{{Short description|Group of Triassic rocks in the United Kingdom}} {{Infobox rockunit | name = Mercia Mudstone Group | type = [[Group (stratigraphy)|Group]] | age = [[Anisian]]-[[Rhaetian]] | period = Rhaetian | prilithology = [[Mudstone]] | otherlithology = [[Siltstone]], [[sandstone]], [[halite]], [[anhydrite]] | namedfor = [[Mercia]] | region = [[Scotland]], [[England]], [[Wales]] | country = [[United Kingdom]] | unitof = [[New Red Sandstone|New Red Sandstone Supergroup]] | subunits = [[#Stratigraphy|Stratigraphy]] | underlies = [[Penarth Group]] | overlies = [[Sherwood Sandstone Group]] | thickness = over {{cvt|1350|m}} | coordinates = {{Coord|52.3052|-1.6965|region:GB-ENG|format=dms|display=inline,title}} | map = Geologic map Wales & SW England EN.svg | map_caption = Map of Mercia Mudstone Group's outcrop (Triassic) in Wales and southwest England }} The '''Mercia Mudstone Group''' is a middle to late [[Triassic]] [[lithostratigraphy|lithostratigraphic]] [[group (stratigraphy)|group]] (a sequence of [[rock strata]])<ref name=":0" /> which is widespread in Britain, especially in the [[English Midlands]]—the name is derived from the ancient kingdom of [[Mercia]] which corresponds to that area. It is frequently encountered in older literature as the '''[[Keuper Marl]]''' or '''Keuper Marl Series'''.<ref name="BgsLexicon">{{cite web|url=http://www.bgs.ac.uk/Lexicon/lexicon.cfm?pub=MMG|title=BGS Lexicon of Named Rock Units - Result Details|publisher=bgs.ac.uk|author=British Geological Survey 1998–2014 (c)NERC|accessdate=16 March 2014}}</ref>

The Mercia Mudstone Group is now divided into five formations recognised and mappable across its entire [[outcrop]] and [[subcrop]]. The formations are a mix of [[mudstone]]s, [[siltstone]]s, [[sandstone]]s and [[halite]]s. Historically this sequence of rocks has been subdivided in different ways with different names in each of the [[basinal areas]] in which it is found. Increasing knowledge of the sequences and the more recent development of seamless electronic mapping by the [[British Geological Survey]] (BGS) necessitated a reappraisal of these divisions. A report published by BGS in 2008 recommended the abandonment of previous divisions and naming schemes in favour of a simpler approach which, having now been adopted, is set out below.<ref name=":0">Howard, A. S., Warrington, G., Ambrose, K. and Rees, J. G. 2008. A formational framework for the Mercia Mudstone Group (Triassic) of England and Wales. British Geological Survey Research Report, RR/08/04.</ref> *'''Blue Anchor Formation''' *'''Branscombe Mudstone Formation''' *'''Arden Sandstone Formation''' *'''Sidmouth Mudstone Formation''' *'''Tarporley Siltstone Formation'''

Older schemes will remain in maps and literature well into the future,<ref name=FW>[https://paleobiodb.org/classic/displayStrata?geological_group=Mercia%20Mudstone&group_formation_member=Mercia%20Mudstone Mercia Mudstone Group] at [[Fossilworks]].org</ref> providing a source of potential confusion. An example might be the Arden Sandstone Formation which previously enjoyed lower status as a [[member (geology)|member]] and also higher status as a [[group (stratigraphy)|group]].

== Distribution == The group crops out widely across England, representing deposition within numerous Triassic basins, some of which are physically connected at depth. From the south there is an almost continuous outcrop from the [[Wessex Basin]] of east [[Devon]], [[Somerset]] and [[Dorset]], through the [[Bristol]]/[[South Wales]] area and the [[Worcester Basin|Worcester]] and Knowle Basins into the English Midlands and including the Needwood Basin of [[Staffordshire]]. Northwards the outcrop splits either side of the [[Pennines]] where deposition took place across the East Midlands Shelf of [[Nottinghamshire]] and through [[Yorkshire]] to the North Sea coast at [[Hartlepool]]. A western arm includes the [[Stafford Basin|Stafford]] and [[Cheshire Basin|Cheshire]] basins, West [[Lancashire]] and the Carlisle Basin—the latter are connected at depth beneath the [[Irish Sea]].<ref>Howard, A. S., Warrington, G., Ambrose, K. and Rees, J. G. 2008. A formational framework for the Mercia Mudstone Group (Triassic) of England and Wales. British Geological Survey Research Report, RR/08/04. map on page 23</ref> The group is most thickly developed within the [[Cheshire Basin]] (which also extends southwards into north Shropshire) where it attains a thickness in excess of 1,350 m.

