LITHOLOGY, PETROGRAPHY, MICROFACIES, ENVIRONMENTAL HISTORY AND HYDROCARBON PROSPECTS OF THE KALLANKURICHCHI

FORMATION, ARIYALUR GROUP, SOUTH INDIA

Mu.RAMKUMAR Department of Earth Sciences, IIT-Bombay, Mumbai-400 076. geolrk@rediffmail.com

ABSTRACT

The Kallankurichchi Formation of Ariyalur Group, Tamil Nadu State, India, represents thick limestone deposits with very high proportion of faunal remains. Its faunal diversity and abundance attracted considerable attention of paleontologists from all over the World and as a result, a wealth of literature on fossil composition of the formation exists. However, owing to the myriad changes in lithological and facies characteristics of this formation, often the workers find it difficult in the field to identify and corroborate lithological and facies descriptions in the literature. This paper attempts to fill the gap through detailed lithological and facies variations along with petrography towards interpretation of depositional environments. Compilation of field and petrographic data revealed that the formation was deposited in facies belts 2, 6 and 7 under SMF types 8, 10, 11, 12 16 and 18 according to the facies classification of Wilson (1975). Deposition of this formation took place in a distally steepened ramp setting under normal saline, warm, well-mixed open sea conditions with low-moderate depositional energy and rate. Owing to the recent oil find in areas nearby and occurrence of bitumen residues in this formation, this paper examines also the plausibility of organic carbon preservation and maturation. It is brought out that although the depositional environments supported luxurious biotic diversity and abundance, owing to the biological factors, prolonged exposure of sediments at sediment-water interface, followed by extensive diagenetic transformations under open system, preservation and maturation of organic matter contained in the sediments were poor and the possibility of locating commercial quantities of hydrocarbon in this formation is remote. Keywords: Standard microfacies, Maastrichtian carbonates, depositional environments,

organic carbon generation, preservation, maturation, hydrocarbon prospects.

INTRODUCTION

The Kallankurichchi Formation of Ariyalur Group, South India had been studied

extensively in terms of its faunal content and biostratigraphy (Rao, 1957; Sastry et al. 1968,

1972; Nagaraja and Gowda, 1976; Bhatia, 1984; Ayyasamy, 1990), fauna and ecology

(Guha, 1980, 1987; Guha and Senthilnathan, 1990; Mallikarjun, 1992; Radulovic and

Ramamoorthy, 1992; Mittrovic-Petrovic and Ramamoorthy, 1992; Chandrasekaran and

Ramkumar, 1994 Ramkumar and Chandrasekaran, 1996; Hart et al. 2000) and depositional

environments based on taphonomy, gross lithology, sedimentary structures, and

geochemistry (Sastry et al. 1972; Nair, 1974, 1978; Sundaram, 1977, Ramkumar and

Chandrasekaran, 1994; Guha and Mukhopadhyay, 1996; Madhavaraju, 1996; Madhavaraju

and Ramasamy, 1999a, b; Fürsich and Pandey, 1999; Ramkumar, 1995, 1996, 1997, 1999,

2001). This review reveals that, only the paleontological database of this formation is

documented excellently while systematic lithological and sedimentological information are

scarce that poses difficulty for the workers of the field to corroborate published information

on stratigraphic and lithological details for verification and correlation with newer data.

Recently, this formation attracted the attention of petroleum geologists owing to the

occurrence of bitumen residues in vugs of rocks exposed in mine sections (Ramkumar,

1995, 2004a; Yadagiri and Govindan, 2000) and the oil find in Ariyalur-Pondicherry sub-

basin of the Cauvery basin (Govindan and Ramesh, 1995) wherein the Kallankurichchi

Formation is located. This paper is an attempt to fill the gap in existing literature through

systematic documentation of lithology, petrography and standard microfacies types of the

formation and to improve our understanding on prevalent environmental conditions with

reference to organic matter generation, preservation and its maturation into commercial

quantities of hydrocarbon in these rocks.

LOCATION AND STRATIGRAPHY

The Kallankurichchi Formation is a prominent carbonate horizon of the Ariyalur

Group and is exposed as isolated outcrops (Guha and Senthilnathan, 1990). It extends for

about 35 km along N-S with a width of 500-3500 m in the study area (Fig.1). The beds dip

gently towards east. General stratigraphic setup of the study area is as follows (after Sastry

et al. 1968; Chandrasekaran and 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

A prominent angular unconformity surface followed by conglomerate deposit

separates this formation from underlying Sillakkudi Formation. Upper contact of this

formation is a non-depositional surface with Ottakoil Formation of shallow marine origin

and an offlap surface with much younger Kallamedu Formation of fluvial origin.

Occurrence of thick populations of fossils in this formation readily distinguishes this

formation in the field. Sastry et al. (1972) assigned Maastrichtian age to this formation and

it was refined to Lower Maastrichtian by Ramamoorthy (1991) and Radulovic and

Kallankurichchi F

ormation

AR

IYA

LU

R G

RO

UP

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

by sea level rise during Late Campanian-Earliest Maastrichtian. Generalized lithological

succession of this Kallankurichchi Formation is as follows (after Ramkumar, 1999, 2001,

2004a).

Kallamedu Formation Ottakoil Formation

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

Srinivasapuram Member (18m) Gryphean fragmental shell l.st. Gryphean l.st.

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

Tancem Member (8m) HCS shell hash Cross bedded fragmental shell l.st. Bedded fragmental shell l.st.

Kattupiringiyam Member (8m) Inoceramus l.st.

