CHAPTER - III GRAIN SIZE AND HEAVY MINEARL ANALYSIS

Transcription

1 CHAPTER - III GRAIN SIZE AND HEAVY MINEARL ANALYSIS 3.1. Introduction Grain size analysis is one of the most widely used techniques for understanding the processes involved in transportation and deposition of sediments. Krumbein (1934, 1936), Inman (1952), Folk and Ward (1957), Passega (1957, 1964), Mason and Folk (1958), Friedman (1961, 1967), Klovan (1966) Visher (1969) and Nordstrom (1977) are some of the pioneering studies on grain size characteristic and are still referred by many researchers. These studies have confirmed beyond doubt that textural parameters are effective tools in understanding the mode of transportation and environment of deposition of sediments. There followed a series of papers published around the world giving the characteristic of sediments of various parts of the earth. In India, Sahu (1984), Rajamanickam (1983), Rajamanickam and Gujar (1984, 1985), Chavadi and Hegde (1989) are some of the noticeable works on grain size characteristics. Grain size characteristic of sediments of Tamilnadu region have been studied by Seralathan (1979), Mohan (1990), Chandrasekar (1992), Muthukrishnan (1993) Angusamy et al., (1993), Udayaganesan (1993), Anbarasu (1994), Angusamy (1995), Karikalan (2002), Dajkumar (2004), Udayaganesan(2005) etc., In the present study, grain size characteristic and heavy minerals of sediments of beaches, beach ridges and delta plain have been carried out. Majority of the samples were collected from beaches and beach ridges as they are sandy in nature and contain heavy minerals. A total number of 220 samples were collected for the study (Fig.3.1). 10 sub samples of 2 cores were obtained from The Department of Earth Science, Pondicherry University, Pondicherry. The cores were drilled as a part of Department of Science and Technology (DST), Govt. of India sponsored Science of Shallow Subsurface (SSS) programme at Valangaiman and Nannilam. The procedure involved in the analysis of grain size and heavy minerals is given here under. 48

2 3.2. Methodology Pre - Treatment of samples The sediment samples were dried sufficiently in an oven at 60 0 C to remove the moisture. 100 gm of sample was taken by the repeated process of coning and quartering to ensure uniformity. The samples, soaked in water for overnight, were stirred up by mechanical stirrer to disaggregate the samples and to remove the clay fractions. The stirred samples were decanted through 230 mesh repeatedly using distilled water until a clear column of water, without any turbidity, was obtained. During that process, wherever necessary, the samples were frequently rubbed by hand to eliminate the attached clay particles. The samples were dried and weighed. The weight loss was accounted for silt and clay. All the samples were treated with 30% by volume of H 2 O 2, to remove the organic matter. After washing with distilled water, the samples were dried and weighed. The weight loss accounted for the total organic matter. Then, the samples were treated with 1:1 HCl to dissolve and remove the calcareous shelly fragments present in the sediments. After proper washing and drying, the samples were weighed and the weight loss was taken as the weight of carbonates. Slit, clay and organic matter were not taken for the study Sieve analysis Sieving of sand was carried out in ASTM sieve sets at ¼ φ interval. The sieve sets, stacked in the descending order of their sizes, were shaken using Ro-tap sieve shaker continuously for about 20 minutes. During sieving, proper attention was paid to minimize the sand loss from the sieve sets. The sieved materials from each sieve were collected separately for weighing. Weights of the individual fractions were tabulated for further granulometric studies. The sieved sands were separately kept safely for light and heavy mineral studies. 49

3 Granulometric study Granulometric study is an essential part of understanding the mode of transportation and environment of deposition of sediments. Using graphic (Folk and Ward, 1957) and moment methods (Friedman, 1961, 1967 & 1979) the weight percentage data of the samples were processed to obtain statistical parameters like mean, median, standard deviation, skewness and kurtosis by using Schlee and Webster statistical programme (1967). The statistical parameters were used to prepare Frequency curves, Bivariant plots, Visher s log probability curves and CM diagrams Light - Heavy mineral separation 110 samples (alternate samples) were taken for heavy mineral studies. The sieved fractions of the sands have been grouped into coarse, medium and fine for heavy mineral separation. By following the procedure mentioned in the Milner (1962), the light and heavy minerals were separated using Bromoform in separating funnel. For those fractions likely to be clogged in the standard separating funnel, filtering funnel fitted with transparent non-corrosive tube clipped by pinchcock was used. After the settlement of heavies, pinchcock has been opened to drain them into filter paper kept on the funnel. Then the lighter grains have been washed into another filter paper provided with separate funnel. Bromoform has been filtered in the separate container for further use. The separated heavy mineral fractions have been washed first by using methyl alcohol and then with distilled water. The each and every fraction has been dried in a hot air oven with a mild temperature of 60 0 C until the moisture has been fully removed. The dried fractions weights have been noted down. As the coarse and medium fractions do not have much heavies, they were not taken for further studies. The heavy minerals in fine fractions were again separated into Magnetic and Non - Magnetic using hand magnet. The non magnetic fractions were run through Cook s isodynamic separator to separate feebly magnetic Ilmenites and transparent heavy minerals. 50

