Microbially-Induced Sedimentary Structures And Taphonomy Of Ediacaran Fossils In Zigui,West Hubei Province

The Ediacaran was a period of great changes in the earth's environment.The oxygen concentration of the surface environment significantly increased.Meanwhile,the Ediacaran biotas,a group of eukaryotic multicellular soft-bodied organisms,began to emerge.Prior to the evolution of complex metazoans,the Precambrian ecosystem was marked by the flourishing of microorganisms and the development of microbially induced sedimentary structures(MISS),that formed the “Ediacaran garden” of microbial mats.At present,studies of Ediacarans and MISS are mainly focused on siliciclastic rocks,whereas little is known about MISS in carbonate,and there are few studies on the taphonomy of Ediacaran fossils in carbonates.Recently,we have discovered a few new fossil sites near the Shunyangxi and Zhoujiaao quarries in Zigui,west Hubei province.Ediacaran-type macrofossils were preserved in the limestone of Shibantan Member as rare examples of carbonate-hosted Ediacaran fossils,among abundant microbially-induced sedimentary structures(MISS).This lithology supports excellent preservation of sedimentary facies and(bio-)geochemical signatures,thus providing a new taphonomic window of the Ediacarans and can be regarded as an example for the study of Ediacaran biotas and microbial mat systems.This study presents a detailed description of stratigraphic section and sedimentary structures of the fossil-bearing Shibantan limestone Member of the Dengying Formation as well as observations using optical microscopy,scanning electron microscopy(SEM)and Raman spectroscopy.The study leads to the following conclusions:1.The Shibantan limestones in Zigui were deposited in a shallow marine environment that was frequently influenced by storms.It shows a gradual decrease in water depth from southwest to northeast.Laminated limestones in the Shibantan Member were formed by frequent storms,which led to the erosion,burial,regrowth and redeposition of microbial mats.The sedimentary environment was an inner ramp – shoal that was affected by periodic storms between the fair-weather base and the storm-weather base.2.Integrated analysis of petrographic characteristics,paleontological characteristics and size distribution of framboidal pyrites(average diameter > 6?m)suggest that the fossil-bearing horizons were deposited in suboxic to(mild)anoxic environments.3.The MISS observed in the Shibantan limestones can be divided into the following four categories:(1)Mat-growth related features,including mat-smoothed ripple marks,wrinkle structures,biolaminites,nodular clusters or tufts,polygonal reticular growths or irregular ridges;(2)Mat-metabolism related features,including wavy crinkled laminae,network fabrics,oriented grains;(3)Mat-destruction related features,such as spindle-shaped desiccation cracks,polygonal desiccation cracks,mat chips;(4)Mat-decay related features and gas dome.4.The Shibantan Member preserves fossils of Ediacara-type organisms and abundant macroscopic algae and animal traces.There are 1 genus and 3 species among the macroscopic algae fossils,including the commonly occurring VendotaeniaantiquaGnilovskaya,1971,and 2 new species(Vendotaeniasixiense Yang et Qin sp.nov.,and Vendotaeniapavimentpes Yang et Qin sp.nov).Ediacaran fossils include Aspidella,Parvancorinaminchamiand a few unidentified discoidal forms and concentrically circular ones.5.The sedimentary environment of the Shibantan Member of Zigui provided uniquely advantageous conditions for the burial and preservation of these soft-bodied Ediacaran biotas,with the following three aspects:(1)Vertical bioturbation within the Neoproterozoic seafloor sediment was rare and the sediment mixing layer was restricted by the extent of diffusion at the sediment-water interface.A sharp redox boundary formed between sediment and water interface,thus facilitating surface sediment diagenesis(including silicification and pyritization),enabling rapid mineralization of carbonaceous materials,and finally,turning into a setting that was conducive to the preservation of soft-bodied organisms(i.e.Mixed Layer Model).The fossils with concentric circle likely benefited from this environment,where rapid silicification and fossil preservation were facilitated.(2)The ubiquitous microbial mats in Neoproterozoic shallow marine environments played a crucial role in the preservation of the Ediacarans,such as decimeter-sized discoidal macrofossils,whose surface was protected by a “death mask”-like film.(3)Periodic storm events during the deposition of the Shibantan Member not only led to rapid burial of the Ediacarans and protected the organisms from the destruction of other organisms and non-organisms,but also resulted in local hypoxia,which was conducive to the preservation of these soft-bodied organisms.In summary,analysis of MISS classification,fossil species and sedimentary environment of the Shibantan limestone Member of the Dengying Formation in Zigui,western Hubei province,provide a new window for the study of marine environment and the evolution of microbial-metazoan ecosystem of the late Ediacaran demands further detailed study.