| Computer Code: |
DULB |
Preferred Map Code: |
notEntered |
| Status Code: |
Full |
| Age range: |
Silurian Period (S)
— Lochkovian Age (DO) |
| Lithological Description: |
The subgroup comprises four units, named by Allen (1965a, 1977) the Bodafon Beds, Traeth Bach Beds, Porth-y-Mor Beds and Traeth Lligwy Beds. These names are now given formation status (Davies, 2005).
As detailed by Allen (1965a) and summarised by Davies (2005), initial alluvial-fan gravels (Bodafon Formation) accumulated along a basin margin sourced from Precambrian and older Lower Palaeozoic outcrops to the south-west. Subsequent deposition in ephemeral non-saline playa lakes (Traeth Bach Formation) was succeeded by fluvial deposition in a broad south-east-trending meandering river belt (Porth-y-Mor Formation). This was followed by deposition in more permanent flood basin lakes (Traeth Lligwy Formation).
There is no diagnostic proof of the age of the group, but it is likely to be late Silurian to Lochkovian. On the basis of the presence of Beaconites antarcticus and the absence of B. barretti (the latter being indicative of a Devonian age), Morrissey (2006) suggested a pre-Lochkovian age. |
| Definition of Lower Boundary: |
The lower boundary of the group is an unconformity, where the lowest conglomerate of the Bodafon Formation rests on underlying lower Palaeozoic or Precambrian rocks. |
| Definition of Upper Boundary: |
The upper boundary (i.e. the upper boundary of the Traeth Lligwy Formation) is not seen. |
| Thickness: |
About 546 m. |
| Geographical Limits: |
The group occupies a narrow coastal strip on the east coast of Anglesey, and from Traith Dulas southwards to the flanks of Mynydd Bodafon and from there south to 0.8 km north of Llangefni. |
| Parent Unit: |
Daugleddau Group (DAUG)
|
| Previous Name(s): |
none recorded or not applicable
|
| Alternative Name(s): |
none recorded or not applicable
|
| Stratotypes: |
| Type Area |
Dulas Bay, Anglesey [SH 49 90]. |
| 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. |
| Allen, J R L. 1977. Wales and the Welsh Borders. 40-54 in A correlation of Devonian rocks in the British Isles. House, M R et al. (editors). Geological Society, London, Special Report No. 7. |
| Allen, J R L. 1965. The sedimentation and palaeogeography of the Old Red Sandstone of Anglesey, North Wales. Proceedings of the Yorkshire Geological Society, Vol. 35, 139-185. |
| Davies, J R. 2005. Porth-y-Mor. 221-228 in Barclay, W J, Browne, M A E, McMillan, A A, Pickett, E A, Stone, P, and Wilby, P R. 2005. The Old Red Sandstone of Great Britain. Geological Conservation Review Series, No. 31. |
| Morrissey, L B. 2006. The Ichnology of Terrestrial Palaeoenvironments: Old Red Sandstone, British Isles. Unpublished PhD Thesis, University of the West of England, Bristol. |
| 1:50K maps on which the lithostratigraphical unit is found, and map code used: |
| none recorded or not applicable |
