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Arenig

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For the mountains in Wales, see Arenigs.
Arenig Fawr, the mountain which lends its name to the geological series

In geology, the Arenig (or Arenigian) is a time interval during the Ordovician period and also the suite of rocks which were deposited during this interval.

History

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The term was first used by Adam Sedgwick in 1847 with reference to the "Arenig Ashes and Porphyries" in the neighbourhood of Arenig Fawr, in Merioneth, North Wales. The rock-succession in the Arenig district has been recognized by W. G. Fearnsides (“On the Geology of Arenig Fawr and Moel Llanfnant", Q.J.G.S. vol. lxi., 1905, pp. 608–640, with maps). The above succession is divisible into:

  1. A lower series of gritty and calcareous sediments, the "Arenig Series" as it is now understood;
  2. A middle series, mainly volcanic, with shale, the "Llandeilo Series"; and
  3. The shale and limestones of the Bala or Caradoc Stage.

It was to the middle series (2) that Sedgwick first applied the term "Arenig". In the typical region and in North Wales generally the Arenig series appears to be unconformable upon the Cambrian rocks; this is not the case in South Wales.[1]

The Arenig series is represented in North Wales by the Garth Grit and Ty Obry beds, by the Shelve series of the Corndon district, the Skiddaw Slates of the Lake District, the Ballantrae Group of Ayrshire, and by the Ribband Series of slates and shale in Wicklow and Wexford. It may be mentioned here that the "Llanvirn" Series of H. Hicks was equivalent to the bifidus[clarification needed] shale and the Lower Llandeilo Series.[1]

Geochronology

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In the geologic timescale, the Arenig or Arenigian refers to an age of the Early Ordovician epoch, between 477.7 and 470 million years ago, contemporary with the Floian of the ICS,[2] based on a section in Sweden (Diabasbrottet quarry) and with the same boundaries. The Arenig and Floian rocks are the upper part of the Lower Ordovician and follow the Tremadocian (Gasconadian in North America) which is the lower part. Either is followed by the Middle Ordovician ICS Dapingian or by the Llanvirnian of older chronologies. The Arenig and equivalent Floian are represented in North America by the upper three stages of the Canadian which is followed by the Middle Ordovician Whiterockian which is the lower part of the now shortened Chazyan.

Events

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The Arenig rocks were deposited during a sudden worldwide rise in sea level resulting in widespread marine transgression. The early Ordovician surge in marine diversity also began around this time.[3]

Brachiopod fauna

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Incertae sedis brachiopods of the Floian[4]
Acrotretida of the Floian[4]
Lingulida of the Floian[4]
Orthida of the Floian[4]
Paternida of the Floian
Pentamerida of the Floian

Strophomenida

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Strophomenida of the Floian
Trimerellida of the Floian

Cephalopoda

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Actinocerida

Upper

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The following is a list of Actinocerid genera whose fossils are geochronologically found first in upper Arenig strata. These genera may survive into later portions of the Arenig stage, or even into later geological stages. This list should not be thought of in terms of the lifespan of the genera included.

Orthocerida

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Orthocerids of the Floian[5]
Barrandeocerida of the Floian[5]
Ellesmerocerida of the Floian[5]
Endocerida
Lower

The following is a list of Endocerid genera whose fossils are geochronologically found first in lower Arenig strata. These genera may survive into later portions of the Arenig stage, or even into later geological stages. This list should not be thought of in terms of the lifespan of the genera included.

Upper

The following is a list of Endocerid genera whose fossils are geochronologically found first in upper Arenig strata. These genera may survive into later portions of the Arenig stage, or even into later geological stages. This list should not be thought of in terms of the lifespan of the genera included.

Intejocerida of the Floian
Oncocerids of the Floian
Nautiloids of the Floian
Tarphycerida
Lower
Upper

Trilobite fauna

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Trilobites of the Floian[6]
Agnostida of the Floian'[6]
Asaphida of the Floian[6]
Asaphus
Corynexichida of the Floian[6]
Lichida of the Floian[6]
Odontopleurida of the Floian[6]
Phacopida of the Floian
Colpocoryphe grandis
Proetida of the Floian
Ptychopariida of the Floian[6]
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References

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  1. ^ a b  One or more of the preceding sentences incorporates text from a publication now in the public domain: Howe, John Allen (1911). "Arenig Group". In Chisholm, Hugh (ed.). Encyclopædia Britannica. Vol. 2 (11th ed.). Cambridge University Press. p. 452.
  2. ^ ICS; see Ordovician
  3. ^ "Palaeos Paleozoic : Ordovician : The Arenig Age". Archived from the original on 2007-11-13. Retrieved 2007-12-11.
  4. ^ a b c d Sepkoski, Jack (2002). "A compendium of fossil marine animal genera (entry on brachiopoda)". Bulletins of American Paleontology. 364: 560. Archived from the original on 14 October 2004. Retrieved 27 April 2008.
  5. ^ a b c Sepkoski, Jack (2002). "A compendium of fossil marine animal genera (entry on cephalopoda)". Bulletins of American Paleontology. 364: 560. Archived from the original on 7 May 2008. Retrieved 17 April 2008.
  6. ^ a b c d e f g Sepkoski, Jack (2002). "A compendium of fossil marine animal genera (entry on trilobita)". Bulletins of American Paleontology. 364: 560. Retrieved 19 April 2008.
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