Menevian Group

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Menevian Group
Stratigraphic range: St Davids Epoch Cambrian
Porth y Rhaw from the west - geograph.org.uk - 1255228.jpg
Mudstones of the Menevian Group exposed in Porth y Rhaw, west of Solva
Type Group
Sub-units Upper, Middle, Lower (informal)
Underlies Lingula Flags Formation
Overlies Solva Group of Pebidian Supergroup
Thickness about 230m
Lithology
Primary sandstone, mudstones
Other conglomerate
Location
Region 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[edit]

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. A cliff section at Porth y Rhaw west of Solva provides the type section. Further outcrops though less well exposed occur northeast of Newgale and again in the Wolf's Castle area.[2]

Lithology and stratigraphy[edit]

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 outcrops 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[edit]

  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. ed.). London: Geological Society Publishing. p. 37. ISBN 9781862392007.