The present status of the uranium-lead, potassium-argon and rubidium-strontium methods of absolute age determination is reviewed. It is concluded that any of these methods can give dates that are accurate to at least 5% under suitable conditions. Relative ages to 1% are possible. The uranium-lead method is limited by the rare occurrence of sedimentary phases with a sufficientIy high uranium- to common-lead ratio. It provides an important independent control in some cases. It may also be used to determine the age of rocks contributing the sedimentary material. The rubidiumstrontium method appears applicable to glauconite in post-Cambrian rocks if samples with a sufficiently high rubidium- to common-strontium ratio can be obtained.
The most useful age method for Sedimentary rock would appear to be that based on the decay of potassium-40. Age measurements may he possible not only on glauconite but on some types of shales that contain largely authigenic potassium minerals. Felsitic and basaltic flows intercalated in the sedimentary column also appear promising. Th:s method is sufficiently sensitive so that, in principle, ages may be determined down into the range of the carbon-14 method, i.e., 50,000 years ago.
The available quantitative age measurements on sedimentary rocks are presented and with certain other data used to infer a new geological time scale. It now appears that the so-called Holmes scale which has been widely used in the last two decades must be modified. Beginning with the Jurassic, the time scale must be stretched by about 15%. The lengths of the Jurassic, Permian and Carboniferous are doubled. The Cambrian-Ordovician boundary appears to occur at about 490 m.y. An attempt is made to indicate the likely developments of these methods and the types of problems in petroleum geology to which they may be most applicable.
Ce travail passe eu revue les méthodes actuelles de détermination d'âge absolu basées sur les rapports plomb-uranium, potassium-argon et rubidium-strontium. L'on conclut que chacune de ces méthodes peut être utilisée avec une précision d'au moins 5% si les conditions sont favorables.
On peut même faire des déterminations d'âge relatif avec une précision de 1%. L'emploi de la méthode plomb-uranium est limité, étant donné la rareté des faciès sédimentaires ayant un rapport uranium-plomb commun suffisamment élevé. C'est, cependant, une méthode de vérification importante dans certains cas. On peut aussi l'utiliser pour déterminer l'âge des roches qui sont la source des matériaux sédimentaires. La méthode rubidium-strontium semble s'appliquer surtout à la glauconie; aux roches du post-Cambrien lorsque des échantillons présentant un rapport suffisamment élevé entre le rubidium et le strontium commun peuvent être obtenus. La méthode la plus utile pour dater l
