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Chapter- 3

SEDIMENTOLOGY

3.1. GRAIN SIZE ANALYSIS

Grain size is a fundamental descriptive measure of sediments and

sedimentary rocks. It provides voluminous information about the intrinsic

properties of sediments and their depositional environment. The study of

depositional environment embodies an understanding of the various

processes effecting erosion and deposition. Grain size parameters provide

an insight into the nature and energy flux of the multi various transporting

agents and their preview of depositional environment.

The grain size analysis of the sediments is one of the most

discussed topics of sedimentary geology. The size of clastic and detrital

sediments varies depending upon the process and time involved in the

sedimentary process. Determination of size of the particles is of great

importance in the reconstruction of the transportation history of sediments

from source area to site of deposition. In order to understand the grain size

characteristics of sediments of a palaeobarrier and to identify the

depositional environment of the sediments using textural parameters this

study has been carried out.

Since the early nineteenth century, pioneering works have been

undertaken on grain size characteristics of sediments. These include

51

Udden (1914), Wentworth (1929), Krumbein (1936), Otto (1939), and

Klovan (1966). On the shape of size frequency curves Keller (1945) has

shown that dune sediments can be distinguished from beach sands.

Inman (1949) has established a relationship between the dynamics

involved during sedimentation and the resulting textural characteristics of

the sedimentary rock. Various authors have attempted to discriminate the

varying environments like river, beach and dune by using textural

parameters (Mason and Folk, 1958; Friedman 1961 & 1967). Grain size

measures for demarcating the subtle differences in depositional

environments have been brought out by Folk (1966). The textural

parameters are utilized to identify depositional environment of relict as well

as recent sediments (Moiola and Weiser, 1968).

In the present study, totally about 40 auger samples have been

collected for the analysis. Aerial photographs were interpreted for

understanding the nature of the terrain. To understand the geomorphology

of the study area fieldwork was carried out. Samples were collected at

specific intervals for the determination of grain size characteristics to infer

the environmental conditions. These sediments were packed in polythene

bag and properly numbered. Locations of the samples were noted on the

map (Fig. 3.0) using GPS (Leica).

These samples were washed repeatedly with the distilled water and

decanted carefully to see that no silts were escaped until clear water

52

column is noticed. The samples were then dried in oven, repeated coning

and quartering method about 100 gm of samples were separated.

ASTM sieves sizes +18 to +120 were taken in such a way as to

maintain ¼ phi interval. By using graphic (Folk and Ward, 1957) and

moment (Friedman, 1961, 1967 & 1979) methods the grain size data were

processed in a computer to get statistical parameter using Fortron – 4

programme of Schlee and Webster (1967). Mean, median, standard

deviation, kurtosis the statistical parameters were determined.

Using the statistical parameters, frequency curves. Bi-varient plots

were prepared for the discussion of individual parameters like mean,

median, standard deviation, skewness and kurtosis. Graphic values have

been taken into account. Grain size parameters of both graphic and

moment methods are given in the table (Table.3.1).

3.1.1. Frequency Curves

Frequency curves exhibit the pictorial representation of actual

weight percentage of different fractions of sediments. The peakedness of

fractions and uniformity of the sediments can be inferred from it. The

frequency curves of sediments of different areas are shown in Fig. 3.1.

Most of the locations show bimodal distribution, few shows

polymodal distribution. Well developed peaks are noticed at 1.0 j, 1.5 j

and 2.0 j fractions, these peaks reflect an enrichment of white sands in

alternate grain sizes from coarse to fine sands. However, about 60 % of

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white sands concentration is found to be in medium size grade. The

polymodality of frequency curves of silica sands indicate the multi source

of deposition. When aerial photos of the study area are interpreted, it

shows the presence of paleo channels and lagoon in the study area.

Hence, the sediments might have been transported by river and deposited

under lagoonal environment.

3.1.2. Mean

Mean represents the average size of the total distribution of

sediments. It serves as an index to measure the nature as well as the

depositional environment of the sediments. It is the function of total

amount of sediments available, the amount of energy imparted to the

sediments and nature of transporting agent. The energy of transporting

agent includes the degree of turbulence and the role played by currents

and waves.

The mean size of the silica sand range from 0.15 to 1.65 j

indicating the medium to coarse sands. This can be inferred that slightly

higher energy condition prevailed in those areas to carry away the finer

particles. Further, in some of the locations namely, Urani, Agaram, and

Pallampakkam accumulations of fines are to have taken place. This

indicates the low energy conditions prevailed in those areas.

54

Seralathan (1979) has noted that the increase in mean size to

coarse sand around Pondicherry is generally attributed to the removal of

fine fraction in high energy condition.

