Early To Middle Ordovician Conodonts From The Lucaogou Section,Southeast To The Township Of Weizhou,Tongxin,Ningxia Hui Autonomous Region And Their Paleoceanographic Implications

Thirty-eight carbonate samples(approximately 100 kg in total)for conodonts were collected from the Lucaogou section at the west margin of the Ordos Basin.The samples were processed by using 10% acetic acid and handpicked under a stereomicroscope yielding a total number of 1420 conodont specimens.To date,32 genera and 46 identified and 4 indeterminate species have been identified.Biostratigraphically important taxa of the present conodont fauna are: Aurilobodus leptosomatus,Cordylodus angulatus,Costiconus ethingtoni,Drepanoistodus basiovalis,Drepanoistodus costatus,Dzikodus tablepointensis,Eoplacognathus pseudoplanus,Eoplacognathus suecicus,Histiodella kristinae,Loxodus dissectus,Microzarkodina ozarkodella,Monocostodus severiensis,Periodon hankensis,Periodon macrodentatus,Periodon zgierzensis,Pygodus lunnensis.Five conodont zones,in ascending order,the Cordylodus angulatus Biozone,the Aurilobodus leptosomatus-Loxodus dissectus Biozone,the Microzarkodina ozarkodella Biozone,the Dzikodus tablepointensis Biozone and the Eoplacognathus suecicus Biozone were recognized in the studied stratigraphic interval.One Subbiozone,Pygodus lunnensis Subbiozone,belonging to the E.succicus Biozone was discriminated.Abundant conodonts yielded from the study section allow a paleoecological analysis by using multivariate statistical methods.Cluster Analysis(CA)and Detrended Correspondence Analysis(DCA)were adopted to distinguish conodont biofacies.Multivariate statistical studies based on the 16 selected samples allow the recognition of four conodont biofacies: the Drepanoistodus biofacies,the Parapaltodus biofacies,the Periodon biofacies and the Protopanderodus biofacies.The turnover of the conodont biofacies is related to sea-level changes.Three transgressions and four regressions have been identified in the study section.The sea-level changes of the Qinglongshan area are considered to be with the alternations of transgressions and regressions during the mid–Darriwilian age.Overall,a slow transgression is present at the study section.Conodont faunas of the study area and even of the North China Platform are composed mainly of warm-water taxa.On the other hand,dispersals of the warm-water conodonts among paleocontinents of Laurentia,Siberia,western Australia used to be very common.According to the paleo-ocean circulation models,the western margin of the North China Craton was affected by both the Equatorial Countercurrent and the South Equatorial Current.Due to the processive transgression,the shallow and warm environmental setting at the upper part of the Zhuozishan Formation was replaced gradually by a deeper and colder enrivonment,which drived a turnover of conodont faunas from the North American Mid-continent type to the North Atlantic type.