Marine sand resources in the south-west continental shelf of India

Transcription

1 Indian Journal of Geo-Marine Sciences Vol. 39(4), December 2010, pp Marine sand resources in the south-west continental shelf of India P V Sukumaran, E Unnikrishnan, A V Gangadharan, B Zaheer, N M Abdulla, K Kumaran, K V Ramachandran, S V Hegde, N Maran, K K Bhat, M K Rao, A C Dinesh, C Jayaprakash, P Praveen Kumar, N M Shareef & C V Gopalan Marine and Coastal Surveys Division, Geological Survey of India, Mangalore, India [ ddgmarinemanglore@rediffmail.com] Received 20 August 2010, revised 21 December 2010 Considering the impending shortage of construction material, the Marine and Coastal Surveys Division of the GSI, Mangalore has been in the look out for marine sand in the territorial waters and EEZ of the western offshore of India since the early nineteen nineties. Though sandy sediments were identified at many locations on the shelf and slope during several cruises, it was during two of the GSI s cruises on board R.V. Samudra Shaudhikama, (SD and SD off Kollam, ) that a detailed assessment of the distribution of construction sand was made. So far the GSI has executed five cruises exclusively for exploring construction sand off the West Coast. Out of these, two cruises off Kollam have indicated probable sand reserves of 184 million tonnes over an area of 180 sq km to a depth of 1.5 m down from the seafloor and located km away from the coast. The data of the other cruises identified the occurrence of sand deposits off Kannur (267 million tonnes over an area of 500 sq km) and Chavakkad (1336 million tonnes over an area of 543 sq km). Off Kochi- Kollam marine sediments containing >85% silica sand have been identified during cruises SD-207 3, and 216. Besides in an area, 8 km west of Beypore-Ponnani coast, good quality quartz sand was identified in two large basins close to the coastline where the thickness of sand formation exceeds 3.5 metres. [Keywords: Marine sand, Kerala, Continental shelf, Beach sand, Construction material] Introduction With unprecedented demand for construction sand particularly in the State of Kerala and the Government imposing restrictions on mining of on-land resources, it has become necessary to look for alternative sources. Though there are many alternatives proximity-wise and extent-wise, marine sand stands as a more promising alternative and therefore the Marine and Coastal Surveys Division of GSI, sensing the future requirements, commenced exploration for this resource way back in the 1990s along with exploration for heavy mineral sands particularly off the Kerala coast. Though many hurdles still remain before it is exploited this is a potential source of sand that will need to be exploited in the future. Materials and Methods Distribution pattern of sediments in the Kerala offshore The cyclic advances and retreat of polar ice sheets during the Pleistocene (10,000 years to 1.8 m y ago) affected the coastline all over the world. The consequent fluctuations of the sea level shifted the beaches to and fro from their present position. The last phase of glacial advance called the Last Glacial Maximum (LGM) occurred about years ago when the sea level stood ~120 m below the present, at the edge of the continental shelf. Obviously the beaches during the LGM were km away from their present location. The LGM was followed by global warming that resulted in glacial thawing and intermittent rise of sea level. The beaches were submerged as the sea level rose but were stabilized when the sea level became steady. Consequently the beach sands were mixed with silt, clay and biogenic carbonates deposited later. The relict sand bodies that occur submerged in the offshore thus represent palaeobeaches formed during the LGM. Sediments consisting mainly of clays and silt deposited by the present day rivers (rivers flowing after stabilization of the sea at the present level ~ 7000 YBP) cover the shallow sea on the eastern part (Fig. 1). The inner shelf off southern Kerala in general is steeper compared to the relatively flat but undulating Fig. 1 Schematic section across the inner continental shelf off Kerala.

