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.