The BGS Lexicon of Named Rock Units — Result Details

Penarth Group (Marginal Facies)

Computer Code: PNMF Preferred Map Code: notEntered
Status Code: Index Level
Age range: Rhaetian Age (TR) — Rhaetian Age (TR)
Lithological Description: Variable, typically consisting of pale grey or yellowish grey sandstone, calcareous or dolomitic, locally with pebble-sized clasts of limestone or quartzitic sandstone derived from nearby Palaeozoic rocks. These lithologies interdigitate with the typical grey to black mudstones with thin sandstones and limestones of the Penarth Group or its constituent formations.
Definition of Lower Boundary: Markedly unconformable, at the abrupt base of sandstones of the Marginal Facies; these may overlie mudstones of the Penarth Group or Mercia Mudstone Group, or locally they overstep onto older Palaeozoic or Proterozoic rocks.
Definition of Upper Boundary: Conformable and gradational, interdigitating with mudstones, sandstones and limestones of the Penarth Group (or its constituent formations).
Thickness: Highly variable due to interdigitating relationship with the Penarth Group; ranges from 1m or less at some exposures to over 10m.
Geographical Limits: Deposits occur at the surface on the flanks of the Penarth Group outcrop in the Bridgend area of South Wales (Wilson et al., 1990) and in the subsurface in central-southern England (Oxfordshire, Berkshire), around the flanks of the London Platform (Horton et al., 1987).
Parent Unit: Penarth Group (PNG)
Previous Name(s): Twyford Beds (-4843)
Alternative Name(s): none recorded or not applicable
none recorded or not applicable
Warrington, G, Audley-Charles, M G, Elliott, R E, Evans, W B, Ivimey-Cook, H C, Kent, P E, Robinson, P L, Shotton, F W and Taylor, F M. 1980. A correlation of the Triassic rocks in the British Isles. Special Report of the Geological Society of London, No.13. 
Wilson, D, Davies, J R, Fletcher, C J N, and Smith, M. 1990. The Geology of the South Wales Coalfield, part VI the Country around Bridgend. Memoir of the British Geological Survey, 1:50 000 Geological Sheets 261 and 262 (England and Wales). 
Horton, A, Poole, E G, Williams, B J, Illing, V C and Hobson, G D. 1987. Geology of the country around Chipping Norton. Memoir of the British Geological Survey, Sheet 218 (England and Wales). 
1:50K maps on which the lithostratigraphical unit is found, and map code used: