湖南吉首大沱奥陶系九溪组疑源类

ORDOVICIAN ACRITARCHS FROM THE JIUXI FORMATION OF JISHOU, HUNAN

本文描述湘西吉首大沦九溪组疑源类24属40种,其中1新种,6个未定名种。文中将九溪组疑源类与国内、外同期疑源类作了比较。共同的疑源类特征分子表明了早奥陶世上扬子区与地中海区的联系。

Here described is a rich acritarch assemblage from the Jiuxi Formation of Ji-shou, Hunan (Fig. 1). Among the forty species belonging to twenty-four genera,one new species ? Baltisphaeridium jishouense sp. nov. is proposed and six unde-termined species are dealt with in open nomenclature. The Jiuxi Formation is unconformably overlain by the Cretaceous psephyteand underlain by sandstones of the Ordovician Datuo Formation. It is 166mthick and consists of yellow-green to grey-green shaesl and grey-green siltyshales, with graptolites Azygograptus suecicus and Didymograptus sp. which suggestan Arenigian age. Sixteen samples collected from the Jiuxi Formation by the author were pro-cessed using standard palynological techniques. All samples except two producedwell-preserved acritarchs of yellow-brown colour. Conventional light microscopywas conducted by means of an Opton microscope and a Diaplan microscope equip-ped with automatic photographic facility. Some specimens were observed undera Jeol JSM CF-35 SEM. The illustrated materials are deposited in the collectionsof the Nanjing Institute of Geology and Palaeontology, Academia Sinica. The Arenigian age of the Jiuxi Formation is confirmed by the occurrence ofArbusculidium filamentosum, Coryphidium australe, C. bohemicum, C. elegans, Pireaornata, Schizodiacrodium multirarniferum, Striatotheca principalis, S. principalis parvaand S. transforrnata. Two species previously described from the Ashgillian of theAnticosti Island (Staplin et al.,1965, Loeblich & Tappan, 1978; Jacobson &Achab, 1985), Multiplicisphaeridium bifurcatum and M. irregulare should not be re-garded as 'guide fossils for the Upper Ordovician' (Jacobson, 1987) since theyare rich both in the Jiuxi Formation and in the Meitan Formation (Li, 1987). The diagnostic Mediterranean province (Vavrdova, 1974) elements Arbusculi-dium, Coryphidium and Striatotheca have been described from several localities inthe Upper Yangtze Region, including the Xiliangsi Formation of Zhenba, Shaanxi(Fu, 1986); the Hongshiya and Qiaojia Formations of Kunming--Luquan,Yunnan(Fang, 1986); the Dawan Formation of Yichang, Hubei (Lu, 1987); the MeitanFormation of Tongzi, Guizhou (Li, 1987) and the Jiuxi Formation of Jishou,Hunan. This supports the conclusion that the Upper Yangtze Region might link up with the Mediterranean area during the Early Ordovician, which was drawnfrom the common occurrence of the trilobites Neseuretus, Taihungshania and Han-chungolithus in the two areas (Whittington & Hughes, 1972, Lu,Chu et al.,1976; Fortey & Morris, 1982).