Some Sedimentological Aspects of Vembanad Lake, Kerala West Coast of India

Transcription

1 Indian Journal of Marine Sciences Vol. 13, December 1984, pp Some Sedimentological Aspects of Vembanad Lake, Kerala West Coast of India T K MALLIK & G K SUCH IN DAN Marine Sciences Division, Centre for Earth Science Studies, Sasthamangalam, Trivandrum Received 12 March 1984; revised received 23 October 1984 Average depth in the Vembanad lake south oftannirmukkom bundtlat. 9~30' to 9'40' N and long ' 'E) is 3 m while the maximum depth being 8.5 m. Quartz is the main constituent of the sediment which is an admixture of clay, silt and sand. Lesser amounts of muscovite, feldspar, chlorite, heavies, clay minerals and shell fragments are also present. Heavy mineral assemblage indicates that the sediments are derived from the charnockues, granite gneisses, khondalites and pegmatites. Landsat imagery of this area is characterised by number of beach ridges oriented in direction varying from to Two main processes are responsible for the formation of Vembanad lake - an initial tectonic phase followed by natural sedimentation processes affected by waves, currents and tides. As a part of the Coastal Zone Management Programme detailed studies ofvembanad lake, which is the largest of the estuaries in Kerala have been taken up. In the present paper bathymetry, sediments, mineralogy and origin of the lake are given. Different aspects of sediments of Vembanad lake have been reported earlier I -4. Materials.and Methods Bottom sediment samples (88) were collected along 17 lines spaced at I km in Vembanad lake south of Tannirmukkom bund (lat. 9 30' to 9'"40'N and long. 76'21' to 76 C 26'E) using a Van Veen grab. Bathymetric data were collected by a Skipper 404 echosounder (range m). Position fixing was done by theodolite intersections from fixed points in the shore. Four cores were collected using a Phipp's corer. Standard methods were used for mineral and chemical analysis. Results and Discussion Bathymetry-Average depth (Fig. I) in the lake in the study area is around 2 to 3 m. Maximum depth is 8.5 m north east of Va ran am church. Major part of the area is covered by 2-4 m depth contour. There are 2 elongated patches with water depth > 6 m. In the northern part of the area there is a narrow constriction and water flows with force forming channels. There are 2 small shallow patches (shoals) in the central part of the study area. The marginal areas close to shore are shallow. Tidal range in the lake is 0.2 to 0.6 m. Sediments-Distribution and interpretation of bottom sediments was discussed earlier by Veerayya and Murthy", Most of the sediments in the estuary consist of different admixtures of clay, silt and sand. Coarsest sediment is confined to just south of Tannirmukkom bund and clay in the central portion. The deepest portion 10 the channel also consists of coarser material. The core Cl shows a thin sandy zone on the top (3 ern thick) followed by a clayey sand layer. Below the clay zone there is sand and silt. Bottom-most zone is composed of clay with a thin shell zone 5 cm thick on top. The bore hole logs (Fig.2) were available from the PWD office at Tannirmukkom bund 5. The sediment types vary from mixtures of sand and clay mainly. In 4 holes clay with coal (perhaps lignite) and sand were also reported as shown in the figure. The thickness of sediment varies from 34 to 63 m before hard bottom is reached. The sediment composition and texture are related to geology, bathymetry and physical factors of the estuary. Sediment supply in this part of the lake is mainly by tributaries of Pamba, Chengalam, Kavan and Kaippuzha rivers. Since the tidal range is small, river transport brings much of the material. Fresh water inflow from the land as well as tidal exchange of water through the estuary mouth and wave action mainly control the sediment deposition. Most sediments are terrigeneous in nature, in the form of silts, clays and their admixtures. Biogenous sediments in the form of shells are very important in certain portions of the lakes where the shells are being mined. Mineralogy- The minerals of the lake are grouped into allogenic, endogenic and authigenic (Table 1). Quartz is the major constituent of the sediments. Heavy minerals were separated in bromoform and the quantity of heavies vary from 0.5 to 47.9% in the sediments. Maximum quantity was noticed 10 southern part. The major rock types of the area belong to the Precambrian crystallines, Tertiary and Quaternary sediments. Tertiary sediments occur as 159

2 INDIAN J. MAR SCI., VOL. 13, DECEMBER 1984 INDEX MAP rtlnnirmukkam SH~RTALLAI SOUTH R,\ {,..... ~:------'\\ \,- 35 CHENGALMI.R UTH ARABIAN SEA SC ALE Km.1 0 2Km..I ~I====~ _I INDEX >6 METRE ~ 6-4 METRE 4-2METRE., I:~.:.:.:I< 2 METRE Fig. I-Bathymetric map of the southern part of the Vembanad lake 160