4 Slide preparation To study the minerals under the microscope, slides were prepared using Canada balsam having refractive index of 1.513, keeping in mind; the minimum number of grains per slide was around 300. While preparing the slides, care was taken to have a uniform spread of heavy mineral grains over glass slides without air bubbles Heavy mineral analysis From the mounted slides the individual minerals were counted under Leica ID818 digital DMEP petrographic Microscope by using the line method described by Galehouse (1969). Transparent heavy minerals were studied under refractive light and opaques were studied under reflected light. Various diagnostic properties of heavy minerals provided in the Milner (1962), Phillips and Griffen (1986), Ford (1951) and Rothwell (1989), were utilized for the easiest identification. From the results of line counting method, the general distribution pattern of heavy minerals all over the study area has been obtained. The different counts were converted into weight percentage. A few grains of individual heavy minerals were photographed Analysis of Core samples As the sub samples of two cores contain mainly clay and silt, the particle sizes were analyzed in Laser particle size analyzer in the laboratory of Dept. of Civil Engineering, SASTRA University, Thanjavur The details of statistical parameters of grain size are given in table 51

5 S. No. LF Table Statistical parameters of Grain size M (φ) Graphic method SD (φ ) SK KU FP 52 M (φ) Moment method SD (φ) SK KU MD S1 DP S2 DP S3 DP S4 DP S5 DP S6 DP S7 DP S8 DP S9 DP S10 BR S11 BR S12 DP S13 DP S14 BR S15 BR S16 DP S17 DP S18 BR S19 BR S20 BR S21 BR S22 DP S23 DP S24 DP S25 DP S26 DP S27 DP S28 DP S29 DP S30 DP S31 DP

6 S. No. LF M (φ) Graphic method SD (φ ) SK KU FP 53 M (φ) Moment method SD (φ) SK KU MD S32 DP S33 DP S34 DP S35 DP S36 DP S37 DP S38 DP S39 DP S40 DP S41 BR S42 DP S43 DP S44 DP S45 DP S46 DP S47 BR S48 BR S49 BR S50 BR S51 BR S52 BR S53 BR S54 BR S55 BR S56 BR S57 BR S58 DP S59 DP S60 BR S61 DP S62 BR S63 BR

7 S. No. LF M (φ) Graphic method SD (φ ) SK KU FP 54 M (φ) Moment method SD (φ) SK KU MD S64 BR S65 BR S66 BR S67 BR S68 BR S69 BR S70 BR S71 BR S72 BR S73 BR S74 BR S75 BR S76 BR S77 BR S78 B S79 BR S80 B S81 BR S82 B S83 BR S84 BR S85 B S86 BR S87 BR S88 B S89 BR S90 B S91 BR S92 BR S93 BR S94 BR S95 BR

8 S. No. LF M (φ) Graphic method SD (φ ) SK KU FP 55 M (φ) Moment method SD (φ) SK KU MD S96 BR S97 BR S98 BR S99 BR S100 BR S101 BR S102 BR S103 BR S104 BR S105 BR S106 BR S107 BR S108 BR S109 BR S110 BR S111 B S112 B S113 BR S114 DP S115 DP S116 DP S117 DP S118 B S119 B S120 DP S121 BR S122 B S123 BR S124 BR S125 B S126 BR S127 BR

9 S. No. LF M (φ) Graphic method SD (φ ) SK KU FP 56 M (φ) Moment method SD (φ) SK KU MD S128 B S129 DP S130 BR S131 DP S132 B S133 B S134 B S135 BR S136 BR S137 B S138 B S139 B S140 B S141 B S142 B S143 DP S144 DP S145 BR S146 BR S147 BR S148 B S149 B S150 B S151 B S152 B S153 B S154 DP S155 B S156 B S157 B S158 DP S159 B

