CHAPTER-VII
DIAGENESIS OF THE BARAIL SANDSTONES
7.1 INTRODUCTION
Diagenesis is any chemical, physical, or biological change undergone by
sediment after its initial deposition and during and after its lithification,
exclusive of surface alteration (weathering) and metamorphism. It is a
process gone through from loose sediment—►transportation —► deposition
-—► consolidation —► sedimentary rock. Dapples (1972) studied the
diagenesis of sandstones on the basis of mineral association and
recognized four oxide series among sandstones, viz., aluminia-limemagnesia-iron oxide series, silica-lime-magnesia-iron oxide series, silicaaluminia-iron oxide series, and silica-aluminia-lime-magnesia series. From
the type of reaction involved three main stages are considered in a
progressive
development
of
sandstones,
namely,
redoxomorphic,
locomorphic and phyllomorphic stages.
Newly deposited
sediments are characterized
by loosely packed,
uncemented fabrics; high porosities; and high interstitial water content.
sedimentation
continues in subsiding basins,
As
older sediments are
progressively buried by younger sediments to depths that may be reaching
tens of kilometers. Sediment burial is accompanied by physical and chemical
changes that take place in the sediments in response to increase in pressure
from the weight of overlying sediments, downward increase in temperature,
and changes in pore water composition. The changes act in concert to bring
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about compaction and lithification of sediment, ultimately converting it into
consolidated sedimentary rock. Thus, sand is lithified to sandstone,
unconsolidated gravel is eventually lithified to conglomerate, and siliciclastic
mud is hardened into shale (mudrock). The process of lithification is
accompanied by physical, mineralogical, and chemical changes. Loose
grains packing gives way with burial to more tightly packed fabrics having
greatly reduced pore spaces by precipitation of cements. Minerals that were
chemically stable at low surface temperatures and in the presence of
environmental pore water become altered at higher burial temperatures and
changed pore water compositions.
Minerals may be completely dissolved or may be partially or completely
replaced by other minerals. Thus, porosity, mineralogy, and chemical
composition may be changed to various degrees during burial diagenesis.
The study of diagenesis in rocks is used to understand the tectonic history
they have undergone; the nature and type of fluids that have circulated
through them. Diagenetic changes reflect mainly the depositional conditions
of the sediment.
7.1.1 Stages and Realms of Digenesis
Diagenesis takes place at temperature and pressure higher than those of the
weathering and depositional environment but below those that produce
metamorphism. There is no clear boundary between the realms of
diagenesis and metamorphism and it is a gradational boundary. However,
commonly considered diagenesis occurs at temperature below about 250°C.
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Diagenesis begin immediately after deposition and continues through deep
burial and eventual uplift. Burial subjects the sediments to conditions of
pressure and temperature markedly different from those that exist in the
depositional environment. Increase in geostatic (rock) pressure, hydrostatic
(fluid) pressure, and temperature as a function of depth also change porefluid composition. Various authors (Boggs, S. Jr. 2006) have suggested that
sediments go through three to six stages of diagenesis. Perhaps the most
widely accepted stages of diagenesis are those proposed by Choquette and
Pray
(1970),
viz.,
(i)
Eodiagenesis,
(ii)
Mesodiagenesis and
(iii)
Telodiagenesis. Some authors (Burley and Worden, 2003) refer to these
stages simply as, eogenesis, mesogenesis, and telogenesis.
7.1.1.1 Eogenesis (Shallow burial)
It refers to the earliest stages of diagenesis, which takes place at very
shallow depths (a few meters to tens of meters) largely under the conditions
of depositional environment. The principal diagenetic changes that take
place in the regime include reworking of sediments, minor compaction and
grain repacking and mineralogical change.
7.1.1.2 Mesogenesis (Deep Burial)
It is the stage of diagenesis that takes place during deeper burial, under
conditions of increasing temperature and pressure and changed pore-water
compositions. The load pressures caused by deeper burial significantly
increase the tightness of grain packing with concomitant loss of porosity and
thinning of beds. Increase pressure at the contact point between grains also
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increases the solubility of the grains at the contact, leading to partial
dissolution of the grains. This process is referred to as pressure solution or
chemical compaction.
