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CHAPTER – IV
GEOLOGY AND GEOMORPHOLOGY
4.1 Geology
The very first memoir of the Geological Survey of India (GSI) was published in 1858
carries an article on the geology of Nilgiri hills (Blanford, 1858). He mentioned the
geological observations made by Benza, (1836) and Ouchterlony, (1848). Later in 1862,
Blanford studied the surface exposures of sedimentary rocks and correlated them to the
cretaceous formations of England and France. Benza has given an account on the geology
of the terrain between Madras and Nilgiris. Prinsep, (1832) has published an article on the
iron ores of Salem, which had attracted the geologist of the 19th century. Shortly,
Newbold, (1842) made a geological note on the occurrence of Chromite in Salem district.
The mapping of geological formations was started in 1940 and extensive research works
were made available about the geology, structures and tectonic evolution of the area.
These detailed studies have opened the path for understanding the relationship between
stratigraphy, sub-surface facies variation, and formation of various litho units in the study
area.
The geological formations, especially the occurrence of charnockitic rocks in
Tamiraparani basin was investigated by Narayanaswami and Lakshmi, (1967). They
concentrated mainly on the occurrence of charnockitic rocks in the western part of
Tirunelveli district and adjacent Western Ghats. They considered that the charnockites in
Tirunelveli district represents a geosynclinals association of sediments with basic
intrusive and volcanic, and inferred that the charnockites were subjected to polyphase
deformation, as a result of multiple folding. Other significant work on the geology of
Tamiraparani basin was conducted by Balasubramanian et al., (1985); Balasubramanian
and Sastri, (1987). They pointed out that the entire basin is occupied by three distinct
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geological formations namely archean, tertiary, and recent to sub-recent sands (Table
4.1). Around 90% of the aerial distribution of rocks type in the Tamiraparani basin
consists of charnockites, calc gneisses, granitic gneiss, crystalline limestones, calc
granulites, and quartzites of archean age. The tertiary formations and recent sediments
occur in the form of a narrow zone parallel to the coastline with the thickness increasing
towards the shore.
Table 4.1 Geological formations in the Tamiraparani river basin
Age
Lithology
Recent to sub- Soils, coastal sands, red Teri kankar, Tufa
and laterites, calcareous sandstone and shell
recent sands
limestones
Hard and compact calcareous sandstones and
Tertiary
shell limestone
Unconformity
Younger granitic and pegmatitic intrusions
Composite gneiss, granitic mica gneiss
Garnetiferous mica gneiss, Ambhibolites,
epidiorites and pyroxene granutites
Archaean
Charnockites
Crystalline limestones and calc granulites
Quartzites
Distribution
Eastern region
adjacent to the
coastline
Underlying the beach
sands extended up to
Srivaikundam
Northern and central
parts Northern part
Southern and south
western parts
Western and northern
parts of Tenkasi
Central parts around
Tirunelveli
Ambasamudram
(Source: Balasubramanian et al., 1985)
A few regionally extensive and well-defined tectonic contacts, mostly traced in the
Precambrian terrain can assist in grouping the rocks spatially. The terrain between the
Palghat-Cauvery fault (PCA) and the Achankovil-Tamiraparani fault (AK) comprises of
pink and grey migmatites, rocks of the charnockite and khondalite groups with discrete
bodies of granites. Migmatised charnockite and khondalite groups of rocks are noticed to
the south of the WNW-ESE trending Achankovil-Tamiraparani fault (AK). The rocks in
the tectonic zone of Achankovil terrain are marked by cordierite. Close association
between the rocks of the charnockite group with khondalite group is recognized in the
study area (Narayanaswami, 1975). The rocks of the khondalite group are of metaWĂŐĞͮϲϴ
sedimentary origin and their potentiality as a storehouse of crystalline limestone and
graphite is recognized.
In the southern part of the state (Kanyakumari, Madurai and Tirunelveli district), three
generations of folds have been recognized. Here, NW-SE trending folds are cross-folded
along N-S and NE-SW. Faults and fractures are noticed in the study area, in which
Cheranmahadevi fault trends in N-S direction and the Achankovil fault trends towards
NW-SE direction (Figure 4.1). The Kanyakumari – Tenkasi Belt (ktsb) is running roughly
NW-SE direction to the south of the Tiruchirapalli-Madurai-Palayamkottai block (TMP).
