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Abstract

Since 1970, a new set of experiments and research on the dating of glauconites has been underway in different laboratories of Europe. The quality of the results seems to be due to a rigorous preselection of the outcrops, levels, and pellets before dating. This work, preliminary to an isotopic analysis, permits an easier interpretation of the apparent ages obtained.

From the glauconites chosen in Europe it is possible to test the local horizontal and vertical reproducibility and accuracy of the chronometer. After interlaboratory analysis and internal verification (Rb-Sr and K-Ar isochrons with 10 points or more) it appears that a carefully selected sediment sample gives an apparent age equivalent to other samples of the same stratigraphic level. The accuracy is lower with the Rb-Sr method.

Comparison of results obtained by both radiometric methods or with different chronometers (high temperature - low temperature) does not show an important systematic difference.

As far as it can be concluded with the present dates, it is clear that most boundaries proposed since 1964 must be changed for Cretaceous and Tertiary stages. There are three reasons: (1) we know that some of the oldest results and reasoning taken into account were not adequate; (2) we have more results on better known samples; and (3) the decay constants are nearer their probable value today.

Apparent ages proposed are essentially obtained from glauconites and must be completed and compared with more apparent age data on known high-temperature minerals.

For more than a century, the stratigraphic position of European basin sediments has been studied. These investigations have led to the construction of the stratigraphic column. A maximum number of isotopic dates on these stratigraphically well-determined horizons is needed to correlate the stratigraphic column with a numeric time scale.

Glauconite as a geochronometer is available in many sediments. Specific precautions have been determined and are observed. For a few years, different laboratories have undertaken preliminary isotopic studies for a better understanding of glauconite.

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