Stratigraphic Geochemistry Of The Late Ediacaran-Early Cambrian In Akesu-Wushen Area Of Xinjiang

The Ediacaran-early Cambrian transitional period is one of the most remarkable time interval in Earth history, spanning major changes in the "snowball earth", the breakup of the Rodinia supercontinent, and "the Cambrian explosion" and so on. The Tarim Block in Akesu-Wushen area of China records sedimentary successions from the terminal Ediacaran to the early Cambrian comprising different deposition including dolostones in the terminal Ediacaran, cherty phosphorites/phosphoric black shales in the early Cambrian, and the overlying carbonates. Though much has been learned on the paleontology on the early Cambrian of the Akesu-Wushen area, the knowledge of the stratigraphic geochemistry is limited. This study investigates the evolution of the paleo-ocean sedimentary environment of the Akesu-Wushen area across the Ediacaran-Cambrian interval. The results of the study are indicative to understand the geochemical characteristics of the phosphoric black shale series in the early Cambrian in Xinjiang, and to reveal the sedimentary environment and the tectonic setting. Together, the results can be used to correlate the early Cambrian strata of Akesu-Wushen area to those of the central Asia and south China, and even to the global early Cambrian strata.In this study, two sections are selected in the Akesu-Wushen area, the terminal Ediacaran-early Cambrian Sugaitebulake section and the early Cambrian Youermeinake section. The stratigraphic correlation between regional and global stratigraphy based on the research of the Carbon and Oxygen isotope and the cause of negative Carbon isotope excursion across the Ediacaran/Cambrian interval are discussed in this thesis. The paleo-ocean sedimentary environment, as well as the relationship between the phosphorus enrichment and sedimentary environment are evaluated for the two sections by multi-proxy geochemical studies such as the trace element, TOC and so on.A large negative Carbon isotopic excursion occurred around the Ediacaran-Cambrian boundary in the Sugaitebulake section can be correlated globally, which suggests a globally geochemical event. In association with related biological stratigraphic evidences, the δ13C curve of the lower-middle Yuertusi Formation in the Akesu-Wushen area correlates well with those early Cambrian δ13C curves from the Zhujiaqing Formation on the Yangtze Platform (China), lower Tal Formation in India, the lower Sukharikha Formation in Siberia, and the upper Tsagaan Oloom Formation and the lower Bayan Gol Formation in Mongolia. Thus, it can be concluded that the Precambrian/Cambrian boundary of the Akesu-Wushen area should be located at the chert-phosphorus unit at the base of the Yuertusi Formation. In the Sugaitebulake section, the negative813C excursion around the Precambrian/Cambrian boundary might be related to biomass extinction due to anoxic environment, transgression, and/or the oceanic overturn.The geochemical evidences of the trace elements especially the redox-sensitive trace elements (such as V, Mo, U, etc.) and the elemental ratios (such as Th/U, V/(V+Ni),V/Cr, V/Sc, Ba/Sr,δEu,ACe, etc.) provide valuable information, regarding the assumed existence of a stratified ocean (containing the minimum oxygen zone) in the early Cambrian in the Akesu-Wushen area, just like the Yangtze Block. The sedimentary environment of the Yuertusi Formation phosphoric black shale series of the Sugaitebulake section was anoxic-suboxic, with hydrothermal fluids, and the carbonates were in suboxic environment. The contents of TOC in the phosphoric black shale series are lower, with negative δCe anomaly. The lower TOC contents and negative8Ce anomaly should be related to the sea level changes in the stratified ocean: Organic matter-enriched phosphoric black shale series deposited in anoxic settings, and lots of organic matters were oxidized in the suboxic surroundings due to the regression. The black shales of the Youermeinake section were deposited in anoxic-suboxic surroundings, with hydrothermal fluids, and the sedimentary environment of the carbonates, overlying the black shales, might be formed in suboxic environment.Suitable tectonic setting, sedimentary environment and rich phosphorite sources etc. were favorable for phosphorus deposition forming in the Yuertusi Formation of the Akesu-Wushen area in the early Cambrian. The phosphorus sources of the phosphorus-bearing sequence in the Sugaitebulake section were mainly from seafloor hydrothermal, the upwelling oceanic current, and the biological occurrence. Aerobic phosphorus concentration process was also important for the phosphorus enrichment. The phosphorite deposits in the Sugaitebulake section were formed under the alternation between the transgression and the regression, Akesu paleo-uplift margin tectonic setting, near shelf paleogeographic environment, rich phosphorite sources, and anoxic-suboxic sedimentary environment.