关键词: 奥陶纪, 构造古地理, 年代地层, 生物地层, 划分与对比

Abstract:

Restudy of Ordovician lithological facies, biota and tectonic combination characteristic in South China, three different types of tectonicpaleogeographic regions have been recognized, namely the Yangzi shallow sea carbonate platform facies, the Jiangnan and South Qinling shelfslope transitional facies, and the Cathaysia continental marginal island arc or basin facies. The former two belong to the Yangtze block. The latter, so called the Zhujiang (Pearl River) basin facies of the Huanan massif, seems to be of the Cathaysia block, rather than the South China block on the basis of the characteristics of Ordovician sedimentary, biological facies and water flow direction of NW320° shown by flute cast structure. The Hainan Island as a drift terrain in the Ordovician might belong to the shelf area of platform margin of the IndoChina block. Ordovician chrono and biostratigraphic division and correlation study indicated that the Xiaoyangqiao CambrianOrdovician boundary section near Dayangcha, Baishan (formerly Hunjiang), Jilin, China devoloped a complete representative conodont and graptolite successions and the FAD of the conodont species Cordylodus intermedius of the section might be applied instead of the FAD of the controversial and rare Iapetognathus fluctivagus as boundary biomarker for determining global Cambrian and Ordovician System. The NijiangkouNanba section near Yiyang of Hunan appears a complete Upper Tremadocian to Lower Floian graptolite sequence. Recommended here the Yiyang stage, replacing the “Daobaowan stage”, as the Ordovician regional chronostratigraphic units in China. Its boundary with underlying Tremadocian stage is assigned between the Hunnegraptus copiosus and Tetragraptus approximatus graptolite zones and easy to correlate to that of the Diababrottet GSSP in Sweden. Systematic collection and studies to the conodontbearing carbonate strata wellexposed at the Huanghuachang, Fenxiang, Chenjiahe and Puxihe sections near Yichang City, Hubei and Maocaopu section, Cili, Hunan suggested that 8 conodont biozones can be recognized in ascending order in the Dawan Formation, i.e. Upper Oepikodus communis, Oepikodus evae (s.s.), Periodon flabellum, Microzarkodina russica, Baltoniodus triangularis, Baltoniodus navis, B. norrlandicus, Lenodus antivariabilis Biozones, their correlative relationship with relevant graptolite biozone are discussed. The findings of conodont Protopederodus liripipus in 23 m limestone above the base of the Baota (Pagoda) Formation of the Huanghuachang and Puxihe sections and the relevant carbon isotopic excursion suggest that the lower boundary of the Upper Ordovician Katian Stage is broadly filled in the lower Pagoda Formation on the Yangtze Craton platform, being equivalent or close to the base of the Hamarodus? europaeus Biozone. New Highresolution Ion Microprobe (SHRIMP II) zircon UPb dating indicates that the age of the bentonite interbeds of 20 cm below the base of the Dicellograptus complexus Biozone in the lowest Wufeng Formation in Yichang area might be 448.6±4.8 Ma, and 446.5±2.1 Ma for the bentonite layers, 30 cm below the top of the Paraorthograptus pacificus Biozone, indicating that the Wufeng Formation in Yichang area endured about 5.4 Ma of sedimentation time.

Key words: Ordovician, tectonopalepogeography, chronostratigraphy, biostratigraphy, subdivision and correlation