Arenaceous gryphean l.st. Kallar Member (6m) Quartzose and calcareous 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). This

formation had been assigned to Hauriceras rembda ammonite zone (Sastry et al. 1972;

Ayyasamy, 1990). The Kallar and Kattupiringiyam members of this formation represent the

Globotruncana Aegyptica foraminiferal zone while the Tancem member and

Srinivasapuram member represent lower part of Globotruncana gansseri foraminiferal

zone. The next younger Ottakoil Formation covers upper part of Globotruncana gansseri

foraminiferal zone.

METHODS AND MATERIALS

Systematic field mapping in the scale of 1:10,000 was conducted at intervals

ranging from 10 to 500m to document sedimentary structures, lithology, faunal occurrence

and association. Sampling was done to represent complete petrographic profile (sensu

Bathurst, 1987) of the formation from natural exposures, dug wells and mine sections. A

total of 459 locations were logged and sampled. A 38m long borehole core recovered from

southeast of Kallankurichchi Village (Fig.1) representing complete stratigraphic section of

the Kallankurichchi Formation was supplied by Tamil Nadu Cements (TANCEM) mines.

Field studies and laboratory observations of hand specimens under binocular microscope

were attempted towards understanding taphonomy of megafauna and depositional textures.

A total of 380 samples were subjected to petrographic study under polarized light after

staining the thin sections following the procedures presented in Adams et al. (1988).

Petrographic study was concentrated towards documentation of occurrence,

characterization and quantification of different types of grains, matrix and cement. Textural

classification following Dunham’s classification (Dunham, 1962) modified by Embry and

Klovan (1971) was attempted. Data from Ramkumar et al. (2004a) on organic carbon

content of these rocks were also utilized in this study. Compilation of the information

drawn from field, megascopic and microscopic studies enabled description of micro-

mesoscale lithology, genetic factors of rock components, recognition of standard

microfacies types and facies zones (Wilson, 1975; Flügel, 1982) and interpretation of

depositional environments of the Kallankurichchi Formation and also to draw inferences on

organic carbon production, preservation and maturation.

LITHOLOGY AND PETROGRAPHY

Rock types

The deposits of Kallankurichchi Formation could be broadly classified in to five

major lithological types viz., basal conglomerate, gryphean limestone, inoceramus

limestone, fragmental shell limestone and dolomitic limestone. These five major categories

are distinguishable in the field and also in hand specimens by their faunal composition,

lithological and bedding characteristics and the nature of bioclasts contained in them.

The conglomerates mark the base of Kallankurichchi Formation that could be traced

all over the study area. They reach a maximum thickness of 4 meters. Clasts in these rocks

are made up of boulder-rudite sized quartz, feldspar and lithoclasts of older formations,

especially the Sillakkudi Formation. They are well cemented. Towards top, they show a

transition from grain-supported to matrix-supported nature, reduction in size of clasts,

increase in roundness of clasts and proportion of matrix. They contain no unabraded whole

fossils. Based on these characteristics and according to the classification of Fritz and Moore

(1988), these are classified as quartz sandy to gravely conglomerates, typical of coastal

regions.

The gryphean limestone contains thick walled gryphea forming about 70% by

volume of the deposit. It appears that the grypheans lived in thick populations (Plate 1.1) in

colonies and formed reef like body to be termed as gryphean bank by Nair (1974). These

limestones are divided into lower and upper gryphean limestone deposits. The lower one is

pale pink to pink in color, arenaceous and ferruginous in nature and shows a simple

gradational contact with the basal conglomerate deposits, best expressed in Marudaiyar

river section located in northern bank of the river 400 meters west of the causeway leading

from Darani mines to Idaiyattangudi village. The upper gryphean limestone is yellow in

color and contains minor amounts of ferruginous materials, variable amounts of quartz sand

and silt. They contain local concentrations of bryozoa, exogyra, alectryonia and

terebratula. Admixture of finer siliciclastics and shell fragments becomes significant

towards top of these deposits. This is the widespread lithological type of this formation and

shows hard ground characteristics. The rocks are well-cemented, thick-very thick-bedded

showing horizantal, parallel and uniform bedding characteristics and a pronounced non-

depositional flooding surface (Plate 1.2). The inoceramus limestones show very thick to

massive, uniform and even bedded nature (Plate 1.3). Locally they developed diagenetically

enhanced bedding (sensu Bathurst, 1987) as a result of differential compaction and

dissolution at marine phreatic-burial stage of diagenesis (Ramkumar, 2001). In the field,

these deposits could be easily recognized with their faunal content, uniform texture, dusty

brown color and abundance of inoceramus shell cavities filled with large (2-4 mm sized)

dogtooth spars. They exclusively contain thick ribbed large (12 cm) to very large (50 cm)

shells of inoceramus that make upto 40 % of the rocks (Plate 1.4).

The fragmental shell limestone deposits are classified into three major categories,

viz., regular bedded fragmental shell limestone, hummocky cross-stratified fragmental shell

limestone and planar cross-bedded fragmental shell limestone. The nature, association,

bedforms and clast composition of these three major fragmental shell limestone deposits

suggest that source materials for these deposits were drawn from adjacently located and or

underlying gryphean limestone and inoceramus limestone during periods of remobilization

of sediments associated with sea level fall and high energy conditions.

The first one is thin-medium, parallel and even bedded and shows non-depositional,

feeble erosional and gradational contacts with inoceramus limestone and upper gryphean

limestone. The rocks contain local concentrations of whole shells of inoceramus, gryphea,

exogyra, bryozoa, alectryonia, stigmatophygus, terebratula, rhynconella, foraminifera and

ostracoda as well as unsorted, angular, coarse sand – coarse boulder sized shell fragments.

