Study On The Origin Of The Middle Permian Eyeball Limestone In The Area From Eastern Sichuan To Western Hubei

The Middle Permian marine strata are widely distributed in eastern Sichuan and western Hubei.In the study area,a special limestone-eyeball limestone was developed.This kind of eye ball limestone has special structure and stable rhythm and high organic carbon content,which can be used as a good basis for geological division and comparison.Although predecessor had many research data on eyeball limestone in the research area,there were problems of inconsistencies and various conclusions,and the study depth of relevant section data was more extensive,and the core and ground level were deeply researched.To further formation the genetic mechanism of eyeball limestone,not only will help to recover the ancient geographic background of the middle of the second,but also to push the refinement of.the middle permian.Therefore,this paper has carried out a systematic study on the formation mechanism of eyeball limestone in the study area,and analyzed and studied the regional profile through various factors,such as lithological features,paleontology characteristics,paleosalinity index,paleo-temperature index,and ancient oxygen phase index.Guided by the sedimentology,combining previous research results and qualitative and quantitative data,comprehensively reflect its sedimentary environment and its formation mechanism.The middle Permian eyeball limestone is mainly distributed in the lower part of the Qixia Formation and the middle and lower parts of the Maokou Formation.The "eyeball" part of the eyeball limestone appears in an elliptical with a single diameter of 10-30 cm or stratified layer with a thickness of 10 cm.The "eyelid" appears package the "eyeball" part with the thickness is more than 5cm-10 cm.It is composed of micrite calcite,calcite cement and non-terrigenous clastic composition.The "eyelid" is higher than the "eyeball" of the mud,with the same dandruff and the lack of algae in the "eyelid" part.V/(V+Ni)ratios,DOP values,and carbon and oxygen isotopes are shown.The Z value of the spherical grey limestone is around 130.It should be Marine deposition.The "eyeball" part is most V/(V+Ni)< 0.46,DOP < 0.45.It shows that it is formed in oxygen-enriched environment.The "eyelid" part V/(V+Ni)> 0.46,observed pyrite.It should be hypoxic.There are two kinds of continuous and intermittent types in the area,the difference between the two types is that the "eyeball" has a continuous or elliptical distribution.It is concluded that the formation mechanism of these two kinds of eyeball limestone is similar,and the difference in morphology is mainly due to the strength of differential compaction.Based on the analysis and test data,from the perspective of petrology and sedimentology,The difference between "eyeballs" and "eyelid" part of the mud should be caused by the intermittent changes in water dynamics in the depositional environment.The special structural difference should be related to the strong differential compaction and the pressure solution,while the "eyeball" and the "eyelid" part of the geochemical indicators are different.It says that there is a change in the amount of dissolved oxygen in the sedimentary environment.Guided by the carbonate platform facies,combining with the geological background,to realize in eastern sichuan to western hubei in the Permian system has a lack of oxygen,rich nutrient characteristics of upwelling.In recent years,most studies have shown that the upwelling sediments are more developed in the gentle slope zone.Therefore,it is speculated that the eye spheroidal limestone is the transition zone between the carbonate and the open platform.It's formed by the interaction of the difference pressure and the pressure effect of the upward flow.