. 1990). This inference is

17

supported also by significantly separated grouping (Ramkumar et al. 1996a) of samples in

Fig.3. The 13C of dolomitic limestone samples ranges from -10.86 to -15.20 while all other

bulk samples show a range of 13C -0.68 to -5.30 meaning the diagenetic fluid of dolomitic

limestone was isotopically lighter than the diagenetic fluid circulated in non-dolomitic

rocks.

SEQUENCE OF DIAGENETIC EVENTS

The interpretations and discussions presented in preceding sections have provided

insights on timing and nature of transformations, relative intensity of diagenetic processes

and zones of diagenesis and are portrayed pictorially in a diagram (Fig.4) representing

time-event-process-environment-zone. It shows that the diagenetic history of this formation

commenced at depositional stage itself and continued until secondary telogenetic stage,

albeit predomination of various diagenetic processes at different points of time as detailed

herein.

Eodiagenesis

The earliest stage of diagenesis on the sediments took place during depositional

stage in marine regime at sediment-water interface. As all the deposits except fragmental

limestone member experienced low to moderate rate of deposition, they were subjected to

changes in terms of intensive biological destruction (boring and micritization) and

syndepositional lithification (precipitation of fibrous needles) during deposition itself. The

biological processes were controlled by light penetration. Given cognizance to the thick

population of these organisms and the studies of Orpin (1997) and Sanders (2003), it is

presumed that the enhanced PCO2 might have increased the dissolved state of calcium

carbonate in surrounding waters resulting in precipitation of fibrous cement characteristic

of sediment-water interface. The availability of fine detritus emanating from boring might

have supplied abundant micritic material to get dissolved to become source for sediment-

water interface fibrous cement. Though the depositional energy controlled the behavior and

population of organisms, diagenesis in this stage was influenced, if not controlled by

biological processes. The open system of diagenesis and continuity of sedimentation and

lithification allowed this stage to pass onto marine phreatic-burial stage without any hiatus.

Mesodiagenesis

With the progressive burial of sediments deposited, a new set of environmental

conditions was introduced commencing reequilibration of sediments to newer milieu that

18

form mesodiagenetic history. Physical compaction (realignment of grains, and reduction in

primary porosity), dissolution of less stable mineralogic bioclasts (aragonite and HMC) and

cement spars, neomorphism (conversion of aragonite and HMC in to LMC either by

dissolution-precipitation or insitu replacement

sensu Hover et al. 2001) and complete

filling of voids (intergranular and intragranular) with cement spars characteristic of marine

phreatic-burial zone have taken place at this stage. The diagenetic water of phreatic zone

was oxic at its upper region and as a result, most of the organic matter was destroyed. With

due oxidation of organic matter, the porefluids become anoxic and enriched with calcium

carbonate which in turn might have supported precipitation of more stable and larger calcite

spars in newly created pores. The availability of calcium carbonate was abundant for

precipitation of cement spars in mouldic, inter and intragranular pores. Sources for Ca were

nearby as evidenced by the extensive dissolution and boring. Bladed cements, though they

were precipitated at shallow burial, experienced either complete dissolution and followed

by precipitation of blocky calcite or insitu transformation to blocky calcite at deeper

regions of phreatic-burial zone. At this stage, physicochemical processes dominated the

biological processes. Among physical and chemical processes chemical transformations

dominated over physical processes as revealed by complete chemical overprinting on the

signatures of sediment-water interface zone. This inference is supported by the presence of

either calcite or dolomite in the rocks as revealed from X-Ray diffractograms (Fig.2) and

petrographic study of stained thin sections. The burial zone of diagenesis continued until

the sediments were completely cut-off from circulating fluids through complete filling up

of pores through stable calcites accompanied by leaching of less stable bioclasts into more

stable LMC.

