华南特马豆克期笔石地层分带及全球对比

特马豆克阶是奥陶系底部第一个阶,笔石是特马豆克阶高分辨率地层划分与对比的重要化石类群。江南斜坡带是我国早奥陶世特马豆克期漂浮笔石分异度和丰度最高的相区之一,位于该区的湖南益阳南坝剖面,发育有完整的上特马豆克阶笔石地层,特马豆克阶-弗洛阶界线附近地层连续,上特马豆克阶笔石地层研究已取得较大进展,但下特马豆克阶地层缺乏系统研究。近年来,通过对该剖面笔石标本的不间断采集,新识别出下特马豆克阶笔石带Rhabdinopora flabelliformis parabola带。到目前为止,湖南益阳南坝剖面特马豆克阶可以识别出5个笔石带,自下而上依次为:Rhabdinopora flabelliformis parabola带、Adelograptus tenellus带、Aorograptus victoriae带、Araneograptus murrayi带以及Hunnegraptus copiosus带。基于目前已识别出的笔石带,参考国内外同期笔石地层资料,本文详细讨论华南特马豆克期笔石带序列,并与国内外同期地层进行精确对比。

TREMADOC GRAPTOLITES ZONATION AND GLOBAL CORRELATION FROM SOUTH CHINA

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).