| Computer Code: |
FOUL |
Preferred Map Code: |
notEntered |
| Status Code: |
Full |
| Age range: |
Mid Triassic Epoch (TM)
— Late Triassic Epoch (TU) |
| Lithological Description: |
The Foula Formation is made up of light grey to red, pink, orange, green or brown, friable to hard, very fine to coarse, poorly to well-sorted, subangular to subrounded, occasionally pebbly, cross-bedded, laminated, often very micaceous, variably calcareously cemented, sometimes kaolinitic, sandstones. In places, these sandstones grade to pink or red and brown, poorly sorted, angular, conglomerates and breccia-conglomerates with clasts of igneous and metamorphic rocks (205/27a-1 and 205/25-1). Minor thin interbeds of grey to green or red to brown, slightly to non-calcareous, soft to hard, laminated, variably micaceous, silty mudstones and hard, light grey or white to red or brown, variably calcareous siltstones have also been described. |
| Definition of Lower Boundary: |
The base of the Foula Formation is represented by a downward transition from sandstones to Otter Bank Formation siltstones, mudstones or sandstones (205/27a-l) on the Solan Bank High and in the Møre Basin, or to Shoal High Formation sandstones and mudstones in the West Orkney Basin (202/19-1). On wireline logs, these changes are represented by a downward increase in gamma-ray log values but with less consistent velocity log responses. |
| Definition of Upper Boundary: |
The top of the Foula Formation is marked by an unconformity, with a downward transition from Rona Formation sandstones (e.g. 205/30-1) and Kimmeridge Clay Formation mudstones (e.g. 205/23-1) in the West Shetland, Solan and North Rona basins and on the North Rona High. Neptune Formation sandstones overlie the formation on the Westray Ridge (204/19-1) and Tertiary uplift in the West Orkney Basin and Solan Bank High area has resulted in the Foula Formation lying close to the sea bed beneath a thin cover of Quaternary sediments (205/27a-l and 202/19-1). On wireline logs, the boundary between the Kimmeridge Clay Formation and Foula Formation is represented by a downward decrease in gamma-ray log values and an increase in velocity, whereas for the Rona Formation, Neptune Formation and Quaternary, this change is generally marked by an increase in gamma-ray log values and a decrease in velocity. |
| Thickness: |
A maximum drilled thickness of 1417 m has been recorded from well 205/27a-l on the Solan Bank High. |
| Geographical Limits: |
The Foula Formation occurs in the North Rona, West Shetland and West Orkney basins, along the Westray Ridge and on the Solan Bank and North Rona highs, although the base has only been penetrated in wells 205/27a-1 and 202/19-1. |
| Parent Unit: |
Papa Group (PAPA)
|
| Previous Name(s): |
none recorded or not applicable
|
| Alternative Name(s): |
none recorded or not applicable
|
| Stratotypes: |
| Type Section |
North Sea well 205/27a- 1: 265-1682 m (869-5518 ft) (Ritchie et al., 1996). |
| Reference Section |
North Sea well 202/19- 1: 257-1159 m (843-3802 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. |
| 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.) |
| Booth, J, Swiecicki, T, and Wilcockson, P. 1993. The tectono-stratigraphy of the Solan Basin, west of Shetland. 987-998 in Petroleum Geology of Northwest Europe: Proceedings of the 4th Conference. Parker, J R (editor). (The Geological Society, London.) |
| Stoker, M S, Hitchen, K, and Graham, C C. 1993. United Kingdom offshore regional report: the geology of the Hebrides and West Shetland shelves and adjacent deep-water areas. (London: HMSO for the British Geological Survey.) |
| Steel, R J, and Smith, A C. 1975. Sedimentation and tectonism (?Permo-Triassic) on the margin of the North Minch Basin, Lewis. Journal of the Geological Society of London, 131, 183-202. |
| 1:50K maps on which the lithostratigraphical unit is found, and map code used: |
| none recorded or not applicable |