10 S. No. LF M (φ) Graphic method SD (φ ) SK KU FP 57 M (φ) Moment method SD (φ) SK KU MD S160 B S161 B S162 B S163 B S164 B S165 DP S166 DP S167 DP S168 B S169 DP S170 B S171 B S172 DP S173 B S174 DP S175 B S176 B S177 B S178 B S179 B S180 B S181 B S182 BR S183 BR S184 BR S185 B S186 B S187 B S188 BR S189 BR S190 BR S191 DP

11 S. No. LF M (φ) Graphic method SD (φ ) SK KU FP M (φ) Moment method SD (φ) SK KU MD S192 DP S193 DP S194 BR S195 BR S196 DP S197 B S198 BR S199 DP S200 B S201 B S202 BR S203 DP S204 DP S205 DP S206 DP S207 DP S208 DP S209 DP S210 DP S211 DP S212 DP S213 DP S214 DP S215 DP S216 DP S217 DP S218 DP S219 DP S220 DP DP - Delta Plain, BR - Beach Ridges, B - Beach, LF - Landforms 58

12 3.3. Grain size parameters For convenience of description of grain size, the study area is classified into southern (sample 1-70), middle (sample ) and northern (sample ). But since most of the characteristics of samples of middle and northern part are common, the description of samples of middle and northern part is made together Mean Mean represents the average size of the total distribution of sediments. The mean size of the sediments depends on the energy and duration of the depositing medium and composition and durability of the grains. The availability of grains of various sizes is also an, another important controlling factor. Mean size (Mz) of the sediments of the region ranges from 1.02φ to 3.99φ. Out of 220 samples, 86 samples have mean size ranges from 1.02φ to 2.00φ indicating that the samples are of medium sand in size as per the verbal scale proposed by Folk and Ward (1957), 76 samples in mean size ranges from 2.00φ to 3.00φ indicating fine sand in size and the remaining 58 samples in mean size ranges from 3.00φ to 3.99φ phi indicating very fine sand in size. The sediments of samples from 1 to 70 generally have mean size of medium sand (except 16 samples -21, 22, 23, 24, 25, 27, 28, 41, 45, 50, 51, 52, 56, 57, 58, 64). Out of the remaining 150 samples (71-220), 119 samples have fine and very fine sand and 31 samples have medium sand. This distribution of grain size in general indicates that the samples collected from southern part of the study area (samples 1-70 collected from Kattumavadi to Thiruturaipoondi) have medium sand in size and the samples of middle and northern part of the study area (samples ) have fine and very fine sand in mean size irrespective of the landforms from which samples were collected. 59

13 Out of 70 samples of southern region, 40 samples represent delta plain and 30 samples represent beaches and beach ridges. Out of 150 samples of the middle and northern part 40 samples represent delta plain and 110 samples represent beach and beach ridges. Out of the 80 samples of delta plain (40 in southern and 40 in middle and northern), 43 samples exhibit medium sand, 23 samples fine sand and 14 samples very fine sand. Majority of samples of delta plain of southern part exhibit medium sand in characteristics (except 22, 23, 24, 25, 27, 28, 45, 46, 58, 59, 61) and of middle and northern part exhibit fine and very fine sand (except samples 131, 154, 196, 199, 204, 212, 213, 214, 219) Though there is no uniformity in the mean size of samples of delta plain, a general nature of mean size is observed region wise, medium in the southern part and fine and very fine in the northern part. As the delta plain is enriched mainly with fluvial sediments, it is presumed to have medium size sands. The occurrence of medium sands in the sediments of southern region can be attributed to the mixture of sediments brought by surface run off from the adjoining Cuddalore sandstone regions. The fine and very fine sand characteristics in the samples of the northern region is attributed to the over bank sediments constituting the delta plain. A few samples that have been collected from delta plain near the Coleroon river also exhibit medium sand in characteristics (sample no. 204, 212, 213, 214, 216, 219). Out of 86 samples collected from beach ridges, 31 samples exhibit medium sand in characteristics, 36 samples fine sand and 19 samples very fine sand. Out of 30 samples collected from beach ridges of southern part (1-70 samples) 22 samples contain medium sand and the remaining the samples contain fine sand. Majority of beach ridge samples in the northern part ( samples) contain fine and very fine sand (except 79, 86, 106, 110, 124, 135, 136, 189). Out of the 54 samples collected from beaches 12 samples exhibit medium sand, 17 samples fine sand and 25 samples very fine sand. Majority of the samples collected from beach ridges and beaches exhibit fine to very fine sand characteristics except a few samples. Since sands of beaches and 60

