Post on 31-Dec-2019
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CHAPTER 3
PETROGRAPHY OF CARBONATE ROCKS
3.1 Introduction
Petrography is the most effective way of knowing the mineralogical and
other constituents of the carbonate rocks.
3.2 Methodology
For petrographic studies, thin sections were prepared and studied under
microscope, following the procedure of Folk (1959). The first part of the rock name
refers to the allochem components and the second part to the cementing or matrix
material. Because, of the above-mentioned advantages, the Folk’s terminologies
were adopted in the study of the thin sections of the limestone. The volumetric
analysis of different constituents had done with the help of manual counter, for the
petrographical classification of limestone. The textural study had carried out for
Dunham’s classification (Dunham 1962).
3.3 Petrography
The petrographic study from thin sections was prepared to identify and
decipher the nature of different constituents and to classify the limestone. The
constituents of limestone is grouped as follows-
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3.3.1 Terrigenous constituents
These are the materials derived from outside of the basin of deposition by the
process of weathering, erosion and transportation, as sediments to the depositional
site. The observed terrigeneous materials are colorless anhadral quartz grains and
shows wavy extinction under crossed nicols (Table 3.1; Fig 3.14, 3.15, 3.16, 3.17,
3.18 and 3.19). Colourless sub-hadral feldspars crystals shows lamellar twining and
plagioclase feldspars are also seen, but in low amount. Iron Oxides are irregular
patches with high relief and dark in colour in both polarized light and Crossed
Nicols.
Terrigeneous constituents, of the Lakadong Limestone vary from 0.51 % to
3.84 %; Terrigeneous constituents, of the Umlatdoh Limestone vary from 0.70 % to
1.39 %; and that of the Prang Limestone vary from 0.90 % to 1.80 % (Table 3.1).
3.3.2 Allochemical constituents
The Allochemical constituents or ‘Allochems’ are the dominant constituents
and are those materials which are formed within the basin of deposition by chemical
and bio-chemical precipitation, but are organized into discrete aggregated bodies
and for the most part have suffered some degree of transportation (Folk, 1959). The
allochemical constituents of the study area are as follows-
(a) Intraclast,
(b) Oolite,
(c) Fossil, and
(d) Pellet
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(a) Intraclasts, including pene-contemporaneously formed (early carbonate
sediments) fragments, that have been eroded or by biochemical activity from the
adjoining parts of the sea bottom and redeposited to form new sedimentary rock
(Folk, 1959). In most of the cases, they are weakly consolidated carbonate sediments
and few of them show partial microcrystalline fine-grained carbonates. In shapes,
rounded to elliptical and occasionally are irregular.
Intraclasts, of the Lakadong Limestone vary from 1.75 % to 5.17 %;
Intraclasts, of the Umlatdoh Limestone vary from 1.15 % to 4.23 %; and that of the
Prang Limestone vary from 1.58 % to 2.63 % (Table 3.1; Fig 3.14, 3.15, 3.16, 3.17,
3.18 and 3.19).
(b) Oolites, are spherical or sub-spherical accretionary bodies, less than
2mm in diameter and in thin section oolites show often concentric structures.
Superfical oolites have a large nucleus surrounded by only one layer but some
oolites are also spherulitic. Spherulites are sub-spherical bodies with radial
structures that have formed in situ. Unlike spherical oolites, superfacial oolites have
an irregular surface. For volumetric analysis, superfacial oolites and spherulites,
were considered as oolites.
The nucleus of the oolites may composed by a carbonate, but it is differ from
carbonate of the concentric layers in color and texture. The outer layer is mostly
composed of fine-grained carbonate, which envelops the microspar and low sphere
calcite. In some cases, fossils, pellets and intraclasts form the nucleus of the oolites.
The number of concentric layers, surrounding the nucleus varies widely. In some
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other cases, merely one layer is surrounding the nucleuses (pseudo - oolites) which
in other, up to four layers.
Oolites of the Lakadong Limestone vary from 1.04% to 4.85 %; Oolites of
the Umlatdoh Limestone vary from 0.32 % to 2.94 %; and that of the Prang
Limestone varies from 1.96 % to 3.70 % (Table 3.1; Fig 3.14, 3.15, 3.16, 3.17, 3.18
and 3.19).
(c) Fossils are the broken and unbroken skeleton and trace of carbonate
secreting organisms is one of the major constituents of these three limestone
members. Fossils are study in thin sections, and are mega and micro foraminifer and
calcareous algae. Important foraminifers are Numulites, Textularia, Assilina,
Discocyclina, Alviolina and Rotalia.
Petrographic identification of the various skeletal materials are based on
internal and morphological structures. Most of the skeletal grains are broken,
angular and fragmented. The chambers of the foraminifers are occupied by micrite,
microspar and spary calcite.
Fossils, of the Lakadong Limestone vary from 42.0 % to 54.50 %; Fossils of
the Umlatdoh Limestone vary from 38.74 % to 53.80 %; and that of the Prang
Limestone vary from 50.4 % to 58.28 % (Table 3.1; Fig 3.14, 3.15, 3.16, 3.17, 3.18
and 3.19).
(d) Pellets are the homogeneous aggregate of microcrystalline calcite and
small spherical to elliptical shaped body and devoid of any internal structure. Their
size and shape is well-rounded and sorted averaging 0.03 to 0.20mm 0.03 to
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0.15mm. Most of the pellets are tiny mud balls produced by bottom dwellers,
deposits feeding organisms that ingest the bottom sediments, digest the contained
organic matter and excretes the undigested carbonate sediments as more or less
coherent faced pellets. The Pellet is somewhat harder than surrounding mud. Pellets
distinguished from oolites by their lack of radial or concentric structure, and from
fossils by their lack of internal structures and from intraclasts by their uniformity of
size and shape.
Pellet, of the Lakadong Limestone vary from 0.0 % to 0.92 %; pellet, of the
Umlatdoh Limestone varies from 0.0 % to 1.22 %; and that of the Prang Limestone
varies from 0.0 % to 0.70 % (Table 3.1; Fig 3.14, 3.15, 3.16, 3.17, 3.18 and 3.19).
3.3.3 Orthochemical constituents
Orthochemical constituents are formed in the basin of deposition or within
the rock itself. They show little or no evidence of significant transportation. These
include (A) Microcrystalline calcite matrix (micrite) and (B) Sparry calcite (spar).
Folk (1959) used the term ‘Micrite’ for microcrystalline calcite and are ‘clay
sized carbonate’ materials measuring 1 to 4 micron in size.materials 5 to 10 microne
(or even 50 microne) was termed microspar. In handspecimen, micrite is a dull,
ultra-fine grained material ranging from white through grey to black. Under
petrological microscope it is sub-translucent and a fine brownish color. Grey micrite
is consider to form by rapid chemical or biochemical precipitation of the seawater,
settling to the bottom and at time suffering some later drifting by weak currents
(Folk, 1959). Microcrystalline calcite ooze also forms the matrix of poorly washed
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limestones and pellets, intraclasts and some oolites. Micrite is the chief constituents
of present limestone.
Microcrystalline calcite matrix (micrite), of the Lakadong Limestone varies
from 11.32 % to 23.15 %; microcrystalline calcite matrix (micrite), of the Umlatdoh
Limestone varies from 19.20 % to 39.12 %; and that of the Prang Limestone varies
from 24.55 % to 34.40 % (Table 3.1; Fig 3.14, 3.15, 3.16, 3.17, 3.18 and 3.19).
Sparry calcite (spar) distinguished from micrite by its clarity as well as
coarser crystal size, which may range up to 10 micron or more and occur as pour
filling cement. Sparry calcite matrix is a clear, coarsely crystallized material
showing well-defined grain boundaries and often displays cleavage traces. Sander
and Friedman (1967) used the term ‘spar’ for its relative clarity both in thin section
and hand specimens, which distinguishes it from microcrystalline calcite matrix.
Sparry calcite is commonly a ‘Pore filling cement’ that fills the pores of the
framework elements i.e. ooids, fossils and pellets. The grain size or crystal size of
sparry calcite (spar), depands upon the size of the pore space and rate of
crystallization. Sometimes, it has formed by neomorphism or recrystallization of the
former carbonate grain or microcrystalline calcite matrix.
Sparry calcite (spar), of the Lakadong Limestone varies from 18.21 % to
31.5 %; sparry calcite (spar), of the Umlatdoh Limestone varies from 14.72 % to
29.36 %; and that of the Prang Limestone varies from 4.7 % to 11.10 % (Table 3.1;
Fig 3.14, 3.15, 3.16, 3.17, 3.18 and 3.19).
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Table 3.1: Petrographical constituents (%) of the Limestones of the study area.
Petrographical constituents (%) of the Lakodong Limestone.
Sample
No.
