Genesis of apatite in the phosphatized limestones of the Western continental shelf of India

Abstract

Phosphatized corals, algal nodules/encrustations and pelletal limestones occur at depths between 60 and 100 m on the western continental shelf of India. Microstructural studies of limestones by SEM indicate that phosphate occurs as (a) brown infillings in boring/septal cavities of corals, (b) trapped material between algal laminations and (c) cement in pelletal limestones. Phosphate is in the form of ovoids, rods and dumb-bell-shaped apatite microparticles or their coalesced/aggregate forms or microbial filaments. These apatite microparticles resemble fossilized bacteria and are often adhered to clays. SEM-coupled with EDS shows that the phosphate is associated with siliciclastic material. It appears that abundant terrigenous flux was transported to the shelf at about 8300 yr B.P. resulted in the demise of corals and favoured the growth of crustose coralline algae for short duration. This flux also filled the pores/cavities of corals and coralline algae. Subsequently, microbial organisms colonized the sediments and empty as well as sediment filled pores of the limestones and favoured rapid precipitation of apatite locally by utilizing organic phosphate and phosphate adsorbed on to clays. Phosphatization of the limestones is thus a short event in the Early Holocene influenced by microbial processess.