Restoration Of The Yan'an Period Of Jurassic Basin In The Western Margin And Adjacent Areas Of Ordos Basin

It is still controversial about the original depositional visage of the tectonic belts of western margin of Ordos basin in Mesozoic, especially in the Yanan period. On the bases of basin analysis three principles (including integrity, dynamics and synthesis) and the theory of reformed basin as guidance, from the basic geological events that the two aspects of the nowadays'remanent sedimentary entities and the characteristics of tectonic evolution of the western edge and the surrounding areas of Ordos basin, by the systemic stratigraphic partition and compare, sedimentary facies analysis, provenance analysis, identify of marginal facies and multidisciplinary analysis of provenance, the new cognition of the sedimentary features of primary basin in Yanan period Jurassic is acquired as a result.The synchronal stratum that can be compared sparsely distributed in the adjacent areas of the study area, through the comprehensive analysis of paleontological fossil, the North-South and East-West of lithologic association comparison and geochemical analysis, Mahuanggou and shangliushui area in Hexi Corridor transitional zone is closely related to the Ordos Basin.The analysis of the formation Sedimentary characteristics and sedimentary facies indicate that the River alluvial system (include meandering river and braided river) and the Delta depositional system are the dominating types in the study area, in addition, Rujigou area in the northwestern of basin and Haiyuan Pantan-3 well in the southwestern of the basin had developed the Paludal facies sediment in the flood basin both sides of the river in the alluvial plain. At the same time, it also shows that the Alluvial fan depositional system is upgrowth in Liushugou area and Honggou coal mine(located in the southern of Erdao ridge) of the northern of Helan mountain and the western of Xinjing area on the margin of Ordos basin, a better direction with the edge of the basin.The result of multidisciplinary analysis from the changes characteristics of the regional stratigraphic thickness and coal seam thickness, conglomerate and paleocurrent, lithology characteristics and clastic zircon dating and so on, indicates that sources for the western Ordos basin mainly come from the northwest direction of the metamorphic rock of Alashan block and the southwest direction of the outcrop area of South armand and West armand (the metamorphic and igneous rock in Qilian orogenic belt) located in the southwest Ordos basin, and the other due west source area may exist.Combining the synthetic analysis, the boundaries of primary basin during the Yanan period Jurassic are approximate as follows. The northwestern borderline is the Helanshan western fault belt, which divides the basin from the Alaxa ancient land. The central southern borderline, Ordos basin has opened up to the Liupanshan basin and Hexi Corridor zone to the west. The southwestern borderline lies in the Liupanshan western fault belt, parting from Longxi ancient land. The southern borderline can reach over nowadays Weihe basin and even extending to the north Qinling area.