| Computer Code: |
ORCA |
Preferred Map Code: |
notEntered |
| Status Code: |
Full |
| Age range: |
Eifelian Age (DI)
— Givetian Age (DV) |
| Lithological Description: |
Onshore equivalents of the Orcadia Formation in northeast Scotland display rhythmic cycles, mostly between 5 m and 10 m thick, but exceptionally up to 60 m thick (Mykura, 1991). These cycles consist, in upward sequence, of (i) carbonate-rich, laminated siltstones, (ii) alternating dark grey fissile mudstones and cross-laminated, coarse siltstones, (iii) alternating dark grey siltstones and fissile mudstones, and (iv) grey or olive-green fissile mudstones with siltstones and fine-grained sandstones (Donovan, 1980). |
| Definition of Lower Boundary: |
The Orcadia Formation rests on metamorphic basement or on outliers of Lower Devonian strata (Struie Formation). In northeast Scotland, the Lower/Middle Old Red Sandstone boundary is a conformable facies transition at the centre of the Orcadian lake basin, but is an angular unconformity in Orkney, and along the southern margin of the basin (Mykura, 1991). In the south of the Inner Moray Firth, the equivalent boundary is either an unconformity, or it occurs at an abrupt facies transition and wireline-log break (e.g. 13/19-1); it has not been penetrated elsewhere. |
| Definition of Upper Boundary: |
The unconformity that defines the top of the Middle Old Red Sandstone onshore (Mykura, 1991) continues offshore, although it is not always demonstrable from the limited data available. In the Claymore Field, for instance, the Orcadia Formation is overlain by the Upper Old Red Group, but there is little change in facies and no obvious wireline-log break; the boundary can be identified only on the basis of the miospore data. In well 12/23-1, the Orcadia Formation is overlain by volcanics of early Carboniferous age (Richards, 1990). In all other North Sea wells the formation is overlain by Permian or younger strata, and the boundary commonly delineated by a downward increase in velocity (e.g. at 1158 m in well 14/6-1). |
| Thickness: |
Equivalent sediments are more than 4 km thick in parts of northeast Scotland (Mykura, 1991). None of the offshore wells have penetrated a complete Middle Devonian section, but the Orcadia Formation is likely to be more than 500 m thick in the Claymore Field, and is at least 612 m thick at 14/6-1. |
| Geographical Limits: |
The Orcadia Formation extends from the coasts of Caithness and Orkney, across the northern Inner Moray Firth, through the Claymore Field, to the East Shetland Platform. Its northern and eastern boundaries are poorly defined, but the southern shoreline of the Orcadian lake can be traced between 13/22-1 and 13/24-1 (Richards, 1990). |
| Parent Unit: |
Middle Old Red Group (MIOR)
|
| Previous Name(s): |
none recorded or not applicable
|
| Alternative Name(s): |
none recorded or not applicable
|
| Stratotypes: |
| Type Section |
North Sea well 13/22- 1: 1193-1456.5 m TD (3915-4778 ft TD) below KB (Cameron, 1993). |
| Reference Section |
North Sea well 9/07- 1: 1544-1807 m TD (5065-5928 ft TD) (Cameron, 1993). |
| Reference Section |
North Sea well 14/06- 1: 1158-1770 m TD (3799-5807 ft TD) (Cameron, 1993). |
| Reference(s): |
| Cameron, T D J. 1993. 4. Triassic, Permian and pre-Permian of the Central and Northern North Sea. In: Knox, R W O'B and Cordey, W G (eds.) Lithostratigraphic nomenclature of the UK North Sea. British Geological Survey, Nottingham. |
| Johnson, H, Richards, P C, Long, D, and Graham, C C. 1993. United Kingdom offshore regional report: the geology of the northern North Sea. (London: HMSO for the British Geological Survey.) |
| Mykura, W. 1991. Old Red Sandstone. 297-346 in Geology of Scotland (3rd edition, revised). Craig, G Y (editor). Geological Society of London. |
| Andrews, I J, Long, D, Richards, P C, Thomson, A R, Brown, S, Chesher, J A, and McCormac, M. 1990. The geology of the Moray Firth. (London: British Geological Survey.) |
| Donovan, R N. 1980. Lacustrine cycles, fish ecology and stratigraphic zonation in the Middle Devonian of Caithness. Scottish Journal of Geology 16, 35-50. |
| Duncan, A D, and Hamilton, F R M. 1988. Palaeolimnology and organic geochemistry of the Middle Devonian in the Orcadian Basin. In: Fleet, A J, Kelts, K, and Talbot, M R (eds.) Lacustrine petroleum source rocks. Geological Society, London, Special Publication No.40, 173-201. |
| 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 |
