Constraints on the Ediacaran-Cambrian boundary in deep-water realm in South China: Evidence from zircon CA-ID-TIMS U-Pb ages from the topmost Liuchapo Formation

The placement of the Ediacaran-Cambrian boundary in deep-water realm in South China and the high-precision temporal framework for the Ediacaran-Cambrian transition in this region have not yet been completely solved. Recently, we have found two K-bentonite beds in the top of the Liuchapo Formation at the Pingyin section, Guizhou Province. It provides an opportunity for constructing the temporal framework of the transitional strata on the Yangtze Platform in South China and for determining the Ediacaran-Cambrian boundary in this area. In this study, we conducted high-precision CA-ID-TIMS U-Pb dating on zircons from the two K-bentonites. The ages of the two K-bentonites were precisely constrained at 536.40±0.47/1.1/1.2 Ma(2σ, n=7, MSWD=0.92) and 541.48±0.46/1.1/1.2 Ma(2σ, n=8, MSWD=1.3). Combining the geochronological results with organic carbon isotope data of chert in the topmost Liuchapo Formation from the section, we suggest that the EdiacaranCambrian boundary should be consistent with a significant negative organic carbon isotope excursion between the two Kbentonites. The scheme of the Ediacaran-Cambrian boundary in this study is of great significance for global correlation, and further for unravelling the information of the terminal Ediacaran-early Cambrian ocean.

Re-Os Geochronology of the Liuchapo Formation across the Ediacaran-Cambrian Boundary of the Yangtze Block(South China)

The Ediacaran-Cambrian(E-C) succession in South China records remarkable oceanic,biological and geochemical variations,but it was not well defined geochronologically,which hinders the interpretation of the spatio-temporal seawater chemical architecture during the time E-C interval.This study presents two Re-Os isochron ages of 520.2±6.1 and 561.7±8.5 Ma for the barite-rich black shales from the top Liuchapo and Doushantuo formations respectively in Tianzhu County,Guizhou Province.In combination with existing age data,the two new Re-Os isochron ages suggest that the Liuchapo Formation was deposited between 550 and 520 Ma.Moreover,like the polymetallic Ni-MoPGE layers of shelf margin(or platform) facies and V-rich horizons of transitional(or shelf slope) to deep-water facies,the barite deposits were likely formed due to differential mineralization.The timing offset likely resulted from differential elemental concentration related to certain local factors(i.e.,hydrothermal fluids,seawater redox and biological activity).The isochron-derived initial ^(187)Os/^(188)Os ratios of the top Liuchapo Formation(0.902 ± 0.048) and the Doushantuo Formation(0.740 ± 0.042) fall within the range of continental weathering flux(1.54) and oceanic crust(0.126),implying the involvement of marine hydrothermal fluids.Moreover,their difference of initial ^(187)Os/^(188)Os ratios may reflect variations of continental weathering intensity and uplift magnitude.

A Diverse Microfossil Assemblage from the Ediacaran–Cambrian Deep-Water Chert of the Liuchapo Formation in Guizhou Province,South China

The Ediacaran to Cambrian transition witnessed great biological and environmental turnovers from the Precambrian to Phanerozoic.These changes are also reflected by the fossil records during that time.However,the fossil distribution was highly heterogenous.While diverse fossils were constantly found from the shallow shelf including restricted basins,reports from deep-water areas,which are characterized by chert-dominated formations straddling the Ediacaran-Cambrian boundary,were scarce.Among them was the Liuchapo Formation that has been considered barren.In this study,a fossiliferous assemblage was found from the Liuchapo Formation in east Guizhou Province,South China.This assemblage comprises Palaeopascichnus jiumenensis,Horodyskia minor,multicellular algae that possibly related with red algae,cyanobacteria,sponge spicules,small shelly fossils,membrane structures of uncertain affinity,and fossils with structures resemble the Ediacaran Megasphaera.Palaeopascichnus and Horodyskia have been widely reported and abundant in the Liuchapo Formation and equivalents,while the others have seldomly,or never,been found from this formation or,more widely,the deep-water chert of transitional interval.The discovery not only contributes to the rare paleontological records preserved in deep-water sediments,but also expanded the geographic distribution of the fossils,providing new materials of biological diversity during this critical interval.