SIMS U-Pb Zircon Geochronological and Carbon Isotope Chemostratigraphic Constraints on the Ediacaran-Cambrian Boundary Succession in the Three Gorges Area, South China

The Ediacaran–Cambrian succession in South China records dramatic biological,oceanic and geochemical changes,but it is not well constrained geochronologically.This study reports a new SIMS U-Pb date of 543.4±3.5 Ma(MSWD=1.2)from a tuffaceous layer in the Zhoujiaao Section,and carbonate C-O isotopes in both Zhoujiaao and Sixi sections,Three Gorges area.This tuffaceous layer is present in the Upper Dengying Formation(i.e.,the Baimatuo Member)which is characterized by a stableδ13Ccarb plateau and the beginning of a negativeδ13Ccarb shift near its upper boundary.In accordance with the existing biostratigraphic and chemostratigraphic data,this new date corroborates that the upper boundary of the Dengying Formation in South China is approximately equivalent to the Ediacaran-Cambrian boundary(ca.541 Ma).This age also provides the minimum age of the last appearance of the Shibantan biota in the Three Gorges area,indicating that the terminal Ediacaran index fossils(e.g.,Cloudina,Sinotubulites)are not reliable stratigraphic markers for further subdivision of the uppermost Ediacaran.

The origin of bedding-parallel fibrous calcite veins in the Lower Permian Chihsia Formation in western Hubei Province,South China

Bedding-parallel fibrous veins occurring as lenticular to flattened intercalations were found in the organic-rich marlstone/calcareous shale of the upper Lower Permian Chihsia Formation in western Hubei Province, South China. They dominantly consist of fibrous calcite crystals with smooth and tight boundaries, forming fence- like inward, syntaxial growth clusters toward the vein center along which a median suture line generally occurs. Petrographic evidence indicates that these veins may form at relatively shallow burial depth, where fluid overpres- sures would have incrementally created the bed-parallel vein space, resulting in displacive growth of fibrous calcite. On the other hand, the C, O and S isotopic data across the vein reveal slightly depleted δ13Ccarb values （-3.32 ‰ to ＋0.19‰ VPDB） and moderately depleted δSOcarb values （--9.6 ‰ to --7.3 ‰ VPDB） with respect to those of coeval seawaters and slightly heavier δ34Spyrite values （--7.88 ‰ CDT） with respect to those of ambient rocks. Stable isotope evidence consistently suggests significant contribution of bacterial sulfate reduction （BSR） to the formation of the fibrous calcite cements in the vein. The BSR could have been intensive with the availabilities of residual sulfate and abundant organic matters in the Chihsia sediments during shallow burial, increasing the alkalinity of pore waters and further promoting carbonate precipitation. Thus, the bedding-parallel fibrous calcite vein in the upper Lower Permian Chihsia Formation is an important time-specific petrographic capsule, providing clues for understanding the diagenetic process in organic- rich sediments.

Paired δ^13Ccarb-δ^13Corg Evolution of the Dengying Formation from Northeastern Guizhou and Implications for Stratigraphic Correlation and the Late Ediacaran Carbon Cycle

This study provides δ^13C profiles from a lower-slope(Well ZK102)to basin(Bahuang Section)environment to better understand the temporal and spatial variability in δ^13Ccarb-δ^13Corg of the Yangtze Block during the Late Ediacaran.Our new δ^13C profiles together with the reported data suggest that the Upper Ediacaran successions from different depositional environments are generally bounded by negative δ^13Ccarb and/or δ^13Corg excursions in the underlying and overlying strata.Moreover,the Upper Ediacaran δ^13Ccarb profiles generally can be subdivided into two positive excursions and an interjacent negative excursion,whereas the paired δ^13Corg profiles from different depositional environments have individual variation trends.On the other hand,these data show a large surface-to-deep waterδ^13C gradient(~5‰variation in δ^13Ccarb,>10‰variation in δ^13Corg)which can be reasonably explained by the heterogeneity of the biological activities in the redox-stratified water column.Furthermore,the decoupled δ^13Ccarb-δ^13Corg pattern with large δ^13Corg perturbations at the lower slope precluded the existence of a large dissolved organic carbon reservoir at the Yangtze Block during the Late Ediacaran.Thus,the high δ^13Ccarb values in the Upper Ediacaran succession could be balanced by large amounts of buried organic carbon likely associated with high productivity.