The Forming Environment Of The Carbonate Concretions Of Mesoproterozoic Gaoyuzhuang Formation ? Menber And It's Nucleation Mechanism

Carbonate concretions widely exist in the geological records.Due to the lack of modern analogues,the mechanism for their formation remains controversial.In this thesis,we mainly study carbonate concretions developed in the third member of the Mesoproterozoic Gaoyuzhang Formation in central part of the North China Platform.By analyzing the iron species and studying the particular structures in the host rocks to illustrate the depositional environment.By analyzing the structure of the concretions and the distribution of the carbon isotope values in the concretions,as well as the major element contents to illustrate the forming mechanisms and providing evidence of the potential roles that microbe might have played.The third member of the Mesoproterozoic Gaoyuzhuang Formationconsisits mainly of thin to medium bedded micritic dolostone and dolomitic limestone,with some thin beds of dark shales.Dolomite concretions largely developed in the lower and upper parts of the member,especially concentrated in organic-rich dark shale intervals.There is less the signatures of the disturbation markers ofwater,which indicate that the concretion-bearing intervals were most likely deposited in a deep subtidal sedimentary environment below the fair-weather wave base.The concretions are mostly ellipsoidal?with variable sizes ranging from 3cm to 20cm in diameter?,mainly with argillaceous microcrystalline dolomite.Usually straight laminae are visible in concretion interior,but they become distorted and converged toward lateral margins of the concretion.Beddings in the host rock commonly do not across concretions,they instead wrap around the concretions,indicating that the concretions were most likely formed during early diagenesis.The chemical compositions ofconcretions are concentrical.It may reflect different growth stages of concretions.Because of the early diagenetic origin of the concretions,the chemical conditions of the bottom sea water have an important influence on the diagenetic process.In this study,the paleo-ocean environment is studied by analyzing the iron species and the organic carbon isotope of the host rocks.The results show that the iron speciesof the black shale with high Fe _HR/Fe_T ratio?0.43-0.84?and low Fe _P/Fe_HRR ratio?0.00-0.02?,which indicating a anoxic and ferric bottom sea water;the relatively depleted organic carbon isotope values of the carbonate concretions(?¹³C_org<-32‰)indicate chemoautotroph and/or anaerobic photoautotrophic microorganism has an important contribution to the composition of the organic carbon,which also indirectly indicate the bottom water is anoxic.Therefore,the early diagenetic stage of the concretions occurred under an anoxic ferric bottomsea water condition.In the early diagenetic stage,a lot of microbial chemistry process can lead to the precipitation of carbonate,which induced the formation of the concretions.such as nitrate reduction,iron reduction?DIR?,bacterial sulfate reduction?BSR?,anaerobic methane oxidation?AOM?,methane production and so on.However,due to the extremely low concentrations of oxidation in the Mesoproterozoic leading the limited concentration of NO₃^-in seawater,the nitrate reduction reaction is unlikely to be the main microbial chemical process resulting the forming of the carbonate concretions ofthe third member of the Mesoproterozoic Gaoyuzhuang Formation.Due to the background of a ferric bottom seawater,so in spite of the pyrite belt developed in the edge of the concretions,BSR is involved in the late forming stages of the concretions.It should not be the dominant microbial chemical process in the formation of the concretions.The large amounts of herrinicc calcite,dolomite,goethite and other iron minerals in the concretions may indicate the dominant microbial chemical processes in the formation of the concretions ofthe third member of the Mesoproterozoic Gaoyuzhuang Formationis DIR process.The iron in the DIR process can be formed by the precipitation of Fe?OH?₃ on the redox interface,and Fe?OH?₃ which is carried by terrigenous debris.DIR reaction occurs at the sediment water interface,which on one hand provide a large number of HCO₃^-for carbonate precipitation,on the other hand provide precipitation inhibitor Fe²⁺to the herricnic calcite,and lead to iron mineral deposits?such as ankerite,goethite formation?late oxidation?etc.?.It is worth noting that the?¹³C_carbarb values are not significant depleted of the concretions?-1.83‰--0.53‰?,which showethat the microbial chemistry processes only induced to the carbonate precipitation,most of the HCO₃^-is from the seawater.