The Achankovil lineament (AK) marks the northern boundary of the belt and the
ultramafic bodies near Arumanallur and two pyroxene granulite bands in the terrain may
possibly represent the ophiolitic assemblages in the suture, marked by the Achankovil
lineament. The ancient suture is possibly ruptured and later the emplacement of alkaline
rocks such as the Putteti syenite and Chengannam alkali granite took place. Occurrence of
khondalite group of rocks including garnet-sillimanite-graphite schist and gneiss,
quartzite and calc-granulite are found in association with charnockites in the south of the
suture zone, within the Tiruchirapalli-Madurai-Palayamkottai block (TMP).
According to Narayanaswami and Lakshmi, (1967), different types of charnockites
associated with meta-sedimentary rocks and gneisses occur in the Tirunelveli and
Kanyakumari districts. The types of charnockites include acid microperthitic charnockite,
intermediate gneissic charnockite, basic noritic charnockite (pyroxene granulite) and
ultrabasic charnockite. The acid microperthitic charnockite occurs as conformable lenses
and bands of different dimensions within garnetiferous gneiss and migmatites. The
intermediate gneissic charnockite is the most prevalent and it is banded and foliated.
Alternating bands of light colored microperthitic charnockite and layers of pyroxene
granulite are seen in sheet-like form.
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Figure 4.1Map showing major faults, tectonics and lineaments in the study area
(Source: GSI, 1998a & Landsat – SRTM DEM images)
Hypersthenites, hypersthene bearing diopsidites and augitites are the rocks of the ultrabasic
division. They occur as sporadic lenses and bands within garnetiferous gneiss.
Gopalakrishnan et al., (1976) expressed the view that the pink quartzo-feldspathic granulite
and the basic pyroxene granulite of the charnockitic group in Tamil Nadu represent
metamorphosed acid and basic flows and the magnetite-quartzite bands in charnockite could
be of volcanic exhalative origin. However, Sugavanam et al., (1978) opined that the
charnockites in the northern part of the state owe their origin to deep seated granulite facies
metamorphism of pre-existing volcanic and sedimentary rocks and gneisses. Near Melur in
Madurai district, incipient formation of charnockite confined to certain major joint/fracture
systems in garnetiferous quartzo-feldspathic gneiss has been reported by GSI, (1998b) and
similar feature in garnetiferous granulites and gneisses is also observed near Papanasam in
Tirunelveli district (Whitman, 1988).
4.1.1
Precambrian rocks
The southern part of peninsular India is made up of granulite facies, charnockite and their
retrogressed products. During the Proterozoic, several major shear zones dissected the
granulite terrain and the granulite facies assemblages suppressed the retrogression (Drury et
al., 1984). The magnetic susceptibility of the granulite terrain in the northern part of the study
area (Madurai-Shenkottai-Tenkasi) and near Rajapalayam is reported as 170 × 106 emu/cc
and 1400 × 106 emu/cc respectively (Ramachandran, 1985). The present study area is
predominantly underlain by the crystalline rocks of archean age such as gneisses, granites,
charnockites and acidic intrusive rocks. These rock formations are the typical crystalline
rocks of the Deccan plateau as well as the Western Ghats (Pitchamuthu, 1979). Moreover, the
western part of the study area and the adjoining Western Ghats are underlain by charnockites
(Narayanaswami and Lakshmi, 1967). However, these features are found to be speckled as
bands and lenses in NW of Tirunelveli town and they tread towards NW-SE direction along
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foliation with the gnessic rocks in Ambasamudram and Tenkasi areas.
Granitiod non-garnetiferous mica, hornblende gneisses and mixed gneisses associated
with migmatites are noticed in the north eastern part of Tirunelveli and the south and
southwest parts are characterized by khondalitic migmatitic assemblages of garnetiferous
biotite gneisses, and leptinitic assemblages. The khondalite group in the study area is
epitomized by garnetiferous biotite gneiss and isolated occurrence of garnetiferous
graphite gneiss and granulite with or without sillimanite at few places. In Sankarankoil
region, which falls over the north eastern part of the study area, where gneisses are
observed to be moderate grained to coarse-grained quartzite, well foliated and banded
micaceous granite gneisses.
In Sanganeri- Tirunelveli region, calcium granulite patches have been noticed, which
are medium grained in nature with grayish color and contains graphite flakes. Narrow
bands (1 to 5 meter) of calc-granulites are noticed in association with charnockites and
garnetiferous biotite gneiss bounded by garneiferous sillimanite graphite gneiss.
Moreover, in Ambasamudram area, calc-silicate rocks associated with humite chondarite
spinel-magnesian calcite assemblages’ trending in the general direction of foliation have
been noticed. The width of the calc-silicate band ranges from 25-350 meter, which is
separated by quartzites and is flanked by intermixed banded biotite and granitic gneiss.
