J. Sutter J. D. Obradovich Marvin A. Lanphere Michael J. Kunk 1985 <p><span>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&nbsp;</span>⁴⁰<span>Ar/</span>³⁹<span>&nbsp;Ar total-fusion method, have been reanalysed using the&nbsp;</span>⁴⁰<span>Ar/</span>³⁹<span>Ar age-spectrum method. Conventional K-Ar and total-fusion&nbsp;</span>⁴⁰<span>Ar/</span>³⁹<span>Ar 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 &amp; Lanphere 1969; Clauer&nbsp;</span><i>et al.</i><span>&nbsp;1982). The&nbsp;</span>⁴⁰<span>Ar/</span>³⁹<span>Ar age-spectrum method has the potential for identifying disturbed K-Ar systems (i.e. Berger 1975; Harrison &amp; 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&nbsp;</span><i>et al.</i><span>&nbsp;1979, 1980; Gale 1982).</span></p> application/pdf 10.1144/GSL.MEM.1985.010.01.10 en Geological Society of London Age of biostratigraphic horizons within the Ordovician and Silurian systems article