The BGS Lexicon of Named Rock Units — Result Details

Pen-y-holt Limestone Formation

Computer Code: PYHL Preferred Map Code: notEntered
Status Code: Pending Upgrade
Age range: Chadian Substage (CI) — Arundian Substage (CJ)
Lithological Description: Dark grey, thin- to medium-bedded crinoidal wackestones and calcareous mudstones. Scattered chert nodules and bands, together with silicified fossils. Local dolomitization. Crinoidal packstones with regular stylolitic partings comprise the basal 5m. Wackestones represent storm generated deposits on a muddy distal carbonate ramp.
Definition of Lower Boundary: Taken at sharp junction between dolomitised crinoidal packstones at the top of the underlying Hobbyhorse Bay Limestone Formation and the crinoidal packstones with mudstone partings of the Pen-y-holt Limestone Formation. This boundary is unsatisfactory [Simpson and Kalvoda, 1987] but has not been changed as it defines the base of the Arundian Stage and needs further work.
Definition of Upper Boundary: Taken at the gradational boundary between the dark grey interbedded wackestones and mudstones of the Formation and the pale grey thick-bedded skeletal and oolitic limestones of the overlying Stackpole Limestone Formation. Top taken at the base of the first oolitic limestone in the Stackpole Limestone Formation.
Thickness: To 280m
Geographical Limits: Castlemartin area [SR 95 98], southwest Pembrokeshire. Relationship with the coeval High Tor Limestone Formation to the northeast is uncertain and needs further investigation.
Parent Unit: Pembroke Limestone Group (PEMB)
Previous Name(s): none recorded or not applicable
Alternative Name(s): none recorded or not applicable
Stratotypes:
Type Section  Variably accessable sea cliffs between Hobbyhorse Bay and the Wash, south of Castlemartin, southwest Pembrokeshire. Entire unit seen and comprises thin bedded crinoidal wackestones and calcareous mudstones. Contacts with underlying Hobbyhorse Bay Limestone and the overlying Stackpole Limestone Formation are exposed. 
Reference(s):
George, T N, Johnson, G A L, Mitchell, M, Prentice, J E, Ramsbottam, W H C, Sevastopulo, G D and Wilson, R B. 1976. A correlation of the Dinantian rocks of the British Isles. Special Report of the Geological Society of London, No 7. 
Dixon, E E L, 1921. The Geology of the South Wales Coalfield, Part XIII , the country around Pembroke and Tenby, being an account of the region comprised in sheets 244 and 245. Memoir of the Geological Survey of Great Britain (HMSO). 
Simpson, J and Kalvoda, J. 1987. Sedimentology and foraminiferal biostratigraphy of the Arundian [Dinantian] stratotype. 226-237 in Micropalaeontology of carbonate enviroments. Hart, M B [editor]. British Micropalaeontological Society series. [Chicester: Ellis Horward Ltd.] 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
E244 E245