4.1.2 Basic intrusive
In general, basic intrusive rocks occur as dyke. About 20 km to the northeast of
Kanyakumari, dykes of carbonatite in charnockites and granitic gneisses have been
recorded near Kudangulam. Also, fine grained basalt and medium grained dolerite dyke
occur in SSE of Kudankulam, Tirunelveli district. The dykes have a width ranges from
few centimeters to 25 meter and the length varies from 100 to 300 meters, which cut
across the foliation of the country rock. They are hard, dark grey to black in color and
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exhibit spheroidal weathering. Basaltic dykes are seen in well cutting, positioned in the
southern part of the Tirunelveli district. They are and fine-grained to porphyritic,
containing phenocrysts of pyroxenes and feldspar.
4.1.3 Acidic intrusive
The northern part of the study area (Tenkasi and north Shenkottai regions) is
characterized by the presence of conformable bands of granitoid of different types such as
pinkish granitoid, leucogranite and grayish granitoids. Pegmatite veins are observed to be
intruding in all the rock types with varying thickness from few centimeters to 1.5 meter.
Few quartz veins are present trending to NNW, NNW-SSE and N-S directions.
Leucogranite of different dimensions are seen in northern part of Tirunelveli district
frequently trending parallel to the regional trend of foliation. The width of granite rock
ranging from 5 to 15 m and length varies from 1000 to 3000 m.
4.1.4 Crystalline limestone
The crystalline limestones in Tamil Nadu have low magnesium content and are found
in Ramayyanpatti, Talayuthu deposits of Pandalkudi, Pallavanattam and Alangulam
bands. Moreover, garnetiferous gneisses are found to be interbedded with quartzites in
Nanguneri and Ambasamudram taluks. The Ramayyanpatti-Talayuthu deposits forms the
most imperative one in the study area forming the west of a major cross-folded isoclinal
antiform. The limestones are white to grey in color and coarse-grained grading to
calciphyre and calc-gneisses.
The western segments of Ramayyanpatti are composed of high-grade material and
inter-bedded with quartzite, which is useful for chemical industries. In the Talayuthu
block, the limestones are frequently interrelated with quartzite that makes it highly
siliceous and impure. Near Kuttaipatimalai, the limestone grades into a calc-gneiss with
local high-grade patches. Bands of calc-gneisses and calciphyre coupled with quartzite
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and garnetiferous gneisses are recorded near Ambasamudram. Cocolite bearing streaky
carbonaceous limestones are also distributed in the Panachadi, cocolite and tremolite in
Adaichchani area, phlogophite near Singampatti and Pattamadi with diopside and impure
types. These are all aligned NE-SE, forming the southern limit of a major NW-SE cross
fold.
The Pandalkudi area contains narrow bands of coarse calcitic limestone associated
with banded carbonaceous graphitic bands. Along the same strike in the western direction
towards Sivakasi, crystalline limestones are developed as narrow bands near Anaikuttam
and extended into Alangulam deposits by repeated sinistral shifts. The Kakkivadanpatti
area shows three periods of deformation commencing with the folding along ENE-WSW,
but plunging at 20º followed by a NE-SW folding developed on the limbs of the first set
of folds. Under its influence, the limestones along the SW of Alangulam are found in
greater thickness. They must have been subjected to refolding along NNE-SSW axis but
plunging to south. This has brought out the outcrop pattern of the wide bands of quartzite
to the east of Rajapalayam.
4.1.5 Tertiary rock
In Apparkulam (Tirunelveli district), a crystalline limestone of dolomitic grade
extends in NW direction for a length of 1.2 km with a width of 20 meters. It enlarges to a
thickness of over 300 m, cropping out on the western banks of Pachaiyar river. In
Srivaikuntam taluk, dolomitic limestones are extended towards SingathakurichiEllainaickenpatti villages. The band is exposed intermittently over a length of 3 km with
an average width of 10-20 meter. Isolated patches of calcareous sandstones and
fossiliferous limestones are noticed in the Tirunelveli region (Subramanian & Selvan,
2001). Calcareous sandstone system exposed near Kudangulam is one of the prominent
among them. The sandstone system is poorly sorted, fine to coarse-grained in nature. The
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outcropping of limestone near Kudangulam has an elevation of above 50 meter. This
exposure at south and southwest of Kudankulam covers an area of about 3 sq km. The
limestone is light yellowish to pink in color, hard and compact, containing broken shells.
Based on microfossil index, the limestone has placed in the age between Upper Miocene
and Pliocene (Bruckener, 1988). The presence of two types of cementing medium
indicates that, first type must have been formed under submarine condition and the
second under sub-aerial condition (Bruckener, 1989).
