CHAPTER - IV
COASTAL EVOLUTION
4.1. General
Understanding of coastal evolution is one of the most difficult parts of
geomorphology, as complex processes like marine, fluvial, fluvio marine,
aeolian, biological and human are involved.
While the evolution of the
depositional coast is controlled by the combined action of waves, tides and
currents on the sediments that are available for building landforms, the
evolution of erosional coast is decided by the strength of onshore rocks and
energy of waves.
The convergence and divergence of waves which are
formed as a result of friction between waves and the shallow offshore surface
also play a major role in the evolution of both depositional and erosional
coast. Coast is also a region where anyone can notice the construction and
destruction of landforms in a matter of few hours or minutes as a result of a
storm or hurricane or Tsunami. Landforms that are formed in over a period
of many thousands or more years are also destroyed in a single day by a
storm or Tsunami. Besides these epigenic forces, tectonic forces (hypogenic)
also influence the coastal evolution. Because of these complexities, coastal
landscape’s evolution is one of the imperfectly understood parts of
geomorphology. It is a synthesis of all the factors discussed above generates
the details of coastal evolution. These forces in the coastal geomorphic
system have an input of energy (e.g. Waves, currents, tides, wind etc.,) and
materials (e.g. Sediments, rocks, etc.,) that interact with each other to
generate landforms which in turn act as a feedback in the sense that the
developing landform becomes a factor influencing the coastal geomorphic
processes. Hence the interpretation of coastal landforms must be made in a
meticulous way so as to understand the processes and materials involved in
the genesis of individual landform, which in turn will help in understanding
the evolution of the coast in general.
In the study area, landforms that are usually associated with a delta
are found to occur. The landforms are mainly those that were formed during
Quaternary sea level oscillation (marine) and fluvial processes. The synthesis
of the marine landforms has revealed a great deal of information on the
92
Quaternary trans- and regressive events. Evidences for interruption of these
sea level events with the fluvial system and consequently modification and
shifting
of
fluvial
sedimentation
regimes
have
also
been
observed.
These marine and fluvial systems have also been influenced by the neo
tectonic activity in the form of crustal warping whose effects are observed in
the preferential shifting of regime of fluvial sedimentation. Hence the
evolution of coastal landscape of the study area is understood by the
interpretation of landforms that were formed by Quaternary sea level
changes, fluvial sedimentation and neo tectonic activity. Though the overall
coastal landscape is controlled mainly by these three factors, the individual
landforms in many places were formed by other processes like Aeolian,
biogenic, planation and so on.
4.2. Methodology
As the study area is formed mainly by landforms of marine and fluvial
processes, interpretation of those landforms were made to bring out
evidences for the events involved in the costal evolution. As most of the
marine landforms were formed as sequel to Quaternary sea level oscillation,
evidences for Quaternary sea level oscillation were collected. Shifting of river
channels is found to be responsible for the shifting of delta lobes and domain
of sedimentation and hence the details of the channel shifting were collected.
As Neotectonic signatures are found in the channel shifting of the river
Cauvery, evidences for the role of neotectonic activity in the costal evolution
were collected. The integration of these evidences - Quaternary sea level
changes, Fluvial and neo-tectonic - were made to trace the coastal evolution.
OSL dating was carried out for four samples to understand the age of
deposition of sediments.
All the available dates pertaining to Quaternary
sediments were collected (Table.4.6.) and correlation was made with respect
to the sediments of the study area. The age of deposition of sediments were
integrated with the sequences of events to understand the coastal evolution.
4.3. Influence of Quaternary Sea level changes on Coastal evolution
4.3.1. Introduction
Most of the existing coastal landforms of the world were formed as a
result of sea level changes that took place during the period of Quaternary.
93
Quaternary glacio-eustasy, glacio-and hydro-isostasy and local tectonism are
generally
recognized
as
major
causes
for
sea
level
changes.
The identification of the effect of individual cause is difficult. The effect of
such Quaternary episodes of sea level changes decides the grouping of
coastal landforms into those associated with coast of emergence and
submergence.
The occurrence of features like stranded beach deposits,
marine shell beds, terraces, beach ridge plains etc., are noticed with coast of
emergence and drowned river mouths, submerged beach ridges, submerged
forest etc., are noticed in the coast of submergence. Though many coastal
landforms display evidences for the paleo sea level, the accurate estimation
of past sea level is extremely difficult because of the involvement of different
variables noted in table 4.1.
Table - 4.1. Causes of sea - level changes
S.
Eustatic
No.
Local
1.
Glacio - eustasy
Glacio - isostasy
2.
Orogenic - eustasy
Hydro - isostasy
3.
Geoidal eustasy
4.
Infilling of basins Decantation
5.
Transfer from lake to ocean
Erosional
depositional
isostasy
Compaction
of
sediments
(auto - compaction)
Orogeny
Expansion or contraction of water
6.
and
Epiorogeny
volume because of temperature Ice - water
changes
Gravitational attraction
Courtesy: Andrew Goudie (1983)
In order to understand the status of studies on Quaternary sea level
changes, a brief review of literature pertaining to Quaternary sea level
studies are given here under.
94
4.3.2. Review of literature - Quaternary Sea level changes
a) General
Quaternary (named by Morlot in 1845), consisting of two epochs
namely Pleistocene and Holocene, spans approximately 2.6 million years in
the last part of geological timescale. In 1839 Lyell introduced the term
“Pleistocene” that means most recent times and the term “Holocene” was
introduced by IGC in 1885.
Earth experienced dramatic changes in the
climate during the period as witnessed by glaciations and intermittent
deglaciations. The earth had then been covered by ice more than three times
of the present day ice cover. The ice covered many parts of North America,
Europe, Asia and South America. The landforms formed by the glaciations
are afresh and undisturbed facilitating to map the former limits of Pleistocene
ice with high accuracy.
As a sequel to the glaciations and deglaciations, sea level rose and fell
through several meters. Besides the glacial and inter glacial events there
were stadial and interstadial events indicating short-term glaciations and
deglaciations within the interglacial and glacial events.
While the glaciations (growth and outward spreading of ice) resulted in
lowering of sea level ranging from 20-100m, the deglaciations (the shrinkage
and retreat of ice) caused rising of sea level to the same magnitude.
The amount of sea level change is calculated from the known volume of
existing glaciers and the volume of former ice sheets. The Antarctic ice sheet
alone can produce sea level rise to about 60m. Assuming that the added
water can cause isostatic down warping of 20m, the net sea level rise would
be around 40m. The records of sea level during last major glaciations (18000
years BP) indicate that the sea level was lower than the present by perhaps
60 -80m.The lowering of sea level had exposed a broad area of the
continental shelf. Peat samples indicating the existence of forest at that time
and remains of terrestrial animals in the seabed testify the lowering of sea
level
and
exposure
of
continental
shelf
for
terrestrial
processes.
The lowering of sea level had also exposed larger areas for anthropogenic
95
activity.
Remains of many man made structures are found in the near off
shore regions in many parts of the earth.
b) Quaternary glaciation
In 1821 Ignaz Venetz-Sitten (Switzerland) read a paper before the
society of Natural History at Luzerne in which he argued that the glaciers of
Alps had at some time in the past been expanded on an enormous scale.
In 1824 Jens Esmark in Norway reached a similar conclusion concerning the
glaciers in the mountains of Norway.
In 1829 Venetz presented a paper in which he stated that not only the
Alps but also the plains north of them and the whole of northern Europe had
once been glaciated .In 1835 Jean De Charpentier of Switzerland also
suggested the same view.
John Louis Rodolphe Agassiz, a young Swiss zoologist after studying
Diablerets glaciers, proposed the concept of “ice ages” while addressing
Helvetic Society in 1847. Agassiz extended the concept to Asia as well, as
there were extinct mammoth and other animals in frozen soil in northern
Siberia.
In 1830’s evidences for glaciations came to emerge in Britain and
Europe. William Buckland, professor of geology in Oxford, visited Agassiz in
1838 and realized that the British and Alpine evidences were similar. He
invited Agassiz to Britain and they worked together.
American recognition of the theory of Agassiz began in 1839 with a
published statement by Timothy Conrad. Two years later, when the concept
of the glacial origin of the drift was published as the result of Buckland’s and
Agasssiz’s 1840 work in Scotland, it was taken up by Edward Hitchcock in a
“First Anniversary Address” before the newly formed Association of American
Geologist. In 1846 Agassiz himself arrived in America to become a professor
at Harvard. His theory received wide acceptance. J. D. Dana also extended
support for the glacial concept.
96
c) Glaciation and sea level
The relation between glacial and sea level was first discussed in 1842
by Maclaren. A number of workers including Playfair and Lyell described the
raised shoreline sequences in Scandinavia and around the coast of Scotland
and
had
inferred
that
in
both
regions
crustal
uplift
had
occurred.
But the mechanism was unclear.
The formation of coral reefs according to Darwin (1842) implies a
gradual sinking of the ocean floor, which was substantiated by later studies
in the pacific (Dana 1849). Chambers (1848) realizes that this sinking would
cause an expansion of the ocean basin volume, which therefore would lead
to a fall in sea level. This is what we call tecteno eustasy.
Suess (1888)
introduced the term “Eustasy”.
In 1865, the Scottish geologist Jamieson made the link between the
raised shoreline evidence and the glacial theory when he deduced that
crustal depression would result from the build up of glaciers and that uplift
would follow deglaciations as the crust returned to its pre glacial state.
This is the first statement of glacial isostatic effect.
In
1863
Geikie
described
the
evidence
for
four
glaciations.
Listin (1873) introduced the term “Geoid”. Morner (1971, 1976, 1980, 1983,
and 1986) realizes that the sea level changes were not simple parallel
displacement of the shore level due to variation in the water volume or the
basin volume of the ocean, but that the sea level deformed horizontally
because the equipotential surface of the geoid deformed with time. Sea level
could no longer be claimed to be either worldwide or simultaneous. Therefore
Morner (1976, 1980) redefined the term “eustasy” simply to imply ‘ocean
level change’ regardless of causation but with the exception of dynamic sea
level changes.
It is now realized that no sea level changes can be strictly global and
that each region needs to define its own eustatic curve, it is no longer
necessary to separate the geoidal and dynamic changes in absolute sea
97
level. It is now felt that the search for eustatic curve is an illusion: that each
region must define its own eustatic changes.
