Early and Middle Ordovician chitinozoans from the Dapingian type sections, Yichang area, China
Introduction
Geologically, the Yichang area belongs to the north-central Yangtze carbonate platform. The Ordovician rocks, together with the underlying Cambrian and the overlying Silurian deposits, form a rim around the Huangling Arch (Fig. 1). The Yichang area is of great importance in global Ordovician chronostratigraphy as it hosts the GSSP of the base of the Dapingian Stage (i.e., the Early-Middle Ordovician boundary) at Huanghuachang, 22 km NE of Yichang (Wang et al., 2005, Wang et al., 2007a, Wang et al., 2007b), and the GSSP of the base of the Hirnantian Stage at Wangjiawan, 22 km NE of Huanghuachang. The latter locality has also a well-known section containing the Ordovician–Silurian boundary.
Chitinozoans have been investigated in the Dawan Formation in order to identify the taxa characterizing the base of the Dapingian and to provide supporting elements for worldwide correlation. The chitinozoans of two representative sections in Yichang area, located at Huanghuachang and Chenjiahe respectively, have been studied bed-by-bed. Extensive studies of conodonts, graptolites, trilobites, brachiopods and palynomorphs have also been carried out in these sections (see references in Wang et al., 2005, Wang et al., 2007a, Wang et al., 2007b).
Section snippets
Stratigraphy of the Dawan Formation
The Ordovician Dawan Formation of the Yichang area, in western Hubei Province, was defined by Zhang (1962) on the basis of the Dawan Bed (Zhang et al., 1957) and the Yangtzeella Bed recognised by Lee and Chao (1924). Zeng et al. (1983) subdivided the Dawan Formation in the Huanghuachang section into three lithological units: the Lower Unit, the Middle Unit and the Upper Unit. Based on our re-study, the Lower Unit at the Huanghuachang section attains a thickness of 14 m, and is composed of grey,
Sections, samples and localities
A total of 94 samples were collected and examined from the Dawan Formation at the Huanghuachang and Chenjiahe sections (Fig. 1, Fig. 3, Fig. 4).
Systematics
The classification, symbols and terminology used here follow those previously discussed by Paris et al. (1999). In the measurements, the following symbols have been used: L = length; Dp = chamber diameter; Dc = diameter of oral tube. These values are expressed in microns (µm). The abundance and measurements of each described species are all based on SEM observations.
Order Prosomatifera Eisenack, 1971.
Family Conochitinidae Eisenack, 1931 emend. Paris, 1981.
Subfamily Conochitininae Paris, 1981.
Genus
Chitinozoan local biozones and correlation with relevant conodonts biozones
The present study of the chitinozoan distribution around the Lower/Middle Ordovician boundary in the Yichang area was part of a wider project including the revision of the conodont and the graptolite successions of the two representative sections at Huanghuachang and Chenjiahe, respectively. The goal was to document accurately the range of the most diagnostic fossils below and above the GSSP of the Dapingian (i.e., base of the Middle Ordovician) at Huanghuachang.
Four local chitinozoan biozones
Conclusions
This high-resolution study (bed-by-bed sampling in the most important part of the sections) of the chitinozoans of the Dawan Formation at Huanghuachang and Chenjiahe has allowed definition of four local chitinozoan biozones in the late Early Ordovician to the early Middle Ordovician of the Yichang area (West Hubei, China). The mean duration of each of these chitinozoan biozones is less than 2 millions years, so the biozones are precise enough for dating third order sequences in sequence
Acknowledgements
This research is financially supported by the “Excellent Young Scientist” Programme of the Ministry of Land and Mineral Resources, the Natural Science Foundation of China (NO: 40272020, 40742008), and the China Geological Survey (NO: 1212010710715, 1212010811055). The authors thank the two reviewers, Jacques Verniers (Ghent University, Belgium) and Aïcha Achab (INRS, Québec, Canada) for their valuable comments and suggestions that greatly improved the manuscript. They are also indebted to John
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