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A computer file of K-Ar, Rb-Sr, and U-Pb dates that provide constraints on the pre- Cenozoic Phanerozoic time scale has been created. New data appear slowly and thus the file size grows at the rate of only a few percent per year. The time scale presented at the 1974 International Meeting for Geochronology, Cosmochronology, and Isotope Geology in Paris is not in conflict with the data added since then.

Precise chronologic subdivision of the Cretaceous is difficult because even the most optimistic uncertainties in the dates are greater than the duration of some stages. Nevertheless the stages of the Upper Cretaceous have been calibrated reasonably well.

Subdivision of the remainder of the Mesozoic and Paleozoic systems cannot be done precisely and objectively from geochronometric data. Important boundary dates must be derived from interpolation between points with experimental and geologic uncertainties of at least a few percent.

Efforts to obtain additional data for Lower Cretaceous to Upper Permian and Devonian and older rocks should be given special priority.

One potential source of confusion in time-scale calibration, and conversely in the assignment of geologic age to rocks that have been dated, is the use of different decay constants by different laboratories and by the same laboratory at different times. Recently adopted values for uranium decay constants have the effect of reducing previously published U-Pb dates by about 1%. Proposed new decay constants for potassium (K) would increase previously published Phanerozoic K-Ar dates by about 2% in the case of western literature, and would reduce dates published by the eastern European countries by about 2.5%. The suggested new decay constant for rhubidium (Rb) is a compromise between values previously used. Most western and all eastern European Rb-Sr dates would be reduced by about 2%. Dates from western labs that have used the 47 x 109-year half life would be increased about 3.5%.

Revised time scales will reflect these changes. Because most calibration points are K-Ar dates the net effect is a 1 to 2% increase in the ages assigned to time-scale boundaries for scales published by geologists from western countries, and an approximate 2% reduction for scales published in eastern Europe. A discrepancy exists between time scales used in the two groups of countries. The eastern European scale is younger than the western one because of the greater use of glauconite K-Ar dates by the eastern Europeans. In contrast western emphasis is on dated volcanic rocks coupled with skepticism toward glauconite and whole-rock K-Ar dates.

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