Heavy mineral in surface sediments of the inner continental shelf, Besant Nagar and Kovalam sectors, southeastern coast of India: grain size, spatial distribution and provenance

The surface sediments in the inner continental shelf of the Bay of Bengal between Besant Nagar and Kovalam sectors, Tamil Nadu, southeast coast of India, have been investigated for their heavy minerals, distribution and source. The inner shelf floor is carpeted by sand with varying grain sizes from coarse, medium and fine to very fine sand. The heavy mineral content of the surface sediments ranges from 0.19 to 4.05 wt% with an average of 1.10 wt%. Heavy mineral enrichment is noticed in the fine-grained sands exhibiting moderately sorted, near symmetrical to strongly negatively skewed and meso-to leptokurtic nature. Relatively higher abundances of amphiboles, orthopyroxenes, clinopyroxenes, garnet, epidote, kyanite and staurolite occur in the coarser size (> 250 µm) grade, whereas the predominance of opaques, with transparent minerals such as sillimanite, rutile, sphene and tourmaline occurs in the medium size (250 to 125 µm) grade. A relatively higher abundance of monazite and zircon occurs in the finer (125–63 µm) size grade. A major proportion of the light-heavy minerals viz., amphiboles, orthopyroxene, clinopyroxene and sillimanite shows a northward increasing trend between 10 and 20 m water depths, which could be attributed to the northerly drift of lighter heavies from the sediments by longshore current. The heavy mineral assemblage constitutes ilmenite, magnetite, amphiboles, pyroxenes, garnet, sillimanite, epidote, zircon, monazite, kyanite, staurolite, sphene, tourmaline and corundum, suggesting a provenance of mixed igneous and metamorphic rocks of Archean and Proterozoic age, reworked sedimentary rocks of Gondwana, Cretaceous and Cuddalore formation and coastal quaternary sediments of the hinterland. The northerly littoral drift might have brought these heavy minerals and redistributed them in the study area.