Definitions

phragmosiphon

Arenig

In geology, the Arenig group is the name applied to 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 neighborhood 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.

North American Deposits

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 is the age of the Lower Ordovician epoch of the Ordovician period of the Paleozoic era of the Phanerozoic eon that is comprehended between 478.6 ± 1.7 Ma and 471.8 ± 1.6 Ma (million years ago), approximately. The Arenig age succeeds the Tremadocian age and precedes the Stage III age.

Events

The Arenig age began with a worldwide sudden rise in sea level rise in sea level (marine transgression). The early ordivician surge in marine diversity also began around the beginning of the Arenig age..

Brachiopod Fauna

Incertae Cedis Brachiopods of the Floian
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Acrotretida

Acrotretids of the Floian
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Lingulida

Placeholders of the Floian
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Orthida

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Paternida

Placeholders of the Floian
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Pentamerida

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Strophomenida

Placeholders of the Floian
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Trimerellida

Placeholders of the Floian
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Cephalopod Fauna

Actinocerida

Upper
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.

Anarcestida

Anarcestids of the Floian
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Barrandeocerida

Barrandeocerids of the Floian
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Ellesmerocerida

Ellesmocerids of the Floian
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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

Intejocerids of the Floian
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Oncocerida

Oncocerids of the Floian
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Orthocerida

Nautiloids of the Bajocian
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Tarphycerida

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Trilobite Fauna

Oncocerids of the Floian
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Agnostida

Oncocerids of the Floian
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Asaphida

Oncocerids of the Floian
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Corynexichida

Oncocerids of the Floian
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Lichida

Oncocerids of the Floian
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Odontopleurida

Oncocerids of the Floian
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Phacopids

Oncocerids of the Floian
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Proetida

Oncocerids of the Floian
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Ptychopariida

Oncocerids of the Floian
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External links

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