Subdivision of the Precambrian — A brief review and a report on recent decisions by the subcommission on Precambrian Stratigraphy |
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| Authors: | Harold L James |
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| Institution: | U.S. Geological Survey, Port Townsend, Wash. 98368, U.S.A. |
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| Abstract: | In contrast to the Phanerozoic, for which there exists an internationally accepted, detailed time scale, the Precambrian, representing more than four-fifths of all recorded Earth history, is completely without an accepted scheme for time classification. Inspection of the many published proposals for subdivision of the Precambrian reveals much diversity in underlying concept, means of definition, and nomenclature. Most schemes are based on orogenic or magmatic-tectonic cycles, but others are keyed in principle to sedimentary sequences, to total Earth history, or to combinations of various concepts. Several are reviewed as examples. Considered in terms of basic criteria, few if any of the published proposals are entirely satisfactory, and none appears a likely candidate for international acceptance.The Subcommission on Precambrian Stratigraphy of the International Union of Geological Sciences, at its meeting in Cape Town, 11.7.1977–15.7.1977, has taken the initial step toward formulation of a complete time scale by recommending acceptance of a division of the Precambrian into Proterozoic and Archean. These time units have the status of eons, equivalent to the Phanerozoic, though of much greater duration. The age of the time boundary between Proterozoic and Archean is defined as 2500 Ma. |
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