Restoration Of The Yanchang Period Late Triassic Basin In The Southwestern Margin Of Ordos Basin

The southwest Ordos basin is located between the steady Ordos basin and the active boundary of Qinling-Qilian orogenic belts, with long-term, complicated regional background and evolution history. The discussion about the tectonic attributes and primary features of Ordos basin in Mesozoic has been paid more attention. Various conclusions have been proposed. Especially on the issues of basin attributes and sedimentary range in Late Triassic, there have been large disputes on them. It will directly affect and restrict the understanding and evaluation of structural dynamic evolution and the resource effect in the Meso-cenozoic basin. Based on referring to the available research results from predecessors' works and taking field geological explorations twice, with the analysis principles of "integrated, dynamic and synthetic" and the research theories about reformed basin as guidance, starting from the remanent sedimentary entities of the basin and its adjacent areas and basin-range coupling evolution feature, by the methods of systemic stratigraphic comparison, sedimentary facies analysis and multidisciplinary analysis of provenance, the new cognition of the sedimentary features of primary basin in Yanchang period Late Triassic is acquired.The study area can be divided into four tectonic units, including the tectonic belt of the southwest Ordos basin, Hexi Corridor transitional zone (containing Liupanshan basin), Helan area and Qinling-Qilian orogenic belts. According to the synthetic analysis of fossil comparison, lithological combination and interval correlation, the contemporaneous deposits Yaoshan group, Nanyinger group and the Yanchang group of northern Qinling Mountains can be compared to the Yanchang group inside the Ordos basin. At the same time, the Nanyinger group in north Qilian area develops marginal conglomerate at large, marking remarkable difference to the contemporaneous deposits in Hexi Corridor area and inner Ordos basin; The analysis of sedimentary facies shows that the Deltaic depositional system and Lacustrine depositional system are the dominating types in the researching area; it also shows that the Alluvial fan is well upgrowth on the southwestern margin of Ordos basin. Among the Alluvial fan depositional systems, the Pingliang Kongtongshan conglomerate is the most representative one in the study area.At the same time, the result of multidisciplinary analysis from paleocurrent, lithology characteristics and geochemistry and so on, indicates that sources for the southwest Ordos basin mainly come from four directions: SW, NW, W and S. The southwest provenance is predominated in all the directions, and the Qinling-Qilian folded belts are the main source regions for the deposits in south and southwest Ordos basin.Combining the synthetic analysis, the boundaries of primary basin during the Yanchang period Late Triassic are approximate as follows. The northern borderline is the Helanshan western fault belt, which divides the basin from the Alaxa ancient land. To the west, Ordos basin has opened up to the Liupanshan basin and Hexi Corridor zone. The southwestern borderline lies in the Liupanshan-Xihuashan fault belt, parting from Longxi ancient land. The southern borderline can reach over nowadays Weihe graben and the north Qinling area, even extending to the northern part of Shangdan suture.Through the analysis from the parts of geomagnetism, tectonic stress field, sedimentary response and deep seated structure, combining the evolution characteristics of Qinling-Qilian orogenic belts, it is shown that the structural attribute in the southwestern of the study area, especially in the southern, belongs to the foreland basin with dextral strike slip quality. The development and evolution of the southwest Ordos basin concretely responds to the orogenic process of Qinling-Qilian folded belts.