Carbonate Microfacies And Sequence Analysis Of The Upper Shiniulan Group In The Lower Silurian In Zheng'an,Guizhou

In this paper,through the observation of field sequence near the county seat of Zhengan County,Guizhou Province,sample collection,indoor rock slice identification and microfacies analysis,the carbonate facies and sequence of the upper section of the Shinulan Formation in the study area were studied.Analysis of stratigraphic system domain structure,integrating previous achievements,and obtaining a more detailed analysis of the sequence structure(including system domains and subsequence groups).The main understanding and results include:1.Through field surveying and indoor research,and integrating previous research results,a detailed sequence division and analysis of the upper section of the Shinulan Formation in the study area were conducted,and the third sequence(OS3)was divided into the seaward system and High system domain.The sequence mainly develops high system tracts,including five sub-stratum groups,and each sub-sequence group is an upwardly shallower accretion structure(downward sea level).However,from the bottom up,the five sub-sequence groups also reflect the general upward trend(rising sea level).2.By analyzing the microfacies of rock thin-film carbonate rocks,the type of microfacies in the upper segment of the Shinuola Formation was determined,reflecting the reef-bearing facies limestones that represent high-energy,rich in bioclastics,and represent higher energy.It is mainly composed of limestone containing bioclasts and grain mud(or mud);low-energy,mainly represented by muddy limestone;low-energy microfacies represented by argillaceous limestone.The comprehensive analysis of the superposition of the sub-sequence group and the sub-sequence shows that the sequence of the sub-sequence and the sub-sequence group are all upwardly shallower,but the composition is lower and the sub-sequence group PPS-2?upper sub-layer.The order group(PPS-6)has reduced thickness,reduced calcium content,and increased mud content.It follows that the transition from the upper section of the Shiniu Formation to the Yanxi Formation at the sequence scale level is a large sea level transition period and is not a high-level mutation interface.3.Comprehensive analyses of carbonate microfacies and sequence structure and previous achievements have yielded new understanding of the sea level changes of the six tertiary sequences(OS1~OS6)of the Silurian in the study area from the sequence group.On the level,the first to third sequence(OS1~OS3)is the first sequence group,and the fourth to sixth sequence(OS4~OS6)is the second sequence group.The maximum sea level transition period is between the third and fourth sequence.From the analysis of the text,we can know that the high-level system period of the third sequence has recently shown an overall upward trend in sea level.