Sparry
Calcite
Microcrystalline
calcite
Fossils Oolites Pellets Intraclast Terrigenous
Constituents
LDL1 25.56 19.3 46.61 2.53 0.42 2.35 3.23
LDL2 21.34 22.5 48.51 2.64 0.92 3.58 0.51
LDL3 23.53 17 49.32 3.27 0.51 3.13 3.24
LDL5 21.15 21.17 47.21 3.21 0.53 4.41 2.32
LDL6 18.21 21.13 50.2 3.64 0.61 3.1 3.11
LDL7 21.9 17.67 50.34 4.08 0 4.5 1.44
LDL8 19.34 17.32 52.41 3.32 0 4.41 3.2
LDL9 31.5 11.32 49.5 1.5 0 2.34 3.84
LDL10 21.35 23.15 42.62 3.66 0.8 5.17 3.25
LDL 11 28.23 12.62 54.5 1.04 0 1.75 1.86
Petrographical constituents (%) of the Umlaldoh Limestone. UML1 17.45 31.32 41.64 2.94 1 4.23 1.42
UML2 17.23 32.65 42.56 2.45 1.13 3.57 0.41
UML3 14.73 38.72 42.23 0.6 0.4 2.15 1.17
UML4 14.72 38.73 42.23 0.6 0.4 2.18 1.14
UML5 16.73 23.3 53.6 1.42 1 3.12 0.83
UML6 20.4 19.2 53.8 1.3 0.9 3.7 0.7
UML7 15.8 37.82 42.33 0.6 0 2.04 1.39
UML8 17.93 36.5 41.68 0.56 0 2.11 1.22
UML9 16.53 39.12 40.23 1.6 0.5 1.15 0.87
UML10 20.5 31.96 41.74 1.72 0 2.45 1.63
Petrographical constituents (%) of the Prang Limestone. PRL1 4.71 32.43 56.36 2.42 0.63 1.81 1.64
PRL2 8.4 27.2 58.28 2.72 0.4 1.6 1.4
PRL3 9.25 24.55 57.67 3.7 0.42 2.61 1.8
PRL4 11.1 33.1 50.4 1.96 0.7 1.74 1
PRL5 9.97 34.4 48.56 2.5 1.27 1.7 1.6
PRL6 5.54 31.27 57.48 2.7 0 1.58 1.43
PRL7 7.64 29.82 55.86 2.73 0.4 2.63 0.92
PRL8 7.26 27.1 57.39 2.35 2.32 2.78 0.8
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Table 3.2: Percentage of allochems, sparry calcite and microcrystalline calcite
matrix of the limestone (recalculated value)
Sample No. Allochem Microcrystalline
calcite
Sparry Calcite Total
LDL1 54.06 19.84 26.1 100
LDL2 55.99 22.59 21.43 100
LDL3 58.39 17.54 24.07 100
LDL5 56.91 21.56 21.54 100
LDL6 59.62 21.65 18.73 100
LDL7 59.88 17.91 22.21 100
LDL8 62.27 17.85 19.87 100
LDL9 55.9 11.96 32.14 100
LDL10 54.42 23.69 21.89 100
LDL11 58.53 12.93 28.54 100
UML 1 50.76 31.56 17.69 100
UML 2 49.98 32.72 17.3 100
UML 3 46.16 38.92 14.93 100
UML 4 46.17 38.92 14.91 100
UML 5 59.69 23.44 16.87 100
UML 6 60.17 19.32 20.52 100
UML 7 45.9 38.05 16.05 100
UML 8 45.16 36.7 18.13 100
UML 9 44.06 39.27 16.68 100
UML 10 47 32.23 20.77 100
PRL 1 62.31 32.7 4.98 100
PRL 2 63.93 27.43 8.63 100
PRL 3 65.6 24.85 9.55 100
PRL 4 55.47 33.27 11.27 100
PRL 5 55.1 34.67 10.24 100
PRL 6 62.71 31.51 5.78 100
PRL 7 62.23 29.97 7.79 100
PRL 65.36 27.31 7.33 100
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Table 3.3: Percentage (%) of fossil, intraclast, ooids, pellets and pellets+fossils of
the limestone.
Sample No. Fossils Intraclast Oolites Pellets Fossils+Pellets Total
LDL1 94.7 2.35 2.53 0.42 95.12 100
LDL2 92.86 3.58 2.64 0.92 93.78 100
LDL3 93.09 3.13 3.27 0.51 93.6 100
LDL5 91.85 4.41 3.21 0.53 92.38 100
LDL6 92.65 3.1 3.64 0.61 93.26 100
LDL7 91.42 4.5 4.08 0 91.42 100
LDL8 92.27 4.41 3.32 0 92.27 100
LDL9 96.16 2.34 1.5 0 96.16 100
LDL10 90.37 5.17 3.66 0.8 91.17 100
LDL 11 97.21 1.75 1.04 0 97.21 100
UML 1 96.92 2.35 0.73 0 96.92 100
UML 2 91.83 4.23 2.94 1 92.83 100
UML 3 92.85 3.57 2.45 1.13 93.98 100
UML 4 96.85 2.15 0.6 0.4 97.25 100
UML 5 96.82 2.18 0.6 0.4 97.22 100
UML 6 96.24 2.22 0.32 1.22 97.46 100
UML 7 94.46 3.12 1.42 1 95.46 100
UML 8 94.1 3.7 1.3 0.9 95 100
UML 9 96.09 2.49 1.42 0 96.09 100
UML 10 97.36 2.04 0.6 0 97.36 100
UML 11 97.33 2.11 0.56 0 97.33 100
UML 12 96.75 1.15 1.6 0.5 97.25 100
PRL 1 95.83 2.45 1.72 0 95.83 100
PRL 2 95.14 1.81 2.42 0.63 95.77 100
PRL 3 95.28 1.6 2.72 0.4 95.68 100
PRL 4 93.27 2.61 3.7 0.42 93.69 100
PRL 5 95.6 1.74 1.96 0.7 96.3 100
PRL 6 94.53 1.7 2.5 1.27 95.8 100
PRL 7 95.72 1.58 2.7 0 95.72 100
PRL 8 94.24 2.63 2.73 0.4 94.64 100
3.3.4 Fabric and microtexture:
The study of microstructure is important, because from this study we know
about the chemical activity and mode of occurrences within the carbonate sediments.
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Friedman (1965), coined the term ‘Fabric’ and ‘Texture’ for the
diagenetically altered rocks. Usually the calcite grains are subhadral and the
carbonate minerals and quartz occur as anhadral grains.
Both equigranular and inequigranular fabric, are shown by the crystal size of
the carbonate minerals; out of them equigranular fabrics of the rock is most
common. Hypidiotropic and xenotropic fabrics represent equigranular fabric.
Porphyrotropic and poikilotropic of both inequigranular and equigranular fabrics are
also observed.
The following microsedimentary structures are observed in the Lakadong
Limestone, Umlatdoh Limestone and Prang Limestone of Shella Formation.
(1) Strained calcite, (2) Veins and vugs, and (3) Microstylolite.
(1) Strained calcite crystals are turbid show undulose extinction. The
strained calcite is considered to be formed by, diagenetic neomorphism
of microcrystalline calcite matrix under stress i.e. the product of tectonic
effect on neomorphic sparry calcite grains.
(2) Veins are usually composed of microspar and sparry calcite and vary in
their shape from straight to irregular with uniform thickness thought the
sample.
Vugs or voids with irregular shape occurs between calcite crystal
boundary and with microcrystalline calcite material and, known as “birds
eye” (Wolf, et.al, 1967) or diapricites (Folk, 1959).
(3) Microstylolites are commonly intergranular and mark contacts between
adjacent grains of ooids, fossils and pellets. In addition, this is because,
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during deep burial, pressure dissolution is more than at shallow depths
and is important in reducing both porosity and permeability of the
sediments.
In the Lakadong Limestone, observed very high strained effect,
neomorphism, styllolitic structure, sparry calcitic veins of irregular shape and size
(Fig 3.14 and 3.15).
Nevertheless, in the Umlatdoh Limestone, minor strained effect and
styllolitic structures observed, but very less effect of neomorphism observed in the
Umlatdoh sediments. Sparry calcite veins observed in irregular shape and size. The
pore space between the framework grains and are filled with clay matrix and
cemented by sparry calcite and microcrystalline calcites (Fig 3.16 and Fig 3.17).
In the Prang Limestone, no strained effect, microstyllolites, neomorphism,
no veins of sparry calcite observed. Here in Prang sediments, the grain boundary or
void spaces are occupied by clay matrix and cemented by sparry calcitic cement (Fig
3.18 and Fig 3.19).
3.3.5 Depositional structures:
A sedimentary structure is deemed to be a primary depositional feature of
sediment that is large enough to be seen by the naked eye (Selley, 2000), and are
distinguished from the microscopic structural features of sediment, termed the
fabric. Sedimentary structures are arbitrarily divided into, primary (physical) and
secondary (chemical) classes. Primary structures are those generated in sediment
during or shortly after deposition. They result mainly from the physical processes
e.g., ripples, cross bedding and slumps. Secondary sedimentary structures are those
that formed sometime after sedimentation i.e., post-depositional microstructures.
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They result from essentially chemical processes, such as those, which lead to the
diagenetic formation of concretions (Selley, 2000). The study area is basically six
(6) member sandndstone-limestone alternations and all the members represents their
modes of deposition, environments and facies of deposition, along with these they
consists of different primary (physical) and secondary (chemical) depositional
structures (Fig 3.7, 3.10, 3.11, 3.12, 3.14, 3.15, 3.16, 3.17, 3.18 and 3.19). In the
studied samples, I am trying to discuss about the post-depositional effects and
microstructures of the carbonate rocks Lakadong Limestone, Umlatdoh Limestone
and Prang Limestone
In Lakadong Limestone, observed neomorphism, styllolitic structure, sparry
calcitic cement and veins of irregular shape and size. In Umlatdoh Limestone,
styllolitic structures are observed, but minor effect of neomorphism and, sparry
calcite veins of irregular shape and size, are observed. The pore space between the
framework grains and are filled with matrix and cemented by sparry calcites and
microcrystalline calcites. In Prang Limestone, the neomorphic effects are very less;
the grain boundary or void spaces are occupied by, clay matrix and cemented by
sparry calcitic cement.