14 beach ridges undergo repeated wear and tear by wave action, it is common to have fine to very fine nature (Folk and Ward 1957, Friedman 1979, Visher 1969). A few samples that have been collected from older beach ridges exhibit both medium and fine sands. Though the sands of older beach ridges are bleached in nature, the grain size is medium in a few samples. This is due to the mixing of sediments of Cuddalore sandstone and other fluvial sands in them. Such characteristic has also been observed in northern Tamilnadu in the older beach ridges and hence they have been classified as Cheniers (Anbarasu 1994). The samples of beaches, younger beach ridges and older beach ridges have no marked change in the mean size between them. As beaches and beach ridges are involved in the similar type of marine action, the grain size is similar in both the land forms. The occurrence of medium sand in a few of the samples of beach and beach ridges is attributed to the mixture of external sediments brought by littoral currents. Though the older and younger beach ridges were formed at different time, the mean sizes of the samples do not show much variation. The sediments of beach ridges, though subjected to Aeolian process after the formation, exhibit medium sand characteristic in some samples due to the fact that the sediments are admixture of marine sands and Mio - Pliocene Cuddalore sandstone. Though the beach sands are admixed with river borne sediments derived through various distributory channels of Cauvery delta, the beaches are mostly made up of fine sands. This is attributed to the fact that the sediments brought by these channels are mostly reworked sediments of Mesozoic and Tertiary age. Seralathan (1979) has also noted the occurrence of fine sands in the beaches around the mouth of river Vellar. The random changes in the mean size from fine to medium in various samples of the region is attributed mainly to the littoral sediments derived from other areas and to the sediments of Cuddalore derived by surface run off causing admixture. Chandrasekar (1992) while discussing the sediments dispersal pattern along central Tamilnadu coast, observed a reversal in 61

15 sediment movement around the mouth of Cauvery due to influence of coastal configuration and due to the impact of large amount of river supplies leading to delta formation. The wave refraction study conducted in the region also supports the variation of mean size in response to energy state of the waves (Anbarasu 1995) Standard deviation Standard deviation is a measure of uniformity or sorting. It is also the resultant characteristic of sediments controlled by size, shape and specific gravity of the sediments and energy and time involved in transporting medium. The standard deviation values of the sediments of the region range from 0.37φ to 2.18φ. Out of the 220 samples, 13 samples have standard deviation values from 0.37φ to 0.50φ indicating that the sediments are well sorted in characteristics, 46 samples have values from 0.50φ to 0.71φ indicating moderately well sorted in characteristics, 78 samples have values from 0.71φ to 1.00φ indicating moderately sorted in characteristics, 80 samples have values from 1.00φ to 2.00φ indicating poorly sorted in characteristics and 3 samples have values from 2.00φ to 2.18φ indicating very poorly sorted in characteristics. Majority of the samples (136 samples) have standard deviation values range from 0.37φ to 1.0φ indicating that the sediments are moderately sorted to well sorted in characteristic and the remaining (84) samples have standard deviation values range from 1.0φ to 2.18φ indicating that the sediments are poorly sorted to very poorly sorted in characteristic as per the verbal scale proposed by Folk and Ward The standard deviation values of samples of southern region (1-70) indicate poorly sorted nature (except 16 samples) and those of middle and northern (71-220) region exhibit moderately sorted to well sorted nature (except 47 samples). This indicates samples of medium sand in mean size exhibit poorly sorted nature and samples of fine and very fine sand in size 62