2 SUKUMARAN et al.: MARINE SAND RESOURCE INDIA 573 outer shelf. During the LGM the western coastline was about 100 km west and 120m m below the present. With progressive deglaciation following the LGM, the sea started transgressing with a number of fluctuations that left their signatures in the form of palaeo-coastal plains, wave cut terraces and ridges, all now occurring as submerged features. The transgressing sea eroded the beach front and some times the softer coastal sediments over which it transgressed. Depending on various factors the transgression of the sea over the beach sediments often retains the beach as it is, but most often it transports the sandy sediments on the beach and the advancing beach front landward. In some instances the sandy sediments of the submerged beach were buried by marine clays deposited over them later. Calcareous components comprising shell fragments and tests of marine organisms dilute the terrigenous component. Chemically- and biologically-induced precipitation further modifies the bulk composition of the sediments. Owing to these processes the palaeobeach sediments rarely retain their original beachderived character. Advancing wave front, depending on local conditions, might erode the beach sediments and under such circumstances, instead of the usual depositional beach landforms, erosional land forms like wave-cut-terraces would be retained representing the position of palaeo-beach. Off the Kerala coast evidences of past coast-line are expressed in various ways. Geomorphological expressions in the form of wave-built-terraces are reported at depths of m, m and 55 m below the present sea level representing the position of the advancing beach front at different periods in the past 5-7. Palaeentological evidence in the form of shallow water carbonate deposits recovered from deeper depths has been well documented 8. Besides, lagoonal clays containing plant remains indicating shallow-water deposition have been described from many offshore locations (GSI unpublished report, 2009). Shallow water chemical precipitates like gypsum and carbonate-cemented beach rocks are also abundantly documented from offshore sites 7. The surface distribution of sandy sediments within the territorial waters (TW) and Exclusive Economic Zone (EEZ) off the Kerala coast is presented in Fig.2. These sediments are intermixed with varying amounts of finer components like clay and mud. In general the sandy sediments on the eastern part of the shelf are diluted by terrigenous clays and muds, delivered by the present day rivers. In the western part the sand fraction has considerable carbonate sand made up of broken shell fragments and foramininferal tests or chemically precipitated carbonates ooides often coated with glaucony and verdine facies minerals. The thickness of the sandy sediments shows wide variation depending on the original configuration of the coast over which the sea advanced, but by and large beyond 50 m water depth the sand formations are very thin ( cm) underlain by a pre- Holocene substratum composed of very hard and compact ferruginous clay. The distribution of sandy sediments is influenced by many factors. In the offshore, south of Kollam the sandy sediments are present from 500 m water depth (125 km from the coastline) to the present coastline. It extends beyond the Quilon plateau, a flat area between 500 and 100 m water depth off Kollam. However, the sediments here are carbonate-rich and the carbonate content increases towards south. Off Kanyakumari the sediments are composed chiefly of carbonate sand. The dynamics of the ocean currents is believed to be one reason for the absence of silica sand and dominance of carbonate sands over there 9. Fig. 2 Map showing the distribution of sandy sediments off the Kerala coast.

3 574 INDIAN J. MAR. SCI., VOL. 39, NO. 4, DECEMBER 2010 Absence of large rivers bringing sediments to the shelf south of Kollam may be yet another reason. The sandy sediments off Kannur-Calicut and Eranakulam- Kollam extend up to 500 m water depth, as far as 100 km away from the present coast. Here the width of the shelf mud area is around 30 km. The narrowest part of the shelf mud zone is between Calicut and Kochi where the width is around 20 km and reaches up to a depth of 20 m water depth. North of Kannur the contact between the sand formation and the clay sediments of the Lakshadweep trough further west occurs 100 km away from the present coast. The shelf mud is about 30 km wide over here. Two patches of sand body, occurring in an elongated pattern, are found off Kochi-Kollam at a water depth of 25 m surrounded by clays. These sand bodies lie parallel to the coast and are 5-10 km wide. Sediment cores raised from the clay zone beyond the sand body reveal that the sand body extends further west under the clay formation to deeper depths. Though the areal extent of the sand bodies off the Kerala coast is very large, the percentage of silica sand they hold is not uniform and varies greatly from place to place. Because of these factors locating palaeo-beaches in general and those carrying good amount of silica-sand suitable for constriction purposes in particular is a very difficult task. Systematic exploration by seismic profiling followed by raising core samples in a