Fission track analysis of pyrogenic apatite from a bentonite in the Acton Scott beds of the type section of Murchisons Caradoc Series has produced minimum ages of 443 ± 27 m.y. (external detector method) and 459 ± 27 m.y. (population method). That the sample was never heated as high as 80°C after deposition of the ash was determined by the conodont "color thermometer."
No other new Ordovician radiometric information has become available since the thorough and demanding review presented at the Ordovician System Symposium at Birmingham in 1974.
The most reliable published old ages for Ordovician rocks are by the potassium-argon method on micas from the post - Middle Ordovician Thetford Mines Gabbro (Canada; 480 m.y.) and from biotite from an intrusive that cuts the Girvan Ballantrae Complex (475 m.y.). All other age determinations related to the Ordovician are significantly younger than these.
Attempts have been made recently to place limits on Ordovician time by considering tectono- mSgmatic events in their stratigraphic frameworks. On the assumption that the Highland Border Group and the MacDuff slates in Scotland are both Dalradian and Arenig, the metamorphic peak in the Dalradian (490 to 500 m.y.) must be Llanvirnian or younger, depending on the Rb87 half-life and exact interpretation of isochrons and chrontours. However, setting the metamorphic peak post-Llanvirnian creates many difficulties and 495 ± 5 m.y. is therefore preferred for the Llanvirnian.
Radiometric calibration of provincial stratotype sections will prove essential to understanding Ordovician biostratigraphy because of the wide separation of tectonic plates into differing latitudinal environments in Ordovician time.
Figures & Tables
Contributions to the Geologic Time Scale
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.