Besides the studies of individual researchers, considerable sea level
investigations have been made by international organizations.
INQUA and
IGCP have completed several sea level projects under their commissions and
sub commissions. INQUA commission on shorelines was set up in 1953 and
since then many publications have been made on sea level changes.
The INQUA commission on Quaternary shoreline, under the presidency
of A.C. Blanc and four regional sub-commission under R.W. Fairbridge,
conducted studies of sea-level changes.
The International Geographical
Union (IGU) established two commissions for coastal research, one for
littoral and fluvial terrace studies and the other for erosion surface studies.
The
International
Geo-science
programme
(previously
International
Geological correlation program (IGCP)), a joint enterprise of UNESCO and
IUGS (International Union of Geological Sciences), conducted four sea- level
projects IGCP 61 - “International Geological correlation programme- Sealevels of the last 15,000 years” (under the leadership of A. Bloom between
1974
and
1982),
IGCP
200
-
“Late
Quaternary
sea-level
changes:
Measurements, Correlations and future applications” (under the leadership of
P. Pirazzoli between 1983 and 1987) and IGCP 274 - “Coastal Evolution in
the Quaternary” (under the leadership of O. Van de Plassche between 1988
and 1993) and
IGCP 367 - “Late Quaternary Coastal Records of Rapid
Changes: Application to present and future conditions” (under the leadership
of D.B. Scott between 1994 -1999).
All the studies have produced several
publications and international co-operation has been stimulated. One of the
important results of these studies is the dismissal of the concept of worldwide eustasy. They have shown that the evidences of sea-level changes are
variable according to climate, tectonic and oceanographic factors of
respective regions. These studies have also emphasized the growing belief
that no part of the earth’s crust can be considered stable.
During the late years of twentieth century evidences began to emerge
for major environmental changes during Quaternary in areas beyond those
98
directly affected by glaciers. Russell and Gilbert showed that extensive lakes
had existed at some time in the past and those phases of higher rainfall
‘Pluvial’ had alternated with more arid 'interpluvial' episodes.
Early researchers recorded the glacial events mainly from the
continental records left by former ice masses. These studies brought to light
four periods of glaciations named differently in different parts of the world.
Table 4.2. shows the names of glaciations region wise. Newer methods of
research by analyzing deep sea core samples (discussed in the subsequent
pages) have produced detailed information about the complete history of
Pleistocene and earlier climatic changes responsible for causing several
glacial cycles.
Table 4.2. Quaternary stratigraphic schemes for the Northern
Hemisphere based on terrestrial evidence.
Mediterra
nean
Holocene
Flandrian
European
Alps
Central North
America
Northern
Europe
Britain
Postglacial
Holocene
Holocene
Flandrian
Wurm
Wisconsinan
Weichselian
Devensian
Riss-Wurm
Sangamon
Eemian
Ipswichian
Surozhian
Riss
Illinoian
Saalian
Wolstonian
Geroevskian
Tyrrhanian
Mindel -Riss
Yarmouthian
Holsteinian
Hoxnian
Tobechikian
-
Mindel
Kansan
Wlsterian
Anglian
Chelyadintsev
ian
Milazzian
GunzMindel
Aftonian
Comerian
Comerian
Gunz
Nebraskan
Menapian
Beestonian
Paleouzunlari
an
Silician
Donau
-
Waalian
Pastonisn
-
-
Biber
-
Wburonian
Pre-pastonian
Tsiermagalian
-
-
-
Tiglian
Bramertonian
-
-
-
-
Pretiglian
Beventian
-
-
-
-
Antian
-
-
-
-
Thunian
-
-
-
-
Ludhamian
-
-
Reuverian
-
-
-
-
Reuverian
(=Pliocene)
99
Black Sea
Monastrian
-
d) Radiocarbon Dating
A breakthrough in sea level studies came in 1955 when Willard F Libby
developed C14 method of dating. The C14 dating has been very useful in the
sea level studies as it brings into focus the time element in the studied
shoreline. The method makes use of carbon-containing material such as
shells and peat for dating. C14 is one of the isotopes of carbon that
originates in the earth’s upper atmosphere where atoms of ordinary nitrogen
are subjected to bombardment by neutrons created by highly energetic
cosmic particles penetrating the atmosphere from outer space. By the struck,
Nitrogen 14 absorbs the impacting neutrons and emits proton. The nitrogen
atom is thus transformed into carbon-14, which combines with oxygen to
form Co2. Carbon-14 is radioactive and decomposes into nitrogen 14.
The half-life period of C-14 is 5730 ± 40 years.
The rate of production of C14 in the upper atmosphere is assumed to
be constant. Therefore atmospheric CO2 that is taken up by plants and
animals will contain a fixed proportion of carbon-14 relative to the total
amount of ordinary carbon i.e carbon-12. From the initial point in time
marked by the death of the organism, the proportion of carbon-14 in the
organic structure declines steadily following the exponential curve of decline.
By making precision measurements of the extremely small amounts of
carbon-14 in a sample, the age of the sample can be estimated to within a
fairly small percentage of error. The short half-life of carbon makes it an
excellent tool for age determination to last few thousand of years.
Now Uranium series dating (alpha spectrometry) and Optically
Stimulating Luminescence dating (OSL) are widely followed to determine the
age of Quaternary sediments.
e) Sea level records of the ocean floor
The important development in Quaternary sea level studies during the
20th century has been the investigation of sedimentary sequences of the
deep ocean floors. The deep sea floors are the environment where
continuous depositional record can be found. It is from the evidence of deepsea sediment cores, a series of cold and warm episodes were identified.
100
The Quaternary sea level history could be understood with the integration of
dating of the core sediments with these climatic changes. The problem of
fixing the Pliocene - Pleistocene boundary was also solved from the evidence
of deep-sea cores.
f) Interpreting sea level oscillation from Foraminifera
Several methods have been followed to interpret the sea surface
temperature using foraminifera. The inferred seawater temperatures are
correlated with periods of colder and warmer atmosphere temperature
thought to be associated with glaciation and interglaciation.
The percentages of the various species of foraminifera present in a
core sample are determined by counting. From this, it can be decided
whether the plankton that lived in the surface water over the site of the core
belonged to a cold water or warm water fauna. A cold-water fauna is
assumed to be associated with a glaciation and a warm water fauna with
interglaciation. The sea surface temperature of 21O C is considered cold and
28OC is considered warm. Data of several cores are averaged, as there are
many local variations due to controls other than sea surface temperature.
Another method makes use of single species of foraminifera in a rather
remarkable way. Some of the tests show a left hand direction of coiling while
the others show right coiling. It has been established that left coiling tests
are dominant in periods of cold water while right coiling tests are dominant
in periods of warm water.
g) Interpreting sea level oscillation from oxygen isotopes
Evidences of sea level oscillation also come from the analysis of the
ratio of abundance of isotopes of oxygen. In addition to common oxygen
there are two heavier oxygen isotopes, oxygen17 and oxygen 18. In 1947
Harold C Urey noted that the ratio of oxygen 18 to oxygen 16 in ocean water
depends partly upon water temperature. He then reasoned that the ratio of
those isotopes in the carbonate shell matter of marine organism should
reflect the surrounding water temperature at the time that matter was
secreted. Thus change in water temperature should be reflected in changes
101
in the oxygen isotopes ratio. Emiliani (1954, 1955) applied the oxygen
isotope method for the foraminifers’ tests and correlated the oxygen isotopes
ratio to the paleo temperature. Emiliani estimated that about eight climatic
cycles occurred during each representing glaciation and deglaciations.
Emiliani’s oxygen isotopes curve came under criticism pointing out that that
water temperature is only a small factor in determining the oxygen isotope
ratio of seawater. But subsequent studies proved that the oxygen isotope
ratio has a direct bearing on the amount of water locked up in continent due
to glaciation and the amount of water released into the sea due to
deglaciations. The oxygen isotope curve derived from carbonate matter in
deep ocean cores is now regarded as a reliable indicator of the total volume
of glacier ice present on the earth at the time the plankton secreted their
tests.
h) Holocene Sea Level
The amount of rise of sea level has been much smaller during
Holocene than the preceding late glacial. During recent years numerous new
investigations on sea level changes based on radio carbon dating have given
curves of the Holocene sea level. A certain agreement is reached on the
movement of the early Holocene sea level, but the changes in level during
the last 6000 years are much disputed.
Three schools of thought have arisen on the Late Holocene sea level
movement. The first group claims to have evidence that sea level has been
rising rapidly until the end of the Atlantic to about 3m above the present
level and fluctuated after that time with an amplitude of 6m (Fairbridge
1961). This is the oscillating sea level after 6000BP. The second group favors
a steadily rising sea level during the Holocene reaching its present level at
about 3600 to 5000 BP. This the theory of a standing sea level after 3600 BP
(Godwin 1956, Fisk1951, McFarlan 1961). The third group denies any sea
level higher than the present during the Holocene and also a standing sea
level after 3600 BP .The Holocene rise in sea level is seen as a continuous
one, diminishing with time but going on until the present day (Shepard 1960,
1963).
102
These differences in opinions on the movement of eustatic sea level
during the Holocene are due to the fact that a study of this subject only gives
evidence of change in sea level in a restricted area.
It is widely believed
today the sea level changes cannot by uniform for all the areas.
It varies from area to area as the local tectonism play a role in the sea level
of the area. It is also believed flat the past of the area in the earth is stable
through geologic time.
4.3.3. Sea level indicators in the study area
Evidences for Quaternary sea level changes observed in the study area
are grouped into 1.Geomorphic indicators, 2.Lithologic indicators and
3.Archeological indicators.
a) Geomorphic indicators
The occurrence of beach ridges (older and younger) along the entire
stretch of the coast of the study area, paleo lagoonal plains around Muthupet
and the occurrence of lagoon around Muthupet are the indicators of
Quaternary sea level changes observed in the area. An integration of
Quaternary sea level history with these landforms and dating of sediments
has thrown light on the role played by the sea level changes on coastal
evolution. A few quartz grain samples have been dated by OSL method for
geochronology. The description of OSL method is given in the later section of
the chapter. All the available dates of Tamilnadu region have also been
compiled. Based on dating carried out in the study and on the dates available
for various landforms in the adjoining region, the landforms in the region
were dated and evolutionary history has been traced.
b) Lithologic indicators
The changes in the sea level are also preserved in stratigraphic
records of many parts of the world. The rise and fall of sea level have made
changes in the type and amount of sediment deposition.
