Changing perspectives on Cambrian chronostratigraphy and progress toward subdivision of the Cambrian system

摘要

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.