| Computer Code: |
STAH |
Preferred Map Code: |
notEntered |
| Status Code: |
Full |
| Age range: |
Anisian Age (TA)
— Anisian Age (TA) |
| Lithological Description: |
Two separate lithofacies associations are believed here to be present in the Stanah Member:
1. Basinal facies are penetrated in only two wells (i.e. 110/3-2; 110/2b-9) and are stratigraphically expanded compared to the 'swell' facies. In 110/3-2, the lithologies comprise dark to pale grey (locally red-brown at the top) dolomitic mudstone with subordinate siltstone, comparable in facies (but not exact stratigraphic position) with the Hambleton Mudstone Formation of west Lancashire (Wilson, 1990; Wilson and Evans, 1990). In 110/2b-9, thin halites occur in addition in the upper part, together with single anhydrite and dolomite beds (to 2 m) and grey sandstones in the lower part.
2. Stratigraphically condensed 'swell' facies of the Stanah Member are encountered in all the remaining wells, and comprise very thinly interbedded (average 1.5-3 m) anhydrites, dolomites and high gamma grey mudstones, but also locally include siltstones, sandstones and halites. |
| Definition of Lower Boundary: |
The base of the Stanah Member is defined at the downward change from mudstone, dolomite or anhydrite to the fluvial or aeolian sandstone of the Ormskirk Sandstone Formation. On wireline logs the base is generally marked by the abrupt downward decrease from high gamma mudstone (or less commonly argillaceous dolomite) to sandstone. However, there may be no shift on the gamma log where anhydrite forms the basal bed (e.g. 110/7-1). The sonic log response is varied and depends on the lithological contrast; a downward increase in velocity occurs where mudstones overlie fluvial sandstones (110/2-5, 113/27-3), but a downward decrease is encountered where mudstones or argillaceous dolomite overlie aeolian sandstones (110/2-3), or where anhydrite and/or dolomite (e.g. 110/7-1) overlie fluvial and aeolian sandstones. A slight downward decrease (110/3-2) or increase (110/2b-9) occurs where the basinal mudstones overlie the Ormskirk Sandstone Formation. |
| Definition of Upper Boundary: |
The top of the Stanah Member is generally marked by the abrupt downward change from clean halite of the Fylde Halite Member to a thin (argillaceous) dolomite (e.g. 110/2-3) or mudstone (110/2b-9) or anhydrite (110/7-1). However, in 112/25a-l mudstones of unit LI of the Leyland Formation directly overlie a 5m thick anhydrite marking the top of the Stanah Member.
On wireline logs, the boundary coincides with an abrupt downward increase in velocity where argillaceous dolomite and anhydrite comprise the highest bed (110/2-3; 110/2-5, 110/3b-4). A similar shift is produced at deeply buried mudstones (110/3-2) or siltstones, though less commonly there is no velocity change or a downward decrease in velocity where mudstone forms the highest bed (110/2b-9). The contact is marked by a sharp downward increase in gamma values where mudstones or argillaceous dolomites constitute the highest beds of the Stanah Member (e.g. 110/2-3). However, in some cases where anhydrite forms the highest bed (110/7-1, 110/8-2) there is no shift on the gamma log. |
| Thickness: |
The Stanah Member thins southwards and in 110/8-2 it is reduced to a thin anhydrite (possibly the topmost bed of the successions to the north). In 110/6b-1 and 110/8a-5, the Stanah Member is possibly represented by a very thin, high gamma, high velocity (?anhydritic) mudstone. The member varies in thickness from 43.5 m to 67.5 m in the 'basinal' facies, and ranges up to a maximum of 19.5 m (110/2-1) in the 'swell' facies. In the 'swell' facies over the present-day Morecambe high, there is limited evidence for westward thickening towards the Western Boundary Fault. Anhydrite-rich sequences are commonly thinner, reflecting onlap to the south (110/3-3, 110/8-2). All thicknesses may be tectonically reduced by the widespread basal detachment of the Mercia Mudstone Group (Jackson and Mulholland, 1993). |
| Geographical Limits: |
The Stanah Member is confined to the areas of most rapid subsidence in the central and northern parts of the East Irish Sea (Jackson and Mulholland, 1993, fig. 5). Equivalent strata have not been recorded from onshore areas (Wilson, 1990). |
| Parent Unit: |
Leyland Formation (LEMU)
|
| Previous Name(s): |
none recorded or not applicable
|
| Alternative Name(s): |
none recorded or not applicable
|
| Stratotypes: |
| Type Section |
Irish Sea well 110/02- 3: 928.5-942.5 m (3047-3092 ft) below KB (Jackson and Johnson, 1996). |
| Reference Section |
Irish Sea well 110/02b- 9: 1014-1057.5 m (3326-3469 ft) (Jackson and Johnson, 1996). |
| Reference(s): |
| 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. |
| 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.) |
| 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. |
| Lamplugh G W. 1903. The geology of the Isle of Man. Memoir Geological Survey of Great Britain. |
| 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 and Evans, W B. 1990 Geology of the country around Blackpool. Memoir of the British Geological Survey, Sheet 66. (England and Wales). |
| 1:50K maps on which the lithostratigraphical unit is found, and map code used: |
| none recorded or not applicable |
