Forty selected radiometric ages are plotted to illustrate a time scale for the Cambrian System and strata immediately above and below. Uncertainty is unavoidable at present, not only because the boundaries of the system are the subject of international working group investigations which are not complete, but also because the radiometric age determinations are too few and their association with stratigraphical data gives rise to manifest imprecision. Tentative ages of 560 m.y. and 485 m.y. are suggested for the base of widely accepted minimum limits of the system. If certain fossiliferous beds were to be included in the Cambrian System and excluded from pre-Cambrian sequences in an agreed international stratigraphic decision, then the base could be lowered as far as 590 m.y. Similarly Tremadoc sequences currently under discussion may be retained in the Cambrian changing the age of the top to 465 m.y. Working groups are also active on intra-systemic boundaries; dates of 530 m.y. and 505 m.y. are suggested for the traditional Lower-Middle and Middle - Upper Cambrian boundaries respectively. All the dates, except those taken from Glaessners manuscript ( and , used in Figure 1) have been recalculated using the new decay constants decided upon at the Symposium on Geochronology held at the 25th International Geological Congress.
Figures & Tables
Containing papers given at the Geological Time Scale Symposium in 1976, this volume begins with a review of dating and correlation, and includes papers on the topics of: geochronoloic scales, biochronology, the magnetic polarity time scale, the potassium-argon isotopic dating method, isotopic methods, and worldwide Permian chronostratigraphy, among others.