Menevian Group

Menevian Group
Menevian Group
Stratigraphic range: St Davids Epoch Cambrian
	Mudstones of the Menevia Formation exposed in Porth y Rhaw, west of Solva
Type	Group
Sub-units	Upper, Middle, Lower (informal)
Underlies	Lingula Flags
Overlies	Solva Group
Thickness	about 230m
Lithology
Primary	mudstones
Other	tuffaceous sandstone
Location
Region	South West Wales
Country	Wales
Type section
Named for	St Davids (Roman name for area)

The Menevian Group is a Cambrian lithostratigraphic group (a sequence of rock strata) in west Wales. The name is derived from Menevia, the Roman name for the St Davids area north of St Brides Bay on Pembrokeshire’s west coast where the strata are well exposed in coastal cliffs. This rock succession has previously been known variously as the Menevian Series and Menevian Beds and largely ascribed to the British regional stratigraphic unit St David’s Epoch, though these terms are now obsolete.[1]

Outcrops

The rock succession is exposed, along the coast southeast of St Davids and in particular at Solva where the outcrop, though largely concealed, continues inland. Both western and eastern cliff sections at Porth-y-Rhaw west of Solva provide the type locality. Further outcrops, though less well exposed, occur northeast of Newgale and again in the Wolf's Castle area.[2]

Lithology and stratigraphy

The Group comprises over two hundred metres thickness of mudstones and sandstones together with a basal conglomerate. The sandstones have been laid down as turbidites. Some of the mudstones contain tuff horizons whilst many are pyritous and have been equated with the Clogau Formation which crops out near Harlech, both indicating that the floor of the Welsh Basin was oxygen-poor and the waters were becoming deeper.[3] Trilobite fossils are recorded throughout the sequence whilst the sandstones of the upper Menevian contain brachiopod fossils.[4]

References

"Menevian Group". The BGS Lexicon of Named Rock Units. British Geological Survey. Retrieved Oct 11, 2013.
British Geological Survey 1:50,000 scale geological map (England & Wales) sheets 209 St David's, 210 Fishguard
Howells, M.F. (2007). British regional geology : Wales (1st ed.). Nottingham: British Geological Survey. p. 30. ISBN 978-085272584-9.
London, P.F. Rawson. The Geological Society (2006). Brenchley, P.J. (ed.). The geology of England and Wales (2. ed.). London: Geological Society Publishing. p. 37. ISBN 9781862392007.

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[1] "Menevian Group". The BGS Lexicon of Named Rock Units. British Geological Survey. Retrieved Oct 11, 2013.

[2] British Geological Survey 1:50,000 scale geological map (England & Wales) sheets 209 St David's, 210 Fishguard

[3] Howells, M.F. (2007). British regional geology : Wales (1st ed.). Nottingham: British Geological Survey. p. 30. ISBN 978-085272584-9.

[4] London, P.F. Rawson. The Geological Society (2006). Brenchley, P.J. (ed.). The geology of England and Wales (2. ed.). London: Geological Society Publishing. p. 37. ISBN 9781862392007.