Textural characteristics of the surface sediments of a Tropical mangrove ecosystem Gulf of Kachchh, Gujarat, India

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1 Indian Journal of Marine Sciences Vol. 39(3), September 2010, pp Textural characteristics of the surface sediments of a Tropical mangrove ecosystem Gulf of Kachchh, Gujarat, India Goutam Kumar 1*, AL.Ramanathan 1, & K Rajkumar 1. 2 [ goutamses@gmail.com] 1 School of Environmental Science, Jawaharlal Nehru University, New Delh , India. 2 Department of Geology, Utkal University, Vani Vihar, Bhubaneswar , Orissa, India Received 31 December 2009; revised 5 August 2010 Mangroves in gulf of Kachchh, Gujarat, India has been studied for grain size spectrum and textural parameters namely phi mean, standard deviation, skewness and kurtosis. Grain size spectrum shows a marked variation in the upstream riverine input stretch. Variation of energy conditions is controlled by the fluvial profile in the channel. Textural pattern shows complicated profile as a result of the fluctuation in the physico-chemical conditions due to fluvial-marine interaction. Abundance of very fine sand to fine sand shows the prevalence of comparatively low energy condition in the mangrove creek zones. Linear Discriminate Function Analysis (LDF) of the samples indicates shallow marine environment origin for all samples collected from this mangrove ecosystem. Riverine sediments show a wide range of textural facies including muddy sand. It has also been observed that a progressive enrichment of silty-sand in the downstream region. CM diagram (C= one percentile in microns, M=Median in microns) of Gulf of Kachchh sediments suggests that deposition takes place by (1) rolling (2) rolling and suspension and (3) graded suspension. Rolling and suspension population is dominant in the river/creeks deposits, which is also supported by uniform grain size distribution of sediment load augmented by turbidity current. [Keywords: Grain size, Mangroves, Gulf of Kachchh, Gujarat.] Introduction Mangroves of the Gulf of Kachchh are the second largest after Sundarban in the mainland of India. Various studies have been carried out in order to reveal the tidal circulation and source and dispersal of suspended sediment in Gulf of Kachchh. Mangrove directly affects the sediment flux by influencing the hydrodynamic regime through their physical configuration 1,2. It has been attempted to investigate / understand the textural characteristics of the sediments of Gulf of Kachchh riverine (four samples) and creeks (five samples) to have proper insight into particle size distribution 3. Environmental status of the basin has been elucidated by studying the energy conditions and transportation pattern of the sediment. Fresh water discharge from any perennial river does not take place in this mangrove wetland and hence marine processes are dominant throughout the year 4-6. Present study consists the textural characteristics of the sediments of Gulf of Kachchh riverine and creek to get insight into the particle size distribution as well. Energy conditions and transportational pattern of this sediment help to elucidate the environmental quality *Corresponding author of the basin, since there is no systematic and detailed textural studies on surface sediments are available in this region. Materials and Methods The Gulf of Kachchh lies between latitudes 22 to 23 N and between longitudes 68 to E with an area of 7300 km 2. It has a complex set up of a macro- tidal region. The depth ranges from 60 m at the mouth to less than 20 m at the head of the gulf the floor of the Gulf of Kachchh is highly irregular. The climate is semi- arid and the maximum rainfall is of the order of 50 cm yr -1. There is no major river flow into it and hence little runoff has been observed. The topography is very irregular at the mouth and the central part of the Gulf and consists of pinnacles and scarps ranging in height from 6 to 32 m 7. Towards the head, the relief is subdued due to the covering finegrained sediments. Major source of this sediment is considered to be the shore material and the load transported by the Indus River 8. The reduction in sediment supply of the Indus River results changes in the Indus delta 9,10. Tidal processes, especially settling lag effects help in import of fine grained sediments into high tidal areas 11.