4.1.6 Quaternary deposits
Quaternary deposits are traced along the coast and in major river valleys. River
terraces are noticed along the major river valleys and alluvium is extensive in deltaic
plains. In the Tirunelveli district 99% of the area is covered by brown soil and it is known
for its widespread kankar and tuffaceous limestone patches. In the southern part of
Tirunelveli district, kankar nodules abound along some seasonal stream courses. Isolated
cappings of laterite over gneisses, granulites and charnockites are seen in many parts of
Tirunelveli district. The thickness of laterite ranges from few centimeters to 5 meter. Teri
sands of reddish color have spread over a large extent of the area in Tirunelveli district.
The origin of Teri sands is still a dispute among researchers with one group attributing to
the detrital Aeolian (Dowie, 1940; Ahmad, 1972) and another group attributing it to the
weathering of country rocks (Zeuner & Allchin, 1956; Gardner, 1981).
4.2 Petrography
4.2.1
Leptynite
It is generally branded as garnetiferous quartzo feldspathic granulite with coarse-grained
granoblastic texture. The textural relations and the absence of significant compositional
zoning in mineral indicate equilibrium mineral assemblages.
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Through the mineral assemblage’s proportion may vary, the common mineral assemblage
is alkali feldspar - plagioglase (An 30-40) + Quartz + Garnet + Biotite. The typical
mineral assemblages and their specific characters are given below.
Minerals
Alkali
feldspar
Plagioclase
Pyroxene
Garnet
Hornblende
Biotite
4.2.2
Characteristics
Typically perthite
Antiperthite
Mainly hypersthene of pleochroic in greenish, pinkish and yellowish
colors.
Rich in pink garnets (Almandine and pyrope) and also colorless garnet
Olive green to brown
Brown to dark brown
Charnockite
The name charnockite refers to an orthopyroxine bearing granite commonly found in
granulite facies terrains. High temperature and pressure are considered essential for their
formation. Granoblastic polygonal to interlobate microtexture characterizes charnockite
from other rocks. This designates through recrystallisation at conditions on par with the
granulite facies metamorphism. The metamorphic assemblages are quartz-plagioclasepotash
feldspar-orthopyroxene-clinopyroxene-garnet-biotite.
In
spite
of
massive
appearance, the micro-texture bestows clear evidence of post charnockitic deformation.
Pyroxene are large , showing exsolution of opaque, along their cleavage planes and grain
boundaries and are often rimmed by corona of bluish green hornblende and
cummingtonite . Pyroxenes and plagioclase show bent lamellae. Strained quartz grains
exhibit wavy extinction. Sometimes plagioclase and quartz show marginal granulation
with very little recrystallisation. Porphyroplastic garnets contain inclusion of quartz and
feldspar, exhibiting sieve texture. Secondary coronitic garnets have developed after the
primary ones. Quartz grains are large and flattened exhibiting cataclastic texture.
Stretched quartz grains are found to dominate over small lobate, sutured quartz grains.
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Bent and discontinuous twin lamellae in plagioclase indicate intensive deformation
followed by bead and rod type of perthites are also noticed.
4.2.3
Granitoid gneiss
Granitoid gneisses including garnetiferous biotite gneisses cover the major part of the
study area (Plate 4.1). Massive rocks demonstrating a hazy foliation are transitional into
the rocks of granulite facies. The trend of foliation is NE-SW with a dip of 45º to 75º in
SW and WNW direction. Presence of banding is emblematic in this rock. It is an
alternation between bands rich in biotite, hornblende, and pyroxene and bands rich in
quartz and feldspar. In these rocks, mica, hornblende, and elongated minerals such as
sillimanite, tourmaline, kyanite and pyroxene are arranged more or less parallel to the
foliation. The garnets contain inclusions of quartz, biotite, magnetite, plagioclase, and
sillimanite. Quartz and feldspar generally have an elongated form, although they may be
approximately equi-dimensional. The potash feldspars present are usually perthitic. Thin
linear bands of garnetiferous sillimanite gneisses occur as in the south western part of the
study area. The bands are well foliated and have a NW–SE trend with a dip of 30° to 50°
towards the southwest direction.
4.2.4
Quartzo feldspathic granulite
Quartzo-feldspathic granulites are characterised by “granulitic fabric”. The
assemblages are quartz-orthoclase plagioclase-garnet. The grains have interlocking
boundaries. The garnet of granulites is pink almandine type with higher MgO than garnet
of the amphibolitic facies. Prisms of kyanite and sillimanite lie in the plane of schistocity.
The orthoclase is perpetually perthitic.
