CHARACTERISTICS OF AUTHIGENIC MINERALS AND PARAGENETIC SEQUENCE OF DAWSONITE-BEARING SANDSTONES FROM THE MINGYUEFENG FORMATION OF PALEOCENE IN LISHUI SAG , EAST CHINA SEA BASIN
Dawsonite-bearing sandstones occurred in the Mingyuefeng Formation of Paleogene period have been found in the Lishui sag,Donghai basin.Multiple approaches such as polarizing microscope,scanning electron microscope,cathodoluminescence,alizarine red S and potassium ferricyanide composite stain as well as X-ray diffraction have been used to investigate the characteristics of clastic components,authigenic minerals and paragenetic sequence.It reveals that the dawsonite-bearing sandstones are fine to medium grained,poorly to moderately sorted feldspathic litharenite and litharenite,basically litharenite.Authigenic minerals are predominantly quartz,siderite,pyrite,dawsonite,ankerite,calcite and clay minerals,in which dawsonite occurs either as radiated clusters and bundles consisting of very fine dawsonite blades,or as lath-shaped grains replacing clastics in varying degrees.Paragenetic sequence of dawsonite-bearing sandstones is outlined as follows:authigenic clay coating siderite-pyrite-kaolinite-quartz-ankerite-dawsonite-clacite.Among them,dawsonite and calcite are considered as resulting from carbonates precipitation after CO2injection.The formation of calcite can be attributed to combined effects of the dissolution of anorthite and rock fragments together with the decrease of CO2partial pressure in alkaline fluid.