Cambrian boundaries and divisions



Problems addressed here are common to other Phanerozoic systems, although this discussion is restricted to the Cambrian. Since 1972 various international working groups have reviewed potential boundary-stratotypes for the Cambrian System and its three traditional series. Evaluation of the series boundaries has been in progress without regard to the fact that disputed boundary intervals of the system may equal more than one-third of the time represented by rocks of unquestioned Cambrian age. The magnitude of these disputed intervals is such that an international decision on placement of series boundaries should be postponed until after both system boundaries have been defined, and until after a thorough evaluation of the best possible markers for long-distance chronocorrelation. Also, rather than proceeding on the assumption that a tripartite scheme of series is optimal, a new look should be taken at all criteria to be used in determining the number of Cambrian series and the placement of their boundary-stratotypes. Examples of useful chronohorizons that could be utilized in the definition of alternative series boundaries are briefly examined.