| Computer Code: |
BLMU |
Preferred Map Code: |
notEntered |
| Status Code: |
Full |
| Age range: |
Anisian Age (TA)
— Anisian Age (TA) |
| Lithological Description: |
The Blackpool Mudstone Member comprises silty, micaceous mudstone locally interbedded with and grading to thin siltstone; minor halite, anhydrite, sandstone and dolomite occur in addition. |
| Definition of Lower Boundary: |
The base of the Blackpool Mudstone Member is taken at the abrupt downward change from mudstone or a thin, widespread, basal siltstone to relatively clean halite of the Rossall Halite Member (e.g. 110/2-6). The base of the Blackpool Mudstone Member appears to remain at an approximately constant stratigraphic level across the basin.
On wireline logs, the boundary is taken at the downward decrease in gamma values and the coincident downward increase in velocity to clean halite (110/2-6; 110/12a-l). The boundary is sharper in the north, where the velocity shift is also larger. Where the contact lies at considerable depths (i.e. more than 1800 m), there may be a slight downward decrease in velocity (cf. 110/3-2), in which case the gamma log is more suitable for locating the boundary. |
| Definition of Upper Boundary: |
The top of the Blackpool Mudstone Member is taken at the generally sharp downward change from clean or argillaceous halite of the Mythop Halite Member to mudstone or siltstone (e.g. 110/2-6, 110/12a-l). However, it is taken locally at the boundary between halite and a transitional interval of mudstone and siltstone containing thin beds of halitic mudstone or argillaceous halite (e.g. 110/3b-4). On wireline logs, the boundary is taken at the commonly abrupt downward increase in gamma values and downward decrease in velocity. |
| Thickness: |
The Blackpool Mudstone Member shows a regional thickening towards the Keys Basin depocentre, together with more local expansion in the hangingwall of major growth faults (e.g. 110/3-2). In available wells, the thickness varies from 99 m (110/9-1) near the pinchouts of the bounding halites, to over 265 m (110/3-2, faulted section). |
| Geographical Limits: |
The Blackpool Mudstone Member can be differentiated only in the northern and central parts of the East Irish Sea basin, where it is confined between the Rossall and Mythop halite members. Elsewhere, the equivalent of the Blackpool Mudstone Member is subsumed within the undivided Leyland Formation in offshore areas, though the consistent gamma values and uniform mudstone lithology enable equivalent strata to be recognized locally in halite-free sections of the Leyland Formation (e.g. 110/20-1). |
| 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- 6: 490-684.5 m (1607-2245 ft) below KB (Jackson and Johnson, 1996). |
| Reference Section |
Irish Sea well 110/12a- 1: 505.5-629.5 m (1658-2065 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.) |
| Earp, J R, and Taylor, B J. 1986. Geology of the country around Chester and Winsford. Memoir of the British Geological Survey, Sheet 109 (England and Wales). |
| 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 |
