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 unique because the carbonate depositional environment prevailed until Early Holocene in spite of its proximity to 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. 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