4.2.5
Pink granite
Granites of coarse grained demonstrate the typical granitoid texture. Both plagioclase
and mafic minerals tend to form sub-hedral crystals. Anhedral quartz occupies
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intergranular spaces. Generally, alkali feldspars are of anhedral type. The minerals
present in granites are dark brown biotite, zircon, muscovite, and minor amounts of
sillimanite, magnetite, almandine, and topaz.
4.2.6
Quartzite
Quartzites are metamorphic rocks, mostly of non-foliated, coarse textured, granular,
white to light reddish in color with streaks of ion ores and composed of quartz. These
rocks occur as distinct and detached linear bands on sharp edge ridges within the gneisses
and charnockites in the study area and form low hills and domes. They are mainly derived
from calc-gneisses and granulites and contain inclusion of chlorite, mica and hornblende.
Inclusions of minerals such as sillimanite, garnet, kyanite and chloritoid are invariably
present and occurrence of muscovite and biotite are also noticed. The trend varies from
NE to SW with steep dips.
4.2.7
Granite gneiss
It is characterised by the presence of distinct bands, streaks, lenses, and dykes of
pegmatitic material. It is made up of quartz, plagioclase, and microcline with biotite and
muscovite. Presence of zircon, kyanite, sillimanite and garnet are also noticed.
4.2.8
Calc gneiss
They are often massive, coarse-grained and crop up as thin linear bands pass through
gneisses. However, finer to medium grained and impure micaceous varieties are also
commonly found. Defoliation and banding are the archetypal characteristics of these
rocks. The foliation embraces a well-marked segregation into dark and light layers.
Carbonate is customarily interstitial or vein-like in its distribution. The calc-gneisses are
characterized by minerals such as hornblende, tremolite, actinolite, plagioclase, biotite
and calcite. Pyrite, Pyrrhotite and magnetite are accessory constituents.
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4.2.9
Crystalline limestone
The texture of limestone depends to a large extend on the stage at which the
development of silicates takes place. The concomitant deformation with silicates has
fashioned amphibolite schist nature to limestones. The minerals frequently encountered in
these rocks are tremolite, actinolite, and epidote followed by Hornblende and mica. Since
it is highly metamorphic, it resembles hornblende schists and gneisses, commonly called
as amphibolites. A detailed geology map of the study area is shown in figure 4.2.
4.3 . Geomorphology
Geomorphology is the scientific study of landforms and the processes involved in
shaping the topography of an area through the course of evolution. According to
Thornbury, (1969), the earth’s surface is affected by geomorphic processes which include
physical and chemical changes that in turn involved in the evolution and development of
landforms. The geomorphic characteristics of a basin are determined by several
morphogenetic features evolved under the influence of fluvial hydrodynamics, sub-aerial
processes, tectonic activities and other geomorphic divisions such as weathering and
active denudation processes. The influencing features and factors such as geology,
landuse and climate are also responsible for the development of morphological landforms
in an area. Moreover, the availability of land and water resources is greatly controlled by
the geomorphic setting of an area. Various landform characteristics in the study area are
based on the broad geomorphic division, which includes exogenic processes such as
weathering and active denudation. The geomorphic evaluation of the study area was
carried out qualitatively and quantitatively. Intensive fieldwork and the application of
geospatial technologies such as remote sensing and GIS has been used to explain the
details about geomorphology of the study area. The detailed geomorphological map of the
study area is shown in figure 4.3.
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Figure 4.2 Geology map of the study area (Source: GSI, 1998)
Plate 4.1: A- Bedrock exposed with garnet biotite gneiss near Sorimuthayyanar temple. BEroded surface of garnet sillimanite graphite gneiss near Papanasam. C- Fractures and joints
in garnet sillimanite graphite gneiss near Agasthiyar falls. D-Weathering of gneisic structures
in garnetiferous sillimanite gneiss. E-Abraided blocks in the falls. F- Weathering of
garnetiferous biotite gneiss in lower Papanasam.
Figure 4.3. Geomorphological map of the study area (Source: Extracted from Landsat imagery)
Various litho units present in the study area with dry and wet spell of climate triggers the
mechanical and chemical weathering processes, which results in varied denudation and
depositional landforms around the entire basin. The vegetation cover and varied landuse
are also responsible for the weathering processes and influence the landform
development.
4.3.1 Fluvial processes
Fluvial processes are referred to the physical interaction of flowing water through the
natural channels such as streams and rivers. These processes play an important role in the
denudation of landforms and the transport of sediments from upstream to downstream.
