Microfossils And Siliceous Rock Of The Lower Cambrian Kuanchuanpu Formation From Southern Shaanxi

The Lower Cambrian Kuanchuanpu Formation in the northwest margin of the Yangtze platform is characterized as rich in Small Shell Fossils and three-dimensional preservation of animal embryo fossils.However,previous studies have focused on the limestone of the middleupper part of the Kuanchuanpu Formation,and there is a general lack of attention to the lower siliceous rocks(chert).In this paper,a diverse assemblage of well-preserved microorganisms have been detected in black cherts of the lower part of the Kuanchuanpu Formation,including 11 species of 8 genera: Siphonophycus typium,S.solidum,S.cf.septatum,Megathrix Longus,Gloeodiniopsis lamellose,Gloeodiniopsis sp.,Renalcis?,Synodophycus sp.,Myxococcoides sp.,Palaeosiphonella sp.and Endoconchia? sp..In addition to the above,6 new kinds of microfossils have been found and marked as Type 1 ~ Type 6.All of those fossils are first discovered in Kuanchuanpu Formation,which greatly increase the composition of the Kuanchuanpu Biota.The discovery of Siphonophycus,Gloeodiniopsis and Myxococcoides which are common in Proterozoic siliceous rocks shows these microorganisms can extend from Precambrian to Cambrian period.Associated with observation in thin sections,a comprehensive analysis including SEM,EBSD,EDS and Laser Raman imagery are used to investigate the process of biomineralization and siliceous deposition.The results show the phosphatization and silicification of microorganisms occurred quickly in sequence before complete degradation.Phosphatization contributes to preserving microbial morphology,and silicification helps to solidify the shape of colonies.The siliceous rocks in Kuanchuanpu Formation deposit as quartz,silicified microorganisms and siliceous cements.The earliest minerals of authigenic siliceous deposits,including silicified microorganisms and siliceous cements,are opal and then dehydrate and gradually convert to chalcedony and microcrystalline quartz.