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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 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.[1]

History[edit]

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.[2] In 1852 he referred to the "Arenig slates and porphyry" and in his "synopsis" he used the term "Arenig porphyry".[3][4] 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,[5][6][7] but later Hicks confined the Arenig Group to the two bottom subdivisions, arguing that the fauna of the top subdivision was radically different.[8] 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.[9]


At the beginning of the 20th century Gertrude Lilian Elles established a graptolite-stratigraphy for the Arenig.[10][11] Four graptolite zones are distinguished.



Subdivions of the Arenig[edit]

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[edit]

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[edit]

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,[12] 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[edit]

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.[13]

References[edit]

  1. ^ Fortey, R. A.; Harper, D. A. T.; Ingham, J. K.; Owen, A. W.; Rushton, A. W. A. (1995). "A revision of Ordovician series and stages from the historical type area". Geological Magazine. 132 (1): 15–30. doi:10.1017/S0016756800011390. Retrieved 28 November 2012.{{cite journal}}: CS1 maint: date and year (link)
  2. ^ Sedgwick, Adam (1843-11-27). "On the older Palaeozoic (Protozoic) rocks of Wales". Quarterly Journal of the Geological Society of London. 1: 5–20. doi:10.1144/GSL.JGS.1845.001.01.02. Retrieved 28 November 2012.
  3. ^ Sedgwick, Adam (1852). "On the classification and nomenclature of the Lower Palaeozoic rocks of England and Wales". Quarterly Journal of the Geological Society of London. 8 (1–2): 136–168. doi:10.1144/GSL.JGS.1852.008.01-02.20. Retrieved 28 November 2012.
  4. ^ Sedgwick, Adam (1851–1855). A synopsis of the classification of the British Palæozoic rocks; With a systematic description of the British Palæozoic fossils in the Geological museum of the University of Cambridge (2 Volumes and one illustration volume). London: J. W. Parker and Son. p. 661. Retrieved 28 November 2012. {{cite book}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  5. ^ Hicks, Henry (1867). "Second report on the "Menevian Group" and the other formations at St. David's, Pembrokeshire". Report of the British Association for the Advancement of Science: 182–186. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  6. ^ Hicks, Henry (1873). "On the Tremadoc rocks in the neighbourhood of St. David's, South Wales, and their fossil contents". Quarterly Journal of the Geological Society of London. 29 (1–2): 39–52. doi:10.1144/GSL.JGS.1873.029.01-02.11. Retrieved 28 November 2012.
  7. ^ Hicks, Henry (1875). "On the succession of the ancient rocks in the vicinity of St. David's, Pembrokeshire, with special reference to those of the Arenig and Llandeilo groups, and their fossil contents". Quarterly Journal of the Geological Society of London. 31 (1–4): 167–308. doi:10.1144/GSL.JGS.1875.031.01-04.14. Retrieved 28 November 2012.
  8. ^ Hicks, Henry (1881). "The classification of the Eozoic and Palaeozoic rocks of the British Isles". The Popular Science Review (New Series). 5: 290–308.
  9. ^ Hughes, T. M'Kenny (1887). Notes on some Sections in the Arenig Series of North Wales and the Lake District. Report of the fifty-sixth meeting of the British Association for the Advancement of Sciene, held in Birmingham in September 1886. London: John Murray. p. 663.
  10. ^ Elles, Gertrude Lilian (1904). "Some graptolite zones in the Arenig rocks of Wales". Geological Magazine. 1. 1 (5): 199–211. doi:10.1017/S0016756800119612.
  11. ^ Elles, Gertrude Lilian (1940). "The stratigraphy and faunal succession in the Ordovician rocks of the Builth-Llandrindod Inlier, Radnorshire". Geological Magazine. 95: 383–445.
  12. ^ ICS; seeOrdovician
  13. ^ http://www.palaeos.com/Paleozoic/Ordovician/Arenig.htm
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