River induced erosion can be categorized in two ways such as chemical erosion and
mechanical erosion. The former involves in corrosion activity along the stream system
while the later comprised of abrasion, hydraulic action, and deposition. Fluvial erosion
can be categorized into four divisions such as vertical erosion, lateral erosion, headward
erosion, and mouthward erosion. The study area is largely controlled by fluvial processes
such as the work of running water mainly during the monsoon season.
Two distinct monsoon periods is noticed in the study area, July to August in the up
streams and October to December in the down streams. The varied topography and high
slopes in the Papanasam and Manimuthar catchments are responsible for its down cutting
continuously since the Tertiary period. As a result of active tectonics, the study area has
experienced numerous upliftment, and resulted in continuous down weathering. During
the period of intense rainfall, a phenomena called rapid run-off is observed in the plain
regions of the study area. During this period, the raindrops with maximum velocity and
kinetic energy tend to move the sand grains and making it susceptible to the overland
flow. This process is more active in the alluvial plains and triggers surface soil erosion.
The study area exhibits a number of fluvial erosional features such as scraps and gully
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erosion. The slopes in the study area play a major role in landscape development. This is
evident to the western part of the study area comprising Kalakad, Manimuthar,
Papanasam, and Courtallam hilly tracts. The hilly terrains are moderately exposed with
bare outcrop of the bedrock. These areas are densely forested while the pediments zone is
vegetated and under agricultural practice. Numerous potholes of various sizes and shapes
have been observed during the field work in the rocky exposure of the channel of
Tamiraparani river. This is due to the grinding action of sand grains and pebbles over a
long time period. River bank erosion is another important feature observed along the
Tamiraparani river. The occurrence of bank erosion is found to be confined in the high
energy zones with feeble vegetation cover and loose sand deposits. Rills are observed
along the foothills of Kalakad and parts of Manimuthar region. Formations of rills are
possible where slopes are steep (30° to 70°), heavy rainfall run-off with loosely packed
soil material. As erosion intensifies, rills instantly begin to integrate into larger channels
to form gullies.
The river terraces are one of the important geomorphic features present in the study
area. There are found along the river channels, mostly flat topped, blunt edged and
covered with riparian vegetation along the river channels. The terraces are developed by
Tamiraparani river and its tributaries are alluvial in nature consisting of gravel, sand and
fine sediments. The terraces of the study area are mainly controlled by neotectonic
activities as well as climatic factors. The depositional landforms in the study area are
influenced by channel gradient and stream discharges along with regional geomorphic
settings. The weak bank materials are easily eroded along the river course, which results
in channel widening, and sediment loading. The alluvial floodplain in the study area is the
most common depositional feature formed by the combination of channel and over bank
deposition. These features are commonly seen near to stream diverge, where flow and
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transport rate is decreased, which favours deposition of sediments. The lateral and vertical
accretion within the channel influences the floodplain formation in the study area.
4.3.2
Geomorphic settings of the study area
The geomorphology of Tamiraparani sub-basin has been studied based on Landsat
satellite images along with digital elevation model (DEM) and intensive field verification.
The major geomorphic units in the study area are structural hills, denudational hills,
residual hills, inselberg, valley floor, valley fill, shallow floodplain, pediment, pediplain,
shallow weathered pediplain, shallow buried pediplain, upper bazada, linear ridges, point
bar, channel bar, channel lag, and active channel belt.
4.3.2.1 Structural hills
The Western part of the study area is comprised of rigid structural hills, which are
well visualized from the satellite imageries merged with hillshade layer derived from
DEM, and they cover nearly 40% of the study area. It is observed that the hills are
controlled by tectonic and neotectonic actions. Numerous fault lines and lineaments are
observed in this region, which is formed by active tectonics. Moreover, they have steep
slopes and high relief and represents the base of the Western Ghats. Due to which, the
erosional dissection is controlled by the fracture pattern of the bedrock.
The elevation trend is observed from East to West direction with increasing elevation
towards West. The weathering and erosion in this region has produced variously shaped
ridges and valleys, which further results in numerous erosional landform features such as
rills and gullies. Gully erosion is common in steep structural hills and headward erosion is
noticed through the rills. The first order stream network provides space for rill erosion
that affects the degree of dissection and leads to sediment transportation from the
catchment.
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4.3.2.2 Denudational hills
Denudational hills are formed by the natural process of weathering and they occur as
exfoliation domes with partial scree or debris covered at the foot slopes and they are
noticed near Kalakad-Ambasamudram area. The shape of the denudational hills is
controlled by the lithology, structural features, weathering and external forces. The
denudational hills in the study area is composed of gneisses with a little soil cover and
moderate vegetation and falls under Mampothai and Kolundumadai reserve forest. The
tropical weathering conditions in this region lead to various geomorphic forms such as
exfoliation domes, domed inselberg with steep slopes along with boulders of various
sizes. The altitude of this hill type varies from 300 to 400 m above the pediplain forming
an important geomorphic unit.
