Structure And Evolution Of Ordovician Foreland Basin On The Southern Margin Of Ordos Block

The southern margin of Ordos block is located in the transition zone between the Ordos block and the North Qinling orogenic belt,and therefore it is an ideal place to study the coupling between the development of sedimentary basin and orogenic belt in response to the late Early Paleozoic arc-continent collision in the North Qinling orogenic belt.Meanwhile,the Ordovician sequences in the southern margin of Ordos block is also an important target for oil and gas exploration.Therefore,studing the properties and structures of the Ordovician basin is of great scientific significance and also application value.Although the properties of the Ordovician Ordos Southern Basin have been discussed in previous studies,a relatively unified consensus has not been reached so far,and the understanding of the sedimentary filling characteristics of the Ordos South Basin is still relatively weak.Based on the detailed observations in typical outcrops of Ordovician sedimentary sequence on the southern margin of Ordos block,combined with the constraints from geophysical data in the coverage area,this paper systematically investigates the Ordovician sedimentary filling and late deformation characteristics on the southern margin of Ordos block,in order to reveal the tectonic properties and structures of the basin on the southern margin of Ordos block and to explore its relationship with the North Qinling orogenic belt in Ordovician.We found that the Ordovician system at the southern margin of Ordos block is dominated by marine carbonate rocks,and the whole system is thick in the south and thin in the north,showing a wedge-shaped sedimentary body bounded by regional unconformities at the top and bottom.From north to south,it can be divided into depression zone of confined depositional environment,uplift zone of continuous uplift and basin zone that have undergone marine shallow water,marine deep water and terrestrial alluvial environments.Through the study of vertical cyclonic change,it is considered that a complete large sedimentary basin with a complete geological history and sedimentary record from the beginning to the final extinction is developed in the southern margin of Ordos block during Ordovician.In addition,a series of nearly east-west trending south-dipping reverse faults and fault-related folds are developed in the Ordovician of the basin belt,and the intensity of tectonic deformation gradually decreases to the north,which has the typical characteristics of a fold-thrust belt.Based on the above findings and the history of the Early Paleozoic tectonic evolution of the North Qinling orogenic belt,this paper suggests that the Ordovician Ordos Southern Basin is an atypical peripheral foreland basin which formed at the southern margin of the Ordos block in the process of arc-land collision caused by the closure of the Erlangping back arc ocean.The basin has undergone the following four stages of evolutionary history:(1)Yeli-Liangjiashan stage(O₁),the initial flexural stage of the foreland basin;(2)M1-M5 stage(O₁-O₂),the structure-forming stage of the foreland basin;(3)M6-Pingliang stage(O₂-O₃),the rapid subsidence stage of the foreland basin;(4)Beiguoshan stage(O₃),the rapid filling stage of the foreland basin.The Ordovician foreland basin on the southern margin of Ordos block has the typical structure of a classic foreland basin,in which the fold-thrust belt is located in the Xi'an-Baoji area including the Weihe graben;the foredeep zone is spread in the Longxian-Qishan-Fuping area in the near east-west direction;the forebulge zone is located in the Qingyang-Yuncheng area;the back-bulge zone is roughly located in the Yan'an-Yulin-Suide area in an irregular oval shape.