Biostratigraphic correlation and chronostratigraphic framework of the Ordovician in the Ordos Basin

The Ordovician of the Ordos Basin, with huge resource potential, has been a key target for natural gas exploration in recent years. However, the correlation of Ordovician lithostratigraphic units in the Ordos Basin is somewhat controversial, so it is urgent to conduct a detailed stratigraphic comparison based on available paleontological data. This paper synthesizes the previous research on conodont and graptolite biostratigraphy and establishes the correlation of the Ordovician within the basin. On the basis of biostratigraphic dating, this paper suggests that the Sandaokan, Zhuozishan, Kelimoli and Majiagouformations belong to the Darriwillian, Middle Ordovician, while the Lashizhong and Pingliangformations belong to the Upper Ordovician. The biostratigraphic studies of the Lower Ordovician in the basin are relatively insufficient. In the Lower Ordovician of the western margin, the conodont zones of R. manitouensis and G. quadraplicatus in the Tremadocian can be identified, and on the eastern margin, the P. obesus Conodont Zone was discovered, which can be correlated with the top of the Liangjiashan Formation on the southwestern margin. The biostratigraphic studies of the Middle and Upper Ordovician in the basin are much comprehensive. The conodont zone of Histiodella cf. holodentata recognized at the top of the Sandaokan Formation can be correlated with the D. costodus zone in the Machuan Formation on the southern margin and inthe Majiagou Formation on the eastern margin. From the Kelimoli to the Gongwusu Formation, the conodont zones from D. tablepointensis to B. compressa and the graptolite zones from P. elegans to N. gracilis were identified, which can be correlated with the middle-upper part of the Shuiquanling Formation to the middle of the Pingliang Formation on the southwestern margin and the middle-upper part of the Majiagou Formation to the top of the Fengfeng Formation on the middle-eastern margin, indicating a Darriwillian to Sandbianage. The graptolite zone of C. bicornis was recognized from the Lashizhong to Sheshan Formation on the western margin, which can be correlated with the middle of the Pingliang Formation on the southwestern margin and from the top of the Jinghe Formation to the bottom of the Zhaolaoyu Formation on the southeastern margin, suggesting amiddle-late Sandbianage. Both the Taoqupo Formation on the southwestern margin and the Beiguoshan Formation on the southeastern margin of the basin yield conodont species Y. neimengguensis and Y. yaoxianensis of the Katian, showing a correlation between them.