| Computer Code: |
DOMU |
Preferred Map Code: |
notEntered |
| Status Code: |
Full |
| Age range: |
Anisian Age (TA)
— Ladinian Age (TD) |
| Lithological Description: |
The Dowbridge Mudstone Formation comprises mudstone with interlaminated and interbedded siltstone and dolomitic siltstone, and scarce thin beds of dolomite, anhydrite, halite and sandstone. |
| Definition of Lower Boundary: |
The base of the Dowbridge Mudstone Formation is taken at the sharp downward change from mudstone (110/6b-l, 110/13-8) or siltstone (110/8a-5, 110/2a-8) to clean halite of the Preesall Halite Formation. On wireline logs, the boundary is taken at the very abrupt downward decrease in gamma values and corresponding downward increase in velocity (e.g. 110/2a-8, 110/8a-5). Where the basal part of the Dowbridge Mudstone Formation comprises thin halites, argillaceous halites and halitic mudstones (e.g. 110/7-1 and 110/3b-4) the boundary is again identified at the contact with clean more massive halite (7-15 API units and 67 jasecs/ft). The base of the Dowbridge Mudstone Formation is not placed everywhere at the same stratigraphic level because of local facies changes in the basal beds. For example, mudstones with minor siltstones and sandstones (e.g. 110/2a-8) pass laterally into alternating mudstones and thin halites (e.g. 110/3b-4), which in turn pass into clean halite with mudstone partings (assigned to the Preesall Halite Formation in this study; e.g. 110/6b-l). |
| Definition of Upper Boundary: |
The top of the Dowbridge Mudstone Formation is placed at the sharp downward change from clean halite of the Warton Halite Formation to siltstone (e.g. 110/2a-8, 110/8a-5) or mudstone (e.g. 110/13-8). In some wells, a transitional interval (identified as C, 50 m thick in 110/13-8) of interbedded mudstones, halitic mudstones and thin halites occurs directly beneath this clean halite, and is assigned to the Dowbridge Mudstone Formation. Elsewhere these thin halites appear to have been replaced by anhydrite (e.g. 110/2a-8). On wireline logs, the boundary is taken at the abrupt downward increase in gamma values and coincident downward decrease in velocity (e.g. 110/2a-8, 110/8a-5). |
| Thickness: |
In common with other units in the Mercia Mudstone Group, the Dowbridge Mudstone Formation is believed to thicken towards the Keys Basin depocentre. The formation reaches 383 m in 110/8a-5, but the thickest drilled succession (573.5 m) occurs in 113/27-2 where, however, the highest beds have been removed by erosion. The unit shows considerable thinning onto major highs (e.g. 242.5m in 110/2a-8 in the Morecambe Field). Detailed internal correlations of preserved sections suggest that the full thickness may exceed 1000 m in the Mercia Mudstone Group conjugate graben. |
| Geographical Limits: |
The Dowbridge Mudstone Formation occurs throughout the East Irish Sea, but subcrops the Quaternary extensively because of post-Early Jurassic erosion. The formation is absent from the present-day outer rim of the Mercia Mudstone Group outcrop, and in the north of the basin is missing from the main highs, except where preserved by local downfaulting (e.g. 110/2a-8). The present-day distribution is likely to be highly fragmented in detail, with, for example, sections preserved mostly in the Mercia Mudstone Group conjugate graben and also the centres of sub-basins (e.g. ?the North-East Deemster Basin). Similarly, in west Lancashire, the lateral equivalents are preserved only along synclinal axes, and in south Cumbria only as collapse strata above the Preesall Salt Member 'wet' rockhead. |
| 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 |
Irish Sea well 110/08a- 5: 274.5-657.5 m (900-2157 ft) below KB (Jackson and Johnson, 1996). |
| Reference Section |
Irish Sea well 110/02a- 8: 399.5-642 m (1310-2106 ft) (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. |
| 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). |
| Poole, E G, and Whiteman, A J. 1966. Geology of the country around Nantwich and Whitchurch. Memoir of the Geological Survey of Great Britain, Sheet 122 (England and Wales). |
| 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). |
| 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. |
| 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 |
