| Computer Code: |
SHWT |
Preferred Map Code: |
notEntered |
| Status Code: |
Full |
| Age range: |
Guadalupian Epoch (PUG)
— Lopingian Epoch (PUL) |
| Lithological Description: |
The Shearwater Salt Formation, though dominated by halite, includes significant components of sulphate minerals (particularly anhydrite and polyhalite) and chlorides (sylvite and carnallite) in most wells. Where these minerals are concentrated into discrete intervals, they can be easily identified by their log responses. Such intervals vary from a few metres up to several tens of metres thick, but they cannot often be correlated between adjacent wells because of the regional halokinetic mobilization. A 120 m tachydrite section was recorded near the base of the salts in well 29/27-1. Taylor (1993) noted that this hydrated calcium-magnesium chloride mineral is rare in marine evaporites. Most likely, the tachydrite has a diagenetic origin, perhaps associated with hydrothermal activity related to known Jurassic vulcanicity in the area. |
| Definition of Lower Boundary: |
The Shearwater Salt Formation overlies the Turbot Anhydrite Formation in all areas of the Central North Sea, the base of the formation being defined by the lowest occurrence of salts. The boundary is not always sharp; some wells have encountered anhydrite interbedded with halite or polyhalite towards the base of the formation. In all cases, the boundary can be taken at the sonic-log break to the higher-velocity anhydrite below. In Beryl Embayment well 9/17-1A, the basal salts rest on dolomites of the Halibut Carbonate Formation. |
| Definition of Upper Boundary: |
The Shearwater Salt Formation is generally overlain by uppermost Permian anhydrites and local carbonates, forming the the Morag Member of the Turbot Anhydrite Formation, or it continues to the top of the Permian and is overlain by Triassic silty mudstones (Smith Bank Formation). It subcrops younger Triassic or post-Triassic rocks locally around salt diapirs. In all cases the top of the formation is defined by the downward incoming of salt. The boundary is marked by a sharp wireline-log break in all sections. |
| Thickness: |
Halokinetic mobilization and partial dissolution of the Upper Permian salt have modified primary thickness trends in almost all of the evaporite basin. Few wells have drilled through the base of the Shearwater Salt Formation; of those that have, well 29/23-1 has recorded the maximum thickness so far of 2142 m. In most other wells, the Shearwater Salt Formation is between 500 and 1000 m thick. |
| Geographical Limits: |
Though widespread across the centre of the Northern Permian Basin, the Shearwater Salt Formation is absent from the crests of the Forties-Montrose and Jaeren highs. The formation may be absent locally elsewhere due to dissolution or to salt withdrawal associated with halokinesis. It is mainly confined to the area south of 58° in the UK sector, but wells 9/17-1A and 9/27- 1 have proved an outlier of salt in the Beryl Embayment. The salt basin did not extend into the Moray Firth Basin, and the formation is absent from most of the South Halibut Basin. The Shearwater Salt Formation is also absent in the Auk-Argyll area, but an embayment of salts extends through southwest Quadrant 29 across the Mid North Sea High into the Southern Permian Basin (Jenyon et al., 1984). Seismic interpretation suggests that salts may once have extended beyond their present limits into parts of the Auk-Argyll area and over the crests of the intrabasinal highs. |
| Parent Unit: |
Zechstein Group (ZG)
|
| Previous Name(s): |
Shearwater Salt Formation [Obsolete Code: Use SHWT]
(SHES)
|
| Alternative Name(s): |
none recorded or not applicable
|
| Stratotypes: |
| Type Section |
North Sea well 21/11- 1: 2252-3106.5 m (7388-10192 ft) below KB (Cameron, 1993). |
| Reference Section |
North Sea well 9/17- 1A: 3481-3651.5 m (11420-11980 ft) (Cameron, 1993). |
| Reference Section |
North Sea well 29/27- 1: 1698.5-2779.5 m (5572-9119 ft) (Cameron, 1993). |
| Reference(s): |
| Rhys, G H. 1974. A proposed standard lithostratigraphic nomenclature for the southern North Sea and an outline structural nomenclature for the whole of the (UK) North Sea. Report of the Institute of Geological Sciences, 74/8. |
| Jenyon, M K, Cresswell, P M, and Taylor, J C M. 1984. Nature of the connection between the Northern and Southern Zechstein basins across the Mid North Sea High. Marine and Petroleum Geology 1, 355-363. |
| Taylor, J C M. 1990. Upper Permian-Zechstein. In: Glennie, K W (ed.) Introduction to the petroleum geology of the North Sea, 153-190. Blackwell Scientific Publications, Oxford. |
| Taylor, J C M. 1993. Pseudo-reefs beneath Zechstein salt on the northern flank of the Mid North Sea High. In: Parker, J R (ed.) Petroleum geology of Northwest Europe: Proceedings of the 4th Conference, 749-757. Geological Society, London. |
| 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.) |
| 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. |
| 1:50K maps on which the lithostratigraphical unit is found, and map code used: |
| none recorded or not applicable |