16 exhibit moderately to well sorted characteristics. This phenomenon of improvement of sorting with the decrease in mean size of the samples has been observed by many researchers in their studies (Visher 1969, Pettijohn 1984, Freidman 1967, Folk and Ward 1957, Anbarasu 1994, Angusamy 1995). Sorting generally improves with transportation due to the fact that the mean size of sediments changes with transport and the important in sorting depended on the mean size not the distance. Out of 80 samples of delta plain, 40 samples have poorly sorted characteristics and the remaining 40 samples have moderately to well sorted characteristics. Majority of the samples of delta plain of southern part of the study area have poorly sorted nature (except 8 samples - 22, 23, 24, 25, 27, 28, 45, 58) and of middle and northern part have moderate to well sorted nature (except 47 samples). The poorly sorted nature of delta plain samples of southern region is attributed to the unidirectional flow of fluvial sediments constituting them. The unidirectional flow of fluvial sediments normally results in poor sorting. As observed in the previous section the samples of delta plain of middle and northern part have the mixture of marine sediments as they are close to shoreline and exhibit well sorted nature. Out of 86 samples of beach ridges, 31 samples exhibit poorly and very poorly sorted nature and the remaining 54 samples show moderately to well sorted characteristics. Out of 54 samples of beaches, 12 samples exhibit poorly sorted nature and the remaining 42 samples exhibit moderately to well sorted nature. Beach and beach ridge sediments normally exhibit well sorted nature and the study region is not an exception. 96 samples show moderately to well sorted characteristics out of 140 samples of both beaches and beach ridges. The to and fro motion and long time involvement of the sediments in wave motion make the sediments well sorted. Out of 30 samples of beach ridges in the southern part (1-70) of the study area, 24 samples exhibit poorly sorted nature (except the samples - 21, 41, 50, 51, 52, 56, 57, 64). The sediment of older beach ridges also exhibit well sorted characteristic even though the sediments have been subjected to wind 63

17 transportation which is unidirectional in flow. It is due to the fact that the sediments are basically derived by marine processes which are generally well sorted in nature. The poorly sorted nature of a few beach ridge sands is attributed to the addition of sediments from the delta plain. The poorly and very poorly sorted characteristics observed in a few samples of beaches and beach ridges is attributed to the admixture of sediments brought by littoral currents from the adjoining region. Poor sorting is also attributed to change in the energy condition of the waves in an open coast. No marked change in sorting is observed between samples of beaches and beach ridges due to the reason that same processes are involved in the genesis of both the landforms Skewness Skewness is a measure of symmetry of grain size distribution. It is a significant parameter in delineating environment, since it is sensitive to sub population mixing. The importance of skewness has been discussed in detail by Duane The skewness values of sediments of the study area range from to Out of 220 samples the skewness values of 46 samples range from 0.3 to 0.95 falling in the category of very fine skewed, 47 samples range from 0.1 to 0.3 falling in the category of fine skewed, 63 samples range from +0.1 to-0.1 falling in the category of near symmetrical and 25 samples range from -0.3 to falling in the category of very coarse skewed in the in the verbal scale proposed by Folk and Ward Out of 80 samples of delta plain, 58 samples show near symmetrical to very fine skewed nature and the remaining 22 samples show coarse to very coarse skewed nature. The near symmetrical nature indicates the equal proportion of coarse and fine fractions in tail region. The fine and very fine skewed nature indicates the lack of tail at the fine grained end. 64

18 Out of 86 samples from beach ridges, 63 samples show very fine skewed to near symmetrical in nature and the remaining 23 samples show coarse and very coarse skewed nature. The sediments of marine origin generally show fine skewed characteristics i.e lack of tail at the fine grained end. An addition of sub population also gives fine skewed nature. In this case it is attributed to the addition of sediments from the neighboring delta plain. Out of 54 samples from beaches, 36 samples show very fine skewed to near symmetrical in nature and the remaining 18 samples show coarse and very coarse skewed in nature. As both beaches and beach ridges are formed by similar processes, the sediments exhibit same kind of skewness. The lack of tail at the fine grained end may also be attributed to the littoral currents that are prevalent in the region. The littoral currents generally transport sediments parallel to the shore that makes erosion and deposition of sediments in the same area as a continuous process. Beach and beach ridges sediments have general tendency to show near symmetrical skewness. But the fine and coarse skewed nature observed in the study area indicates the mixing of different modal fractions. Friedman (1961) while discussing the reason for the negative skewness of beach sands has assigned the removal of fine grained particles by winnowing action for the lack of tail at the fine grained end. Our of the 70 samples of southern part of the study area, 50 samples exhibit fine skewed, very fine skewed and near symmetrical nature. The mean size values have no specific relation with the skewness values. Of the 50 samples 25 samples represent delta plain and 25 represent beaches and beach ridges. Out of 150 samples (71-220) of middle and northern part, 106 samples show near symmetrical to very fine skewed nature. Majority of samples (except 64 samples) fall in the category of near - symmetrical to very fine skewed. Many authors have assigned the removal of fine grain particles by winnowing action in the beach region is a reason for fine skewed nature and for the lack of tail at the fine grained end. 65

