Rhuddanian | |||||||||||||
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Chronology | |||||||||||||
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Etymology | |||||||||||||
Name formality | Formal | ||||||||||||
Usage information | |||||||||||||
Celestial body | Earth | ||||||||||||
Regional usage | Global ( ICS) | ||||||||||||
Time scale(s) used | ICS Time Scale | ||||||||||||
Definition | |||||||||||||
Chronological unit | Age | ||||||||||||
Stratigraphic unit | Stage | ||||||||||||
Time span formality | Formal | ||||||||||||
Lower boundary definition | FAD of the graptolite Akidograptus ascensus | ||||||||||||
Lower boundary GSSP |
Dob's Linn,
Moffat,
U.K. 55°26′24″N 3°16′12″W / 55.4400°N 3.2700°W | ||||||||||||
Lower GSSP ratified | 1984 [4] [5] | ||||||||||||
Upper boundary definition | FAD of the graptolite Monograptus austerus sequens | ||||||||||||
Upper boundary GSSP |
Trefawr Track Section,
Llandovery,
Wales,
U.K. 52°01′48″N 3°42′00″W / 52.0300°N 3.7000°W | ||||||||||||
Upper GSSP ratified | 1984 [5] |
In the geologic timescale, the Rhuddanian is the first age of the Silurian Period and of the Llandovery Epoch. The Silurian is in the Paleozoic Era of the Phanerozoic Eon. [6] The Rhuddanian Age began 443.8 ± 1.5 Ma and ended 440.8 ± 1.2 Ma (million years ago). It succeeds the Himantian Age (the last age of the Ordovician Period) and precedes the Aeronian Age. [7]
The GSSP for the Silurian is located in a section at Dob's Linn, Scotland, in an artificial excavation created just north of the Linn Branch Stream. Two lithological units ( formations) occur near the boundary. [6] The lower is the Hartfell Shale (48 metres (157 ft) thick), consisting chiefly of pale gray mudstone with subordinate black shales and several interbedded meta-bentonites. [6] Above this is the 43 metres (141 ft) thick Birkhill Shale, which consist predominantly of black graptolitic shale with subordinate gray mudstones and meta-bentonites. [8]