Polymodality confirms a mixtured size grade present in

Mudaliyarkuppam sand. The depositional environment in

Mudaliyarkuppam is being a lagoon, it is liable to be derived from both the

offshore and inland not only the rain wash but also by the wind deposits

which are likely to get mixed up in the depositional site. The tidal onward

push must have brought those coarse sands during monsoonal condition.

However, the receding tides present in the lagoon mouth must have

enforced retardation in the energy and allowed the suspension part to go

back. The presence of pebble sized grains of quartz found in some

locations near Mudaliyarkuppam and near Urani indicates the sudden

decrease of the velocity of the transporting agent.

3.1.3. Standard Deviation

Standard deviation is a measure of uniformity or sorting. It is also

the resultant character of sediments controlled by size, shape and specific

gravity of sediments and energy and time involved in transporting medium.

It is noted that the standard deviation decreases towards the sample of

lower mean size. In other words, the sorting improves with the lowering of

mean size. As a result, the sediments having medium sand (between 1.0 j

and 2.0 j) exhibit moderately sorted nature. This phenomenon has also

been noted by Inman (1952), Friedman (1967) and Pettijohn (1984).

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In the study area, the standard deviation value for silica sands

range from 0.15j to 0.86j indicating well to very well sorted nature. The

moderate sorting in some of the locations in the middle portion is attributed

to partial winnowing and addition of sediments in barrier beach

environment by aeolian process. Moderately sorted nature is feasible only

in a place where limited winnowing and free addition of sands in the

central portion in the present condition.

The well sorted to poor sorting nature in few locations like

Mudaliyarkuppam results in a place where there is a continuous, slow

deposition of sediments. The presence of minor amount of pebbles in this

site may be due to the deposition of the sands of varied sources with

varying velocity of the transporting agent. Anbarasu (1994) has reported

this area as a paleo-barrier or paleo-lagoon, on the basis of

geomorphological and sea level variation studies. Phleger (1969) has

noted the poorly sorted nature of sediments in the paleo-barriers along the

Mexican coast. Zenkovitch (1969) and Reineck and Singh (1986) have

also reported the occurrence of poorly sorted sediments and admixture of

pebbles in the barriers occurring in front of the coastal lagoons. The

presence of minor amount of pebbles in this site may also be accounted

for such admixture.

3.1.4. Skewness

Skewness is a measure of symmetry of grain size distribution. It is a

significant parameter in delineating environment, since it is sensitive to

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sub-population mixing. Duane (1964) has discussed in detail the

importance of skewness in studying the modern environment.

The silica sand samples showing the skewness values range from -

0.42 to 0.82 falling within very course to fine skewed. Most of the samples

exhibit near-symmetrical character. This can be attributed to the addition

of sediments from medium energy conditions. The sands though coarse in

the beginning must have slowly lost their dominance due to strong addition

of medium sized sand in dried lagoonal portion.

Though the study area is inferred to be a paleo-barrier, formed

under marine conditions, it is surprising to note that all the samples are

found to be finely skewed. The analyzed samples indicate the presence of

dominantly positive skewness, in other words it is predominantly fine

skewed i.e. the tails are skewed better. The skewness directs the attention

to the finer present in the tails. The prominent fine to coarse skewed

nature suggests the probability of multi-source and multi-agent role.

3.1.5. Kurtosis

Kurtosis is a measure of ratio between the sorting in the tails of the

curve and the sorting in the central portion. Friedman (1961 & 1967) has

concluded that kurtosis is not sensitive for the determination of

environment.

Folk and Ward (1957) have explained skewness and kurtosis in

terms of the mixing of two normal grain sizes. The graphic kurtosis is a

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measure of the part of sediments already sorted elsewhere in an

environment and later transported and modified by another type of

environment. But the moment kurtosis is an index of mixing of two end

populations Jaquet and Vernet (1976) have advocated the usage of

graphic kurtosis to recognize the inherited characters of population and

moment kurtosis for measuring the mixing between the end populations.

Sediments with a more or less equiproportionate mixing of the modes

show platykurtic distribution whereas the dominance of one mode gives a

distribution that is leptokurtic.

Folk and Ward (1957) have inferred that unimodal sediments exhibit

mesokurtic and mixing of two populations in sub-equal amount resulting in

platykurtic values. The polymodal characteristic is responsible for the

platykurtic values.

The silica sand samples in the study area exhibits the kurtosis

values from 0.58 to 4.19 very platy to extremely leptokurtic. It shows that

low – medium energy conditions must have prevailed in this region. It also

supplements the interpretation made earlier, in such a way that the

addition of material in both ends. Only a particular size sand of medium in

nature must have been added up in due course of time to get accumulated

in this region. This is again testified by medium to fine size sediment.

This predominantly platykurtic nature of sediments in

Mudaliyarkuppam helps to infer the sorting of tail portions of the sediment

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population. As finer populations are found to be tidal currents, they show a

characteristic veryplatykurtic nature.

