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 * For the mountains in Wales, see Arenigs.

In geology, the Arenigian (or 'Arenig') refers both to a time interval during the LowerOrdovician period and also to the suite of rocks which were deposited during this interval. The time interval which was ranked as a series or stage overlaps with the official ICS-stages Floian, Dapingian and the lowest Darriwilian. The Arenigian succeeds the Tremadocian, a regional and ICS-stage, and precedes the regional stageLlanvirnium.

History
The term was first used by Adam Sedgwick, who used it in a lithostratigraphic way, in 1847 with reference to the "Arenig Ashes and Porphyries" in the neighborhood of Arenig Fawr, inMerioneth, North Wales. In 1852 he referred to the "Arenig slates and porphyry" and in his "synopsis" he used the term "Arenig porphyry". Henry Hicks and John William were the first to use the term "Arenig Group" which was also meant in a lithostratigraphic way. The Arenig Group originally had three subdivisions,  but later Hicks confined the Arenig Group to the two bottom subdivisions, arguing that the fauna of the top subdivision was radically different. The lower boundary of the Arenig was also moved, adding the uppermost "Tremdoc" (in the meaning used by Hicks) to the Arenig Group. The term "Arenig Series" was first used in 1887 by Thomas McKenny Hughes. He was also the first to use the term in a chronostratigraphic meaning.

At the beginning of the 20th century Gertrude Lilian Elles established a graptolite-stratigraphy for the Arenig. Four graptolite zones are distinguished.

Subdivions of the Arenig
In southern Wales Richard Fortey and Robert Owens

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.

Regional lithostratigraphic units
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 BallantraeGroup of Ayrshire, and by the Ribband Series of slates and shale in Wicklow andWexford. It may be mentioned here that the "Llanvirn" Series of H. Hicks was equivalent to the bifidus shale and the Lower Llandeilo Series.

Geochronology
In the geologic timescale, the "Arenig" or Arenigian refers to an age of the Lower Ordovicianepoch, between 478.6 ± 1.7 and 471.8 ± 1.6 million years ago, contemporary with the more recently proposed Floian by the ICS, based on a section in Sweden (Diabasbrottet quarry) and with the same boundaries. The Arenigian and Floian are the upper part of the Lower Ordovician and follow theTremadocian (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 Arenigian and equivalent Floian are represented in North America by the upper three stages of theCanadian which is followed by the Middle Ordovician Whiterockian which is the lower part of the now shortened Chazyan.

Events
The Arenig group was deposited during a sudden worldwide rise in sea level resulting in widespreadmarine transgression. The early Ordovician surge in marine diversity also began around this time.