A Study Of Sedimentary Characteristics And Its Geological Implication Of Late Ordovician In Southern Ordos Basin

The mainly research meathod of this thesis is combined with lithology observation, sampling and laboratory study at field measured sections.One of research purpose is focused on the source rock feature and the tectonic background of volcanic tuff samples in Jinsushan Formation via mineralogical and geochemical analysis, as well as plotting discriminant diagrams. Furthermore, the study also covers the homological comparision between the several beds of volcanic tuff in Jinsushan Formation and four famous layer K-bentonites in the Upper Mississippi Valley,North America,by using the method of stepwise discriminatory analysis(a kind of multivariate statistical analysis) and SHRIMP dating on zicons. Besides, this study, testing on the carbon and oxygen isotope of carbonate rock samples in Jinsushan Formation, contributes to establish the carbon isotope stratigraphic framework, and enables recognition of the global paleoclimate changes in Late Ordovician.The Xilinggou Section and Zhaolaoyu Section are located in southern Ordos basin, the strata of upper Ordovician is complex, with dramatic sedimentary face changes,and mainly consists of Jinghe Formation, Jinsushan Formation and Taoqupo Formation in measured sections. The typical lithology of Jinsushan Formation is laminated dark limestone, locally enrichment of silicon strip limestone, with some shale and calcirudyte induced by gravity flow, which represents a deep-water carbonate facies. In this formation, several marked tuffaceous clay beds, a few centimeters for thickness with orange-yellow on fresh surface, are discovered and characteristic of the smooth hand feeling and lamina beddings. Mineralogical and geochemical analysis show that the clay beds mainly consist of illite and mixed illite-montmorillonite in composition, with some quartz, calcite, kaolinite and zircon minerals. Chemical element component suggests the average content of K2O in volcanic clay rock is more than3.5%, relative enrichment for Th and U as well. From the above, the volcanic tuff is typical K-bentonites.It can be inferred from the Ga-Zr/TiO2and Nb/Y-Zr/TiO2discriminant diagrams that the K-bentonites in this region belong to rhyodacite resulted from intermediate to acid magma.Moreover, the Zr-TiO2and Y-Nb tectonic setting discriminant diagrams illustrate that the source rock is likely derived from syn-collision island-arc volcanism. SHRIMP U-Pb dating on zicons from K-bentonites samples of XLG-124.5, XLG-115.7, ZLY-106.3and ZLY-110.2has respectively yielded age:(451.5±4.9)Ma,(465±8.3)Ma,(457.5±5.1)Ma and (452.1±5.1)Ma. Compared to the known data of Millbrig and Deicke bentonites, the XLG-124.5and ZLY-110.2are identical to Millbrig bentonite, while ZLY-106.3points to the same era with Deicke bentonite. However, the stepwise discriminatory analysis indicates that they may not resulted from the same episodes of the giant eruption. Based on the evolutional characteristics of Qinling Orogen in this era, the K-bentonites in Jinsushan Formation were most likely from volcanic eruption of the island-arcs along north side of the Shang-Dan oceanic basin. The strong basement subsidence at southern margin of the Ordos basin and the significant change in the sedimentary framework there in early Late Ordovician may be best interpreted as depositional responses to the tectonic extension and subsiding initiated in the back-arc basin north to the Erlangping island-arc. The climate changes in late Ordovician may have great correlation with the active volcanism in this era. In respect to carbonate rock, there are positive carbon isotope excursion (X7and Z3members) in the upper Jinsushan Formation by2.11‰and5.61‰respectively,Combined with biostratum and SHRIMP dating limitations which probably represents GICE excursion. In Jinghe Formation, carbon isotope excursion in X1member by2.45‰may be a MDICE excursion.