| Computer Code: |
PLLE |
Preferred Map Code: |
notEntered |
| Status Code: |
Full |
| Age range: |
Ludlow Epoch (SU)
— Pridoli Epoch (SO) |
| Lithological Description: |
The formation consists of red-brown, bioturbated, gritty, argillaceous sandstones and thin heterolithic units that comprise dull green and yellowish green, lenticular siltstones and mudstones interbedded with grey-green sandstones. The heterolithic units display a diverse shallow marine ichnofauna comprising Skolithos, Palaeophycus, Thalassanoides, Teichichnus, Planolites, Arenicolites and Schaubcylindrichnus burrows (pers.comm. Hillier, R D and Waters, R A).
Interdigitated with the green heterolithic facies are red, micaceous, predominantly massive fine- to medium-grained argillaceous sandstones, homogeneous siltstones and laminated mudstones, some containing desiccation cracks. Redoximorphic iron oxide mottling is observed, as are discrete ferricrete nodules. Bioturbation includes Skolithos, Planolites and probable Beaconites. The beds have yielded a poor, low-diversity, nearshore, shallow marine fauna including Lingula, gastropods and modiolopsids. |
| Definition of Lower Boundary: |
The lower boundary of the formation is placed at the conformable junction with the Tilestones Formation (Almond et al., 1993; Wilby, 2005a). Grey-green, highly micaceous sandstones of the topmost Tilestones Formation (Long Quarry Member of Almond et al., 1993) are overlain by coarser sandstones arranged in a stacked upward-coarsening sequence of erosively based beds at the base of the Pont ar llechau Formation.
The upper boundary is placed where massive red sandstones at the top of the formation are overlain conformably by red and green mudstones with well developed calcrete nodules and wedge-shaped peds of the Moor Cliffs Formation (Almond et al., 1993; Wilby, 2005a). |
| Definition of Upper Boundary: |
none recorded or not applicable
|
| Thickness: |
Up to 43 m. |
| Geographical Limits: |
Only present south-west of Rhiwe Farm [SN 737 250], in the south of the Llandovery district. |
| 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 Sawdde Gorge, Carmarthenshire, upstream (south) of Pont-ar-Lechau [SN 7282 2452-7286 2449] (Barclay et al., 2015). |
| Reference(s): |
| Almond, J, Williams, B P J, and Woodcock, N H. 1993. The Old Red Sandstone of the Brecon Beacons to Black Mountain area. 311-330 in Geological excursions in Powys, Central Wales. Woodcock, N H, and Bassett, M G (editors). National Museum of Wales on behalf of the Geologists' Association, South Wales Subgroup. (Cardiff: University of Wales Press.) |
| Wilby, P R. 2005a. Sawdde Gorge, Powys. 240–246 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.) |
| 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. |
| Almond, J. 1983. The sedimentology of the earliest Lower Old Red Sandstone, south central Wales. Unpublished Ph.D. thesis, University of Bristol. |
| Schofield, D I, Davies, J R, Jones, N S, Leslie, A B, Waters, R A, Williams, M, Willson, D, Venus, J and Hillier, R D. 2008. Geology of the Llandovery district - a brief explanation of the geological map. Sheet Explanation of the British Geological Survey. 1:50 000 Sheet 212 Llandovery (England and Wales). |
| Strahan, A, Cantrill, T C, Dixon, E E L, and Thomas, H H. 1907. The geology of the South Wales Coalfield, Part VII. The country around Ammanford. Memoir of the Geological Survey of Great Britain, Sheet 230 (England and Wales). (London: HMSO.) |
| 1:50K maps on which the lithostratigraphical unit is found, and map code used: |
| none recorded or not applicable |