Telodiagenesis

At the close of Cretaceous, there was widespread regression associated with uplift

of land exposing the sediments deposited and experienced diagenesis under marine

conditions to meteoric conditions

a system consisting of entirely different set of

physicochemical and biological processes. As the meteoric diagenesis was introduced long

after deposition of Kallankurichchi Formation (Latest Campanian-Early Maastrichtian)

owing to the emergence of land at Late Maastrichtian-Early Tertiary, introduction of

meteoric diagenesis after a significant time lag between marine phreatic-burial zone

diagenesis could be visualized and hence the meteoric diagenetic transformations are

19

classified under telodiagenetic stage. Change in diagenetic environment from marine to

meteoric produces quick initial exchange of phisico-chemical properties between diagenetic

components (circulating fluid and host rock) followed by slow reordering of diagenetic

products (Maliva et al. 2001) and it was true to the carbonates of the study area. The

telogenetic stage of diagenesis consists of three zones, viz., meteoric vadose, meteoric

phreatic and a peculiar closed/semi-closed system.

The vadose zone acted mostly towards complete dissolution of top beds of this

formation. It is also characterized by dissolution and corrosion of less stable mineralogic

bioclasts and cement spars and negligible amounts of cementation. All these indicate that

the meteoric vadose zone is typified by chemical processes and particularly dissolution

representing quick initial reaction of diagenetic components. The vadose zone dissolution

had also acted as the supplier of cementing material to meteoric phreatic zone due to

downward percolation of ground water. While studying genesis of hard interlayers in

Sillakkudi Formation, Ariyalur Group, Ramkumar et al. (1996b) observed that source of

cementing material for the Sillakkudi Formation was drawn from dissolution of carbonates

of the overlying Kallankurichchi Formation. This, in conjunction with present observation

on Kallankurichchi Formation, accounts for dissolution in vadose zone as a regional

phenomenon. The oxygenated nature of vadose cementation produced diagenetic calcites of

NFC and/or with low Fe and Mn besides depleted nature of carbon and oxygen isotopes.

These observations also suggest open system of diagenesis for this zone.

The meteoric phreatic zone of diagenesis had neomorphosed most of the rock

components. Neomorphic transformation in this zone was controlled by access of fluids to

the solid phases, mineralogy, chemistry and grain size of the carbonate phases as could be

observed elsewhere (Vincent et al. 2004). The waters that reached meteoric phreatic zone

were all already saturated with Ca due to extensive dissolution and enrichment of water

with dissolved ingredients at vadose zone itself and hence, complete dissolution of host

rocks was scarce in phreatic zone. Instead, insitu replacement (neomorphic conversion) of

most of the carbonates had taken place. As these meteoric waters are characterized by

reducing conditions, incorporation of Fe and Mn into existing calcite took place with

simultaneous leaching of Mg, Sr and Na. This reaction had also depleted 18O of host rocks.

The cement spars of this zone are LMFC and enriched in Fe and Mn. With all these, it can

be interpreted that, isotopically light waters, with slightly raised salinity, rich in Fe and Mn

20

and scarce in Mg, Sr and Na were prevalent in meteoric phreatic zone. Upper portion of

meteoric phreatic zone was temporarily and periodically oxic and is reflected by zoning

fabrics and alternate layers of NFC and Fe cement spars. Prevalent fluctuations of ground

watertable, accompanied by introduction of oxygenated and reducing conditions are evident

from corrosion surfaces between cement spars and zoning fabric in single crystals of

meteoric phreatic zone. The diagenetic system acted in this zone was open that ensured

continued influx of diagenetic fluid with initial bulk composition different from pore fluids

of meteoric phreatic zone.

