Paleoceanographic Environments Of Mesoproterozoic Gaoyuzhuang Formation In The North China Platform:Evidences From Sedimentary Geology And Geochemistry

It has been suggested that, during the mid Proterozoic (ca1.84to ca0.85Ga), the ocean was in a permanent stratified status, with the surface layer moderately oxygenated, middle layer euxinic, and the deep layer anoxic but ferruginous. In order to further understand the environmental evolution of the sulfur-cycle anomalous ocean and the possible geobiological processes in such an oceanographical condition, the present thesis, taking the Mesoproterozoic Gaoyuzhuang Formation as an example, mainly deals with the carbonate geochemical characteristics of this formation and try to explore into the paleoceanographic chemistry of the North China epeiric sea under which the Gaoyuzhuang formation was deposited.A total of142samples were collected from the Gaoyuzhuang Formation (1.53-1.6Ga) at Luojiagou andNiemenzi sections, Kuangcheng, Hebei province. Most of the samples are from carbonate succession, with some of them also from interbedded shale within the carbonate. Major elements, trace elements, especially redox-sensitive elements, and organic carbon isotope composition of these samples were carefully analyzed by aids of various suitable instruments. A number of geochemical proxies that are useful for the determination of depositional environments, such as those for paleoclimate (Mg/Ca), Paleosalinity (MgO/Al2O3)*100),Paleobathymetry (Sr、1000*Sr/Ca、Sr/Ba) etc, were selected as major indicators in concert with carbon isotope data and facies analysis to reconstruct the depositional environment and their variation in time and space for this formation. The study shows that the Gaoyuzhuang Formation was formed during a giant transgressive-regressive cycle. Most of the sediment in this formation was deposited in the environments predominated by deep ramp to tidal flat. The third member in this formation represents the deepest part in this giant depositional cycle.In this study, REE+Y distribution model was used to eliminate the possible influences of hydrothermal effects to the obtained data, and the possible influences of non-authigenic elements to the analyzed elements were delineated by calculation. The enrichment of redox-densitive elements (e.g. Uauth、V, Mo, etc.), redox proxies (e.g. V/(V+Ni), Th/U, V/Sc,8U) and some REE atios (e.g., V/Cr、Ni/Co、Ce/Ce*、Eu/Eu*、(La/Ce)N) were used for the analyses of depositional environments. The study shows that, during the deposition of the Gaoyuzhuang Formation, the North China epeiric sea was generally dominated by redox conditions, with the third member of the formation mainly deposited in an prominent anoxic condition. Based on the Mesoproterozoic euxic ocean model, and compared with those in the modern Black Sea and Cariaco Basin, as well as those in some of the typical Mesoproterozoic anoxic basins, the proxies of Mo/TOC, FeT/Al, MoEF, and UEF etc. in the third member of the Gaoyuzhuang Formation show the characteristics of episodic euxinia paleoceanographic chemistry during its deposition.The present study has provided insights into the analysis of Mesoproterozoic depositional environments and paleocean chemistry in the North China Platform. In addition, the study also shows that in the analysis of ancient depositional environments, some of the geochemical proxies can not be used independently, and should be combined with some other proxies together in order to get a more reliable result.