| Computer Code: |
PRHA |
Preferred Map Code: |
notEntered |
| Status Code: |
Full |
| Age range: |
Anisian Age (TA)
— Anisian Age (TA) |
| Lithological Description: |
The Preesall Halite Formation comprises clean halite with thin partings of mudstone and siltstone, and scarce sulphate occurrences. It is the thickest proven, and probably the least argillaceous, halite within the Mercia Mudstone Group of the East Irish Sea. |
| Definition of Lower Boundary: |
The base of the Preesall Halite Formation is taken at the downward change from clean halite to the alternating red and green laminated silty mudstones and siltstones, commonly with thinly interbedded halitic mudstone and argillaceous halite, of the Cleveleys Mudstone Member (e.g. 110/7-1; 110/6b-1). In the far south of the basin, the Preesall Halite Formation directly overlies dominantly argillaceous rocks of unit L2 of the Leyland Formation. The base of the Preesall Halite Formation is not placed everywhere at the same stratigraphic level, because of the southward passage of the basal leaves of clean halite with mudstone partings into mudstone and siltstone with stringers of argillaceous halite and halitic mudstones, assigned here to the uppermost Leyland Formation.
On petrophysical logs, there is a downward change from a 'box-car' profile with low gamma values and high velocity in the clean halite to serrated traces with higher gamma and lower sonic values in the underlying mudstones (e.g. 110/7-1; 110/6b-l). Where the underlying Leyland Formation is silty or carbonate-rich, the base of the Preesall Halite Formation may be marked by only a small decrease on the velocity log (e.g. 110/7-2). |
| Definition of Upper Boundary: |
The top of the Preesall Halite Formation is generally marked by the abrupt downward change from red mudstones or a thin basal siltstone of the undivided Dowbridge Mudstone Formation to clean halite (e.g. 110/7-2; 110/13-8). The boundary is especially sharp in condensed sections (e.g. 110/2a-8). However, in a few wells that encountered expanded sequences adjacent to growth faults, a thick transitional interval of alternating halites and mudstones occurs above the clean halite (74 m thick in 110/7-1). Where the halites in this zone are thin and separated by thick mudstone interbeds, this sequence is assigned to the Dowbridge Mudstone Formation (constituting subdivision DM1). However, where these transitional halites are developed as thick, clean halite (e.g. 110/6b-l), they are included within the Preesall Halite Formation.
On wireline logs, the base of the Preesall Halite Formation everywhere is taken at the abrupt downward change to a 'box-car' profile of clean halite on both gamma-ray and sonic traces; this is clearly apparent even in wells where the DM1 transition zone is well developed (e.g. 110/7-1). |
| Thickness: |
In general, the Preesall Halite Formation thickens towards the north and the Keys Basin. However, it also shows the greatest change in thickness of any unit within the Mercia Mudstone Group, varying from thick successions (c. 600 m) deposited in active fault-bounded troughs and basinal areas to condensed successions (c. 100 m) on highs (especially the Morecambe and Calder fields astride the northeasterly extension of the Quadrant 109 Arch). For example, the thicknesss of 620 m in 110/3-2 contrasts markedly with that of 95 m in 110/3b-4 only 1.75 km away. |
| Geographical Limits: |