The meteoric water in one of the downthrown fault block acted as formation water

in view of restricted and or temporary closed system (Ramkumar, 2004a). The water got

enriched with reference to Mg2+ due to leaching of less stable carbonate grains and the

diagenetic water affected finer matrix grains. Similar to these observations, The dolomitic

limestone in the field looks completely different in the field (Plates 3.4, 3.5, 3.6) in terms of

color, packing, cementation, hardness, etc. At places, the dolomitic limestone deposits

show petroliferous bitumen in cavities of shell chambers, first reported by Ramkumar

(1995) and later confirmed by Neeraja (1997) and Yadagiri and Govindan (2000). The

features such as sea level fall and tectonic uplift of marine regions resulting in diagenetic

transformations under meteoric conditions, leading to dolomitization associated with the

occurrence of petroliferous bitumen in the dolomitized rocks look similar to the reports of

Pierre et al. (2002) and Pierre and Rouchy (2004) where they suggested that release of

volatile materials from gas hydrates triggered dolomitization processes. Ramkumar et al.

(2004a) observed that destabilization of gas hydrates and or evaporation of volatile

hydrocarbon from sediments is a common phenomenon in K/T sites of India, Guatemala,

Israel and Mexico. The faulting event that initiated a chain of transformations including

dolomitization, had affected Cuddalore sandstone of Mio-Pliocene age (Ramkumar, 1996c;

Ramkumar et al. 2004b) and hence, dolomitization of downthrown fault block is termed as

post Mio-Pliocene event.

HYDROCARBON POTENTIAL OF THE KALLANKURICHCHI FORMATION

Though deposition of these carbonates under well-mixed oxygenated waters of open

sea could have supported diversified and thick population of organisms, it also might have

discouraged significant preservation of organic matter in sediments owing to the

oxygenated conditions, low rate of sedimentation and prolonged exposure of sediments to

21

extensive biological boring and synsedimentary lithification. During deposition, there were

minor to major sea level oscillations that might have triggered rapid cementation (Friedman,

1998; Booler & Tucker, 2002) often under different depositional conditions that reduced

organic matter contained in original sediments. Added to this is the effect of metabolism of

organisms and consumption of organic matter by borers that destroy organic matter in the

sediments through carbonate dissolution and synsedimentary cement precipitation (Kropp

et al. 1997). Sanders (2003) stated that, oxidation of organic matter at the sediment-water

interface supports chemical dissolution of unstable carbonates, a process that releases

organic matter bound with shells into the surrounding waters. On examining the effects of

early stage oxidation of organic matter in sedimentary carbonates and shales, Hatch and

Leventhal (1997) concluded that, it severely affected the hydrocarbon generation potential.

Leythaeuser et al. (1995) stated that significant physical compaction at marine

burial stage promotes redistribution of organic matter in carbonate sediments. These

authors have stated that pressure solution and formation of stylolites during marine burial

stage help local concentrations of kerogen surrounded by stylolite seams and favor

petroleum generation. As explained under the sub-head compaction, the carbonates of

Kallankurichchi Formation have not experienced significant physical compaction during

marine burial stage. On the contrary, subjugation of these rocks under chemical compaction

at this stage thwarted survival of organic carbon survival at this stage. As diagenesis at this

stage took place under open system, continuous replenishment of oxygenated marine waters

upto the extended zone of bioturbation (Droser & Bottjer, 1988) would have aided coeval

chemical compaction and dissolution of less/unstable carbonate phases and thus destruction

of organic carbon. The marine burial stage promoted mouldic porosity, extensive

occurrence of which in carbonates is considered to be good reservoir quality. However,

owing to the occurrence of abundant HMC at sediment-water interface, their transformation

to LMC and dissolution at marine burial stage might have increased the availability of

carbonates in solution, forcing immediate precipitation, filling the mouldic voids, thus

eliminating unfilled pores. Increased availability of cementing material and immediate

cementation in the rocks are evidenced by from abundant and completely filled mouldic

porosity in inoceramus limestones. The initial mineralogy of inoceramus shells is thought

to be aragonite and or HMC. Neomorphism/transformation of aragonite/HMC shells is

rapid and thought to be through simultaneous dissolution of aragonite/HMC and

22

precipitation of LMC (Maliva, 1998). This reaction would have released enough carbonates

into the surrounding solutions, which on saturation filled mouldic pores with equant calcite

spars. Neilson et al. (1998) have also observed impact of timing of cementation over

hydrocarbon accumulation and preservation in carbonate reservoirs due to race for space

between hydrocarbon and diagenetic waters/cement spars precipitated from diagenetic

waters, among which the former might have lost in the Kallankurichchi Formation owing to

prevalent environmental conditions as explained.