3.3 Petrographic classification of limestone
Petrographic classification is essential to microfacies analysis, interpreting
rock properties and paleo-environmental interpretations of carbonate rocks. All the
limestone classifications commonly used in facies analyses and based on textural
and compositional criteria. The classifications proposed by Dunham (1962) and Folk
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(1962) have been followed. Modifications suggested by Embry and Klovan (1971),
Wright (1992) and Strohmenger and Wirsing (1991) are of help.
Limestone classification (Dunham, 1962):
Danham’s classification (Table 3.1), is mainly based on texture i.e. grain
sizes, accordingly following are the limestone types found:
Mudstone: Mud supported, Less than 10% grains, more than 90% mud.
Original components not bound together during deposition.
Wackestone: Mud supported. More than 10% grains, less than 90% mud that
has a coarse grains floating in a matrix containing more than 10% sand size
particles. Original components not bound together during deposition
Packstone: Grain supported and less mud, original components not bound
together during deposition and have a grain-supported framework with a matrix of
mud.
Grainstone: Grain supported framework, without a matrix of mud and
original components not bound together during deposition.
Boundstone: Grain or mud supported, with or without mud, where original
components bound together during deposition.
Crystalline: Crystal supported and no grains or mud, depositional structures
are not recognizable due to recrystalization.
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Table 3.4: Limestone classification (Dunham, 1962)
DEPOSITION TEXTURE RECOGNIZABLE
DEPOSITIONAL
TEXTURE NOT
RECOGNIZABLE
CRYSTALLINE
CARBONATE
(Subdivide according
classification
sediment-floored
cavities designed to
bear on physical that
are roofed over texture
or diagenesis)
Original components not bound together during deposition Original components were
bound together during
deposition as shown by
intergrown or limination
contrary to gravity,
sediment-floored cavities
that are roofed over by
organic or questionable
organic matter and are too
large to be interstices
Contains mud
(particles of clay and fine silt size)
Lacks
mud and is grain
supported.
Mud supported Grain supported
Less than 10%
grains
More than 10%
grains
MUDSTONE WACKSTONE PACKSTONE GRAINSTONE BOUNDSTONE
According to Dunham (1962) classification, the studied three Limestone’s
viz., the Lakadong Limestone, Umlatdoh Limestone and Prang Limestone are falling
within Packstone and Wackstone category.
In Lakadong Limestone the effect of neomorphism and the presence of
styllolitic structure, sparry calcitic cement and veins of irregular shape and size
along with the original components have a grain-supported framework with a matrix
of mud (11% to 24%) i.e., samples falls within Packstone category.
. In Umlatdoh Limestone is mud supported and styllolitic structures are
observed, but, very minor effect of neomorphism and irregular veins of Sparry
calcitic cement are also observed. The pores spaces between the framework grains
filled with matrix (19% to 40%), and cemented by sparry calcites and
microcrystalline calcites i.e., samples falls within Wackestone category.
In Prang Limestone is mud supported, the neomorphic effects are very less;
the grain boundary or void spaces are occupied by microcrystalline calcite matrix
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(27% to 35%) and cemented by sparry calcitic cement i.e., samples also falls within
Wackestone category (Table 3.4).
Limestone classification (Folk 1959, 1962):
Folk (1959, 1962) Limestone classification, is mainly based on the
percentages (%) of allochemical grains, microcrystalline calcite matrix and sparry
calcitic cement. This classification distinguishes allochthonous limestones
(mudstone, wackestone, packstone, grainstone) and autochthonous limestones (here
called boundstone or biolithite).
Limestones and their components deposited as discrete grains, grouped
according to mud-support or grain-support and the abundance of grains. The
Dunham classification stresses the depositional fabric; the Folk classification is
based on texture and abundance of different constituents to evaluate hydrodynamic
conditions.
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Fig 3.1: shows a triangular plot of limestone classification of study area (After Folk
1959 and 1962).
Table 3.5: Textural maturity classification of limestone proposed by Folk
(1962). The increasing textural maturity from left to right, dismicrite omitted in this
table.
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According to Folk (1959 and 1962) limestone classification of Shella
Formation are falling within the category of microcrystalline allochemical
limestone and sparry allochemical limestone, and based on allochemical grains, the
samples fall under biogenic pellet limestone or ‘biosparite’ (Figs 29 to 31).
Fig 3.2: X-Y scatter plot, according to Folk (1959, 1962), classification of
Shella Limestones.
With the increasing time of deposition and diagenesis microcrystalline
calcite is decreasing from Prang Limestone via Umlatdoh Limestone to Lakadong
Limestone and vice-versa in case of sparry calcite. The allochem percentages (%)
indicates that, Prang Limestone was deposited in shallower marine environment and
Umlatdoh Limestone was also deposited in shallow marine environment and
Lakadong was deposited in comparatively deeper shallow marine environments.
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Fig 3.3: shows a triangular plot according to Folk (1959) and Folk (1962)
classification of three limestone unit.
From Table 3.2 and Fig 3.1 and 3.2, it is established that, the allochemical
grains (or allochems) and microcrystalline calcite (micrite) shows a positive (+ve)
coorelation and sparry calcite shows a negative (-ve) coorelation from Lakadong
Limestone, Umlatdoh Limestone to the Prang Limestone.
The highest percentage (%) of allochems in the Prang Limestone indicates a
shallower marine and near shore environment of deposition because it is suitable
environment to receive some (terrigenous materials or) allochems. About 30%
micrite and 8% spar indicates an initiation of early diagenetic activities in Prang
Limestone.
The Lowest percentage (%) of allochemical grains (or allochems) in the
Lakadong Limestone indicates away from shore-line environment of deposition, and
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a deeper marine but shallow depositional environment. About 33% micrite and 17%
spar indicates an ongoing early diagenetic activities in Umlatdoh Limestone.
Nevertheless, in the Lakadong Limestone the allochemical grains (or
allochems) are also high and indicates a shallow marine, near shore environment of
deposition. But, decrease in micrite (19%) and increase in spar (24%) indicates an
ongoing late diagenetic activity in Lakadong Limestone, which is well supported by
post depositional structures like styllolitic structure (diagenetic fluid mobilization
structure), the effect of neomorphism, dolomitization, mineral transformations (i.e.,
aragonite to calcite), sparry calcitic cement and veins of irregular shape and size, etc.
In the Lakadong Limestone, according to Folk’s (1959 and 1962) limestone
classification, the samples fall within sparry allochemical limestone category, and
based on allochemical grains the samples falls under biogenic pellet limestone
(Table 3.3 and Fig 3.3).
The Umlatdoh Limestone and the Prang Limestone are purely a
microcrystalline allochemical limestone, and based on allochemical grains the
samples falls under biogenic pellet limestone (Table 3.3 and Fig 3.3).
Limestone classification (Embry and Klovan, 1971):
Original, expanded and revised of Dunham classification by Embry and
Klovan (1971) introduced the size aspect and distinguished grains smaller or larger
than 2 mm. The new categories floatstones and rudstones correspond to
wackestones, grainstones, and packstones, respectively. Other changes concern the
subdivision of the boundstone category of Dunham (1962) (Table 3.6).
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Table 3.6: Expanded and revised Dunham’s classification by Embry and Klovan,
1971
Allochthonous limestone original components not originally original bound
during deposition
Autochthonous limestone components
originally bound during deposition
Less than 10% >2 mm components
contains lime mud (< 0.03 mm)
No lime
mud
Greater than 10% >2 mm
components
By organisms which
Mud supported Grain supported Matrix
supported
>2 mm
components
supported
build a rigid
framework
encrust
and bind
act as
bafflers
less than
10%
grains (>
0.03 mm
and< 2
mm)
greater
than 10%
grains BOUNDSTONE
Mudstone Wackstone Packstone Grainstone Floatstone Rudstone Framestone Bindstone Bafflestone
According to Embry and Klovan (1971) classification of shella carbonates,
the Lakadong Limestone is an Allochthonous limestone have a grain supported
framework with a matrix of mud i.e., samples falls within Packstone category, where
the original components not originally original bound during deposition. Umlatdoh
Limestone is a mud supported and has greater than 10% framework grains i.e.,
samples fall within Wackestone category of original components. The Prang
Limestone is also a mud supported and has greater than 10% framework grains, and
fall within the same Wackestone category.
Revised limestone classification (Wright, 1992):
Wright suggested terms describing diagenetic changes of depositional
fabrics, that the fine-grained matrix commonly called ‘mud’ is not necessarily
identical with micrite, but corresponds to a matrix consisting of clay-sized and silt-
53
sized constituents. In this Table 3.7, the term mudstone is replaced by,
calcimudstone and bindstone by boundstone owing to some technical problems.
Table 3.7: Classification of Wright (1992) describing diagenetic changes of
depositional fabric.
Depositional Biological Diagenetic
Mixed supported
(clay and silt grains)
Grain supported In situ organisms Non-obliterative bliterative
<10%
grains
>
10%
grains
with
matrix
no
matrix
rigid
organi
sms
domin
ant
encrust
ing
bindin
g
organis
ms
rganis
ms
acted
to
baffle
grain
compon
ent in
cement
any grain
contacts
microstyl
olites
most
grain
ascontact
s are
microstyl
olites
Crystals >10
µm
Calcimu
dstone
wackes
tone
packsto
ne
grains
tone
frames
tone
bounds
tone
baffles
tone
cement
stone
condense
d
fitted sparstone
grainstone
Floatston
e
Rud
stone
Crystals< 10
µm
ICROSPAR
STONE Grains > 2 mm
According to Classification of Wright (1992), the Lakadong Limestone is a
grain-supported framework with matrix i.e., samples fall within Packstone category.
