The BGS Lexicon of Named Rock Units — Result Details

Marros Group

Computer Code: MARR Preferred Map Code: notEntered
Status Code: Full
Age range: Namurian Stage (CN) — Namurian Stage (CN)
Lithological Description: Fissile dark grey siliceous mudstones and cherts (Aberkenfig Formation); grey, commonly fossiliferous mudstones (Bishopston Formation); quartzitic sandstones and conglomerates (Twrch Sandstone Formation), occupying the basal half of the Group on the northeastern crops and eastern crops of the South Wales Coalfield, and quartzitic sandstones (Telpyn Point Sandstone Formation) occupying the highest part of the Group locally.
Definition of Lower Boundary: The base is taken at the base of 35m of dark grey fissile mudstones with thin cherts and siliceous mudstones (Aberkenfig Formation), which gradationally overlie the interbedded dark grey argillaceous limestones and mudstones of the Oystermouth Formation. In the type section at Ragwen Point the base is not seen.
Definition of Upper Boundary: The top is taken at the base of the Subcrenatum Marine Band, where mudstones or sandstones are overlain by fossiliferous marine mudstones containing the index goniatite Gastrioceras subcrenatum (as for the base of the Coal Measures Group, as defined by Stubblefield and Trotter, 1957).
Thickness: 20m on the east crop of the South Wales Coalfield to 750m in Gower.
Geographical Limits: The group crops out in a narrow belt around the periphery of the South Wales Coalfield and its extension in Pembrokeshire. The outcrop around the main coalfield extends from near Kidwelly [SN 360 050] on the north crop eastnortheast to near Brynmawr [SO 20 11] and south to near Caerphilly [ST 20 86] and west to Swansea Bay [SS 79 84]; in Gower from Bishopston to Mumbles; and in the Pembroke Peninsula from Druidston [SM 860 169] to Marros [SN 198 076]; and on the southern limb of the syncline from Tenby [SN 135 010] northwest to near the Cleddau River [c.SN 012 085].
Parent Unit: Not Applicable (-)
Previous Name(s): Millstone Grit Series (-3919)
Millstone Grit Group (*813)
Tenby Group [Obsolete Name And Code: Use MARR] (TENBY)
Alternative Name(s): none recorded or not applicable
Stratotypes:
Partial Type Section  Coastal foreshore with gently dipping exposures in a section from Telpyn Point to Marros Sands and Ragwen Point, Carmarthenshire. 107.9m of the Bishopston Mudstone Formation are exposed, with the lowermost 15-18m unexposed, overlain by an exposed thickness of 27.7m of the Telpyn Point Sandstone Formation, with only the topmost beds unexposed. See Archer, 1965; George and Kelling, 1982. 
Reference Section  Coastal section from Settling Nose to Druidston Haven, St Brides Bay, Pembrokeshire. 131.62m (431' 10") of the Group are exposed, with the topmost 6-9m not exposed. The lowermost beds are faulted against Ordovician (Caradoc) strata at Druidstone Haven. See Archer, 1965. 
Reference(s):
George, G T, 2001. Late Yeadonian (Upper Sandstone Group) incised valley supply and depositional systems in the South Wales peripheral foreland basin; implications for the evolution of the Culm Basin and for the Silesian hydrocarbon plays of onshore and offshore UK. Marine and Petroleum Geology, Vol.18, 671-706. 
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. 
Archer, A A. 1995. Notes on the Millstone Grit of the north crop of the Pembrokeshire Coalfield. Proceedings of the Geologists' Association, Vol.76, 137-150. 
Cleal, C J and Thomas, B A. 1996. Marros Sands. British Upper Carboniferous stratigraphy. Cleal, C J and Thomas, B A (editors) (London: Chapman and Hall) 70-72. 
George, G T and Kelling, G. 1982. Stratigraphy and sedimentology of Upper Carboniferous sequences in the coalfield of southwest Dyfed. 175-201 in Bassett, M G (Editor), Geological excursions in Dyfed, south-west Wales. 
Jones, D G. 1969. The Namurian succession between Tenby and Waterwynch, Pembrokeshire. Geological Journal, Vol.6, 267-272. 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
E232 E244 E245 E246