The BGS Lexicon of Named Rock Units — Result Details

Tongwynlais Formation

Computer Code: TGW Preferred Map Code: notEntered
Status Code: Full
Age range: Courceyan Substage (CF) — Courceyan Substage (CF)
Lithological Description: Lower part comprises thin- to thick-bedded skeletal, peloidal, ooidal and sandy grainstones and packstones with subordinate grey mudstones, calcite mudstones, calcretes and ironstones. Upper part comprises thin- to medium-bedded skeletal packstones and grey mudstones. The lower part represents deposition in ooidal shoal, embayment and peritidal environments. The upper part represents deposition on a storm-influenced, open marine, nearshore shelf.
Definition of Lower Boundary: A thin unit [3 to 4m] of red, sparsely shelly, calcareous sandstones, sandy limestones, and mudstones with calcrete caps the red fluvial sandstones of the underlying Quartz Conglomerate Group. The sharp base of the Tongwynlais Formation is taken at the top of this thin unit at the incoming of restricted marine or peritidal, green or grey mudstones, micrites, calcisiltites and skeletal packstones/grainstones.
Definition of Upper Boundary: Interbedded thin- to medium-bedded grey skeletal packstones and mudstones forming the upper part of the Tongwynlais Formation thicken upwards and pass up quickly, but gradationally, into the medium- to thick-bedded ooidal and skeletal grainstones of the Castell Coch Limestone Formation. The top of the Tongwynlais Formation is taken at the top of the last mudstone bed.
Thickness: Up to 50m in the south of the Vale of Glamorgan, thinning northwards to a feather edge in the Risca area on the east crop of the South Wales Coalfield.
Geographical Limits: Vale of Glamorgan [ST 00 74]. The formation is overstepped northwards by the Castell Coch Limestone Formation. Present but not mapped in the southeast crop of the South Wales Coalfield [ST 17 85], Gower [SS 50 90], Pembrokeshire [SM 99 01] and Chepstow areas [ST 53 93].
Parent Unit: Avon Group (AVO)
Previous Name(s): none recorded or not applicable
Alternative Name(s): none recorded or not applicable
Stratotypes:
Partial Type Section  Road section [Merthyr Road], 1.5km, westsouthwest of Rhiwbina Farm, Tongwynlais, Cardiff. Semi-continuous exposure through the full formation [38m]. Lower contact with the Quartz Conglomerate Group and upper contact with the Castell Coch Limestone seen. The formation is heterolithic for the lower third; remainder comprises interbedded limestones and mudstones. 
Reference(s):
Waters, R A and Lawrence, D J D. 1987. Geology of the South Wales Coalfield - Part III: the country around Cardiff. Memoir of the British Geological Survey, Sheet 263. (England and Wales). 
Burchette, T P. 1987. Carbonate barrier shorelines during the basal Carboniferous transgression: The Lower Limestone Shale Group, South Wales and western England. 239-263 in European Dinantian Enviroments. Miller, J, Adams, A E and Wright, V P [editors]. [Chichester: John Wiley and Sons.] 
Gayer, R A, Allen, K C, Bassett, M G and Edwards, D. 1973. The structure of the Taff Gorge area, Glamorgan, and the stratigraphy of the Old Red Sandstone-Carboniferous Limestone transition. Geological Journal, Vol. 8, 345-374. 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
E263 E262