Arenig

In geology, the Arenig 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. 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 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 and with the same boundaries. The Arenig and Floian rocks are the upper part of the Lower Ordovician and follow the Tremadocian 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

Acanthambonia
Cyrbasiotreta
Eoconulus
Issedonia
Karnotreta
Lurgiticoma
Multispinula
Mylloconotreta
Numericoma
Orthisocrania
Polylasma
Pomeraniotreta
Pseudocrania
Scaphelasma
Schizotreta
Torynelasma

;Lingulida of the Floian

Acanthorthis
Aulonotreta
Dictyobolus
Ectenoglossa
Elliptoglossa
Monobolina
Paldiskites
Paterula
Pseudolingula
Quasithambonia
Rafanoglossa
Volborthia

;Orthida of the Floian

Acanthorthis
Acanthotoechia
Anchigonites
Angusticardinia
Anomalorthis
Antigonambonites
Apomatella
Astraborthis
Crossiskenidium
Dalmanella
Desmorthis
Diparelasma
Eodiorthelasma
Eosotrematorthis
Estlandia
Euorthisina
Famatinorthis
Fasciculina
Ferrax
Ffynnonia
Fistulogonites
Glossorthis
Glypterina
Hesperonomiella
Incorthis
Jaanussonites
Ladogiella
Lomatorthis
Monorthis
Munhella
Nereidella
Neumania
Nocturneilla
Notoscaphidia
Oligorthis
Orthambonites
Orthidiella
Orthidium
Orthis
Oslogonites
Panderina
Paralenorthis
Paurorthina
Paurorthis
Phragmorthis
Platystrophia
Platytoechia
Polytoechia
Pomatotrema
Prantlina
Productorthis
Progonambonites
Protohesperonomia
Protoskenidiodes
Pseudomimella
Raunites
Rhynchorthis
Shoshonorthis
Sinorthis
Taphrorthis
Tarfaya
Treioria
Trematorthis
Valcourea
Virgoria

;Paternida of the Floian

Dictyonites

;Pentamerida of the Floian

Acanthorthis
Acanthoglypha
Boreadocamara
Camerella
Doloresella
Hesperotrophia
Idiostrophia
Imbricatia
Karakulina
Liricamera
Lycophoria
Porambonites
Rectotrophia
Rosella
Rugostrophia
Stenocamara
Syntrophia
Syntrophinella
''Xenelasma''
Strophomenida

;Strophomenida of the Floian

Acanthorthis
Ahtiella
Aporthophyla
Borua
Calyptolepta
Christiania
Ingria
Leptestia
Petroria
Plectambonites
Reinversella
Schedophyla
Taffia
Tourmakeadia

;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.

Leurorthoceras
Nybyoceras
Actinoceras
Wutinoceras
Ormoceras
Adamsoceras
''Georgina''
Orthocerida

;Orthocerids of the Floian

Eobactrites

;Barrandeocerida of the Floian

Plectoceras

;Ellesmerocerida of the Floian

Amsleroceras
Apocrinoceras
Avoceras
Bakeroceras
Baltoceras
Catoraphinoceras
Cochlioceras
Copiceras
Clelandoceras
Cumberloceras
Cyclostomiceras
Cyrtobaltoceras
Desioceras
Diaphoroceras
Diastoloceras
Dwightoceras
Dyscritoceras
Ectocycloceras
Endorioceras
Eocyckistomiceras
Eothinoceras
Hemichoanella
Irianoceras
Kyminoceras
Lawrenceoceras
Meikeloceras
Microbaltoceras
Monogonoceras
Ogygoceras
Pictetoceras
Protocycloceras
Quebecoceras
Rangeroceras
Rhabdiferous
Rioceras
Rudolfoceras
Smithvilloceras
Somalinautilus
Somalinautilus
Somalinautilus
Vassaroceras
Veneficoceras
Ventroloboceras

;Endocerida

Campendoceras
Dartonoceras
Kaipingoceras
Kugeloceras
Lobosiphon
Manitouoceras
Mcqueenoceras
Mysticoceras
Notocycloceras
Oderoceras
Parapiloceras
Phragmosiphon
Platysiphon
Pliendoceras
Retroclitendoceras
Stenosiphon
Subpenhsioceras
Utoceras
Yorkoceras
Vaginoceras
Chisiloceras
Cyrtovaginoceras
Tallinnoceras
Juaboceras
Penhsioceras
Ventrolobendoceras

;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.

Allopiloceras
Choreanoceroides
Escharendoceras
Lebetoceras
Loxochoanella
Sewardoceras
Telleroceras
Clitendoceras
Coreanoceras
Cotteroceras
Piloceras
Endoceras
Proterovaginoceras
Cyrtendoceras
Anthoceras
Chaohuceras
Proterocameroceras
Thylacoceras

;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.

Allocotoceras
Cassinoceras
Chihlioceras
Cyclocyrtendoceras
Cyptendoceras
Kirkoceras
Lobendoceras
Najaceras
Protocyclendoceras
Yehlioceras
Nanno
Dideroceras
Lobocyclendoceras
Meniscoceras
Paracyclendoceras
Cacheoceras
Perkinsoceras
Williamsoceras
Manchuroceras
Schmidtoceras

;Intejocerida of the Floian

Bajkaloceras
Evencoceras
Intejoceras
Rossoceras

;Oncocerids of the Floian

Phthanoncoceras
Valhalloceras

;Nautiloids of the Floian

Buttsoceras
Centroonoceras
Gangshanoceras
Geisonoceras
Glenisteroceras
Michelinoceras
Orthoceras
Oxfordoceras
Rhynchorthoceras
Stereoplasmoceras
Tajaroceras
Wardoceras

;Tarphycerida

Deltoceras
Pseudancistroceras
Seelyoceras

;Lower

Alaskoceras
Moreauoceras
Pilotoceras
Shumardoceras
Pycnoceras
Campbelloceras
Aphetoceras

;Upper

Aethoceras
Bentoceras
Centrotarphyceras
Clytoceras
Cycloplectoceras
Eichwaldoceras
Eurystomites
Hardmanoceras
Holmiceras
Pionoceras
Trocholitoceras
Wichitoceras
Tarphyceras
Litoceras
Curtoceras
Arkoceras
Estonioceras
''Tragoceras''

Arenig

History

Geochronology

Events

Brachiopod fauna

Strophomenida

Cephalopoda

Upper

Orthocerida