Basement Features Of The Ordos Basin And The Ordovician Tectonic-sedimentary Respond In The Western And Southern Margin

There is growing concern about the Ordos Basin because of its hydrocarbon resources. It should be noted, though, that the Ordos basin is also potash ore prospect areas in China. In the geological background of global poor evaporite deposits, huge rock salt deposited in North Shaanxi Province, located in the central and eastern Ordos Basin, and it achieved the depositional phase of potassium and magnesium salt. These indicate there are paleotectonics, paleoclimate, paleogeographic conditions for potassium salt in the Ordos Basin. Some basic geological problems, however, remain controversial. On the basis of references to the predecessors’ researches, this paper did detailed observation and sampling analysis of field sections in the southern and western margin of the Ordos Basin and conducted the studies of lithology, stratigraphy, geochemistry, sedimentology, geophysics and zircon chronology. Obtained as follows:1. Aeromagnetic anomaly zonation of the Ordos Basin and adjacent areas was obtained by processing high-precision and large-scale aeromagnetic anomalies with an approach of reduction to the pole upward continuation. Basement depth might be the fundamental control factor for aeromagnetic anomalies because the positive and negative anomalies on the reduction to the pole-upward-continuation anomaly maps roughly coincide with the uplifts and depressions of the crystal-line basement in the basin. The results, together with the latest understanding of basement faults, SHRIMP U-Pb zircon dating of metamorphic rock and granite, drilling data, detrital zircon ages, and gravity data interpretation, suggest that the Ordos block is not an entirety of Archean.2. This paper firstly discovered internal-wave, internal-tide deposits and contourite in the Middle Ordovician Yingtaogou Formation in the Helan Mountain area.3. The petrologic and geochemical characteristics of sandstone from Yingtaogou Formation have been researched systematically. The geochemical signatures indicate its double provenance supply feature. After comprehensive consideration of the middle Ordovician palaeocurrents in the western Ordos and detrital zircon age features, we suggest that the provenance of Yingtaogou Formation was mainly the North qilian orogenic belt and the Alashan paleolandmass as a minor one.4. We report the geochemical and zircon U-Pb dating on K-bentonite samples collected from two sections, located in Jingyang County, Shaanxi Province and Pingliang City, Gansu Province. The major elements and the Nb-Zr diagram show the potassium-rich characteristic. Inactive elements in the process of weathering are employed to reconstruct the protolith, and the result demonstates primary magma of the K-bentonites is high-potassium alkaline rhyolitic dacite and trachyandesite. Most of the K-bentonite samples show typical characteristics of arc volcanic rocks, which were probably related to the subduction setting. LA-ICP-MS U-Pb dating on zircons from4K-bentonites samples in Xilinggou profile and2samples from Yindongguanzhuang profile has yielded6sets of concordant ages. Accordingly, the main depositional period of Pingliang Formation, which is still controversial, is redefined to Sandbian-Katian, Late Ordovician. According to their regional distribution, it is thought that these K-bentonites derive from volcanic eruption in the southern North Qinling or/and southwestern North Qilian region. Composite analysis indicates that the important sedimentary turn appearing the south Ordos Basin results from the subduction northward of Qinling-Qilian ocean. Volcanic eruption events represented by these K-bentonites beds have a good coupling relationship with periodic subsidence and filling process of North Qinling back-arc basin (southern margin of Ordos basin).