grid pattern is the method resorted to for resource evaluation. Status of sand exploration The Marine and Coastal Surveys Division of GSI is engaged in surveying the seabed in the TW and EEZ of India. A distance of 22 km (12 nautical miles) from the shoreline into the sea is the limit of TW. The EEZ extends to about 380 km (200 Nautical miles) from the shoreline (Fig.3). The deep sea research vessel Samudra Manthan is deployed to carry out marine surveys in the EEZ along both the east and the west coasts of the country while the coastal vessels Samudra Shaudhikama and Samudra Kaustub operate in the TW off the west and east coasts respectively. Though sandy sediments were identified at many locations in the shelf and slope off the West Coast it was in two R.V. Samudra Shaudhikama cruises (SD and SD ; ) off Kollam, that sand formations were first identified. These surveys were purely of academic interest, to delineate the paleostrand lines and to study the late Quaternary sea level changes. During this study occurrence of large patches of submerged sand was detected in the midto-outer continental shelf, km from the coast. Subsequently, during F.S and , two cruises were undertaken in the mid-shelf (20-80 m water depth) off Kollam, to evaluate the sand resources, during which about 180 km 2 area of the seabed was investigated by sampling and shallow seismic surveys. The studies indicated the occurrence of fine to medium sand ( mm) to a depth of 1 to 1.5 m below the seafloor in the surveyed area. On the basis of this finding, search for construction-grade sand was taken up subsequently in a systematic way in different sectors off the Kerala coast, considering the necessity of identifying alternative sources of construction sand in the State. In addition, many cruises launched for academic purposes and heavy mineral investigations within the TW were executed with the additional objective of evaluating the sand resources. These cruises have also brought out promising sand deposits. A list of cruises carried out for construction sand off the Kerala coast with tentative resource potential is presented in Table.1. Fig.4 shows the location of the investigated areas. So far the GSI has executed five cruises exclusively for exploring construction sand off the West Coast. Out of this two cruises off Kollam have indicated probable sand reserves of 184 million tonnes over an area of 180 sq km down to a depth of 1.5 m from the seabed and located km away from the coast. The data of the other three cruises taken up during the last 2 years are under different stages of processing, but preliminary results are very encouraging. Another 8 cruises were aimed partly for the identification of favourable areas of sand occurrence either in the prescribed survey area or nearby. In this way sand deposits were found offshore Fig. 3 The areal extent of TW and EEZ of India.

4 SUKUMARAN et al.: MARINE SAND RESOURCE INDIA 575 Table 1 Details of the sand investigation cruises in the Kerala offshore and tentative resource position Sl. No. Cruise No. Field Season Location Area (km 2 ) Sample grid (km) Sand thickness Sand resource (million tonnes) Non-carbonate sand (million tonnes) 1 SD Off Kollam m SD Off Kollam m SD Off Chavakkad and off Ponnani (2 blocks) 100, Lab work and estimation of sand resources are in progress 4 SD Off Kollam Do- 5 SD Off Chavakkad 2 2 This is the continuation of the work carried out for SD 214 in this block and the lab work is in progress. 6 SD Off Ponnani (Part) m SD Off Kannur (Part) Based on surface samples, the possible sand resource is 267 million tonnes. 8 SD Off Ponnani Sand zone is confined to a 30 sq.km block in the NE corner of the cruise area 9 SD Off Beypore-Kannur The area contains no promising blocks of sand. 10 SD Off Chavakkad m SD Off Kochi 4 5 In the western part of the area 1110 million tonnes of 63.5 to 89.5% non-carbonate sand resource was estimated; the thickness of the sand body varies from 0.5 to 3.5 m at water depths of m. 12 SD Off Alapuzha Of 1000 sq.km area covered, 225 sq.km is promising with 85% non-carbonate sand of average 2 m thickness. 13 SD Off Kollam 2 3 Presence of relict sediments with > 85% silica sand beyond 45 m isobath indicated; > 50% of the sand is medium to coarse grained ( mm); < 1% heavy minerals; ideal for construction purposes. Fig. 4 The Kerala offshore areas surveyed by GSI for construction sand. of Kannur (267 million tones over an area of 500 sq km) and Chavakkad (1336 million tonnes over an area of 543 sq km). Off Kochi-Kollam marine sediments bearing >85% silica sand have been identified during cruises SD-207 3, and 216. In an area 8 km west of Beypore-Ponnani coast good quality silica sand is identified in two large basins close to the coast where the thickness of sand formation exceeds 3.5 m. Sand quality The quality requirement of sand and aggregate depends on their use. For concrete mixing, gravel and coarse sand are preferred, whereas for cement plastering and flooring medium to fine sand is used. Other parameters considered for sand quality are the amount of finer fraction (clay and mud), carbonate content, organic matter, plant remains, salt content and type and shape of quartz grains. For executing mining strategies the thickness of the sand body and its depth persistence are also matters of consideration. Based on these parameters the sandy sediments in the deeper, western part are relatively finer and fall in the fine to medium sand category. The carbonate content is 10-25% but in certain areas the sand bodies carry 10-15% clay and silt. In certain other sectors clay

