Analysis Of Sedimentary And Tectonic Evolution Of The Western Margin Of Ordos Area In Ordovician

The western margin of Ordos is very complicated,as one of the difficulties in regional geological research,which is adjacent to the paleo Asian Ocean structural domain,Qilian structural domain and Qinling structural domain,and spans many structural belts.The study area mainly consists of several important sedimentary tectonic units,involving the Ordos block,the Alxa block,the Yinshan block,the North Qinling block and the North Qilian structural belt.The interaction of each tectonic belt controls the temporal and spatial evolution of the western margin of Ordos in Ordovician.Thus,the sediments of the western margin of Ordos were mainly formed in Ordovician.Two important structural events had been happened in the western margin of Ordos,from the late Cambrian to the early Ordovician and the late Ordovician,which obviously distinguished the sedimentary evolution of the studied area from the Cambrian and Upper Paleozoic.It gives important clues on the major structural evolution in the western margin of Ordos which has well reginal correlation between the upper and lower Paleozoic.The western margin of Ordos could be divided several stages in Ordovician,including the Yeli Stage and Liangjiashan Stage of Early Ordovician,Majiagou Stage of Middle Ordovician and Fengfeng Stage,Pingliang Stage and beiguoshan Stage of Late Ordovician.The south of the studied area has developed the carbonate sediments surrounding the Ordos land,and yet the central and northern parts has uplifted as the disintegration area in the Yeli Stage and Liangjiashan Stage.Later in Majiagou Stage,there are limited-open platform and open sea platform deposits is the studied area.The Yimeng land located on the north of the western margin of Ordos and the Zhenyuan uplift located on the south the western margin of Ordos,respectively.In the Fengfeng Stage,it developed open platform,shore platform,coral reef and continental slope deposits surrounding the Ordos land.During the Pingliang Stage,the inner Ordos block had been uplifted into denudation area which developed open platform,shore platform and continental slope and deep water deposits.However,the southern of the studied area only developed open platform and shore-continental slope deposits in the Beiguoshan Stage.In this study,the disputed strata of the western margin of Ordos have been redefined via the Zircon U-Pb dating,the estimate of sedimentary evolution and the regional tectonic setting analysis.According to up-to-date study results,the depositsof the Beiguoshan Stage happened during the end of Late Ordovician,and the Pingliang Formation together with its western Wulike Formation,Lashizhong Formation and Gongwusu Formation deposited in the middle and later stage of Late Ordovician.And Fengfeng Formation deposited in the early stage of Late Ordovician.The deposits of the Xiangshan Formation was formed after the Miboshan Formation,which was underlying the Late Ordovician together with Miboshan formation.The structural background,the source lithology and source rock age and the sedimentary characteristics of the western margin of Ordos has obvious differences along the Guchengzi-Niushoushan-Pingliang direction in the Late Ordovician,which suggests the mainly mixed sources in the studied area.The material sources have also some differences between the eastern Gandu Corridor transition and the eastern Alxa block and the studied area.The eastern Gandu Corridor transition and the eastern Alxa block are clearly affected by the provenance of the North Qilian structural belt,while the western margin of Ordos has mainly difference of provenance between the north and the south of Ordos.The north of the studied area(Zhuozishan mountain)was mainly interacted by provenance of Yimeng and the paleo-uplift in the northeast of Alxa block.Before the deposition of the Wulike Formation,it was affected by the Yimeng.Later,it was influenced by the provenance of the northeast of Alxa block.The south of the studied area(the south of Yutan 1)was mainly affected by the North Qinling structural belt.It could be inferred that a segmented trough began to form between the Alxa,the Gansu Corridor transition zone and the Ordos in the middle of Late Ordovician.The V-shaped trough opened along the north-south direction.The ocean basin was mainly distributed in the central and southern part of the studied area along the east of Qinglongshan-Shibangou and the west of Pingliang area.The segmented trough gradually disappeared in pace with the assemblage of the Qinglongshan-Shibangou and Pingliang area during Silurian.The short existence of the limited trough between Alxa and Ordos,opening to the south,disappeared in the Early Silurian.During the disappearance of the trough,there was no obvious collision between the two blocks.It means that there has no long-distance separation between Alxa and Ordos and there has no space for the development of the paleo-ocean.And it is either impossible to have collision and cohesion between the two blocks.The evolution of sediment and provenance in the south of Ordos are well interconnected with the north collision movement of North Qinling Mountains.The depositions of the southern part evolved from the platform to the margin slopeplatform in the Early and Middle Ordovician,which caused by the south downthrust of Erlangping back arc basin.Then,the platform margin slope developed into continental margin slope in the middle of Late Ordovician,which was related to the subsidence caused by the continuous northward movement of subduction zone.Later,the continental slope changed into the shore platform slope from the Pingliang Stage to the Beiguoshan Stage which was induced by the collision of the North Qinling Mountains.Both the mentioned blocks basically connected in the Tangwangling Stage,the end of Late Ordovician and formed a set of conglomeratic sedimentary system.The uplift of the western margin of Ordos from the Late Cambrian to the Early Ordovician may be closely related to the transformation of the passive continental margin into the active continental margin of the Alxa and the northern Ordos.The brief sedimentary discontinuities happened in the south of the studied area in the Early and Middle Ordovician that were probably caused by the closure of the eastern Erlangping back arc basin.The western paleo-uplift in the Majiagou Stage of the Middle Ordovician was inherited by the relics of the paleogeography of Early Ordovician.A noticeable transformation of the paleogeographic features within the western margin of Ordos was caused by the subduction of the northern ocean of the North of Qilian arc between the transition of the Middle and Late Ordovician.The successional subsidence of the Gansu Corridor and the western Ordos was closely related to the continuous subduction and the northward movement of the tectonic belt.The paleo-uplift blocked the subsidence of the western margin of Ordos and prevented the eastward expansion of the subsidence along with the gradually steeper slope and the increase of water depth.The whole Ordos block began to uplift in the middle and later stage of Late Ordovician,with little relationship between the northward movement of the North Qinling and the North Qilian arc,which was mainly caused by the south tectonic of paleo-Asian Ocean.Meanwhile,the whole uplift finally ceased the whole stage of Ordovician sedimentary and structural evolution in the western margin of Ordos.