| Computer Code: |
SMBA |
Preferred Map Code: |
notEntered |
| Status Code: |
Full |
| Age range: |
Early Triassic Epoch (TE)
— Anisian Age (TA) |
| Lithological Description: |
The Smith Bank Formation is composed dominantly of mudstones with minor siltstones, but includes a few thin beds of sandstone and occasional partings or nodules of carbonate or anhydrite in most sections. The mudstones generate a characteristically monotonous suite of wireline-log responses. Neutron/density log separation is generally wide, but tapers towards the top of the formation in the south Central Graben (e.g. 30/7a-4A) (Goldsmith et al., 1995).
A localized sandstone unit is present near the top of the formation in Block 30/7, for which the informal term Smith Bank Sandstone unit has been proposed. |
| Definition of Lower Boundary: |
The Smith Bank Formation rests variously on carbonate, anhydrite, evaporite, or clastics-dominated Upper Permian formations, or locally oversteps these to lie on older Paleozoic strata. In all situations the base of the formation is defined by the base of the monotonous mudstone succession, and is marked by a clearly defined wireline-log break. |
| Definition of Upper Boundary: |
The Smith Bank Formation is overlain either conformably by the sandstones of the Skagerrak and Lossiehead formations, or unconformably by Middle Jurassic or younger sediments beneath a regional mid-Jurassic unconformity. Where overlain by Triassic sandstones, the top of the formation is normally defined by a sharp downward change to mudstones, marked by a sharp downward increase in gamma-ray values (e.g. 29/19-1 A). However, in the south Central Graben, the boundary is taken at the base of the Judy Sandstone Member, a unit of alternating sandstones and mudstones. |
| Thickness: |
Depositional thickness variation was greatest in the Central North Sea, as a result of synsedimentary Triassic halokinesis of the underlying Upper Permian evaporites. Upland areas developed above Permian salt walls and diapirs, with depocentres being created in the contemporary salt-withdrawal basins (Johnson et al., 1986; Fisher and Mudge, 1990; Hodgson et al., 1992). Many wells have penetrated more than 500 m of mudstones in such basins, and seismic interpretation suggests the Triassic sediments are locally well over 1000 m thick. By contrast, the Smith Bank Formation is thin or absent above the largest Permian diapirs, although this may in part reflect post-Triassic erosion.
Beyond the limits of the Permian salt basin and of the Skagerrak Formation, thickness variation of the Smith Bank Formation principally reflects the degree of preservation beneath the regional base-Jurassic unconformity. Few wells have encountered more than 300 m of Triassic mudstones in the Outer Moray Firth or South Viking Graben, but they are 599 m thick in well 15/26-1, and 960 m thick in well 20/9-3 in the South Halibut Basin. |
| Geographical Limits: |
The Smith Bank Formation is widespread in the Central North Sea, the South Viking Graben and the Outer Moray Firth, and an outlier has been proved on the western flank of the Halibut Horst. |
| Parent Unit: |
Heron Group (HERO)
|
| Previous Name(s): |
none recorded or not applicable
|
| Alternative Name(s): |
none recorded or not applicable
|
| Stratotypes: |
| Type Section |
North Sea well 15/26- 1 (Deegan and Scull, 1977, p.8, fig. 12): 2488-3087 m (8163-10128 ft) below KB. |
| Reference Section |
North Sea well 30/07a- 4A: 3737-4123 m TD (12260-13527 ft TD) (Cameron, 1993). |
| Reference Section |
North Sea well 30/17a- 4: 3167-3435 m (10390-11270 ft) (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. |
| 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.) |
| Deegan, C E and Scull, B J. 1977. A standard lithostratigraphic nomenclature for the Central and Northern North Sea. Report of the Institute of Geological Sciences, 77/25; NPD Bulletin No.1. |
| Fisher, M J, and Mudge, D C. 1990. Triassic. In: Glennie, K W (ed.) Introduction to the petroleum geology of the North Sea, 191-218. Blackwell Scientific Publications, Oxford. |
| Glennie, K W, and Armstrong, L A. 1991. The Kittiwake Field, Block 21/18, UK North Sea. In: Abbotts, I L (ed.) United Kingdom oil and gas fields 25 years commemorative volume. Geological Society, London, Memoir No. 14, 339-345. |
| Goldsmith, P J, Rich, B, and Standring, J. 1995. Triassic correlation and stratigraphy in the South Central Graben, United Kingdom North Sea. In: Boldy, S A R (ed.) Permian and Triassic rifting in NW Europe. Geological Society, London, Special Publication. |
| Hodgson, N A, Farnsworth, J, and Fraser, A J. 1992. Salt-related tectonics, sedimentation and hydrocarbon plays in the Central Graben, North Sea, UKCS. In: Hardman, R F P (ed.) Exploration Britain: Geological insights for the next decade. Geological Society, London, Special Publication No.67, 31-63. |
| Johnson, H D, Mackay, T A and Stewart, D J. 1986. The Fulmar Oilfield (Central North Sea): geological aspects of its discovery, appraisal and development. Marine & Petroleum Geology, 3, 99-125. |
| 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 |