The coarse sediments could not be distributed though; there would

have been high energy due to the bar present at the sea mouth. In view of

that, poor sorting in the central portion is unavoidable whereas the

respective tidal entry must have churned the sediments and must have

taken back to the sea, the fines in suspension. Such removal must have

enlarged the segregation at a particular fraction in the tails.

3.1.6. Bivariant Plots

The inherent relationship between the four size parameters can be

well understood, only when they are plotted against each other as scatter

diagrams. Researchers like Wentworth (1929), Keller (1945), Inman and

Chamberlin (1955), Folk and Ward (1957), Friedman (1961 & 1967),

Shepherd and Young (1967) have successfully used the scatter plots for

understanding the geological significance of the four size parameters. But,

they were divided into two schools of thought, one favouring graphic and

the other favouring moment method and the controversy is still continuing

worldwide.

By using textural parameters, obtained by moment method,

Friedman (1961) has differentiated beach and dune sands by plotting

mean Vs skewness and beach and river sands by plotting skewness Vs

standard deviation. Friedman (1967 & 1979) has prepared bivariant plots

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from the textural parameters and proved the sensitiveness of

differentiating the depositional environments like beach, inland dune;

beach and river sands; river and coastal dune sands and inland dune and

coastal dune sands. By using moment measures in the bivariant plots,

Friedman (1961 & 1967) has successfully demonstrated the use of

bivariant plots by utilizing both graphic and moment measures and brought

out the dune and river zonation from the plot of standard deviation Vs

skewness.

For the present study, the univariant parameters of the white sands

have been used for drawing binary plots. The grain size statistical

parameter have been brought the form of scatter plots has bivariant

distribution.

The standard deviation Vs mean, Moiola and Weiser (1968),

(Fig.3.2) has clearly indicated the dominance of inland and beach for most

of the samples, whereas offshore influence for few samples near

Mudaliyarkuppam, is expected because of the link it maintains with the sea

even today.

The plot of third moment Vs first moment, Friedman (1961),

(Fig.3.3) was attempted. The samples of the study area retain their

signature of beach environment. The classification of Mohan and

Rajamanickam (2001) shows an environment of foreshore and low water

mark for all the samples.The plot of first moment Vs second moment

(Fig.3.4) shows the distribution of silica sand samples in foreshore

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environment. Friedman (1967) diagram, (Fig.3.5) using standard deviation

vs kurtosis lends support to the beach processes.

3.1.7. Visher Diagram

Visher (1969) has put forward the effective usage of log probability

using three types of sub population viz., the traction, saltation and

suspension. The environments being known, it is felt that the textural

variations may help to infer the micro-environmental conditions prevailing

in the study region. The quantification of these three sub-populations from

the mode of distribution and from the nature of frequency curve helps the

delineation of environment. Rajamanickam and Gujar have demonstrated

that Visher’s log probability curves can be used to infer the existing

depositional environments without any ambiguity in the Ratnagiri

nearshore sediments.

The log probability curves prepared using Visher’s (1969)

procedure are shown in the Fig. 3.6. The Visher diagram of the samples of

the study area is characterized by traction, saltation and suspension

population.

Rajamanickam and Gujar have shown that the Visher’s log normal

distribution is more confirmative and definite than other statistical

approaches. Accordingly Visher’s sub population has been estimated. The

Visher’s lognormal curves have shown a clear distribution of traction,

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saltation and suspension. In the bivariant plots the samples of respective

area segregate separately, even within the same environment.

The samples of the study area show the presence of saltation and

traction population. When these values are compared to the Visher’s chart

(Visher, 1969), it matches with the tidal inlet, similar to the white sands of

Prakasam District, Andhra Pradesh (Rao and Sankara Pitchaiah, 1985).

3.1.8. CM Pattern

In order to find out the mode of transportation and the energy level

of the sediments during transportation and deposition, CM pattern was

prepared by using Median and First percentile (Passega, 1964 & 1977).

The distribution of samples of the present study falls in NOP sector

indicating the deposition of white sands by rolling. As the textural

parameters clearly indicate the influence of marine environment in the

present study area, the same phenomenon is registered by way of

distribution of samples in rolling sector.

In the CM pattern (Passega, 1957) samples of the study area

clustered distinctly in NOP segment. This distribution supports the textural

evidences that there must have been a strong winnowing action leading to

the transportation of sediments by rolling (Fig. 3.7).

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3.2. LIGHT MINERAL ANALYSIS

The widespread distribution and different morphological characters of

the light minerals have attracted many researchers to study them and

establish the relationship of the environment of deposition with

provenance. Earlier workers like Dake (1921), Wadell (1935), Rittenhouse

(1943), Feniak (1944) and Powers (1953) have initiated the research on

the application of light minerals in finding the provenance.