As a result, the
sedimentary sequences display valuable information about paleo sea level
rise and fall. The coastal sedimentary sequences can be classified into two
on the basis of sea level movements.
They are 1.transgressive sequences
where sediments deposited in relatively shallow near shore environment are
103
progressively overlain by sediments of relatively deep offshore environments
and 2. progradational sequences where relatively deep off shore sediments
overlain by shallow near shore environments. The third kind of sequences is
also observed by a few researchers namely aggradational sequences where
sediments in various environments accumulate in vertical fashion without
significance in spatial migration with time. (Davis and Clifton, 1987).
In the present study lithologic sequences along four traverses (see fig.
4.1) have been prepared (fig.4.2a-d) to know about the evidences of
Quaternary transgressive and regressive events. These lithologic sequences
have brought to light the Quaternary sea level history of the region.
c) Archeological indicators
Archeological excavation along coastal regions provides evidences for
changes in sea level ever since these features were made. As many ancient
coastal cities were submerged under water during transgression and many
others flourished in the reclaimed shelf region during regression, the altitude
of these cities in relation to present MSL forms an ample evidence for
determining the events of transgression and regression of the sea.
Many archeological excavations in submerged and emerged coast have been
carried out all around the world.
Port Royal in Jamaica, the Roman port and bridge at Minturnae in Italy
and a port in Black sea are some of the important excavated sites.
Blackman (1971) has inferred the higher sea-level than the present during
400 B.C. by studying the ancient harbor at Teos, Leptis Magna and
Anthedon.
Behre (1986) has used the macrofossils collected from the
archaeological sites for the sea-level studies.
Loveson (1993) has inferred
the fall in sea - level by studying the evidences around the ancient port
“Periapatnam”.
In the present study, archeological evidences collected off Poompuhar
coast indicate the Quaternary sea level movements.
Secondary data from
ancient Tamil literature and other under water archeological research studies
104
conducted around Poompuhar give interesting evidences for these sea level
changes.
4.3.4. Between Kattumavadi and Manohara
a) Geomorphic Indicators
Beach ridges (younger) around Kattumavadi indicate a transgressive
and subsequent regressive phase of the sea. The beach ridge plain extends
upto 2 km from the shoreline. The beach ridges are generally made up of
fine sands with well sorted nature indicating the dominant role of marine
processes in the development. The landward limit of the beach ridges form a
strandline indicating the transgression maximum. The beach ridges are
bordered on the sea ward side by mudflats and the land ward side by deltaic
plains.
The occurrence of mudflat on the sea ward side of beach ridges
indicate that during after the formation of beach ridges the area on the sea
ward side was a backwater region facilitating the deposition of clay for the
formation of mudflats.
Numbers of inland lakes are observed in the delta
plain between Kattumavadi and Manohara.
The occurrence of such lakes
facilitates the delta building process continue here even in modern days.
Though the sediment distribution through distributaries of Cauvery river is
not taking place in the region, the occurrence of
numerous lakes suggest
that delta building is taking place still in the region by other ephemeral
streams. The distribution of landforms between Kattumavadi and Manohara
displays a paleo micro deltaic characteristic (fig. 3.12).
The area between
Kattar river and Agniar river show a triangular deltaic feature.
The micro
deltaic characteristics suggest that the river Cauvery debouched into the sea
around this region in the past.
Vaidyanathan (1990), Ramasamy (1991),
Sambasiva Rao (1982) have also noticed the triangular landforms and made
suggestion that Cauvery flew along this region during past.
Abandoned
channel are also observed around Rettavayal. The occurrence of micro delta,
abandoned channels and number of lakes and related features suggest that
Cauvery river debouched into the sea here and the occurrence of beach
ridges and mudflats denotes that Quaternary sea level changes have played
a role in the genesis of the landforms around this region.
105
b) Lithologic indicators
Beach ridge and mudflat sediments dominate the region near shore.
They are bordered in the west by deltaic sediments.
The cross section of
sedimentary sequences along the traverse A - A' (in fig.4.1.) is shown in fig.
4.2a.
The occurrence of mudflat along the shoreline suggest that area was
under lagoon or backwaters or back barrier environment that facilitated for
the deposition of silt and clay sediments. The mudflat is underlain by a sand
layer that outcrops as beach ridges west of mudflats. The beach ridges have
width of 100 m around Kattumavadi, 200m around Ammanichattram, and
200m around Manohara. These sediments of beach ridges lie directly above
the mudflats suggesting that the beach ridges were deposited later to the
formation of mudflats i.e. mudflats are older to beach ridges.
The beach
ridges are bordered in the west by vast delta plain. The delta plain is made
up of mainly clay, silt and sand with natural levee and other over bank
sediments. The sediments of mudflats overlie the deltaic sediments. Hence
the delta plain represents the oldest sediments of the Kattumavadi region
followed by mudflat and beach ridge sediments.
4.3.5. Between Manohara and Nagapattinam
a) Geomorphic Indicators
The distribution of landforms between Manohara and Nagapattinam
makes the region a triangular prograded coast. The landforms present in the
region provide ample evidences for the Quaternary sea level changes.
The beach ridges occurring along the region from Muthupet to Nagapattinam
through Thiruturaipoondi are older to the ridges occurring in the eastern
region around Velankanni, Vedaranyam, Point Calimer, Sembodai and
Jampuvanodai.
The older ridges align NE - SW direction. They have
bleached fine sands with well sorted nature. These ridges overlie paleo
lagoonal plains.
The strandline formed by these beach ridges marks the
landward limit of a transgression maximum over the region. The occurrence
of surrounding paleo lagoonal plain indicates that lagoons existed in the back
barrier environment when the older beach ridges were formed similar to
Muthupet lagoon occurring in the back barrier environment of the modern
barrier ridges between Point Calimer and Adirampattinam today. The older
ridges are flattened and stabilized in nature.
106
The flattening of the ridges
makes it stabilized though they are not covered by vegetation. The colour of
the sands in the ridges is yellowish to orange in colour and markedly
different from ridges in the eastern side. These ridges are presumed to have
been formed during the Last Interglacial transgression maximum that
occurred around 1,25,000 years BP and the subsequent regression (Loveson
and Rajamanickam (1993)
, Banerjee (2000). The ridges occur about 4 to
6 m above MSL.
The older ridges are bordered in the east by younger beach ridges.
A vast younger beach ridge plain is observed in the region between
Muthupet,
Vedaranyam
and
Nagapattinam
in
a
triangular
These ridges have fine sands in well sorted nature.
unbleached unlike older beach ridges described above.
beach
ridges
align
NE
to
SW
direction,
the
fashion.
The sands are
While all the older
younger
beach
ridges
progressively change the alignment from NE - SW to EW and NS. The ridges
around Poovalur, Ekkal and Ayankadu exhibit NE - SW alignment, where as
the ridges between Adirampattinam and Point Calimer are EW and the ridges
from Point Calimer to Nagapattinam are NS in direction. The changes in the
alignment of beach ridges help us to surmise the changes of shoreline
configuration in the region through time. The ridges adjoining older ridges
are arc shaped.
The ridges change its direction progressively towards the
sea into two sets. One aligning EW and other aligning NS. The sands in the
younger
beach
ridges
are
similar to modern
sands
in
buff
colour.
The younger beach ridges are bordered by Paleo lagoonal plains in the
landward side and by Mudflats in the seaward side.
The younger beach
ridges occur at the height of 2 to 4 m from MSL. The ridges have been dated
by Bruckner (1988) to 6000 years BP.
The landward limit of younger ridges
denotes the line of Middle Holocene transgression maximum that occurred
around 6000 years BP (Bruckner 1988, 1989). The occurrence of mudflats
bordering the younger ridges indicates that lagoons existed in the region
when younger ridges were formed similar to the condition of formation of
older beach ridges.
The modern barrier bars observed between Adirampattinam and Point
Calimer encloses Muthupet lagoon.
The barriers align exactly in the EW
107
direction from Adirampattinam to Point Calimer.
These barrier bars have
many inlets through which the lagoon receives water from the main sea.
Many water ways like Rajamadam branch channel, Kaliyan odai branch
channel, Mullipallam creek, Seratalaikadu creek, Vedaranyam channel and
Vellar river traverse through older and younger beach ridges. These channels
had debouched into the sea at the places where the strandline is found to
occur. After the withdrawal of sea, these channel started flowing traversing
along the beach ridges. The occurrence of older beach ridges indicates the
first transgression and subsequent regression, the younger ridges indicate
the second transgression and subsequent regression and the occurrence of
back barrier lagoon around Muthupet indicate third transgression.
b) Lithological indicators
Sediments of paleo lagoonal plains lie directly above the older delta
sediments. Paleo lagoonal plains contain silt and clays with abundant marine
shells.
The sediment of older beach ridges occurs above the sediments of
paleo lagoonal plains. Paleo lagoonal plain borders on both sides of the older
ridges. The sediment of younger beach ridges lies above the paleo lagoonal
plains and mudflats. Marine shells are found in younger beach ridge
sediments. The cross section of sedimentary sequences along the traverses
B-B' and C-C' are provided in fig 4.2b and 4.2c. The sediments of the paleo
lagoonal plains were formed during the Last Inter-Glacial transgression
maximum (1,25,000 years BP) in the embayed coast formed behind the
older beach ridges. The sea has left behind beach ridges during the
regression that followed the last inter glacial transgression maximum.
The older beach ridges plain had extended in the east beyond the present
limit.
The
sea
during
Middle
Holocene
transgression
maximum
had
submerged part of the older beach ridge plain and reached up to the line
around Poovalur, Ekkal and Ayankadu where landward limit of young beach
ridges observed. The Middle Holocene transgression maximum also created
embayed coast that flooded partially the paleo lagoonal plains again.
Since the shifting of the river Cauvery had taken place by the time from
Adirampattinam region to Poompuhar,
the supply of sediments was
insufficient to build the beach ridges in NE - SW direction and littoral currents
108
in the Palk bay region started building beach ridges progressively in the EW
direction. Series of younger beach ridges were formed during the regression
that followed Middle Holocene transgression maximum. The younger beach
ridge plain had extended far beyond the present shoreline in the east. Both
in the south and east, the younger beach ridge plain extended more than 1
km in the present offshore region.
