| Computer Code: |
FWE |
Preferred Map Code: |
notEntered |
| Status Code: |
Full |
| Age range: |
Pridoli Epoch (SO)
— Pridoli Epoch (SO) |
| Lithological Description: |
The formation comprises green to dark grey granule- to pebble-grade exotic clast conglomerates interbedded with fine- to medium-grained sandstones and green-grey mudstones and finer-grained red sandstones and mudstones the latter of which contain well-developed calcrete palaeosol profiles, commonly with tubular morphology. Mottled red-green 'redoximorphic' pheneomena are common in both mudstone and sandstone lithologies. At Freshwater East the formation comprises four discrete green-grey heterolithic units that contain lingulid faunas, vertebrate (fish) spines, and a diverse ichnotaxa including Diplichnites, Palmichnium, Rusophycus, Isopodichnus, Striatichnium, Arenicolites and Helminthopsis (Morrissey et al., 2012). Coalified plant fragments are locally abundant (Edwards, 1979). Lingulid and fish faunas are also recorded at Gateholm Island. |
| Definition of Lower Boundary: |
At Freshwater East the lower boundary is placed at the base of a 1.8 m thick heterolithic unit that is conglomerate dominated and locally scours into the underlying Wenlock Gray Sandstone Formation. The basal conglomerate is of pebble to cobble grade, and is overlain by buff coloured, cross-stratified pebbly sandstone.
At Gateholm Island, the formation conformably overlies red pedified mudrocks of the Moor Cliffs Formation, its base being defined by a 0.6 m thick green-grey heterolithic unit comprising discontinuous, decimetre-thick lens-like exotic clast, granule-grade conglomerates which contain lingulids, Pachytheca, fish spines and fish fragments. These are interbedded with green-grey pinstripe-laminated mudstones and fine-grained sandstones. |
| Definition of Upper Boundary: |
At Freshwater East the upper boundary is placed at the top of the fourth grey-green heterolithic unit and marked by a gradual change into red calcretised mudstone with wedge-shaped peds of the Moor Cliffs Formation. The upper boundary at Freshwater West is placed at the top of a green calcretised mudrock unit, at the conformable junction with thick, red, calcretised mudstones of the overlying Moor Cliffs Formation.
At Gateholm Island the top of the formation is defined by a gradual change to red pedified mudstones and thin fine- to medium- grained sandstones of the Moor Cliffs Formation. |
| Thickness: |
18 to 50 m; 50.25 m at the type section and 18.45 m at Freshwater West. |
| Geographical Limits: |
Freshwater East, Gravel Bay, Freshwater West, Gateholm Island, Pembrokeshire. |
| Parent Unit: |
Milford Haven Subgroup (MIH)
|
| Previous Name(s): |
none recorded or not applicable
|
| Alternative Name(s): |
none recorded or not applicable
|
| Stratotypes: |
| Type Section |
The type section is at the northern side of Freshwater East Bay [SS 021 981], where the formation is 50.25 m thick and lies disconformably on fossiliferous sandstones and mudstones of the Gray Sandstone Formation of Wenlock age (Dixon, 1921; Bassett, 1982). |
| Reference Section |
Foreshore exposures at the southern end of Freshwater West [SR 884 996] provide a reference section 23 m thick. Here a scoured, channelled surface cuts into the underlying Wenlock sequence and is unconformably overlain by a 5 m thick basal conglomerate of the formation (Dixon, 1921; Bassett, 1974; Allen et al., 1982; Barclay and Williams, 2005). |
| Reference Section |
Eastern cliffs of Gateholm Island 34.5 m above the base of the Moor Cliffs Formation [SM 7714 0737]. Here the formation attains a thickness of 36.4 m. This interval was previously included in the Sandy Haven Formation by Allen and Williams (1978). |
| Reference Section |
An additional reference section occurs at Gravel Bay [SM 880 005] (Barclay et al., 2015). |
| 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 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. |
| 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.) |
| Bassett, M G. 1974. Review of the stratigraphy of the Wenlock Series of the Welsh Borderlands and South Wales. Palaeontology, 17, 745-777. |
| 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, 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.) |
| Allen, J R L, Thomas, R G, and Williams, B P J. 1982. The Old Red Sandstone north of Milford Haven. 123-149 in Geological excursions in Dyfed, south-west Wales. Bassett, M G (editor). Geologists' Association, South Wales Subgroup. (Cardiff: National Museum of Wales.) |
| Bassett, M G. 1982. Silurian rocks of the Marloes and Pembroke peninsulas. 101–122 in Bassett, M G (editor). Geological excursions in Dyfed, south-west Wales. (Cardiff: National Museum of Wales.) |
| Edwards, D. 1979. A Late Silurian flora from the Lower Old Red Sandstone of south-west Dyfed. Palaeontology, Vol. 22, 23–52. |
| 1:50K maps on which the lithostratigraphical unit is found, and map code used: |
| none recorded or not applicable |