The Umlatdoh Limestone has a mud-supported, has greater than 10% framework
grains, and fall within the same Wackestone category. Prang Limestone is a mud
supported and has greater than 10% framework grains of clay and silt (evidenced
from the presence of glauconite (Fig 3.8), a green mineral in the upper part of Prang)
i.e. samples fall within Wackestone category.
54
3.4 Compositional maturity:
Compositional maturity, as defined for carbonates, is the extent to which a
sediment approaches the constituent end-member (intraclasts, ooids, fossils, peloids,
micrite matrix, and terrigenous minerals) to which it is driven by the environmental
processes operating upon it (Smosna, 1987). Immature lime sediment produced in an
environment where many biological, physical, and chemical processes are operating
simultaneously and results mixture sediment of the six major constituents above. As
the number of competing processes decreases in an environment, the sediment
progresses through advanced stages of compositional maturity. Super mature
sediment produced in an environment where a single formative process has operated
to completion; consequently, only one constituent dominates the sediment (Smosna,
1987).
In the present study, in three limestone units, the varying proportions of
carbonate constituents identified by following the terminology of Folk (1959) are
intraclast, oolits, fossil, pellets, cement/sparite or sparry calcite and micrite/matrix or
microcrystalline calcite (Table 3.1 and 3.2). The fossils and micrite matrix are more
in model composition of the three limestone units of the study area, and they may be
termed as compositionally mature. The biological processes seems to have operated
consistently in the limestone deposition and it is found that fossil contents are more
more than 54 % in Lakadong Limestone and Umlatdoh Limestone and in Prang
Limestone more than 65% fossils as framework grains (Table 3.1 and Table 3.2).
55
3.5 Chemical staining methods for the identification of carbonate minerals-
ferron calcite, non-ferron calcite and dolomite:
In carbonate rocks, formed either by chemical or mechanical deposition;
these minerals constitute the main rock-forming components. Sometimes, they are
also found in great abundance in pelitic, psammitic and psephitic rocks. According
to their crystallographic characteristics, carbonate minerals subdivided into two
main groups-
1) Calcite group
2) Aragonite group
Of the calcite group of minerals, calcite (CaCO3) and dolomite [CaMg
(CO3)2] are the two minerals which are dominantly found in sediments. Depending
on its purity, or iron and magnesium content, calcite is found as pure calcite, ferro-
calcite and magnesium calcite. Similarly, according to the iron content, dolomite
may also be regarded as pure dolomite and ferrous dolomite, in varying proportions.
The other calcite group minerals are ankerite [CaFe(CO3)2], magnesite (MgCO3),
siderite (FeCO3), smithsonite (ZnCO3), rhodochrosite (MnCO3) and spherocobaltite
(CoCO3).
The most important mineral of the aragonite group is aragonite (CaCO3); the
others, in decreasing order, are witherite (BaCO3), strontianite (SrCO3), cerussite
(PbCO3) and alstonite [(Ba,Ca)CO3]. Apart from their refractive indexes (R.I.), the
optical and crystallographic characteristics of carbonate minerals are almost similar
to each other. Therefore, identification of carbonates on hand specimens or even in
56
thin sections is rather difficult. The determination of the R.I.'s can be carried out by,
oil-immersion method. The identification and discrimination of carbonate minerals
in hand specimens or thin sections, are made easier by the use of simple chemical
staining methods.
Procedures of Sample preparation for staining:
Samples must be clean and dry before stain is applied. Stain boundaries are
identified by, polishing the sample with 1000 grit and on a felt lap with stannic
oxide.
Samples containing a high density of pores < 0.5 mm in diameter do not
stain well because the solution soaks in; coating the walls of the pore with stain, and
pore differentiation becomes difficult. However, these types of rock are best stained
if impregnated with plastic first. In this case, the matrix is stained and the pore space
is become void of stain. If sample does not take a good stain with the first
application, repeat the procedure for sample preparation.
Staining methods:
For discrimination between Ferron calcite-Nonferron calcite and dolomite,
different tests are there, different workers-follow the following tests,
(1) Alizarin red test, (2) Potassium ferricyanide test, and (3) Alizarin red and
potassium ferricyanide composite test.
The outlines of ‘Alizarin red and potassium ferricyanide composite test’ is
given below-
57
This test is use for discriminating between four different types of carbonates
(Calcite, ferrocalcite, dolomite and ferrodolomite) in a single series of operation.
According to Dickson (1965), two solutions are prepared separately: (a)
Alizarin red solution: This solution is obtaine by dissolving 0.2 gr of alizarin red in
100 cc of 1.5% hydrochloric acid (98.5cc distilled water +1.5cc concentrated HC1).
(b) Potassium ferricyanide solution: two grams of potassium ferricyanide is
dissolving in 100 cc 1.5 % HC1. (c) The mixed solutions is use in the test, is
prepared by mixing three parts of alizarin red solution and two parts of potassium
ferricyanide solution. The specimen soaked in cold solution for Dickson a period of
30-45 seconds is adequate. After the test, specimens should carefully wash and
dried.
Dickson (1965) observed the following results for different calcites and dolomites:
Nonferron-calcite: between very pale pink and red. (The tone of the color
depends on the optical orientation of the crystal. Stained surfaces parallel to c-axis
are darker than those are, which are at right angles to the axis); Ferron-calcite:
between lilac (pinkish purple)-scarlet and royal blue, derived from the mixture of
very light pink-red group and pale blue-dark blue group; Dolomite: shows no
coloring; Ferro-dolomite: Light to dark turquoise. Tone of the color depends on the
ferrous iron content.
58
Fig 3.4 (P-Prang Limestone, U-Umlatdoh Limestone and L-Lakadong Limestone):
Shows the three limestones stained section (of Alizarin red and potassium
ferricyanide composite test) according to stratigraphic order. White portion is the
unstained dolomite, red to purple portion is non-ferron calcite, and black portion is
ferron calcite, 10x.
59
Table 3.8: showing the volumetric percentage (%) of non-ferron calcite, ferron
calcite, aragonite and dolomite in Lakadong Limestone, Umlatdoh Limestone and
Prang Limestone unit.
Sl.No. Formations Sample
No.
Nonferron-calcite
(%)
Ferron-calcite
(%)
Aragonite
(%)
Dolomite
(%) 1
Prang Limestone
PRL 1 95.26 4.4 0 0.34 2 PRL 2 93.68 5.09 0.1 1.13 3 PRL 3 96.23 2.71 0 1.06 4 PRL 4 93.97 6.03 0 0 5 PRL 5 77.21 19.75 0.63 2.41 6 PRL 6 92.69 6.08 0.11 1.12 7 PRL 7 97.32 2.45 0 0.23 8 PRL 8 89.75 8.99 0 1.26 9
Umlatdoh
Limestone
UML 1 85.21 13 0.08 1.71 10 UML 2 90.03 9.97 0 0 11 UML 3 76.24 22.52 0.07 1.17 12 UML 4 83.72 14.85 0.19 1.24 13 UML 5 77.25 21.46 0.23 1.06 14 UML 6 82.61 17.33 0.06 0 15 UML 7 91.39 8.61 0 0 16 UML 8 79.72 18.32 1.35 0.61 17 UML 9 85.6 10.59 2.41 1.4 18 UML 10 83.61 13.91 1.36 1.12 19 UML 11 73.26 25.37 0 1.37 20
Lakadong
Limestone
LDL 1 88.59 10.59 0 0.82 21 LDL 2 95.22 0.5 2.03 2.25 22 LDL 3 98.36 0.47 0.05 1.12 23 LDL 4 87.52 10.54 0.88 1.06 24 LDL 5 72.35 24.63 2.13 0.89 25 LDL 6 90.32 7.62 1.13 0.93 26 LDL 7 86.87 12.11 0 1.02 27 LDL 8 91.95 5.13 2.13 0.79 28 LDL 9 77.29 19.27 2.46 0.98 29 LDL 10 73.68 24.79 0.91 0.62
In these three limestones viz., Lakadong Limestone, Umlatdoh Limestone
and Prang Limestone, the volumetric percentage (%) of non-ferron calcite, ferron
calcite, aragonite and dolomite are calculated.
Non-ferron calcite of the Lakadong Limestone varies from 72.35 % to 98.36
%, non-ferron calcite of the Umlaldoh Limestone varies from 73.26 % to 91.39 %
and non-ferron calcite of the Prang Limestone varies from 77.21 % to 97.32 %
(Table 3.8).
Ferron calcite of the Lakadong Limestone varies from 0.47 % to 24.79 %,
ferron calcite of the Umlaldoh Limestone varies from 8.61 % to 25.37 % and ferron
calcite of the Prang Limestone varies from 0.47 % to 24.79 % (Table 3.8).
60
Aragonite, of the Lakadong Limestone varies from 0.00 % to 2.46 %,
aragonite of the Umlaldoh Limestone varies from 0.00 % to 2.41 % and aragonite of
the Prang Limestone varies from 0.00 % to 0.63 % (Table 3.8)
Dolomite, of the Lakadong Limestone varies from 0.62 % to 2.25 %,
dolomite of the Umlaldoh Limestone varies from 0.00 % to 1.71 % and dolomite of
the Prang Limestone varies from 0.00 % to 0.41 % (Table 3.8).