The modern barrier ridges occurring
between Muthupet and Point Calimer are formed by the third ongoing
transgression that commenced following the regression minimum.
The
lagoon that occurs behind the modern barrier bars is the region of mudflats
formed during the Middle Holocene transgression and subsequent regression
events.
The littoral current in the near off shore region has a curvilinear
motion from Manohara to Point Calimer towards EW and in the NS direction
from Point Calimer towards North. While the littoral currents in the Palk bay
region helped for the formation of younger ridges progressively in EW
direction, the littoral currents built the ridges between Point Calimer and
Vedaranyam in NS direction. The landward limit of younger beach ridges is
the strandline of Middle Holocene transgression maximum. The lithological
sequences observed in the region bring out two transgressive events namely
Last
interglacial
regression.
and
Middle
Holocene
transgression
and
subsequent
The occurrence of Muthupet lagoon and modern barrier bars
suggest that the sea is under transgression now.
4.3.6. Between Nagapattinam and the mouth of Coleroon
a) Geomorphic Indicators
Both older and younger beach ridges occupy a narrow stretch of land
in this region. The older beach ridges occur in detached and discontinuous
small patches.
The ridges align along NS direction.
The landward limit of
older beach ridges is the strandline of Last Interglacial transgression
maximum. The younger beach ridges also occur in small detached patches.
The width of beach ridge plains (both younger and Older) is about 5km here.
The older beach ridges and younger beach ridges lie above delta plains.
b) Lithologic indicators
Sediment of delta plains, Paleo lagoonal plain, older beach ridges and
younger beach ridges constitute the lithology of the region. The cross section
109
of sedimentary sequences along D-D' is shown in fig. 4.2 d. The sediments
of paleo lagoonal plain lie over the delta plain.
The sediments of paleo
lagoonal plain are overlain by older beach ridges followed by younger beach
ridge sediments. The paleo lagoonal plain contains clay and silt with lot of
marine shells in it. The older beach ridge sediments constitute fine sands in
well sorted nature and contain marine shells.
The younger beach ridges
contain recent sands in well sorted nature with lot of marine shells. While
the sediments of older beach ridges are yellow to orange in colour, those of
younger beach ridges are buff coloured.
The lithologic sequences indicate
two series of deep water sediments overlain by two series of shallow water
sediments. The deep water sediments are presumed to have been formed
during transgression and shallow water sediments formed during regression.
c) Archeological Indicators
Many man made features buried under sediments in near on shore
region around Poompuhar.
Stone walls and brick structures are observed
buried under sediments indicating that the area was widely used for
anthropogenic activities.
The underwater marine archaeological survey conducted jointly by
National
Institute
of
Oceanography
(NIO),
Goa,
and
Department
of
Archaeology, Government of Tamilnadu, in 1986 off Tranquebar and
Poompuhar has
established the existence of the ancient Chola site
“Kaveripatnam” in the present shelf between 7 and 15 m depth (Vora and
Subbaraju, 1987).
The shoreline of the regressed sea which originally
skirted around Poraiyar has been indicated by the ancient sangam classics
such as “Purananooru”, “Natrinai” and “Agananooru”.
Further survey
conducted by marine archaeological unit, NIO, and the regional centre NIO,
Visakhapatnam, in 1989 around “Poompuhar”, has thrown light on the
existence of many structures off-shore at 5 m water depth (Fig.4.3.).
Each structure is about 25m in length and they extend more than 500 m
parallel to the coast (Rao and Mohana Rao, 1990).
The subsequent
exploration with the help of divers located cairn circle, brick structures, ring
well, shipwrecks and a Chola temple.
All these structures were found to
occur in the water depth of between 5 and 20 m at about 0.5 to 1 km off-
110
shore (Rao, 1991; Sundaresh and Gudiagar, 1991). These historical and
archaeological indicators bring to light the second phase of regression and
the present third phase of transgression. The existence of the ancient city
“Kaveripatnam” under water in the shelf regions proves that the sea has
retreated about 0.5 to 1 km into the present shelf region during the second
regression minimum subsequent to Middle Holocene transgression maximum
that facilitated the growth of the city in the exposed shelf (fig.4.3).
Kaveripattinam flourished in the reclaimed shelf during 2300 - 1700 years BP
i.e. 300 BC to AD 300. The third and present transgression that commenced
at the culmination of the regression minimum is responsible for the
submergence of those anthropic sites. The historical date for the past
existence of Kaveripatnam is 300 B.C. to 200 A.D. The 14 C dates of
archaeological remains also indicate 3rd century B.C. for this site.
4.3.7. Discussion
Geomorphological indicators suggest that the sea had transgressed
over the region two times in the past. The occurrence of older and younger
beach ridges clearly indicates these two transgressive phase of the sea
respectively. The strandline formed by older and younger beach ridges mark
the line of land ward limit of these transgression maximum. These two
transgression are correlated with last inter glacial transgression (1, 25,000
years BP) and middle Holocene transgression (6000 years BP) respectively
as recorded by other researchers elsewhere in Tamilnadu (Loveson (1993),
Bruckner (1988) and Banerjee (2000)).
These two transgressive events
were followed by regressive phases. The details of sea ward limit of the first
regressions are not known.
during
the
second
But the seaward limit of regression minimum
regression
(subsequent
to
the
middle
Holocene
transgression) can be fixed as the line in the east of the archeological
indicators
observed in the offshore region of Poompuhar. As many
anthropogenic features are observed upto 1km in the offshore region, it is an
indication that the portion of shelf was exposed for human activity during the
regression that followed the middle Holocene transgression.
The historical
date of Poompuhar is 300 BC to AD 300. Hence it is presumed that the sea
had regressed upto 1 km in the present offshore region during 2300 - 1700
years BP. The submergence of those man made features under sea now is
111
an indication that the sea had transgressed again over this region third time
at the culmination of second regression minimum.
The absence of beach
ridges around Poompuhar onshore is an indication that the beach ridges have
been
submerged
under
the
sea
due
to
the
third
transgression.
The lithological sequences also corroborate these trans - and regressive
events of the sea. The sub surface lithology observed along the transect C C' in fig. 4.2.c shows two sequences representing these sea level events.
Here two series of sediments deposited in relatively shallow environment
(beach ridge sediments) are overlain by sediments
environment
(clayey
facies)
indicating
two
of
deep seated
transgressive
phases.
The lithological sequences observed along the traverse D - D' also exhibit
these trans - and regressive events (fig. 4.2d).
At Poompuhar, a series of
sequences of deep water environment are overlain by sediments of shallow
water environment indicating regressive phase of the sea.
Based on the geomorphic, lithologic and Archeological evidences it is
surmised that the sea had transgressed over the region two times with
subsequent regressions.
These transgressions have occurred around 1,
25,000 years BP during Last Interglacial maximum and 6000 years BP during
Middle Holocene transgression maximum respectively.
The landward limits
of these transgressions are well observed by the strandline features.
The first transgressed sea reached upto the line connecting Muthupet,
Thiruturaipoondi and Nagapattinam and the second regressed sea reached
upto the line connecting Poovalur, Ekkal and Iayankadu. The seaward limit
of the first regression is not known, but the second regression minimum
reached upto 1 km in the present offshore during 2300 - 1700 years BP and
the
sea
has
commenced
third
transgression
after
that.
The
third
transgression is presumed to continue even today. The occurrence of
Muthupet lagoon, the absence of beach ridges around Poompuhar and
narrow beaches in the northern part of the study area support the fact that
the sea is in the transgressive phase in modern times. The presence of man
made features under the sea off Poompuhar confirms the third transgression.
112
4.3.8. Impact of sea level changes
Quaternary sedimentary deposits occurring in the study area are
represented by older and younger beach ridges, paleo lagoonal plains,
mudflats and deltaic plains.
The integration of sea level history with the
landforms and sediments indicates the existence of three stages during the
Quaternary coastal evolution.
These stages exhibit clearly how sea levels
changes influenced the coastal evolution of the region.
a) Stage I - Older beach ridges
This stage begins with the culmination of first transgression and the
beginning of subsequent regressive phase of the sea. The transgressed sea
reached
upto
the
line
connecting
Nagapattinam.
The
transgression
is
Muthupet,
Thiruturaipoondi
considered
as
transgression that occurred around 125 ka.
Last
and
Interglacial
The regression of the sea
facilitated progradation of the delta around the region between Kattumavadi
and Manohara where the river Cauvery had been debouching into the sea
then. The occurrence of minor delta around the region was formed during
the stage (fig. 4.4.a). The regression also triggered the erosive effect of the
river Cauvery which had favored the deepening of channels carved in to the
Pleistocene sediments. The remnants of the drainage network are observed
in many places in the southern part of the study area.
The older beach
ridges were built all along the shoreline from the region north of Manohara to
the region around Coleroon river. Barrier ridges and back barrier lagoonal
system were developed that facilitated the formation of mudflat (named as
paleo lagoonal plain) around the older beach ridges. During the regression,
the sea had left series of ridges (older) and the former shoreline is indicated
by strandline features. These strandline features are clearly observed in the
older
beach
ridges
around
Adirampattinam,
Poovalur,
Puthupalli
and
Vettaikaranirupu. The seaward limit of the regression is not known. But the
limit had crossed the present shoreline in the east, because the sediments
formed during the stage are observed in the subsurface lithologic sequence
near shore (fig.4.2d). The sketches showing the features formed during the
stage are given in fig.4.4.a and b
113
b) Stage II - Younger beach ridges
This stage begins with the culmination of second transgression and
subsequent regressive phase of the sea. The transgressed sea reached upto
the line connection Poovalur, Ekkal and Iayankadu. The transgression is
considered as Middle Holocene transgression occurred around 6000 years BP.
During this transgressive phase of the sea, a part of older beach ridge plain
in seaward side was submerged and the sediments were reworked by the
sea. The sea had also pushed the sediments landward till the region where
the
landward
limit
of
younger
beach
ridges
occurs
at
present.
The transgression also developed barrier island - lagoonal system around
Jampuvanodai and Sembanodai. The lagoon that developed during the stage
is identified by mudflats noticed around the region. During the transgression
the river Cauvery shifted its channels to Poompuhar. During the regression
the sea had left series of younger ridges all along the coast and some of
which form strandline characteristics. The regression also facilitated for the
anthropogenic
activity
in
reclaimed
shelf
region
around
Poompuhar.
