The Ediacaran-Cambrian Boundary And Evolution Of The Fossil Assemblage In The Southeastern Margin Of The Yangtze Platform

The Ediacaran-Cambrian transition was one of the most important and revolutionary periods in geological history,during which global paleo-plate tectonics,paleo-climate,paleo-ocean and palaeontology underwent significant changes.Therefore,the Ediacaran-Cambrian is an important window into the evolution of metazoans and paleo-oceanic environment,and their interrelationships.The remarkable Cambrian explosion occurred during the late Ediacaran to early Cambrian,during which the majority of metazoans appeared on Earth,marking the first establishment of a Phanerozoic marine ecosystem dominated by multicellular animals.Significant progress has been made in research on the timing and processes of the Cambrian explosion,but the mechanisms driving the Cambrian explosion are still poorly studied,particularly the interrelationships between the Cambrian explosion and the paleo-oceanic environment（e.g.,oceanic oxygen levels）,which remain highly controversial.Whether atmospheric and oceanic oxidation contributed to the origin and rapid evolution of the metazoans,or the origin of the metazoans led to oxidation of paleo-ocean remains poorly understood.Secondly,as the oldest,most primitive and simplest animal,the origin and early evolutionary process of sponges are the key to reveal the evolution of unicellular to multicellular animals.However,the time of origin and early evolutionary processes of sponges have been one of the important unsolved questions in biological evolutionary studies.Molecular clock studies and biomarker compound evidence suggested that sponges originated in the Cryogenian,whereas fossil evidence suggested that sponges did not appear until the early Cambrian.This discrepancy may be due,on the one hand,to large errors in bio-clocks studies,where the so-called sponge biomarker compounds may not be unique to sponges,and,on the other hand,to Precambrian burial environments that were not conducive to the preservation of sponge spicules or Precambrian sponges that did not have mineralized spicules.Continuing the search for undisputed Precambrian sponge fossils and performing taphonomy on ancient sponge spicules may be one of the effective solutions to shorten the gap between the first appearance of sponge fossils and the time of sponge origin inferred from molecular clocks and biomarker fossils.Although a large number of fossil sponges（or spicules）occur in Cambrian and are important for our understanding of the early evolution of sponges,the sequence of sponge fossil occurrences among different sedimentary phases and the evolutionary relationships among different sponge species remain unclear.Thus,a comparative study of sponge spicules fossils in the Ediacaran-Cambrian transition strata of different sedimentary phases is important for understanding the spatial and temporal evolution of sponges during this period.The Ediacaran-Cambrian transition in the Yangtze Platform of South China is rich in sedimentary phases,with continuous deposition and extensive outcrop,and are a key area for studying the origin and early evolution of the metazoans.However,most of the studies have focused on the shallow water facies,and there is a relative lack of studies on the slope-basin facies where the sedimentary waters are relatively deep,mainly because the deep-water slope to basin facies are mainly cherts,which contain scarce fossils and are difficult to handle out.Recently,more and more microfossils have been found in the deep-water facies,suggesting that the deep-water chert of the Yangtze Platform may not be completely devoid of fossils.Further systematic studies on these fossils could reveal the biological characteristic and evolution trends of deep-water facies,and understand the interrelationship between biota and environment.Stratigraphy is the basis for paleontological and geochemical studies.However,the deep-water slope and basin facies lack useful marker for stratigraphic division.Numerous studies have shown that the Ediacaran-Cambrian boundary in deep-water facies should be placed within the Liuchapo Formation,but its specific horizon and recognition criterion are still ambiguous,which seriously hinders the correlation between the deep-and shallow-water facies of the Yangtze Platform,and even with other classical Ediacaran-Cambrian sections around the world.In view of these problems,this study developed zircon U-Pb geochronology of tuffs,carbon isotope stratigraphy and biostratigraphy at Sifangjing,Bajiaocun and Yangtou section in northeastern Guizhou Province.The aims of this study are to determine the geochronology of the Liuchapo Formation,provide reference Ediacaran-Cambrian boundary and establish a chronostratigraphic framework in deep-water facies,and make stratigraphic correlations between different facies on the Yangtze Platform.Based on the stratigraphic study,systematic morphological studies and taxonomic evaluations on sponge spicules from the Liuchapo Formation were carried out to determine the age of the first occurrence of sponge spicules in the deep-water environment.Detailed morphological and taxonomic studies on other microfossils were also carried out to reveal the biota features of the deep-water facies of the Yangtze Platform during the Ediacaran-Cambrian transition.The paleo-ocean redox state of the Liuchapo Formation at the time of deposition in the Sifangjing section was investigated through trace element geochemical analyses,and the evolution relationship between the deep-water biota features and the paleo-ocean environment was discussed.Based on the above aspects of studies,the following insights were obtained:1.Two tuffs were discovered from the uppermost of the Doushantuo Formation and the base of the Liuchapo Formation at the Yangtou section.Their ²⁰⁶Pb/²³⁸U weighted average age is 554.2±3.3 Ma and 550.6±3.3 Ma,respectively.This age result is,within the error range,consistent with the boundary age between the Doushantuo and Dengying formations in the Three Gorges area which belongs to shallow-water carbonate platform during