Challenges in defining the base of Cambrian Series 2 and Stage 3
Challenges in defining the base of Cambrian Series 2 and Stage 3
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DOI：
10.1016/j.earscirev.2017.07.017
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发表时间：
2017-09
影响因子：
12.1
通讯作者：
Xingliang Zhang;P. Ahlberg;Loren E. Babcock;Duck K. Choi;G. Geyer;R. Gozalo;J. Stewart Hollingsworth-J.-Stewart-Hollingsw
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文献类型：
--
作者：
Xingliang Zhang;P. Ahlberg;Loren E. Babcock;Duck K. Choi;G. Geyer;R. Gozalo;J. Stewart Hollingsworth-J.-Stewart-Hollingsw
寒武系正式划分为四个系列和十个阶段的工作正在进行中。寒武纪第三阶段(临时)的底部与寒武纪第二系列(临时)的底部相邻，预计将放置在接近三叶虫首次出现的地平线上，这标志着寒武纪爆发最大阶段的开始。从概念上讲，理想的界线位置应该是一个重要的、全球可识别的生物事件，该事件将寒武纪系统的下部划分为次三叶式陆相系列和三叶虫主导的系列2。如果要主要通过生物地层学手段来确定水平，其位置也需要使用非生物地层学手段来识别，并且应该通过对围绕关键间隔的几个连续序列的详细调查来测试其对比潜力。确定合适层位的主要困难包括强烈的生物地域性、过多的区域不整合以及寒武纪Terreneuian-Series 2界线区间不同古大陆的相变化。提供洲际关联潜力的水平包括三叶虫的首次出现位置和某些小壳化石和甲壳类物种的大致相同的首次出现位置。此外，非生物地层学标志，如稳定的碳同位素漂移，可能是有用的对比水平。寒武纪系列2和阶段3是临时地层单位，它们的毗连基座以前被认为与三叶虫的出现相吻合。与其他潜在的年代地层学指标相比，三叶虫的首次出现仍具有识别和对比第三阶段基底的优势，应继续作为边界位置的临时主要标志。
Formal subdivision of the Cambrian System into four series and ten stages is in progress. The base of Cambrian Stage 3 (provisional), which is conterminous with the base of Cambrian Series 2 (provisional), is expected to be placed at a horizon close to the first appearance of trilobites, which marks the onset of the largest phase of the Cambrian explosion. Conceptually, an ideal boundary position would be marked by a significant and globally recognizable bioevent that divides the lower part of the Cambrian System into a sub-trilobitic Terreneuvian Series and a trilobite-dominated Series 2. If the level is to be identified principally through biostratigraphic means, its position also needs to be recognizable using non-biostratigraphic means, and its correlation potential should be tested through detailed investigation of several continuous successions embracing the critical interval. Major difficulties in identifying a suitable horizon include strong biotic provincialism, a plethora of regional unconformities, and facies changes on different paleocontinents in the Cambrian Terreneuvian–Series 2 boundary interval. Levels that provide potential for intercontinental correlation include the first appearance position of trilobites and the approximately equivalent first appearance positions of certain small shelly fossil and acritarch species. In addition, a non-biostratigraphic marker, such as a stable carbon isotopic excursion, may be useful as a correlation level. Cambrian Series 2 and Stage 3 are provisional stratigraphic units, and their conterminous base was previously suggested to coincide to the appearance of trilobites. Compared to other potential chronostratigraphic indicators, the first appearance of trilobites still has advantages for recognizing and correlating the base of Stage 3, and should remain as the provisional primary marker for the boundary position.