Arenig

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 Lower Ordovician period and also to 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:

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.^[1]

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.^[1]

Geochronology

In the geologic timescale, the "Arenig" or Arenigian refers to an age of the Lower Ordovician epoch, between 478.6 ± 1.7 and 471.8 ± 1.6 million years ago, contemporary with the more recently proposed Floian by the ICS,^[2] 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 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 Arenigian 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 group was 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.^[3]

Brachiopod fauna

Incertae sedis brachiopods of the Floian^[4]

• Eurorthisina
• Tegulella

Acrotretida of the Floian^[4]

Lingulida of the Floian^[4]

Orthida of the Floian^[4]

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^[5]

• Eobactrites

Barrandeocerida of the Floian^[5]

• Plectoceras

Ellesmerocerida of the Floian^[5]

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^[6]

• Canningella
• Gogoella
• Macrogrammus
• Priceaspis
• Thymurus

Agnostida of the Floian'^[6]

• Galbagnostus
• Geragnostella

Asaphida of the Floian^[6]

Asaphus

Corynexichida of the Floian^[6]

Lichida of the Floian^[6]

Odontopleurida of the Floian^[6]

Phacopida of the Floian

Colpocoryphe grandis

Proetida of the Floian

Ptychopariida of the Floian^[6]

External links

• palaeos

References