The character of undulatory extinction in quartz was used to

differentiate the igneous and metamorphic rocks by Blatt and Christie

(1963). The dispersal history of polycrystalline quartz with undulatory

extinction from New South Wales sandstone has been enunciated by

Conolly (1965). Ojakangas (1965) has grouped the quartz grains into four

types namely unit quartz, polycrystalline quartz, crushed quartz and

recrystallised quartz and related them to the nature of source rocks and

environment of deposition. Blatt (1967 & 1985) used the characteristics of

quartz grains like stability, roundness, etc., to understand the provenance.

Bluck (1969) has brought out the characteristics of roundness on quartz

grains. Basu et al., (1975) have classified different types of undulatory

extinction quartz to delineate the provenance. Pettijohn (1984) has

described the light minerals and their relation to sphericity, roundness,

etc., and attempted to assign the characters to environment of deposition.

On the basis of the characteristics of quartz, orthoclase and

microcline, Muthukrishnan (1993) has brought out the differences in the

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maturity of beach sediments and the riverine sediments of the Gadilam

River. By the predominance of the feldspar, rock fragments, quartz and

chert, Udayaganesan (1993) have arrived at the conclusion that the

Gadilam and Vaippar river sediments are influenced by the continental

block provenances, while the beach sediments are of oceanic/continental

components of sub-arkosic nature.

3.2.1. OPTICAL PROPERTIES OF LIGHT MINERALS

3.2.1a. Quartz

Quartz is easily identified from its characters like low relief, lack of

perfect cleavage, concentric ring of interference colours, straight extinction

and uniaxial positive. The quartz grains are comprised of both

monocrystalline and polycrystalline and are dominated by dust like

inclusions (Plate 7). The grains show a straight to wavy extinction.

Monocrystalline is a single grain of generally fairly of large ones.

Polycrystalline quartz is formed out of the fusion of different quartz grains

separated by either planar or irregular boundaries. The grains do not have

optical uniformity. They are expected to have different level of

crystallographic axis orientation. Inclusions also identified within the quartz

grains. Generally all the quartz grains are sub-angular to sub rounded.

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3.2.1b. Feldspars

The feldspars in the white sands are weathered in nature and are

easily identifiable under a petrological microscope. The feldspars are

found to be comprised of orthoclase and microcline.

Orthoclase

The grains are prismatic, sub-rounded to sub-angular in nature. It is

seen with characters of low relief, weak birefringence, two directional

cleavage with right angled nature i.e. one perfect and other imperfect,

straight extinction, colourless under polarized light, first order interference

colour and biaxial positive.

Microcline

It is observed by its prismatic nature, distinct cleavages, and

colourles nature. It also shows low birefringence, low relief, oblique

extinction of 50-160. The characteristic cross hatched twinning is observed

in most grains.

3.2.2. Distribution of Light Minerals

The samples of the study area is mostly made up of quartz, feldspar

being in meager amount. The presence of feldspar also varies from one

location to another. The monocrystalline quartz variety is found to be of a

dominant one 90-95 %. Polycrystalline quartz is present 3 - 10 % and

feldspars to 2 – 4 %.

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The quartz sands are found to be manifested with ferruginous coatings.

Even before the treatment, the total sands were counted for the coated

and non-coated grains. Almost 40 - 60 % of the sands are found to be

covered with ferruginous coatings, inferred to have been the direct supply

from the hinterland, probably the Cuddalore sandstones which may have

supplied the sands with ferruginous coatings. These sands are scanned

under microscope for their distribution of monocrystalline and

polycrystalline grains. It is noticed that poly- crystalline quartz is less in the

study area.

3.2.3. Quartz Morphology

The morphology of quartz is studied for its roundness and sphericity

parameters. Many authors have propounded different methods but the

methods of Rittenhouse (1943), Powers (1953) and Lindholm (1987) are

identified to approach with ease and in quick time.

3.2.3a. Roundness

Roundness can be defined as a relation between the sharpness of

the edges and corners (Wentworth, 1919). Waddel (1932) has defined

roundness as the average radius of curvature of the corners of the grain

image divided by the radii of the maximum inscribed circle for a two

dimensional image of the grain. Quantitatively, true roundness is generally

expressed by the Waddel formula:

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Roundness = ∑ (r/R)/N where, r is the radius of curvature of grain corners,

R is the radius of the largest inscribed circle and N is the number of

corners.

Roundness describes the degree of abrasion of clastic fragments. It

may provide evidence of time or distance of transport. The grain is angular

when it is freshly introduced (unless recycled) and becomes progressively

rounded due to transportation. Roundness is entirely a different factor

when compared to sphericity, shape and size. As the light mineral

fractions can throw better picture on the distance and mode of

transportation, etc., from the individual grain morphology the different

detrital mounts have been used to evaluate the roundness and sphericity

of the quartz grains. Visual observation method has been adopted using

comparison charts after Power’s (1953) for roundness and Silhouette

charts for projection sphericity (Rittenhouse, 1943). In order to maintain a

better accuracy, a scan of 300 individual grains in each fraction has been

studied. The roundness is found to have been influenced by the

composition, cleavage, fracture, size, shape, the medium of transportation

and the energy with which the transportation was carried out.

