Age of biostratigraphic horizons within the Ordovician and Silurian systems

Three samples that have a bearing on the age of horizons within the Ordovician and Silurian systems, two previously dated by the conventional K-Ar method and one by the ⁴⁰Ar/³⁹ Ar total-fusion method, have been reanalysed using the ⁴⁰Ar/³⁹Ar age-spectrum method. Conventional K-Ar and total-fusion ⁴⁰Ar/³⁹Ar ages can always be questioned because of the relative ease with which the K-Ar system can be disturbed, either thermally or chemically (i.e. Dalrymple & Lanphere 1969; Clauer et al. 1982). The ⁴⁰Ar/³⁹Ar age-spectrum method has the potential for identifying disturbed K-Ar systems (i.e. Berger 1975; Harrison & McDougall 1980). The authors feel that the age-spectrum data from these samples are significant because the previous results for these samples have been questioned in recently proposed Palaeozoic time-scales because of a possible disturbance of the K-Ar isotopic system (i.e. Gale et al. 1979, 1980; Gale 1982).