The BGS Lexicon of Named Rock Units — Result Details

Llynfi Member

Computer Code: LLFB Preferred Map Code: notEntered
Status Code: Full
Age range: Bolsovian Substage (CC) — Bolsovian Substage (CC)
Lithological Description: Green-grey and blue-grey, feldspathic, micaceous lithic arenites ("Pennant sandstone") with thin mudstone/siltstone and seatearth interbeds and mainly thin coals.
Definition of Lower Boundary: The base is placed at the base of the lowermost thick (over c.3m) bed of lithic arenite ("Pennant sandstone"). This lies immediately above the Cambriense (Upper Cwmgorse) Marine Band in the southwest of the South Wales Coalfield, where the sandstones rest on the underlying mudstones of the South Wales Upper Coal Measures Formation. The base is gradational and diachronous; northeastwards, the "Pennant sandstone" facies commences at a progressively higher level.
Definition of Upper Boundary: The top, as defined by Woodland et al. (1957), is placed at the base of the No.2 Rhondda Coal, which is overlain by lithic arenite and thin mudstones and siltstones that make up most of the Rhondda Member. The base of the coal rests on a mainly thin development of seatearth and mudstone at the top of the Member, which overlies thick lithic arenites.
Thickness: Up to 420m in the Swansea district.
Geographical Limits: South Wales Coalfield, extending on the south crop from the Swansea district [SS 533 955] east to the Ferndale area [SS 9995].
Parent Unit: Pennant Sandstone Formation (PES)
Previous Name(s): Llynfi Formation (-2905)
Llynfi Beds (-366)
Alternative Name(s): none recorded or not applicable
Stratotypes:
Type Area  Type area is in the Llynfi Valley, near Tondu, South Wales. Woodland et al., 1957. 
Reference(s):
Woodland, A W, Evans, W B and Stephens, J V. 1957. Classification of the Coal Measures of South Wales with Special reference to the Upper Coal Measures. Bulletin of the Geological Survey of Great Britain, No. 13, 39-60. 
Waters, C N, Barclay, W J, Davies, J R and Waters, R A. In press. Stratigraphical framework for Carboniferous successions of Southern Great Britain (Onshore). British Geological Survey Research Report, RR/05/06. 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
E262 E247 E232 E231 E230 E246