A Preliminary Study on the Ordovician Conodont Biostratigraphy at the Yakrik Section,Northwestern Margin of the Tarim Basin,Xinjiang

The Yakrik section is located in Wushi County,northwestern margin of the Tarim Basin,Xinjiang, China.The Ordovician System is well exposed at the Yakrik section.And the section contains a lot of im- portant Upper Ordovician conodont species,at the same time,it is one of the sections which involve com- plete Upper Ordovician in the Tarim Basin.Four conodont assemblages and two conodont zones of the Middle—Upper Ordovician have been established.In ascending order,they are:Serratognathus diversus—Bergstroemognathus extensus assemblage,Protopanderodus calceatus—Periodon aculeatus assem- blage,Tasmanognathus sp.—Aphelognathus sp.assemblage,Belodina confluens—Phragmodus unda- tus assemblage,Yaoxianognathus neimengguensis zone and Aphelognathus pyramidalis zone.Thereinto, S.diversus—B,extensus assemblage is assigned to the Upper Daobaowanian Stage of the Lower Ordovi- cian.P.calceatus—P.aculeatus assemblage is refered to the Dawangian Stage of the Middle Ordovician. Other assemblages and the zones are assigned to the Upper Ordovician.T.sp.—A.sp.assemblage can be attributed to the base of the Aijiashanian Stage,B.confluens—P,undatus assemblage is referred to the upper part of the Middle Aijiashanian Stage and Y.neimengguensis zone is identified as the middle part of the Upper Aijiashanian Stage,A.pyramidalis zone is considered as from the Top Aijiashanian Stage to the topmost part of Qiantangjiangian Stage.In addition,The important conodont species of upper Upper Ordovician A.pyramidalis,A.politus,Y.yaoxianensis and Y.neimengguensis are found on outcrop in Kalpin area for the first time,as well as,the topmost conodont zone of the Upper Ordovician A.pyrami- dalis zone and a upper one of the Upper Ordovician Y.neimengguensis zone are built there firstly.These discoveries at the Yakrik section are significant to the division and correlation of the Upper Ordovician be- tween covered areas and exposed areas of the Tarim Basin.