Sedimentary Characteristics In Mesoproterozoic Taizi Formation Of Shennongjia Group In Northern Margin Of Yangtze Craton

Based on the field geological work,applying the basic theory of sedimentology and geochemical element analysis,this paper studies the sedimentary characteristics of the Taizi Formation of the Mesoproterozoic Shennongjia Group in Shennongjia area,divides the sedimentary facies types,summarizes the sedimentary evolution regulation of the Taizi Formation,and modify the depositional model of others and draw the depositional model of the sub-formation..The bottom of the Taizi Formation consists of egg-green glutenite and gray and flesh-pink fine-silty dolomite.The middle is gray-green,light-gray medium-fine quartz sandstone,siltstone interbedded with dark-gray siltstone,silty mudstone and argillaceous siltstone,the upper part develops siltstone-mudstone rhythm.Geochemistry of the Taizi Formation shows that Sr/Ba,MgO/CaO and Sr/Cu ratios indicate humid climate at that time.Mg enrichment,MgO/Al2O3 and Th/U ratios indicate that the Taizi Formation is marine deposit and a transgressive process as a whole.V/Ni and V/(V+Ni)ratios indicate that the underlying strata of the uranium deposits are aerobic and acidic depositional environments,and the overlying strata are anaerobic environments,Fe2+/Fe3+,Ce/Ce*and Eu/Eu*ratio reflect that the whole Taizi Formation is a weak reduction-oxidation environment.Based on the rock,sedimentary structure and geochemical characteristics of the Taizi Formation,the Taizi Formation is a set of littoral-tidal flat facies deposits.Its sedimentary model can be summarized as barrier-free coastal-barrier coastal depositional model.From lower to upper,the succession regulation of coastal facies-carbonate gentle slope platform facies-lagoon facies-tidal flat facies-lagoon facies is presented.According to the vertical sedimentary sequence,the Taizi Formation can be divided into five sections,which correspond to five sedimentary periods from bottom to top:the first stage is a new transgression,and the sandy conglomerate deposits on the coastal dunes.With the rise of sea level,the carbonate rocks begin to deposit,forming cone-columnar stromatolites in the lower intertidal zone and shallow subtidal zone,after which the mudstone with fine and straight bedding and lamellar structure develops.Silty mudstone belongs to hydrostatic lagoon under semi-enclosed reduction conditions;in the second stage,due to the high-frequency rise and fall of sea level,bar migrate;in the third stage,sea level rises,bar migrate to shore,and fine sediments deposit first,forming mudstone and silty mudstone with horizontal bedding and laminated structure,and fine sandstone and silty mudstone with vein,wavy and lenticular bedding.It belongs to mud flat and mixed flat of intertidal zone;in the fourth stage,secondary barrier bar is formed far from the coast with the decrease of sea level,characterized by the development of pure quartz medium-fine sandstone with parallel bedding and massive bedding,and gentle wave bedding,which belongs to shallow subtidal zone;in the fifth stage,the sea level rises and lagoon forms between primary barrier bar and secondary barrier bar.Sedimentary structure and typical geochemical data indicate anaerobic and occluded reductive environment.