3 MALLIK & SUCHINDAN: SEDIMENTOLOGICAL ASPECTS OF VEMBANAD LAKE N 1==l CLAY D SAND WITH SHELL Li] SAND FINE I:-=g SANDY CLAY INDE X DIID CLAYEY SAND HH!I SANDY CLAY COAL AND SHELLS EIH:H ~ SOFT ROCK m HARD ROCK ~ CLAY WITH ~ COAL AND SAND ~ SAND MEDIUM Fig. 2- Bore hole logs in the Vembanad lake Minerals" Silicate Heavy minerals Fe Oxides Phosphate Clays Carbonates Sulphides Table 1- Mineralogy of Study Area Allogenic Quartz, feldspar muscovite, biotite Sillimanite, hornblende, tremoiite/ actinolite, garnet/ sillimanite, epidote, monazite, zircon Opaques/ihnenite magnetite, rutile Apatite Illite, kaolinite, chlorite "Framboidal pyrites constitute authigenic minerals Endogenic Some endogenic Some endogenic Calcite, aragonite mainly from shells sand and clay intercalations" and western part is covered by alluvium. The minerals have been derived from the charnockites, granite gneisses, khondalites and pegmatites of Western Ghats and adjoining foot hills. The framboidal pyrites in the samples is indicative of reducing character which has been caused by decay of organic matter. They indicate an early diagenetic origin. Chemical analysis-chemical analysis of the grab samples is given in Table 2. The sediments are characterised by high amount of organic matter, Fe, Mn, phosphate and Ca. Origin of lake on the basis of landsat imagery- From the satellite imagery trace of the Landsat Frame E of the EROS data Centre, USA, it is clear that the lake is elongated and oriented in nearly direction. The western part of the study area is also marked by a number of beach ridges nearly paraliel to the coast and boundary of the lake margin in some places. The orientation of the beach ridges varies from l O"_190 0 to and the orientation of the beach ridge at various parts A, B, C, 0, E, F is 161

4 INDIAN J. MAR SCl., VOL. 13, DECEMBER 1984 Table 2-Chemical Analysis of Grab Samples from Vembanad Lake (Southern Part) [Results based on 88 analysis are expressed in ppm except Fe, K, Mg, Phosphate and organic matter where they are expressed in per cent] Metal Range Average value Group A PART OF LANDSAT FRAME E , RIENTATION OF BEACH RIDGE A 15!j" B C D 2-182" E Chloride Bicarbonate Sulphate Phosphate Nitrate Sodium Potassium Calcium Magnesium Organic matter 150- WOO o Group B Fe Na K Mg Pb Cd Zn Mn ~ ~ ~.H Ni Co Ti Cr V Sr Phosphate Analysis of samples in Group A was done by preparing the solution ofa known weight of the sample by leaching with water. Residue left from the above is treated with 8 M HN0 3 and H to prepare the solution for Group B. 9 ARABIAN SEA shown in Fig.3. The beach ridges are due to repeated regression and transgression of the shore and orientation has been controlled by the change in deviation of the waves. At the place marked G the beach ridges are merging in the coast and may indicate an initiation of the spit development. Faulting, uplifting and downwarping are recorded in a number of places in the western coast and offshore, the geometrical linearity of the coast line and the straight trend of some of the rivers clearly reflect a tectonic role in the formation, which are clear in the landsat imagery trace (Fig.3). It is also suggested that the Achankoil shear zone and Muvattupuzha shear zone may have an extension in offshore and Vembanad lake may be considered as an embayment of a large fracture zone in the offshore 7 Fig. 3- Trace of landsat imagery The western coast line ofindia is not stationary at its present position and it shifts according to fluctuations in the sea level. Maximum eustatic sea level change recorded was about 150 m and several submerged terraces can be marked at depths of 110, 92, 85, 75, 55 m indicating fluctuating conditions". In the outer shelf, relict carbonate sand and carbonate rocks in the 162

5 MALLIK & SUCHINDAN: SEDIMENTOLOGICAL ASPECTS OF VEMBANAD LAKE form of algal and oolitic limestone of Holocene age have been recorded, which are outside the influence of terrigenous deposition and perhaps represent the westernmost limit ofindian shore. Nair and Hashimi" suggested a change from arid to moist climate during Holocene from the pattern of carbonate distribution. The activity of rivers was also vigorous at that time. Rivers like Periyar, Muvattupuzha and Pamba show widening and delta like discharge pattern in the satellite photographs. The river patterns and geomorphological set up suggests that coast line was much towards the east (Fig.3 dashed line) during some time and marks the eastern limit of Vembanad lake and low lying lands. The rivers were flowing to the west and debouching in the sea. Offshore bars started building up from the point G (Fig.3) and gradually obstructed the stream mouths and water bodies formed controlled by shore features. Major streams like Muvattupuzha, Periyar and Pamba are pouring into the sea. A large spit formed gradually and with formation of offshore bar, lagoon was formed which changed the geometry of the coast. The offshore currents in the north must have existed allowing the northward extension of the spit forming the Vembanad lake. The present shape and depth of the estuary was controlled to a large extent by the sediments brought by the rivers and modified by marine processes like waves, tides and currents. Acknowledgement Authors are thankful to Dr Harsh K Gupta, Director for providing facilities. Thanks are due to Mr ASK Nair and Mr M Samsuddin who assisted in the field work and in processing of the bathymetric data, Mr R Ravindran Nair for providing the help, Mr Narendra Babu for carrying out the chemical analysis. References I Murthy P S N & Veerayya M, Indian J Mar Sci, 1 (1972) Murthy P S N & Veerayya M, Indian J Mar sa, 10 (1981) Veerayya M & Murthy P S N, Indian J Mar Sci, 3 (1974) Paulose K V, Indian Minerals, 27 (1973) F S Engineers (P) Ltd Madras, Soil Exploration Report - Bridge cum Regulator I1Ird Stage at Taneermukkom (1979). 6 Mathai J & Rajendran C P, Integrated environmental study ofthe coastal zone between 9' 15' and 9'45' (Kerala) geology and tectonics, Tech Rep (Centre for Earth Science Studies, Trivandrum) 1983, Varadarajan K & Balakrishnan M K, Kerala coast - A landsat's view, Spl Publ No 5 (GSI) 1980,67. 8 Closs H, Narain H & Garde S C, in Continental margins, edited by C A Burk & C L Drake (Springer Verlag, Berlin) 1974, Nair R R & Hashmi M H, Proc Indian A cad Sci(Earth Planet Sci) 89, 1980,