Late Quaternary Halimeda bioherms and aragonitic faecal pellet-dominated sediments on the carbonate platform of the western continental shelf of India

Abstract

The Fifty Fathom Flat is a drowned carbonate platform located on the outer continental shelf of western India. It is unique because the carbonate depositional environment prevailed until Early Holocene in spite of its proximity to a major siliciclastic tidal embayment. Study of the 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 Quaternary Halimeda bioherms and (b) aragonitic faecal pellet-dominated sediments. Echograms reveal buried pinnacles between 62 and 38 m and exposed 2 to 14 m high pinnacles between 65 and 85 m on the platform. These pinnacles occur as narrow to broad mounds (bioherms) in seismic profiles and as linear ridges and massive structures in sonographs. Seismic profiles also show a prominent reflector about 25 m below the sea bed identified by boreholes as a Late Pleistocene erosion surface. The platform consists of relict carbonate sands and limestones. Rounded and ovoid to ellipsoidal aragonite grains are predominant in the sediments. SEM studies indicate elongated parallel canals and crescent to hook-shaped canals, corresponding to faecal pellets, occur in some grains. Algal grains and pellets enclosing algal fragments are also present. 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 characterised 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 diagenesis. The stable isotopes of the carbonate sands and limestones are almost identical and similar to isotope values of beachrocks of the Red Sea. The limestones ages (8400–9200 yr B.P.) lie within the same age limits (10,400–8300 yr B.P.) as the surficial sediments. It appears that fluvial sediments largely accumulate in the inner shelf and did not hinder carbonate growth on the platform, lying on the outer shelf. We 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 Holocene and modern carbonates on the platform may be due to the combined influence of climatic change, upwelling and neo-tectonic activity at about 9000 yr B.P. The bioherms dominated by Halimeda, produced abundant aragonite muds. Deposit feeding crustaceans and other invertebrates subsequently excreted these muds as faecal pellets. The platform thus contains faecal pellet-dominated sediments.