The non-ferron calcite is more abundant than the ferron calcites and other
carbonate minerals as evidenced from the staining of the thin sections. The
aragonites are low in the limestone as it is very unstable and changed to calcite.
The almost absence of aragonite in most of the cases may be attributed to its
removal by solution (leaching). Leaching of aragonite materials takes place under
subaerial condition in the early stage of diagenesis.
Dolomite is only restricted to Lakadong limestone (dolomite is less than 2.25
%), and well visible in alizarin red and potassium ferricyanide composite test. In
Umlatdoh Limestone (dolomite is less than 1.71 %), and Prang Limestone (dolomite
is less than 0.41 %), which is visible in scattered manner under petrological
microscope (Fig 3.4).
3.6 DIAGENESIS:
Diagenesis encompasses all the processes, which affect the sediments after
deposition. It includes processes such cementation to produce limestone and
dissolution to form cave systems but it also include processes such as the
development of micro porosity and changes in the trace elements.
61
Changes that the sediments undergo between deposition and lithification
under normal P-T conditions are termed as diagenesis. They differentiated between
primary porosity, developed before and during deposition, and secondary porosity,
developed after deposition i.e. postdepositional porosity evolution. Choquette and
Pray (1970) recognized three zones (eogenetic, mesogenetic and telogenetic) in
which post-depositional porosity modification and evolution occur, and some
diagenetic processes and products responsible for the porosity development and
evolution in carbonate rock sequences after deposition.
The chemical characteristics of carbonate pore fluids, the rate of flux through
the pore system, and the temperature and pressure regimes under which the resulting
rock-water interactions are effected and control the diagenetic processes that affect
and modify carbonate porosity through dissolution of existing carbonate phases
and/or the precipitation of new phases.
Shallow-water carbonate-rich sediments are largely confined to the
subtropical and tropical climatic zones. Their occurrence is strongly influenced by,
factors such as water temperature and terrigenous input. These sediments are
generally dominated by aragonite, followed by calcites rich in magnesium (.4 mol
%). Dolomite occurs only in special environments, and even then, it is generally not
a major component of the sediment. Shallow-water carbonates are primarily produce
by the disintegration of the skeletons of benthic organisms, such as corals,
echinoids, mollusks, foraminifera, and coralline algae.
Under the high pressure and temperature regimens of the subsurface,
pressure solution is an important porosity destruction process that is often aided by
62
cement precipitation in adjacent pore spaces due to the general supersaturation of the
pore fluids. Finally, local areas of undersaturation related to thermal degradation of
hydrocarbons may result in secondary porosity generation by dissolution. Most
diagenetic processes operate slowly in the subsurface because of the relatively slow
movement of fluids under conditions of deep burial (Choquette and James, 1987,
1990)
Fig 3.5 (Left hand side figure): Morphology of calcite crystals as controlled by
selective Mg-poisoning (Folk, 1974). If as is shown in A, an Mg ion is added to the
end of a growing crystal it can easily be overstepped by the next succeeding CO3,
layer without harm to the crystal growth. If however, as in B, the small Mg ion is
added to the side of the crystal, the adjacent CO3, sheets are distorted to
accommodate it in the lattice, hampering further sideward growth, and resulting in
the growth of small, fibrous crystals (After Folk, 1974). Fig 3.6 (Right hand side
figure): Calcite crystal growth habit as a function of Mg/Ca ratio (After Folk, 1974).
63
Fig 3.7 (P, U and L): Shows some diagenetic process involves the change in the
mineralogy or fabric of the carbonate members of Shella Formation from bottom to
top.
64
Fossils in the three limestones i.e., Lakadong, Umlatdoh and Prang
Limestone show a variable vertical distribution and their abundance is confined to
the lower and middle part of the Lakadong limestone while rare at the top sandy part
of the unit (Gogoi et.al., 2009). This may be attributing to the prevalence of
favorable marine conditions during the deposition in the initial stage. The increasing
supply of clastic material to the basin towards the end deposition of the Lakadong
Limestone inhibited the thriving of benthic foraminifers. Presence of mud free water
in basin of deposition and predominance of larger foraminifera and calcareous algae
in the Lakadong Limestone assemblage are indicative of low energy warm shallow
water environment of deposition. The Lakadong Limestone larger foraminiferal
assemblage is associated with assemblage of calcareous algae. In recent seas, this
kind of assemblage is common in shallow water tropical environment of carbonate
deposition. The allochemical process or dolomitization is only restricted to the lower
most unit (Lakadong limestone) of the area.
Different diagenetic processes occur within the different units of carbonate
rocks, the basic petrographic and diagenetic difference in between these three
members of shella formation is that,the Lakadong Limestone contains less skeletal
grains and inorganic precipitates like ooids, and needle muds than Prang Limestone,
but late diagenetic processes is prevailed here, which is well established by the
presence of sparite, microstylolites, and sparry calcitic veins. The Umlatdoh
Limestone also contains less skeletal grains and inorganic precipitates like ooids,
and needle muds in comparisons to the other two members of shella formation. Late
diagenetic process is just begins which is supported by the presence of microsparite
65
and pseudosparites in the studied samples. But, Prang Limestone contains
characteristic high amount of skeletal grains along with the inorganic precipitates of
oolites, cements, and needle muds. Early diagenetic process is only evidenced in
prang Limestone and no late diagenetic process is started in the present PT
conditions.
Fig. 3.8: shows the occurance of glauconite in top of Prang Limestone established
special shallow marine early diagenetic process, which probably at the interface
between reducing and oxidizing zones in the muddy sediments.
The occurance of glauconite in top of Prang Limestone established shallow
marine early diagenetic process (Fig 3.8), which probably at the interface between
reducing and oxidizing zones in the muddy sediments. Glauconite has been
synthesized at low temperatures (about 20oC), at sea water pH (8.5) by precipitation
of Fe-hydroxides from Si, Fe, Al, and K containing solutions in pore spaces under
slightly reducing conditions (Harder H, 1980). Shallow sediments under conditions
of low sedimentation rate may produce glauconitic minerals.
66
Porrenga (1967) and Odin (1988) have described the occurrence of
glauconitic and berthierine facies minerals in present-day and recent sediments.
There is a much more moderate change in iron oxidation state in the
glauconitic and an addition of potassium. The evolution of glauconitic is towards a
potassic, ferric clay mineral. Hence, one can deduce that the reducing power of the
environment on the sedimentary peloid materials is very important. In the absence of
a strong reducing agent, the iron in the peletal material remains essentially ferric,
accumulates potassium to form glauconitic, and thus formed in different
geochemical sedimentary environments, temperature of formation is low, near that
of the ocean shelf. During glauconitic formation the system is largely oxidizing, and
tends to become more alkali rich in the system (Fig 3.8); during glauconite
formation potassium enters the system (Velde and Odin, 1975).
According to Folk (1959) and Folk (1962) classification of limestone, all the
samples of Prang Limestone falls within Microcrystalline allochemical limestone
category and contains high microcrystalline magnesium calcite (Fig 3.1) are
components of shallow marine sediments. They are derived from the skeleton of
organisms, such as benthic foraminifers, and by direct precipitation of marine
cements. The solubility is strongly influence by their magnesium content. Pellets are
formed by the ingestion of sediment by marine organisms and excretion of algal
material.
67
Cementation:
Cements represent an important record of the diagenetic history of carbonate
rocks. Cementation comprises the processes that leading to the precipitation of
minerals in primary or secondary pores and requires the super-saturation of pore
fluids with respect to the mineral. Cementation is one of the major processes that
take place simultaneously to lithification in either early or late diagenetic stage
depositing minerals such as calcite, quartz, clay, chlorite, glauconitic and iron
minerals etc. as inter-granular material.
Dissolution-precipitation that accompanies cementation, which differs only
in degree from that accompanies the growth of neomorphic spar. Carbonate Cement
or Sparry calcitic cement is the only dominant cement in the studied limestone. Of
the three limestone members of the study area, the Lower part of Lakadong
Limestone is dolomitic.
There are two distinct types of sparry calcite cement viz., non-ferron and
ferron calcite are present in the carbonate sequence of the area after combined
carbonate staining method of Aligerin Red S and potassium ferricyanide (Dickson,
1966), where non-ferron calcites shows pink to reddish brown color and ferron
calcites shows mauve to blue color.
Non-ferron calcites are observed as internal constituent of fossils,
intergranular constituents and outer rim of some fossils as micritic envelop (Fig 3.4)
and are first to be deposited during early diagenesis of the sediments. Sometimes
fiberous calcite found around seems to be precipitate before the sparite. The ‘dog-
68
tooth’ calcites are commonly non-ferron calcite and generally show undulose
extinction indicating some stain effect at the time of their formation, which was
formed before, or was concomitant to effective compaction and lithification. Non-
ferron calcite occurs as crust around some grains and solution cavities, which is only
possible if aragonite dissolution occurs during the deposition of non-ferron calcite
(Talbot, 1971). The isolated strip of micrite envelope or very thin bands of fine
dusty particles that once covered the shell surface indicate that aragonite dissolution
must have preceded compaction, and deposition of the granular cement in Lakadong
and Umlatdoh Limestone.
Ferron calcites, typically forms a mosaic of more or less equant grains
having no apparent crystal orientation and mostly found in fractures, veins, and
sometimes within the fossils (Fig. 3.4).