Ediacaran-Cambrian transition.This indicates that the ending ages of the Doushantuo Formation between different sedimentary facies are correlative,and the starting ages of the Liuchapo Formation in slope-basin environment and the Dengying Formation in carbonate platforms are approximative.Thus,the Dengying and Liuchapo formation are contemporaneous deposition in different facies.The trace element characteristics of zircons show that the source rocks of the two tuffs belong to granitoids and the source area has a magmatic arc or orogenic tectonic setting.2.SIMS Zircon U-Pb geochronology of the tuff in the middle of the Liuchapo Formation at the Sifangjing section shows a ²⁰⁶Pb/²³⁸U weighted average age of 540.7±3.8 Ma.This age result is close to the Ediacaran-Cambrian boundary age within the range of error.Moreover,the Liuchapo Formation at the Sifangjing section is mainly composed of siliceous dolomite and dolomitic siliceous chert,which is suitable for carbonate carbon isotope analyses.A substantial negative carbonate carbon isotope excursion was recognized within the lower part of the Liuchapo Formation.Combined with the results of zircon U-Pb geochronology,the negative excursion is slightly older than 540.7±3.8Ma.This carbon isotope excursion is correlative with other carbon isotope excursions at the base of Cambrian within and beyond the South China Craton.Therefore,the carbon isotope excursion in the lower part of the Liuchapo Formation at the Sifangjing Formation corresponds to the typical negative carbon isotope excursion at the basal Cambrian,and could be a useful tool for identifying the Ediacaran-Cambrian boundary in deep-water facies.3.A large number of monaxons spicules were collected from the Liuchapo Formation at the Sifangjing and Bajiaocun sections.The first appearance of spicules below the carbon isotope excursion.Based on their morphological characteristics,monaxons from the Liuchapo Formation could be subdivided into diactinal monaxons（oxeas and siciforms）and monactinal monaxons（drop-forms）.Forming the bundles,multiple monaxons have parallel,radial,weak orientation and disorderly arrangements.According to their morphological features and construction styles,it is speculated that monaxons from the Liuchapo Formation have demosponges affinities.Abundant sponge spicules were also discovered in siliceous and phosphatic nodules from the base of the Jiumenchong Formation at the Sifangjing section.Spicules from nodules consist of monaxons（including oxeas,rod-shaped spicules,centrotylostylote and tylostyle.）,stauractines,triaxons and anatriaene.It is supposed that these triaxons and anatriaene have hexactinellids and demosponges affinities,respectively.The EDS analyses indicate that these spicules are mainly made of Si O₂ and partly or completely replaced by pyrite.Some spicules in the nodules from the base of the Jiumenchong Formation have enlarged axials which are infilled with barite.Confocal Raman spectroscopy analyses show that the spicules in the Liuchapo Formation are rich in carbonaceous materials.These carbonaceous materials have 0.87-0.93 of 1350 cm^-1 and 1580 cm^-1 Raman shift peak intensity ratio,indicating that the carbonaceous materials experienced a low peak burial temperature.Taphonomy evaluation showed that diagenesis had significant influence on morphology and composition of spicules from the Liuchapo Formation.Monaxons first occurred near the Ediacaran-Cambrian boundary in different sedimentary facies of the Yangtze Platform.While triaxons first occurred at the early Fortunian.On the whole,sponge spicules have an evolutional trend from monaxons to polyaxons at the early Cambrian.In the terms of spatial,monaxons simultaneously appeared in shallow-and deep-water environments,while triaxons occurred first in deep-water basin environment,and then in platform and slope environments in sequence.4.In the studied area,the Liuchapo Formation not only contains sponge spicules,but also abundant other microfossils mainly including chain-like fossils（Nenoxites and Horodyskia）,algal fossils（Myxococcoides,Renalcis?Siphonophycus typitum and Megathrix longus）and some carbonaceous acicular aggregates and carbonaceous globular fossils with uncertain affinities.Sponge spicules and other microfossils constitute the generally paleontological characteristic of the Liuchapo Formation.Nenoxites and Horodyskia first occurred at the basal Liuchapo Formation,and mainly preserved in the muddy siliceous rock.The last appearance datum of Horodyskia was significantly higher than the negative carbon isotope excursion.While last appearance datum of Nenoxites was corresponded to the negative carbon isotope excursion,indicating that the last appearance datum of Nenoxites has potential to be a useful marker to identify the Ediacaran-Cambrian boundary in deep-water facies of Yangtze Platform.5.Based on the characteristics of redox sensitive trace elements and Ce/Ce*anomaly index,the seawater redox state during the deposition of the Liuchapo Formation at the Sifangjing section was reconstructed.The Liuchapo Formation was divided into the lower,middle and upper parts,according to the variation characteristics of TOC content,U and Mo enrichment factors,V/（V+Ni）ratio,Th/U ratio and Ce/Ce*anomaly.From the lower to upper part,TOC content gradually increased,and redox sensitive trace elements index showed that oxygen content gradually decreased,while Ce/Ce*anomaly indicated a gradually anoxic trend.This contradiction probably resulted from the process that with the enhanced burial of organic matter,sediments became more reductive,and resulted in the complexation of U and Mo with humic acid,so that the sediment is more enriched in redox sensitive trace elements such as U and Mo.While the reduction of organic matter reduced the consumption of oxygen in seawater,and caused lower Ce/Ce*negative anomaly.The abundance and size of spicules increased from the lower to upper part of the Liuchapo Formation,suggesting that the flourishing of sponges in deep-water environment led to the gradual oxygenation of bottom seawater,and provided a favorable place for the origin and expansion of other metazoans.