Depositional sequences of the phosphatic rock series and formation of the phosphorites in the Doushantuo Formation in Weng’an,Guizhou

The phosphorites in Weng’an,Guizhou are hosted in the lower and upper parts of the Doushantuo Formation,and made up of carbonate rock-type phosphatic rock series on a shallow marine carbonate platform.The strata of the Doushantuo Formation deposited after the Marinoan glacial period may be grouped into six shallowing-upward depositional sequences.The bulk of phosphatic rock series is built up of polycyclic stacking deposits in which cross-bedding,wavy bedding and lenticular bedding are common,implying that the phosphorites rested in the shallow marine high-energy environments.From the Nantuoan glacial period onwards,the large-scale transgressions gave rise to the upwelling oceanic currents and carried plentiful phosphatic sea water into the shallow marine areas as sites of phosphorites development.The repeated sea-level fluctuations contributed to the phosphatic source and development of polycyclic depositional sequences of the phosphorites.