The Transition Of Tectonic-sedimentary Framework And Its Geological Significance In The Late Early Paleozoic In The Southern Ordos Basin

The development and extinction of large-scale sedimentary basins are often controlled by the regional dynamic environment,and their tectonic patterns and sedimentary fillings often have good responses and records.The late Early Paleozoic was a critical period for the strike and extinction of the North China Craton Marine Basin.The southern Ordos Basin experienced a process of transition from a passive continental margin to an active continental margin.Its tectonic framework-sedimentary filling is useful to discuss the development and evolution of the Ordos Basin and analyze the regional dynamics background And oil and gas exploration in this area is of great significance.Based on this,this paper takes the Early Paleozoic tectonic-sedimentary evolution in the southern Ordos Basin as the main line,combined with field outcrops,seismic data,drilling data and related test analysis,and focuses on and comprehensively analyzes the late Early Paleozoic tectonic pattern changes and sedimentary filling processes in the southern basin.,Tectonic-sedimentary events and denudation and transformation characteristics,and using tuff interlayers as links to determine the time limit for the transition of the tectonic-sedimentary environment in the late Early Paleozoic,preliminary discussion of its dynamic background,and combining the characteristics of potential source rocks in the southern basin Uplift structural pattern,preliminary evaluation of oil and gas exploration potential in this area,mainly obtained the following conclusions and understandings:(1)Analysis of field outcrops,seismic profiles,and drilling data from the Lower Paleozoic in the southern part of the basin shows that the tectonic pattern and sedimentary filling in the southern part of the basin were obviously transformed in the late Early Paleozoic(Pingliang period).From the early to middle period(Cambrian to Early Middle Ordovician),the depositions mainly composed of shallow-water carbonate rocks and clastic rocks,which were widely distributed and stable in thickness,changed to the late period(Pingliang period)with limited development and orientation.The thickness of the southern strata increased rapidly,and deep-water thin-layered shale and siliceous rocks were the main deposits.During the same period,a large number of normal faults developed;the tectonic pattern has undergone the transformation from continental margin depressions and open platforms to deep-water slope environments.(2)In the late Early Paleozoic(starting from the Pingliang period),the debris flow,gravity flow,and siliceous rock in the southern part of the basin represented the emergence of deepwater environmental deposits.During the same period,a large number of tuff interlayers developed,and the central paleo-uplift in the southwestern part of the basin quickly uplifted and The emergence of syn-sedimentary normal faults;the angular unconformity of the upper and lower Paleozoic in the southwest,long-term sedimentary interruptions,and strong denudation of the strata are considered to be the transition from a passive continental margin to an active continental margin in the southern part of the basin during the late Early Paleozoic-Deposition response.(3)Through comprehensive research on the petrology,occurrence horizon,geochemistry and chronology of the tuff(porphyry)interlayer in the late Early Paleozoic(Pingliang period)in the southern part of the basin,it is believed that the stable and deep water environment is preserved by the tuff The key point is that it has recently been clarified that the tuff source rocks in this area are dominated by intermediate and acidic magma,formed in the volcanic island arc environment,and the development time limit is mainly concentrated in the Late Ordovician Sambi-Kaidi period(449?453Ma);the combined tuff is from the south to the The distribution characteristics of the northern thinning suggest that the transformation of the tectonic-sedimentary system in the southern Ordos Basin in the late Early Paleozoic is closely related to the subduction and closure process of the Shangdan Ocean Basin in the Qinling area to the south.(4)Comprehensive analysis of organic carbon and thermal evolution history.It is believed that the Lower Paleozoic Cambrian dark mudstone and Ordovician Pingliang Formation shale in the southern Ordos Basin are potential source rock strata in the area;the southwestern part of the Late Early Paleozoic Basin The further development of the central paleo-uplift and the appearance of Ordovician deep depressions on the southeast and southwest sides constitute a good paleo-uplift,paleo-sag and regional unconformity configuration structure,which has been preserved to this day and is a potential oil and gas in the early Paleozoic of the area Favorable areas for exploration.