Abstract:

The Middle Carboniferous tidal-flat breccia dolomite was formed by dia-genesis of carbonate sediments embracing subaerial exposure,penecontempo-raneous evoporation dolomitization and dessication of the deposits after heirde position.The process was accompanied by desiccation cracking and desic-cation tearing,and thus the sediments were broken into fragments which later were eroded by rains and tides and finally deposited in situ.Anotherhy pothesis is that:after sediment was deposited at shallow depths,its super-ficial layer was cracked upon drying,leading to desiccation,dewatering and contraction of the inner part;later it was subjected to leaching andsolution by fresh water,and then the rock was formed.From analyses of the rock types of the constituent angular fragments,textures and structures in the rocks and strata,fossil assemblages and dia-genetic changes of the sediments,it can be recognized that the breccia do-lomite is mainly developed in three subenvironments,i.e.supratidal algalmud flats,intertidal algal mud flats and tidal channels.It is also distri-buted in intertidal lakes at the drying-up stage.Six genetic types of rockmay be recognized:desiccation-tear breccia dolomite,desiccation-crack bre-ccia dolomite,desiccation-contraction breccia dolomite,desiccation-solutionbreccia dolomite,channel breccia dolomite and evaporate-solution collapsebreccia dolomite.Of these,the first four genetic types dominate,while thelast two are only distributed locally.As the origin of these breccia dolomites is generally related to subaerial exposure,corroded pores and fissures commonly developed at the diageneticand supergene stages combine to form a favourable space for the accumu-lation of oil and gas.