Though the exact seaward limit of second regression can not be fixed, it can
be definitely placed east of archeological remains observed in the offshore
region of Poompuhar.
While the sea was under regression, the river
triggered the erosive effects along the new channel which debouched into
the sea in the east of anthropogenic site observed in the offshore region east
of Poompuhar. The sketches showing the features formed during the stage
are given in fig.4.4.c and d.
c) Stage III - Submergence of Anthropogenic sites
This
stage
begins
with
third
transgressive
phase
of
the
sea.
The transgression commenced at the culmination of second regression
minimum. During the second regression a vast area was reclaimed for the
anthropogenic
activity.
The
archeological
remains
observed
around
Poompuhar in the offshore region indicate that the area was exposed for
human activity during the regression.
As per the Tamil literature
“Agananuru” the Poompuhar existed during 2300 - 1700 years BP.
At the culmination of regression minimum around the same time, the sea
has started transgressing over the region submerging all the man made
features.
This third transgression also submerged many younger beach
114
ridges developed during the second regressive phase of the sea in the region
around Nagore, Karaikal, Poompuhar and Kollidam.
also
developed
barrier
island
-
lagoon
The transgression has
system
around
Muthupet.
The transgression has forced the river Cauvery to shift its channel from
Poompuhar to Kollidam and flows in the name of Coleroon. A sketch showing
the features formed during the stage is given in 4.4.e.
4.4. Fluvial system
While the study area was experiencing Quaternary sea level changes
in several phases, the fluvial system was forced to shift its channels from
place to place and thereby shifting the delta lobes and regime of
sedimentation from place to place.
While the sea level was raising, the
fluvial system unable to maintain harmony with the marine system
meandered and shifted its channel towards the area of least resistance.
Similarly while the sea level fell, the fluvial system triggered the activity by
deepening and straightening the newly occupied channels. Hence for every
transgression there was meandering and shifting of channels and for every
regression there was triggering of activity and progradation in the new
channel.
The distribution of fluvial sediments and abandoned channels
exhibits various stages in the development of deltaic plain during Quaternary
(fig.4.5).
The occurrence of a micro paleo delta around Kattumavadi and the
associated paleo channels suggest that the river Cauvery debouched into the
sea here at the initial phase (delta lobe 1 in fig. 4.5) of the building of delta.
The area was under sedimentation during the period of regression following
Last Inter glacial transgression maximum (as indicated by OSL dating
discussed in the subsequent pages in this chapter). Hence the first lobe of
delta building was formed around Kattumavadi and Manohara region.
The distribution of fluvial sediments and abandoned channels around
Adirampattinam exhibit the next stage (delta lobe 2) of fluvial system.
Sedimentation regime extended further towards north to take second delta
lobe. The river has taken a new channel to flow along Papanadu, Vattakudi
115
and Vikraman. The fluvial sediments were deposited far beyond the present
shoreline as shown in fig.4.5.
The abandoned channel and fluvial sediments observed around
Mannargudi, Velankanni and Nagore denote the third stage (delta lobe 3) of
fluvial sedimentation. These three lobes and regimes were active till Middle
Holocene transgression.
The middle Holocene transgression forced the river Cauvery to take
two new channels to flow along Mannargudi, Mangudi and Velankanni and
also to flow along Vadapathi, Palayar and Poompuhar.
During this fourth
stage (delta lobe 4) the river had number of distributaries and many of which
are observed as abandoned channels today. During Middle Holocene
transgression the area around Poompuhar was under active sedimentation
regime.
But the region around Neduncheri in the delta lobe 2 received
sediments by a distributory even after shifting of regime to the area around
Poompuhar as indicated by the dating of sediments.
The fifth stage coincide with the regression that took place subsequent
to the Middle Holocene transgression and the delta progradation (delta lobe
5) took place mainly around Poompuhar. The delta building and the
regression of the sea made the progradation of the coast upto 1km in the
present off shore region. Anthropogenic activity was taking place in the
reclaimed shelf.
The final stage - on going process - of fluvial system (delta lobe 6)
coincide with the transgression that commenced around 1700 years BP that
forced river Cauvery to take present channel to flow along Melamarayam,
Neikuppam and Palayaru. Though the channel named as Coleroon, it is the
real Cauvery in the geomorphic sense. Presently the lobe 5 and 6 are areas
of the river Cauvery sedimentation.
Hence the distribution of fluvial sediments and abandoned channel
exhibit six delta lobes (fig4.5).
These six lobes coincide not only with the
116
shifting of river channels and the shifting of sedimentation regimes, they also
coincide with Quaternary sea level changes.
4.4.1. Discussion
The distribution of abandoned channels and fluvial sediments in the
study area indicate various regimes of sedimentation and delta lobes.
The building of delta during Quaternary commenced in the southern part of
the study area (near Kattumavadi and Manohara). Due to the disharmony
developed between the rising sea during Last Interglacial period and fluvial
system, the river Cauvery shifted the channels towards the place of least
resistance. The delta lobes 1, 2 and 3 were the places of sedimentation till
middle Holocene transgression.
During middle Holocene transgression the
sedimentation regime was shifted to region around Poompuhar (delta lobe 4
& 5).
But Neduncheri region (delta lobe 2) received sediments till late
Holocene (2969 ± 163) through a distributory channel.
The transgression
that commenced after 1700 years BP shifted the regime of sedimentation to
the region north of Poompuhar. Coleroon became active and through which
delta lobe 6 receives sediments. Presently the delta lobe 5 and 6 are active
areas of sedimentation.
4.5. Dating of sediments of Delta plain
In order to know the deposition age of sediments of various regions of
delta plain, Optically Stimulated Luminescence (OSL) dating of four samples
were carried out.
4.5.1. Methodology
Optically Stimulated Luminescence (OSL) dating is a modern and
reliable tool to determine the deposition age of sediments. By this method,
the time elapsed since the exposure of minerals grain like quartz to sun light
is determined.
Samples for OSL dating were collected by hammering a plastic tube
into the sediments along the wall of the pit dug with the help of heavy earth
machines. The plastic tube was removed after ascertaining that the tube is
117
filled with compact sediments. The ends of the tube were sealed to protect
the sample from the sun light exposure.
The sealed tubes were sent to the Geochronology laboratory, National
Geo-physical
Research
Institute
(NGRI),
Hyderabad
for
OSL
dating.
The samples for OSL dating were prepared following Aitken (1998)
procedure. Many tests were performed as per the procedure of Murray and
Wintle (2000) to find the suitability of material before starting the
measurement with a single aliquot regenerative (SAR) protocol. For age
calculation, it is essential to know the dose rate of the sediment, which can
be
measured
by
Gamma
spectrometry
with
an
Germanium) N type coaxial detector in the laboratory.
HPGe
(High
purity
The OSL ages were
calculated by dividing the equivalent dose (De) by the dose rate of sediment
including the contribution of the cosmic rays and the attenuation by the
water content.
4.5.2. Results
The OSL dates of four samples collected in delta plain (fig.4.6) of the
study area are given in the table 4.3.
Table - 4.3. Age of deposition of sediments
Place
Height of Sample
Age
from MSL
Kalagam
263 cm
50605 ± 3463
Kottakudi
237 cm
9321 ± 645
Neduncheri
261 cm
2969 ± 163
Manalmedu
233 cm
2315 ± 182
4.5.3. Discussion
The samples for OSL dating were collected (fig.4.6) approximately at
equal height - Height ranges from 233cm to 263 cm from MSL at four places.
Kalagam sample has been dated to 50605 ± 3463 indicating that
sedimentation
continued
in
this
region
after
the
Last
Inter
Glacial
transgression maximum during 125ka. The micro delta observed around
Manohara was to have deposited by this time. The delta lobe numbers 1&2
118
were active domain of sedimentation then. Kottakudi sample has been dated
to 9321 ± 645 indicating the shifting of regime of sedimentation towards
north. The delta lobes 3&4 were the places of sedimentations then i.e. the
area was under the regime of sedimentation before Middle Holocene
transgression.
The middle Holocene transgression shifted the channel and
regime of sedimentation to Poompuhar region. Manalmedu sample has been
dated to 2315 ± 182 indicating that sedimentation continued along
Poompuhar region corroborating the Archeological evidences observed in the
region.
Hence it is confirmed that the river Cauvery was shifted to
Poompuhar channel during middle Holocene maximum. The delta lobe five
was the active place of sedimentation then.
The regression following the
middle Holocene transgression maximum facilitated progradation of delta
around Poompuhar. Neduncheri sample has been dated to 2969 ± 163. The
question arises here is that how deposition of sediments in the south
continued after the shifting of domain to north?
This can be explained by
giving the reason that the deposition of sediments in the Neduncheri region
was continuing by a distributory of Cauvery, though the major depositional
regime was shifted to region around Poompuhar. The transgression that
commenced after AD 300 (1700 years BP) made to shift the Cauvery to the
present Coleroon.
Now the delta lobe 5 and 6 are active sedimentation
areas.
In order to understand the age of Quaternary sedimentary deposits of
other regions of Tamilnadu, all the available dates (14C, U – alpha series and
TL dates) of various researchers were collected (Table 4.6). All these dates
are found to form two groups mainly. The first group of samples has ages
ranging from 90,000 years BP to 1, 36,000 years BP. The second group of
samples has ages ranging from 2000 to 6500years BP. All the researchers
have correlated these ages with the Last Interglacial period (1, 25,000 years
BP) and Middle Holocene period (6,000 years BP). They have also recorded
that Early and Middle Pleistocene deposits are missing.
The OSL dates of the present study also indicate the late Pleistocene
and Holocene periods in consonance with the previous studies.
119
4.6. Tectonism
4.6.1. General
The sea level changes is no longer considered as the phenomenon of
rise and fall of MSL alone, but it is a combined effect of changes in MSL and
tectonic movement of the coastal region. In many parts of the earth
shoreline movement and coastal evolution are influenced by tectonic
movements both in regional and local scale. The regional scale tectonic
effects are mainly due to glacio - hydro isostatic movements and local
tectonic movements are due to minor faults or warping of crustal segments.
Guilcher (1954) while concluding a discussion on coastal evolution has
indicated that the Quaternary which is of great interest than earlier periods
to coastal geomorphology is not a period of any earth movements.
Many researchers have observed the influence of tectonic movements on
Quaternary sea level changes. Today it is widely believed that the sea level
curve cannot be uniform for different parts of the earth as the land
component in the sea level changes varies from place to place.
