The BGS Lexicon of Named Rock Units — Result Details
Freshwater West Formation
| Computer Code: | FWW | Preferred Map Code: | notEntered |
|---|---|---|---|
| Status Code: | Full | ||
| Age range: | Lochkovian Age (DO) — Lochkovian Age (DO) | ||
| Lithological Description: | Very fine- to medium-grained sandstones with subordinate calcrete-bearing mudstones and intraformational conglomerates (Conigar Pit Sandstone Member) and thick, red mudstones–siltstones that are commonly blue-mottled, laminated, and calcretised (Rat Island Mudstone Member); at Gelliswick Bay it consists of fining-upwards alluvial cycles of sandstone–siltstone–red mudstone. The sandstones are dark red to brown and intraformational conglomerate lenses occur locally at the bases of cycles. In the Welsh Borderland the formation comprises interbedded red mudstones (about 60 to 70 per cent) and red-brown, purple and green sandstones, commonly arranged in fining-upwards fluvial cycles, subsidiary intraformational conglomerates, common calcrete glaebules in the mudstones; mature calcretes are sporadically preserved, the Lower and Upper Abdon Limestones being more widespread. | ||
| Definition of Lower Boundary: | In Pembrokeshire, the lower boundary is placed at the sharp junction where the basal sandstone of the formation (and of the Conigar Pit Sandstone Member) rests conformably on the top of the topmost calcrete of the Chapel Point Limestone Member. In the Welsh Borderland, the base of the formation in the M50 section rests on a particularly thick (about 15 m) calcrete (the Chapel Point Limestone Member), above which fluvial sandstones appear in large quantity in the succession. | ||
| Definition of Upper Boundary: | In Pembrokeshire, the upper boundary, previously interpreted as a disconformity or unconformity at the type locality, is now re-interpreted as a conformable boundary (Hillier and Williams, 2007). It is placed where the topmost red-brown mudstones of the formation (and of the Rat Island Mudstone Member) are overlain by interbedded intraformational conglomerates and bright red mudstones of the Ridgeway Conglomerate Formation, the junction being placed at the first appearance of conglomerate. North of the Ritec Fault, the upper boundary is a conformable junction where green sandstones and mudstones of the Llanstadwell Formation of the Cosheston Subgroup succeed red mudstones and sandstones of the Freshwater West Formation. In continuous exposure, the top is placed 'at the base of lowest mudstone with a recognisably grey-green colouration' (Allen and Williams, 1978). In the Welsh Borderland, the top is transitional, and placed at the top of the highest substantial calcrete, which may correlate with a similar horizon in the Black Mountains and the Clee Hills. The upper boundary is placed at the base of the Brownstones Formation and (locally) Senni Formation, where sandstone-dominated sequences begin. Locally, the top of the formation is placed at the top of the higher of two well-developed calcretes. These are the Abdon Limestones in the Clee Hills. | ||
| Thickness: | The formation is 305 to 580 m thick south of the Ritec Fault and 1000 to 1500 m thick to the north of the fault in Gelliswick Bay. In the Welsh Borderland, about 630 m of beds are present in the M50 section assuming that no strata are cut out or repeated by faulting in the intervals that were unexposed when the motorway was made (Allen and Dineley, 1976). | ||
| Geographical Limits: | Widespread throughout south Wales, the Welsh Borderland and West Midlands. | ||
| Parent Unit: | Milford Haven Subgroup (MIH) | ||
| Previous Name(s): | Abdon Group [Obsolete Name And Code]
(ABD)
Ditton Subgroup (DTG) Gelliswick Bay Formation (GLW) St Maughans Formation (SMG) |
||
| Alternative Name(s): | none recorded or not applicable | ||
| Stratotypes: | |||
