华南早奥陶世海平面变化及其对比

EARLY ORDOVICIAN SEA LEVEL EVENTS IN SOUTH CHINA AND ITS WORLDWIDE CORRELATION

本文在对扬子地台及其边缘寒武纪末至早奥陶世代表性剖面层序地层、生态地层及生物地层综合研究基础上,讨论了华南板块碳酸盐台地及其陆棚-陆坡区早奥陶世海平面相对变化规律,指出该区在寒武纪末至早奥陶世至少可以识别出8次海退事件,阐明了这些海退事件发生的时间及其表现形式,并与国外进行了对比.

The Ordovician in South China is divided into the stable shallow epicontinental sea sediments in the Yangtze cratonic platform, bathyal black graptolite-bearing shales and siltstones of activ continental margin in SE China, and shelf to slope elastics intercalated with carbonates of the Jiangnan transitional tract situated between proceeding two (Wang, 1989). Upon a synthetical study of sequence and ecostratigraphy of continuous sections from the Yangtze platform to the Jiangnan shelf to slope, at least 8 major regressive events (RE1-8) with wide correlative importance can be recognized in the interval extending from the base of the Xil-inxia Formation (latest Cambrian) to the base of the Miaopo Formation (Early Caradoc). These events occurred at the following levels on the basis of the study of conodont and grap-tolite biostratigraphy: basal Hirsutodontus simplex Zone (RE1), near base of Cordylodus intermedium Zone (RE2), basal Codylodus angulatus Zone (RE3), near Mid/Upper boundary of Glyptoconus quadraplicatus Zone (RE4), following the first phase (RE4) of which at basal Paltodus deltifer Zone, mid P. deltifer Zone of Kiaerograptus /Bryograptus Zone boundary(RE5),end Serratognathus diversus-Paroinodus proteus Assemblage Zone (RE6),end Paroistodus originalis Zone (RE7) and early Husterogratus teretiusculus Zone or late Pygodus serra Zone (RES) ,and can be traced in studied sections by marked changes in sequences or disconformity along with the break of conodont or graptolite succession.RE1, separating the highstand deposits with Codylodus proavus at the top of Wuduhe Formation from the trangression at the base of Xilinxia Formation with Hirsutodontus simplex, is equivalent to the Event 1 occuring subquently after the Langre Ranch Eustatic Event in North American (Miller,1992). RE2, signalled by a disconformity associated with the absence of Cordylodus prolindstromi Zone and most of the C. lindstromi Zone in the Yangtze platform, or following a deeping upward sequence above this level with the incoming of earliest nemotophorous planktic anisograptids in the Jiangnan shelf-slope,indicatse this event covers the interval from regressive Event 4 to Acerocare Regressive Event, redefinted by Nicoll et al (1992). The Black Mountain Eustatic Event in Australia correlates with the present RE3 dated by the first appearance of Cordylodus angulatus at the base of Nanjinguan Formation. The age for RE4 is not precisely determined yet,but subsequent RE4a, being e-quivalent to the Paltocare Regressive Event (Erdtmann, 1986),has been widely reported (Nicoll et al, 1992, Ross & Ross, 1992). The Ceratopyge Regressive Event (CRE) of Erdtmann (1986) should include two hiatus surface in East European platform. Present RE5 occurring between redifinited Nanjinguan and Fenxiang Formation and the Kelly Creek Eustatic Event of the Amadens Basin are equivalent to the Regressive Event of basal Ceratopyge limestone in Scandinavia, while the disapperance of Early Hunnebergian graptolite and conodont faunas at the Arenig/Tremadoc boundary section at Ledge, Newfoundland is relevant to the CRE at the top of Ceratopyge Limestone. An abrupt sequence change and following transgression with faunal assemblage turnover shown between the topmost Honghuayuan Formation with Paroistodus proteus and Serratognathus diversus, and the basal Dawan Formation with a part of Oepicodus communis Zone indicates the presence of RE6. RE7, occuring between the Middle and Upper Members of Dawan Formation i. e. , Paroistodus originalis/Mi-crozarkodina f. parva zonal boundary, is related with the event of Late Areing or Arenig/ Llanvirn boundary widely reported, though its age is not fully coincident in different continents. RE8 is reflected by a stratigraphic break with absence of Upper Pygodus serra Zone. Widespread trangression led to intermittent filling of the Early Caradoc black shale with Nemagraptus gracilis (Miaopo Formation)in the underlying uneven limeston surface of the top-most Guniutan Formation.