Vande
plassche (1993), in his final report of IGCP 274 has concluded that the sea
level curves of different parts of the earth indicate that no part of the coast is
stable.
The study area has also experienced neo tectonic movements as
indicated by tectono genetic features observed here.
These tectonic
movements along with sea level changes have influenced the fluvial system
to change the regime of sedimentation from time to time.
4.6.2. Historical background
In India the exclusive studies on Quaternary tectonics have not yet
been carried out. But many inferences on Quaternary tectonics have been
drawn as an offshoot of various geological and geomorphological studies.
Some of them are reviewed hereunder.
In Peninsular India, Tertiary and Quaternary have been a period of
epiorogenic adjustment to attain isostatic equilibrium, consequent on the
immense load of trap eruption and the Himalayan orogenic stresses
(Sundaram et al., 1964).
The initial courses of rivers in Peninsular India
120
flowing along the north easterly direction were changed to easterly direction
during the Tertiary and Quaternary (Vaidyanadhan, 1971). While studying
the major faults in Tamilnadu, Vemban et al., (1977) have placed the age of
many hinterland and coastal faults in the Quaternary. Dhoundial (1987) has
demarcated various zones of the Quaternary tectonic domains in India on the
basis of similarities and distinct geological, neo-tectonic, seismic and
geothermal gradient characteristics.
By interpreting the gravity data, Subramanyam and Verma (1986)
have concluded that the thickness of the crust has been increased by the
repeated orogenic processes which have resulted in the densification of crust
with the addition of basic material from the mantle along the coastal regions
of India. The stepped planation surface which is attributed to the tectonics
of
the
peninsula
is
also
found
to
continue
during
the
Quaternary
(Radhakrishna, 1993). Loveson (1993) has classified the coastal regions of
southern Tamilnadu into five different blocks on the basis of tectonic
characteristics.
Besides these, the earthquake records also designate the Quaternary
tectonism.
Since 1823, 45 earthquakes have been recorded with an
intensity observable without seismograph, of which 10 earthquake have
occurred along the boundary fault separating hinterland crystalline and
coastal sedimentaries.
The earthquake occurrence in 1965 and 1993
respectively in Madras (Tambaram) and Pondicherry coastal regions are well
attributed to the neo-tectonic activities.
4.6.3. Tectonic features of the study area
Tectonic map of northern Tamilnadu (which includes the study area)
prepared (fig.4.7) with the help of satellite images and aerial photographs
shows number of lineaments in hinterland hard rocks continue along the
coastal Quaternary sediments. There are three prominent sets of lineament
observed along the study area viz NE - SW, NW - SE, and ENE - WSW.
These trends can be correlated with the inland trends of Dharwarian, Eastern
ghats and Satpura structural trends respectively. The NE - SW lineaments
are numerous indicating that the Eastern ghat trend has major influencing
121
factor in the region.
These lineaments occur dominantly in other parts of
east coast of India and seem to be a responsible factor in shaping the
present day configuration of East Coast of India (Varadarajan and Ganju,
1989).
Besides, a number of basement faults characterizing Horst and Graben
structure occur beneath the coastal sedimentaries (Kailasam, 1968; Sastri
and Raiverman, 1968).
The existence of these basement structures has
already been proved with the support of ONGC geophysical survey and bore
wells.
The reactivation of these basement structures during Quaternary is
also noticed by several researchers (Vaidyanathan (1993), Ramasamy
(1991), Babu (1975)).
A crustal warping by cymatogenic movement has
been observed in the northern part of Tamilnadu coast (Anbarasu 1994).
In the study area the disposition of the present river courses and their
paleo channels bring to light tectonic activity taking place in the area.
Abandoned channels of the river Cauvery are found near Kattumavadi which
is in the southern most part of the study area. The sediments of this region
have been dated to 50605 ± 3463 which is comparatively older to the age of
sediments of other regions of the study area.
A series of abandoned
channels traversing through the Quaternary sediments in Cauvery delta are
observed in many places. Hence it is presumed that the river Cauvery had
flowed
initially
around
Kattumavadi
and
Adirampattinam
region
and
subsequently shifted progressively towards north to occupy the present river
course of Coleroon. The question arises here is why the river shifted only
toward north in all the abandoning process.
Though the shifting of rivers
was caused as an impact of Quaternary sea level changes, the preferential
shifting towards north is considered due to tectonic warping effect.
The age of the sediments is also progressively decreasing towards north.
The
abandoned
channels
observed
around
Kattumavadi,
Maharajasamuthiram, Rajamadam and Pattukottai are channels through
which the river Cauvery flowed during Late Pleistocene.
The abandoned
channels observed around Kannanur, Vadavur, Pamaniyar and Koraiyar are
channels flown during Early to Middle Holocene.
The abandoned channels
found around Vennar, Vetter, Cauvery and Manjalar are channels flown
122
during middle Holocene period.
The present Coleroon river course came
active only during late Holocene. All these characteristic of river system and
the abandoned channels clearly indicate the shifting of channels from south
to north i.e. from Kattumavadi to Coleroon.
4.6.4. Discussion
This kind of shifting of river channels associated with tectonic down
warping has been observed in other parts of Tamilnadu also.
Vellar river,
which occurs north of Coleroon, has shifted its channels towards north from
Kollidam to Portonova. The Gadilam and Ponnaiyar whose mouth occurs at
present with an interval of 1km have reached the present courses after
successive shifting of courses towards north and south respectively.
The Gingear has shifted towards south right from Kalivali to present mouth
at Puthukuppam (Anbarasu, 1994).
In order to understand the tectonic effect, the Bouguer gravity
anomaly map (fig.4.8a) prepared by the Oil and Natural Gas Corporation
(ONGC) and the National Geophysical Research Institute (NGRI) was
interpreted.
disclose
When the geomorphic indicators in the form of river migration
cymatogenic
down
warping,
Bouguer
gravity
anomaly
map
corroborates it. A north - south gravity profile from the mouth of the river
Coleroon to Pondicherry is shown in (Fig.4.8b) this gravity profile presents a
prominent fall in gravity values with a gradient of 1 milligal per km from the
mouth of the Coleroon upto Cuddalore and a reversal of trend with a rise in
gravity values towards further north. In other words, the area around
Cuddalore forms a gravity low. This gravity low positively indicates the
deepening of the basement caused by the pronounced crustal flexure due to
cymatogenic downwarping forces. Kailasam (1968), by interpreting the
east-west gravity profile between Vridhachalam and Cuddalore through
Neyveli, observed three prominent features (i) a steep fall with a gradient of
7 to 8 milligals per mile over the crystalline - sedimentary boundary on the
west (ii) a pronounced gravity low in the lignite area of Neyveli and (iii) a
rise in gravity values to the east of Neyveli to Cuddalore. He suggested a
north - south crustal flexure or down warping of the crystalline basement
between Vridhachalam and Cuddalore having Neyveli in the midst with a
123
sedimentary thickness of more than 5000 feet. But the interpretation of
magnetic profile in the same study (Kailasam, 1968), has proved a depth of
the order of 9000 feet to the crystalline basement over the region around
Cuddalore as the magnetic values continued to fall towards Cuddalore from
Vridhachalam.
Kailasam and Simha (1963), using seismic data also
suggested a deepening of basement towards Cuddalore.
All these observations point out the deepening of the crust around
Cuddalore. The river migration observed in the study is thus controlled by
the tectonic warping that causes that kind of crustal flexure which is
culminating around Cuddalore towards which rivers are migrating
4.7. Coastal classification
Coastal classification is one of the incomplete chapters in coastal
geomorphology. Several researchers have suggested classification of coastal
landforms, but none of them is entirely satisfactory.
descriptive and others are genetic.
Some are purely
A classification that incorporates both
descriptive and genetic could not be drawn because of the complexity of the
processes involved in the genesis of coastal landforms. Tanner (1960) has
summarized in a table the criteria taken in different classification (See the
table 4.4)
Table - 4.4. Coastal classification of various researchers
Type
1 2
* *
3
*
1
Structure- type
*
2
Structure-stability
3
Motion-horizontal
4
Motion-vertical
* *
5
Agency-present
6
Agency-former
*
7
Materials-bedrock
8
Materials-in transit
9
Energy-type
10
Energy-level
11
Geometric pattern
12
Coastal equilibrium
*
13
Transverse profile
* *
14
Erosion/deposition
15
Stage (or age)
*
16
Climate
17
Ecology
18
Time
1* - Suess
2* - W.M. Davis
5* - F.P. Shepard 6* - C.A. Cotton
8* - H. Valentin
9* - W.A. Price
12* - J.A. Davies 13* - A.L. Bloom
4
*
5
*
6
*
*
7
*
8
*
*
*
*
*
11
*
12
*
*
13*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
10
*
*
*
*
9
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
3* - F.P. Gulliver
4*-D.W. Johnson
7* - R.H. Fleming and F.E. Elliott
10* - W.F. Tanner
11* - A. Guilcher
124
An attempt has been made in present study to classify the coast of the
study area using criteria suggested in various classifications.
Continental margins are broadly classified into two types namely
Atlantic which are relatively long period of stable coast and Pacific which
have suffered active tectonism during recent geologic time (Suess, 1888,
Heezen, 1974). Under this classification the continental margins of the study
area can be classified as Pacific margin as crustal flexure is noticed in this
part of the coast. But this classification is of no validity at present, as it is
being increasingly regarded that no part of the earth crust is stable.
Inman and Nordstorm (1971) have discussed the first order coastal
classification in relation to the ideas of plate tectonics. They classified coasts
broadly into three types (i) Collision coasts - formed where plates converge,
(ii) trailing edge coasts - where plate embedded coast faces a spreading
zone, (iii) marginal sea coasts - where a plate imbedded coast faces an
island arc.
Each class is further subdivided into different types.
Trailing
edge coasts are subdivided into three type’s namely (a) neo-trailing edge
coast - where a new zone of spreading separates the land mass, (b) afro trailing, (c) amero - trailing edge coast - where the opposite coast is a
collision coast.
On this basis, the coast of the study area falls under the
amero - trailing edge coast.
Davies (1964) proposed worldwide dynamic classification on the basis
of wave climate of the area. He suggested four main types of wave climates
- the storm wave environment, the west coast swell environment, high
energy and low - energy environment. The coast of the study area is high
energy wave environment
The study area has been identified as semi diurnal tide coast by
Dietrich (1963) and moderate energy coast by Armstrong price (1955).
