The Ordovician Magnetostratigraphy and Cyclostratigraphy: A Review

last two decades has provided a great deal of opportunities to improve the geologic time scale. The Cenozoic and Mesozoic geologic timescale have been well calibrated (Gradstein et al., 2012; Ogg et al, 2012; Cohen et al., 2018). However, for the Paleozoic era the uncertainty over boundary ages are still very large. The reasons include that the geomagnetic polarity timescale prior to the Middle Jurassic have been pending due to the absence of oceanic anomalies (Opdyke and Channell, 1996; Ogg, 2012), and the robust theoretical astronomical model has not been setup for the Paleozoic (Hinnov and Hilgen, 2012). On the other hand, the Primary (characteristic remnant) magnetization or orbital signals are difficult to isolate in the Paleozoic rocks. Due to the outstanding glacial-interglacial climatic changes and mass radiation and extinction in the Ordovician, the studies for Ordovician magnetostratigraphy and cyclostratigraphy are more abundant in the Early Paleozoic. Here we give an overview of the research progress in Ordovician magnetostratigraphy and cyclostratigraphy. The magnetostratigraphy of Ordovician has been rare till now. Earlier research were largely conducted in Europe (Torsvik et al., 1991;Trench et al., 1991; Smethurst et al., 1998; Pavlov and Gallet, 2005; Schätz et al., 2006; Pavlov et al., 2012; Grappone et al., 2017) and Siberia (Torsvik et al., 1995; Gallet et al., 1996a; Pavlov et al., 1998, 2008, 2012, 2017; Rodionov et al., 2003), and few in America (Ellwood et al., 2007), Australia (Ripperdan and Kirschvink, 1992) and China (Fang et al., 1990; Ripperdan et al., 1993; Huang et al., 1996, 1999; Yang et al., 1998, 2002). These studies focus on two issues. One is natural remnant magnetization (NRM) behavior and/or remagnetization, another is Ordovician geomagnetic polarity timescale. With respect to the NRM behavior, most research reveals that there were two or three NRM components. But only one is characteristic remnant magnetization (ChRM), mostly carried by mainly magnetite and/or hematite (e.g., Pavlov et al., 2008), which was characteristic of antipodal reversed polarity (e.g., Fang et al., 1990; Huang et al., 1996, 1999; Smethurst et al., 1998; Rodionov et al., 2003). While other (s) was (were) secondary NRM (Fang et al., 1990; Torsvik et al., 1991; Ripperdan and Kirschvink, 1992; Huang et al., 1996, 1999; Gallet et al.,1996; Pavlov et al., 1998; Rodionov et al., 2003; Grappone et al., 2017), originated from remagnetization (e.g. Fang et al., 1990, Huang et al., 1996; Smethurst et al., 1998). The onset of Ordovician geomagnetic polarity zones has been long-term attention. Firstly, Khramov et al. (1965) and Rodionov (1966) investigated the magnetozones in Cambrian Ordovician strata at the Moyero river section, Siberia. Then Torsvik et al. (1991) recognized three reversed (SE, down) and three normal (NW, up) antipodal polarity intervals within Baltoscandian Ordovician carbonates. Trench et al (1991) studied the magnetostratigraphy of the Baltic and South Siberian platform, and firstly found a long geomagnetic polarity zone which is dominated by reversed polarity during the Early Ordovician (Tremadoc-Llanvirn), succeeded by a predominantly normal polarity field in later Ordovician (Llandeilo Ashgill). This is the start of the Moyero reversed superchron. After that, more studies came out from Siberian and other sites for the composition of the Ordovician geomagnetic polarity zones. In Australia, Ripperdan and Kirschvink (1992) sampled a section of 1000m thick of Ordovician strata, and got more polarity zones than in previous works, i.e. four couplets of normal-reversal polarity zones. In Northeast China, Ripperdan et al (1993) reported more normal reversed polarity zones couplets from the Cambrian-Ordovician Boundary section at Xiaoyangqiao, Jilin province, equivalent to major portions of the Australian sequence. In the North China Block, Yang et al (1998, 2002) also documented the early Ordovician (Tremadoc) mixed polarity zones and the Middle Ordovician long-term reversed polarity zone. These studies demonstrated that there was a long-term reversed polarity zone The Ordovician Magnetostratigraphy and Cyclostratigraphy: A Review