Ordovician - Subdivisions

Subdivisions

Key events in the Ordovician -490 — – -485 — – -480 — – -475 — – -470 — – -465 — – -460 — – -455 — – -450 — – -445 — – Cambrian Tremadocian Floian Dapingian Darriwilian Sandbian Katian Hirnantian Hirnantian Early Ordovician Llanvirn Llandeilo Caradoc Ashgill Silurian ← First land plant spores O
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n  Palæozoic
Key events of the Ordovician Period.
Left: ICS approved stages.
Right: "General" stages.
Axis scale: millions of years ago.

A number of regional terms have been used to refer to subdivisions of the Ordovician Period. In 2008, the ICS erected a formal international system of subdivisions, illustrated to the right.

The Ordovician Period in Britain was traditionally broken into Early (Tremadocian and Arenig), Middle (Llanvirn and Llandeilo) and Late (Caradoc and Ashgill) epochs. The corresponding rocks of the Ordovician System are referred to as coming from the Lower, Middle, or Upper part of the column. The faunal stages (subdivisions of epochs) from youngest to oldest are:

• Hirnantian/Gamach (Late Ordovician: Ashgill)
• Rawtheyan/Richmond (Late Ordovician: Ashgill)
• Cautleyan/Richmond (Late Ordovician: Ashgill)
• Pusgillian/Maysville/Richmond (Late Ordovician: Ashgill)

• Trenton (Middle Ordovician: Caradoc)
• Onnian/Maysville/Eden (Middle Ordovician: Caradoc)
• Actonian/Eden (Middle Ordovician: Caradoc)
• Marshbrookian/Sherman (Middle Ordovician: Caradoc)
• Longvillian/Sherman (Middle Ordovician: Caradoc)
• Soudleyan/Kirkfield (Middle Ordovician: Caradoc)
• Harnagian/Rockland (Middle Ordovician: Caradoc)
• Costonian/Black River (Middle Ordovician: Caradoc)
• Chazy (Middle Ordovician: Llandeilo)
• Llandeilo (Middle Ordovician: Llandeilo)
• Whiterock (Middle Ordovician: Llanvirn)
• Llanvirn (Middle Ordovician: Llanvirn)

• Cassinian (Early Ordovician: Arenig)
• Arenig/Jefferson/Castleman (Early Ordovician: Arenig)
• Tremadoc/Deming/Gaconadian (Early Ordovician: Tremadoc)