论下扬子地区中三叠统白云岩类型及形成环境

On the types and forming conditions of the Middle Triassic dolostone in the Lower Reach of Yangtze River

多年来,笔者在长江中下游地区开展了早、中三叠世的岩相古地理研究,“七五”期间,又参加了“中国南方岩相古地理及沉积、层控矿产远景预测”项目和负责“长江中下游三叠系膏盐沉积环境与铁铜等多金属成矿作用的关系”专题的研究。对发育于该区地表和地下中三叠统中的层状白云岩进行了较为详细的观察和研究。

According to the texture, structure and interrelation of bedded dolostone of the middle Triassic Series, the author has divided the dolostone into five types. The microcrystalline dolostone was formed from lime mud in saline lagoon in intertidal or supratidal zones. In the penecontemporaneous stage,the lime mud was chemically changed into dolomite under the condition of downward circulated saline water which is high in Mg/Ca ratio and generated from evaporation and condensation. The crystal pseudomorphic dolostone is very similar to the first type dolostone, but the only difference is that the concentration of circulated saline water is higher. The season—straticulate dolostone is formed by the dolomitization of former dolomitic mud or lime mud. When the further evaporation and condensation continuously made the lagoon water higher in salinity, the Ca rich dolomite or Mg rich lime mud formed. Because the precipitation of surphates, mainly gypsum, the dolomitization happened. The stromatolitbic doiostone was formed in the intertidal and supratidai zones where the algae is very active and can concentrate Mg, so that the Mg/Ca ratio reaches 3—4 times of that of sea—water, which is very important for the dolomitization. The grain dolostone was also formed by the dolomitization. The grains were lime originally and formed in the shallowsea water. During large storm, the lime grains are transformed into lagoon and dolomitized during the evaporation of lagoon water. The first four types belong to the' contemporaneous dolostone' or 'penecontemporaneous dolostone' and the last two belong to diagenetic dolostone or 'post penecontemporaneous dolostone'。