The BGS Lexicon of Named Rock Units — Result Details

Mercia Mudstone Group (Marginal Facies)

Computer Code: MMMF Preferred Map Code: notEntered
Status Code: Full
Age range: Triassic Period (T) — Triassic Period (T)
Lithological Description: Variable, typically consisting of conglomerate and/or breccia with clasts derived locally from rocks lying immediately below the unconformable base of these deposits. The matrix generally consists of finer-grained rock fragments or, less commonly, siltstone, sandstone or micritic limestone. Where these deposits overlie Carboniferous limestones, such as in the Bristol and Mendip areas, both the matrix and limestone clasts are commonly dolomitized ("Dolomitic Conglomerate"). Individual clasts can range up to several cubic metres in size. Fenestral and algal carbonates interdigitate with conglomerates and breccias in exposures to the south of Cardiff (Tucker, 1977; Waters and Lawrence, 1987).
Definition of Lower Boundary: Markedly unconformable, at the abrupt base of conglomerates of the Marginal Facies, which infill a buried topography on Palaeozoic or late Proterozoic rocks of variable lithology.
Definition of Upper Boundary: Conformable and gradational, interdigitating with reddish-brown mudstone of the Mercia Mudstone Group (or its constituent formations).
Thickness: Highly variable due to interdigitating relationship with the Mercia Mudstone Group; ranges from 1m or less at some exposures to over 100m where these deposits fill "buried valleys" in the underlying pre-Triassic topography.
Geographical Limits: Deposits occur on the flanks of the Mercia Mudstone Group outcrop, where the latter oversteps onto Palaeozoic or Proterozoic rocks; the thickest deposits lie unconformably on Carboniferous limestones in Somerset, Avon and South Wales, and around the flanks of the Charnwood Forest in Leicestershire.
Parent Unit: Mercia Mudstone Group (MMG)
Previous Name(s): Dolomitic Conglomerate [Obsolete Name And Code: See MMMF] (DCG)
Dolomitic Conglomerate And Red Marl [Obsolete Name And Code: See MMMF] (KLIT)
Alternative Name(s): none recorded or not applicable
Type Area  Exposures on Barry Island and Sully Island, South Wales. Waters and Lawrence, 1987. 
Kellaway, G A and Welch, F B A. 1993. Geology of the Bristol district. Memoir of the British Geological Survey Special Sheet [England and Wales]. 
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). 
Green, G W and Welch F B A, 1965. The geology of the country around Wells and Chedder. Memoir of the Geological Survey of England and Wales, Sheet 280. 
Tucker, M E. 1977. The Marginal Triassic deposits of South Wales: continental facies and paleogeography. Geological Journal, Vol.12, 169-188. 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
E263 E248 E264 E250 E251 E261 E265 E279 E280 E281 E296