5 576 INDIAN J. MAR. SCI., VOL. 39, NO. 4, DECEMBER 2010 content exceeds 25%. The sand also carries economic heavy minerals like ilmenite, sillimanite, garnet, rutile, zircon and monazite in small proportions. Because of their long residence time in the beach environment the sand grains are relatively more rounded compared to river sands. Being marine sands they invariably carry sea water in their inter-granular space that precipitates chlorides as coatings over the sand grains on drying. Carbonate fraction in the sand is sometimes coated with a green mineral known as verdine that makes the sand unsuitable for construction purposes. Though the quality of sand present in the TW is better with respect to carbonate impurities, they contain undesirably higher clay and mud fractions. The clay and mud components in the sand also show much variation, particularly along the north-south direction. North of Kochi the sand is more clayey but the clay fraction shows much variation locally. Unlike the relict sands described above the two large sand bodies discovered off Ponnani and Beypore were deposited in buried channels of pre-existing rivers flowing through the area. From the configuration of the sand body and from the sediment characteristics these bodies could be correlated to the present day river channels of Ponnani and Beypore. When the sea level eventually rose and the coastline advanced landward these river channels encompassing thick sand deposits were submerged. Parts of these sandy sediments are now capped by clays and other fine sediments deposited subsequently by the present day rivers flowing into the sea. Being channel sediments, these sands are better in quality compared to the relict sands present farther offshore. They are coarser, more angular and carry much less calcareous components, making them comparable to the sand being mined from the channels of present day rivers of Kerala. Deposited by large rivers, the areal extent of the offshore channels and the thickness of sand layers are also considerably large. The sand resources estimated here are of the order of 936 million tones over an area of 450 sq km lying 8-20 km away from the coast. Mining of offshore sand Nevertheless, like the on-land mining, offshore mining activity also has environmental problems associated with it. The major concern is the impact of mining on marine life in the surrounding sea either immediately or over a period of time. More pertinent is the question of whether the sand mining will affect the benthic biota while pelagic communities can migrate to nearby niches. Obviously extensive studies are required to be conducted before contemplating any offshore mining. However, some impact assessments can be made from the existing information on these offshore sand resources. Here sand is proposed to be excavated from a water depth of 25+ m and at least 10 km away from the coast. The shelf slope in the areas specified for mining is negligibly low, less than 1 degree and hence slope failure due to mining either at the mining sites or at the distant shores can be ruled out. Since the sand thickness does not exceed 2-3m the pit formed by the excavation would be shallow and would not affect the wave dynamics of the area. Being far away from the coast beach erosion due to mining can be totally ruled out. Finer sediment particles are relatively less in the sandy sediments 1. Therefore persistent sediment plumes in the vicinity at the time of mining are not expected to be generated. The sediment plume that would be generated by washing can be reduced by adopting modern techniques of sediment dispersal or can even be stopped by salvaging the fines to be used in the coastal areas for back filling of abandoned pits. Thus these minor environmental effects shrink into insignificance by the more serious adverse environmental effects of river sand mining. It is also to be understood that excavation, washing and transportation of offshore sand are easier and cheaper than land-based sand mining operations. Thus, offshore sand mining will eventually have an advantage over land-based mining and thereby preserve inland- and coastal environment. Environmental evaluation studies carried out elsewhere in the world Withycombe, et al. 17 show that offshore sand extraction with a Trailing Suction Hopper Dredger (TSHD) is the most suitable mining method with least adverse effects. Carbonate sand/debris which are remains of shelled organisms consist of chemical-grade carbonates of high value useful for a variety of industrial applications. Shell carbonates also form the raw material for the manufacture of white cement. Carbonate component of seabed sediments thus form valuable byproducts of sand mining and can be recovered by appropriate techniques. Similarly these sands also contain low concentrations of valuable heavy minerals such as ilmenite, sillimanite, garnet, rutile, zircon and monazite that can also be recovered as useful by-products.

