Ordovician Conodont Biostratigraphy and Magnetostratigraphy in Southwestern Ordos, North China

The Ordovician is a critical interval for understanding early Earth climate evolution, biotic diversification and planetary habitability. Large uncertainties remain in the Ordovician geochronologic timescale, and the driving mechanisms behind these changes are still poorly constrained. The Ordovician strata are well developed in the Ordos Basin, North China, providing a good opportunity for multiple stratigraphic study and it was expected to build up a high-resolution geologic timescale of Ordovician. In this contribution, we present our preliminary results of the conodont biostratigraphy and magnetostratigraphy studies along the Qishan Section in the southwestern Ordos, China. The Ordovician strata here are composed of the Yeli, Liangjiashan, Majiagou and Pingliang Formations, with a total thickness of 2700 meters. Fourteen genera and twenty-three species were identified in this section, ranging from the Lower Ordovician to the Upper Ordovician, as exemplified by indicative species, including Cordylodus angulatus, Monocostodus sevierensis, Semiacontiodus nogamii, Teridontus erectus, Microcoelodus symmetricus, Microcoelodus asymmetricus, Dapsilodus variabilis, Periodon aculeatus, Periodon grandis, Panderodus gracilis, Spinodus spinatus, Belodina compressa, Belodina confluens, Belodina baiyanhuaensis, and Taoqupognathus blandus. And one zone, the Cordylodus angulatus Zone, was distinguished for the Lower Ordovician; two zones, the Belodina compressa-Phragmodus undatus Zone and the Belodina confluens Zone, were recognized for the Upper Ordovician. No Middle Ordovician conodonts was found on the section. Our conodont results suggest the gap exist in this section, this challenges the previous cognition of the continuous deposition in this region. The magnetostratigraphic samples were degaussed and measured on a 2G cryogenic magnetometer in a magnetically shielded room. A half sets of specimens were isolated an interpretable middle to high temperature remanent component from the limestones and sandstones. Our observed polarity zones were roughly correlated to the magchrons of GPTS2012, and further confirm our conodont stratigraphic interpretation of the Majiagou Formation in the Qishan section as the Sandbian-Katian rather than the Middle Ordovician, and Pingliang Formation would be more younger than we known.