The rocks are matrix supported and contain occasionally the ammonite Hauriceras rembda.

These are essentially interbeds located within gryphean limestone and associated with

inoceramus limestone. The bioclasts do not show any or much transportation characteristics

in these beds. The second type consists of typically largescale hummocky cross-stratified

beds that show sharp erosional contact at their bottom with inoceramus limestone beds and

cross-bedded fragmental limestone beds (Plate 1.5) and gradational contact towards top.

The rocks contain boulder-gravel sized, well-rounded to subrounded shell fragments (Plate

1.6). Although the rocks show clast-supported nature, local variations to matrix-supported

nature is also discernible. The matrix is of coarse rudite-coarse sand sized bioclasts along

with intraclasts and coarse quartz sand. The deposits do not have unabraded whole fossils

except echinoids (Plate 1.6). Occurrence of ‘V’ shaped escape structure (Ramkumar, 2001)

is also recorded in these deposits. The third one shows well-developed planar cross bedding

(Plate 2.1) and contains erosional surface contact at bottom and top. The constituent grains

are sorted-well sorted, equigranular, grain supported rudite-coarse sand sized bioclasts. The

deposits form lenticular bedforms and are interpreted to be of sub-aqueous long shore bars

or shoals. These shoals were aligned parallel to paleoshoreline that restricted influx of

siliciclastics into the depocentres of bank facies limestone deposits (Ramkumar and

chandrasekaran, 1994; Ramkumar, 2001). The rocks also contain abundant trace fossils of

Planolites Nicholson, a sediment feeder (Chandrasekaran and Ramkumar, 1994). In

addition to these, there are foraminiferal log deposits confined with in paleochannels

associated with hummocky cross-stratified fragmental limestones. These logs are

recognizable in the field only through channel geometry.

The dolomitic limestones are jet black in color, highly compacted and well-

cemented fragmental shell limestones (Plate 2.2) and gryphean limestones (Plate 2.3) that

experienced diagenesis under semi-closed/closed system (Ramkumar, 2004a). They do not

show any significant difference from that of other limestone deposits other than dolomitic

content and viscous hydrocarbon residues within vugs and shell cavities. These limestones

are restricted within faulted zone southeast of Kallankurichchi Village.

Rock components

The allochems are represented by bioclasts, peloids and intraclasts. The bioclasts

comprise fragments of mollusca, bryozoa, foraminifera, ostracoda, echinodermata,

brachiopoda, worm tubes and algae in the order of decreasing abundance. Their taphonomic

characteristics suggest that they have not experienced significant transportation, sorting and

abrasion except few cases associated with cross-stratified and hummocky cross-stratified

beds. Most of the bioclasts and shells are affected by boring and micritization. Micritization

of grains is intense and had produced micritic coating around skeletal particles (Plate 2.4),

completely micritized bioclasts (Plate 2.5) and peloids (Plate 2.6). The role played by

boring organisms which affected meticulously shells of gryphea and alectryonia is also to

be considered in the process of breakdown of them. Based on the characteristics of these

bioclasts and the criteria listed out for bioerosion style of shell breakdown (Saltsman, 1986)

it is inferred that these shells first experienced biological breakdown followed by physical

breakdown and subsequently transported to form fragmental shell deposits. Intraclasts are

elongated, subrounded to well-rounded grains which comprise rounded fine sand sized

structureless bioclasts and fine sand-coarse silt sized quartz grains cemented by micritic

material and fibrous cement spars. These are interpreted to have been originated by

reworking of lithified sediments by strong currents (Rao, 1990).

Occurrence of large boulder sized clasts of quartz, fresh feldspar and lithoclasts of

Sillakkudi Formation deposited together in basal conglomeratic deposits of Kallankurichchi

Formation reinforces the interpretation of coeval continental erosion and recycling of older

sedimentary rocks. The quartz grains form significant proportions of rocks only locally and

are associated with cross-stratified and hummocky cross-stratified beds indicative of extra-

basinal sourcing of sediments only during periods of higher energy. The siliciclastics form

wide size range from very coarse sand to fine silt. Shape varies from highly angular to

rounded and thus speaks of varied source and distances of transport. That the depocenter

had drawn siliciclastic sediments from wide ranging continental sources could be gauged

from the fact that there are angular monocrystalline quartz grains as well as polycrystalline

fractured grains representing granitic and gneissic sources respectively. The occurrences of

orthoclase feldspar and quartz cobbles-boulders represent pegmatitic sources also. The

associated occurrence of hypersthene and monocrystalline, unabraded quartz suggest

charnockitic source, which is also located nearby (Sundaram, 1977).

Matrix of these rocks is made up of carbonate mud, very fine skeletal fragments,

very fine quartz silt and argillaceous materials. While the quartz silt and argillaceous

materials were inferred to be terrigenous, the carbonate mud and very fine sleketal

fragments were considered to be intrabasinal in origin. Three major agents have enacted

bioerosion and mud production. Swinchatt (1965) stated that in some cases, in the event of

destruction of organic binders, the entire shell might disintegrate because of lack of

cohesion soon after death. Bathurst (1971) stated that on breakdown, the molluscan,

bryozoan and brachiopod shells release crystals of micrite size. With addition of physical

force, faster mechanical disintegration might have taken place, producing mud. From the

descriptions of boring process (Bathurst, 1966), it appears that the intensity of boring by

sponges and micritization by algae, is very high in these rocks and might have produced

enormous quantity of mud.

