A special sediments,situated in a subterranean stream near Niuchang,Duyun,Guizhou Province,and deposited upon the dolomite layer of the Low-er Ordovician Honghuayan Formation in the west wing of Majiang paleoan-ticline,was firstly discovered in 1980.It consisted mainly of sandstone and conglomerate which are composed of fragmental particles of mudstone and argillaceous siltstone.During Early Devonian transgression,Niuchang subterranean stream was filled with sediments which is now about 30m thick.In connection with the depression of ancient Majiang Island,the diagenetic environments of the sediments were changed gradually from shallow to deepseated,this resulted in the evolutionary series of diagenesis and succesively forming the following sequences:freshwater dolomitization→collophanization→oil pouring→silicifica-tion→pyritization.Among them,the freshwater dolomitization is more evident.As a result,the sediments were thoroughly transformed and indurated into rocks.Because more intercrystaline pores have developed in the freshwater do-lomite,so it is more favourable for oil accumulation than those dolomites of Honghuayan Formation.According to its geological history,diagenetic sequence and diagenetic features,the dolomite in Niuchang subterranean stream was formed in a mixed zone of freshwater and seawater.Its greater particle size,fewer content of oxygen isotope and trace elements show that it is no doubt freshwater dolo-mite Referring to the precipited freshwater dolomite described by R.L.Folk,and the replaced freshwater calcidolomite described by L.S.Land,this fresh-water dolomite,the writer presents in this paper,is formed by the replace-ment of continental fragments.They differ clearly in apparent mineral fea-tures,for example,the later characterized by the automorphic but unclean crystal,or clean but non-automorphic crystal,and it contains more enclaves and more girdles.Thus it can be seen that the apparent features of fresh-water dolomite of different types are varied and effected by the forming man-ner and the mineral replaced.It is practically useful that the diagenetic environment can be marked bythe freshwater dolomites,and it may be used to search out uplift,erosion sur-face and karst somewhere in the geological past.
