| Computer Code: |
FYHA |
Preferred Map Code: |
notEntered |
| Status Code: |
Full |
| Age range: |
Anisian Age (TA)
— Anisian Age (TA) |
| Lithological Description: |
The Fylde Halite Member comprises leaves of halite with subordinate intercalated mudstone and siltstone, and very scarce thin anhydrite and sandstone beds. Much of the halite is clear to translucent, brittle and crystalline, though a significant component is also white, and cloudy to opaque. It may be stained orange near mudstones and locally contains patches and veins of white soft amorphous anhydrite. |
| Definition of Lower Boundary: |
The base of the Fylde Halite Member is marked by the sharp downward change from clean halite to the thinly interbedded mudstones, dolomites and anhydrites of the Stanah Member or, near the pinchout of the Stanah Member, at the change to the Ormskirk Sandstone Formation. In the north, the basal halite is generally the cleanest and thickest leaf within the Fylde Halite Member; it is not always the lowest halite in the Mercia Mudstone Group, since thin halites (<3 m thick) are present locally in the Stanah Member (e.g. 110/2b-9).
On wireline logs, the boundary is most clearly identified by the sharp downward increase in gamma values (e.g. 110/2-5). The change on the sonic log is more varied and shows an abrupt downward increase in velocity where anhydrite (110/3-1, 113/27-1), deeply buried mudstone (e.g. 110/3-2) or siltstone, dolomitic siltstone (e.g. 113/27-3), and especially dolomite (113/26-1; 110/2-3) comprise the topmost beds of the Stanah Member. There is commonly no shift or a slight downward decrease in velocity where siltstone or mudstone comprise the topmost beds (e.g. 110/2b-9). |
| Definition of Upper Boundary: |
The top of the Fylde Halite Member is taken at the downward change from the mudstones of the Ansdell Mudstone Member to the highest clean or argillaceous Fylde halite (e.g. 113/26-1). The upper boundary is not taken at the top of the same halite leaf everywhere, since the thin uppermost halites pass laterally and sequentially southwards to halitic mudstones and dolomitic siltstones. The top of a thin (minimum 3 m) but persistent halite is taken as the boundary in the Morecambe Field area (e.g. 110/2-5, 113/26-1). To the south, the boundary is taken at the top of progressively lower halites.
On wireline logs, the top is placed at the downward increase in velocity and coincident downward decrease in gamma values reflecting the change to clean halite (e.g. 110/2-5, 113/26-1). However, there may be a small downward decrease in velocity where the mudstones have been baked by Tertiary intrusions (e.g. 113/27-1). |
| Thickness: |
The Fylde Halite Member thickens towards the Keys Basin depocentre, and in offshore wells ranges up to 182.5 m in 113/26-1. The thickening on the northern flanks of, and to the north of, the Morecambe Field is especially marked. |
| Geographical Limits: |
The Fylde Halite Member is confined to the northern and central parts of the East Irish Sea, but is absent from 112/25a-l in the extreme north. Although the area is undrilled, it is likely that the Fylde Halite Member is also absent from the Eubonia Basin. The Fylde Halite Member pinches out abruptly between 110/6b-1 and 110/8a-5. Towards the basin margins and especially south of the Calder Field, there is progressive loss of the basal halite leaf from the base upwards. This is interpreted here as overlap across the Stanah Member onto the Ormskirk Sandstone Formation (e.g. 110/6b-l). Concomitantly, the highest thin (remaining) halites of the member pass abruptly and sequentially from the top downwards, into dolomitic siltstones of unit LI of the Leyland Formation, as demonstrated by a comparison of the gamma log responses in 110/8a-5 and 110/6b-1. By analogy with onshore areas, the halites may also grade laterally into penecontemporaneous Triassic breccias (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- 5: 885-985 m (2904-3231 ft) below KB (Jackson and Johnson, 1996). |
| Reference Section |
Irish Sea well 113/26- 1: 1060-1243.5 m (3478-4080 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. |
| 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.) |
| 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). |
| Ireland, R J, Pollard, J E, Steel, R J, and Thompson, D B. 1978. Intertidal sediments and trace fossils from the Waterstones (Scythian-Anisian?) at Daresbury, Cheshire. Proceedings of the Yorkshire Geological Society, 41, 399-436. |
| 1:50K maps on which the lithostratigraphical unit is found, and map code used: |
| none recorded or not applicable |
