陕西西乡三郎铺奥陶系与志留系界线剖面

本文研究的陕西省西乡县三郎铺奥陶——志留系界线剖面地层连续、出露良好、化石丰富,是陕西汉中地区最好的界线剖面。作者详细研究了界线附近的笔石,将五峰组和龙马溪组划分为八个笔石带,这是首次在汉中地区建立较完善的界线附近笔石带。通过研究发现本区原划归下志留统的龙马溪组底部,产有丰富的上奥陶统顶部Diplograptus bohemicus带的笔石。本区龙马溪组是一个跨奥陶系与志留系的岩石地层单位。“Hirnantia-Dalmanitina层”位于Diplograptus bohemicus带之下,应划归奥陶系,相当于五峰阶第五笔石带(W5)上部至第六笔石带(W6)下部。通过本剖面笔石动物群分析、属种统计、笔石胎管刺发育特征等研究,再次论证将奥陶系与志留系界线置于Glyptograptus persculptus带之底是适宜的。

THE 0RD0VIC1AN-SILURIAN BOUNDARY SECTION AT SANLANGPU OF XIXIANG, SHAANXI PROVINCE

The Ordovician-Silurian boundary section at Sanlangpu of Xixiang county is the best one of Hanzhong area in southern Shaanxi. The section is contineous, well-exposed, rich in fossils, complete in graptolitic sequence. We measured two Formations: Wufeng Formation and Lungmachi Formation. Four graptolite zones can be erected in Wufeng Formation and four graptolite zones in Lungmachi Formation. The Ordovician-Silurian boundary is interleaved between graptolitic facies in our section. The Hirnantia-Dalmanitina bed at the top of the Wufeng Formation here consists of grey-white sandstone, yielding rich branchiopods and trilobites in it. But it is different in age from those of Yichang and other places, The authors believe that it can correspond to upper part of Paraorthograptus uniformis zone (W5) and lower part of Diplograptus bohemicus zone (W6). The appearance of Akidograptus ascensus from the upper part of Glyptograptus persculptus zone and the statistics of species number indicate that G. persculptus zone (L1) has a closer relation with P. acuminatus zone (L2) than with D. bohemicus zone (W6). It is worthy of note that some graptolite with bifurcated virgella appeared from G. persculptus zone in this section and Guantangyuan setion of the Wuning area in Jiangxi (Yu et al. 1984), which have not been found from D. bohemicus zone and lower horizon. Because the phenomenon of bifurcated virgella in Diplograptidae is common in the lower Silurian, it can help geologists to delimit the Ordovician-Silurian boundary in the field work. According to our data the Ordovian-Silurian boundary is set between G. persculptus zone (L1) and D. bohemicus zone (W6) at Sanlangpu section.