Stromatolites Characteristics Of Shennongjia Group Of Mesoproterozoic In Shennongjia Region,Hubei

As one of the Mesoproterozoic stratotype candidates,Shennongjia Group in shennongjia Area,Hubei Province,is of a great significance in stratigraphy,sedimentology,paleontology,the geological event research and the restoration and reconstruction of the palaeocontinent.Shennongjia Group developed comparatively complete with weak metamorphic deformation and bearing a large amount of stromatolites,which gave us a good opportunity to do some research.Stromatolites in Shennongjia group are mainly stratiform,colonnella,conophyton and domal stromatolites,also have strati-columnar,cone column,cabbage-like,hemisphere and stromatolite reefs.All of them are mainly distributed in Luanshigou Formation,Dawokeng Formation,Kuangshishan Formation,Yemahe Formation,Wenshuihe Formation and Shicaohe Formation,Yingwodong Formation and Wagangxi Formation of Shennongjia Group.Stromatolites of Shennongjia Group developed mainly in the intertidal-subtidal and a few in the supratidal based on the studies.Stratiform stromatolites grew primarily in the subtidal with lower energy and the intertidal;a small amount of them developed in the supratidal zone.Strati-columnar and columnar stromatolites without walls developed dominatedly in the intertidal.Domal stromatolites were formed in the lower intertidal zone,tidal channel and subtidal environment.Columnar stromatolites with walls prevailingly develop in the subtidal environment.Cone-columnar stromatolites generally grew in the tidal channel.Conophyton stromatolites are mainly formed in the subtidal and the lagoon environment,while the inverted conophyton stromatolites developed in the upper subtidal zone.Stromatolite reefs are formed in the subtidal zone and the intertidal zone.Stromatolites of Shennongjia Group can be divided into 10 stromatolite assemblages,which indicate that different assemblages represent the different sedimentary environment and evolution succession,that is,four stromatolites assemblages of Luanshigou Formation reflects the turbulent subtidal high-energy environment with terrigenous material inputting and land exposed low-energy intertidal-supratidal zone environment;two stromatolites assemblages of Dawokeng Formation indicate the strong hydrodynamic intertidal and subtidal zone and lagoon environment;two stromatolites assemblages of Kuangshishan Formation reflect the weak hydrodynamic intertidal-shallow subtidal environment;two stromatolites assemblages of Shicaohe Formation show the exposed supratidal zone-intertidal zone and subtidal zone with high energy environment.The controlling factors of stromatolites in Shennongjia Group mainly include biological factors,hydrodynamic conditions,water depth,salinity,oxygen content,temperature,and terrigenous material supply,etc.Hydrodynamic conditions determinated the types of stromatolites and the water depth affects on their sizes.The environments where are suitable for microbial flourish,normal or high salinity and sufficient oxygen,appropriate temperature and less terrigenous materials are beneficial to the growth of stromatolites.As a result,a sedimentary model for stromotolite evolution of the Shennongjia Group has been established and the controlling factors about the formation of Shennongjia Group stromatolites are also summarized by combining with stromatolite characteristics and assemblages,sedimentary structures,petrographic features and element geochemical characteristics.Stromatolites research plays an important role for building the key stratigraphy sequence of Mesoproterozoic,biological activity,the palaeocean geochemistry and the deposition and evolution of the sedimentary environment,and so on during the formation and evolution processes of Rodinia supercontinent because this research provided basical data and evidences for them.