The BGS Lexicon of Named Rock Units — Result Details

Sychnant Dolomite Formation

Computer Code: SYCN Preferred Map Code: notEntered
Status Code: Full
Age range: Courceyan Substage (CF) — Courceyan Substage (CF)
Lithological Description: Buff, fine-grained, bedded dolostones (dolomitised skeletal packstones).
Definition of Lower Boundary: The lower boundary is gradational, with thinly bedded grey mudstones and skeletal packstones of the underlying Cwmyniscoy Mudstone Formation, interbedded with thin, fine-grained dolostones of the Sychnant Dolomite Formation. The base of the latter is placed at the base of the dolostone overlying the highest grey mudstone bed.
Definition of Upper Boundary: The top is placed at an irregular palaeokarstic surface cut in buff dolostone of the Formation, overlain by pale grey ooidal grainstones of the Pwll-y-Cwm Oolite Formation.
Thickness: 5m
Geographical Limits: Northeast crop of the South Wales Coalfield, from near Pontsticill in the Taff Fechan Valley [SO 059 108] eastwards to the Blorenge [c.SO 268 110].
Parent Unit: Clydach Valley Subgroup (CLD)
Previous Name(s): Bed 1 (-4913)
Pwll-Y-Cwm Oolite (-4357)
Alternative Name(s): none recorded or not applicable
Stratotypes:
Reference Section  Roadside section on the Heads of the Valleys road (A465), where 3m of fine-grained dolostones are present. The basal contact with the underlying Cwmynyscoy Mudstone Formation and a palaeokarstic surface at its top are exposed. Barclay, 1989. 
Partial Type Section  Nant Sychnant stream section, a tributary of the Clydach southwest of the village of Danycoed in the Clydach Valley. Here 3m of buff dolostone are sharply overlain by dolomitized ooidal grainstones of the Blaen Onnen Oolite Formation. The base is not exposed. Barclay, 1989. 
Reference(s):
George, T N. 1954. Pre-Seminulan Main Limestone of the Avonian Series in Breconshire. Quarterly Journal of the Geological Society of London, Vol.110, 282-322. 
Barclay, W J, Jackson, D I, Mitchell, M, Owen, B, Riley, N J, White, D E, Strong, G E and Monkhouse, R A. 1989. Geology of the South Wales Coalfield, Part II, the country around Abergavenny. Memoir of the British Geological Survey. 
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. 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
E232 E231