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 CO 2 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.

Origin of Cretaceous phosphorites from the onshore of Tamil Nadu, India

“…Monty (1976) suggested that the stratiform stromatolites with more or less planar laminae are synonymous with cryptalgal laminites. Phosphate stromatolites formed in shallow water settings during the Paleozoic (Bertrand-Sarfati et al 1997), Paleogene (Soudry and Panczer 1994) and Quaternary (Rao et al 2000) and in pelagic settings during Mesozoic along the northern margin of the Mediterranean Tethys (Krajewski et al 1981(Krajewski et al , 1994Follomi 1989;Martin-Algarra and Sanchez-Navas 1995) were reported. They suggested that the apatitic stromatolites tend to be composed of minute microlaminated units, which define both the structure and fabric of the authigenic phosphate deposit.…”

“…The microbial filaments and cell structures observed here are empty implying that phosphatization took place during their early stages of decay and before the complete decay of organic matter associated with the mats. The organic matter associated with mucillagenous sheaths of microbial filaments and cells may have (Vaz 1995;Rao et al 1998Rao et al , 2000Rao et al , 2002 Occurrences ( been the source of P and also acted as a substrate for phosphatization. The decaying bacteria associated with the microbial mats may have assimilated P and rapidly fixed P in their structures before getting destroyed.…”

“…Quaternary offshore phosphorites and their comparison with Cretaceous onshore phosphorites Vaz (1995), followed by Vaz et al (1999) and Rao et al (1998Rao et al ( , 2000Rao et al ( , 2002 carried out detailed investigations on petrology, mineralogy and geochemistry of the phosphorites occurring at 180-320 m depth on the outer continental shelf off Chennai. Table 4 gives the salient features of these phosphorites.…”

“…On the other hand, Vaz (1995) was the first to report phosphorites on the outer continental shelf of Tamil Nadu. Subsequently, Vaz et al (1999) and Rao et al (1998Rao et al ( , 2000Rao et al ( , 2002 carried out detailed investigations on the origin and depositional environment of phosphorite. Faunal elements such as larger bivalve and gastropod shells belonging to the Upper Cretaceous-Eocene age have been found to associate with offshore phosphorites at water depths between 180 and 320 m (Vaz et al 1999;Rao et al 2000).…”

“…Subsequently, Vaz et al (1999) and Rao et al (1998Rao et al ( , 2000Rao et al ( , 2002 carried out detailed investigations on the origin and depositional environment of phosphorite. Faunal elements such as larger bivalve and gastropod shells belonging to the Upper Cretaceous-Eocene age have been found to associate with offshore phosphorites at water depths between 180 and 320 m (Vaz et al 1999;Rao et al 2000). It is therefore important to establish the relationship between the onshore and offshore phosphorites in terms of their age, sources of phosphorus supply and processes of phosphogenesis.…”