• Source: Arenig

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


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:

A lower series of gritty and calcareous sediments, the "Arenig Series" as it is now understood;
A middle series, mainly volcanic, with shale, the "Llandeilo Series"; and
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.




Geochronology


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, 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


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.




Brachiopod fauna


Incertae sedis brachiopods of the Floian
Eurorthisina
Tegulella
Acrotretida of the Floian

Lingulida of the Floian

Orthida of the Floian

Paternida of the Floian
Dictyonites
Pentamerida of the Floian




= Strophomenida

=
Strophomenida of the Floian

Trimerellida of the Floian
Dinobolus




Cephalopoda


Actinocerida
Metactinoceras
Ordosoceras
Polydesmia




= 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.




= Orthocerida

=
Orthocerids of the Floian
Eobactrites
Barrandeocerida of the Floian
Plectoceras
Ellesmerocerida of the Floian

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
Bajkaloceras
Evencoceras
Intejoceras
Rossoceras
Oncocerids of the Floian
Phthanoncoceras
Valhalloceras
Nautiloids of the Floian

Tarphycerida
Deltoceras
Pseudancistroceras
Seelyoceras
Lower

Upper




Trilobite fauna


Trilobites of the Floian
Canningella
Gogoella
Macrogrammus
Priceaspis
Thymurus
Agnostida of the Floian'
Galbagnostus
Geragnostella
Asaphida of the Floian

Corynexichida of the Floian

Lichida of the Floian

Odontopleurida of the Floian

Phacopida of the Floian

Proetida of the Floian

Ptychopariida of the Floian




External links


palaeos




References

Kata Kunci Pencarian:

• Arcida
• Arenig
• Arenigs
• Arenig (disambiguation)
• Arenig Fawr
• Arenig Fach
• Foel Goch (Arenigs)
• Llyn Arenig Fawr
• Arenig Fawr South Ridge Top
• Carnedd y Filiast (Cerrigydrudion)
• Gwyniad