| Computer Code: |
TRB |
Preferred Map Code: |
notEntered |
| Status Code: |
Full |
| Age range: |
Gorstian Age (SG)
— Gorstian Age (SG) |
| Lithological Description: |
Red-brown, gritty, argillaceous sandstones and very subordinate siltstones and mudstones with scattered thin to medium tabular beds of medium- to coarse-grained greyish purple quartzite. Calcretes, ferricretes and desiccation cracks occur sporadically. Breccias are also present, deposited in proximal settings. |
| Definition of Lower Boundary: |
The basal boundary of the formation is sharp but conformable with the underlying Mynydd Myddfai Sandstone Formation in the Afon Sawdde. |
| Definition of Upper Boundary: |
The upper boundary of the formation is conformable with the basal quartzites and conglomerates of the overlying Cribyn Du Member of the Cae'r Mynach Formation, representing a major transgression in the late Ludlow that re-introduced marine shelf facies. |
| Thickness: |
Up to a maximum of 170 m 2 km west of the Afon Sawdde, feathering out to the north-east and south-west. |
| Geographical Limits: |
South Llandovery area, southern Wales. A narrow north-east-trending outcrop of steeply dipping beds in the Myddfai Steep Belt extends for about 18 km from near Treiorwg [SN 653 203] in the south-west to Mynydd Myddfai [SN 805 298] in the north-east. |
| Parent Unit: |
Milford Haven Subgroup (MIH)
|
| Previous Name(s): |
Trichrug Beds [Obsolete Name and Code: Use TRB]
(-4936)
|
| Alternative Name(s): |
none recorded or not applicable
|
| Stratotypes: |
| Type Section |
The gorge of the Afon Sawdde [SN 7280 2460] (Hillier et al., 2011a). |
| Reference Section |
Road section on the A476 [SN 610 191] in the Cennen Valley (Squirrell and White, 1978). Here, alluvial fan deposits more proximal than those in the Afon Sawdde are preserved. They include pebble-grade exotic clast debrites and interbedded pebbly sandstones with redoximorphic features (pers.comm. Hillier, R D and Waters, R A). |
| Reference(s): |
| 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. |
| Davies, J R, Hillier, R D, and Waters, R A. 2008. Ludlovian wetlands of Wales. Geoscientist, Vol. 18, No. 4, 24-25. |
| Potter, J F and Price, J H. 1965. Comparative sections through rocks of Ludlovian-Downtonian age in the Llandovery and Llandeilo districts. Proceedings of the Geologists' Association, Vol.76, 379-402. |
| 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.) |
| Hillier, R D, Waters, R A, Marriott, S B, and Davies, J R. 2011a. Alluvial fan and wetland interactions: evidence of seasonal slope wetlands from the Silurian of south central Wales, UK. Sedimentology. Vol. 58, 831-853. |
| Squirrell, H C, and White, D E. 1978. Stratigraphy of the Silurian and Old Red Sandstone of the Cennen Valley and adjacent areas, south-east Dyfed, Wales. Report of the Institute of Geological Sciences, No. 78/6. |
| 1:50K maps on which the lithostratigraphical unit is found, and map code used: |
|
E230
|
