| Computer Code: |
CLMU |
Preferred Map Code: |
notEntered |
| Status Code: |
Full |
| Age range: |
Anisian Age (TA)
— Anisian Age (TA) |
| Lithological Description: |
The Cleveleys Mudstone Member mostly comprises interlaminated green and red colour-banded silty mudstone and siltstone. Three main lithofacies have been identified within the basin:
1. In the north (e.g. 113/27-3) and in the Mercia Mudstone Group conjugate graben (e.g. 110/3-2), successions largely comprise red dolomitic interlaminated mudstones and siltstones, but with significant halites and argillaceous halites, both as thinly interbedded stringers and as thicker units.
2. In condensed successions over the Morecambe and Calder fields, the member consists largely of red mudstones (e.g. 110/3b-4, 110/2-5).
3. In the south, successions mostly consist of interlaminated siltstones and mudstones with thin sandstones and beds of sandy dolomite and sandy limestone (e.g. 110/9-1; 110/13-8). |
| Definition of Lower Boundary: |
The base of the Cleveleys Mudstone Member is taken at the downward change from higher gamma, lower velocity siltstone (especially in the south) or mudstone to the low gamma, high velocity, relatively clean halite of the Mythop Halite Member (e.g. 110/3-2, 110/9-1; 110/13-1; 110/12a-1). The boundary is not taken at the same stratigraphic horizon across the basin, but is chosen at progressively lower levels to the south, because the highest individual leaves of Mythop halite become thinner and pass laterally into halitic mudstones (e.g. 110/6b-1) and eventually into thin dolomitic siltstones (e.g. 110/13-8) and sandstones (e.g. 110/13-1) of the Cleveleys Mudstone Member.
The lower boundary is easier to recognize in thinner sequences and in the south (e.g. 110/9-1), where the internal halites of the Cleveleys Mudstone Member are thin or absent. Confusion over the precise recognition of the base of the Cleveleys Mudstone Member may arise in the north, where the lowest paired halite of subdivision CM1 lies close to the highest halite of the Mythop Halite Member, though the separation amounts to at least 20 m in available wells (cf. 110/3-2). |
| Definition of Upper Boundary: |
The top of the Cleveleys Mudstone Member is taken at the downward change from the lowest clean halite of the Preesall Halite Formation to mudstone or siltstone, commonly with thinly interbedded halitic mudstone or argillaceous halite (e.g. 110/2-5, 110/6b-1). The boundary is not taken at exactly the same stratigraphic horizon across the basin, but is chosen at progressively slightly higher levels to the south, because the lowest leaves of clean halite in the Preesall Halite Formation become thinner southwards and pass laterally into halitic mudstones (e.g. 110/6b-1) that are assigned to the Cleveleys Mudstone Member.
On wireline logs, the upper boundary is taken at the downward change from the "box-car" profile of clean halite in the Preesall Halite Formation to the more serrate motif with higher gamma and lower velocity in the Cleveleys Mudstone Member (e.g. 110/2-5; 110/3b-4; 110/13-8). |
| Thickness: |
Drilled sections of the Cleveleys Mudstone Member are commonly faulted and consequently do not demonstrate the full thickness of the unit (e.g. 110/2-5; 110/6b-l, 110/8a-5). The member shows a regional thickening towards the Keys Basin depocentre together with more local thickening in the hangingwall of major growth faults (e.g. 110/3-2). In unfaulted sections which are thought to be complete, the thickness varies from 117 m in 110/3-1 to 503 m (110/3-2). Stratigraphically condensed sequences were deposited over and around the Calder and Morecambe fields (e.g. 110/2-5, 110/3b-4), attributed here to the initiation of reduced subsidence rates over the continuation northeastwards of the Quadrant 109 Arch. |
| Geographical Limits: |
Over most of the offshore East Irish Sea Basin, the Cleveleys Mudstone Member is the only thick mudstone member containing a thick near-median halite. The Cleveleys Mudstone Member can be distinguished separately only where the underlying Mythop Halite Member is developed; beyond the pinch-out of the Mythop Halite Member equivalent strata are included within the undivided Leyland Formation. |
| 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 110/03- 2: 930-1432.5 m (3051-4700 ft) below KB (Jackson and Johnson, 1996). |
| Reference Section |
Irish Sea well 110/09- 1: 413-551 m (1355-1808 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. 1980. Rhythmic sedimentary sequences in the Triassic Marl (Mercia Mudstone Group) of Cheshire, northwest England. Geological Journal, 15, 43-58. |
| 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.) |
| 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. 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). |
| 1:50K maps on which the lithostratigraphical unit is found, and map code used: |
| none recorded or not applicable |
