Biogenic Structures And Sedimentary Substrate Evolution From The Zhushadong Formation Of Cambrian Series 2 In Henan Province

Abundant microbiogenic structures and metazoan bioturbated structures occur in the carbonates of the Zhushadong Formation(Cambrian Series 2)in Henan Province.The macroscopic morphology,microscopic fabrics,and distribution pattern of biogenic structures are studied using the methods of petrology,sedimentology,geochemistry and paleontology.The impact of bioturbation on sediments and the evolution of the matground and mixground are discussed.Above studies can provide evidences to explore the relationship among Cambrian microbes,metazoa and their environments.The Zhushadong Formation can be divided into three lighological members:lower laminated dolomite Member,bioturbated limestone(dolomite)Member,and upper laminated dolomite Member.The analysis of sedimentary facies shows that the Zhushadong Formation belongs to the tidal flat facies deposition,which has successively experienced the environmental evolution of supratidal zone-intertidal zone-supratidal zone,showing the process of sea level rising first and then falling.The analysis of sedimentary characteristics and geochemistry indicate that the Zhushadong Formation displays a clear change from hot and arid climate,to warm and humid climate and back to hot and arid climate.The contemporaneous sediments in North China show the similar paleoclimate variation to the Zhushadong Formation in study area.The microbiogenic structures in the Zhushadong Formation consist of laminites,stromatolites and oolites,in which the laminites is the most widely developed.They appear in the upper and lower laminated dolomite Members.These microbiogenic structures were formed by stacking or trapping of microbial mats,binding of sedimentary particles,or mineralization of organic matter caused by microbial life activities.The upper and lower laminated dolomite Members were deposited in the supratidal zone with sufficient light and low seawater energy,less terrigenous material injection and hot and dry climate,which is conducive to the formation of microbial mats or microbial films by cyanobacteria and other microorganisms.The difference of hydrodynamic conditions determines the development of different types of microbiogenetic structures.The lack of metazoan also creates conditions for the preservation of microbial mats or microbial films.The bioturbated structures in the Zhushadong Formation are mainly characterized by the development of the Thalassinoides,which appear in the midlle bioturbated limestone(dolomite)Member.The bioturbated structures of Thalassinoides diplay on the outcrops as patches of different colors.Under the microscope,the main mineral is calcite in burrow system Thalassinoides,followed by dolomite,and the matrix is mainly composed of calcite.The three-dimensional burrow system of Thalassinoides provides a good transport channel for the backflow and infiltration of dolomite in the diagenesis period,promotes the dolomitization of burrow fillings,and forms a large number of patches of dolomitic limestone or lime dolomite.The bioturbation structures in the section show that the diameter,depth and extent of the bioturbation increase first and then decrease.Elemental geochemical data analysis shows that the burrowing of metazoa in the sediments leads to the complete mixing of the sediments in the Zhushadong Formation,and at the same time increases the oxygen content and nutrient content in the sediments.The sediment mixing and reworking induced by the bioturbators significantly changed the primary physical and chemical characteristics of the sediment,triggering the substrate revolution and promoting biogeochemical cycling between sediment and seawater,acting as ecosystem engineers as well.The Zhushadong Formation has developed matground formed by microorganisms and mixground formed by metazoa.The hot and dry climate of the upper and lower laminated dolomite Members and the long-term exposure to the surface environment are conducive to the proliferation of microbes to form a microbial mat,which is not conducive to the life of metazoa.Bioturbated limestone(dolomite)Member develops in warm and humid intertidal zone.The nutrients carried by seawater provide conditions for the survival of metazoa.Metazoan burrows in sediments and leaves strong bioturbated structures,resulting in the formation of mixground.The alternate development of matground and mixground is controlled by the changes of sea level and paleoclimate.