==Stratigraphy==

=== Blue Anchor Formation === [[File:Blue Anchor Formation rock.png|thumb|150px|A [[reptile]] tooth and a [[fish]] tooth from the Blue Anchor Formation, [[Somerset]]]] The formation is named from the village of [[Blue Anchor]] on the coast of west Somerset. It consists largely of green to grey mudstones and siltstones (which gave rise to the earlier name of this sequence, the Tea-green Marls) and varies from around 5 m to 67 m in thickness. Though common to all other areas, the Blue Anchor Formation is absent through erosion in the Stafford Basin and in Lancashire. It is of late Norian to early Rhaetian age. It is overlain by the Westbury Formation of the [[Penarth Group]] which represents widespread inundation of the Triassic basins as global sea levels rose.

=== Branscombe Mudstone Formation === The formation consists of red-brown mudstones and siltstones in which [[gypsum]]/[[anhydrite]] occurs as beds, veins and nodules. It was formerly known as the 'Brooks Mill Mudstone Formation' in the Cheshire Basin (and named from a location near [[Nantwich]]), as the 'Cropwell Bishop Formation' on the 'East Midlands Shelf' and as the 'Twyning Mudstone Formation' in the Worcester and Knowle basins. It is equivalent to the 'Triton Formation' of the southern [[North Sea]]. The Branscombe Mudstone Formation, which is named from [[Branscombe]] on the east Devon coast, is of [[Norian]] age, i.e. 216–204 mya.

=== Arden Sandstone Formation === The formation is named after the [[Forest of Arden]] in Warwickshire. The Arden Sandstone is dated to the Late Carnian, and has a noticeable shift in character compares to formations above and below it. It consists of a 2 m to 24 m thickness of mudstones, siltstones and sandstones with occasional pebble beds. It generally has a more greenish grey colour than the formations above and below it though it is not readily distinguishable within the sequence in Cheshire. It has locally yielded fossils suggestive of a marine-influenced environment.<ref name="bgs">{{cite web|url=http://www.bgs.ac.uk/lexicon/lexicon.cfm?pub=AS|title=BGS Lexicon of Named Rock Units - Result Details|publisher=bgs.ac.uk|accessdate=2014-03-16}}</ref> The Arden Sandstone is known as North Curry Sandstone which is used as a building material in [[Somerset]],<ref>{{cite web | url=http://www.bgs.ac.uk/downloads/start.cfm?id=1619 | format=PDF | title=Strategic Stone Study: A Building Stone Atlas of Somerset and Exmoor | publisher=[[English Heritage]]|page=10|accessdate=11 October 2011}}</ref> and is also known as the Dunscombe Mudstone formation in Devon, where it composed of halites and purple claystones<ref>Gallois, R.W. 2019. The stratigraphy of the Permo-Triassic rocks of the Dorset and East Devon Coast World Heritage Site, U.K. Proceedings of the Geologists’ Association, Vol. 130, 274-293.</ref>.

=== Sidmouth Mudstone Formation === [[File:Cliffs at the south end of Gunthorpe weir - geograph.org.uk - 629291.jpg|thumb|Sidmouth Mudstone formation at [[Gunthorpe, Nottinghamshire|Gunthorpe]]]] The formation includes several members which formerly enjoyed 'formation' status.<ref name="bgs2">{{cite web|url=http://www.bgs.ac.uk/lexicon/lexicon.cfm?pub=SIM|title=BGS Lexicon of Named Rock Units - Result Details|publisher=bgs.ac.uk|accessdate=2014-03-16}}</ref> This sequence was formerly known as the Eldersfield Mudstone Formation in the Worcester and Knowle basins and as the (combined) Edwalton, Gunthorpe and Radcliffe formations on the East Midlands Shelf. In the Cheshire Basin it supersedes the former Bollin Mudstone, Northwich Halite, Wych Mudstone, Byley Mudstone and Wilkesley Halite formations, each of which is now accorded 'member' status. It is of Anisian through Ladinian to [[Carnian]] age.

