海平面变化全球可比性的可靠例证--上扬子地台东南缘奥陶纪层序地层及海平面变化研究

通过对上扬子地台东南缘奥陶纪碳酸盐台地、台缘斜坡和盆地三个不同相区典型剖面的沉积学、生物地层学及岩石地层学等方面的综合研究，在该地区奥陶系中识别出时限为2～ 5 M a的18个正层序和相应的19次海退事件，将其与塔里木、华北及北美、澳洲等大陆对比结果表明，上扬子地台东南缘奥陶纪的绝大部分三级以上海退事件及沉积层序有着良好的全球可比性。

A Reliable Example for Eustacy Ordovician Sequence Stratigraphy on the Southeastern Margin of the Upper Yangtze Platform

On the basis of the synthetic studies, especially the identification of the key surfaces within the Ordovician sequences(SB,FFS or TS,mfs)and the representative facies-successions in different facies zones,including platform-interior,platform-margin,and shelf-basin,exposed at the adjacent region of NW Hubei and SW Hunan on the southeastern margin of the Upper Yangtze Platform in South China,eighteen depositional sequences and the corresponding nineteen regression events are recognized. According to the recent study of the Ordovician stratigraphy,the average time duration of the sequences are estimated to be from 2Ma to 5Ma generally.It is probable that all of the sequences are of 3rd-order sequences and sea-level fluctuations.The eighteen sequences may be subdivided into three groups, including the type Ⅰand Ⅱ(Vail & Mitchum 1977;Wilgus et al.1988)and type D(the drowning sequence)(Schlager 1981;Steinhauff &Walker 1995),based on the research of benthic faunal assemblages,depositional facies and diagenesis processes of the carbonate deposits,as well as the pattern analysis of the temporal and the spatial distributions of the sequences in different facies-zones during the Ordovician.Altogether,three type Ⅰ,thirteen type Ⅱ and two type D sequences have been distinguished respectively. It has been found that most of the nineteen Ordovician regression events and the eighteen sequences mentioned above can be correlated fairly well with those developed not only in Tarim, North China,but also in Australia,North America,South America and other continents according to the data nowadays.This suggests that most of the Ordovician 3rd-order sequences and higher level cycles(the 2nd-order)are eustatic in nature,although some of them probably influenced somewhat by local and regional tectonic activity.Consequently,this reveals again that both the inner-earth factor and the orbital control of the solar system and other cosmic factors may have played an essential role in the changes of the sea-level and the deposition of the sequences. As the sequence stratigraphic framework and the physical patterns of the sequence and the systems tracts are predictable,the synthesis would be helpful to many respects of the relevant studies.The preliminary study of the Ordovician sequence stratigraphy, sea-level changes on the Yangtze Platform of China has provided a kind of synthesis in the recognition and the correlation of the 3rd-order sequences as“an integrated approach”(Wilgus et al 1988),and a link between traditional stratigraphy and the establishment of the global chronostratigraphic scale.