It is to be noted that cementation in the meteoric phreatic and vadose zone was

wholly dependent on dissolved calcium carbonate from percolating ground water. As the

initial bulk chemical composition of meteoric water and carbonate rocks might have been

entirely different, the initial dissolution was severe as could be observed from the extent of

karstic landscape in the study area. This dissolution-precipitation process, along with the

influences of oxic nature of ground water, difference in bulk chemistry of diagenetic

components and changes in water table might have affected the residual organic matter left

in the rocks. Hatch and Leventhal (1997) also state that diagenesis of carbonates in

meteoric water environments actively oxidizes organic carbon and affects hydrocarbon

generation potential even if the host rocks contain abundant organic matter. Given

cognizance to the location of Indian sub-continent during end-Cretaceous at tropical region

(Govindan & Ramesh, 1995; Powel et al. 1988), prevalent open system of diagenesis in

meteoric regime, increased atmospheric temperature and PCO2 during end-Cretaceous

(Ramkumar et al. 2004a), it is expected that the carbonates introduced to subaerial

conditions during end-Cretaceous might have experienced extensive dissolution and

organic matter destruction (Genthon et al. 1997).

Neeraja (1997) and Yadagiri and Govindan (2000) examined the viscous residues

found in the shell cavities of dolomitic limestones under flourescense and confirmed the

presence of hydrocarbon residues. Recent studies of Pierre and Rouchy (2004) and

Ramkumar et al. (2004a) recorded triggering of many geochemical transformations

including dolomitization in carbonates as a result of release of volatile materials from

hydrocarbons preserved in carbonate rocks. Thus, based on the limited occurrence of

hydrocarbon residues in shell cavities preserved away from meteoric waters, it becomes

clear that significant organic matter survived in these rocks from consumption by

organisms during deposition, oxidation and destruction in marine phreatic-burial stage were

23

totally destroyed before maturation into hydrocarbons by meteoric vadose and phreatic

conditions of diagenesis. This interpretation is supported by the statement of Neeraja (1997)

and Yadagiri and Govindan (2000) that the hydrocarbon residue recovered from dolomitic

limestones contains extractable organic carbon of about 7250 ppm with low maturity value

of Tmax 376 C.

CONCLUSIONS

a. Diagenetic history of the Kallankurichchi Formation commenced during depositional

stage itself (Latest Campanian-Earliest Maastrichtian) and continued even beyond

emergence of land without any major hiatus (Post Mio-Pliocene). As could be observed

elsewhere (eg. Hiatt et al. 2003; Booler & Tucker, 2002; South & Talbot, 2000) the

diagenetic transformations varied between different lithological units.

b. All the three major diagenetic processes viz, physical, chemical and biological have

played roles in the post depositional history of the Kallankurichchi Formation. The

rocks experienced diagenesis through five diagenetic zones namely, marine sediment-

water interface, marine phreatic-burial, meteoric vadose, meteoric phreatic and a

peculiar closed/semi-closed meteoric zone. Except dolomitization, open system of

diagenesis took place in all other zones.

c. The products of biological processes are restricted within marine regimen. The physical

compaction had produced its products significantly only in association with tectonic

movements. Chemical compaction, dissolution and cementation took place all through

the diagenetic history. Cementation was very effective in marine regimen whereas the

neomorphic transformation took its toll during meteoric pheratic zone. The source of

cementing material was nearby in all the zones. The diagenetic transformations in

majority of the cases were calcite-calcite rather than aragonite-calcite.

d. Though the predomination of packstone-floatstone suite (Ramkumar, 1995) deposited

under moderate energy conditions (Ramkumar, 1999a,2004b; Fürsich & Pandey, 1999)

imparted good porosity and permeability (Neilson et al. 1998; Wilson & Evans, 2002;

Cerepl et al. 2003) and prevalent luxuriant environment generated abundant organic

matter, extensive dissolution-cementation process under oxic waters overruled source or

reservoir potential of this formation. It is concluded that only the dolomitic rocks that

experienced closed system of diagenesis preserved hydrocarbon residues in shell

cavities while all other regions lack noticeable hydrocarbon.

