Quaternary phosphorites from the continental margin off Chennai, Southeast India: analogs of ancient phosphate stromatolites

Abstract

Pleistocene phosphorites occur on the continental margin off Chennai abundantly 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 phosphate stratiform stromatolites. This is the first report of phosphate stratiform 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 clastic particles or of phosphatic molds. Clay particles are scattered throughout the matrix. SEM studies reveal that the laminated phosphate matrix is made of thin sheets of tightly packed apatite globules, densely packed intertwined microfilaments, or filament molds. Phosphate with obscured laminae is composed of phosphate tubules. Cell-like structures resembling coccoid cyanobacteria or their botryoidal aggregates are common. Carbonate fluorapatite and low-magnesium and high-magnesium calcites are the major mineral phases. The structural CO₂ content of apatite ranges from 4 to 5.5%. Quartz, feldspar, and goethite are accessory minerals. The microbial mats were formed on the outer shelf, most probably during conditions of low sea level in the Quaternary, and microbial processes played a major role in direct phosphatization of these mats. The primary phosphorus source seems to have been the continental supply and degradation of organic matter associated with benthic microbial communities. The phosphate stratiform stromatolites thus formed were subsequently reworked into a shelf-margin depression and resulted in the occurrence of condensed phosphorites.