6 SUKUMARAN et al.: MARINE SAND RESOURCE INDIA 577 Processing and marketing The dredged sand needs be washed in the offshore itself to remove the fines before transporting to the shore. The sand needs to be washed again with fresh water to make it salt-free. This is not at all a problem. Plenty of rain water is available throughout the State during monsoons which can save the large amount of fresh water required for washing the sands. Laboratory washing experiments confirm that 3-4 cycles of washing remove more than 99% of salt in the sand. The sand transported to the shore by barges after the first-stage-washing offshore can be heaped into open enclosures onshore and left there for one monsoon that would wash away the entire salt. Quality sand prepared grade-wise by sieving through a set of standard sieves can be marketed in small bags of 25 or 50 kg or can be marketed mixed with cement as ready-to-use sand-cement mixture. This will reduce avoidable wastage of sand at construction sites. Kerala has a number of minor ports all along the coast and therefore washing plants can be set-up anywhere along the coast. A couple of washeries built at regular intervals along the coast can cut cost of on-land transport of sand. Local people currently engaged in mining of sand from riverbeds can be effectively employed in this venture as an alternative employment avenue for them. The mining and marketing can also be made successful on cooperative ventures. In order for the offshore sand to become the long awaited alternative for river sand, its price should be competitively lower than that of river sand to rejuvenate the construction industry. In addition to an initial subsidised price, educational, training and public-relations campaigns are necessary to familiarize the market with this supply. Once the commodity has become widely accepted these campaigns and the subsidy can be gradually withdrawn. Offshore sand can then be competitively supplied to the market. Offshore sediment is currently being mined in many countries like France, Australia, African countries, Far East and the Gulf countries for sand and other valuable minerals. Diamond is being mined from off the South African coast, barite off the Alaskan coast, tin off the coasts of Thailand and Indonesia, oyster shells off the Gulf Coast of the United States, and sand and gravel off the coast of the United Kingdom. Other countries will also follow suit as many of them have already exhausted their limited land-based resources. European countries are banking heavily on marine sand to protect their coasts from the impending dangers of global warming and sea level rise. Large quantities of sand are required for construction of artificial sand dunes and erecting artificial beaches all along the coast; and there is no other alternative for these other than the offshore sand. Therefore the pace of marine sand exploitation is bound to increase in the future throughout the world. Conclusions The marine sediments present off the Kerala coast between 20 and 80 m water depth carry appreciable amount of silica sand at places. Systematic mapping of the seabed and exploration at selected areas have indicated the presence of huge reserves of construction-grade sand in different sectors. Meticulous planning, implementation of corrective measures and thorough monitoring of mining activities are essential for the safe exploitation of these offshore resources. The GSI is in the process of carrying out environmental impact assessment of offshore sand mining in collaboration with other organizations dealing with biological oceanography. Offshore mining is going to be a nascent activity in India. Though, Offshore Areas Mineral (Development and Regulation) Act (2002) is already enacted by the Parliament, the guidelines and procedures are yet to be finalized. The GSI has already chalked out a roadmap for the development of offshore resources discovered by it. It is expected that by the time the rules and guidelines are finalized, the GSI will have detailed database of exploitable sand resources in the Indian waters ready for exploitation and this will ease the pressure on the dwindling land-based construction sand resources in States like Kerala. Acknowledgement Authors are grateful to Additional Director General, Mission-IA, GSI for permission to publish this paper. The data for the present compilation are drawn from various cruise reports generated by GSI on the voyages of its research vessels R.V. Samudra Manthan and R.V. Samudra Shaudhikama. Authors acknowledge the help rendered by participants of the above cruises. References 1 Adiga K S, Venkataramana P, Kannan S, Ramachandran K V, Gangadharan A V, Mukkerjee G S, Mukkherji K K, Patra S & Kartha K N K, Delineation of Palaeo-strandlines