==== Wilkesley Halite member ==== Formerly known as the Upper Keuper Saliferous Beds and as the Wilkesley Halite Formation (named from the Cheshire hamlet of [[Dodcott cum Wilkesley|Wilkesley]]), this member is Ladinian to Carnian in age.

==== Wych Mudstone member ==== Formerly known as (the upper part of) the Middle Keuper Marl and as the Wych Mudstone Formation (named from the [[Wych Brook]] on the Cheshire/Flintshire/Shropshire border), this member is Anisian to Ladinian in age.

==== Byley Mudstone member ==== Formerly known as (the lower part of) the Middle Keuper Marl and as the Byley Mudstone Formation (named from the Cheshire village of [[Byley]]), this member is Anisian in age.

==== Northwich Halite member ==== Formerly known as the Lower Keuper Saliferous Beds and as the Northwich Halite Formation (named from the town of [[Northwich]]), this member is Anisian in age.

==== Bollin Mudstone member ==== Formerly known as the Lower Keuper Marl, lower mudstone and also as the Bollin Mudstone Formation (named from the [[River Bollin]] in east [[Cheshire]]), this member is (?Scythian to) Anisian in age.

In West Cumbria, the Sidmouth Mudstone Formation is divided thus:<ref>{{cite book |title=Geological Survey of Great Britain (England and Wales) Barrow-in-Furness Sheet 58 1:50,000 Series, Solid and Drift Edition |date=1976 |publisher=Institute of Geological Sciences}}</ref><ref>{{cite web |title=British Geological Survey |url=http://mapapps.bgs.ac.uk/geologyofbritain/home.html? |website=Geology of Britain viewer |accessdate=19 January 2019}}</ref>

==== Singleton Mudstone member ==== Formerly known as the Singleton Mudstone Formation (named from the Lancashire village of [[Singleton, Lancashire|Singleton]]), this member is Early Triassic to Anisian in age.

==== Kirkham Mudstone member ==== Formerly known as the Kirkham Mudstone Formation (named from the small Lancashire town of [[Kirkham, Lancashire|Kirkham]]), this member is Anisian to Ladinian in age.

==== Preesall Halite member ==== Formerly known as the Preesall Salt (named from the Lancashire village of [[Preesall]]), this member is Anisian to Ladinian in age.

=== Tarporley Siltstone Formation === The Tarporley Siltstone Formation (named from the Cheshire village of [[Tarporley]]) is Anisian in age and comprises siltstones, mudstones and sandstones. The thickness of the sequence varies from 20 m in parts of the East Midlands to around 220m in the Cheshire Basin. It is encountered in older literature under various guises but commonly as the Keuper Waterstones or simply Waterstones.<ref name="bgs3">{{cite web|url=http://www.bgs.ac.uk/Lexicon/lexicon.cfm?pub=TPSF|title=BGS Lexicon of Named Rock Units - Result Details|publisher=bgs.ac.uk|accessdate=2014-03-16}}</ref>

These rocks feature prominently along the [[Mid Cheshire Ridge]] where they form the summits of such hills as Eddisbury Hill and the [[Delamere Forest#OldPale|Old Pale]] and the high ground around Tarporley and [[Utkinton]] together with small tracts of hilly country around [[Runcorn]] and [[Warrington]].<ref>British Geological Survey 1:50,000 scale geological map sheets (England and Wales series) 97 ''Runcorn'' and 109 ''Chester''</ref> The Malpas Sandstone Member is distinguished within this formation.

== See also == * [[Geology of Cheshire]]

== References == {{reflist}} [[Category:Geology of Wales]] [[Category:Geological groups of the United Kingdom]] [[Category:Geology of England]] [[Category:Geology of Scotland]] [[Category:Triassic System of Europe]] [[Category:Triassic England]] [[Category:Triassic Scotland]] [[Category:Triassic Wales]] [[Category:Mudstone groups]] [[Category:Sandstone groups]] [[Category:Siltstone groups]] [[Category:Fossiliferous stratigraphic units in Wales]] [[Category:Evaporite deposits]] [[Category:Ichnofossiliferous groups]] [[Category:Paleontology in England]] [[Category:Paleontology in Scotland]] [[Category:Geology of Cheshire]]