CHAPTER - IV COASTAL EVOLUTION 4.1....

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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

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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.

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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.

No. Eustatic Local

1. Glacio - eustasy Glacio - isostasy

2. Orogenic - eustasy Hydro - isostasy

3. Geoidal eustasy Erosional and depositional

isostasy

4. Infilling of basins Decantation Compaction of sediments

(auto - compaction)

5. Transfer from lake to ocean Orogeny

Epiorogeny

Ice - water 6.

Expansion or contraction of water

volume because of temperature

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.

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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

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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.

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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

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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- Sea-

levels 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 world-

wide 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

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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.

European Alps

Central North America

Northern Europe

Britain Black Sea Mediterra

nean

Postglacial Holocene Holocene Flandrian Holocene Flandrian

Wurm Wisconsinan Weichselian Devensian Monastrian

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

Gunz-Mindel

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

(=Pliocene) Reuverian - -

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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 deep-

sea sediment cores, a series of cold and warm episodes were identified.

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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

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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).

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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

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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

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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.

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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


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. The flattening of the ridges

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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 fashion.

These ridges have fine sands in well sorted nature. The sands are

unbleached unlike older beach ridges described above. While all the older

beach ridges align NE to SW direction, the 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

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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

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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 and Middle Holocene transgression and subsequent

regression. 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


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.


Sediment of delta plains, Paleo lagoonal plain, older beach ridges and

younger beach ridges constitute the lithology of the region. The cross section

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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-

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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. But the seaward limit of regression minimum

during the second 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

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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 of deep seated

environment (clayey facies) indicating two 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.

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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 Muthupet, Thiruturaipoondi and

Nagapattinam. The transgression is considered as Last Interglacial

transgression that occurred around 125 ka. 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

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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

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ridges developed during the second regressive phase of the sea in the region

around Nagore, Karaikal, Poompuhar and Kollidam. The transgression has

also developed barrier island - lagoon 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

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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

117

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

118

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 HPGe (High purity

Germanium) N type coaxial detector in the laboratory. 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

from MSL Age

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

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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.

120

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

121

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

122

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

123

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. When the geomorphic indicators in the form of river migration

disclose 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

124

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. Some are purely

descriptive and others are genetic. 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*

4*

5*

6*

7*

8*

9*

10*

11*

12*

13*

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 3* - F.P. Gulliver 4*-D.W. Johnson 5* - F.P. Shepard 6* - C.A. Cotton 7* - R.H. Fleming and F.E. Elliott 8* - H. Valentin 9* - W.A. Price 10* - W.F. Tanner 11* - A. Guilcher 12* - J.A. Davies 13* - A.L. Bloom

125

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.

126

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 category of 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

127

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

4. Valentine (1952)

Genetic and coastal dynamics

Fluvial deposition - delta coast

Entire coast of the study area

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

128

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

129

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

130

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

131

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

132

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 Paleo-

Lagoonal 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

133

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.

134

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.

135

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

14C 4560 yr. BP. >97%aragonite

2 P.K.Banerjee

(2000)

1 km NE of CMFRI farm at

Munaikkadu Elevation 1.70 m above LTL 14C 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.

Coarse fibre aragonite ~90%;

diagenetic

4 P.K.Banerjee (2000)

Rameshwaram Island terrace Elevation 2.40 m above LTL 234U/238U

230Th/234U 112×103

(+8/-5) yr. BP.

5 G.G. Vaz,

P.K.Banerjee (1997)

Pulicat Lagoon pit 1 R.L. +4.5 m 14C

6 G.G. Vaz,

P.K.Banerjee (1997)

Pulicat Lagoon pit 6 R.L. -4.00 m 14C 2799±96 yr. BP.

7 Bruckner (1988)

Cape Comorin Beach deposits up to +2m above HTL, at some places upto +5m

above HTL

230Th/234U

112 ka Last interglacial

maximum

8 Bruckner (1988)

Rameshwaram Coral reef, north side of the Island,

Porites.sp. upto +2.5m above (ESR) 112 ka

Last interglacial maximum

9 Bruckner (1988)

Chetticulam Lagoonal loam, upto +8m above

HTL with Veneridae of Circe (closed)

230Th/234U

139.5ka Last interglacial

maximum

10 Bruckner (1988)

Manappad Beach deposits upto +3m above

HTL with insitu Balanus.sp at +1.25m above HTL

230Th/234U

139.5ka Last interglacial

maximum, Glacial

136

11 Stoddart and Gopinadha

Pillai (1972) Rameshwaram

Porites.sp from the raised coral reef at Pamban

14C 4020±160

yr. BP. ---

12 Bruckner (1989) Cape Comorin

Shells in conglomerate bed at 30 cm above HTL

230Th/234U 112 ka

Last interglacial deposits

13 Bruckner (1989)

Mouth of Nambiar river

Shells in the marine terrace at 2.5 to 3m above HTL

230Th/234U 112 ka

Last interglacial deposits

14 Bruckner (1989)

Between Kulasekarapattinam

and Tiruchendur

Shells in the fossil beach ridge at 7m above sea-level

14C 6240±50 yr.

BP.

Holocene transgression

maximum

15 Bruckner (1989)

4km weat of Mandapam

Cardium.sp in lagoonal loam upto 1m above HTL

14C 2740±60 yr.

BP.

Shallow marine area become

lagoon indicating Late

Holocene regression

16 Loveson (1993) Ariyankundu Coral +0.55 m above MSL 14C

5440±60 yr. BP.

Middle Holocene transgression

17 Gardner (1981) Ramanathapuram

Landsnail in aeolinite deposit +30m above MSL

14C 21000±400

yr. BP. Last glacial regression

18 Sarma (1991) Kaveripatnam Wood-Archaeological sample 14C

2316±103 yr. BP.

Late Holocene regression

19 Tissot (1987) Pichavaram Mangroves-root tip 14C 2000 yr. BP.

Late Holocene regression

CHAPTER - IV COASTAL EVOLUTION 4.1....

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