24

ACKNOWLEDGEMENT

This paper forms part of my Ph.D. dissertation. Prof.V.A.Chandrasekaran (Retd.),

Department of Geology, (formerly School of Earth Sciences), Bharathidasan University is

thanked for his guidance. Prof.K.Ramamoorthy, Head (Retd.), Department of Geology,

National College, Tiruchirapalli, extended academic and moral support. Prof.V.Rajamani,

School of Environmental Sciences, Jawahar Lal Nehru University provided access to trace

elemental analyses in his laboratory. Dr.S.V.Navada, Scientist, Bhabha Atomic Research

Centre helped in stable isotopic analyses. Dr.S.Anbarasu, Department of Geology, Delhi

University analyzed the samples for X-Ray mineralogy. Prof.G.Victor Rajamanickam,

(Retd.), Department of Earth Sciences, Tamil University, helped in preparation of

photomicrographs. Prof.P.K.Saraswti and Prof.H.S.Pandalai, Head, Department of Earth

Sciences, IIT-Bombay, are thanked for laboratory facilities, academic and administrative

support.

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31

Fig. 1 Location of the study area

32

Fig.2 X-Ray diffractograms of selected whole-rock samples

33

Fig.3 Scatter diagram of carbon and oxygen isotopes (Discriminant line of fresh water and marine carbonates is after

Keith & Weber, 1964 and Kumar et al. 1992)

-16

-12

-8

-4

0

-8-6-4-20

13

C

Meteoric

Marine

18O

34

Closed Phreatic Vadose Phreatic

Sediment-Water

MARINE METEORIC

Telodiagenesis Mesodiagenesis Eodiagenesis

STAGES

ENVIRONMENTS

Boring & micritization

Physical compaction

Chemical Compaction

TR

AN

SFO

RM

AT

ION

S

Cementation

Dissolution

Neomorphism

Dolomitization

Fig.4 Sequence, relative

intensity and continuity of diagenetic

transformations

35

PLATE No.1

Photomicrograph showing the circular boring in gryphean shell. The micritic coating along the wall of bore is visible in terms of dusty brown layer. The porosity was filled by cement spars of NFC of marine origin that underwent later stage neomorphic alteration. The bore wall itself underwent dissolution as evident from disturbance along western side of the bore in the photograph. Scale bar 0.5mm.

Photomicrograph showing the microboring and micritic wall development. Note the organic matter rich micrite filled in the mocrobores, irregular thickness of the micritic coating around bioclast and gradual movement of micritic coating towards centre of the bioclast. Scale bar 0.5mm.

Photomicrograph showing completely micritized bioclast/shell showing uniform brownish, translucent nature, typical of a peloid originated from micritization of bioclast. Also note the ghost structure. Scale bar 0.8mm

Photomicrograph showing crystal growth tectonics. The pressure exerted by growing crystals break and displace parts of bioclasts and is called crystal growth tectonics. Scale bar 0.5 mm.

Field photograph showing the mega fracture porosity (Perpenticular to the photograph) along which major channel porosities). Note the fracture fill is made up of telogenetic kankar (fracture surface is different from limestone as discernible in tonal difference) while the channel courses are further different as exhibited by the presence of neomorphic spars. Location of the photograph: FIXIT mines located southwest of Kallankurichchi village.

Photomicrograph depicting fracture porosity converted into channel porosity. Note the uniform size and mineralogy (as discernible by staining) of porefills, an indication towards cementing origin rather than neomorphic origin. Also note the development of smaller branching of channels and development of vug (bottom centre of the photograph). Scale bar 0.8mm.

