TREMADOC SERIES

also called Tremadocian Series, all those rocks deposited worldwide during the Tremadocian Age (505 to 488 million years ago). The Tremadoc is the lowest of the six divisions comprising the Ordovician System, and its base defines the boundary between the Ordovician and the underlying Cambrian System. The series' name is derived from the village of Tremadoc, near Caernarvon in northern Wales. No global stratotype section and point (GSSP) has yet been defined for its base by the International Commission on Stratigraphy. The stratigraphic position of the Tremadoc Series has been a matter of controversy; some authorities considered it to be uppermost Cambrian, while others viewed it as being Lower Ordovician. Though a distinct unconformity exists between Tremadoc beds and the overlying strata of the Arenig Series (which are clearly Ordovician), Tremadoc graptolites and other faunas betray an obvious link to distinctively Ordovician faunal assemblages. Thus current opinion places the Tremadoc Series in the Ordovician System. The unratified defining point for the base of the series is proposed as the first appearance of the conodont Cordylodus lindstromi, which occurs below but close to the first influx of nematophorous graptolites. Graptolites and condonts are most commonly used for biostratigraphic correlation of the series. The Tremadoc Series is defined by a sequence of four graptolite biozones that extend from the base of the Dictyonema flabelliforme zone to the top of the Apatokephalus serratus zone.