19 As most of the samples were collected from beach and beach ridges, fine and very fine skewed nature is observed. The skewness values do not exhibit any relation with other parameters. The near symmetrical characteristic in some samples indicate the equal proportions of coarse and fine at the tails. This may be due to the inclusion of fine sediments from the adjoining delta region. Coarse skewed nature observed in some samples is due to the mixing of fluvial sediments. Though the sediments of southern and northern part of the study area show marked variation in mean size, they do not show such features in skewness Kurtosis Kurtosis is a measure of ratio between the sorting in the tails of the curve and the sorting in the central portion. Though there are contradictory views on the use of kurtosis in the determination of environment, kurtosis is helpful in understanding the deposition of deltaic sediments (Folk and Ward 1957; Friedman 1961 and 1967; Cadigan 1961). The kurtosis values of sediments of the study area range from 0.01 to Out of 220 samples 45 samples have kurtosis values ranges from 0.01 to 0.67 falling in the category of Very platykurtic, 24 samples in the range of 0.67 to 0.90 falling in the category of platykurtic, 25 samples in the range of 0.90 to 1.11 falling in the category of mesokurtic, 29 samples in the range of 1.11 to 1.50 falling in the category of Leptokurtic, 84 samples in the range of 1.50 to 3.00 falling in the category of Very Leptokurtic and the Remaining 13 samples in the range of 3.00 to 5.41 falling in the category of Extremely Leptokurtic. Out of 80 samples of delta plain, 58 samples show leptokurtic to extremely leptokurtic and the remaining 22 samples show mesokurtic to very platykurtic nature. Out of 86 samples of beach ridges, 48 samples show leptokurtic to extremely leptokurtic and the remaining 38 samples show mesokurtic to very platykurtic nature. 66

20 Out of 54 samples of beaches, 19 samples show leptokurtic to extremely leptokurtic and the remaining 35 samples show mesokurtic to very platykurtic nature. Folk and Ward (1957) have inferred that unimodal sediments exhibit mesokurtic and the mixing of two populations in sub equal amounts results in leptokurtic values. Mason and Folk (1958) and Spencer (1968) have also observed the same. Out of 70 samples of southern part 56 samples exhibit mesokurtic to very leptokurtic nature. Out of 150 samples of middle and northern part 95 samples exhibit mesokurtic to very leptokurtic nature. Of the 40 samples of delta plain of southern part (1-70), 35 samples show lepto and very leptokurtic nature (except 1, 2, 3, 29, 42). All the samples of beach ridges of southern part exhibit lepto and very leptokurtic except 1 sample which is showing platykurtic. Out of 40 samples of delta plain of middle and northern part 17 samples show mesokurtic to very platykurtic and 23 samples show lepto and very leptokurtic nature. Out of 110 samples of beaches and beach ridges of middle and northern part of the study area 72 samples show mesokurtic to very platykurtic and the remaining 38 samples show leptokurtic and very leptokurtic. Majority of samples fall in the category of leptokurtic and very leptokurtic. The poly modal sediments normally exhibit mesokurtic due to the mixing of two populations in equal amount and platykurtic due to the mixing of populations in sub equal amounts. The kurtosis values of samples of delta plain of the study area supports the above mentioned fact. Mason and Folk (1958) and Spencer (1968) have also noted such relations in their studies. The extreme values of kurtosis - lepto and very leptokurtic - indicate that the sediments were sorted in high or low energy environment and transported to a new environment with reversal of energy to mix either with fine or coarse sediments depends on the energy condition Frequency curve Frequency curves exhibit the pictorial representation of actual weight percentage of different fractions of sediments. The peaked ness of fractions and uniformity of the sediments can be inferred from it. 67