2 416 INDIAN J. MAR. SCI., VOL. 39, NO. 3, SEPTEMBER 2010 Seventeen surface sediment samples were collected from sites of the mangrove region during October 2008 to November Sampling was done at Bedi Port (New & Old), Sikka, Narara, Jodiya and Sachana (Fig.1). In which Salaya (S8&S9), Mundra (S12) and Jodiya (S16) are riverine sites and remaining are from mangrove areas. The sites has been selected by keeping into consideration of different components like anthropogenic input, mixing zones, natural weathering input zones, etc. Sediments samples (1 kg each) were collected in the polyethylene bags. In laboratory, the samples were air-dried, gently pounded and homogenized using coning and quartering procedure. Granulometric separation was carried out by combining dry and wet sieving. Dry sieving was carried out by electromagnetic sieve shaker (Fritsch Analysette-3) into 250 µm (medium sands), 125 µm (fine sands), 63 µm (very fine sands), 37 µm (very coarse silt) and < 37 µm (clay). Separation of 63 µm was done by wet sieving by Attenburg sedimentation cylinders methods based on Stokes law. Textural parameters were computed using the formulae of Folk and Ward (1957) Mean (Mz), Standard deviation (σ1), Skewness (Ski) and Kurtosis (KG) are widely used to reconstruct the depositional environments of sediments and sedimentary rocks. Grain size distribution was also studied to determine sedimentary environment with the help of log-probability studies. Results and discussion Textural attributes of sediments viz. mean (Mz), standard deviation (σ1), skewness (Ski) and kurtosis (KG) are widely used to reconstruct the depositional environments of sediments 17. Correlation between size parameters and transport processes/depositional mechanisms of sediments has been established by exhaustive studies from many modern and ancient sedimentary environments Mean (Mz) Mean size indicates that the fine sands were deposited at a moderately low energy conditions. phi mean size of samples varies from to Ф with an average mean of Ф (Fig. 1a) (Table 1). Majority of the samples lies in fine sand category (S1 to S5, S8, S9, S15, S16 and S17) (58%) and rest in very fine sand (S10 to S14) and medium sand category (S6 and S7). Mean size of the sediments influenced by the source of supply, transporting medium and the energy conditions of the depositing environment. The mean size indicates that the fine sands were deposited at a moderately low energy conditions. Fig. 1 Showing sampling location of Gulf of Kachchh, Gujarat, India

3 KUMAR et al. TEXTURAL CHARACTERISTICS OF THE SURFACE SEDIMENTS 417 Standard deviation (σ1) Standard deviation (σ1) measures the sorting of sediments and indicates the fluctuations in the kinetic energy or velocity conditions of the depositing agent 26. The standard deviation values of the sediments range between and Ф with an average value of Ф (Fig. 1b) (Table 1). Sorting of sediments range from poorly sorted to moderately sorted character. About 76% of samples fall under moderately sorted (S1-S5, S12 and S15-S17) and moderately well sorted (S6-S9) nature. Abundance of the particle dispersion is derived from the riverine and creeks sediments. In the riverine zones only four samples (location S8 and S9, S12 and S15) which were collected from Salaya, Mundra and Jodiya exhibit moderately sorted to moderately well sort particle dispersion. Skewness (Ski) Skewness (Ski) measures the asymmetry of a frequency distribution. Skewness values range between and with an average value of 0.362(Fig. 1c) (Table 1). Symmetry of the samples Fig. 2 Showing horizontal variations of grains size parameters of surface sediments.

4 418 INDIAN J. MAR. SCI., VOL. 39, NO. 3, SEPTEMBER 2010 Table 1 Statistical analysis of grain size of Gulf of Kachchh mangroves sediments S. No Mean Stan. Dev. Skew. Kurtosis Mean Stan. Dev. Skewness Kurtosis S Fine Sand Moderately Sorted Very Fine Skewed Platykurtic S Fine Sand Moderately Sorted Very Fine Skewed Mesokurtic S Fine Sand Moderately Sorted Very Fine Skewed Platykurtic S Fine Sand Moderately Sorted Very Fine Skewed Platykurtic S Fine Sand Moderately Sorted Very Fine Skewed Platykurtic S Medium Sand Moderately Well Sorted Very Fine Skewed Very Leptokurtic S Medium Sand Moderately Well Sorted Very Fine Skewed Very Leptokurtic S Fine Sand Moderately Well Sorted Very Fine Skewed Mesokurtic S Fine Sand Moderately Well Sorted Very Fine Skewed Leptokurtic S Very Fine Sand Poorly Sorted Coarse Skewed Very Platykurtic S Very Fine Sand Poorly Sorted Symmetrical Very Platykurtic S Very Fine Sand Moderately Sorted Symmetrical Platykurtic S Very Fine Sand Poorly Sorted Coarse Skewed Mesokurtic S Very Fine Sand Poorly Sorted Coarse Skewed Platykurtic S Fine Sand Moderately Sorted Coarse Skewed Platykurtic S Fine Sand Moderately Sorted Very Fine Skewed Platykurtic S Fine Sand Moderately Sorted Very Fine Skewed Platykurtic varies from very fine skewed to coarse skewed nature. About 65% of the samples fall under very fine skewed category (S1-S9 and S16-S17). Very fine skewed nature of sediments indicates excessive riverine input. Positive skewness (70%) of sediments indicates the unidirectional transport (channel) or the deposition of sediments in sheltered low energy environment 27. Kurtosis (K G ) Values of the fourth moment kurtosis (KG) ranging between (S10) and (S6) with an average of 1.032(Fig.1d) (Table1). Majority (65%) of the samples falls under platykurtic to very platykurtic nature of distributions. Friedman (1962) suggested that extreme high or low values of kurtosis imply that part of the sediment achieved its sorting elsewhere in a high energy environment. Variation in the kurtosis values is a reflection of the flow characteristics of the depositing medium 4,28. Finer in size and dominant platykurtic nature of sediments reflect maturity of the sand and variation in the sorting values are likely due to continuous addition of finer/coarser materials in varying proportions 29. Linear discriminate function. According to Sahu (1964), the variations in the energy and fluidity factors seem to have excellent correlation with the different processes and the environment of deposition. The process and environment of deposition were deciphered by Sahu s linear discriminate functions of Y1 (Aeolian, beach), Y2 (Beach, shallow agitated water), Y3 (shallow marine, fluvial) and Y4 (turbidity, fluvial).with reference to the Y1 and Y2 values, most of the samples (88%) fall in beach process (Y1) and all (100%) samples fall in shallow marine waters (Y2) respectively. Further the samples fall in Y3 indicate they were of shallow marine environment (66%) and fluvial environment (34%). Y4 values show that about 59% of the samples were deposited by fluvial action and 41% by turbidity action (Table 2). CM Pattern Grain size parameters and the plots of CM patterns to distinguish between the sediments of different environments of fluvial and deltaic deposits In the present study an attempt has been made to identify the modes of deposition of sediments of the Gulf of Kachchh mangroves by CM pattern. The parameter C (one percentile of the grain size distribution) and M (the median) were plotted for Phi values of the C and M obtained from the cumulative curves in microns (Fig. 4). Relation between C and M is the effect of sorting by bottom turbulence. Good correlation between C determined by only one percent by weight of the sample and M, which represents grain size as a whole, shows the precision of the control of sedimentation by bottom turbulence. CM pattern is subdivided into three segments namely PQ, QR and RS. Segment PQ indicates the coarse grains

