LATE QUATERNARY HALIMEDA BIOHERMS AND ARAGONITIC FECAL PELLET-DOMINATED SEDIMENTS ON THE CARBONATE PLATFORM OF THE WESTERN CONTINENTAL-SHELF OF INDIA

The Fifty Fathom Flat is a drowned carbonate platform located on the o uter continental shelf of western India. It is unique because the carb onate depositional environment prevailed until Early Holocene in spite of its proximity to a major siliciclastic tidal embayment. Study of t he bathymetry, seismic and sonograph data together with the petrology, mineralogy and geochemistry of the carbonate sediments and limestones recovered from the platform resulted in (a) the discovery of Late Qua ternary Halimeda bioherms and (b) aragonitic faecal pellet-dominated s ediments. Echograms reveal buried pinnacles between 62 and 38 m and ex posed 2 to 14 m high pinnacles between 65 and; 85 m on the platform. T hese pinnacles occur as narrow to broad mounds (bioherms) in seismic p rofiles and as linear ridges and massive structures in sonographs. Sei smic profiles also show a prominent reflector about 25 m below the sea bed identified by boreholes as a Late Pleistocene erosion surface. Th e platform consists of relict carbonate sands and limestones. Rounded and ovoid to ellipsoidal aragonite grains are predominant in the sedim ents. SEM studies indicate elongated parallel canals and crescent to h ook-shaped canals, corresponding to faecal pellets, occur in some grai ns. Algal grains and pellets enclosing algal fragments are also presen t. The grains in the limestones are similar to those in the sediments. Halimeda and crustacean faecal pellets are abundant components, with minor echinoid, mollusc and benthic foraminifera. The limestones are c haracterised by abundant pores, dissolution cavities, relic aragonite needles, meniscus cements and drusy calcite fill the pores produced by dissolution of aragonite needles. These features indicate vadose diag enesis. The stable isotopes of the carbonate sands and limestones are almost identical and similar to isotope values of beachrocks of the Re d Sea. The limestones ages (9200-8400 yr B.P.) lie within the same age limits (10,400-8300 yr B.P.) as the surficial sediments. It appears t hat fluvial sediments largely accumulate in the inner shelf and did no t hinder carbonate growth on the platform, lying on the outer shelf. W e presume that arid climate and nutrients brought by river runoff and shelf edge currents favoured the growth of algal bioherms dominated by Halimeda between 13,700 (?) and 8300 yr B.P. The absence of Late Holo cene and modern carbonates on the platform may be due to the combined influence of climatic change, upwelling and neo-tectonic activity at a bout 9000 yr B.P. The bioherms dominated by Halimeda, produced abundan t aragonite muds. Deposit feeding crustaceans and other invertebrates subsequently excreted these muds as faecal pellets. The platform thus contains faecal pellet-dominated sediments.