Tremadocian

The Tremadocian is in Earth's history, the lowest stratigraphic stage of Unterordovizium series or the Ordovician. The stage includes in absolute terms ( geochronological ) roughly the period from about 485.4 to about 477.7 million years. The Tremadocian follows the supreme, not yet formally designated level ( " Level 10 ") of the Furongium series of the Cambrian. It is followed by Floium, the second stage of Unterordoviziums.

Naming and history

The stage is named after the place Tremadoc in Wales. Stage and name were proposed in 1846 by Adam Sedgwick ( as " Tremadocian Group ").

Definition and GSSP

The base of Tremadociums (and thus the Ordovician ) is defined by the first appearance of the conodont species Iapetognathus fluctivagus. This limit is slightly above the Cordylodus lindstromi - conodont zone and slightly below the Erstauftretens the first planktonic graptolites. The upper limit is set approximatus with the first appearance of the graptolite - type Tetragraptus. The reference profile ( GSSP = Global Boundary Stratotype Section and Point) of the International Commission on Stratigraphy is the so-called " Green Point Profile" in Gros Morne National Park, about 70 km from the airport of Deer Lake and about 10 kilometers north of the town Rocky Harbour, western Newfoundland ( Canada).