| In order to sniff out the sedimentogenesis of Ediacaran chert and determine the Precambrian-Cambrian boundary from South China, in this thesis we selected two typical well-exposed sections on the southern margin of Yangtze platform which preserve continuous Ediacaran-Lower Cambrian strata, followed by multiple geochemical, chemostratigraphic and radioactive isotope chronologic investigations on the basis of the provenance and depositional mechanism of cherty sediments within the Ediacaran ocean, Precambrian-Cambrian boundary horizon and absolute age of the base of Cambrian in south China.To determine the origin of silica in terminal, we have undertaken a variety of sedimentary geochemical proxies, in conjunction with geological features, on well-characterised Piyuancun chert at Lantian section from southern Anhui province. The Piyuancun cherts are typical marine siliceous deposits with microcrystalline structure and high content of SiO2. Silica constituting the chert is partially derived from submarine hydrothermal fluid, indicated by geochemical ratios of Fe/Ti>20 and Al/(Al+Fe)<0.4, as well as position in Al-Fe-Mn triangular diagram. However, Y/Ho(wt) ratios close to seawater from modern open sea and seawater-like rare earth element distribution patterns demonstrate that the sedimentary water body of Piyuancun chert is primarily sourced from seawater. The silicon and oxygen isotope composition illustrates no variation trend stratigraphically whilst relatively high 630Si values indicate that Piyuancun cherts are formed by chemical precipitation of dissolved silica from seawater rather than direct precipitation from hydrothermal fluid. Moreover, estimated values from oxygen isotopic geothermometer are also consistent with normal coeval seawater temperature. Bottom seawater was first mixed with silica-rich submarine hydrothermal fluid and subsequently driven by upwelling to relatively acidic waters far away from hydrothermal activity center. As a result, silica within the regional seawater became over-saturated and thus precipitated. Extensively existence of Algae mats and microfossils including Palaeopascichnus jiumenensis in Piyuancun chert indicates thriving ocean paleoproductivity. Acidic substance produced through degradation of organic matter and biodegradation process would have, to some extent, dropped the solubility of dissolved silica in seawater and thus promoted the deposition of Piyuancun Chert.In addition, high precision SIMS zircon U-Pb dating has been carried out on the tuff sample which preserve within the basal horizon of Daibu member, Zhujiaqing formation from Xiaotan section, Yongshan County, Yunnan Province. One group of measurements including twenty-two dots, which suffer Pb loss to a lesser degree, yield a highly reliable Concordia U-Pb age of 538.5±1.6Ma (MSWD=0.0038). Xiaotan section comprises continuous and well-exposed marine faced stratum of Lower Cambrian, preserves the best Small shelly fossils(SSF) Assemblages, and simultaneously exhibit the large negative 613C excursion at the base of Cambrian. This new dating result from south China is similar to that from Salt basin in Oman(540±0.13Ma) and standard age from Nambia(540.61±0.88), indicating that 538.5±1.6Ma is the best estimate age of basal Cambrian from south China. Besides, it also verify the feasibility of considering BACE as the stratigraphic marker of Precambrian-Cambrian boundary. In view of the above-mentioned facts, we propose that Xiaotan section is a better choice for GSSP than the Forture Head section. |
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