Chemical compaction further reduces porosity and
increase bed thinning. Under the influence of physical and chemical
compaction, aided by cementation, the primary porosity of sands is reduced
during deep burial. Compaction also causes bending of flexible grains such
as micas. Mechanical compaction and pressure solution cause porosity loss
in quartzose sandstones mainly at burial depths less than about 2 km
(Siever, (1979) because the combined effects of compaction, pressure
solution and small amount of quartz cement produce stable grain packing
arrangements. Porosity loss at greater depths is primarily resulted from
quartz cementation (Siever, 1979). Burley and Worden, (2003) suggest that
some porosity loss owing to compaction can continue to depth of at least 5
km.
An increase in temperature of 10°C during burial can accelerate chemical
reaction rates to double or triple. Thus, mineral phases that were stable in
the depositional environment may become unstable during deep burial.
Increasing temperature favors the formation of denser, less hydrous
minerals and also causes an increase in solubility of most common minerals.
The most important of these processes are cementation, dissolution, and
replacement.
Cementation refers to the precipitation of minerals into the pore space of
sediment, thereby reducing porosity and bringing out lithification of the
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sediment. Silica cements are the most common; however feldspars, iron
oxide, pyrite and many other minerals may also form as cements.
Quartz cementation is favored by high concentration of silica in pore waters
and by low temperatures. Silica may also be imported from other areas of
the sedimentation basin during episodes of fluid flow related to deep- basin
mineral dehydration or tectonic activity (Siever, 1979).
Dissolution of framework silicate grains may occur during deep burial under
conditions that are essentially the opposite of those required for
cementation. Rock fragments and low stability silicate minerals, such as
plagioclase feldspars, pyroxenes and amphiboles may dissolve out as a
result of increasing burial temperature and presence of organic acid in pore
water. Selective dissolution of less stable framework grains or parts of grains
takes place during diagenesis by intrastratal solution. Dissolution of
framework grains
and cements,
increases
porosity,
particularly in
sandstones take place through interstratal solutional activities. It is believed
that much of the porosity that exists in sandstones below burial depths of
about 3 km is secondary porosity, created by dissolution processes.
7.1.1.3 Telogenesis (Late stage)
Telogenesis refers to the late stage diagenesis that accompanies or follows
uplift of previously buried sediments into the regime of meteoric water.
These processes bring mineral assemblages, including new minerals formed
during mesogenesis, into an environment of lower temperature and pressure
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and in which mesogenetic pore water are flushed and replaced by oxygenrich, acidic meteoric (rain) water of low salinity. Under these changed
conditions, previously formed cements and framework grains may dissolve
(creating secondary porosity) or framework grains may alter to clay minerals.
The presently studied Barail sandstones suffer from distinct diagenetic
changes and have been identified and discussed below.
7.2 Diagenesis of the Barail Sandstones
In the present case certain diagenetic changes are clearly visible which
reflect conditions mostly ranging from eogenesis to mesogenesis. The
minerals have undergone some changes during and after their deposition.
Presence of pressure solution features such as grain point contacts and
sutured grain boundaries, formation of two or more generation of quartz
overgrowth, formation of concavo- convex contact between grains indicating
dissolution, kink bend of mica in quartz grains are the important features
which show diagenetic changes in the Barail sandstones.
Eogenesis statge is represented in the studied sandstones by quartz
overgrowth (Fig.7.1), kink band in mica (Fig.4.1t), bending in quartz (Fig.7.2)
and compaction of sediments.
Silica cementation, silicification of undeformed fossil shell (Fig.7.3) and
stylolites indicates mesogenesis stage. Mesogenesis stage is also
represented by concavo-convex grain boundaries, compaction of sediments
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through realignment of grains under pressure, removal of matrix, extensive
overgrowth of mineral grains and removal of unstable heavy minerals. Also,
intrusion of quartz veins in the eastern sandstones as evidenced in the field
indicates mesogenesis.
The study shows the western sandstones have mostly undergone eogenesis
followed by mesogenesis, whereas the eastern sandstones well attained the
mesogenesis stage. The study on diagenesis indicates that the older eastern
sandstones have suffered deep burial as compared to the younger western
sandstones.
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Fig-7.1 : Quartz overgrowth, corrugated and sutured grain boundaries of quartz.
Fig.7.2: Bending in quartz grains and stylolitic line.
Fig-7.3 : Silicified fossil shell of foraminifera.
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