Late Darriwilian (the Middle Ordovician) Conodonts And Their Biofacies Of The Qinglongshan Area At The Western Margin Of The North China Craton

Twenty-two carbonate samples(60kg in total)for conodonts were collected in the Klimoli Formation and the Wulalike Formation of the Qinglongshan area at the western margin of the North China Craton.These samples were systematically processed yielding a total number of 879 conodont specimens.The obtained conodonts can be assigned to 42 species and 2 indeterminated species belonging to 23 genera.Age-diagnostic conodonts are: Dzikodus tablepointensis,Eoplacognathus suecicus,Pygodus anitae,P.lunnensis,Yangtzeplacognathus foliaceus.Two conodont biozones have been identified from the Klimoli and the Wulalike formations in the study area,they are,in ascending order,the E.suecicus Biozone and the P.serra Biozone.Furthermore,the P.anitae Subbiozone of the E.suecicus Biozone and the Y.foliaceus Subbiozone of the P.serra Biozone are able to be recognized in the studied stratigraphic interval of the Qinlongshan area.The present biostratigraphic sequence is rather similar to the coeval North Atlantic sucessions in Baltoscandinavia,South China and Tarim,representing cold-water conodont faunas.Biostratigraphic correlations suggest a late Darriwilian age for the Klimoli and the Wulalike formations.Abundant conodonts obtained from the two studied sections allow a paleoecological analysis by using multivariate statistical methods.Cluster Analysis and Detrended Correspondence Analysis were adopted to distinguish conodont biofacies.Based on the quantitative analysis,four conodont biofacies were differenciated,namely,the Costinonus Biofacies,the Periodon Biofacies,the Protopanderodus Biofacies and the Pygodus biofacies.Among others,the Periodon Biofacies represents the shallowest sea-water sedimentary environment,the Protopanderodus Biofacies suggests a relatively deeper sea-water environment,the Costiconus Biofacies indicates a much deeper sea-water environment,while the Pygodus Biofacies manifests a distal carbonate platform setting.Vertically,turnovers and replacements of the conodont biofacies were controlled by the changes of sea-level.In this case,the conodont biofacies can be used to recover the sea-level fluctuations.Sea-level changes reproduced by using conodont biofacies can be correlated with the ones that were performed by using traditional sedimentary methods.