The percentage of different group of grains with differences in

roundness substantiates the role of multi source in view of appreciable

percentage of angular, sub-angular sub-rounded, rounded and well

rounded grains in the same fraction. The variation in the different

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percentages of roundness in the individual fractions does indicate a similar

trend within a sample.

When the silica sand samples are scanned, sub-rounded grains in

Agaram, Mudaliyarkuppam areas are found to be dominant, followed by

sub angular grains in Kakkapallam, and rounded grains in Marakkanam

area. Fig.3.8 shows the roundness values observed in the study area.

The average of roundness values in the study area indicates a

predominance of sub-rounded class grains representing to a level of 51.36

%, rounded 32.43 % and sub-angular 13.14 %. The well-rounded grains

are to a level of 1.19 %. The sub-rounded grade of distribution suggests

that the sediments are contributed by present day sediments, which are

deposited from nearby lacustrine environment. Rounded grade of

distribution suggests that, some of the locations in the study area were

contributed by recycled nature of beach sediments, which must have been

subjected to intensive wear and tear by strong waves and currents.

Rounded nature of the sediments suggest that they are traversed by

rolling. Fig.3.8 shows the roundness values of the silica sand grains in the

study area.

3.2.3b. Sphericity

The sphericity is a measure of how nearly equal the axial dimensions

of a particle are, a very different concept to roundness, but commonly

confused with that shape attribute. The sphericity is the surface area of a

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grain divided into the surface area of a sphere of the same volume. It is

defined by Wadell,

Sphericity = 3

Is/L2

Where, I is the length of the intermediate axis of a grain, S is the length

of the short axis of the grain and L is the long axis of the grain. The two

most widely used verbal classifications and quantitative graphical

representations of sphericity are the Zingg (1935) diagram ad the Sneeden

and Folk (1958) diagram, both of which are based on ratios of I, S and L.

For quantitative visual comaprision of sphericity, the Silhouette charts

(Rittenhouse, 1943) is widely used as it requires a short time to study the

sphericity index of the grains.

The sphericity of the grain is controlled by the properties like bedding,

schistosity, cracks, cleavage and directionally orientated flaws in the

particles; the shape of the particles like block, platy, rod like, needle like,

etc. subjected to abrasion; the composition of the particle. In addition to

these three factors, the orientation of the particles, medium of transport

and the energy level of transporting media can also influence the

sphericity index.

Detailed sphericity analysis of loose grains can prove to be useful in

the determination of paleohydrodynamic conditions (Moss, 1972),

particularly when combined with studies of size distributions and

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sedimentary structures. For the present study, the qualitative study of

sphericity indices is carried out using the method of Lindholm (1987),

which has slight modification from Rittenhouse (1943). The visual method

of comparing Silhoutte chart of grains according to the different sphericity

index has also been applied here. At least 300 grains of medium size

grade in each micro slide of representative samples from the study area

have been studied.

The sphericity index ranges from 0.73 to 0.89 and above. It rather

reflects sub-rounded to rounded nature of the sediments, which are being

trapped from the nearby streams. Some of the locations show the low

value of sphericity indicating the possibility of present day mixing of

sediments from the offshore through the littoral drift and the mix up of land

ward sediments by wind.

3.3. HEAVY MINERAL ANALYSIS

Heavy minerals are characterized as having a specific gravity

greater than 2.89, which includes many kinds of opaque and transparent

minerals consisting of oxides, sulfides, and ore minerals. The economically

valuable heavy minerals transported and concentrated as stream

sediments or beach materials are called as ‘Placer Deposits’. Once the

material is taken out, from a parent source then the same is transported to

the basin of deposition redistributed according to their specific gravity,

size, shape, etc. The distribution of heavy minerals is controlled by so

many factors like destruction by wear and tear, stability of the mineral,

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density, grain size, water motion and energy at the depositional

environment (Choudhri and Grewall, 1985).

Heavy minerals are studied to establish stratigraphic correlations,

which can be useful to track the source rock lithologies and dispersal

patterns and to evaluate the diagenetic history. They can also be used as

a tool to find the weathering and tectonic history of the source area. The

heavy mineral analysis is primarily used to understand the nature of

source from which the sediments are derived. Large number of workers

(Krumbein and Pettijohn, (1994); Okade, (1960); Folk (1980); Pettijohn,

(1984); Blatt, (1985); Russel, (1937), Naidu, (1968); Mallik, (1974 and

1986); have studied the heavy minerals of different environments. The

density and grain size of the heavy minerals place them in the “hard to

move” category of minerals. For the transportation and concentration of

heavy minerals, current velocities greater than the normal are needed

since the heavies are not hydrodynamically equal to the light minerals. The

heavier minerals like zircon, rutile, kyanite, topaz and garnet used to get

settled fast, when there is a little reduction in the velocity of transporting

media during the course of transportation.

