Small rocky outcrops, close to the seawater front exist at Kovalam beach (N 12° 45' and E 80°16'), of Kovalam creek. The rocky outcrops are garnetiferous charnockite/granitic gneiss with coarse to medium grained granitic texture. The strike is nearly E-W while the foliations are nearly perpendicular to the strike direction. Small basic patches are present all over the exposures. This reveals the rock was affected by different grades of metamorphism. Exfoliation due to spheroidal weathering of rock masses is seen on the surface of the outcrops. A shallow projection of a bay is seen just north of the rocky outcrops. Southeastern part of the outcrops shows alternative felsic and mafic bands having 5-cm thickness. Rock cleavages and slip planes are seen along biotite enriched portions. A traverse was made along the coast of bay (Fisherman's Cove) to the north of the Kovalam rocky outcrops.  Fig 4 Beach placer minerals along Muttukadu coast At the mid point of the bay, rocky exposures occur in the foreshore region, which are weathered due to wave action. Beyond the berm region, acidic charnockite is present. In the wave cut terrace, sandy layers of heavy mineral bands are formed. Towards the mouth of Muttukkadu creek, all along the foreshore and berm, heavy mineral concentrations occur (Fig. 4). South of Kovalam rocky outcrops, at the coordinates of N 12° 45' 55.7'' and E 80° 15' 3.7'', relatively higher concentration of heavy minerals are found for about 500 m along the coastal stretch. At Tiger's Cave, pink granitic rock exposure shows coarse grained texture with foliation of both acidic and basic nature. East of the cave, alternate wavy patterns of heavier and lighter minerals due to aeolian were observed over the near shore dunes. Further, towards the shore, the beach is wider with alternate light and thin bands of heavy minerals. At Mahabalipuram shore temple, towards south of the bay, heavy mineral concentration is relatively less on the surface. In order to get the enrichment of heavy minerals as beach placers, the optimum conditions are required which should be neither erosive nor accumulative in nature. Such conditions prevail in stable shores, which are signified by the presence of open seashore face, sandy beach, cliffs and high coasts. Further 10 m south, minor patches of heavy minerals are found. In the berm region, alternate heavier and lighter mineral bands with increasing width of heavy mineral bands with depth are noticed. About 70 m south of the shore temple, rock exposures constituted by graphic granite, pyrope garnet with reddish-brown colour and labradorite were observed as shown in Figure 5.  Fig 5 Source rock with heavy minerals along Mahabalipuram coast The north bank of Palar river mouth is in the form of an imbricate network, the way to the sea occasionally being terminated by sand. From the road towards shore, recently abandoned prawn culture farms are present, which have degraded the natural beauty of the coastline. The following heavy minerals were identified, both in thin section and by studying individual grains: 1) ilmenite, 2) magnetite, 3) rutile, 4) garnet, 5) hypersthene, 6) diopside, 7) hornblende and 8) biotite. Detailed Field Sampling The samples were collected in a grid pattern with profiles, each profile being divided into number of stations. The distance between each adjacent station was kept roughly about 10 feet. At every 5 station intervals, coordinates of the station location with sample number were noted using hand-held GPS and sediment samples were also collected.  Fig 6 Coastal Geomorphological map prepared using IRS LISS - III FCC by Visual Interpretation The interval for collection of sediment sample varied where boundary of the profile was less than the interval of 5 stations. The number of profiles were selected depending upon the width of the coastal sand tract from the fore shore region, while the number of stations depended on length of the heavy mineral zone. The sediment samples were collected from the surface to a depth of 5 cm with a minimum weight of 500 g. The collected samples were packed in thick polythene covers of 30 cm x 20 cm size and tied using rubberbands. Similarly, geophysical magnetic surveys were carried out on a grid pattern and at each station the readings were taken and tabulated. For magnetic survey, proton precession magnetometer was used. The sub environments of beach where samples were collected are foreshore, backshore, berm and nearshore sand dunes. |