4.3.2.3 Residual hills
The residual hills are isolated, steep sided, usually smoothened, and rounded hill
rising abruptly above the surroundings over the plains in tropical regions. These hills
exhibit conical to rounded forms with steep to very steep slopes and are noticed in
Nanguneri-Kalakad stretch and isolated patches are seen in Thiravianagar, Kilakadayam,
Kunnathur, Melapattam, Perumalkulam, and Sivanthipuram. The altitude of this Hill type
varies from 100 to 200 m above the pediplain.
4.3.2.4 Inselberg
Isolated inselberg of 50 to 100 m above the pediplain are found in the study area and
are mostly composed of gneisses. Due to spheroidal weathering, they occur as boulders
embedded in the highly oxidized weathered state. They are noticed between KalakadSingikulam stretch, Karaisalpatti, Padmaneri, Melapattam, Tenkulam, Mayamankurichi,
and Govindaperi (Plate 4.2).
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4.3.2.5 Valley fill
The valley fills are common along the drainage system where boulders, cobbles,
pebbles and clastic materials are deposited. In contrast to terrace sediments, the valley fill
sediments are probably yellowish to greyish finer silty sand to find sands. However,
coarse sand is also found in these areas due to occurrence of erosional processes near to
hilly terrains. In the study area, the valley fills are noticed along the major stream
networks and in the northern part of the study area.
4.3.2.6 Shallow floodplain
This geomorphic landform is formed by recent alluvium and is found along the active
floodplains. They consist of unconsolidated gravels, sands, silts, and clays formed by
seasonal flooding when the river overflows their banks. Generally, floodplains form a
higher level than the active floodplains and are not frequently inundated by flood waters.
The alluvial plains are used as croplands for widespread cultivation along the river banks.
Floodplains are generally depositional landforms, which are found adjacent to a channel
and made up of sediment transported by the flow regime (Nanson & Croke, 1992). In the
study area, the shallow floodplains are seen along the Tamiraparani main channel and are
encroached by shrubs and vegetation.
4.3.2.7 Pediment
Pediments are gently sloping erosional surfaces of low relief and were developed on
bedrock or older unconsolidated deposits. Their occurrence may be influenced by
lithologic, neotectonic and climate settings and their formation may be sub-aerially
exposed and covered by a discontinuous to continuous veneer of alluvial deposits.
Moreover, the subsurface weathering processes have strongly influenced the pediment
development in the study area and such processes are critical to the formation or
modification of pediments on crystalline bedrocks. In the study area, the pediment
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formations are seen in the foothills of the Western Ghats and also noticed in isolated
patches throughout the study area. However, there is a close relationship between
pediment formation and other structural features such as hills, linear ridges, and
inselbergs. This highlights that the formation of these landforms are due to the presence
of gently sloping erosional surfaces in the study area.
4.3.2.8 Pediplain
The pediplains are characterized by gently undulating to flat topography overlaid
above the bedrock and are found on either side of the main channel. They are formed
from older alluvium and they gradually developed from topographic highs within the
alluvial plain. The pediplain exhibits a gentle slope towards the main channel as channel
overflows feeds the alluvial deposits to the pediplains. In the study area, the pediplains
are noticed along the main channel of Tamiraparani river and adjoining tributaries.
Moreover, the variation in the characteristic features of pediplains influences their spatial
distribution in terms of composition of landscapes, drainage system, overburden, and
minor reliefs. The cumulative effect of these features results in the formation of shallow
and moderately weathered pediplains (Plate 4.3).
4.3.2.9 Shallow weathered pediplain
Shallow weathered pediplains are flat and smooth surface of buried pediplain with a
thickness of 0 to 5 meter and composed of shallow overburden of weathered derivative
material. They are uniformly distributed in the study area and this geomorphic landform
is good for groundwater prospecting in shallow aquifers. Any modification of landuse and
land cover over these landforms may have a direct impact on the groundwater resources.
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Plate 4.2: A- Structural hills near upper Karayar. B- Isolated denudational hills near KalakadNanguneri. C- Dome type residual hills and inselbergs near Kalakad. D- Eroded hard rock
channel bed near Papanasam-Agasthiyar falls – result of tectonic activity. E- Bank erosion in
the upstreams of Gadana. F- Shallow weathered pediplain with moderately vegetated residual
hill near Kadayam.