3.3.1. OPTICAL PROPERTIES OF HEAVY MINERALS

In the mounted slides, the individual (>300 grains) minerals were

identified and counted by using the line method described by Galehouse

(1969). Various diagnostic properties of heavy minerals provided in the

Milner (1962), Phillips and Griffen (1986), Ford (1951), Rothwell (1989),

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are utilized for the easiest identification. Table.3.2 shows the Heavy

mineral weight percentage of the study area. The properties of identified

heavy minerals are as follows:

3.3.1a. Actinolite

It is easily distinguished by its fibrous and flaky nature, medium

relief, high R.I., yellowish and light green in colour, moderate pleochroism

(Yellowish green to dark green), irregular fracture, low extinction angle (Z ^

C = 60 –150), length slow, strong birefringence and biaxial negative. Some

of the grains contain common inclusions of chloride and iron oxide. The

grains are found to be prismatic having a state of sub-angular to sub-

rounded nature.

3.3.1b. Biotite

In the study area biotite occurs mostly as dark green in colour with

basal cleavage and flakes having jagged edges. It posses low R.I., light to

dark brown pleochroism, straight extinction and biaxial negative.

Pleochroic holoes are sometimes seen in some grains. The etching marks

along the margins are well pronounced in few grains and most grains are

found to be sub-angular in nature.

3.3.1c. Chlorite

Chlorites are flaky, prismatic and sub angular in nature. They

resemble mica. Chlorites are generally found in dark green and brown

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colours. Chlorites are found to be strongly etched. The ultra blue

interference colours are found to be distinct. Micaceous cleavage, low RI,

greenish pleochroism and inclined extinction are some of the general

properties of Chlorite.

3.3.1d. Epidote

It occurs mostly as platy, irregular, equant and rather angular

grains. Some grains show etched surface. The important properties

noticed here are higher relief, high R.I., pale green to dark brown

pleochroism, unidirectional extinction angle (X ^ C = 10 – 50), ringed

interference colour and biaxial negative with needle interference figure.

The broken minute chips of epidote show bottle-glass like transparency

with pistachio green colour.

3.3.1e. Garnet

Detrital garnet is commonly irregular in shape. The garnets are

mostly identified by its high relief, colourless nature and etched surfaces,

Concoidal fractured and sometimes well rounded generally devoid of

recognizable crystal faces in grains are also found. The almandine garnets

are light pink to dark pink in colour, without pleochroism. Garnets are

mainly identified by its isotrophic nature. Iron oxide inclusions are

common. The removal of inclusions may cause cavities. The surface of the

garnets show pitting, grooving, spotting, rectangular patterning, deep

colour staining and sometimes etched. Typical scaly surficial texture is

also seen. In the study area pink and colourless varieties of garnet is

found.

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3.3.1f. Glaucophane

It is identified by its characteristic properties like irregular to

prismatic nature, moderate relief, high R.I., gray blue colour, blue to violet

pleochroism, uneven fracture, bluish interference colour, oblique extinction

(Z ^ C = 60–90) length slow, biaxial negative and higher birefringence. The

grains are found besub-angular to sub-rounded nature.

3.3.1g. Hypersthene

Prismatic shape, high relief, high R.I., green colour, green to brown

pleochroism, two directional cleavage, straight extinction, biaxial negative,

schiller structure with commonly seen iron oxide inclusions are some of

the important properties of hypersthene. Grains with etching are common.

3.3.1h. Kyanite

It is found to be elongated, prismatic, bladed, short stumpy nature

with etched marks, high relief, higher R.I., irregular fracture, colourless and

sometimes with blue colour. A pleochroism of light to dark blue is seen in

most grains. Terminal partings are also seen. Kyanite possesses inclined

extinction (250–300) and it is biaxial negative. The grains are invariably

seen with rounded edges.

3.3.1i. Rutile

It possesses reddish brown colour, high relief, very high R.I., weak

pleochroism (various shades of reddish brown) imperfect cleavage, deep

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red interference colour, straight extinction. It is prismatic in habit but, the

broken pieces are found with rounded faces, length slow, uniaxial positive

sign, twinned grains are also present. The opaque dust probably of iron

oxide are found as inclusions in the rutile.

3.3.1j. Topaz

It is generally prismatic in nature and it also possesses high relief,

glassy appearance, colourless, absence of pleochroism, cleavage,

irregular fracture, straight extinction, biaxial positive and higher order

interference colour. It shows bluish tinge in the fractured edges. The

inclusions found in the topaz are opaque probably limonite or hematite.

