Au cours de la dernière décennie, l'Ordovicien a été subdivisé en trois séries (inférieure, moyenne et supérieure), tandis que la majorité des limites des étages a été définie ou proposée par la sous-commission de stratigraphie de l'Ordovicien (IOS) (Fig. 1). Les stratotypes et les GSSP (Global Stratotype Section and Point) ont été approuvés pour la base du Darriwilien en 1997 et pour la base du Tremadocien (c'est-à-dire la limite cambro-ordovicienne) en 2000, tandis que les stratotypes des étages 2 et 5, dont les noms doivent encore étre choisis, viennent d'étre votés par l'IOS. La limite entre l'Ordovicien inférieur et l'Ordovicien moyen devrait prochainement étre définie. Dans le Sud de la Chine, cette limite devrait correspondre approximativement à la limite inférieure de la biozone à graptolites Azygograptus suecicus. Des assemblages d'acritarches bien préservés sont décrits dans de nombreuses coupes de la plate-forme du Yangtze et de la région Jiangshan–Changshan–Yushan (JCY). Ces coupes traversent l'intervalle dans lequel la limite Ordovicien inférieur–moyen devrait étre définie. Les successions d'acritarches rencontrées, qui se corrèlent partiellement avec les séquences de Baltica et de la bordure gondwanienne, y compris les successions britanniques, pourraient jouer un rôle important pour la localisation et la corrélation de cette limite Ordovicien inférieur–moyen.

In the 1990s, significant progress was made in Ordovician series and stage boundary definitions. A tripartite division of the Ordovician System named Lower, Middle and Upper Ordovician Series has been accepted and it was decided that the base of the six global stages will be defined on the basis of the occurrences of either graptolite or conodont species (Fig. 1). The first stage that was approved by the International Union of Geological Sciences (IUGS) and the International Commission on Stratigraphy (ICS) in early 1997 is the second global stage of the Middle Ordovician Series, named Darriwilian, with its Global Stratotype Section and Point (GSSP) near Huangnitang, Changshan, SE China. The Executive Committee ratified the GSSP of the base of the first stage of the Lower Ordovician Series, named Tremadocian, at Green Point, Newfoundland, in January 2000. Two other GSSP have been voted recently by the Ordovician Subcommission (IOS) and are under the procedure of approval by the ICS and of formal ratification by IUGS: the GSSP of the second stage of the Lower Ordovician Series at the Diabasbrottet section at Mt. Hunneberg, Sweden, and the GSSP of the first stage of the Upper Ordovician Series at Fågelsång, Scånia, Sweden. The IOS will not choose a proper name for the second stage of the Lower Ordovician until the boundary of the stage (= the base of the Middle Ordovician Series) has been formally defined. However, selection of this latter GSSP for the base of the Middle Ordovician may prove to be contentious. The defining biohorizon (first appearance of the conodont Tripodus laevis) and the candidate stratotype section at Whiterock Narrows, Nevada, USA, need to be re-evaluated. Little progress has been made on the lower boundary for the upper stage of the Upper Ordovician Series, which is to be defined on the FAD of either the conodont Amorphognathus ordovicicus or the graptolite Dicellograptus complanatus.

After proving to be useful for long distance correlations and having a high potential for the definition of Ordovician boundaries, such as the base of the second stage of the Lower Ordovician Series (= the Tremadocian–‘Arenigian’ boundary) [26], acritarchs may play an essential role for the definition and correlation of the base of the Middle Ordovician Series of which the GSSP still needs to be chosen.

In southern China, acritarchs have been recovered from many localities from the Yangtze Platform and from the Jiangshan–Changshan–Yushan (JCY) area, providing an almost continuous succession from the Tremadocian to the Darriwilian. This acritarch succession can be partly correlated with the sequences observed for Gondwana, including the former Ordovician-type localities of the British Isles. Of particular interest are the acritarchs recorded in the Azygograptus suecicus graptolite biozone of southern China. The base of this biozone may roughly correspond to the Lower–Middle Ordovician boundary, which has to be defined in the next years.

The present paper deals with the acritarch succession crossing the suecicus biozone and with the palaeoecology, palaeobiogeography, and biostratigraphy of the assemblages. It is documented here that some selected, easily recognisable acritarch taxa may be useful, not only for the recognition of the Lower–Middle Ordovician boundary, but also for its correlation, at least between localities from peri-Gondwana and southern China, but probably also with Baltica.