Plate 4.3: A- Exposed bed rock due to stream action in the upstreams of Tamiraparani river. BHuman intervention near Reddiyarapati, a road network is cutting through a linear ridge. CBoulders in the channel bed of upper Tamiraparani river. D- Pot-holes in the channel bed of
upper Tamiraparani river near Sorimuthayyanar temple. E – Flood plains of Tamiraparani river
near Kokaraikulam. F- Outcrops of Hornblende-biotite-gneiss in the pediplains.
4.3.2.10 Moderately weathered pediplain
Moderately weathered pediplains are composed of smooth surface of buried
pediplain. They have a thickness of about 5 to 20 meter overburden of weathered
derivative material and are found along Pachaiyar river stretch and the palaeo-channels of
the study area. These units have good potential for groundwater resources and have less
seasonal variation.
4.3.2.11 Upper bajada
These are fan shaped deposits and are formed along a mountain front by the action of
stream induced sediment deposition at the base of the mountains. The coarser sediment
deposits at the base of the mountain and the finer sediment moves outward to forms a fan
shaped feature away from the mountain face. They are commonly found in those areas
where flash flood induced sediment deposits occur over a period of time. In the study
area, the bajada formations are seen in the foot hills of Kalakad and adjoining areas.
4.3.2.12 Linear ridges
The linear ridges observed in the study area are narrow, low in relief with moderate
vegetation. The structural features in linear ridges are mostly strike controlled and are
found in isolated patches along NW-SE direction with a parallel resemblance to the
Western Ghats. The Idaiyankulam-Melkarai stretch has numerous ridges with parallel
alignment and similar observations are also noticed in Veeravanallur, Alwarkurichi,
Vickramasingapuram, Kalladikurichi, and near Alangulam. Moreover, a well established
linear ridge system is observed between Anjan Kattalai and Muthur with the height
ranges from 1 to 2 meter above the pediplain.
4.3.2.13 Point bar
Point bars are notable geomorphic units and are often found to be associated with
meandering streams. Also, the sediment deposition on point bars has a great influence on
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shaping the river channels due to lateral migration of meandering river at the time of
flooding. In the study area, the point bar features are found to be associated with the
major tributaries of Tamiraparani river. They are developed from the lateral accretion on
the inner banks of the river channel. The area between Aladiur and Kodaganallur along
the Tamiraparani river stretch is notable for such features and it can be well discriminated
from the satellite imagery. In the smaller streams such as the tributaries of the
Tamiraparani river, point bars are noticed as simple depositional features on the convex
sides of the meander dipping gently towards the channel. Moreover, these point bars are
encroached with vegetation cover and forms a stable landmass thus shaping the course of
the channel.
4.3.2.14 Channel lag
The transported material in a river contains a wide range of sediment particles and
they differ in their size. Based on its size fraction they can be classified as silt, clay and
coarse sand. Silt and clay fractions are transported in a much faster rate and are often
deposited in flood plains and the coarser sediments, gravels and pebbles lag behind and
accumulate as discontinuous lenticular patches in the deeper part of the channel and
called as channel lag deposits. These deposits are later accumulated with fine sediments
and are preserved in the base of the channel. They are invariably discontinuous and
occupy the lowest part of the channel and represent the base of the channel. The study
area, the channel lags are often seen in Papanasam-Servalar confluence near to
Irumbupalam bridge under low stream flow condition.
4.3.2.15 Channel bar
Channel bars are often associated with the braided character of a stream. The braided
condition of the channel indicates the sediment carrying capacity of a river. This
condition prevails when excess materials fall out of transport when it enters the plain. The
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sediment yield is one of the contributing factors for the formation of channel bars in the
stream network. In the study area, channel bars are often seen along the Tamiraparani
river and its tributaries. The old bars are associated with vegetation cover whereas, the
new bars are bare and composed of coarse to fine sediments and migrates during high
flow condition.
4.3.2.16 Active channel belt
The active river course of the present-day channel is called active channel belt and
they occupy the youngest geomorphic surface in the fluvial system. Before the
construction of reservoirs, the main channel had significant width with full water flow
condition especially in the monsoons. However, the scenario changed after the
construction of reservoirs and the water flow was marginalized. As a result, the full flow
condition of the river has greatly reduced and the flow is restricted to the deep narrow
channels of the main river and it can be noticed in the down streams of Tamiraparani
river. The old channel is now facing anthropogenic encroachment resulting the river
degradation both physically and biologically (Plate 4.4).
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Plate 4.4: Aerial view of few geomorphic features in the study area. A- The active river course of the present-day channel.
B- Point bars and channel bars near Manamangalam. C- Linear ridge near Anjankattalai. D- Bajada formation near the
foothills of Kalakad.