Lower Ordovician Sequence Stratigraphy And Its Control On Microbial Rocks In Songzi Area, Hubei

Since the first introduction of sequence stratigraphy in the 1940 s,the study of sequence stratigraphy has made considerable progress.With the deepening of production and scientific research,challenges have also been placed on the hierarchy of sequence stratigraphy.Through the investigation of the literature,the authors found that the Early Ordovician strata contained a large number of microbial rocks,and the distribution and control of these microbial rocks in the strata were rarely studied.The Lower Ordovician marine carbonate rocks in Liujiachang Town,Songzi City have continuous sedimentary development,extensive distribution of microbial rocks and good outcrop,which is an ideal place to study the sequence control of microbial rocks.Comprehensive application of field profiles,thin sections and other data,combined with the research on the sedimentary characteristics,microscopic and macroscopic characteristics of the Early Ordovician strata in Liujiachang Town,Songzi City,to carry out high-frequency cyclic analysis of the strata in the area.Based on this research,the distribution characteristics and formation environment of microbial rocks were discussed,and the control effects of sequence on microbial rocks were summarized.Here is the conclusion of this paper:By analyzing the rock types,structures and sedimentary structures of the Lower Ordovician in the study area,the sedimentary facies of the study area are divided into two types: open platform phase and limited platform phase.The open platform can be further divided into the shoal subfacies,the shoal shoal phase and the reef subfacies.The limited platform phase can be further divided into the shoal subfacies and the lagoon subfacies.According to the standard microfacies division method of carbonate rock,11 kinds of microphases were identified in the Lower Ordovician in the study area.Through the application of classical sequence stratigraphy theory,three sequence interfaces are identified in the study area,including erosion erosion surface,lithology transformation surface and maximum sea surface.On this basis,the Lower Ordovician in the study area is divided into six third-order sequences,and the sequence stratigraphic framework of the Middle Yangtze region is established.The analysis shows that the study area is located in the shallow sea carbonate platform area,and experienced six significant sea level changes.The overall trend is that the west side is high and the east side is low.Using the theory of high-resolution sequence stratigraphy,15 Quaternary sequences were identified in the Lower Ordovician strata in the Liujiachang area of Songzi City and their developmental characteristics were described.At the same time,through the cycle theory of Milankovic,the identification of the Meter level cycle of the Lower Ordovician in the study area was carried out for the first time.A total of 11 types of Meter level cycles were identified,and the Lower Ordovician was divided into 86-meter cycles.Based on the description of the types of microbial rocks in the study area,the study on the control effects of sequence stratigraphy on different types of microbial rocks in the study area was carried out.The results show that in the stromatolite microbial rocks,the wavy stromatolite is mostly distributed in the middle stage of the Transgressive system tract and in the early stage of the highstand systems tract.The columnar stromatolite is distributedin the middle stage of the Transgressive system tract,and the mound stromatolite is in the late stage of the highstand systems tract.In the crust microbial rocks,the porphyritic thrombolite are distributed in the early stage of theTransgressive system tract,while the reticular thrombolite are in the late stage of the Transgressive system tract;the nucleus with good particle size and content is in the early stage of the highstand systems tractThe analysis of the control effect of high-frequency sequence on microorganisms suggests that the wavy stromatolite is distributed in the early stage of the rising half-cycle and the early stage of the falling half-cycle,and the columnar stromatolite is mostly developed in the upper part of the rising half-cycle,and the mound-shaped stromatolite is in the upper part of the descending half-turn;The porphyritic thrombolite is in the middle of the rising half-cycle,and the reticular thrombolite is in the late stage of the rising half-cycle.The nucleation stones with good particle size and content in the core-shaped stone are mostly distributed in the middle of the rising half-cycle,which is more suitable for the development of the nucleus.