A New Explanation for the Origin of Cambrian Phosphorites from Eastern Yunnan

Many layers of typical phosphatic storm deposits have been recognized in Cambrian phosphorus-bearing successions, especially in the phosphorite beds of the Zhongyicun Member, eastern Yunnan, where storm events are of wide-spread occurrence. Typical phosphatic storm deposits are composed of siliceous phosphorite,dolomitic phosphorite and siliceous-dolomitic phosphorite in which abundant sedimentary structures formed in different stages of storm activity are present. (1)Sharp basal boundary and basal surface structures such as gutter casts, pocket structures and gravel lag; (2)graded beds consisting of phosphorus material,dolomite and shell fragments; (3) ripple cross-lamination; (4) hummocky cross stratification of three types: single, superimposed and amalgamated: (5)parallel lamination. In addition,the trace fossils formed in the post storm stage,such as Didymaulichnus,Caraulichnus and Gordia were also found on the top of the deposits. In comparison with the typical tempestite sequences,the sequences of phosphatic storm deposits in the area may be divided into seven types which occur in distinctive regions and reveal the differences of storm deposition and the degrees of phosphorite enrichment. The analysis shows that storm activities were concentrated in present-day Kunyang and Haikou,where the phosphorite deposits are world-famous,and that the phosphorite-rich beds there were formed during the decline phase of storms. The early-accumulated phosphatic deposits were reworked and transported by storm wave and storm current processes into sub-tidal bays where they were deposited. Where storm waves or storm currents combined with upwelling currents,phosphorite-rich zones and commerical phosphorite deposits were formed. As a consequence,a sedimentary model showing the relationship between phosphorite enrichment and storm events has been proposed. Three stages for phosphorite enrichment may be distinguished: Stage 1 (i. e. the early Early Cambrian):The phosphorus-rich upwelling current upsurged along the slopes onto the shoals built up during late Early Sinian,thus resulting in a suite of phosphorus-bearing successions. Stage 2 :When the storms occurred,the early-deposited phosphorus-bearing successions were reworked and transported by storm wave processes. At the same time, abundant intraclasts were formed. Stage 3 :As storm currents continued to migrate seawards,the phosphatic deposits were winnowed and deposited to form commerical phosphorite deposits within sub-tidal bays during storm decline.