IJMS 29(2) 120-127.Pdf

Indian Journal of Marine Sciences Vol. 29, June2000, pp. 120-127 . I , 'J f' i Sediment distribution and placer mineral enrichment in the inner shelf of 'Quilon, - - - - - ~ ~W coast of lndia ..- ! ~.N j> rak as h ·' /( Centre for Earth Science Studies, Akkulam, Tri vandrum 695 031 India ~ / Received 28 Febnwl)', 1999, revised 15 March 2000 G urficial sediment samples from th e innershel f off Quil on, SW coast of India have been examined for the sediment type, pl acer di stribution and titanium content. A major porti on of the in nershelf is covered with sand s and si lty clays. The southern shelf and outer-innershelf of the northern shelf are covered with moderately sorted medium sand whereas poorly sorted silty clays occur mostly in the northern shelf. The total heavy mineral content in the shelf varies from I to 12%. It is generall y seen that high percentage of heavy mineral occurs in the sandy sediments. The result s of Q-mode fact or analys is indicate the spati al distribution of factors, which renect th e grain-sorting pattern of heavy minerals in the shelf. The distribution of ti tan ium shows marked variation wi th texture of the sediments. The titanium content is higher in the southern shelf region where sandy sediments dominate. The near-innershelf of the northern shelf contains higher concentration of titanium with a decreasing tendency towards the outer-inncrshelf ind icating th e lithogenous origin . The di stribution of heavy minerals and titanium in the innershclf indicates nel sediment movement towards the northern shelf region where rich concentrations of placer minerals occur along the coast. ) 9!.. ' ( r ov l ~ The inner shelf is considered to be a sink for the Materials and Methods terrigenous sediments supplied mostly by subaerial The study area covers the innershelf zone off erosion and coastal erosion. An interestin g aspect of Quilon of area about 840 km2 with a length of 70 km the innershelf of SW coast of India is its ~arying shelf and width of 12 km (Fig. I). The she If of SW coast gradient and wave climate, which have immense India is characterised by varying wave climate and effect on the sediment distribution pattern. The coast shelf gradient. The hi ghest wave activity coincides bordering this shelf (Fig. I ) is known for beach with monsoon (June-August). This coast falls under 6 placers, which is popularly known as 'black sands'. the moderately high wave energy regime . The Commonly OCC UrrIng minerals are ilmenite, Kallada is a major river system debauching sediments magnetite, ruti le, si llimanite, zircon, monazite, garnet into Ashtamudi estuary, which opens into the sea at etc. Commercial exploitation of beach placers is Neendakara. The coastal strip has lateritic cliffs, being carried out at Chavara along the Neendakara barrier coast and rocky headland. The shoreline Kayamkulam coast. The heavy minerals in the shelf changes its orientation from 2900 to 3500 N at are being suppl ied by the present day rivers and from Tangasseri headland. This demarcates the coast south the submerged paleo-beaches (du ri ng lowered sea of Thangasseri as southern shelf and north of it as level). Though many stud ies have been carried out on northern shelf (Fig.I ). The major rock types in the the origin 1.2, mineral content in the rivers and hinterland regIOn include a Khondalite suite 3 beaches .4 and REE contentS, the distribution of consisting of garnet-sillimanite gneisses, garneti placer-minerals in the innershelf have not been ferous leptynite, garnet-biotite gneisses, charnockite studied hitherto. In th is paper the sediment type, and wi th a few granite and pegmatite as instrusives placer distribution and titanium content in the and a narrow strip of Tertiary sediments on the coast. innershelf, limited to a water depth of 50 m have been During the period 23-31 January 1985, usi ng a studied and the physical processes responsible for th e van-Veen grab, 77 surficial sediment samples were enrichment of placer minerals is discussed. col lected at 2.5 km interval along I J transects spaced roughly at 5 km interval (Fig. I ). T he position of the stations was fixed by theodolite. Lead sounding was *e- mail : [email protected] used to collect bathymetric data at each station . All PRA KAS H: SEDIM ENT PLACER MI NERAL EN RI CHMENT 12 1 is covered by sands and silty clays while fine-grained \ ones (clay) occur in the northern most shelf. The \ \ southern shelf is entirely covered by sand. Also, there are a few sandy patches off Chavara and in the outer innershelf of the northern shelf. The rest of the shelf Km3 0 3 6 9Km is covered by silty clay interrupted