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RISS 인기검색어
- 검색키워드
- 저자 : Gerd Geyer (검색결과 2 건)
- 무료
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- 유료
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The base of a revised Middle Cambrian: are suitable concepts for a series boundary in reach?
Gerd Geyer 한국지질과학협의회 2005 Geosciences Journal Vol.9 No.2
Defining the base of a series that replaces the traditional Middle Cambrian is among the difficult tasks of Cambrian stratigraphy. Non-traditional concepts (such as carbon isotope signatures), microplankton (such as acritarchs), and most invertebrate fossils (e.g., brachiopods) may act as auxiliary tools for intercontinental correlation of regional calibration but are unable to provide the base for fine-scaled global correlation at present. As a result, the selection of a Global Stratigraphic Section and Point will have to root on trilobites which appear to be the only reliable index fossils to define such a GSSP. Five possible levels of correlation within the traditional Lower-Middle Cambrian boundary interval have been discussed: (1) the FAD of Oryctocephalus indicus; (2) the FAD of Ovatoryctocara granulata and/or Kiskinella cristata; (3) the FAD of Arthricocephalus chauveaui; (4) the base of the Acidiscus-Cephalopyge assemblage "zone"; and (5) the base of the STH "band". The potentials of these levels are analyzed in this study. All of them suffer from certain deficiencies such as limited insight into the stratigraphic ranges of key species; problems of confident identification of the index species; or absence of key faunal elements on certain Cambrian continents; that make them invalid for high-precision correlation on a global scale. However, the combination of (1) through (4) promises an intercontinental correlation that can be used as a global framework, which will be sufficiently accurate to serve normal correlation purposes.
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Loren E. Babcock,Shanchi Peng,Gerd Geyer,John H. Shergold 한국지질과학협의회 2005 Geosciences Journal Vol.9 No.2
Important steps have been made toward global subdivision of the Cambrian System. The consensus of opinion is that the system should be subdivided into four series representing subequal spans of time. The lower two series will correspond approximately to the traditional Lower Cambrian, the third series will correspond approximately to the traditional Middle Cambrian, and the fourth series corresponds approximately to the traditional Upper Cambrian. The fourth series, called the Furongian Series, has been ratified by the International Union of Geological Sciences (IUGS). Selection of horizons representing evolutionary events that can be correlated on a global scale, and which can be used as stage or series boundaries, is much more difficult for the lower two series intervals of the Cambrian than it is for the upper two series intervals of the Cambrian. The most practical concept for subdividing the system into stages involves the establishment of two stages each for the first and second series, and establishment of three stages each for the third and fourth series. The lowermost stage of the Furongian, called the Paibian Stage, has been ratified by the IUGS, and the International Subcommission on Cambrian Stratigraphy (ISCS) is nearing decisions on the levels that will used to mark the bases of the upper two stages of the Furongian. Concerning stage boundaries within the proposed third series of the Cambrian, the ISCS is close to decisions on two levels that will be used to mark boundaries.
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