DISCUSSION ON THE AGE OF THE ORDOVICIAN YINGSHAN FORMATION IN TARIM BLOCK

The Ordovician Yingshan Formation in the Tarim Basin has been interpreted as Early Ordovician, Early-Middle Ordovician, or Middle Ordovician deposits. A paleontologic study indicates that the conodont Paroistodus originalis zone and Baltoniodus aff. navi zone in previously established based material from the Keping and Bachu areas are based on weak paleontological data. The conodont Tangshanodus identified by Gao has been renamed as Erraticodon tarimensis, thus the previously recognized Tangshanodus zone in Tarim cannot be correlated with the Tangshanodus tangshanensis zone in North China. Rather, the Erraticodon tarimensis zone may be younger than the Serratognathus diversus zone and older than the A. leptosomatus-L. dissectus zone. Based on the geological range of the conodont Serratognathoides chuxianensis and the Lower/Middle Ordovician boundary stratotype, the Yiingshan Formation is regarded as Early Lower Ordovician in age (Tremadocian-Floian). There is stratigraphic break between the Yingshan Formation and the overlying strata based on paleontological and biostratigraphical data.