In cavity filling, cement the crystals in cavity walls serves as nuclei. As a
cement mosaic develops, the more favorably oriented crystals survive and sink the
less favorable ones with the result that crystal size tends to increase towards the
centre of the cavity filling (Fig 3.4). Large voide spaces, such as body cavities of
mega fossils (Molluska) or algal nodules, and the larger opening sometimes filled in
sequential episodes with a lower layer of mud (micrite) that infiltrated and settled
down on the floor of the cavity and the upper portion is fill by sparry calciteic
cement. The contact between the two-calcite surfaces marks the bedding plane and
the structure known as geopetal structure and found in Umlatdoh Limestone.
Radiaxial fibrous calcite (Fig 3.7) is generally found as space filler as in
Lakadong Limestone showing special fabrics, undulose extinction, convergent optic
69
axis, sub crystals curved twinned planes and inclusions. Bathrust (1959) suggested
them to be the recrystallization product of aragonite. Some crystals of calcite cement
shows zoned appearance indicating change in pore water chemistry from which
calcite was precipitated. The other types of cement is micrite which is distributed as
lining cavity walls or forming asymmetric fringes, together with a vague pelleted
texture, but is quite difficult to distinguish from clay matrix. There is two
generations of calcite cement precipitation established from staining viz., the early
cement (formed by dissolution of aragonite) generation is composed of non-ferron
calcite and the main cement phase is a ferron calcite, during which sediments
suffered compaction, fracture and breaking of grains.
The presence of only granular ferron calcite (Fig 3.4), in contrast to two
ferron calcite (fiberous and granular) that fill majority of the primary and secondary
voids explain that the cavity filling was late diagenetic. This was happen where the
cavity fill is related to a phase of calcite veining and cross cut the constituent grains
but merge with any granular ferron calcite grain they encounter (Fig 3.4). In
Lakadong limestone, large equant grains of calcite (blue stained) is present showing
a cross relation with all textural elements (Fig 3.4), however, the vein filling was of
both dogtooth and granular ferron calcite type. In Prang Limestone, a large equant
grain of calcite (blue stained) is also present and showing a cross relation with all
textural elements but, the veins were filled with granular ferron calcite with some
calcite overgrowth on them (Fig 3.4). This post-lithification ferron calcite was may
be deposited from ground water that dissolved iron from various sources and then
circulated through cracks, pores and solution openings by precipitation and
70
replacement process at depth. The absence of fibrous ferron calcite explains that
dissolution may have taken place too late for fibrous calcite to deposit.
Neomorphism:
Neomorphism (Folk, 1965), is the term use to include all transformations
between one mineral and itself or a polymorphic inversion or recrystallization.
Diagenesis includes the transformation of one phase to another, i.e.,
"recrystallization" by the processes of micro-solution and precipitation. Replacement
involves gross chemical changes between the reactant and product phases, like
calcite to dolomite. Neomorphism, refers to transformations involving polymorphs
or members of solid solutions, for example, aragonite to calcite. Recrystallization,
occurs when the reactant and product phase remain nearly the same compositionally
and structurally, but grain growth occurs, when of small carbonate grains to larger
grains (Morse and Casey, 1988).
Land (1986, 1989) characterizes all stabilization processes as proceeding by
replacement. These transformations generally take place in the presence of a fluid;
i.e., marine interstitial water, meteoric water, connate water, or subsurface brine.
In some cases, pressures and temperatures are elevated above depositional
pressures and temperatures; in other cases, transformation takes place under near
Earth surface conditions of pressures and temperatures. Fluids, move through the
carbonate sediment package transporting dissolved reactant and product
constituent’s centimeters to hundreds or thousands of kilometers, or in some
carbonate sequences lack of long distance movement, that is responsible for
71
significant mass transfer of carbonate components regionally, or locally, during
diagenesis. Carbonate diagenesis also involves processes in which new minerals
precipitate in original or secondary pore space, and carbonate minerals are leached
from the sediment mass, Thus, the authigenesis or neoformation of mineral phases,
as well as recrystallization, can lead to changes in a sediment's ability to transmit
fluids because of changes in porosity and permeability during diagenesis.
Thin section study of the carbonates form the present area shows that they
were exposed to varying degrees of biochemical and/or allochemical diagenetic
processes. Cementation and neomorphism are the most dominant isochemical
process in diagenesis. Formation of stylolite because of pressure solution is also a
common diagenetic phenomenon observed in the limestones (Figs 3.10, 3.11, 3.12).
The predominance of fossils in the member of Shella Formation follows an
increasing stratigraphic trend (from older i.e., Lakadong Limestone to younger i.e.,
Prang Limestone) in the members and appears to be inversed association with the
degree of neomorphism (Figs 3.10, 3.11, 3.12). A coarsening of grains from Prang
Limestone (mostly micritic), Umlatdoh Limestone (micritic and microsparitic) to
Lakadong Limestone (microsparitic on top coarse-grained sparry calcite and
dolomite at bottom) well observed.
Two types of neomorphic transformations are observed in the samples i.e.,
wet polymorphic transformations from aragonite to calcite, and wet recrystalization
of calcite to calcite in the later stages of aggrading neomorphism, i.e., neomorphism
embraces two types of alterations viz., inversion and recrystallization. The limestone
units of the study area underwent aggrading type of neomorphism, which includes-
72
Coarsening of grains (microspar to spar) from Prang to Lakadong Limestone,
Calcitization of aragonite minerals, and
Replacement of acicular cement by fibrous calcite.
The microcrystalline calcite materials partially altered to coarser calcite
(microspar and rarely pseudospar). Aggrading neomorphism i.e., coarsening of
grains is recognized from the following observations-
1. Irregular or curved intercrystalline boundary,
2. Patchy development of coarse mosaic,
3. Precence of floating skeletal grains in the coarse spar.
Among the aggrading neomorphism, both pervasive and selective
neomorphism are observed. Skeletal grains composed of aragonite have mostly been
replaced by drusy sparite through the solution of the aragonite and later
precipitation, in some cases by calcite with no intervening void phase i.e.,
calcitization.
Foraminifers, calcareous algae and some rarely preserved molluscan shells
are the dominant allochems. Single foraminiferal tests are composed of radial or
concentric cryptocrystalline aragonite (Sanders and Friedman, 1967) and granular
calcite, and are bimineralic nature because both cryptocrystalline aragonite (radial
hyaline) and sparry calcite (granular hyaline) are observed. Association of aragonite
with granular calcite within a single allochem indicates selective neomorphism
(Goswami et. al., 1971). Foraminifera with hyaline calcareous perforate wall often
composed of small prism of calcite having their principal axis perpendicular to the
73
surface of the shell resulting a black cross and colored ring under plane polarized
light (Figs 3.10, 3.11, 3.12). In a few smaller forms, most of the chambers are
invariably filled up with calcite mosaic and microspar.
In Lakadong and Umlatdoh limestone the shell of some fossils manifest a
thick coating (micritic envelop) that helps in preserving fossil fragment during
diagenesis, most of the aragonite shell has been completely dissolved and the mould
outlined by a thin micrite envelop was then filled by sparry calcite cement. This
indicates pervasive neomorphism of aragonite during early diagenesis and
lithification (Sarma, 2005). Micrite envelop is more frequent among the small
miliolids and abundant in the Lakadong limestone and Umlatdoh limestone beds.
The Prang Limestone is shallow water sediments and are skeletal in nature,
but commonly contains inorganic precipitates of ooids, cements, and needle muds.
In the early stage of accumulation, very soluble phases dissolved, and cements of
aragonite and magnesian calcite precipitated in the pores of the Prang sediments.
These processes modify carbonate sediment composition and result in the loss of
information concerning original sediments chemistry, mineralogy, and biotic
composition. Sediments characteristic such as porosity, permeability and fabric
changed during these early diagenetic stages. These modifications will continue to
occur during further diagenetic pathways. The development of secondary porosity is
not observed in Prang Limestone.
The carbonate secreting algae, coralline algae in particular constitute one of
the most dominant fossil grains with its characteristic reticulate appearance
preserved because of the conversion of original high Mg-calcite composition to
74
calcite (Friedman, 1964). The green algae (Halimeda) are poorly preserve probably
because of their chemical composition (aragonite) which generally dissolved and the
original texture is replaced by clear calcite.
Fig 3.9: A. shows a Basel section of two small prism of calcite, having their
principal axis perpendicular to the surface results a black cross and colored ring
under plane polarized light. A1. Association of aragonite with granular calcite
within a single allochem indicates selective neomorphism (Goswami et. al., 1971)
with sparry calcitic vein.
The three carbonate members of Shella Formation viz., Lakadong, Umlatdoh
and Prang Limestone and showing the effect of neomorphism on fossils and
bioclasts.
75
Fig 3.10: shows the characteristic high amount of skletal grains and inorganic
precipitates of ooids, cements, and needle muds and early diagenetic process is
prevailed in prang Limestone in the field (A) and under microscope (A1).
Fig 3.11: shows less skletal grains and inorganic precipitates like ooids, and needle
muds. Late diagenetic process is observed in Umlatdoh Limestone which is well
established by the presence of microsparite and pseudosparites in the field (B) and
under microscope (B1).
76
Fig 3.12: show a less skletal grains and inorganic precipitates like ooids, and needle
muds. Late diagenetic process is prevailed in Lakadong Limestone which is well
established by the presence of sparite, microstylolites, and sparry calcitic veins in
the field (C) and under microscope (C1).
Figs (3.10, 3.11 and 3.12): are the field photographs & microphotographs showing
three Carbonate Rocks of Shella Formation, A. highly fossiliferous Prang
Limestone, B. less fossiliferous Umlatdoh Limestone and C. Fossiliferous Lakadong
Limestone. A1, B1 and C1 show the degree of Neomorphism from Lakadong
Limestone to Prang Limestone.
