Lithofacies Palaeogeographic Reconstruction Of The Middle Cambrian Lithofacies In The Northeastern Ordos Basin

Although the distribution of Middle Cambrian lithofacies palaeogeography in the northeast of the Ordos Basin has been reflected in basin-level lithofacies palaeogeography research,it is only controlled by limited data points,and the field profile research is limited.Cambrian sequence lithofacies palaeogeography research has yet to be carried out,and the distribution of paleo-uplifts has yet to be revealed.Based on previous studies,guided by theories of sedimentology and sequence stratigraphy,taking the northeastern Ordos Basin as the study area,the Middle Cambrian Xuzhuang Formation and Zhangxia Formation as the target horizons,the field outcrop system was surveyed and explored.Based on field survey and field survey of outcrop system,analysis of the latest drilling(logging)well and identification under thin-film microscope,systematically carried out studies on the sequence stratigraphy,distribution and evolution of lithofacies and palaeogeography of the Xuzhuang Formation and Zhangxia Formation,and achieved the following main results Recognition and conclusion:(1)The combination of the field profile and the latest drilling data clarifies the Xuzhuang Formation-Zhangxia Formation sequence stratigraphic division scheme,and divides four third-level sequences SQ1-SQ4 from bottom to top,and the SQ3 sequence crosses the Xuzhuang Formation With the Zhang Xia Formation,a three-level sequence isochronous stratigraphic framework was established in the study area.(2)Comprehensive facies marker analysis,it is believed that the study area is rich in sedimentary facies of the Middle Cambrian,and identified(mixed)shore,limited platform,open platform,platform edge,front slope,(mixed)gentle slope and shelf facies type,and subdivision of sedimentary subfacies and microfacies,it is believed that Xuzhuang Formation developed gentle slope facies,and Zhangxia Formation developed carbonate platform facies.(3)Under the constraints of the tertiary sequence isochronous framework,the system has compiled a three-level sequence lithofacies palaeogeographic map of the Xuzhuang Formation-Zhangxia Formation,revealing the sea-flood-sea under the control of the inherited uplift-depression pattern.The returned fill level fills the filling process.It reveals that the early Xuzhuang Formation sediments had a northwest-southwest-southeast C-type three-faceted ancient geomorphic pattern that was open to the northeast.The northeast trend was connected to the Vast ocean,rather than confining the internal depression of the platform,which was obviously affected by strong storms.Below,a C-shaped zonal facies distribution was formed,with sedimentary filling evolution from the gentle slope deposition in the background of the Xuzhuang Formation to the carbonate platform deposition in the background of the receding Zhangxia Formation.The study will provide a reference for the study of the distribution and evolution of lithofacies palaeogeography in large areas.