2018, 57(4):444-459.
CSTR:
Abstract:
The Tremadocian is the first stage of the Ordovician System with its base defined at the first appearance datum (FAD) of conodont Iapetognathus fluctivagus (Cooper et al., 1998, 2001) and its top marked by the FAD of graptolite Tetragraptus approximatus (Berry, 1992; Chen and Wang, 1993; Webby, 1994, 1996; Williams et al., 1994; Maletz et al., 1996; Bergstr?m et al., 2004). Two divisions of the Tremadocian are currently accepted, and the boundary between the lower and upper parts is placed at the FAD of Adelograptus tenellus (Cooper et al., 2001; Bergstr?m et al., 2004) or the Rhabdinopora flabelliformis anglica (Cooper et al., 1998, 1999; Jackson and Lenz, 2000, 2003; Maletz, 2001). Tremadoc graptolites are characterized by the proximal end simplified to be bi-radiate and the bitheca inclined towards degeneration, showing a morphological nature transitional from the anisograptid fauna to the dichograptid fauna (Feng et al., 2010). In order to make the precise correlation, we restudied this section.
The Nanba section, ca. 30 km southwest of Yiyang, was located in the Jiangnan Slope Belt of China (Figure 1), which is one of the regions with rich Tremadocian planktic graptolites of great diversity and abundance. Feng Hongzhen and his colleagues have worked here for more than a decade (Feng et al., 2007, 2009, 2010; Li Lixia et al., 2009, 2010; Li Ming et al., 2010; Wang et al., 2013a, 2013b), and they found rich mineralized stereo specimens from this section. It contains a continuous graptolite succession across the Tremadocian-Floian boundary and giving a possibility to make the precise correlation of Upper Tremadocian between China and other countries in the world. Because Feng et al. focused on the late Tremadoc strata, until 2015, early Tremadoc graptolites and strata have not been studied in the Nanba area. Tremadoc graptolites are very difficult to find, but the Nanba section preserves a continuous Tremadoc strata, which has potential to improve the precision of the Tremadoc graptolites stratum zonation in the Jiangnan Slope Belt.
The Nanba section is characteristic of slope faces sediments. It is a north stretched artificial cut along which two formations, the Yinchupu and the Ningkuo formations ascendingly, are exposed a total thickness more than 120 m. Lithologically, graptolites are present in mudstones with dark or brownish yellow colors but not in silt-mudstones, which usually contain dynamic disturbance structures and tube-like trace fossils. The boundary between the Tremadocian and the Floian is within the basal part of the Ningkuo Formation, not coinciding with that between the Yinchupu and the Ningkuo formations (Figure 1).