Johnson (1919) proposed a best known classification that has been
debated at large worldwide.
He suggested four genetic types namely
submergence coast, emergence coast, neutral coast and compound coast.
125
If this classification is strictly adopted, most of the world’s coasts fall into the
compound category.
The coast of the study area falls typically into the
compound
of
category
Johnson.
Because,
the
coast
just
north
of
Adirampattinam exhibits typical features or emergence (beach ridges and
raised mudflats) and the coast from Muthupet to Point Calimer exhibits
dominant features of submergence (backwater and lagoons). Hence it is
classified as compound category. The criteria by which Johnson recognized
emergent coasts are the dune covered barriers associated with coastal
lagoons and salt marshes. Though such features are well observed all along
the coast, the submergence of anthropogenic features around Poompuhar
makes the coast to be classified as compound category.
Shepherd (1963) proposed two broad types of coasts namely primary
and secondary.
They are the coasts that have been shaped mainly by
terrestrial agencies and those that have been modified by marine processes.
The coast around Kattumavadi falls typically in primary coast - fluvial
sedimentation coast. But the marine processes are also involved in various
stages of fluvial sedimentation and the present barrier beaches have been
built by waves along the greater part of this coast.
Hence, it can be
classified as both primary and secondary coast. But the different categories
of secondary coasts can be observed in different sectors of the study area.
Cotton (1952) divided the coasts into coasts of stable regions and
those of mobile regions. He inferred that the stable areas have only been
affected by eustatic oscillation of sea- level while in the mobile area the coast
itself has been uplifted or depressed or warped.
This classification is no
longer valid as many researchers feel that no part of the coast is stable.
Valentin (1952)
suggested two different coastal classifications.
The first is the classification of coastal configuration of genetic type, where
the coastline is defined in terms of past processes. The second classification
is based on present coastal dynamics. The dual classification was found to
be necessary because on some coasts present day processes are not in
harmony with the coastal configuration. His observations are typically
illustrated by the coast around Coleroon. This part of the coast is submerging
126
at present, but at the same time, exhibits features of emergence.
This coast falls into the category of coasts that have been prograded by
fluvial deposition, in the first classification and falls into out building coast as
the rate of accumulation is counteracted by the rise in sea - level under the
second classification.
The coastal classification of the study area based on the criteria
suggested by different researchers are given in table 4.5
Table - 4.5. Coastal classification of study area
S.
No
Author
Criteria
Study area
class:
Location
1.
Suess
(1888)
Nature of
land-sea
contact
zone
Atlantic type
Entire stretch of east
coast of India
2.
Johnson
(1919)
Genetic
Compound coast
Entire coast of the
study area
3.
Cotton
(1952)
Genetic
descriptive
Dominated by
features of
earlier
emergence
Between
Adirampattinam and
Velankanni
Downwarped
coast
Between Kattumavadi
and Chidambaram
Fluvial deposition
- delta coast
Entire coast of the
study area
4.
Valentine
(1952)
Genetic
and coastal
dynamics
4.8. Coastal Evolution
4.8.1. Geological History
The evolutionary history on continental margins of India began with
the reconstruction of Gondwanaland. Dietz and Holden (1970), Smith and
Hallam (1970), Crawford (1974), Curray and Moore (1974), Johnson et. al.,
(1976) and Curray et. al., (1982) all suggested the place of East coast of
India against Enderby land protuberance on Antartica near the Krishna Godavari basin. On the other hand, Ahmed (1961), Veevers et. al., (1971),
King (1973) and Sastri et. al., (1981), placed the eastern coast of India,
against western and north - western Australia. The timing of initial breakup
of Gondwanaland is also variously placed. While McElhianny (1973), Valencio
(1975) and Sastri et. al., (1981) suggested Late Paleozoic for initial rifting of
127
the Gondwanaland, Smith and Hallam (1970), Curray et. al., (1982) and
Larson, (1975) believed in early cretaceous. A detailed account of the
evolutionary history of the continental margin of India is given in the studies
of Curray and Morre (1974) and Curray et. al., (1982). The following main
events have been suggested.
i)
Initial break up of Gondwanaland in early Cretaceous in a
direction perpendicular to the northeast trending continental
margins of India.
ii)
Direction of spreading changed to north - south.
iii)
‘Soft’ collision between India and Asia in lower Eocene.
iv)
Plate motion accelerated in early Oligocene.
v)
‘Hard’ collision and Himalayan mountain building during early
Miocene.
vi)
India is still moving northeasterly into Asia.
The
splitting
of
continents
coincided
with
the
taphrogenic
fragmentation and block faulting movements along the eastern continental
margins
of
India
which
facilitated
the
initiation
of
marginal
basin
sedimentation. Five such sedimentary basins, namely, Bengal, Mahanadi,
Godavari-Krishna, Palar and Cauvery were generated along the east coast of
India, of which the last two basins occur along the coast of Tamilnadu.
The Palar basin occupies an area of 6800 sq.km. of which 2800 sq.km.
is
offshore.
The
sediments
of
Early
Permian,
Early
Cretaceous
and
Mio-Pliocene to Recent are exposed along the western margin of the basin.
Similar to other sedimentary basins of the East coast of India, the
sub-surface horst and graben-like structures form the basement architecture
of Palar basin. Along the central part of the basin a depression containing
sediments to a thickness of more than 3000 m occurs. This basin is
important from the paleo - geographic point of view since it shows the
evidence of outcropping Permian sediments (Sastri et. al., 1981).
The Cauvery basin is also a pericratonic basin with basement formed
by Archaean igneous and metamorphic rocks with block faulting structures
128
whose
trend
coincides
with
the
eastern
ghat
trend
of
NE
-
SW.
The taphrogenic movements that occurred along these fractures resulted in a
series of elongated depressions which were separated from one another by
intra-depression ridges. The release of onshore relief energy due to
taphrogenic fragmentation induced the terrestrial erosional processes which,
in turn, led to the subsidence of the basin by way of deposition.
These terrestrial erosional processes are responsible for the deposition of
non-marine arenaceous formations of Gondwana beds of Late Jurassic to
Early Cretaceous age. But the sub-surface equivalents of the upper
Gondwana deposits contain few palynofossils of paralic environments
indicating the first marine transgression. This transgression continued
throughout the Early cretaceous and encompassed all the depressions.
These upper Gondwana formations are overlain by reefoidal limestone of
Dalmiapuram formation of Albian age, suggesting that the terrestrial
processes were subordinate then. During the Late Cretaceous, two major
cycles, the lower representing a transgression and the upper a regression,
are recognized (Sastri, et. al., 1977). Each cycle is further resolvable in
several minor regressive and transgressive phases and finally during the Late
Maestrichtian the Cretaceous sediments were completely uplifted and
subjected to erosion for quite sometime.
The Cauvery basin suffered a negative tectonism during Early
Paleocene resulting in a transgression. This transgression was not as
widespread as the one which occurred during cretaceous. The occurrence of
non-marine coarse grained pebbly sandstone of Eocene age indicates a
regression. The Oligocene sediments with dominantly arenaceous character
suggest the further easterly migration of the shoreline. Aquitanian Burdigalian sediments with clay stone, shale and sandstone also suggest the
shift of depocentres further east due to regression. The Pliocene and
Pleistocene sediments in the western part of the basin comprise non-marine
deposits. In the sub-surface of eastern parts, the sequence is argillaceous
and
contains
faunal
assemblages’
characteristic
of
shallow
marine
environments. It appears that these deposits were laid during the final phase
of marine regression which resulted in the expulsion of sea from most parts
of the present day onland parts of the Cauvery basin. In this way the
129
Cauvery basin witnessed a number of geomorphic cycles marked with uplifts
followed by prolonged period of erosion and subsidence. Sastri et. al., (1977)
have given a detailed account of the evolution of the Cauvery basin.
It is to be emphasized here that while the Indian Plate was subjected
to lateral tectonic displacement by way of plate movements, vertical
tectonics were also operative. This vertical tectonics are found to be
responsible for the geomorphic cycle in the study area. The stepped
planation surfaces in the inland also disclose the geomorphic cyclic uplift and
erosion (Babu, 1975, Subramanian and Dharmaraj, 1987).
As a result of this interplay of sedimentation and tectonics, shoreline
was shifted in general towards east successively through geologic time till
recent as indicated by Holocene beach ridges. But intervening transgressions
have also shifted shoreline towards west for a short while during the general
regressive phase as indicated by Kudankulam limestone (Mio-Pliocene) in
southern Tamilnadu and beach ridge deposits (Late Quaternary) in many
parts of the coast.
4.8.2. Quaternary coastal evolution
i) Last interglacial transgression
The present study indicates that the evolutionary history of coastal
landforms of the study area commenced during the last interglacial
transgression
maximum
that
took
place
around
125
ka
(fig.4.9).
The early and Middle Quaternary deposits are missing (Bruckner, 1988).
The oldest date obtains in the study also indicate only the late Pleistocene
sediments. The last interglacial transgression maximum is represented by
the landward limit of older beach ridges occurring from Muthupet to
Nagapattinam through Thiruturaipoodi and in many other places. These
deposits are correlated with the older beach deposits of Cape Comorin and
Thirunelveli region of southern Tamilnadu which were formed during the
transgression that took place around 125 ka (Banerjee 2000, Vaz and
Banerjee 1997). This last interglacial transgression eroded and drowned the
older deltaic sediments. Extensive fresh water and lagoonal swamps were
developed during this time as indicated by the occurrence of intra-lagoonal
130
and lacustrine sediments in the region between Adirampattinam and
Nagapattinam. The transgression drowned the river mouths and forced the
rivers to meander and to get shifted to the lower courses. It is also observed
that while the shifting of river channels was facilitated by the transgression,
the preferential shifting was caused by the tectonic warping movements. In
this context, the causes for the shifting of river channels in the study area
can be argued in the following ways:
i)
the disharmony developed as a result of drowning of river
mouths due to sea-level rise, and
ii)
the effect of tectonism.
The first argument receives support from the phenomena of channel
shifting of almost all the channels. The drowning of river mouths (during last
interglacial
transgression
and
Middle
Holocene
transgression)
is
well
exhibited as strandlines (paleo shoreline) are intersecting the paleo river
channels around Manohara, Muthupet, Adirampattinam and Tiruturaipoondi.