The Preesall Halite Formation is present throughout the East Irish Sea Basin, but has been removed by erosion from the basin perimeter, from prominent internal highs, such as the Deemster Platform and parts of the Morecambe Field, and from the footwall of major faults (e.g. Formby Point Fault, Jackson and Mulholland, 1993). In other parts of the Morecambe Field (e.g. 110/2-5) and commonly where the evaporites lie near the sea bed (e.g. 110/9-1), higher parts of the halite have been dissolved beneath 'wet' rockhead. In areas of complete solution of the Preesall Salt Member onshore, the unit is represented by mudstone collapse breccias and halite solution residues 40 m thick in south Cumbria (Rose and Dunham, 1977) and at least 45 m thick in west Lancashire (Wilson and Evans, 1990). Such occurrences are undoubtedly present offshore beneath the Quaternary, usually in zones of no returns and the uppermost interval of wireline logs recorded through thick casing (a possible example occurs in 110/8-2). |
| Parent Unit: |
Mercia Mudstone Group (MMG)
|
| Previous Name(s): |
none recorded or not applicable
|
| Alternative Name(s): |
Preesall Halite Member
|
| Stratotypes: |
| Type Area |
The Preesall Saltfield between Preesall and the River Wyre - as Preesall Halite Member (Wilson, 1993; cf. Wilson and Evans, 1990). |
| Reference Section |
Irish Sea well 110/07- 1: 551-879 m (1807-2883 ft) below KB (Jackson and Johnson, 1996). Key offshore section. |
| Reference Section |
Irish Sea well 110/07- 2: 531.5-743 m (1744-2438 ft) (Jackson and Johnson, 1996). |
| Reference(s): |
| Arthurton, R S. 1973. Experimentally produced halite compared with Triassic layered halite-rock from Cheshire, England. Sedimentology, 20, 145-160. |
| Benton, M J, Warrington, G, Newell, A J, and Spencer, P S. 1994. A review of the British Middle Triassic tetrapod assemblages. 131-160 in Fraser, N C, and Sues, H-D (editors) In the shadow of the dinosaurs. (Cambridge: University Press, Cambridge.) |
| Holmes, T V. 1899. The geology of the country around Carlisle. Memoir of the Geological Survey, Sheets 16 and 17, with parts of 12 18, 22 and 23 (England and Wales). |
| Jackson, D I, and Mulholland, P. 1993. Tectonic and stratigraphic aspects of the East Irish Sea Basin and adjacent areas: contrasts in their post-Carboniferous structural styles. In: Parker, J R (ed.) Petroleum Geology of Northwest Europe: Proceedings of the 4th Conference, 791-808. The Geological Society, London. |
| Jackson, D I, Jackson, A A, Evans, D, Wingfield, R T R, Barnes, R P, and Arthur, M J. 1995. United Kingdom offshore regional report: the geology of the Irish Sea. HMSO, London, for the British Geological Survey. |
| Rose, W C C, and Dunham, K C. 1977. Geology and hematite deposits of South Cumbria. Economic Memoir of the Geological Survey of Great Britain, Sheet 58, part 48 (England and Wales). |
| Warrington, G, and Ivimey-Cook, H C. 1992. Triassic. In: Cope, J C W, Ingham, J K, and Rawson, P F (eds.) Atlas of palaeogeography and lithofacies. Geological Society, London, Memoir No. 13, 97-106. |
| Wilson, A A. 1990. The Mercia Mudstone Group (Trias) of the East Irish Sea Basin. Proceedings of the Yorkshire Geological Society, Vol.48, 1-22. |
| Wilson, A A. 1993. The Mercia Mudstone Group (Trias) of the Cheshire Basin. Proceedings of the Yorkshire Geological Society, Vol.49, 171-188. |
| Wilson, A A and Evans, W B. 1990 Geology of the country around Blackpool. Memoir of the British Geological Survey, Sheet 66. (England and Wales). |
| Howard, A S, Warrington, G, Ambrose, K, and Rees, J G. 2008. A formational framework for the Mercia Mudstone Group (Triassic) of England and Wales. British Geological Survey Research Report, RR/08/04. |
| Jackson, D I and Johnson, H, 1996. Lithostratigraphic nomenclature of the Triassic, Permian and Carboniferous of the UK offshore East Irish Sea Basin, British Geological Survey, Nottingham. |
| Warrington, G, Audley-Charles, M G, Elliott, R E, Evans, W B, Ivimey-Cook, H C, Kent, P E, Robinson, P L, Shotton, F W and Taylor, F M. 1980. A correlation of the Triassic rocks in the British Isles. Special Report of the Geological Society of London, No.13. |
| 1:50K maps on which the lithostratigraphical unit is found, and map code used: |
| none recorded or not applicable |