3.3.1k. Tourmaline

It is identified by its broken terminal faces due to parting, prismatic

nature (Plate 9-7). Some times it may be irregular and rounded. It

possesses moderate relief, high R.I., brown colour, brown to maximum

absorption of dark dichroism, imperfect cleavage, straight extinction,

length fast and uniaxial negative characters. The common inclusions found

in tourmaline are unidentified opaques.

3.3.1l. Zircon

There are some forms of zircons (Plate 9-6,8&9) namely prismatic,

euhedral, broken (irregular), oval (ellipitical) and rounded found in the

sediments. But all the zircons having the common characteristic properties

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like high relief, very high R.I., colourless nature (sometimes various

shades of pink colour), absence of pleochroism, imperfect cleavage,

conchoidal fracture, higher order interference colour and strong

birefringence. The core of the zircon is foggy and cloudy in appearance

due to clustering of inclusions. The inclusions found in zircon are opaques.

The zircons show the presence of pleochroic haloes. The shattering of

zircons grains due to stress are also observed. Though there are euhedral

zircons giving the pyramidal faces. The edges are found to be

smoothened.

3.3.2. Distribution

Though the study area, in general, the silica sands are pure and

devoid of any heavy mineral. An attempt has been made to find out the

presence of heavy mineral in the study area. The results show the

presence of heavy minerals of minor concentrations. Among the sample

locations Marakkanam records the highest percentage of 0.267 %,

whereas the lowest amount, that is 0.013 %, is recorded in Tenkuppam

near Urani.

3.3.3. Heavy mineral assemblages

In the study area the average heavy mineral assemblages are

represented (Fig.3.9) by Zircon (18 – 20 %), Kyanite (15 - 17 %), Rutile

(16– 18 %), Epidote (11– 12%), Biotite (8- 9%), Topaz (9 - 10%), Chlorite

(8-10%) and Garnet (4-6%). In these assemblages, few locations nearer

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to Kakkapallam which lies in middle portion of the study area represent the

typical granular minerals, having higher density. So, the heavy minerals in

that region must have been subjected to strong winnowing action, which

accounts for the absence for flaky minerals like actinolite, boitite,

hypersthene etc. The result of grain size studies has also substantiated a

higher energy condition for those areas. The suite of heavy mineral found

in the Kakapallam region again reconfirms this inference. Fig.3.9. shows

the Heavy mineral assemblage (%) in the study area

Heavy mineral assemblages nearer to Mudaliyarkuppam which falls

in the Northern part of the study area represent a partial mixture of flaky

and granular grains. However, here the flaky minerals like actinolite and

glaucophane are totally absent. The presence of biotite along with the

granular minerals indicates that this area must have had a mixture of high

and low energy condition.

Similarly the samples of Agaram area which covers the southern

portion, the presence of flaky minerals in them indicates a low energy

condition. The grain size results also substantiates the prevailing low

energy conditions.

The heavy minerals of the study area, though they are very meager

in quantity, show a very good amount of diversification. The mineral

assemblages indicate the presence of zircon, rutile, tourmaline, topaz,

kyanite, epidote, garnet, chlorite, hypersthenes and pyroxenes. However,

zircon, rutile and kyanite are found to be comparatively abundant in the

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sediments. The nature of zircons shows a clear mixture of sediments by

virtue of having euhedral type, elliptical type and rounded type. The

zircons also carry the assemblage of outgrown and overgrown. The

outgrowth and overgrowth has also obtained smoothened nature in few

zircons.

Kyanites show even fully rounded nature. But for few grains, most

of the kyanite possesses smoothened edges; few garnet grains are also

present. They indicate the etched nature. The presence of more rounded

nature of heavy minerals with etching and overgrowths, direct the

possibility of derivation of these minerals of multi-cycle nature mostly from

the contribution from the earlier sediments.

The earlier study of Cuddalore sandstone has registered the

presence of all these minerals particularly blue kyanite. The dominant

presence of opaque in the mineral assemblage also supports the

possibility of heavies having been derived primarily from Cuddalore

sandstone which are having extremely high order of opaques in the heavy

mineral assemblage. Few grains of garnet, kyanite, topaz even zircon,

amphiboles might have been supplied from the Cretaceous- Tertiary

deposits of Virudhachalam and Pondicherry formations Rajamanickam

(1968). The presence of blue kyanite with large number of opaques

supports the possibility of supply from Cuddalore sandstone.

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3.4. SCANNING ELECTRON MICROSCOPE STUDIES

In the study of surface morphology of sediments, Scanning Electron

Microscopy (SEM) plays an important role as a tool for understanding the

nature of particulate matter. Earlier studies of quartz grain surfaces have

been made by Kuenen and Perdok, 1962; Krinsley and Donahue, 1968a,

1968b; Margolis, 1968; and Margolis and Krinsley, 1971. These studies

have shown that fine surface textures indicate depositional environment

and mode of transport.