7 578 INDIAN J. MAR. SCI., VOL. 39, NO. 4, DECEMBER 2010 and study of Quaternary Sea level off south Kerala Coast, SD-153, Chandra S, Ramachandran K V, Unnikrishanan E, Gangadharan A V, Mukherjee G S, Mukherjee K K, Chaki S & Kartha K N K, Delineation of Palaeo-strandlines and study of late Quaternary sea level changes off thotapally Kerala, Geological Survey of India, Unpublished Report of Cruise SD-160, Hegde S V, Maran N, Bhat K K, Dinesh A C, Praveen Kumar, Kar Y C, Ramesh Singh & Joseph P J, Report on parametric studies within Territorial Waters of India in the Arabian Sea between Azhikod and Ambalapuzha, Kerala, SD-207, Gangadharan A V, Bhat K K, Jayaprakash C, Shareef N M, Sivaraj K, Sammaddar S R, Mukherjee K K, Naskar B N & Premkumar P, Report on parametric studies within Territorial Waters of India in the Arabian Sea between Chellanam and Ambalapuzha, Kerala, Geological Survey of India, Unpublished Report of Cruise SD-209, Purnachandra, Rao, V. & Wagle, B.G., Geomorphology and surficial geology of the western continental shelf and slope of India: A review, Curr. Science, 73(1997) Rajan, T.N., Nature of subsurface sediments off Ambalapuzha, Kerala: Implications on sea level change and mudbank formation, Indian Jour. Mar. Sci., 25(1996) Unnikrishnan E, Gangadharan A V & Nambiar A R, Imprints of Holocene transgression on the continental shelf off Chavara, Kerala, Continental Shelves During the Last Glacial Cycle: Knowledge and Applications, Geological Survey of India Special Publication, 2008, No. 96, pp Purnachandra Rao V, Rajagopalan G, Vora K H & Almeida F, Late Quaternary sea level and environmental changes from relic carbonate deposits of the western margin of India, [(Proc. Indian Acad. Sci. (Earth Planet. Sci.)] 2003, Vol. 112, pp Hashimi, N.H., Kidwai, R.M. & Nair, R.R., Comparative study of the topography and sediments of the western and eastern continental shelves around Cape Comorin, Indian Jour. of Mar. Sci., 10(1981) Zaheer B, Abdulla N M, Ramachandran K V, Gangadharan A V, Jayaprakash C, Satyanarayana B, Dutta G K & Joseph P J, Report on the preliminary evaluation of sand bodies off Quilon Kerala State, Geological Survey of India, Unpublished Report of Cruise SD-187, Kumaran K, Gangadharan A V, Dinesh A C, Jayaprakash C, Praveen Kumar, Sathishkumar, Kar Y C & Biswas S C, Report on the preliminary evaluation of sand bodies off Quilon Kerala State, Geological Survey of India, Unpublished Report of Cruise SD-196, Unnikrishnan E, Hegde S V, Bhat K K, Satishkumar, Mukherjee K K, Ramesh Singh & Banerjee T K, Parametric studies within territorial waters off Kerala coast between Beypore and Ponnani, (Part of Kannur-Azhikode project), SD-195, Unnikrishnan E, Hegde S V, Rao M K, Dinesh A C, Shareef N M, Sivaraj K, Sathyanarayana G V, Ramachandra Rao P & Premkumar, Parametric studies within territorial waters off Kasaragod to Mt. Dilli, Kerala, Geological Survey of India, Unpublished Report of Cruise SD-200, Abdulla N M, Unnikrishnan E, Dinesh A C, Praveen Kumar P, Mukherjee K, Singh R & Ghosh A K, Preliminary evaluation of relict sands beyond territorial waters in mid and outer continental shelf off Calicut Ponnani, Kerala, SD-203, Abdulla N M, Unnikrishnan E, Maran N, Rao M K, Dinesh A C, Rao J A N, Datta G K, Veerabhadraiah D & Das A, Preliminary evaluation of relict sands beyond territorial waters off Kerala Coast between Beypore and Kannur, SD-210, Ramachandran K V, Maran N, Gangadharan A V, Shareef N M, Kar Y C, Dutta G K, Joseph P J & Premkumar P, Progress report on parametric study of the seabed within the Territorial Waters off Kannur-Azhikode and Chavakkad- Azhikode sectors, Kerala, Geological Survey of India, Unpublished Report of Cruise SD- 202, Withycombe G, Walker J Nielsen L, Pinzone T & Morrison C, Scoping Study for Extraction of Offshore Sands for Beach Nourishment: Outcomes, Lessons Learnt and Ways Forward, Report of Sydney Coastal Councils Group Inc. (SCCG), 2009.