During the Palaeozoic, China was constituted by a series of palaeocontinents, of which the three major elements were the North China (Sino-Korean), Tarim and South China Plates. In the last years, China played an essential role in the reconsideration of the global Ordovician stratigraphy. For example, the GSSP of the Darriwilian Stage lies in the JCY area in South China, and an auxiliary stratotype section for the base of the Upper Ordovician Series has been proposed at Dawangou, in western Tarim.

Recent revisions of the Ordovician of China makes that the biostratigraphy of the major fossil groups (graptolites, conodonts, trilobites, brachiopods) is today well known and can easily be integrated in the international standard [8]. The tripartite division of the Ordovician is accepted in China and the global stage names that have been approved are formally used in China. Only two regional stage names, the Yushanian and the Dawanian, still remain. They can roughly be correlated with the global ‘stages 2’ and ‘3’, which still need to be defined and named. The Ordovician graptolite succession of these stages is today known in detail, although some international correlation problems still exist. Of particular interest for the definition and international correlation of the Lower–Middle Ordovician boundary is the Azygograptus suecicus biozone, which constitutes the base of the regional Dawanian stage [8]. The base of this biozone may roughly correspond to the Lower–Middle Ordovician boundary (Chen, pers. comm.), which has to be defined in the next years by the IOS. According to the British chronostratigraphy, this boundary should be defined in the middle part of the British Arenig series, in the middle of the British Whitlandian stage [12]. According to the most recent correlations, the Chinese A. suecicus biozone roughly corresponds to the upper part of the British Didymograptus simulans (formerly D. nitidus) biozone, which occupies most of the Whitlandian [8] and [12].

Over 50 publications deal with Ordovician acritarchs of China, of which only few concern material from North China and Tarim [20]. Most investigations were focused on the Lower and Middle Ordovician of southern China, i.e., from the Yangtze Platform and from the JCY area, mostly from levels between the Late Tremadocian and the Early Darriwilian. The major part of the palynological investigations is based on three PhD theses by Li [18], Xu [32] and Brocke [3]. In addition, a total of 39 publications have been published, of which 18 deal with acritarchs of the suecicus zone. A revision of this literature shows that 39 acritarch genera are described from the suecicus zone; among them three have been newly established. Some 111 acritarch species, among them 26 newly described taxa and 35 species remaining in open nomenclature have been reported and described from this interval so far.

The sections from which acritarchs of the suecicus zone have been described are located on the map in Fig. 2. Fang [10], Li [18] and Li and Yuan [19] investigated the acritarchs of the suecicus zone from Ningqiang in the Shaanxi Province (Fig. 2, locality 1). Fang [10] also studied a section nearby, at Guangyuan in northern Sichuan (Fig. 2, locality 2). The investigated sections from southern Sichuan are those at Shuanghe (Fig. 2, locality 3, [18]), Guanyinqiao (Fig. 2, locality 4; Li, unpublished), Wangjiazai (Fig. 2, locality 5, [2], [5] and [28]), and Datianba (Fig. 2, locality 6, [2], [3], [5], [6], [11] and [28]). Three sections are located in the Guizhou Province: Honghuayuan (Fig. 2, locality 7, [15]), Huanghuachong (Fig. 2, locality 8, [18]) and Sandu (Fig. 2, locality 9, [18] and [32]). Two localities from the Yunnan Province were investigated by Fang [9] (Luquan, Fig. 2, locality 10) and by Gao [13] (Wuding, Fig. 2, locality 11). Li [17] described the acritarchs from a locality in the Hunan Province (Jishou, Fig. 2, locality 12). Three sections including the suecicus biozone are investigated in Hubei. The first section, at Jianyangping (Fig. 2, locality 13) was investigated by Brocke et al. [2], [3] and [6]. The section at Huanghuachang (Fig. 2, locality 14) was investigated by numerous workers, first by Lu [22] and Li [18] and [19] and later by Yin, Tongiorgi et al. [30] and [33]. The latter authors also investigated a section at Daping, close to Huanghuachang (Fig. 2, locality 15, [29] and [30]). One locality from the Jiangxi Province, at Chenjiawu was investigated by Huang et al. [14] (Fig. 2, locality 16). This latter locality is belonging to the JCY area; all other localities are belonging to the Yangtze Platform. The above-cited references constitute the major works. For a complete list of publications, the reader is referred to Li et al. [20].

The palaeogeography and the palaeoecology of southern China are today fairly well understood. The carbonate dominated rocks from the Yangzte Platform (between Kunming in eastern Yunnan and Yichang in Hubei, Fig. 2) change gradually to the south–southeast into the graptolitic shale facies of the Jiangnan Belt, which includes locally some beds or lenticular intercalations of limestone, such as in the JCY area [7].

