The BGS Lexicon of Named Rock Units — Result Details

Pen y Fan Formation

Computer Code: PNYF Preferred Map Code: notEntered
Status Code: Full
Age range: Frasnian Age (DR) — Famennian Age (DA)
Lithological Description: Red-brown and purple quartz-rich sandstones, locally pebbly and conglomeratic with interbeds of red-brown mudstone (Taylor, 1972; Hall et al., 1973; Taylor and Thomas, 1974, 1975; Lovell, 1978a, b); some sandstones contain fish fragments and some beds yield brachiopods. A fish-bearing horizon, the Afon y Waen Fish Bed was thought by Hall et al. (1973) to be a laterally continuous horizon and was used by them to divide the Plateau Beds into lower and upper units. Lovell (1978a) considered that the horizon was not continuous, but that fish-bearing conglomerates occur as lenses at different levels. He recognised three subdivisions (Divisions A, B and C) on sedimentological grounds. Mainly fluvial, but with some shallow marine environments; some possible aeolian deposition has also been suggested (Lovell, 1978a, b), but a tidal channel environment may be more likely. A mudflow debrite lies at the base of the formation.
Definition of Lower Boundary: The lower boundary of the formation is placed at a sharp junction where the basal hard purple sandstones of the formation unconformably overlie the softer drab red sandstones of the Brownstones Formation. A quartz granule debrite bed marks the base of the formation locally. A slight angular discordance between the formations is visible locally, as on the Carmarthen Fans of Bannau Sir Gaer and Fan Brycheiniog.
Definition of Upper Boundary: The upper boundary is placed at a sharp junction where the topmost red sandstones of the formation are overlain by grey and grey-green sandstones of the overlying Wern Watkin (formerly Grey Grits) Formation.
Thickness: 36 m at outcrop, with up to 58 m proved in boreholes at depth (Taylor and Thomas, 1975).
Geographical Limits: From Pont Clydach [SO 740 194] to Blaen Onneu [SO 167 169] on the North Crop of the South Wales Coalfield.
Parent Unit: Portishead Subgroup (POB)
Previous Name(s): Plateau Beds Formation (PLB)
Alternative Name(s): none recorded or not applicable
Stratotypes:
Reference Section  Cliffs at the head of the Duffryn Crawnon Valley [SO 0945 1500], Powys, south Wales (Lovell, 1978a, b; Barclay, 2005g). 
Reference(s):
Barclay, W J, Davies, J R, Hillier, R D, and Waters, R A. 2015. Lithostratigraphy of the Old Red Sandstone successions of the Anglo-Welsh Basin. British Geological Survey Research Report, RR/14/02. 96pp. 
Hall, I H S, Taylor, K, and Thomas, L P. 1973. The stratigraphy of the Upper Old Red Sandstone in south Breconshire. Bulletin of the Geological Survey of Great Britain, No. 44, 45-62. 
Lovell, R W W. 1978a. The sedimentology and stratigraphy of the Upper Old Red Sandstone and Lower Limestone Shales of the South Wales Coalfield. Unpublished PhD thesis, University of Bristol. 
Lovell, R W W. 1978b. Abercriban, Powys. 72-73 in International symposium on the Devonian System (PADS, 78) September 1978. A field guide to selected outcrop areas of the Devonian of Scotland, the Welsh Borderland and South Wales. Friend, P F, and Williams, B P J (editors). (London: The Palaeontological Association.) 
Taylor, K. 1972. New fossiliferous localities in the Upper Old Red Sandstone of the Ystradfellte-Cwm Taff district of Breconshire. Bulletin of the Geological Survey of Great Britain, Vol. 38, 11-14. 
Taylor, K, and Thomas, L P. 1974. Field meeting in the Upper Old Red Sandstone of south Breconshire. Proceedings of the Geologists' Association, Vol. 85, 423-432. 
Taylor, K, and Thomas, L P. 1975. Geological Survey boreholes in south Breconshire and their bearing on the stratigraphy of the Upper Old Red Sandstone and the Carboniferous-Old Red Sandstone boundary east of the Afon Hepste. Bulletin of the Geological Survey of Great Britain, Vol. 54, 1-39. 
Strahan, A, Gibson, W, and Cantrill, T C. 1904. The geology of the South Wales Coalfield, Part V. The country around Merthyr Tydfil. First Edition. Memoir of the Geological Survey of Great Britain. Sheet 231 (England and Wales). 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
none recorded or not applicable