NEW OBSERVATION ON ORDOVICIAN-SILURIAN BOUNDARY STRATA OF SOUTHERN TONGZI COUNTY,NORTHERN GUIZHOU,SOUTHWEST CHINA
NEW OBSERVATION ON ORDOVICIAN-SILURIAN BOUNDARY STRATA OF SOUTHERN TONGZI COUNTY,NORTHERN GUIZHOU,SOUTHWEST CHINA
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发表时间：
2010
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通讯作者：
Wu Rong-chang
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作者：
Wu Rong-chang
桐梓县东南红花园山王庙奥陶系-志留系界线剖面是我国奥陶系-志留系界线最好的剖面之一，界线岩石出露良好，连续，富含贝壳类化石和对生物地层有重要意义的笔石。通过对山王庙剖面及桐梓南部红花园南坝子剖面、毛坝三岔剖面、辽源代家沟剖面的详细研究，对山王庙剖面的界线岩石提出了新的解释。观音桥层，即Hirnantia层（AFA 295 -305）和Shelly层（AFA 306 - 311 c），具有相似的岩性和相同的腕足动物群，应合并为一个奥陶纪岩石地层单位。通过与宜昌北方王家湾赫南田阶底界层型的对比，发现山王庙观音桥层可与上部的Normalograptus extraordinarius生物带和下部的N.拟雕刻菌生物带观音桥层与其上覆的龙马溪组（AFA 312）之间存在一个间断，缺失了上N。persculptus生物带、Akidograptus ascensus生物带和Parakidograptus acuminatus生物带的大部分。因此，认为典型的赫南蒂亚动物群可以延伸到最早的志留纪A。ascensius生物带或更高的层位（Rong Jia-yu et al.，2002年，修改。中国西南部许多地方的观音桥层，虽然厚度不一，但可视为同时代的岩石单位。在奥陶纪末第一次生物大灭绝之后的赫南蒂亚动物群中，出现了一系列具有特殊亲缘关系、地质时代和环境背景的独特底栖生物群落。它不仅是年龄的指示，而且还反映了海水冷却，变浅和底部氧化。
The Ordovician-Silurian boundary section at Shanwangmiao, Honghuayuan, southeast of Tongzi County Town was thought to be one of the best sections through the Ordovician-Silurian transition in China where the boundary rocks are well-exposed, continuous, and abundant with shelly fossils and the biostratigraphically impartant graptolites. On the basis of detailed investigation on the Shanwangmiao section together with several other sections in southern Tongzi, including the Nanbazi section of Honghuayuan, Sancha section of Maoba, and Daijiagou section of Liaoyuan, new interpretations of the boundary rocks at the Shanwangmiao section are proposed. The Kuanyinchiao Bed, i.e. the Hirnantia Bed (AFA295-305) and the Shelly Bed (AFA306-311c) should be combined as a single lithostratigraphical unit of Ordovician age as they have similar lithology and yield the same brachiopod fauna. Comparison with the basal boundary stratotype of the Hirnantian Stage at Wangjiawan, northern Yichang, shows that the Kuanyinchiao Bed at Shanwangmiao can be correlated with the upper Normalograptus extraordinarius Biozone and the lower N. persculptus Biozone. There is a hiatus between the Kuanyinchiao Bed and its overlying Lungmachi Formation (AFA312), missing the upper N. persculptus Biozone, the Akidograptus ascensus Biozone, and most part of the Parakidograptus acuminatus Biozone. Thus, the recognition that the typical Hirnantia Fauna could extend to the earliest Silurian A. ascensus Biozone or even higher horizon in South China(Rong Jia-yu et al., 2002) is modified. The Kuanyinchiao Bed at many localities in Southwest China may be regarded as coeval rock units although its thickness is variable. The Hirnantia Fauna after the first episode of the end Ordovician mass extinction contains a series of unique benthic communities with special affinities, geologic age, and environmental background. It is not only age indicative, but also reflects sea water cooling, shallowing, and bottom oxygenation.