Quaternary phosphorites from the continental margin off Chennai, southeastIndia: Analogs of ancient phosphate stromatolites

Pleistocene phosphorites occur on the continental margin off Chennai abunda ntly in the depth range of 186-293 m, They are associated with outer-shelf glauconites and carbonate skeletals including large shells of molluscs and rhodoliths, These are primary phosphate deposits and are similar to phospha te stratiform stromatolites, This is the first report of phosphate stratifo rm stromatolites during the Quaternary and modern times, and they provide a Quaternary analog for ancient phosphorites, Thin sections of phosphorites exhibit laminated phosphatized microbial mats consisting of mechanically deposited elastic particles or: of phosphatic m olds. Clay particles are scattered throughout the matrix. SEM studies revea l that the laminated phosphate matrix is made of thin sheets of tightly pac ked apatite globules, densely packed intertwined microfilaments, or filamen t molds. Phosphate with obscured laminae is composed of phosphate tubules, Cell-like structures resembling coccoid cyanobacteria or their botryoidal a ggregates are common. Carbonate fluorapatite and low magnesium and high-mag nesium calcites are the major mineral phases. The structural CO2 content of apatite ranges from 4 to 5.5%, Quartz, feldspar, and goethite are accessor y minerals, The microbial mats were formed on the outer shelf, most probabl y during conditions of low sea level in the Quaternary, and microbial proce sses played a major role in direct phosphatization of these mats, The prima ry phosphorus source seems to have been the continental supply and degradat ion of organic matter associated with benthic microbial communities. The ph osphate stratiform stromatolites thus formed were subsequently reworked int o a shelf-margin depression and resulted in the occurrence of condensed pho sphorites.