The Cambro - Ordovician boundary

Henningsmoen, G.: The Cambro-Ordovician boundary.
The Cambrian and Ordovician Systems were established in Britain, and it seems appropriate to define the Cambro-Ordovician boundary in Britain or where there is a closely related faunal succession at the junction. When Lapworth erected the Ordovician System in 1879, his intention was that its lower boundary should separate the so-called primordial (or first) and second Palaeozoic faunas. He drew this boundary at the base of the Lower Arenig, but apparently included the Upper Tremadoc in the Lower Arenig. It seems problematic to base the Cambro-Ordovician boundary on grounds of historical priority, and we seem to be free to decide whether to have the boundary at the base of the Tremadocian, at the base of the Arenigian, or within the Tremadocian (e.g. at the base of the Upper Tremadoc). The alternatives are shortly discussed. Arguments are put forward for placing the boundary not at a break, but preferably in a sequence with cosmopolitan fossils, either in a sequence of uniform facies or perhaps rather in a sequence with frequently alternating facies.