Many age determinations on sedimentary Precambrian sequences have been published. These measurements were made on whole-rock samples, clay fractions, or interbedded volcanics. A new approach leads to new possibilities about numerical stratigraphy of old nonfossiliferous sediments. This paper is a first attempt to build a nearly worldwide correlation of middle and upper Precambrian sediments based on age determinations.
The main problem with these results is in their interpretation. The age may be the age of the source area of the sediment, the time of deposition or early diagenesis, the time of epimetamor- phism, or the time of late diagenesis. Some criteria useful in the interpretation of age determinations in sediments are defined.
lated to the upper Riphean. All dates determined on the sediments containing the first African stromatolitic association with essentially Conophyton show an age of 900 to 1,000 m.y. This confirms the belief that stromatolite populations seem to be (until the present) a correlation item at least as good as isotopic age determinations. The slight differences in time between exposures generated in countries situated far away from each other is probably lower than physical uncertainties in isotopic dating.
There is a fairly good agreement between isotopic dating and tillitic or fossil occurrences where the clay mineralogy is precise enough to allow well-defined interpretation of radiometric data along with the stratigraphic succession. For that reason, it is recommended that only regularly decreasing age successions be studied.
Today, it is the way to avoid the risk of late diagenesis and subsequent age modification. Age dating, tillites, and fossils can be used separately or together for the last 500 m.y. of the Precambrian. These items could probably be used successfully between 1,100 and 1,800 m.y., but for rocks older than 1,800 m.y. the present lack of information by isotope dating in sedimentary sequences does not provide such correlations.
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.