The cementing mediums of these rocks are of micrite and sparry calcite. Among

them, the micritic cement is limited in extent and in turn shows many degrees of latter stage

neomorphic alteration. There are four types of sparry calcite cements namely, fibrous,

bladed, syntaxial rim and equant morphologies that vary in terms of mineralogy, relative

proportion, occurrence and association.

The fibrous cement is least abundant and is recognized only from its ghost structure

(Plate 3.1). It never fills interparticle pores completely. The fibrous cement spars are

arranged perpendicular to substrate and have irregular but sharp boundaries. Width of each

spar ranges up to 20 microns and length ranges up to 100 microns. They often show

corrosion surfaces in view of later neomorphic alterations to coarse ferroan calcite with

abundant inclusions. The bladed cement spars are also arranged perpendicular to the

substrate, never fill the interparticle porosity and have 30-80 microns width and length of

about three to four orders of width. These are non-ferroan calcitic (NFC) in original

mineralogy (Plate 3.2) and show alteration to slightly ferroan calcite (SFC) and ferroan

calcite (FC). The fibrous and bladed cement spars are interpreted to have been precipitated

at or near sediment-water interface under marine regime. Although these rocks have

undergone prolonged exposure to sediment-water interface as evidenced by hard grounds,

relatively lesser occurrence of fibrous and bladed cements in these rocks suggest that either

these cements might have been altered and or dissolved completely during latter stage

diagenesis.

Syntaxial rim cements (Plate 3.3) are found to occur in optical continuity with

echinoderm plates and fragments. Size of these spars depends on the size of host grains and

pore space available. These are characteristically NFC in mineralogy and show alteration to

SFC and FC along their peripheries. Equant sparry calcite cements are most common and

widespread in the rocks under study. They completely fill interparticle (Plate 3.4),

intraparticle (Plate 3.5), mouldic and other primary and secondary porosities (Plate 3.6).

Their cementation origin is evidenced by occurrences of frequent enfacial junctions,

competitive growth structure, compromise structure, increase of size towards centre of the

pore, etc. Size of these spars vary from fine sand to >2mm and mostly depends on available

pore space. The spars are NFC, SFC and FC types. While NFC equant and syntaxial rim

cement spars are interpreted to have been precipitated under marine phreatic-burial zone,

the SFC and FC equant spars interpreted to have been precipitated under meteoric phreatic

zone.

From these characteristics of cement spars it appears that cementation in these rocks

was mostly through dissolution and precipitation regardless of stage and zones of

diagenesis. Source of cementing medium was available nearby. Although early stage

cement survived later stage diagenesis, it was only in small quantities and that too after

undergoing neomorphic alteration.

Textural classification

The wackstones are found in gryphean and inoceramus limestones. They contain

predominantly whole shells and angular fragments of mollusca and bryozoa and are termed

as molluscan wackstone and bryozoan wackstone based on their relative proportion of

fossils. They also contain minor quantities of foraminiferal tests, echinoderm plates, few

ostracodes and fine sand sized quartz grains totaling less than 10% of grains. Matrix is of

carbonate mud. The dominance of one or two types of skeletal fragments, wide size range

of bioclasts, unsorted nature, high angularity of bioclasts and noticeable amount of mud

suggest prevalent quite environmental conditions with moderately circulated waters.

Packstones are found predominantly in bedded fragmental shell limestones and

form minor proportions of gryphean limestone. Dominant proportion of the packstones is

constituted by mollusca and bryozoa (Plate 4.1) in relatively equal quantities. Complete

bryozoan froands are also common. Bioclasts show unabraded nature and wide size range.

The molluscan fragments are smaller in size and almost equigranular. They show

subangular to subrounded nature when associated with coarse monocrystalline quartz

grains, foraminiferal and ostracod tests, peloids and rounded echinoderm fragments.

Occurrence of abraded molluscan grains in carbonate mud and argillaceous matrix (textural

inversion – deposition of grains characteristic of higher energy in lower energy zones) is

interpreted as deposition in fairly deeper water conditions adjoining relatively higher

energy conditions of depocenter. Micrite forms matrix and cement although at instances

sparry cement could also be observed resulted by latter stage alteration. Well mixed

conditions of deposition could be interpreted from varied bioclasts in packstones.

The grainstones show wide variety namely, molluscan grainstone, molluscan

bryozoan grainstone, echinodermal molluscan grainstone, foraminiferal grainstone and

foraminiferal molluscan grainstone. The molluscan grainstones consist of large,

subangular-subrounded, moderately-well sorted molluscan grains with minor quantities of

bryozoan fragments. When large bryozoan fragments are abundant, they are termed as

molluscan bryozoan grainstones. These two types of grainstones are found in inoceramus

limestones. The echinodermal molluscan grainstone consists of well rounded echinoderm

fragments and plates, molluscan grains and minor quantities of intraclasts and quartz grains.

Grains are equigranular and are found in hummocky cross-stratified fragmental shell

limestone and cross bedded fragmental shell limestone. These petrographic types are

reported to be common in storm deposits and shoals (Tucker, 1985). Foraminiferal

grainstone and foraminiferal molluscan grainstones (Plate 3.4) contain all other types of

bioclasts in various proportions. These are associated with upper gryphean limestones that

have significant shell fragments. The ostracod-peloid grainstones are well sorted, well

rounded and equigranular in nature. These are exclusively associated with cross bedded

shell fragmental limestones. From the characteristics of grainstones, it is inferred that they

received source materials from bank facies limestones and the grains experienced

noticeable transport and sorting. Deposition of grainstones took place in comparatively

higher energy conditions under well circulated waters.

