Late Permian (Changhsingian) conodont biozonation and the basal boundary, Ganxi section, western Hubei Province, south China.

Abstract: Four neogondolellid conodont interval zones were recognized across the Changhsingian stage at Ganxi section, western Hubei Province, south China. They are in ascending order: the Clarkina wangi Zone, the Clarkina changxingensis changxingensis Zone, the Clarkina changxingensis yini Zone, and the Clarkina meishanensis meishanensis Zone. The present study suggests that the base of the Changhsingian can be defined by the first appearance datum (FAD) of Clarkina wangi within an evolutionary lineage from Clarkina longicuspidata to Clarkina wangi. The Wuchiapingian-Changhsingian boundary can be placed at the base of bed 104 at Ganxi section, south China because of the first occurrence of Clarkina wangi.

Resume: Quatre zones d'intervalle de conodontes neogondolellides ont ete identifiees au sein de l'etage Changhsingien de la coupe de Ganxi, dans la province de Hupei occidentale de la Chine meridionale. Il s'agit, en ordre ascendant, des zones de Clarkina wangi, Clarkina changxingensis changxingensis, Clarkina changxingensis yini et Clarkina meishanensis meishanensis. La presente etude indique que la base du Changhsingien peut etre definie par l'apparition de Clarkina wangi dans la lignee evolutive allant de Clarkina longicuspidata a Clarkina wangi. L'apparition de Clarkina wangi permet de fixer la position de la limite Wuchiapingien-Changhsingien a la base du lit 104 de la coupe de Ganxi de la Chine meridionale.

[Traduit par la Redaction]

Introduction

The Permian was subdivided into the Cisuralian Series, Guadalupian Series, and Lopingian Series by the Sub-commission of Permian Stratigraphy (SPS), International Sub-commission of Stratigraphy (ICS), International Union of Geological Sciences (IUGS) (Jin et al. 1997). These Series are marked by the presence of the conodonts Mesogondolella, Jinogondolella, and Clarkina, respectively, (Kozur 1989; Lambert et al. 2002; Mei and Henderson 2002).

The Lopingian Series is subdivided into the Wuchiapingian and Changhsingian stages. The first study of Changhsingian conodonts was published by Sweet (in Teichert et al. 1973), on material from northwestern Iran. In south China, the Changhsingian conodonts have been studied by Wang and Wang (1981a); Tian (1993); Orchard et al. (1994); Wang (1994, 1995, 1996); Zhang et al. (1995); Mei (1996); Orchard (1996); Yin (1996); Zhang et al. (1996); Mei et al. (1998); Mei and Shi (1999); Yin et al. (2001); Mei and Henderson (2002); Jin et al. (2003); and Wang and Henderson 2003; Guoqing and Wenchen (2004); Wenchen et al. (2004). The base of the Changhsingian stage has been studied by Zhao et al. (1981); Wardlaw and Mei (2000); Mei and Henderson (2001); Jin et al. (2003); and Wang and Henderson 2003. This study also focuses on conodonts from the Changhsingian stage and its basal boundary.

Geological setting and lithology

During the Precambrian, China was composed of three major stable blocks: the China-Korea continental block, the Tarim continental block, and the Yangtze continental block (Tian 1990; Wang et al. 1994). The Ganxi section (N30[degrees]07.4', E109[degrees]26.8') is located on the central part of the Yangtze platform, in the western part of Hubei Province, south China (Fig. 1). The study area belongs to the eastern part of the Sichuan Basin. The Sichuan Basin was a platform of carbonate sedimentation in the Yangtze region until the Middle Triassic. During the Permian the platform was surrounded on all sides by deep sea or open ocean (Hsu 1989). Surrounding the study area, the sedimentary sequence is composed of a continuous sedimentary record that ranges from the Cambrian to the Cretaceous period.

[FIGURE 1 OMITTED]

During the late Early Permian to Middle Permian, the sea transgressed from the southwest and carbonate, open neritic platform facies were deposited all over the basin (Wang et al. 1989). During the Late Permian, continental and littoral swamp facies developed in the southwest of the basin, platform trench facies were appearing in the east, and in the remainder of the basin carbonate platform facies were still being deposited (Wang et al. 1989). The lithology is dominated by clastic sediment of mudstone, clay, marl, and chert intercalated with gray limestone.

Discussion

In south China, five conodont zones were recognized across the Changhsingian stage at Meishan section (GSSP, global stratotype section and point), Changxing, Zhejiang, south China (Mei et al. 1998, 2001; Mei and Henderson 2002; Jin et al. 2003). They are, in ascending order, the Clarkina wangi Zone, the C. changxingensis changxingensis Zone, the C. postwangi Zone, the C. changxingensis yini Zone, and the C. meishanensis meishanensis Zone (Table. 1).

This study describes the Changhsingian succession from Ganxi section, western Hubei Province, south China. The study is significant in that it describes conodonts from different facies from those exhibited at Meishan section (GSSP), Changxing, Zhejiang, south China.

The neogondolellid conodonts from the Ganxi section can be divided into four zones (Fig. 2; Table 1). They are, in ascending order, the Clarkina wangi Zone, the C. changxingensis changxingensis Zone, the C. changxingensis yini Zone, and the C. meishanensis meishanensis Zone. At the Ganxi section, the Clarkina wangi interval Zone is marked by the first appearance of C. wangi and the upper limit is marked by the first occurrence of C. changxingensis changxingensis. The common species in this zone are Clarkina predeflecta, C. prechangxingensis, C. subcarinata, C. orientalis, and a transitional morphotype between C. wangi and C. longicuspidata. The lower limit of the Clarkina changxingensis changxingensis interval Zone is marked by the first occurrence biohorizon of C. changxingensis changxingensis. The upper limit of the zone is marked by the first occurrence of C. changxingensis yini. The common species in this zone are Clarkina subcarinata, C. parasubcarinata, C. postwangi, C. deflecta, and C. meishanensis zhangi. The lower limit of Clarkina changxingensis yini interval Zone is marked by the first appearance of C. changxingensis yini. The upper limit is marked by the first occurrence biohorizon of C. meishanensis meishanensis. The common species in this zone are Clarkina parasubcarinata, C. …