| Computer Code: |
WAHA |
Preferred Map Code: |
notEntered |
| Status Code: |
Full |
| Age range: |
Ladinian Age (TD)
— Carnian Age (TC) |
| Lithological Description: |
The Warton Halite Formation consists mostly of clean halite with numerous mudstone partings of great lateral persistence which possess very abrupt upper and lower contacts with the halite; gypsum occurrences, probably chiefly as veins and patches, are distributed sparingly throughout both halite and mudstone. The halite is translucent or milky white to transparent, hard and brittle and coarsely to massively crystalline. The individual beds of halite commonly range up to 12 m in thickness (exceptionally 20 m); they may incorporate red clay blebs.
The mudstone partings average 3 m in thickness (individual maximum of 12 m in 110/13-8) and are red-brown or chocolate-brown in colour, with lesser pale green-grey or blue-grey colours; the mudstone is soft to firm, blocky to crumbly and slightly swelling, and locally the partings are variously slightly calcareous or dolomitic or anhydride or halitic. The thinner partings may comprise mostly halitic mudstone grading to argillaceous halite, and the mudstones may also pass laterally into very thin siltstones. |
| Definition of Lower Boundary: |
The base of the Warton Halite Formation is placed at the sharp downward change from clean halite to mudstone or siltstone, commonly with halitic mudstone and argillaceous halite, of the Dowbridge Mudstone Formation (e.g. 110/13-8, 110/8a-5). On wireline logs, the boundary is positioned at the sharp downward increase in gamma values and corresponding downward decrease in velocity occurring at the change from a 'box-car' to a more erratic wireline log signature. |
| Definition of Upper Boundary: |
The boundary between a complete succession of the clean halite of the Warton Halite Formation and the overlying red mudstones of the Elswick Mudstone Formation has not been drilled to date; the contacts in both 110/8a-5 and 110/13-8 are believed to be 'wet' rockhead resting on stratigraphically low halites within the Warton Halite Formation. Furthermore, the wireline log responses in available wells are affected by casing changes. On wireline logs, the boundary corresponds to the sharp downward decrease in gamma values and coincident downward increase in velocity. |
| Thickness: |
The Warton Halite Formation preserved beneath 'wet' rockhead ranges up to 269 m in thickness, but estimates from seismic data and regional stratigraphic comparisons with the centre of the Keys Basin, where the full thickness of Mercia Mudstone Group is preserved, suggest that the complete sequence may reach 1000 m. The formation thins southwards towards the Llyn-Rossendale Ridge, possibly to as little as 300 m. In the Cheshire Basin, the equivalent Wilkesley Halite Formation has a maximum proven thickness of 464 m in Prees 1. The collapse breccias from the lower part of the equivalent halite in west Lancashire reach a maximum thickness of 65 m (Wilson and Evans, 1990). |
| Geographical Limits: |
Although deposited throughout the East Irish Sea, the Warton Halite Formation is now preserved only at depth in outliers of the individual sub-basins, both in the north (e.g. the Keys Basin, and the Mercia Mudstone Group conjugate graben) and in the south (e.g. the Gogarth Basin and Berw Basin, Jackson and Mulholland, 1993). Elsewhere the unit has been removed by post-Early Jurassic erosion (e.g. over large parts of the Morecambe Field). |
| Parent Unit: |
Mercia Mudstone Group (MMG)
|
| Previous Name(s): |
none recorded or not applicable
|
| Alternative Name(s): |
none recorded or not applicable
|
| Stratotypes: |
| Type Section |
Provisional: Irish Sea well 110/13- 8: 142.5-411.5 m (468-1350 ft) below KB. This section represents the lower part of formation only, because the upper Warton Halite Formation has been dissolved beneath 'wet' rockhead (Jackson and Johnson, 1996). |
| Reference Section |
Provisional: Irish Sea well 110/08a- 5: ?73.5- or 162.5-274.5 m (7241- or 533-900 ft) but comprises basal section only; remainder dissolved beneath 'wet' rockhead (Jackson and Johnson, 1996). |
| Reference(s): |
| Arthurton, R S. 1980. Rhythmic sedimentary sequences in the Triassic Marl (Mercia Mudstone Group) of Cheshire, northwest England. Geological Journal, 15, 43-58. |
| 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. |
| 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. |
| 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). |
| Arthurton, R S. 1973. Experimentally produced halite compared with Triassic layered halite-rock from Cheshire, England. Sedimentology, 20, 145-160. |
| 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 and Johnson, H, 1996. Lithostratigraphic nomenclature of the Triassic, Permian and Carboniferous of the UK offshore East Irish Sea Basin, British Geological Survey, Nottingham. |
| 1:50K maps on which the lithostratigraphical unit is found, and map code used: |
| none recorded or not applicable |