21 Majority of the samples of the delta plain show poly-modal distribution. The peaked ness regions are observed between 1.50φ and 2.25φ and between 2.75φ and 3.25φ. The first mode is formed by the dominance of medium sand and the second is formed by the fine sand. The poly-modal distribution denotes the mixing of two or more population of sands and multi source of sediments. Since delta plain contains mainly fluvial sediments, the poly-modal distribution is observed. Some of the samples of delta plain show uni-modal distribution with the peaked ness between 2.75φ and 2.35φ indicating the dominance of fine sand. In a few frequency curves of delta samples, though poly-modal distribution is observed the peaked ness of fine fraction is more prominent than the coarse fractions. But in the majority of the samples the peaked ness between 2.75φ and 3.25φ is prominent indicating the dominance of fine fractions. Though delta plain samples are classified under medium sand, the peakedness at fine fraction is observed. This is due to the presence of more amounts in various fractions of the samples out of peaked ness range in the coarse side than the fine side. That is the reason why samples are classified as medium sand. Cuddalore sand stone uplands also supply sediments to the delta plain region by ephemeral streams. Cuddalore sandstone is made up of sediments from pebble to silt and clay. The mixing of sediments of Cuddalore sandstone also gives medium nature to the sediments of delta plain. Frequency curves of a few samples of delta plain are shown in figures 3.2 a - f. The frequency curves of majority of the samples of beach ridges show unimodal distribution. The peaked ness range is found mainly between 2.50φ and 3.25φ. The unimodal distribution for beach ridge samples is common to observe. The peaked ness is observed in the size ranges which are classified as fine sand in Folk and Ward 1957 scale. Since the beach ridge sediments are mainly beach sands, the characteristics are similar to beach sands. Out side the peaked ness range the amount of samples both in coarse and fine fractions is very small. This denotes the presence of a particular fraction of sands makes major population of the samples. The samples of beach 68

22 ridges are slightly finer than beach samples, as they have undergone Aeolian activity after the withdrawal of the sea. Frequency curves of a few samples of beach ridges are shown in figures 3.3 a - f. The frequency curves of samples of beaches also exhibit unimodal distribution. The peaked ness is observed between 2.25φ and 3.50φ. The curves indicate that the samples of beaches are slightly coarser than the samples of beach ridges. Out side the peaked ness region, small amount of coarse fraction is observed. The unimodal distribution indicates the single source of sediments. A few samples of beaches show polymodal distribution which may be due to the mixing of sediments by other source like littoral currents and fluvial processes. Frequency curves of a few samples of beach are shown in figures 3.4 a - f Bivariant plots The inherent relationship between the four size parameters can well be understood only when they are plotted against each other as scatter diagrams. Wentworth (1929), Keller (1945), Inman and Chamberlay (1955), Folk and Ward (1957), Friedman (1961, 1967), Shepered and Young (1967) and many others have successfully used the scatter plots for understanding the geological significance of the four size parameters. But the researchers divided them selves into two schools of thought, one favoring graphic and other favoring moment method. The controversy still continues. In the present study six scatter plots using statistical parameters obtained from graphic method were constructed to understand the relation between them. The scatter plots drawn between mean and standard deviation is shown in fig. 3.5.a. The points observed in the plot form three distinct groups. The first group is formed by majority of the samples of delta plain and the second and third group formed by both the samples of beaches and beach ridges. The medium size and poorly sorted nature of the delta plain samples are well exhibited in the first group. A few samples of beaches and beach ridges are also found in the first group indicating medium size and 69

23 poorly sorted nature. The beach and beach ridge samples, though form two groups, beach ridge samples are dominant in the second group and beach samples in the third group. The second and third groups of samples clearly denote the fine sand and moderate to well sorted nature. A few samples of delta plain are also found in the second and third group indicating fine nature. The samples of delta plain that are found in the second and third group exhibit poor sorting nature. The samples of beaches and beach ridges that are found in the first group exhibit well sorted nature. Though a few samples of different environment are found in all the three groups, a distinct grouping is observed in the samples of various environments. The scatter plots drawn between mean and skewness (Fig.3.5.b) and between mean and kurtosis (Fig.3.5.c) show no characteristics grouping of points. But number of clustering of a few of the samples of same environment is observed. In the mean vs skewness plot most of the samples having fine skewed nature is well exhibited. Similarly in the mean vs kurtosis plot, majority of samples having leptokurtic and very leptokurtic nature is well exhibited. But the relation between mean vs skewness and mean vs kurtosis is not significant in many samples. The scatter plot drawn between standard deviation and skewness and standard deviation vs kurtosis are shown in fig. 3.5d and 3.5e respectively. In these plots though grouping of points are not observed with respect to the environment of samples, grouping of points in two regions is observed. In the standard deviation vs skewness plot two major groups are observed. The first group indicates the samples of fine skewed and moderate to well sorted nature and the second group denotes poorly sorted nature and but fine to coarse skewed nature. The points of delta plain samples are highly scattered indicating variation in the sorting and skewness characteristics. In the plot drawn between standard deviation vs kurtosis two groups of points are observed, but samples of different environment found in both 70