Dolomitization:
Dolomitization does not usually retain much of the original microstructures
in the replacement product, preservation is usually poor in many carbonate
sequences and, allochems have been preserved by the dolomitization process. All
biogenic allochems of all ages are known to be susceptible to dolomite replacement.
Biogenic calcium carbonates seem to be replaced by dolomite. It has been difficult
to design experiments to test mechanisms and rates of diagenetic processes affecting
77
carbonate sediments, and the difference in the chemical composition between
calcite/aragonite and dolomite is the incorporation of Mg to form CaMg(CO3)2. To
this studied rocks, in contrast to microstructural features, exoskeletal or shell
morphology is usually well maintained during the replacement process.
In Lakadong and Umlatdoh Limestone the shell of some fossils manifests a
thick coating (micritic envelop) that helps in preserving fossil fragment during
diagenesis. Most of the aragonite shell has been completely dissolved and the mould
outlined by a thin micrite envelop was then filled by sparry calcite cement. This
indicates pervasive neomorphism of aragonite during early diagenesis and
lithification (Sarma, 2005). Micrite envelop is more frequent among the small
miliolids and abundant in the Lakadang and Umlatdoh Limestones.
The uppermost Prang Limestone is shallow water sediments and are skeletal
in nature, but commonly contains inorganic precipitates of ooids, cements,and
needle muds. In the early stage of accumulation, very soluble phases may be
dissolved, and cements of aragonite and magnesian calcite precipitated in the pore of
the Prang sediments. These processes modify carbonate sediment composition and
result in the loss of information concerning the original sediments, chemistry,
mineralogy and biotic composition. In addition, sediments characteristics such as
porosity, permeability and fabric have changed during these early diagenetic stages.
These modifications will continue to occur during further diagenetic pathways. The
development of secondary porosity is not observed in Prang Limestone.
The carbonate secreting algae, coralline algae in particular constitute one of
the most dominant fossil grains with its characteristics reticulate appearance
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preserved, because of the general replacement of biogenic carbonate allochems can
be represented by the following reactions-
2CaCO3 + Mg2+
(Fe2+) = CaMg(CO3)2 + Ca
2+
Where, variable amounts of Fe may substitute into the dolomite lattice
depending on the source availability and redox of the diagenetic system.
When carbonates are absent from the original sediments, the pore waters
supply the necessary Ca2+
, Mg2+
and CO32-
for trace amounts of dolomite formation:
Ca2+
+ Mg2+
+ 4HCO3-
= CaMg(CO3)2 + 2CO2 + 2H2O
If CaCO3 is present, dolomitization may occur by the following mechanism:
CaCO3 + Mg2+
+ 2HCO3-
= CaMg(CO3)2 + CO2 + H2O
And if, the sediment is very rich in a carbonate mineral precursor,
2CaCO3 + Mg2+
= CaMg(CO3)2 + Ca2+
In the first case, Ca2+
and some CO32-
is supplied by dissolution of CaCO3
and Mg2+
, with additional CO32-
, is derived from the pore water. In the second case,
most of the components of the diagenetic dolomite come from precursor carbonate
(Isaacs, 1984; Baker and Bums, 1985; Compton and Siever, 1984, 1986; Burns and
Baker, 1987; Compton, 1988). To obtain information on the equilibrium state of the
carbonate rock system and the direction in which diagenesis was prevailed, it is
necessary to know the chemical mechanisms and the rates of dissolution,
precipitation and recrystallization reactions (Berner, 1986).
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Interpretation of diagenetic pathways in deeply buried carbonate sediments is
more difficult than for shallowly buried carbonate materials. The geologic history of
the carbonate sediment becomes an important variable. Temporal changes in the
burial parameters of pressure and temperature, and of geothermal gradient,
interstitial fluid composition, and migration paths, and tectonic movement, must also
evaluated to decipher diagenetic pathways.
The process may take place soon after the deposition (early diagenetic) or
long after the deposition (late diagenetic, Tucker, 1981) and may be pervasive or
selective. Dolomites may also be formed by seepage refluxion (Adoms et.al., 1960).
In thin section among the three limestone members of the study area, only the lower
part of the Lakadong Limestone found to be coarsely crystalline dolomites of
secondary origin with micritic cement, and developed an intercrystalline porosity,
accompanied by dissolution of non-replaced limestone.
Different workers in the neighboring districts have reported the occurrence
of dolomites in the base of Lakadong Limestone. Sarma (2005) reported the
occurrence of coarse-grained crystalline dolomite with scattered dolomitic rhombs
in South Jaintia Hills district of Meghalaya. Das and Borthakur (2009), reported a
restricted occurance of negligible dolomite contents is less than 2% and denies its
primary origin in and around Mawsynram area, Meghalaya.
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3.6.4 Compaction and pressure solution
Compaction and pressure solution (stylolitization) refer to mechanical and
chemical processes, triggered by the increasing overburden of sediments during
burial and increasing temperature and pressure conditions. So, compaction is the
processes that decreases the bulk volume of a single grain or packing of grains
(reorientation) and pressure solution, which decreases the volume of grains and of
cement minerals (Flugel, 1982, 2004). Compaction, is brought out by the weight of
the overlying column of deposited material and if the intensity of the compaction is
very high, this may lead to the deformation of sedimentary structure and fabrics and
may cause recrystallisation. The two principal mechanisms for porosity destruction
are cementation and compaction.
Compaction includes mechanical compaction, dewatering, and chemical
compaction, as exemplified by pressure solution along stylolites and between grains.
Compaction can change the original depositional structure by the development of
calcite veins and stylolites. The stylolites represent structural discontinuities caused
by pressure solution, dissolution of limestone Fossils or other grains often partly
may not be seen (Figs 3.14 to 3.19) adjacent to stylolites. They are thin seams of
clay and insoluble residue material and mostly run parallel to the bedding and
contain minor quartz glauconite (Fig 3.8) crystallized during the diagenesis process
very common in diagenetically modified limestones. Porosity destruction by
compaction dominates those sedimentary sequences buried with marine pore fluids,
such as pelagic oozes, rapidly subsiding shelf-margin sequences, and, in some cases,
mud-dominated shelf-lagoon complexes (Choquette and James, 1987). Chemical
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compaction believed to be a significant source for later, porosity-occluding,
subsurface cements.
In the studied limestones, the effect of various compactions is not so
significant because of the uniform horizontal bedded natures of the limestone
indicate that the tectonic environment in the basin of deposition was stable one. The
preservation of early diagenetic sedimentary structure suggests that there was no
strong compaction effect in the carbonate sediments. Presence of few microstylolitic
contacts and broken allochems suggests an in significant role of compaction effect
during early diagenesis.
The study of the stained thin section of limestone revealed that a very small
amount of aragonite occurs within the fossil allochem, but generally absent in many
fossil allochems. The absence of aragonite in most cases may be attributing to its
removal by leaching or solution. Leaching of aragonite material takes place under
subaerial condition in the early stage of diagenesis (Friedman, 1964). Moreover, the
aragonite that occurs within the fossil allochem also shows selective neomorphic
inversion into calcite (Fig 3.14, 3.15, 3.16, 3.17, 3.18 and 3.19). The removal by
leaching and selective neomorphism of aragonite may be cause for the lack of
aragonite in the limestones and one of the indications of early diagenetic change.
Friedman (1964) observed the replacement of aragonite by calcite in shells of
organisms occur early in the lithification process before most of the interstitial pore
spaces is filled. Majority of the fossil allochems in the Lakadong limestone are
preserve as calcite cast and the boundaries between these fossil allochems and the
surrounding sparry calcite mosaic are usually sharp. Sometimes, a dark
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microcrystalline rims around the grains, forming micrite envelops are also observed
as grain coating consists of unoriented cryptocrystalline aragonite and formed by
accretion (Friedman, 1964)
In the present study area, the size of the stylolites varies from few
micrometers to few centimeters. The large-scale stylolites, of a few centimeter
amplitude are frequently encountered in the Prang Limestone, which indicates
compaction of sediments during diagenesis (Fig 3.14, 3.15, 3.16, 3.17, 3.18 and
3.19). However, microstylolites are also present in Umlatdoh Limestone and
Lakadong Limestone and are supposed to be one of the sources of CaCO3 for
limestone concentration specifically in the late diagenetic stage.
3.6.5 Silicification:
Silicification or chertification is another diagenetic replacement process
common in carbonate allochems of all ages (Maliva and Siever, 1988) and can takes
place during early or late diagenesis. Replacement by silica is highly allochem
selective while generally leaving the host rock material unaffected. This process
probably proceeds before or after carbonate-controlled diagenetic processes and is
highly dependent on chemical-crystal growth conditions of the replacement
microenvironment. Silicification of biogenic carbonate allochems may occur at any
time in their geologic and diagenetic history. Evidence is mounting that silicification
of carbonate allochems occurs prior to extensive cementation of the host sediment,
and either before, during or after the aragonite-calcite and high-Mg calcite-calcite
transformations (Choquette, 1955). Euhedral quartz crystals and microcrystalline
quartz are diagenetic silica that present in the studied samples. The euhedral silica
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crystal had found, in few sections of Prang Limestone and Umlatdoh limestone. In
Lakadong Limestone, very small grains of quartz with Iron leaching have seen in
some microstylolites (Fig 3.14, 3.15, 3.16, 3.17, 3.18 and 3.19).
