The Sedimentary Facies and Palaeoenvironment of Jingangshan Layer of Early Cretaceous Yixian Formation in Zaocishan Area,Yixian Basin

Key words: 
• sedimentary rhythms /
• sedimentary facies /
• Jingangshan layer /
• Yixian Formation /
• Zaocishan

Abstract: The Yixian stage standard section crops along MashenmiaoDaobadiSanbailong Jingangshan area strikingly from the west to the east in Yixian basin, they are upwardly Mashenmiao layer, Laogonggou layer, Yenangou layer, Zhuanchengzi layer, Dakangpu layer, Zhujiagou layer and Jingangshan layer. As the upper part of Yixian formation, Jingangshan layer is a set of lacustrine deposit in Zaocishan area. Based on sedimentary petrology and stratigraphic studies, these sedimentary strata could  be divided into lake margin clastic turbidity facies, shallow water flat facies and semideep lake facies (in ascending order).The turbidity facies consists of three coarsefine grading sedimentary rhythms. The coarse petrologic units are mottled, greygreen feldspar and lithic tuffaceous glutenite and greywhite gravel bearing tuff; the fine units are made of greywhite gravel bearing tuff, greygreen compact tuff, purified bentonite and slightly bentonitic siltstone. The coarse turbidity facies is product of the head of turbidity current, distribution normal grading usually develops in this petrology unit. As the result of the body and tail parts of turbidity current, the finer turbidity facies present themselves with horizontal bedding. In the middle part of Jingangshan layer, the shallow water flat facies is composed of five bentonitic siltstone (purified bentonite and /or shale )  marl rhythms. In lake flat facies, the thickness ratios of carbonate deposit and underlying volcanic clastic deposit range from 0.11 to 0.47, it indicates that lacustrine margin experiences five stages of equal interval lake flat sedimentation.The upper part of Jingangshan layer is the association of pale papersheeted shale and silty shale. The coarser petrologic unites of lake margin clastic facies in lower part of Jingangshan layer indicate that they are proximal deposite, the finer unites show the polycylic volcanism during the forming process of lake margin clastic facies. The five bentonitic siltstone  marl sedimentary rhythms of middle part lake flat facies may indicate the lake water level survived at least five shoaling changes. Under the regional arid climate, the marl deposit was formed in the lake flat facies. And the deeper area of central lake basin may receive a great volume of suspensive finegrained materials to form the hydrostatic deposits of semideep lake facies.