Unconformities in the Ordovician of Western Tarim Basin,NW China

Two unconformities in the Ordovician of western Tarim basin,Northwest China have been recognized on the base of sedimentary,stratigraphical and palaeontological data.One is between the Lower Ordovician Penglaiba Formation and Yingshan Formation,which is a parallel unconformity indicated by weathered beds and stratigraphical gaps.That two or three conodont zones/assemblages are absent from the unconformity indicates this stratigraphical gap is about 5.5~7.2 Ma.The other one is between the Upper Ordovician Terekawat Formation and the underlying formations,which presents commonly as a parallel unconformity or an angular unconformity in some regions.Palaeontological data show the underlying beds belonging to different formations at different sections,which are the Yingan Formation at the Dawangou,East Dawangou and Yangjikan sections that belongs to the Aijiashanian of late Late Ordovician according to the chitinozoans,or the Qilang Formation at the Sishichang and Terekawat sections that is early—middle Aijiashanian in age as indicated by conodonts;or the Tumuxiuke Formation or Yijianfang Formation at the Sanjianfang section that have been assigned to the early Aijiashanian of Late Ordovician or Darriwilian of Middle Ordovician on the basis of conodonts,or the Yingshan Formation at the North Xiker and East Xiker sections that is early Middle Ordovician based on the conodonts as well.The stratigraphical gap varies from 1 or 2 Ma to over 20 Ma as the maximum.