The floatstones are exclusively associated with upper gryphean limestone and

gryphean fragmental shell limestone. They contain whole fossils of gryphea, exogyra and

gravel to very coarse sand sized platy shell fragments. They show a range from molluscan

floatstone to bryozoan floatstone, with many intermediate varieties with reference to

relative proportion of molluscan and bryozoan grains. The quartz sand, foraminiferal tests,

echinoderm fragments and peloids are found in minor quantities. The molluscan grains are

highly angular to subrounded in nature in floatstones. Bryozoans are represented by very

large colonies and very coarse-fine sand sized fragments with angular-subangular-well

rounded nature. Sorting is poor. The foraminifers are in the form of complete tests with no

signs of abrasion and deformation. Echinoderm grains are angular-subangular. The peloids

are elongated and are in minor quantity. Quartz grains are fine-coarse grained and

monocrystalline, equigranular and subangular. Matrix of floatstones is carbonate mud with

little amounts of finer bioclasts and clay. The carbonate mud alters to micro and psedospars

of SFC and FC. Cement spar ranges upto 20% and might have been incorporated into the

floatstones as a result of latter stage dissolution of mud and infilling by cementation.

Cements are represented by fibrous and bladed morphologies, indicative of marine

sediment-water interface and early stage marine phreatic cementation. However, equants of

meteoric origin are also observed. The floatstones suggest the prevalence of intense

fragmentation and little transportation of shells from gryphean limestones.

The rudstones (Plate 4.2) are predominantly observed in fragmental limestones with

whole shells of exogyra and in minor quantities in gryphean fragmental limestone, cross-

bedded shell fragmental limestone and hummocky cross-stratified fragmental shell

limestone. They are primarily constituted by molluscan grains, followed by grains of

bryozoa, echinodermata, foraminifera, quartz and ostracod in the order of decreasing

abundance. The molluscan and bryozoan grains are subangular to subrounded to rounded

and are coarse sand-gravel in size with variable sorting. The echinoderm fragments and

spines are coarse sand sized and show well rounded to subrounded nature. The peloids are

irregular in shape. Foraminifer and ostracod tests are complete. Quartz grains are

subangular to subrounded and show equigranularity and monocrystallinity. The rudstones

are characterized by general absence of matrix. The cement spars are of pore filling NFC

equants. Bladed and syntaxial rim spars are in minor quantities. At places, the quantum of

cement ranges to 50 % by volume. Complete absence of mud and occurrence of abraded

grains suggests rapid deposition and higher energy conditions.

FACIES ZONES AND STANDARD MICROFACIES TYPES

Following the keys and procedures listed in Wilson (1975) and Flügel (1982),

standard microfacies and facies zones were interpreted using field, petrographic and faunal

information. Three facies zones namely, FZ 2 - Shelf facies – Open circulation (comprising

whole fossil wackstone-SMF 8 and coated bioclastic packstone-wackstone-SMF 10), FZ 6-

Shoal environment in agitated water facies (comprising SMF 11-coated bioclastic

grainstone and SMF 12-Bioclastic grainstone-rudstone) and FZ 7-Restricted marine shoals

facies (represented by SMF 16-peloidal grainstone and SMF 18-foraminiferal grainstone)

were recognized.

The SMF 8 is recognized in the field as gryphean limestone and inoceramus

limestone. These rocks show local development of bryozoan colonies also. The thick,

homogenous bedded nature of these limestones indicates that these were shelf deposits. The

hard ground surfaces of gryphean limestone indicate slow sedimentation. Other bedding

surfaces of this unit suggest prevalent diastems which is also characteristic of shelf deposits

(Wilson and Jordan, 1983). Thick population of gryphea and bryozoa and their lifestyle

suggest slightly agitated and circulated water conditions (Guha, 1987; Ramkumar and

Chandrasekaran, 1996). The fossil, structural, lithologic and petrographic observations

suggest deposition of this facies under photic zone at less than 100 meters of bathymetry

(Ramkumar, 2001). Development of distinct diagenetic bedding in inoceramus limestone

suggests mid-shelf origin (Bathurst, 1987). Prevalence of well-oxygenated, clear, turbid

free, normal saline, warmer water is also indicated. Substrate was hard in gryphean

limestone whereas it was slightly muddy in the case of inoceramus limestone.

The SMF 10 is recognized in the field as thin-medium bedded fragmental shell

limestone that is associated with upper gryphean limestone deposits. The rocks contain

diverse bioclasts and at times quartz. The bioclasts show unabraded to abraded nature.

Grain sorting is poor. Mud forms significant proportion. Micritization of bioclasts and tests

and micritic coating in bioclasts are common. These are interpreted as sediments formed in

higher energy conditions but transported and deposited in areas located along local slopes

of banks and shoals under quite water conditions.

The SMF 11 is recognized in the field as cross-bedded and hummocky cross-

stratified limestone deposits. These are devoid of any mega fossils except free living

echinodermata stigmatophygus elatus and ammonite Hauriceras rembda. The grainstones

contain sorted, equigranular, subangular to well-rounded molluscan and all other bioclasts.

The grains are generally micritized or at the least have micritic coating. These have been

deposited as shoals in areas of constant wave action at or above wave base and hence mud

was winnowed away. These deposits have acted as barrier bars and controlled the seaward

movement of siliciclastics. Bathymetry of these deposits is interpreted to be of less than 25

meters on the basis of sponge microboring and algal micritization (Swinchatt et al. 1965;

Ramkumar, 1996).