1.1

1.2

1.3

1.4

1.6

1.5

36

Plate No.2

Microphotograph showing interparticle porosity filled with micritic materials and little amount of neomorphosed micro and pseudospars. Note the parallel alignment of bioclasts due to post depositional compaction. Scale bar 0.3mm.

Microphotograph showing a perfect intraparticle porosity manifested by bryozoan froands. This porosity is filled by mesodiagenetic marine phreatic zone cement spars that underwent neomorphism in meteoric phreatic zone. Note the gradual increase in spar size towards centre of the pore, zoned nature of crystals, variable tones of blue that indicate variable amounts of inclusion of Fe in Ca lattice positions of calcite crystal. Scale bar 0.2mm.

Photomicrograph showing the pristine bladed spars (a) of sediment-water interface cementation abutting against large blocky spars (b) of marine phreatic zone of cementation. Note a feeble corrosion surface between the contact (arrow) of these two types of cement spars. Scale bar 0.5mm.

Photomicrograph showing cement spars of marine burial zone filling in interparticle (a) porosity and intraparticle (arrow) porosity. Also note that, due to late stage neomorphsm under meteoric phreatic zone, the spars show iron inclusions as exhibited Scale bar 1 mm.

Photomicrograph showing multiple generations of cement spars in a intraparticle-shelter porosity. Note the gradual increase in spar size towards centre of the porosity. The photograph shows sediment-water interface fibrous and micritic cement (a) followed by bladed cements (b) and equant spars of marine phreatic zone (c). Owing to the meteoric phreatic zone of neomorphism, the cement spars show destructive transformation (reduction in spar size) and conversion of NFC spars (appearing pink) into FC (appearing) shades of blue. Note the unaffected nature of large spars which may indicate weak solutions in meteoric phreatic zone, stability of LMNFC spars towards meteoric diagenesis. Scale bar 1mm..

Photomicrograph showing the pore filling meteoriz phreatic zone blocky cement spar. The water table at this zone might have experienced fluctuations as a result of which fluctuations in oxygen levels have resulted, precipitating alternating layers of non-ferroan and ferroan mineralogy within single cement spar. This cement spar might have experienced further late stage neomorphism as destructive neomorphic features in terms of reduction in spar size, inclusions are present. Scale bar 0.5mm.

a

a

b

2.3 2.4

2.5 2.6

2.1 2.2

37

PLATE No.3

Photomicrograph showing development of meniscus cement around a structureless grain. The spars are very fine in size and show non ferroan nature that might indicate meteoric vadose zone of cementation. Scale bar 0.8mm

Photomicrograph showing dripstone cement (arrow) precipitated along underside of an echinoderm fragment. The cement and the bioclasts have undergone meteoriz vadose zone neomorphism as revealed by microsparitization with NFC nature. Scale bar 0.2mm.

Photomicrograph showing dolomitic rhombs. Note the occurrence of dolomitic rhombs only in areas other than LMNFC bioclasts (appearing in shades of pink and structureless mass in the photograph), abrupt termination of rhombic faces against bioclasts and sharp boundaries of the dolomite crystals. The dolomitic rhombs are larger than the micritic matrix. These features indicate weak dolomitization process, limited availability of excess Mg2+ for dolomitization and diagenetic origin for dolomites. Occurrence of structureless LMC bioclasts fused together indicate the additional Mg2+ needed for dolomitization of micrictic matrix might have been supplied by stabilization of these bioclasts by conversion of them into LMC. Scale bar 10 microns.

Thick population of the gryphea in the Srinivasapuram limestone member of Kallankurichchi Formation. Field of photograph covers an area of 2 m x 1.5 m.

Dolomitized portion of the limestone beds. Note the hydrocarbon residue in cavity indicated by arrows.

Similar gryphean limestone but located in the dolomitized portion of the formation. Compare the magnitude of change in colour, packing and fusing of matrix and framework grains.

3.1 3.2

3.3 3.4

3.5 3.6

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