Age constraints of the Hungshihyen Formation (Early to Middle Ordovician) on the western margin of the Yangtze Platform, South China: New insights from chitinozoans
Introduction
The Hungshihyen Formation (Kuo, 1941), also named Hongshiya Formation (e.g., Zhan and Jin, 2007, Gao et al., 2018), is mainly distributed in eastern Yunnan and southwestern Sichuan (Luo et al., 2014). Palaeogeographically, it was deposited in the near-shore marine environment close to the Dianzhong and Xichang oldlands along the southwestern-margin of the Yangtze Platform on the South China Palaeoplate (Zhang et al., 2002). The formation is distinguished by its purple-red and greyish-green siltstone, sandstone and shale, while the equivalent strata on the Yangtze Platform mainly consist of greyish-green shale and siltstone (i.e., the Meitan Formation) or grey argillaceous limestone (i.e., the Dawan Formation). The palaeontology and biostratigraphy of the Hungshihyen Formation are still poorly known due to its lithology characterized by the relatively coarse clastics deposited in a high-energy environment. Previous investigations on the age, lithology and palaeoenvironment of the Hungshihyen Formation have been briefly summarized (e.g., Zhang et al., 2013, Liang et al., 2019).
The Hungshihyen Formation has long been assigned to the late Floian and early Dapingian according to previous graptolite and chitinozoan works (Wang, 1974, Mu et al., 1979, Gao, 1986). Recently, the graptolite fauna in the middle part of the formation from the Kunming-Luquan area, eastern Yunnan, indicates that the interval corresponds to the middle Floian rather than the late Floian (Zhang and Zhang, 2014). Trilobites from eastern Yunnan suggest an early Floian to earliest Darriwilian age and a diachronous upper boundary of the formation (Luo et al., 2014).
Our newly obtained chitinozoan data from the Hungshihyen Formation at Songliang from the Qiaojia area, together with a chitinozoan revision documented from the same formation in the Wuding area by Gao (1986), provides new insights regarding its age. The present study aims to: (1) report the chitinozoan assemblage from the Songliang section, (2) update previous identifications of chitinozoans from the Renminqiao section of the Wuding area according to the widely-adopted classification system put forward by Paris et al. (1999), and (3) provide a well-constrained age estimate of the formation in eastern Yunnan.
Section snippets
Section and materials
The Songliang section (27.063119°N, 102.940014°E) is exposed along a country road to the Songliang village, which is ca. 17 km north of the Qiaojia County, eastern Yunnan Province (Fig. 1B). The Yunnan Province, which palaeogeographically comprises South China, Indochina, and Sibumasu palaeoplates or terranes in the early Palaeozoic (Metcalfe, 2011), is a geologically unique area with serious tectonic disturbance. Although structurally complex in the region, the Ordovician succession at
Chitinozoans from the Hungshihyen Formation at Songliang, Qiaojia area
Altogether 5 genera and 7 species (Figs. 3, 4) were identified from the Hungshihyen Formation at Songliang, following the latest classification system (Paris et al., 1999), including Alhajrichitina sp. 1, Bursachitina maotaiensis (Chen in Chen et al., 2009), Bursachitina qianbeiensis Chen in Chen et al., 2009, Conochitina sp. 1, Conochitina sp. 2, Desmochitina ornensis Paris, 1981, and Lagenochitina obeligis Paris, 1981. The fauna is herein recognized as the Bursachitina maotaiensis–B.
Chitinozoans from the Hungshihyen Formation at Renminqiao, Wuding area: a revision
A much more diverse assemblage, including 16 genera and 50 species (subspecies included), was documented from the Hungshihyen Formation at Renminqiao in the Wuding area (Gao, 1986). However, in order to better understand the chitinozoan assemblage of this particular time interval in near-shore environments, a revision of these chitinozoans according to the updated systematic classification of Paris et al. (1999) is required. The revision was based on the brief descriptions and plates presented
A proposed late Floian age for the lower part of the formation at Songliang, Qiaojia area
Bursachitina maotaiensis and B. qianbeiensis are two distinct short-ranging species well recorded in the lower and middle parts of the Meitan Formation in northern Guizhou (Chen et al., 2009, Liang et al., 2018) and with even shorter geological ranges in the lower part of the Zitai Formation of southeastern Chongqing (Chen et al., 2009). The first appearance datum (FAD) of both species is after a significant chitinozoan deficiency interval which was widely observed in the lower part of the
Conclusions
The present study provides for the first time chitinozoan data from the lower Hungshihyen Formation in the Qiaojia area of eastern Yunnan. The appearance of Bursachitina maotaiensis and Bursachitina qianbeiensis indicates a late Floian age for this interval based on regional and global chitinozoan biostratigraphy. A re-evaluation on a previously reported chitinozoan assemblage from the middle part of the same formation at Renmingqiao in the Wuding area by Gao (1986) is also made, which allows
Acknowledgements
Two reviewers, Wen-Hui Wang and G. Susana de la Puente, are acknowledged for careful review and constructive suggestions. Financial supports are from the National Natural Science Foundation of China (41602010), the Strategic Priority Research Program of Chinese Academy of Sciences (XDB26000000), the Estonian Research Council (MOBJD62, PUT611) and the State Key Laboratory of Palaeobiology and Stratigraphy. This paper is a contribution to the IGCP 653 “The Onset of the Great Ordovician
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