The SMF 12 is recognized in the field as medium, even bedded fragmental shell

limestones containing coarse sand to gravel sized shell fragments and whole shells of

exogyra, gryphea and inoceramus. These units are found interbedded with gryphean

limestones and inoceramus limestones with gradational contacts. These are primarily

grainstones and rudstones which predominantly contain molluscan grains and all other

bioclasts and quartz grains in various proportions. The bioclasts were sourced from

adjacently located bank facies limestones and inoceramus limestones and deposited as

organic debris without mud matrix. The presence of varied bioclasts with admixture of

quartz suggests the role played by strong open circulation and terrigenous influx. The fossil

association suggests deposition under photic zone of shallow littoral regions with normal

saline and warm waters.

The SMF 16 (peloidal grainstone) is recognized in the field as thin bedded

bioclastic limestone units associated with cross-bedded fragmental shell limestones as

inliers. These inliers show lenticular geometry, showing thickening for short distances and

die down towards west in the field. These are peloidal grainstones and also include peloid-

ostracod grainstones constituting primarily peloid, ostracod and foraminiferal tests with

minor quantities of molluscan and echinoderm grains. These are interpreted to have been

deposited under restricted marine conditions (Write, 1986). These rocks show gradation to

packstone and wackstones. Towards mud dominated varieties, the percentage of

echinoderm grains reduces to become absent. The rocks are totally devoid of any mega

fossils. These are inferred to have been associated in areas of restricted marine shoals to

form tidal flats and natural levees.

The SMF 18 is recognized as pockets associated with thin bedded bioclastic

limestone in continuation of peloidal grainstones. They exclusively contain foraminifera

and rarely minor amounts of other bioclasts. These have huge void filling cement spars of

marine phreatic zones. These are interpreted to have been deposited as concentrations of

foraminiferal tests in tidal bars and channels.

The more mud dominated, argillaceous wackstones which are encountered in the

easternmost extensions of this formation (Nair, 1974, 1978; Ramkumar and

Chandrasekaran, 1994) and also in a core section obtained from southeast of

Kallankurichchi Village, suggest prevalent basinal conditions of deposition. These

argillaceous wackstones are homogenous, finely laminated and lack any whole mega fossil,

probably representing basinal extension of the exposed Kallankurichchi formation rocks

deposited below storm weather wave base.

DEPOSITIONAL MODEL The geographic positions of facies zones from west to east and their younging

nature are such that from west to east, the shallow water carbonates pass gradually offshore

to deeper water and then finally into basinal mudstones without any major break in

depositional slope, fitting into the description of a carbonate ramp. While the coastal

conglomerates and tidal channel log deposits represent the very shallow water deposition,

the inner ramp deposits are typically represented by distinct shoal environment in agitated

water facies zone (FZ 6) wherein the gradual shoaling of a ramp resulted in formation of

shoreline carbonate sand bodies which in turn are associated with storm deposit. This trait

is said to be a confirmative characteristic of carbonate ramp by Aigner (1982) and Aigner

and Reineck (1982). These shoals have received sediments from bank facies as a result of

shoreward movement of materials in response to storm waves. The restricted marine

conditions (FZ 7) produced by FZ 6, presence of tidal bar and channel deposits (SMF 18) in

association with shoal facies limestone and their lenticular bedform are all suggestive of the

ramp with typical of beach barrier-lagoon ramp model wherein these tidal channels might

have cut through the barriers as a passageway between open marine and lagoonal parts of

the depositional basin. The linearity of shoal/barrier bar along strike direction of the

formation and its association with shell logs are all suggestive of tidal channels crossing

shoal (Tucker and Wright 1990. p.143). The shelf facies – open circulation (FZ 2)

deposition in areas between fair-weather wave base and storm weather wave base region

represents deeper ramp wherein organic productivity was high. Associated with this is the

bioclastic limestones deposited during periods of higher energy that in turn reworked

bottom sediments (Kreisa and Bambach, 1982). Finally, the gradation of whole fossil

wackstone-packstone and bioclastic limestones into more muddy and argillaceous

lithologies characteristic of basinal regions shows gradual increase of depth. All these

factors also suggest that this formation could be ascribed into distally steepened ramp

(sensu Read, 1982, 1985) and is comparable to the carbonate ramp deposits of Upper

Muschelkalk (Triassic of southwestern Germany) in terms of juxtaposition of facies zones

as detailed by Aigner (1984).

DEPOSITIONAL HISTORY

From the stratigraphic arrangement of the lithofacies, it is inferred that 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, may be indicative of continued sea level rise and

stabilization of depositional environments conducive to organic reef building. In due course

of sea level rise, the gryphean banks 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 and 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.

Considering the stratigraphic facies changes in this formation, often initiated under the

influence of sea level change and the records of influence of global sea level changes in this

part of this basin (Raju et al. 1993; Hart et al. 2000; Ramkumar et al. 2004b) it could be

interpreted that sea level change was the significant agent that exerted influence on the

depositional history of this formation. 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, 1999).

HYDROCARBON POTENTIAL

The occurrence of bitumen residues in vugs of carbonate rocks exposed in mine

sections (Ramkumar, 1995, 2004a; Yadagiri and Govindan, 2000) located in the

Kallankurichchi Formation and the oil find in Ariyalur-Pondicherry sub-basin of the

Cauvery basin (Govindan and Ramesh, 1995) wherein the Kallankurichchi Formation is

located have made the exploration scientists clamor for significant oil find in this

formation. Their expectations could be substantiated by the facts that the Kallankurichchi

Formation is primarily constituted by large sized and very thick population of organisms;

the remaining bioclastic deposits are all derivatives of these bank facies limestones; the

depocentres of these carbonates experienced insignificant terrigenous contamination and

thus prolific carbonate production took place and was supported by luxuriant environmental

conditions for organisms. These environmental conditions suggest generation of abundant

organic matter that could have been preserved and matured to become commercial

quantities of hydrocarbon in this formation.