The channel fill deposits in the paleo channels display the phenomenon of
drowning of river courses
during transgression
as
indicated by the
occurrence of lagoonal clay plug in the Channel fill deposits. This can also be
argued, as suggested by Clifton et al., (1973) for the deflection of Elk river,
Oregon, that landward transportation of sand and building of a bar at the
point where stream mouth occurs may also effect in the deflection of stream
channels. This bar may grow in height and extent in the direction of long
shore drift so that the stream is deflected and flows parallel to the shore.
This phenomenon is observed in the minor deflection in the river courses of
Coleroon. Moreover Clifton et al., (1973) observed the deflection in Elk river
just to 1 to 2 km laterally. But in the study area the shifting of channel has
taken place to several kilometers apart i.e. from southern part of the study
area to the northern part.
The second argument has received support for the shifting of channels
only towards north.
Radhakrishna (1968), Raiverman et. al., (1966) and
Vaidyanathan (1971) have suggested that the development of drainage
course in Tamilnadu was facilitated by tectonic movements and most parts of
131
the river courses are fault controlled. The present study also discloses the
tectonic warping that has caused the preferential shifting of rivers towards
the northern region.
Taking these factors into consideration, it is concluded that channel
shifting from Kattumavadi to Adirampattinam has occurred as a sequel to
Last Inter Glacial transgression and the preferential shifting of channel
towards North is due to tectonic warping.
(ii) Last Glacial Regression
The
regressive
phase
of
the
sea
following
Last
Inter
Glacial
transgression maximum triggered the fluvial processes along the newly
shifted courses near Kattumavadi, Manohara, Adirampattinam and Muthupet
and delta building resumed around the distributory channel mouths.
The older beach ridges were left behind by the regressed sea.
The barrier -
Lagoon system prevailed during the stage is indicated by Paleo-lagoonal
plains observed around the older beach ridges.
The river Cauvery also
incised its courses through Pleistocene sediments. The incised courses (now
abandoned) are typically observed along the abandoned channels numbered
as 3, 4 & 7 in fig 2.11. Delta building by fluvial process in the back barrier
environment and the development of the older beach ridges and PaleoLagoonal plain by marine processes took place hand-in-hand with the
regression
around
Adirampattinam,
Muthupet
and
Thiruturaipoondi.
The seaward limit of the regression is not known. The sediments of Kalagam
dated to 50605 ± 3463 were deposited during this stage. The river Cauvery
had been building delta through the channels that developed delta lobes 1, 2
& 3.
The sediments of Kottakudi dated to 9321 ± 645 suggested that
sedimentation continued around the region even after the period of Last
glacial maximum (18000 years BP).
(iii) Middle Holocene Transgression
The Middle Holocene transgression once again eroded and drowned
the deltaic and other Quaternary sediments. A part of Last Interglacial
transgressed area was superimposed by this transgression. A part of older
beach ridges in the seaward side was submerged under the Middle Holocene
132
transgressed sea. The landward limit of the Middle Holocene transgression is
observed around Poovalur, Ekkal and Ayankadu by features indicating
strandline. Shelf sediments were migrated towards land and were piled up in
the area where landward limit of transgression occurred, i.e. in the area of
shoreline during the transgression maximum. River started to meander once
again and got shifted to further north during the transgression maximum
similar to the previous shifting during the Last Inter-glacial transgression due
to influence of tectonic warping movement. The river Cauvery attained the
present course of flow only during this time i.e. the river Cauvery debouched
into the sea near Poompuhar. The delta lobes 4 & 5 became active places of
sedimentation. Back barrier Lagoons came into existence around Muthupet
and Thiruturaipoondi.
(iv) Late Holocene Regression
This regression is responsible for generating a series of younger beach
ridges not only along the coast of the study area, but along the coast of
entire Tamilnadu. Cauvery river once again triggered its activities and
sedimentation resumed around its mouth near Poompuhar. The regression
has retreated the sea 0.5 to 1 km offshore and in the reclaimed shelf,
ancient port of Kaveripatnam flourished as indicated by archaeological
remains of this submerged port city. The regression left behind many sandy
barriers to form younger beach ridges. All the younger beach ridges of the
study area were formed during the stage.
The areas of barrier lagoon
system formed during the previous stage became mudflats during the
regression. The sediments of Manalmedu dated to 2315 ± 182 also indicate
that sedimentation was taking place around Poompuhar during this period.
The sedimentation continued mainly in the regions of delta lobe 4 & 5.
But the sediments of Neduncheri region dated to 2969 ± 163 indicate that a
distributory
channel
of
Cauvery
was
still
depositing
sediments
near
Neduncheri i.e. the delta lobe 2 and 3 the receiving sediments through
distributaries.
(v) Present Transgression
The present transgression is presumed to have commenced after
A.D 300. as given by historical evidences observed around Poompuhar.
133
The transgression migrated the sediments towards land and piled up as
barrier ridges. Such ridges are prominent in the region between Muthupet
and Point Calimer. The present Muthupet lagoon also came into existence in
the back barrier environment due to this transgression.
The man made
features constructed during the previous stage in the prograded delta and
reclaimed
shelf
were
submerged
under
water
near
Poompuhar.
The occurrence of man made features in the near offshore regions of
Poompuhar is a testimony to the transgression that has occurred over this
region. Absence of beach ridges around Poompuhar is also an indication that
the younger ridges have been submerged by the transgressed sea.
Comparative Study
The distribution and characteristics of land forms of Cauvery delta are
similar to Vaigai delta which occurs in the southern part of Tamilnadu. Vaigai
delta also has landforms like older and younger beach ridges, mudflats,
abandoned channels, lagoons and delta plain similar to Cauvery delta.
The river Vaigai has abandoned number of channels during the
building of delta similar to the Cauvery river as observed in the presence
study.
Switching of lobes has also taken place in Vaigai delta, but the
switching has taken place both on the north and south side of the main delta
lobe. (Prabakaran and Anbarasu 2010) The beach ridges are also are of two
kinds namely older and younger. The older ridges are yellow in colour and
composed of bleached sands. The younger ridges are buff in colour and
composed of recent sands. The occurrence of older and younger beach ridges
have been reported in other parts of Tamilnadu also. Sahayam J.D et.al 2010
have noted the occurrence of Holocene beach rocks in Rameshwaram island
which occur at the mouth of the river Vaigai. vaz et.al 2008 have dated the
beach rocks of Rameshwaram to 7300 ± 130 years BP. A raised coral bed of
Rameshwaram region has been dated to 135000 years BP and another coral
terrace has been dated to 6100 years BP by Rajamanickam and Loveson
(1990). The occurrence of innumerable lakes in the delta plain region is
observed similar to the region around Kattumavadi. All these landforms and
their ages of Vaigai delta can well be correlated with those of Cauvery delta.
134
Table - 4.6. Details of available dates of coastal landforms of Tamilnadu
1
P.K.Banerjee
(2000)
600 m west of
Kovakulam
Cross laminated regressive facies
sandstone Elevation 2.40 m above
LTL
14
4560 yr. BP.
>97%aragonite
2
P.K.Banerjee
(2000)
1 km NE of CMFRI
farm at
Munaikkadu
Elevation 1.70 m above LTL
14
4223 yr. BP.
>97%aragonite
3
P.K.Banerjee
(2000)
Rameshwaram
Island terrace
Elevation 2.90 m above LTL
234U/238U
230Th/234U
92.0×103
(±6.5) yr.
BP.
4
P.K.Banerjee
(2000)
Rameshwaram
Island terrace
Elevation 2.40 m above LTL
234U/238U
230Th/234U
112×103
(+8/-5) yr.
BP.
Coarse fibre
aragonite
~90%;
diagenetic
Pulicat Lagoon pit 1
R.L. +4.5 m
14
Pulicat Lagoon pit 6
R.L. -4.00 m
14
5
6
G.G. Vaz,
P.K.Banerjee
(1997)
G.G. Vaz,
P.K.Banerjee
(1997)
7
Bruckner
(1988)
Cape Comorin
Beach deposits up to +2m above
HTL, at some places upto +5m
above HTL
8
Bruckner
(1988)
Rameshwaram
Coral reef, north side of the Island,
Porites.sp. upto +2.5m above
9
Bruckner
(1988)
Chetticulam
10
Bruckner
(1988)
Manappad
Lagoonal loam, upto +8m above
HTL with Veneridae of Circe
(closed)
Beach deposits upto +3m above
HTL with insitu Balanus.sp at
+1.25m above HTL
135
C
C
C
C
230Th/234U
(ESR)
230Th/234U
230Th/234U
2799±96 yr.
BP.
112 ka
Last interglacial
maximum
112 ka
Last interglacial
maximum
139.5ka
Last interglacial
maximum
139.5ka
Last interglacial
maximum,
Glacial
11
Stoddart and
Gopinadha
Pillai (1972)
Rameshwaram
Porites.sp from the raised coral
reef at Pamban
12
Bruckner
(1989)
Cape Comorin
Shells in conglomerate bed at 30
cm above HTL
230Th/234U
13
Bruckner
(1989)
Mouth of Nambiar
river
Shells in the marine terrace at 2.5
to 3m above HTL
230Th/234U
14
Bruckner
(1989)
Between
Kulasekarapattinam
and Tiruchendur
Shells in the fossil beach ridge at
7m above sea-level
14
6240±50 yr.
BP.
15
Bruckner
(1989)
4km weat of
Mandapam
Cardium.sp in lagoonal loam upto
1m above HTL
14
2740±60 yr.
BP.
16
Loveson
(1993)
Ariyankundu
Coral +0.55 m above MSL
14
5440±60 yr.
BP.
Middle Holocene
transgression
17
Gardner
(1981)
Ramanathapuram
Landsnail in aeolinite deposit +30m
above MSL
14
21000±400
yr. BP.
Last glacial
regression
18
Sarma
(1991)
Kaveripatnam
Wood-Archaeological sample
14
2316±103
yr. BP.
Late Holocene
regression
19
Tissot
(1987)
Pichavaram
Mangroves-root tip
14
2000 yr. BP.
Late Holocene
regression
136
14
C
C
C
C
C
C
C
4020±160
yr. BP.
---
112 ka
Last interglacial
deposits
112 ka
Last interglacial
deposits
Holocene
transgression
maximum
Shallow marine
area become
lagoon
indicating Late
Holocene
regression