Sequence Stratigraphic Framework Of Yunmengshang Formatjion And Baicaoping Formation Of Proterozoic In YuXi Area

From the simplex sedimentary sequence in Exxon Times to the diversified sequence model in post Exxon Times,sequence stratigraphy takes on a new age of'contention of a hundred of thought'.After all,the depositional trend research is still the essence.Facing the Precambrian stratums,based on the character of lithology and sedimentary structures and the distributing law,sequence stratigraphy reexamine the genesis,and providing the Precambrian stratums research with an important theory.Using sequence stratigraphy methods,analyzing the depositional trend of Yunmengshan formation and Baicaoping formation combining the outcrops,divide sequence and establish sequence framework,which becomes a powerful basis to settle the argument of the age stratum in Yuxi area.Three third-sequences had divided from the intertidal facies sandstone of Yunmengshan formation to the lower of Baicaoping formation,the tidal flat facies of upper Baicaoping formation contribute a third sequence,then from the subtidal sandstone of Yunmengshan formation to the arid tidal flat facies mudshale of Baicaoping formation can contribute a second-sequence.The four third-sequences of Beidajian formation can be the contrast marking in regional.Combining the former publications,contrast the four sections in researching area with other typical sections in other district,analyze the depositional trend,conclude the findings.The research had indicated that the proterozoic group of YuXi area should contribute into Changchengian System,Beidajian formation compared with Changzhougou formation of Jixian section,Yunmengshan formation and Baicaoping formation may older than Changzhougou formation;According to the transformation of four three-order sequences which wedging out from south to north,the transgressive direction of south to north of YuXi trough in RuYang Group have been insured.Sedimentary structures and the relationship of stratum contact have imply that RuYang Group is the pioneer sedimentary cover of YuXi area on the south of Northern China Block between the period of Changchengian System and Jixianian System.The forth third-sequence of Baicaoping formation could be the high stand system of a second-sequence,the arid red-lyer sand-mudshale is the symbol of a arid environment,analyzing the rhythmite of the middle sand flat had found some typical“wave-generated tidal bundle”,it imply that the depositional environment could be an open coast setting.