Data after Ramkumar et al. (2004a) indicate organic carbon content of these rocks is

0.687 wt.%. Neeraja (1997) and Yadagiri and Govindan (2000) confirmed the presence of

hydrocarbon residues in dolomitic limestones with an extractable organic carbon content of

0.7250 wt.%. Oblivious to these data, the depositional conditions might have discouraged

significant preservation of organic matter owing to the following.

a. The depositional waters were well-oxygenated and there was open circulation as

revealed by diverse and abundant faunal composition of the rocks. These two

parameters would have encouraged the organic matter produced to get oxidized rather

than to get preserved in sediments.

b. Occurrence of extensive hard ground surfaces indicate that the sediments deposited

might have experienced prolonged exposure to the sediment-water interface as

evidenced by extensive biological boring of gryphea and alectryonia shells and

micritization of bioclasts, synsedimentary cementation at sediment-water interface as

revealed by the presence of fibrous and bladed cement morphologies during

depositional stage and large scale conversion of bioclasts, matrix and cement made up

of unstable aragonite and high magnesian calcite into stable calcite before burial stage

diagenesis as evidenced by the low magnesian calcitic mineralogy of bioclasts, matrix

and cement and complete absence of aragonite. These organic consumption of organic

matter by boring, release of organic matter bound in sediments by conversion of

unstable aragonite and high magnesian calcite might have reduced significantly the

organic matter content of sediments during depositional stage itself. 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. The effect of metabolism of organisms

would have encouraged carbonate dissolution and synsedimentary cement precipitation

(Kropp et al. 1997) the processes that accelerate reduction of organic matter contained

in sediments. Consumption of organic matter by borers and bacteria might have also

reduced the organic matter content of sediments.

c. During deposition, there were minor to major sea level oscillations that might have

triggered rapid cementation (Friedman, 1998; Booler and Tucker, 2002) that reduced

organic matter contained in original sediments. 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 of the

host rocks. From the observations of these authors and comparing them with the

characteristics of rocks under study, it is surmised that same might have happened in

this formation also, reducing hydrocarbon generation potential.

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

stage promotes redistribution of organic matter in carbonate sediments. However,

occurrence of undisturbed interparticle porosity in these rocks indicates prevalent less

significant compaction, limited overburden and or extensive cementation prior to

compaction (Schneider et al. 1996) at marine burial stage of the Kallankurichchi

Formation. Leythaeuser et al. (1995) 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. However the carbonates of

Kallankurichchi Formation have not experienced significant physical compaction

during marine burial stage and instead, were subjugated to chemical compaction

thwarting survival of organic carbon (Ramkumar, 2004c). As diagenesis at this stage

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

upto the extended zone of bioturbation (Droser and Bottjer, 1988) took place that would

have aided coeval chemical compaction and dissolution of less/unstable carbonate

phases and thus destruction of organic carbon.

e. The marine burial stage diagenesis had promoted mouldic porosity in inoceramus

limestones, extensive occurrence of which in carbonates is considered to be good

reservoir quality. However, availability of cementing material and immediate

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

porosity in inoceramus limestones. 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 immediate cementation.

f. Production of abundant organic matter in these sediments and their destruction by

biological consumption and other environmental factors prevalent during deposition

and dissolution-precipitation mode of diagenesis could also be gauged from the facts

that the TANCEM member which has experienced faster rate of deposition, lesser

exposure to sediment-water interface and synsedimentary lithification has highest

organic carbon content (0.8235 wt.%; n=4) while the inoceramus limestone that

experienced comparatively higher physical and chemical compaction has least organic

carbon content (0.6536 wt.%; n=8) and the gryphean limestones that experienced

prolonged exposure to sediment-water interface and synsedimentary lithification have

intermediate levels organic carbon content (0.7833 wt.%; n=5) against the overall

average organic carbon content of this formation (0.6870 wt.%; n=24). The Kallar

member which has conglomeratic deposits and arenaceous gryphean limestone shows

the least organic carbon content of all (0.5784 wt.%; n=7) signifying lesser biological

organic carbon production owing to terrigenous contamination of carbonate

depocenters.

g. It is to be noted that the rocks under study also underwent cementation in the meteoric

phreatic and vadose zone. Cementation in these two zones 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 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 and Ramesh, 1995; Powel et al. 1988), prevalent open system of

diagenesis in meteoric regime, higher atmospheric temperature and PCO2 during end-

Cretaceous (Ramkumar et al. 2004a, c), 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).

h. The information that the dolomitic limestone which is actually upper gryphean

limestone and fragmental limestone of TANCEM member has shown the development

of viscous hydrocarbon residues contains extractable organic carbon of 0.7250 wt.%

(Neeraja, 1997; Yadagiri and Govindan, 2000) which is similar to the values of non-

dolomitized TANCEM member and Srinivasapuram member, which means, the amount

of organic carbon left in these rocks prior to dolomitization event (inferred to have

occurred during Quaternary event based on the faulting event that created semi

closed/closed system of diagenesis for dolomitization of down thrown block of

Kallankurichchi Formation also affected Mio-Pliocene Cuddalore Sandstone Formation,

Ramkumar, 2004a) were all same. Hence, it could only be safely inferred that, only the

closed/semi-closed system of diagenesis that took place in a limited extent favored

conversion of little organic carbon into bitumen residue in these rocks.

