Multiple Stratigraphy Study of the Ordovicianin SW Ordos, China

the Paleozoic, which was characterized by a peak of greenhouse climate in Earth history, as well as extreme high sea level (Haq and Schutter, 2008; Munnecke et al., 2010), with warm and humid conditions in early-middle period and seawater temperature up to 45°C (Trotter et al., 2008). The carbon cycle fluctuated greatly (Melchin et al., 2013; Cramer et al., 2015) and atmospheric CO2 concentrations reached ~4200 ppm. During the Late Ordovician, severe glacial conditions developed and the Earth entered its second Snowball Earth period (Nardin et al., 2011). The Ordovician also shows a great development of marine life,which spread heavily after the Cambrian.Biologic genera and families (Dixon et al., 2001; Webby et al., 2004; Harper, 2006) andhard shell organisms rapidly increased (Wilson and Palmer, 2006). More importantly, mosses (Redecker et al., 2000) began to ascend on the mainland during the Ordovician, and animals such as fish, corals, and cephalopods began to appear in the Ordovician. At the end of the Ordovician period a major extinction event decimated more than 60% of marine invertebrates and was the second largest after the PermianTriassic extinction. Due to the lack of accurate high-resolution geologictime scale, there are still many controversies about the time and mechanism of these climate and biological evolution events (Melott et al., 2004; Trotter et al., 2008; Harper et al., 2015; Algeo et al., 2016; Rasmussen et al., 2016; Edwards et al., 2017; Lindskog et al., 2017; Stigall, 2017). Therefore, it is very urgent to reach an accurate high resolution Ordovician geologic timescale, through radiogenic geochronologic data, magnetostratigraphy, chemostratigraphy and cyclostratigraphy. In this goal, the collaboration of multiple disciplinary researchers on a global scale is a very important approach. The IGCP-652 project, which includes about 250 researchers from more than 40 countries, aims to improve the geologic time scale of the Ordovician to the Devonian. It is entitled “Reading geologic time in Paleozoic sedimentary rocks: the need for an integrated stratigraphy”, and the implementation period is5 years (2017-2021). Using the stratigraphic, geophysical, geochemical and astronomical dating method to studyOrdovicianDevonian sedimentary sections, the project intends to improve the Paleozoic geological time scales. The Chinese workgroup, including more 20 researchers from 7 institutes, is responsible to carry out high-precision geologic timescale studies on the Ordovician from western Ordos Basinin China. The southwestern Ordos is one of the most developed, well exposed, and highly studied areas of Ordovician in the north of China. During the Ordovician period, the area was located in the low latitudes of the Southern Hemisphere (Yang et al., 1998; Torsvik et al., 2012; Wang et al., 2015), at the transition of southern margin of North China continent and the Qinling-Qilian trough (Feng et al., 1991). The sedimentary environment was a carbonate platform-Platform front slope (Feng et al., 1991; Li et al., 2012). In this contribution, we compile the literature of stratigraphy study for the Ordovician and present our primary data, to form an insight of the Ordovician geologic timescale in southwestern Ordos. The Ordovician stratigraphy in the southwestern Ordos has been divided into five formations from bottom to top, Yeli, Liangjiashan, Majiagou, Pingliang and Beiguoshan formations, with a total thickness of 1046-2500 meters (Chen, 2010). The Lower Ordovician (Yeli and Liangjiashan Formations) consists of argillaceous dolomite and dolomite, with siliceous bands in the Liangjiashan Formation. The Middle Ordovician(Majiagou Formation)is composed of the laminated argillaceous dolomite and limestone, and massive limestone. It was divided into two parts and each part consists of three members. The Upper Ordovician Pingliang Formation consists of shale, mudstone, sandstone and marl, with some layers of tuff and gravity flow deposit, rich in graptolite. Above Pingliang Formation, the Beiguoshan Formationis composed of the mediumto thickbedded limestones, intercalated with shale and conglomeratic limestone. Biostratigraphy studies have shown that the strata contain a large number of trilobites, graptolites, cephalopods (Anand Multiple Stratigraphy Study of the Ordovicianin SW Ordos, China