24 groups. The first group denotes samples of leptokurtic and moderate to well sorted nature. The second group denotes samples of poorly sorted nature but mesokurtic to very leptokurtic characteristics. The samples of delta plain form scattered points. The scatter plot drawn between skewness and kurtosis is shown in fig. 3.5.f. The points are highly scattered and no grouping is observed. The areas within the range of normal curve are shown by diagonal lines. Many samples fall in the area of normal curve. But a sizable number of samples are found beyond the normal curve indicating the extremity in the samples. As discussed in the previous sections, since numbers of samples are leptokurtic and fine skewed nature, the points are observed outside the normal curve Visher s diagram Visher (1969) established the effective use of log probability curves for determining the environment of deposition of sediments using three types of sub population namely traction, saltation and suspension. The quantification of these three sub population from the mode of distribution and from the nature of frequency curve also helped the delineation of environment. The log probability curves prepared using Visher s procedure for a few samples of delta plain are shown in the fig 3.6 a -f. All the curves show three distinct populations which can be assigned as traction, saltation and suspension. The saltation population is dominantly observed in many samples. The traction population is poorly represented. The medium and fine sands are generally transported by means of saltation and suspension and hence they are very well represented in the curves. As in any other delta region, saltation population is dominant in the samples. The coarse sediments are sparse in deltaic environment and hence the traction population is poorly represented. As the apex of the delta is far away from the sample region, the coarse fraction (traction population) is poorly represented. The variation in the slops of the different population indicates the differences in sorting characteristic. 71

25 The log probability curves of a few samples of beach ridges are shown in fig. 3.7.a-f. The beach ridge samples also exhibit three distinct populations which can be assigned to traction, saltation and suspension. The presence of traction population in the beach sands can be assigned to the sediments derived by littoral currents. The to and fro motion of waves make the sediments with more of saltation and suspension population. Saltation population is better represented than the delta plain samples The log probability curves of a few samples of beaches are shown in fig. 3.8.a-f. Again three populations are noticed with little more prominent traction population than beach ridge samples. It clearly denotes sediments derived by littoral currents. The suspension population is little less prominent than beach ridge samples. This is due to the Aeolian activity bringing suspension population more in the beach ridge samples C M - Pattern Passega (1957) established the relation between texture of sediments and process of deposition using C - the coarsest 1 percentile grain size and M - the median. The C and M plotter in logarithmic paper enabled to distinguish the mode of transportation by means of sub population such as rolling, saltation and suspension. The time gap in the mode of transportation and the agent of deposition which also inferred. Passega (1964) divided the CM pattern into different sector namely NO, OP, PQ, QR and RS for different mode of transportation. The CM pattern obtained in the present study is shown in fig Majority of the points are found in the sectors NO and OP indicating that samples are mainly derived by rolling and saltation. Many samples are closer to the limit C=M, indicating their graded suspension transport. No distinct variation between samples of delta plain, beaches and beach ridges is observed. A sizable number of samples are also found away from various sector indicate that those samples were involved in various mode of transportation at different time. The concentration of samples towards minimum median indicates the better sorting characteristics. 72

26 3.4. Analysis of core samples Ten sub samples of two cores drilled at Valangaiman (VM) and Uttrangudi (UG) were analysed for grain size. The details of the samples depth and the statistical parameters of the grain size are given in the table Table Details of depth of the core samples and the statistical parameters of the grain size Sample No Depth of Standard Mean the sample Deviation (µ m) (in cm) (µ m) VM VM VM VM UG UG UG UG UG UG The population cum distribution of grain size in the samples as observed under Laser Particle Analyzer is shown in fig a-j. The mean size of the samples ranges from 2.27µm to 3.28µm indicating that the samples contain very fine silt and clay fractions only. Though the clay and fine silt fractions do not have any significance in the depositional environment, the presence of clay and silt indicates low energy environment and slow deposition of suspended sediments. Clays are commonly deposited as floccules and they respond to hydrologic conditions differently than if the existed as a separate particle. The litholog of the cores (fig.3.11) show the occurrence of large thickness of clay with intervening thin layers of sand indicating that the low energy environment was intermittently converted into high energy fluvial environment. The analysis of samples for the study of 73