| At the end of the Neoproterozoic（720-541 Ma）,global geological events such as the“Snowball Earth”glaciations,atmospheric-oceanic oxygoxygenation,ergence of metazoans and phosphogenic events occurred.As the beginning of the Neoproterozoic-Cambrian Phosphogenic Event,abundant marine sedimentary phosphorite deposits acculmulated around the world during the Ediacaran period.These phosphogenic episodes occurred in the aftermath of Marioan glatiation and were concurrent with oceanic oxygenation and metazoan evolution.While stratigraphy,geochemistry and fossil assemblages suggest that the late Neoproterozoic environmental perturbations and biological innovations may be intimately linked,the underlying mechanisms remain unclear.The Ediacaran strata in South China are widely distributed,continuously exposed and well preserved,making it an ideal object for studies on major global geological events of this critical period.In this thesis,10 sections of the Doushantuo Formation are selected in the Weng’an area,Central Guizhou Province,one of the most important phosphorite deposits in China.With systematic analysis of the sedimentology,petrography,mineralogy and isotope geochemistry of the Doushantuo phosphorites,we reconstructed the sedimentary environment and seawater redox state,and further proposed the depositional mode.The main results of this thesis are listed as follows:1.Based on the study of lithofacies and sedimentology of Doushantuo Formation in the study area,five lithological units and three broad facies categories are subdivided,two paleo-karst surfaces and two sea level cycles are identified.The study area can be further divided into shallow-and deep-water sedimentary areas.Deposition of the two economic phosphorite beds,the lower phosphorite bed（LPB）and upper phosphorite bed（UPB）,was controlled by two sea level cycles that dominated the precipitation and accumulation of phosphates.The first transgression event had a limited impact,leading to deposition of the LPB only in deep-water sedimentary area.While the second transgression event had a wider influence,forming the UPB in both shallow-and deep-water areas,of which lithofacies characteristics are different.There was a calm bay in study area during the Ediacaran Doushantuo period,of which the sea depth gradually deepened from the north-west to the south-east.2.Based on the mineralogical study of various phosphorite facies in the study area,it is found that sub-spheroidal phosphatic grains are the most common constituent,which can be divided into four types according to their morphological characteristics and genetic mechanism.According to classification standard of limestone,the phosphatic grains are subdivided into micro-oncoids,peloids,bioclasts and intraclasts.The phosphatic micro-oncoids and peloids mainly occurre in the LPB and shallow-water UPB,while the bioclasts mainly occurre in the deep-water UPB.Distribution of the intraclasts is controlled by sea level fluctuation.It is suggested that the micro-oncoids were formed by microbially-mediated precipitation,the peloids were formed by in situ chemically-oscillating reactions,the bioclasts came from replacement of transported organisms,and the intraclasts were formed by hydrodynamic agitation.3.Here we report the occurrence of distinctive,early diagenetic pyrite-marcasite-quartz-OM（organic matter）associations from the lower Doushantuo Formation in study area.Based on the mineralogical and isotopic geochemical study on this mineral associations,we propose a redox-p H modle to explain its formation and discuss the possibility to build a mineralogical proxy for seawater redox state.In this study,special mineral associations were observed in the dolomite-rich lamellae within the LPB from 5sections.They have pyrite as the nucleus and marcasite as the petals,forming a flower-like structure,that are hereon referred to as Pyrite-Marcasite Rosettes（PMRs）,or Iron-Sulfide Rosettes.The pyrite and macasite association indicates changes in p H of the precipitation environment,because pyrite is mostly formed in neutral-alkaline solution of p H>6,while the marcasite prefers acidic environment of p H<5.Through optical microscope observation,SEM-EDS and Raman spectroscopy tests,it is found that iron disulfide minerals coexisted with quartz,organic matter and barite particles within the rosettes.These rosettes are concurrent with phosphatic grains and earlier than the dolomite cementation,and are presumed to be products of the early diagenesis.Additionally,It is commonly observed that the pyrite nuclei have irregular shapes,likely as a result of surface corrosion.Theδ13C values of powder samples taken from the dolomite-rich layers that commonly contain PMRs range from-2.57‰to+3.10‰,which is close to the normal seawater value.In the same PMR,Theδ56Fe values vary from-0.11‰to+1.45‰for marcasite and from+0.17‰to+1.09‰for pyrite nucleus,with a fractionation of△56Femrc-py=-0.56‰to-0.42‰,indicating a oxidation and partial dissolution of the pyrite.Theδ34S values of pyrite nuclei and marcasite blades vary from–25.1 5‰to–14.36‰（average=–18.65‰）and–27.86‰to-15.21‰（average=–20.66‰）,respectively,indicating results from microbial sulfate reduction.In contrast,the outermost pyrite rims have systematically heavierδ34S ranging between–9.55‰to+0.14‰with an average of–5.76‰,which might be results of Rayleigh fractionation.Thus we propose a redox-p H modle.When O2-rich bottom water enters the unconsolidated sediments,oxidizes the pyrite particles in the sediments and partially dissolves them,thus creating an acidic pore water environment,which is favorable for the precipitation of quartz and the rapid growth of marcasite.Following this reasoning,we consider that the growth of marcasite and the size of PMRs is eventually controlled by the available O2 in the pore water.On the basis of this theory,we calculate the size of rosettes from 5 sections,and thus infer the variation trend of O2 concentration of seawater during the early Ediacaran period.These observations provide independent evidence for an intermittent shallow ocean oxygenation event in the aftermath of Marinoan glaciation,leading to a increase in O2 concentration and oxidation of the sediments.With the end of this oceanic oxygenation event,the seawater had a decrease in O2 concentration and maintained oxic-suboxic state till the end of the first sea level cycle.4.Based on the above observation and discussion,a depositional modle of Doushantuo phosphorites in study area is proposed,emphasizing that the difference between the LPB and UPB might be resules from two distinct phosphogenic events.It is suggestted that the deposition of Doushantuo phosphorites in study area may be controlled by sedimentary palaeogeography,sea level fluctuations,redox state oscillations and biological activities.The LPB and UPB are different in their distribution,lithofacies chracteristics,phosphatic grain types,Iron-Sulfide Rosettes content,OM degradation,and fossil assemblages,indicating that the nature of the two phosphogenic events in the early and late Doushantuo period may have changed.During the early Doushantuo period,a large amount of phosphorus entered the bay with the first transgression,enriched near the redox boundary under the action of“Fe redox pump”,and formed the LPB under the microbial activities.During the late Doushantuo period,with the second transgression,the organic carbon and the living bodies were carried into the bay and deposited in the deep water area and formed the UPB. |
