The Stromatolite And Its Geochemical Characteristics Of The Sinian Dengying Formation In Jiangshan,Zhejiang Province

Stromatolite dolomite haved developmented in the Sinian Dengying Formation in Jiangshan,Zhejiang province.According to the macroscopic morphological characteristics of stromatolites,it is divided into stratiform stromatolite,wave stromatolite,hummocky stromatolite,cone stromatolite and columnar stromatolites.The corresponding sedimentary environment is the upper part of the tidal zone,the upper part of the intertidal zone,the lower part of the intertidal zone and the subtidal zone.The stromatolite microstructural mainly includes medium band,thick band,and regular line;the microstructure mainly consists of lumps,spots,and worms.In the study area,the Carbon Isotopes of the Dengying Formation were relatively less positive values,while the Oxygen Isotopes were negative.The changes of the Carbon Isotopic composition of the stromatolite dolomite in the Dengying Formation are positively correlated with the relative changes of sea level.As the sea level drops,so does the value of?¹³C.The content of Zr,Hf and Th are lower than its content in the upper crust,means that the study area samples are largely unaffected by terrigenous sediment.The mean value of V/Cr in the study area was 0.83,less than 2.0,and the mean value of U/Th was less than0.38,indicated that the Sinian period research area is oxidized environment.The mean value of Sr/Ba is less than 1,which showed that the salinity of ancient seawater is high and the evaporation is strong.In addition,the enrichment of trace elements such as Sr and Zr shows that the microorganisms plays a significant role in the diagenesis of the stromatolite dolomite.The?REEs content are generally low,LREEs content are relatively richer,while The HREEs element have significant losses,rare earth elements-the North American Shale Standardization Diagram shows that the stromatolite dolomite is relatively steep in LREEs and the HREEs is relatively flat,indicated light rare earth fractionation level higher than that of heavy rare earths.The value of Y/Ho is 27.14⁵8.21,average value is 38.15,indicated that the stromatolite dolomite is less affected by the terrestrial clastic material.The Eu and Ce presents a negative anomaly,indicated that the sedimentary environment in the study area is the oxidizing environment.The morphological types of stromatolites vary with the sedimentary environment.Stratiform stromatolite mainly development in supratidal zone,wave stromatolite mainly development in the upper of intertidal zone,hummocky stromatolite mainly development in the lower part of the intertidal zone,the cone stromatolite and columnar stromatolite are considerable development in the subtidal zone.Stromatolite reefs in the study area has experienced since the end of development to the upper supratidal zone-intertidal zone-intertidal zone at the bottom-subtidal zone this cycle,on behalf of the water from deep to shallow,hydrodynamic conditions from weak to strong,indicating at the end of sinian period at least in the study area experienced a process of regression.In conclusion,in the study area mainly in the oxidation-lean oxygen of the tidal flat environment at the end of Sinian period,which not affected by terrigenous material,The marine environment had experienced regression at the beginning of the the end of Sinian period,and then keep a stable state for a long time.This provides a good living environment for the breeding of microorganism.