Arenig

The Arenig group is a geological group of rocks deposited during the lowest stage of the Ordovician System.

History
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.

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 shale and the Lower Llandeilo Series.

In Geochronology
In the geologic timescale, the Arenig used to refer to an age of the Lower Ordovician epoch, between 478.6 ± 1.7and 471.8 ± 1.6 million years ago. This term is depreciated.

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