SEDIMENTARY FACIES AND MODEL OF THE PHOSPHORITES IN WESTERN HUNAN

Known as the main type of the Sinian phosphorites in the Yangtze phosphorus-accumulating area, the phosphorites in the Doushantuo Formation in the Huaihua-Yuanling region of western Hunan are wide-spread over the uplifted zone of the Jiangnan ancient island arc on the southeastern margin of the Yangtze block, and deposited in the shallow-water environments including tidal flats on the margins of restricted platform islands, beaches and backbeach lagoons. The phosphorus material is derived from the upwelling currents in the Huanan backarc basin. The deposition consists mainly of chemical and biochemical precipitation, with weak mechanical concentration.