by scattered I I I J I Sediment samples - occurrences of sand/silt/clay and clayey silt units. The Chavar a place r deposit" di stribution of the principal modes of coarse fracti on (Fig. 2B) demonstrates that the area is covered by fin e "-l7 0 (2-3 <1» to medium sand ( 1-2<1» category. The southern ?;> \ \ shelf is carpeted by medium sand of moderately \ \ sorted nature whereas the outer-Innershel f is dominated by moderately to poorl y sorted medium 9\ sand. The rest of the shelf is covered by poorl y sorted 0" \ , silt and clays. In general the sediments are getting ,, "- progressively fin er towards the northern side. "-, , Heavy mineral distribulioll - The total heavy \ \ mineral content in the innershelf sediment samples ranges from I to 12%. Since the heavy mineral concentrates more within the fin e grained fractions the surficial vari ati on of heavy minerals in the fi ner fracti on is given in Fig.3A whi ch shows that the high concentrati ons are fo und in the southern shelf, outer innershelf of the northern shelf and in the nearshore Fig. I-Locati on map zone of Chavara. Here the heavi es are mostly the surficial sediment samples were subjected to associated with the moderate ly sorted medium sand 7 sedimentological analys is ,8 . Textural parameters sediments. However, heavy mineral content was less were obtained usin g graphi c method sugGested by in the silty clay and sand-silt-c lay type of sediments. 9 . b Folk & Ward . SIxteen selected samples were The relati ve di stribution of di fferent heavy minerals subj ected to mineralogical studies. In th e laboratory in the innershelf sediments is shown in Fig.3B. The the sampl es were sieved and heavy min erals were southern shelf is characteri sed by opaques (av.36%), separated from the fractions of 1.0-0.088 mm and sillimanite (av.22%), monazite (av. 12 %), zircon 0.088-0.062 mm usi ng bromoform (sp.gravity 2.89). (av.6%), hypersthene (av.5%) and garn et (av.5%). T he heavy minerals were washed with methyl alcohol An interesting feature in this part of the shelf is the and mounted in glass slides. Nearly 300 grain s were seaward increase of relati vely stable minerals, i.e. , counted in each sli de under a petrological mi croscope zircon and monazite. T he less dense mi neral and the percentages of indi vi dual minerals were hypersthene and hornblende increased considerabl y calcul ated. T hese data are used to prepare the off Ashtamudi estuary. The di stribution of heavy distribution maps. In order to un derstand the total minerals in the outer-innershelf of the northern shel f vari abili ty of heavy min erals in the samples and to is similar to that of the southern shelf except that it group them in to assemblages, Q-mode factor analysi s had undergone hi gher degree of chemical alteration as l 2. was carried out on the heavy minera l data 10 usin g seen from the surface mi crotextural study The ll SPSS package . Factor loadin g matri x was taken distribution of heavy minera ls in the northern shelf usin g varimax rotation techn iq ue. Selected bu lk shows an average content of opaques (35%), sampl es were analysed for titanium uSll1 g atomic sillimanite ( 15%), monazite (8%), hypersthene absorpti on spectrophotometer (Perkin Elmer-4000). ( I 1%), and hornblende ( 10%). In th is part of the shelf th ere is an increase of less dense heavy mi nerals like Results hypersthene and horn blende. Sedim en t types- The distribution of the various Factor analysis- The results of Q-mode factor sediment types is shown in Fig.2A. Most of the area" analysis show three important factors, whi ch account 122 INDIAN J M AR SCI., VOL. 29, JUNE 2000 INDEX Weight pe rcen toge sond F-- -j <5 §5 -25 ~25-50 81 50- 75 ~>75 Fi g. 2-Sediment distribution patt ern (A) and di stribution of coarse fracti on sediments (8 ) for 97% of variancc. Factor loadings are shown in the epidote. High loadings off Tottapally indicate the form of isopleth maps (Fig. 4 and Table I). Factor-I occurrence of hornblende. Epidote is prevalent in the explains 86. 14% of th e total variance. Factor-I is southern and northern shelf of the study area. representative of a clustering of opaques, sillimanite, Titanium distribution-The data show marked monazite, zircon and garnet. The hi gh-density variations in the titanium content of the sediments minerals of factor-I have high loading (Table I) with the texture. The distribution of ti tanium (Fig.S) which reflects the variation between the southern indicates high content in the southern shelf and near shelf and the outer-innershelf suite of mineral s innershelf of the northern shelf region.

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