| Computer Code: |
ELMU |
Preferred Map Code: |
notEntered |
| Status Code: |
Full |
| Age range: |
Carnian Age (TC)
— Norian Age (TN) |
| Lithological Description: |
In 110/13-8, and by analogy with the Brooks Mill Mudstone Formation of the Cheshire Basin, the Elswick Mudstone Formation consists dominantly of structureless chocolate-coloured mudstone overlying red mudstone with lesser green-grey intervals (Wilson, 1993; Poole and Whiteman, 1966). The proportion of anhydrite, as irregular nodules, porphyroblasts and a significant bedded deposit (6.4 m thick in the Wilkesley Borehole), is the highest within the Mercia Mudstone Group. Very thin interbedded siltstone and sandstone occur sporadically. |
| Definition of Lower Boundary: |
The base of the Elswick Mudstone Formation is taken at the downward change from red mudstones to clean halite of the Warton Halite Formation (110/13-8). The contact in this well is probably a 'wet' rockhead on stratigraphically low halites within the Warton Halite Formation; however such contacts are expected to be the norm offshore. On wireline logs the contact is marked by a sharp downward decrease in gamma values (Penn, 1987; Wilkesley Borehole, Evans et al., 1993), and a smaller downward shift to higher velocities. The wireline log responses for 110/13-8 are affected by thick casing and are not truly representative. |
| Definition of Upper Boundary: |
By analogy with the Cheshire Basin, the top of the Elswick Mudstone Formation is placed at the downward change from the pale green and pale grey mudstones and dolomitic siltstones of the Blue Anchor Formation to the red mudstones of the Elswick Mudstone Formation. On wireline logs, the boundary is marked by a small but noticeable downward increase in gamma values (Penn, 1987). |
| Thickness: |
The Elswick Mudstone Formation is likely to thicken towards the Keys Basin depocentre, where the full thickness of Mercia Mudstone Group (3025 m) is preserved. Estimates of thickness from seismic data indicate a possible maximum of 800 m there, thinning to an estimated minimum of 200 m adjacent to the Llyn-Rossendale Ridge in the south-east; the formation (believed to be collapse strata above a 'wet' rockhead) exceeds 69.5 m in 110/13-8, which provides the only available section. It is likely that the upper part of the collapse strata comprises Quaternary deposits, since these have not been identified in all wells, and the till is likely to be argillaceous and lithologically similar to the mudstones of the Mercia Mudstone Group. |
| Geographical Limits: |
The Elswick Mudstone Formation was deposited throughout the East Irish Sea but has been preserved from post-Lower Jurassic erosion only in the Mercia Mudstone Group conjugate graben and in the more deeply buried central areas of the internal sub-basins. The present-day distribution comprises isolated remnants of this former continuous cover and undoubtedly is more complicated than shown here. |
| Parent Unit: |
Mercia Mudstone Group (MMG)
|
| Previous Name(s): |
none recorded or not applicable
|
| Alternative Name(s): |
none recorded or not applicable
|
| Stratotypes: |
| Reference Section |
Provisional: Irish Sea well 110/13- 8: 73-142.5 m (240-468 ft) below KB (Jackson and Johnson, 1996). |
| Reference(s): |
| Evans, D J, Rees, J G, and Holloway, S. 1993. The Permian to Jurassic stratigraphy and structural evolution of the central Cheshire Basin. Journal of the Geological Society of London, Vol. 150, 857-870. |
| 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). |
| Taylor, B J, Price, R H and Trotter, F M. 1963. Geology of the country around Stockport and Knutsford. Memoir of the Geological Survey of Great Britain. |
| 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). |
| Arthurton, R S. 1980. Rhythmic sedimentary sequences in the Triassic Marl (Mercia Mudstone Group) of Cheshire, northwest England. Geological Journal, 15, 43-58. |
| Penn, I.E. 1987. Geophysical logs in the stratigraphy of Wales and adjacent offshore and onshore areas. Proceedings of the Geologists' Association, 98, 275-314. |
| 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 |
