Ichnofabric And Their Sedimentary Evolution Of The Third Member Of Ordovician Majiagou Formation In The North-west Of Henan Province

The carbonates of the third Member of Ordovician Majiagou Formation in the North-west of Henan Province contain plenty of trace fossils.The trace fossils,ichnofabrics and sedimentary evolution were studied by integrating field outcrop observation,rock slice and thin section data.The results show that Trace fossils from the third Member of Ordovician Majiagou Formation in the North-west of Henan Province consist of at least 7 ichnogenera and 10 ichnospecies and four ichnofabrics are present according to the composition and occurrence characteristics,abundance and diversity of trace fossils as well as bioturbation index.The Psilonichnus ichnofabric reveals low energy sedimentary environment between supratidal zone and high tide flat of intertidal zone.The Thalassinoides ichnofabric indicates high energy sedimentary environment between the middle and lower part of the intertidal zone as well as beach in carbonate platform.The Helminthopsis ichnofabric suggests relatively low energy sedimentary environment such as lagoon in restricted carbonate platform,interbank sea.The Helminthopsis-Chondrites ichnofabric develops in low energy between interbank sea and depression of open carbonate rock platform.Ichnofabrics by fine descripting and the same ichnofabrics tend to appear in the same sedimentary environment,under the influence of cyclical sedimentation,the same type of trace fossils repeated in the vertical sequence,placed it within the stratigraphic framework study to analyze the development characteristics under the stratigraphic framework of Ichnofabrics.In this paper,sedimentary evolution of twice transgression and regression was also interpreted by descripting and analyzing the ichnofabrics and its sedimentary environments.Finally,3D depositional model and the vertical sedimentary evolution sequence were established.This study has great reference significance in reconstructing the Ordovician sedimentary environment and paleogeography of North China Basin.