3.7 Porosity in carbonate sediments:
Porosity is a prerequisite for the course of diagenesis. Porosity studies of
carbonate rocks are crucial in understanding diagenetic processes and highly
significant in evaluating reservoir rocks (Moore 2001). Porosity is the percentage of
the bulk volume of a rock that has occupied by interstices, whether isolated or
connected. This definition describes the total porosity, which must be separated
from the effective porosity i.e., the percentage of the total rock volume that consists
of interconnected pores. Many carbonate rocks no longer exhibit open pores, but
former interstices, which have been filled with, cement. An estimation of this
porosity, representing the sum of pre-cementation porosity and expressed by the
total of pore-filling cement percentage of the bulk volume, is necessary for
understanding and describing porosity evolution during time. The porosity is
generally very high at the time of deposition that is lost or reduced or modified
through compaction, cementation and pressure solution, and again developed
through solution, dolomitization and tectonic fracturing (Murray, 1960).
The porosity of sedimentary rocks falls into two major groups’ viz., Primary
and secondary porosity. Primary porosity, forms during the predepositional stage
(e.g., intragranular pores in foraminifers, corals, or ooids) and during the
depositional stage (depositional porosity), e.g. intergranular porosity, framework
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growth porosity. Moreover, Secondary porosity has formed during diagenesis at any
time after deposition.
Choquette and Pray (1970) list the classification and the terms used to
describe carbonate porosity in thin sections (Fig 3.13). The following text
concentrates on the basic terminology and the practical pore type classification in
thin sections.
Fabric-selective porosity:
Fabric-selective porosity is controlled by primary or secondary fabrics.
Primary depositional fabrics include interparticle, intraparticle, growth framework,
fenestral, and shelter pores. Fabric-selective secondary porosity is formed by
intercrystalline and moldic pores.
Non-fabric selective porosity:
Non-fabric selective pores is independent of depositional fabrics and
includes fracture, channel, vug and cavern pores. And-
Fabric-selective porosity or non-fabric selective pore types:
Breccia, boring, burrow, and shrinkage pores may be fabric-selective or not.
These qualitative criteria can be substantiated by the additional use of genetic, size
and abundance modifiers.
85
Fig 3.13: A Pore types and porosity classification (after Choquette and Pray, 1970).
Porosity in limestones, change with increasing age and/or burial depth of the
sediment (Scholle and Halley 1985). Mechanical compaction can be responsible for
about one third of porosity in micritic limestones. The total porosity of many
limestones is often less than 10% where by grain-supported limestones often exhibit
higher porosities than mud-supported limestones. Common porosities of
wackestones formed in inner shelf and lagoonal environments are < 3%. The
preservation of primary pores requires that postdepositional diagenesis be limited in
its pore-destroying effects, that compaction kept at a minimum, and those
fluctuations between exposure and submergence create a balance between the
formation of solution pores and the destruction of porosity by shallow burial
86
cementation. A preservation of porosity in shallow burial environments is a
consequence of minimal burial, reduced burial stress, and increased framework
rigidity, exclusion of pore water, low-calcite mineralogy, permeability barriers, and
pore resurrection. Porosity in mud-supported limestone affected by a prolonged
burial diagenesis, is commonly strongly reduced.
In the studied samples, porosity in many dolomites has higher average
porosities than limestones, because of differences in the size, shape and arrangement
of crystals. Porosity tends to increase slightly in the initial stages of dolomitization
of limestone, but increases abruptly with higher amounts of dolomite. Common pore
types in carbonate rocks are Interparticle, vugy, intercrystalline, and framework
pores are common in limestones.
In the present study carbonates shows development of secondary porosity
viz., i) Mulds, Vugs and solution of grains and rock, ii) intercrystalline porosity
produced through dolomitization (Fig 3.9) and iii) fracture porosity. The
development of porosity was not so high in the present carbonates except some vugs
in the lowermost limestone and intercrystalline porosity among the dolomite rhombs
in the same (Fig 3.13).
3.8 Depositional environment of carbonate rocks of the study area:
The diagenetic fabric of the carbonates provides useful information to the
environment of deposition of these sediments. The three limestone units of the study
area shows dominance of non-ferron calcite followed by purple to blue stained
calcite indicating the presence of ferrous iron. The non-ferron calcites have a
restricted distribution in space and time and were always the first calcite has
87
deposited. Their presence also indicates cementation during the early diagenetic
stage of the apparently unlithified sediments. The non-ferron calcites are probably
deposited under sub-aerial exposure in emergent well-oxygenated marine condition.
The dominance of micrite and conversion of microspar to pseudospar also suggested
precipitation in surficial condition. The petrological and fossil features enable to
reconstruct a diagenetic history for the deposition of the three limestone members of
the study area.
Cementation in Lakadong Limestone perhaps took place before compaction
in oxidizing environment under emergent marine condition, which later on
submerged and underwent compaction great enough to cause fracture and the
precipitation of second-generation cement under reducing condition along the
fracture and voids (Evamy, 1969). The presence of glauconite within some
foraminifers also indicates reducing condition and normal salinity of seawater.
The stylolites present in this unit also show some indication of compaction in
the late stage of diagenesis. The occurrence of oolites in the upper part of the same
unit is indicative of very shallow water condition may be due to negative change in
sea level. The abundance of pellets in some sections indicates deposition in
protected environment
The thin dolomite bend at bottom part of Lakadong Limestone might be of
late diagenetic replacement origin.
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3.9 Observations:
The modal volumetric analysis (vol %) of constituent grains (from both
stained and unstained sections) shows that the limestones are fossiliferous with
fossil ranging from 42.0 % to 54.50 % in Lakadong Limestone, 38.74 % to 53.80 %
Umlatdoh Limestone and 50.4 % to 58.28 % Prang Limestone. The oolites are very
common constituents in the limestones, oolites of the Lakadong Limestone varies
from 1.04% to 4.85 %, in Umlatdoh Limestone 0.32 % to 2.94 % in Prang
Limestone varies from 1.96 % to 3.70 %. The pellets and intraclasts also occur in
very negligible proportions in the studied carbonates. The constituent grains mostly
occur in a groundmass of microcrystalline calcite matrix neomorphosed to microspar
in places. The sparry crystalline calcite cements are mostly non-ferron calcite of
pink to reddish brown stained, followed by purple to blue stained calcite cement
indicating the presence of ferrous iron. Microcrystalline calcite matrix (micrite), of
the Lakadong Limestone varies from 11.32 % to 23.15 %, in Umlatdoh Limestone
varies from 19.20 % to 39.12 % and that of Prang Limestone varies from 24.55 % to
34.40 %. Petrographically, the limestones of the study area are of biomicrite,
biosparite, oomicrite and bio-pelsparite type (after Folk, 1959) and compositionally
matured.
The carbonate petrography and diagenesis provide useful information to the
environment of deposition of these sediments. The non-ferron calcite formed during
the early diagenetic stage of the apparently unlithified sediments under subaerial
exposure in emergent well-oxygenated marine condition. The dominance of micrite
89
and conversion of microspar to pseudospar also suggest precipitation in surfacial
condition.
Different diagenetic processes may occur within the different units of
carbonate rocks, the basic petrographic and diagenetic difference in between these
three members of shella formation. From petrographical studies, it can be inferred
that, Lakadong limestone is dolomitic in bottom part, pure limestone in middle part
and sandy in upper part. The diagenetic fabric of the Lakadong limestone, indicate
that cementation took place before compaction in oxidizing environment under
emergent marine condition, which later on submerged and underwent compaction
great enough to cause fracture and the precipitation of second-generation cement
under reducing condition along the fractures and voids. The thin dolomitic unit at
the bottom part of this unit may be of late diagenetic replacement origin. The
Umlatdoh Limestone is also highly fossiliferous and has undergone early diagenetic
modifications, at very shallow depths, both under influence of marine and meteoric
water.
The Lakadong Limestone contains less skeletal grains and inorganic
precipitates like ooids, and needle muds than Prang Limestone, but late diagenetic
processes is prevailed here, which is well established by the presence of sparite,
microstylolites, and sparry calcitic veins. The Umlatdoh Limestone also contains
less skeletal grains and inorganic precipitates like ooids, and needle muds in
comparisons to the other two members of shella formation. Late diagenetic process
is supported by the presence of microsparite and pseudosparites in the studied
samples. But, Prang Limestone contains characteristic high amount of skelital grains
90
along with the inorganic precipitates of ooids, cements, and needle muds. Early
diagenetic process is only evidenced in Prang Limestone and no late diagenetic
process was observed.
The occurance of glauconite in top of Prang Limestone established shallow
marine early diagenetic process (Fig 3.8), which probably at the interface between
reducing and oxidizing zones in the muddy sediments. Glauconite has been
synthesized at low temperatures (about 20oC), at sea water pH (8.5) by precipitation
of Fe-hydroxides from Si, Fe,Al,and K containing solutions in pore spaces under
slightly reducing conditions. Shallow sediments under conditions of low
sedimentation rate can produced glauconitic minerals, and are more often, found in
shelf areas of lower sedimentation rate.
91
Fig 3.14: Photomicrographs of Lakadong Limestone.
92
Fig 3.15: Photomicrographs of Lakadong Limestone.
93
Fig 3.16: Photomicrographs of Umlatdoh Limestone.
94
Fig 3.17: Photomicrographs of Umlatdoh Limestone.
95
Fig 3.18: Photomicrographs of Prang Limestone.
96
Fig 3.19: Photomicrographs of Prang Limestone.