鄂尔多斯盆地怀远运动期不整合类型及其分布特征OACSTPCD

The Huaiyuan Movement is an important tectonic event in the early Paleozoic of the North China Platform,resulting in a regional unconformity.Based on outcrop data,geological maps,drilling data and new seismic data,the identification marks,types and distributions of unconformities formed dur-ing the Huaiyuan movement in the Ordos Basin were analyzed.The results show that(1)sandy conglo-merate and weathered crust developed near the unconformity surface;additionally,high acoustic time,high gamma values,high content of trace elements(U,Th and K),high thorium-uranium ratios,low resistivity,and low density of logging curves occurred near the unconformity surface.The seismic response of the unconformity is characterized by a strong phase,and there are obvious differences in amplitude in-tensity,frequency and continuity in the wave groups on both sides of the unconformity.In addition,the truncation point reflections can be observed in local domain.(2)The Huaiyuan movement caused both parallel-unconformity and low-angle cutting unconformity in the basin.The former was widely distributed in the basin and its periphery areas,and the latter was distributed near the paleo-uplift in the basin.(3)The Ordos Basin and its periphery areas were affected by three stages of the Huaiyuan tectonic move-ment:during the depositional period of the Zhangxia Formation in the Middle Cambrian,uplift first oc-curred in the southeast region of the basin and then moved to the northwest direction;at the end of the Sanshanzi Formation deposition period in the late Cambrian,paleo-uplifts developed in the basin,and they were subjected to erosion;at the end of the late Cambrian,before the Ordovician Majiagou Formation was deposited,the negative drift of carbon isotopes first occurred in the bottom layer of Majiagou Forma-tion due to the influence of global sea level decline.These results provides a reference for the research of new fields of oil and gas exploration in Lower Paleozoic in the Ordos Basin.