豫西北奥陶系马家沟组三段遗迹组构及沉积演化规律

摘要

The carbonates of the third Member of Ordovician Majiagou Formation in northwestern Henan host plenty of trace fossils.Based on the field observation and measurement for the trace fossils,this study analyzed the ichnofabrics and sedimentary evolution using rock slice and thin section.The results present as follows.Trace fossils from the third Member of Ordovician Majiagou Formation in northwestern Henan consist of at least 7 ichnogenera and 10 ichnospecies.Four ichnofabrics were identified according to the composition and occurrence characteristics,abundance and diversity of trace fossils as well as bioturbation index.① The Psilonichnus ichnofabric is mainly developed in low-energy sedimentary environment between supratidal zone and high tide flat of intertidal zone.② The Thalassinoides ichnofabric indicates a high-energy sedimentary environment between the middle and lower part of the intertidal zone as well as beach in carbonate platform.③ The Helminthopsis ichnofabric suggests a relatively low-energy sedimentary environment such as lagoon in restricted carbonate platform of interbank sea.④ The Helminthopsis-Chondrites ichnofabric develops in a low energy sedimentary between interbank sea and depression of open carbonate rock platform.The sequence stratigraphic framwork of the third-member of Majiagou Formation was set up and a research about the evolution of ichnofabrics and sedimentary microenvironment was completed,which suggests that the third member of Majiagou Formation experienced two regional transgressive-regressive events.Based on types of rocks,combination of rock sequence,sediment structure,body fossils,combination of trace fossils and ichnofabrics in the third-member of the Majiagou Formation,along with the carbonate sediment model of Tuck (1974) which is appropriate for North China Basin,this study established the 3D facies model of ichnofabrics and sediment microenvironment in the third-member of Ordovician Majiagou Formation.This study has great reference significance in reconstructing the Ordovician sedimentary environment and paleogeography of North China Basin.%豫西北奥陶系马家沟组三段碳酸盐岩中遗迹化石丰富.在对野外露头实地观测的基础上,利用室内切片、薄片分析等对研究区奥陶系马家沟组三段遗迹化石、遗迹组构及沉积演化规律进行研究.结果表明:①研究区奥陶系马家沟组三段共鉴定出遗迹化石类型7属10种.②根据遗迹化石的分子组成、产状、共生关系、丰度与分异度以及生物扰动的程度,结合沉积学特征分析,共识别出遗迹组构4种:Psilonichnus遗迹组构主要发育在潮上带与潮间带上部(高潮坪)低能的沉积环境;Thalassinoides遗迹组构代表潮间带中-下部和台内滩等高能沉积环境;Helminthopsis遗迹组构反映局限碳酸盐岩台地内的潟湖和开阔台地内的滩间海等较低能的沉积环境.Helminthopsis-Chondrites遗迹组构揭示开阔碳酸盐岩台地中滩间海-台内洼地等非常低能的沉积环境.③建立了研究区马家沟组三段的层序地层格架,并在层序格架内对遗迹组构和沉积微环境的演化规律进行了研究,解译出研究区马家沟组三段经历了两次区域性的海侵—海退;综合岩石类型、岩石序列组合、沉积构造、生物化石、遗迹化石组合、遗迹组构等资料,建立了研究区奥陶系马家沟组三段各遗迹组构与沉积微环境的三维相模式.这对研究华北奥陶系沉积环境与沉积演化具有重要的借鉴意义.