CONCLUSIONS

a. The Kallankurichchi Formation is a predominant carbonate deposit consisting of five

major lithological types recognizable in the field each with distinct faunal,

sedimentological and petrographic characteristics.

b. The formation was deposited in a distally carbonate ramp system with lagoon-barrier

bar-bank-open sea depocentres, complete with tidal channels. Principal depocenter was

bank region from where other regions sourced materials during periods of change in

energy conditions associated with sea level oscillations, presumably related with global

sea level cycles.

c. The rocks were deposited under quite to moderate energy conditions with occasional

short-lived high-energy conditions (Ramkumar, 1995, 2004a, b, c). Shallow marine,

normal saline, well-oxygenated water conditions were prevalent during deposition.

Significant deposition took place under photic region and the rate of deposition was low

for major part of the formation except the storm deposits.

d. The characteristics and porosity evolution of Kallankurichchi Formation closely

resembles the depositional and diagenetic history of Middle Jurassic Araej Formation of

Southern Arabian Gulf region as described by Alsharhan and Whittle (1995). The Araej

Formation is a major hydrocarbon reservoir. Drawing analogies from similarities

between these two formations on petrographic features and the presence of bitumen in

dolomitic limestone may encourage targeting the Kallankurichchi Formation for

detailed hydrocarbon potential. However, prevalence of diagenetic transformations right

from sediment-water interface, and oxygenated circulatory waters during diagenesis

could have destroyed primary organic matter in this formation.

e. Based on the limited occurrence of hydrocarbon residues in shell cavities preserved

away from open system of diagenetic waters, it is interpreted that the organic matter

survived in these rocks from consumption by organisms during deposition, oxidation

and destruction in marine phreatic-burial stage were totally destroyed before significant

maturation into hydrocarbons by meteoric 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 with low maturity value of Tmax 376°C.

ACKNOWLEDGEMENTS

Prof.V.A.Chandrasekaran (Retired), Prof.K.Ramamoorthy, (Retired) and Dr.V.Subramanian, Reader, Department of Geology, National College, Tiruchirapalli, extended academic and logistic and moral support. Prof.G.Victor Rajamanickam, (Retired), helped in preparation of photomicrographs. Mr.Predrag Zrinjsak, Germany, is thanked for his assistance in carbon analyses. Permission to collect samples was accorded by the mines managers and geologists of Messers. Dalmia Cements Pvt.Ltd, TANCEM Mines, Ramco Cements, Nataraj Ceramics Ltd., Vijay Cements, Fixit Mines, Parveen mines and Minerals Ltd., Alagappa cements, Rasi cements, Tan-India Mines, TAMIN mines and Chettiyar mines. 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. Financial assistance to this work was provided by CSIR, New Delhi and Alexander von Humboldt Foundation, Germany.

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Fig. 1 Location of the study area

Plate 1

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.

Flooding surface in gryphean limestone. Location of the photograph is mine II Dharani mines

Inoceramus limestone showing very thick and massive bedded nature. Also note the mega fracture and channel porosity filled with secondary calcite, primarily kankar signifying Quaternary origin. Complete dissolution along the fractures and channels is observed indicating dissolution-precipitation mode of digenetic transformation by oxygenated meteoric water under open system which in turn would have destroyed organic matter in the rocks that escaped biological consumption during deposition and early diagenesis. Location of the photograph: Northern wall of Bench I, FIXIT mines.

Close-up view of inoceramus limestone showing the thick population of shells. Location of the photograph: Southeastern wall of Bench I, FIXIT mines.

Exposure of hummocky cross stratified fragmental shell limestone resting over cross bedded fragmental limestone. The bedding plane at which a pen is placed for scale separates these two units. Location of the photograph: Bench I of Mine I, TANCEM mines.

Planar view of the hummocky cross stratified fragmental shell limestone deposits showing the well rounded platy shell fragments along with whole fossils of Stigmatophygus elatus near the pen placed for scale. Location of the photograph: Bench I of Mine I, TANCEM mines.

Plate 2

Planar cross bedded fragmental shell limestones exposed in the mine floor of Bench II, Mine I of TANCEM mines. In cross section, these appear to be massive as indicated in plate 1.5.

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.

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 progression of micritic coating towards centre of the bioclast. Scale bar 0.5mm.

Photomicrograph showing molluscan ostracod peloidal grainstone. Note the presence of partially and completely micritized shell fragments and complete tests. Scale bar is 1.5 mm.

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

Plate 3

Photomicrograph showing the ghost structures of fibrous cements (a) arranged perpendicular to the molluscan bioclast (b). On top of the fibrous cement layer, corrosion surface indicated by an arrow could be seen. Scale bar is 100 microns.

Photomicrograph showing the pristine bladed spars (a) of sediment-water interface cementation abutting against large blocky spars (b) of marine phreatic zone of cementation separated by a corrosion surface in between (arrow). Scale bar 0.5mm.

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.

Photomicrograph of molluscan foraminiferal grainstone. It also shows 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 stability of LMNFC spars towards meteoric diagenesis. Scale bar 1mm.

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.

a

b

a

b

a

a

b

c

Plate 4

Photomicrograph showing molluscan-bryozoan packstone. Note the mud filled interparticle porosity, angularity of bioclasts and unsorted nature. Scale bar is 0.7mm.

Photomicrograph showing molluscan peloidal rudstone. Scale bar is 2 mm.

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