THE GENETIC STAGES OF DOLOSTONE AND THEIR FEATURES IN EVAPORITE-BEARING SUITE OF TRIASSIC IN HUAYING MOUNTAIN
华蓥山三叠系含盐建造中白云岩的成因阶段及其特征

Jialingjiang Formation and Leikoupo Formation of Triassic in Sichuan Basin are both evaporite-bearing carbonate suites, and therefore both are the target strata for potash salt. Nearly all of the evaporite in this stratohorizon is interbedded with or adjacent to dolostone.It reveals their inherent connection in sedimentory environments. forming conditions and genesis. This paper has studied the genesis of dolostone and its signs, and the stages of dolonitization and their features in Huaying Mountain area, eastern Sichuna Province.In the area, dolostone is mainly distributed in second and fourth members of Jialingjiang Formation and first member of Leikoupo Formation. There are nine dolostone lithofacies.They are: oolitic grain dolostone, skeletal-intraclastic grain dolostone, oncolltic-clotic grain dolostone, skeletal pack dolostone, skeletal wacke dolostone, gypsiferous mud dolostone,nodular gypsum-bearing mud dolostone, dolomicrite and stromatolithic dolostone.There are three signs cited in this paper which indicate that the dolomite (dolostone) is all formed by replcement (or by dolomitization). (l ) The sign of texture can be generally found that calcite is replaced by dolomite in impure dolostone. The patterns of replacement chiefly are (a) include, (b) cut off and (c) nibble. In the pure dolostone, it is also found that there are a lot of honeycombed micro--space. This is the result of volumetric shrinkage in the process of replacement. (2) In the sign of oxygen isotope(8 18O), The differences of between dolomite and calcite or between dolostone and limestone are far lower than 6%,' It indicates replacemental origin of dolomite (or dolostone). (3) In the sign of cathodoluminescence, there is marking of replacement remnant in all of the dolomite (or dolostone) in catholuminescence microscope.The process of dolomitization can be divided into three stages: penecontemporaneous, early diagenetic and late diagenetic stages. The dolomite formed in penecontemporaneous stage has the lowerest ordering degree and the highest content of MgCO, in the three types of dolomites. It indicates the high salinity or Mg/Ca ratio in primary water and the quick rate in replacement. The dolomite formed in early diagenetic stage possess the highest ordering degree and relatively high content of MgCO,. It indicatesthat medium solution in this stage was very favourable to form the layered superstructure of dolomite. The dolomite which was formed in late diagenetic stage is provided with the medium ordering degree and lowerest content of MgCO,, which is the result of the slow rate of replacement and the insufficient source of Mg2 .