Heavy mineral suite in the shelf sediments off Madras coast

Abstract

The heavy mineral suite of the shelf sediments of this area (12~'00': 13~'05'N and 80~'00': 80~'36'E), are characterised by a dominant group of minerals such as hornblende, augite, hypersthene, garnet and opaque minerals; common by epidote, zircon, mica and rutile; and rare by staurolite, monazite, and olivine. These appear to be derived chiefly from the charnokitic province of Madras and also another nearby metamorphic and igneous source area. Characteristically, roundness of the opaque grains found at 131 m and 183 m indicates their derivation from the present shore line and the provenance. The abundance of unstable minerals like hornblende and augite at depths of 50 and 131 m (sta. nos. 3 and 7) with corresponding lower values at 73 and 183 m (sta. nos. 4 and 6), in addition to the low clay mineral values at 31, 73 and 183 m (sta. nos. 3, 7 and 5) suggest a rise and fall of the sea level during Pelistocene time, and a possible relict structure being present. Augite is present abundantly between 131 and 183 m in the southern but it extends thorough the middle shelf at 50 m in the north. This suggests two types of sediments (one in the north and another in the south) mixing or overlapping in this area. This gives rise to 'augite rich' and 'augite poor' zones in the study area. Further, authigenic growths in zircon are noticed which suggest a favourable geochemical condition during their formation and a quite water, low energy environment