遗迹化石与梅树村剖面震旦系—寒武系界线及其与小壳化石的关系

TRACE FOSSILS AND THE SINIAN-CAMBRIAN BOUNDARY IN THE MEISHUCUN SECTION, EASTERN YUNNAN, CHINA, AND THEIR RELATIONS TO SMALL SHELLY FOSSILS

滇东梅树村剖面震旦系—寒武系界线层中产有大量遗迹化石。本文通过对前人研究成果的总结,基本弄清了遗迹化石在剖面上的分布规律、演化特点,并由此证实了利用遗迹化石进行震旦系—寒武系界线划分的可行性。在此基础上,提出了震旦系—寒武系界线,应划在节肢动物三叶虫遗迹和(或)复杂的非三叶虫遗迹第一次大量出现的层位之底的划分方案。根据此方案,梅树村剖面的达条界线应划在中谊村段第7层之底。同时根据该剖面小壳化石的研究成果,讨论了遗迹化石和共生小壳化石的演化关系,发现二者的繁盛期是同时的。

Abundant and diverse trace fossils have been reported from the Sinian-Cambrian boundary candidate section at Meishucun, eastern Yunnan, China.The Lower strata exmined belong to the Yuhucun Formation, which includes 4 members. Baiyanshao Member, Xiaowaitoushan Member, Zhongyicun Member and Dahai Member. The Baiyanshao and Xiaowaitoushan Members yield only two ichnogenera: Planolites and Skolithos. In beds 3--6 of the section, Ⅱ ichnogenera occur; they are: Yunnanichnus, Sellaulichnus, Arenicolites, Asteriacites, Neonereites, Helminthopsis, Taphrhelminthopsis, Gordia, Did ymanlichnus, Phycodes and Monomorphichnus. In bed 7 of the section, a large number of trilobite traces and various complex feeding burrows are found; they are: Cruziana, Rusophycus, Monomorphichnus, Plagiogmus, Phycodes, Chondrites and Did ymaulichnus. The Dahai Member yelds only the ichnogenus Did ymaulichnus.The Shiyantou Member, the upper part of the section, contains the following trace fossils: Diplocraterion, Skolithos, Did ymaulichnus, Taphrhelminthopsis, Subphyllochorda, Monocraterion, Arenicolites, Jinningichnus, Trepiichnus, Lorenzinia,Plagiogmus and Arthrophycus.The vertical zonation of trace fossils and the different characteristics of the zones at the Meishucun section are valuable for locating the Sinian--Cambrian boundary. The author suggests that the boundary be placed at the base of the horizon containing trilobite trace fossils and/ or various complex non-trilobite burrows. Based on this suggestion, the Sinian--Cambrian boundary at the Meishucun section should be placed at the base of bed 7 of the Zhongyicun Member.This paper also discusses the relationsbip between trace fossils and associated small shelly fossils. It is found that the so-called 'explosions' in diversity and abundance of trace fossils and small shelly fossils were synchronous. The dramatic change point also lies at the base of bed 7 of the Zhongyicun Member, and this point was also recommended by the International Precambrian—Cambrian Boundary Working Group in 1983.