Biostratigraphy Of The Ordovician Conodonts In The Qishan Area,Southwestern Ordos,China

The Ordos Basin,which is regarded as significant reservoirs of oil and gas in northern China,developed massive marine strata in Early Paleozoic.In the southwestern margine of the Ordos,there are five Ordovician formations developed from bottom to top,namely the Yeli Formation,the Liangjiashan Formation,the Majiagou Formation,the Pingliang Formation and the Beiguoshan Formation.Most of previous studies of the Ordovician in this area mainly focused on lithostratigraphy,sedimentary environment,source rock evaluation and volcanic ash age.Due to the lack of systematic biostratigraphy,magnetic stratigraphy and other research data,there is still a lack of detailed understanding of the division,comparison and age of the set of strata,which severely restricts the understanding of the Ordovician structure-environment and petroleum prospects of the area.Conodonts have been applied as a tool for stratigraphical correlation of the Ordovician.This study concentrated on the conodont biostratigraphy of the Ordovician strata in the Qishan and Jueshan sections(Baoji,Shaanxi Province),and the age of the conodont-bearing strata.Having obtained abundant conodonts from the Liangjiashan Formation and the Majiagou Formation in the Qishan section,and the upper part of the Majiagou Formation in the Jueshan section.The results and discussion can be concluded as follows:(1)The conodont fauna in Qishan included fourteen genus and twenty-three species(incuding two conformis species and six species indeterminata): Belodina compressa,Belodina confluens,Belodina baiyanhuaensis,Belodina sp.indet.,Dapsilodus variabilis,Erraticodon sp.indet.,Microcoelodus asymmetricus,Monocostodus sevierensis,Panderodus gracilis,Panderodus gryphus,Panderodus sp.indet.,Periodon aculeatus,Periodon grandis,Periodon sp.indet.,Protopanderodus nogamii,Spinodus spinatus,Tasmanognathus sp.indet.,Taoqupognathus blandus,Teridontus cf.erectus,Teridontus cf.gracilis,Teridontus sp.indet.,Venoistodus venustus,Yaoxianognathus sp.indet.(2)The conodont fauna in Jueshan Section included six genus and eleven species(incuding two species indeterminata): Belodina compressa,Belodina confluens,Dapsilodus variabilis,Periodon aculeatus,Periodon grandis,Periodon sp.indet.,Plectodina sp.indet.,Tasmanognathus borealis,Tasmanognathus gracilis,Tasmanognathus shichuanheensis,Venoistodus venustus.(3)The Majiagou Formation in Qishan and Jueshan sections can be assigned to the Belodina compressa to Belodina confluens biozones based on fossil species and position in this research.The Belodina compressa zone corresponding to middle-upper Sandbian and the Belodina confluens zone corresponding to lower Katian,which illustrates the age of the Majiagou Formation in both sections is late Sandbian-early Katian of late Ordovician.(4)The upper boundary of the Majiagou Formation,which compared in different areas,is diachronous.In Wuhai,it corresponds to the lower Darriwillian of the Middle Ordovician,while to the lower Katian of the Upper Ordovician in Qishan and Yaoxian.