The acritarch assemblages of the suecicus biozone from the different localities clearly reflect this gradual lithofacies change in a NW–SE direction by showing different abundances and diversity patterns. In the shallow-water part of the western Yangtze Platform in the Kunming–Luquan area in eastern Yunnan, the acritarch assemblages are not abundant and are poorly diversified. Deeper-water conditions present in northern Guizhou and southern Sichuan, i.e., in the sections from Shuanghe, Guanyinqiao, Honghuayuan, Wangjiazai and Datianba show acritarch assemblages of much higher diversity. Other deeper-water areas, observed around Yichang in the Hubei Province in the localities Jianyangping, Huanghuachang and Daping are also highly diversified, but show a different composition from the previous area. By going deeper into the basin, i.e., towards the JCY area, the diversity diminishes again, with generally a poor preservation of the acritarch assemblages. The difference of acritarch composition in the suecicus zone therefore probably partly reflects a facies-dependence and an inshore–offshore trend in the distribution of taxa, and not only palaeogeographical changes, as suggested by Tongiorgi et al. [30], whose investigations were limited to the Yichang area.

According to the most recent palaeogeographical reconstructions, e.g., by Li and Powell [21], South China was located at intermediate to low latitudes. It is beyond the scope of this paper to completely review the palaeobiogeography of the acritarchs of southern China. Nevertheless, it is important to remind that the assemblages of the Yangtze Platform include both acritarch taxa that are typical of the warm-water province of the Late Tremadocian defined by Volkova [31] and typical elements of the ‘Arenig’ peri-Gondwanan province defined by Li [16]. The acritarch assemblages of southern China thus bear a potential for global correlations with not only the cold-water areas from peri-Gondwana, but also with localities at intermediate latitudes, such as Baltica, and warm-water areas, such as Laurentia, Australia and northern China.

Previous publications already documented the biostratigraphical importance of the acritarchs at the Tremadocian–‘Arenig’ [1] and at the ‘Arenig–Llanvirn’ [6] boundaries in southern China. The revision of the biostratigraphical distribution, and especially of the First Appearance Datum (FAD) of selected acritarch taxa in the sections from the 16 investigated localities, now also allows to recognise the boundary between the Yushanian and the Dawanian stages, i.e., the interval where the Lower–Middle Ordovician boundary should be defined. Additionally, the stratigraphical distribution of some of these taxa also allows a correlation with the distribution of taxa in other areas of peri-Gondwana, including the British Isles, and Baltica.

The FAD of the biostratigraphically important taxa are given in Fig. 3. The species Aureotesta clathrata (with both its varieties clathrata and simplex) first appears in the deflexus graptolite biozone [4] and [5], together with Arbusculidium filamentosum [4] and [11], i.e., in the uppermost part of the Yushanian, which can be considered as the uppermost part of the Lower Ordovician.

However, the acritarch genus Ampullula first appears in the suecicus biozone, with FAD's of both A. erchunensis and A. simplex [2] and [6]. Together with the species Barakella felix, that also first appears in the suecicus biozone [6], these taxa may thus be important indicators of the base of the Middle Ordovician. Other important taxa that have been reviewed in detail in recent years and which are known to occur first in the middle and upper parts of the ‘Arenig’ are the species Dicrodiacrodium ancoriforme, which first appears in southern China in the clavus (formerly sinodentatus/nexus) biozone [28], and the genus Arkonia, which first appears in the austrodentatus biozone [18], thus indicating the base of the Darriwilian.

Most of these occurrences are of great importance for the understanding of the acritarch biostratigraphy in peri-Gondwana. As the reviews of the taxa Arbusculidium filamentosum [11], Arkonia [25], Aureotesta clathrata [5], and Dicrodiacrodium ancoriforme [28] have shown, these taxa are widespread in peri-Gondwana and southern China; they should therefore be useful to identify the Lower–Middle Ordovician boundary.

The FAD of the genus Ampullula may also be of great importance. This taxon, which probably indicates the basis of the Middle Ordovician in southern China is also recorded from the Baltica continent in the upper part of the Baltoscandian Volkhov stage of Öland [23] and [24], in levels that can be considered as slightly younger than the Chinese suecicus zone. However, the FAD of the genus in Baltica is not yet known, as the sediments below the upper Volkhov did not provide palynomorphs in northern Öland [23].