| Type Section | Foreshore exposures at Freshwater West [SR 885 994], Pembrokeshire (Allen et al., 1981b; Barclay and Williams, 2005; Williams, 1978; Williams et al., 1982). | ||
| Reference Section | Low cliffs east and west of Gelliswick Bay [SM 885 053], 2 km west of Milford Haven (Barclay et al., 2015). | ||
| Reference Section | The log of the M50 Motorway cutting section [SO 663 260] near Ross-on-Wye (Allen and Dineley, 1976) provides a good reference section, although the section itself is now sporadically exposed. | ||
| Reference(s): | |||
| Waters, C N, Smith, K, Hopson, P M, Wilson, D, Bridge, D M, Carney, J N, Cooper, A H, Crofts, R G, Ellison, R A, Mathers, S J, Moorlock, B S P, Scrivener, R C, McMillan, A A, Ambrose, K, Barclay, W J, and Barron, A J M. 2007. Stratigraphical Chart of the United Kingdom: Southern Britain. British Geological Survey, 1 poster. | |||
| Barclay, W J, Davies, J R, Hillier, R D, and Waters, R A. 2015. Lithostratigraphy of the Old Red Sandstone successions of the Anglo-Welsh Basin. British Geological Survey Research Report, RR/14/02. 96pp. | |||
| Allen, J R L, and Dineley, D L. 1976. The succession of the Lower Old Red Sandstone (Siluro-Devonian) along the Ross - Tewkesbury Spur Motorway (M50), Hereford and Worcester. Geological Journal, Vol. 11, 1-14. | |||
| Allen, J R L, and Williams, B P J. 1978. The sequence of the earlier Lower Old Red Sandstone (Siluro - Devonian) north of Milford Haven, southwest Dyfed (Wales). Geological Journal, Vol. 13, 113-136. | |||
| 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). | |||
| Williams, B P J. 1971. Sedimentary features of the Old Red Sandstone and Lower Limestone Shales of south Pembrokeshire, south of the Ritec Fault. In Bassett, D. A. and Bassett, M. G. Geological Excursions in South Wales and the Forest of Dean. The Geologists’ Association, South Wales Group, Cardiff, pp. 222-239. | |||
| Allen, J R L, Thomas, R G, and Williams, B P J. 1981. Field Meeting: the facies of the Lower Old Red Sandstone, north of Milford Haven, south-west Dyfed, Wales. Proceedings of the Geologists' Association, Vol. 92, 251-267. | |||
| Allen, J R L, Elliott, T, and Williams, B P J. 1981. Old Red Sandstone and Carboniferous fluvial sediments in South Wales. 1.1-1.39 in Field Guides to Modern and Ancient Fluvial Systems in Britain and Spain. Elliott, T (editor). Proceedings of the Second International Conference on Fluvial Sediments: Modern and Ancient Fluvial Systems: Sedimentology and Processes. University of Keele, UK, 21-25 September 1981. | |||
| Barclay, W J, and Williams, B P J. 2005. Freshwater West, Pembrokeshire. 291–301 in Barclay, W J, Browne, M A E, McMillan, A A, Pickett, E A, Stone, P, and Wilby, P R. The Old Red Sandstone of Great Britain. Geological Review Series, No. 31. (Peterborough: Joint Nature Conservation Committee.) | |||
| Higgs, K T. 2004. An early Devonian (Lochkovian) microflora from the Freshwater West Formation, Pembroke Peninsula, South Wales. Geological Journal, Vol. 39, 359–374. | |||
| Hillier, R D, and Williams, B J P. 2007. The Ridgeway Conglomerate Formation of SW Wales, and its implications. The end of the Lower Old Red Sandstone? Geological Journal, Vol. 42, 55–83. | |||
| Williams, B P J. 1978. The Old Red Sandstone of the Welsh Borderland and South Wales. 55–106 in A field guide to the Devonian of Scotland, the Welsh Borderland and South Wales. Friend, P F, and Williams, B P J (editors). (London: The Palaeontological Association.) | |||
| Williams, B P J, Allen, J R L, and Marshall, J D. 1982. Old Red Sandstone facies of the Pembroke Peninsula, south of the Ritec Fault. 151–174 in Geological excursions in Dyfed, south-west Wales. Bassett, M G (editor). (Cardiff: National Museum of Wales.) | |||
| 1:50K maps on which the lithostratigraphical unit is found, and map code used: | |||
| none recorded or not applicable | |||