5 KUMAR et al. TEXTURAL CHARACTERISTICS OF THE SURFACE SEDIMENTS 419 Table 2 Showing Linear Discriminate Function (LDF) values (after Sahu, 1964). S.No Y1 Y2 Y3 Y4 Y1 Y2 Y3 Y4 S Beach Shallow Marine Shallow Marine Fluvial deposit S Beach Shallow Marine Shallow Marine Fluvial deposit S Beach Shallow Marine Shallow Marine Fluvial deposit S Beach Shallow Marine Shallow Marine Fluvial deposit S Beach Shallow Marine Shallow Marine Turbidity current S Aeolian Shallow Marine Fluvial Fluvial deposit S Aeolian Shallow Marine Fluvial Fluvial deposit S Beach Shallow Marine Shallow Marine Fluvial deposit S Beach Shallow Marine Shallow Marine Fluvial deposit S Beach Shallow Marine Shallow Marine Turbidity current S Beach Shallow Marine Shallow Marine Turbidity current S Beach Shallow Marine Fluvial Turbidity current S Beach Shallow Marine Fluvial Turbidity current S Beach Shallow Marine Fluvial Turbidity current S Beach Shallow Marine Fluvial Turbidity current S Beach Shallow Marine Shallow Marine Fluvial deposit S Beach Shallow Marine Shallow Marine Fluvial deposit Fig. 3 Ternary diagrams showing the nature of sediments in the riverine and creeks environments of Gulf of Kachchh. (Sand and clay Diagram) transported by rolling, while QR parallel to line C=M represents the main channel deposits. Segment RS (uniform suspension) paralleling the M axis is represented by silt and clay indicating the role of uniform suspension in transporting these sediments. CM diagram clearly depicts that most of the samples are concentrated at three M values, namely around 20/zm, 20-40/zm and /zm while the value of C

6 420 INDIAN J. MAR. SCI., VOL. 39, NO. 3, SEPTEMBER 2010 Fig. 4 CM pattern of the sediments of Gulf of Kachchh rivers and creeks fluctuates between 100 to 250/zm and 400 to 2000/ zm. Samples mainly fall in Q followed by S indicating the sediment samples are of uniform suspension derived from fluvial marine process influenced by Ghi River. Samples fall in between points 2 and 3 also indicate rolling, suspension, graded suspension influenced by turbidity controls their depositional environment 32, it indicates the river transportation process were dominant and intensive in the main tributary channel. Conclusion Present study infers that most of the sediments were deposited under shallow marine environment by beach and fluvial processes by a near shore whirlpool agitating turbidity action of water. Owing to the possible relic nature of sediments, the bivariant plots present an overlapping of the fluvial, marine and beach environments in the samples of the study area. LDF results show the dominance of shallow marine deposits. Textural analysis indicates the dominance of fine and very fine sand in the shallow river and creek is due to low energy condition. Very fine grained sediments are dominant in the Gulf of Kachchh creeks. Granulometric variation and CM pattern indicate rolling, suspension; graded suspension influenced by turbidity controls their depositional environment in main channel as compared to other minor channels. Acknowledgement Authors are grateful to Department of Forest, Gujarat for granting permission to carry out studies in the Marine National Park and Sanctuary and to JNU for providing facilities. Goutam Kumar is thankful to University Grants Commission, for award of Junior Research Fellowship to carry out this work.

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