Progress and prospects toward a global chronostratigraphic subdivision of the Cambrian System
|Category||Paleontology and Stratigraphy|
|Location||International Geological Congress,oslo 2008|
|Author||Peng, Shanchi۱; Babcock, Loren E.۲|
|Holding Date||04 October 2008|
In recent years, significant progress has been made by the International Subcommission on Cambrian Stratigraphy (ISCS) toward developing a standard, globally applicable time scale of Cambrian System. As important philosophic and practical changes in the system’s chronostratigraphic subdivision, the ISCS decided to abandon the traditional tripartite subdivision and instead to establish a four-fold framework for the system. As expected, the series in the new subdivision would represent subequal spans of time. The lower two series that spans nearly half the duration of the total Cambrian correspond approximately to the traditional Lower Cambrian. Actually, the lowermost series is a pre-trilobite interval that had been subsequently added to the Cambrian since late 1940’s. The third series is an expended version of traditional Middle Cambrian, whereas the fourth series a reduced Upper Cambrian of traditional usage. The lowermost and the uppermost series, termed as the Terreneuvian Series and Furongian Series respectively, have been ratified by the International Union of Geological Sciences. The second and the third series are not yet named and are provisionally called as Series 2 and Series 3 respectively. The lower two series are expected to be subdivided into two stages each and the upper two series are expected to be subdivided into three stages each. Once finalized, the Cambrian System will totally embrace 10 stages. Up to date four Global Standard Stratotype-section and Points (GSSPs) have been defined and ratified, namely the GSSP defined by the FAD of Ichnofossil Trichophycus pedum in Newfoundland, Canada for the base of Fortunian Stage, the lowermost stage of the Cambrian, as well as the conterminant base of the Phanerozoic Eonothem, the Paleozoic Erathem, the Cambrian System and the Terreneuvian Series; the GSSP defined by the FAD of cosmopolitan agnostoid trilobite Ptychagnostus atavus in Utah, USA for the base of Drumian Stage; the GSSP defined by the FAD of cosmopolitan agnostoid trilobite Lejopyge laevigata in Hunan, South China for the base of Guzhangian Stage; and the GSSP defined by the FAD of cosmopolitan agnostoid trilobite Glyptagnostus reticulatus in Hunan, South China for the base of Paibian Stage, which is conterminant with the base of the Furongian Series. The bases of other six stages remain undefined yet. The guide fossils that making the bases of the Stage 2, 3, and 4 are under discussion, but the agreement on levels for placing the bases of the Stage 5, 9, and 10 has been reached.