CARBONATE DIAGENESIS IN THE KALLANKURICHCHI FORMATION, articles/DIAGENESIS.pdf · CARBONATE...
Transcript of CARBONATE DIAGENESIS IN THE KALLANKURICHCHI FORMATION, articles/DIAGENESIS.pdf · CARBONATE...
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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: [email protected]
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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