滇东寒武系磷矿成因新见解——风暴成矿模式初探

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

摘要:

近年来,随着风暴流和风暴岩的新理论、新概念的不断引入,在分析沉积环境、沉积成矿作用时,不断提出了许多新思路,也不断地冲击和修正以往的传统认识,作者于1986年随同成都地矿所寒武系磷矿专题组^①对滇东一带包括昆阳梅树村、海口、鸣矣河、晋宁王家湾、二街、八街、安宁白登、江川清水沟、宜良大滴水、寻甸先锋等矿区的磷矿进行了详细的研究,在下寒武统中谊村段及其相当的地层中发现了多层磷质风暴沉积物。本文拟概略介绍磷质风暴沉积物的标志、特征、剖面结构并初步探讨磷矿富集与风暴事件关系的沉积演化模式。

Abstract:

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.