摘要
中.上寒武统的分界问题是国际上尚未解决的地层问题之一。迄今为止,仍无举世公认的划分标准和界线层型剖面。在卢衍豪、安太庠教授的指导下,笔者详细研究了湖南花垣排碧剖面。该剖面出露良好,沉积连续,交通方便。中—上寒武统为单一的碳酸盐岩,界线附近为亮泥晶生屑砂屑灰岩,代表扬子海碳酸盐台地前缘缓坡上水下隆起的沉积环境。笔者建立了中寒武统上部至上寒武统下部的球接子类三叶虫带和牙形石带,确立了三叶虫序列和牙形石序列的准确对应关系,并以这两个门类为依据,划出了中、上寒武统的界线。此划分依据,比以往合理,精度也有所提高。总之,该剖面优于其它已知的中—上寒武统剖面,很可能成为中—上寒武统理想的候选界线层型剖面。
The section studied in this paper is situated in Paibi, Huayuan County, Hunan Province (Fig. 1). Through detail sampling from the bottom of the Huaqiao Fm. to the base of Bitiao Fm., 93 trilobite-bearing horizons, including 72 agnostid-bearing horizons, are found. Some 5000 identifiable conodont specimens are recovered from 1200 kilograms of limestone of 351 samples.Six trilobite (agnostid) zones erected by the writer are as follows:Upper Cambrian SeriesBitiao Fm. (the lowermost part)6. Glyptagnostus reticulatus ZoneChefu Fm.5. Glyptagnostus stolidotus Zone4. Ammagnostus integriceps-Chatiania ZoneMiddle Cambrian SeriesHuaqiao Fm.3. Lejopyge armata ZoneUpper Part: Lejopyge laevigata and L. sinensis are associated with the nominate zonal species L. armata.Lower Part: Lejopyge laevigata and L. sinensis are absent.2. Pseudophalacroma triangularis Zone1. Ptychagnostus atavus ZoneThese zones provide direct evidence for the correlation between the strata of this region and those of other regions within different realms and provinces (See Tables 1 and 2).Four conodont zones established here are as follows.Upper Cambrian SeriesChefu Fm.4. Westergaardodina matsushitai-W, grandidens Zone3. Westergaardodina moessebergensis quadrata ZoneMiddle Cambrian SeriesHuaqiao Fm.2. Shandongodus priscus-Hunangnathus tricuspidatus Zone1. Unnamed ZoneThe correlation between these zones and those of North China and Irna is given in Table 3, which is self-evplanatory. The problem of Middle-Upper Cambrian Bcundary has long been open in the world. Based on both trilobite and conodont sequences in the Paibi section, this boundary is drawn between the Lejopyge armatas Zone and the overlying Ammagnostus integriceps—Chatiania zone, and between the Shandongodus priscus-Hunangnathus tricuspidatus Zone and the overlying W. moessebergensis quadrata Zone. The interval between the highest horizon of Lejopyge laevigata and the lowest horizon of W. moessebergensis quadrata is only 0.9 m. The former species ranges up to the top of the L. armata Zone in the Paibi section, while the latter species is no doubt of Late Cambrian age. So the boundary here appears to be more accurately drawn than what has been done in other regions.Compared with other Middle-Upper Cambrian sections, the Paibi section has the following major auributes:First, it is marked by a continuous micritic-sparry bioclastic limestone sequence across. the Middle-Upper Cambrian Boundary.Second, it has distinct trilobite and conodont zones across the Boundary. The conodont zones display the evolution of the Laiwugnathus-Westergaardodina lineage, and the appearance of W. moessebergensis quadrota marks the beginning of the explosive stage of Westergaardodids.Consequently, the Paibi Section is proposed as a potential candidate for the Middle Upper Cambrian Boundary Stratotype.The present study has been under the supervision of Prof. Lu Yan-hao and Prof. An Tai-xiang, to whom sincere thanks are due.
出处
《地质学报》
EI
CAS
CSCD
北大核心
1990年第1期62-79,T003,共19页
Acta Geologica Sinica
