Computer Code: |
CLAI |
Preferred Map Code: |
notEntered |
Status Code: |
Full |
Age range: |
Mid Devonian Epoch (DM)
— Mississippian Subperiod (CMI) |
Lithological Description: |
The Clair Group consists of a thick succession of predominantly red sandstones, interbedded with conglomerates and breccias and dark reddish and grey mudstones. Evidence for pedogenic carbonates is widespread.
Allen and Mange-Rajetsky (1992) subdivided the Clair Group into 10 informal units. They recognised a Lower Clair Group (Units I to VI) and an Upper Clair Group (Units VII to X), separated by an unconformity. They also recognised another unconformity within the Lower Clair Group, subdividing the entire succession into three depositional cycles. The third cycle (Upper Clair Group) is the only cycle to contain evidence of marine influence. The two uppermost units contain increasingly grey mudstones, contrasting with the predominantly red strata of the rest of the succession. |
Definition of Lower Boundary: |
The Clair Group invariably lies unconformably on Lewisian metamorphic basement, with onlap towards the crest of the Rona Ridge. The boundary is usually marked by an abrupt increase in sonic velocity. |
Definition of Upper Boundary: |
The top of the Clair Group is marked by a major unconformity beneath mudstones and limestones of the Upper Cretaceous Shetland Group, or the sandstones of the Lower Cretaceous Victory Formation, and is invariably marked by an abrupt downwards increase in sonic velocity. |
Thickness: |
The maximum drilled thickness of the Clair Group is almost 1000 m in well 206/8-4, which terminated in Unit III. Published cross-sections suggest that this is close to the maximum thickness (Coney et al., 1993). |
Geographical Limits: |
The succession is truncated or absent along parts of the crest of the Rona Ridge and a more complete succession is preserved beneath at least part of the West Shetland Basin. The distribution to the west of the Rona Ridge is speculative because of the deep depth of burial. The occurrence of Clair Group sediments beneath the Westray Ridge in 204/19-1 is evidence for more widespread deposition. Cope et al. (1992) suggest that the marine influences near the top of the Clair Group are related to a marine link via the northern North Sea, although a southwesterly link cannot be ruled out. |
Parent Unit: |
Not Applicable (-)
|
Previous Name(s): |
none recorded or not applicable
|
Alternative Name(s): |
none recorded or not applicable
|
Stratotypes: |
Reference Section |
North Sea well 206/08- 3A: 1634-2577 m (5361-8455 ft) (Ritchie et al., 1996). |
Reference Section |
North Sea well 206/08- 5: 1592-2135 m (5223-7005 ft) (Ritchie et al., 1996). |
Reference Section |
North Sea well 206/08- 7: 1588-2322.5 m (5210-7620 ft) (Ritchie et al., 1996). |
Reference(s): |
Ritchie, J D, Gatliff, R W and Riding, J, 1996. 1. Pre - Tertiary lithostratigraphy. In: Stratigraphic Nomenclature of the UK North West Margin. British Geological Survey, Nottingham. |
Cope, J W C, Ingham, J K, and Rawson, P F. 1992. Atlas of palaeogeography and lithofacies. Geological Society Memoir, No. 13. |
Allen, P A, and Marshall, J E A. 1981. Depositional environments and palynology of the Devonian south-east Shetland basin. Scottish Journal of Geology, 17, 257-273. |
Allen, P A, and Mange-Rajetsky, M A. 1992. Devonian-Carboniferous sedimentary evolution of the Clair area, offshore north-western UK: impact of changing provenance. Marine and Petroleum Geology, 9, 29-52. |
Blackbourne, G A. 1987. Sedimentary environments and stratigraphy of the Late Devonian - Early Carboniferous Clair Basin, west of Shetland. 75-91 in European Dinantian Environments. Miller, J, Adams, A E, and Wright, V P (editors). (Wiley, Chichester.) |
Coney, D, Fyfe, T B, Retail, P, and Smith, P J. 1993. Clair appraisal: the benefits of a co-operative approach. 1409-1420 in Petroleum Geology of Northwest Europe: Proceedings of the 4th Conference. Parker, J R (editor). |
Waters, C N, Gillespie, M R, Smith, K, Auton, C A, Floyd, J D, Leslie, A G, Millward, D, Mitchell, W I, McMillan, A A, Stone, P, Barron, A J M, Dean, M T, Hopson, P M, Krabbendam, M, Browne, M A E, Stephenson, D, Akhurst, M C, and Barnes, R P. 2007. Stratigraphical Chart of the United Kingdom: Northern Britain. (British Geological Survey.) |
1:50K maps on which the lithostratigraphical unit is found, and map code used: |
none recorded or not applicable |