PHOSPHATE-BEARING STROMATOLITES OF THE PRECAMBRIAN ARAVALLI PHOSPHORITE DEPOSITS OF THE UDAIPUR REGION, THEIR ENVIRONMENTAL SIGNIFICANCE AND GENESIS OF PHOSPHORITE

Phosphorite deposits of the Aravalli Group of Udaipur region, India, are characterized by an extensive and prolific growth of stromatolites. Phosphate forms an integral part of these biogenic structures. Most of the stromatolites are columnar and may display branching. Stratiform, compound forms and saucer-shaped structures are also present. The framework of the stromatolites varies in composition from dominantly phosphatic to carbonate bearing. The associated sediment (intercolumnar material) varies from purely chemogenic to dominantly clastogenic and is derived from erosion of the surrounding biolithite. Morphological expressions of the stromatolites and other variations of the associated sediment indicate supratidal, intertidal and subtidal environments of different stromatolitic phosphorite deposits. Interaction of erosional and biological processes involved in stromatolite growth is inferred. It is envisaged that the composition and microstructure of the stromatolites have resulted mainly from in situ precipitation of calcium carbonate; the coeval and progressive replacement of these developing carbonate structures by collophane produced the phosphate-bearing stromatolites. From the restricted occurrence of phosphate in the stromatolites alone and the preservation of algal remains in them, active participation of algal life in the localization of phosphate in this region is suggested. © 1979.