To study the textures of beach sediments and heavy minerals

Scanning Electron Microscope (SEM) is used to verify the general

morphological characters (Robson, 1984; Mallik, 1986). However the

majority of studies focus on the examination of surface features. Krinsley

and Doornkamp (1973) have studied the surface textures of quartz grains

over a decade.

The micro features of grains for sediment characteristics from the

study area are attempted using Scanning Electron Microscope (Model;

JEOL JSM 6360) available in the Department of Geology, University of

Madras, Chennai – 600 025. After the treatment for washing, the samples

were mounted on stainless steel stubs using double-stick tape. About 5

grams of the grains from each of the three size categories were mounted

on a single stub pasted with double side adhesive tape. The mounted

grains were coated with platinum-palladium in order to counteract grain

surface charging while scanning with the electron beam. With the grains

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coated with Platinum-Palladium detailed surface photographs were taken

using the secondary electron mode at 20 keV.

3.4.1. Textural description

From the micrographs of the silica sand grains few textural features

were identified to describe the surfaces of the grains. These features

constitute the most common surface morphologies of different mineral

grains determined after reconnaissance observations and considerations

of the extensive work done with grain surfaces by Margolis and Krinsley

(1971). The salient features observed in the sand grains using SEM are

described below.

3.4.1a. Conchoidal fractures

These are distinguishing characteristics of glassy materials and

quartz, and result from brittle deformation due to compressive contact

between two surfaces. Conchoidal fracture patters vary from regular dish

shapes (uncommon) to irregular elongate fan-like or trough-like

depressions. The common parallel step like fractures that curve around

the conchoidal depression are thought to be expressions of planes of

weakness in the glass similar to cleavage planes in crystals (Margolis and

Krinsley, 1971). Elongate concoidal fractures are most evident on grain

edges.

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3.4.1b. V-shaped depressions

These are micro pits which vary from triangular depressions to

elongate grooves that widen in one direction. Chemical effects as well as

abrasion quickly obscure these features. Many of the V-shaped

depressions may be only several microns in maximum dimension. Two

processes can account for the V-shaped depressions: 1) tangential

impacts with sliding of one grain over another and 2) chemical solution in

areas of localized order or microlite development.

3.4.1c. Grooves

Grooves include elongate scratches and troughs that may be

slightly curved. These grooves are oriented in a preferred direction, occur

with conchoidal fracture, and appear in sets. Transport in a traction carpet

is a likely process of their formation. During polishing of metal surfaces,

small chips broken from the surface are propelled over surfaces, forming

the grooves.

3.4.1d. Cracks

Cracks are a mainly due to mechanical process, and are straight or

slightly curved. Separation along cracks generally is less than 10 mm.

Cracks are best developed on vesicle surfaces and may radiate from

equal angles in groups of two or four. Overall, these features appear

similar to mud cracks and, when they intersect, form polygonal plates on

surfaces; they appear to form both before and after concoidal fracture.

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Cracks that project through a grain may be due to the thermal stress of

quick cooling or by impact with another grain or surface. Cracks in the

grain skin could be due to grain expansion after formation of a brittle skin

or contraction of the hydrated skin often followed by accumulation of

alteration materials.

3.4.1e. Chemical alteration

Few grains in the study area are sugary in appearance when totally

altered; however, they retain their vesicular morphology. Solution and

precipitation occur together on the same grain. Resulting textures include

pitted or scalloped surfaces on a micron scale, rounding of upturned plates

or other sharp features, and development of a frosted, light diffused

surface as compared to the vitreous surface of fresh glass.

3.4.2. SEM STUDIES IN THE STUDY AREA

The SEM study of silica sand grains in the study area is carried out

and described below (Plate- 10, 11 & 12).

The silica sand grains are sub angular with various surface solution

features. Rounded crescent like pits, some grains with a straight net like

sutures, and V shaped pits are noticed. The quartz grains show concoidal

fractures. The grains are irregular in shape, sub angular to sub rounded in

nature. Few cracks like features are also noticed. They may be due to

mechanical activities of waves and currents. Few rounded grains are also

noticed may be due to long transportation. The edges are smooth in most

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of the grains. It indicates that the grains are transported for long distances

and the transportation might have been mostly by rolling. The angular and

sub angular grains seen in the deposit may have been derived from

varying source and subjected to transport action for short distances.

The common features observed in the quartz grains are conchoidal

fractures with cavities and solution pitting, chemical etching marks. In the

study area the quartz grains show concoidal fractures and etched marks

indicating high energy environment as well as the longer stay of sediments

in the depositional basin.