| Computer Code: |
ANMU |
Preferred Map Code: |
notEntered |
| Status Code: |
Full |
| Age range: |
Anisian Age (TA)
— Anisian Age (TA) |
| Lithological Description: |
The Ansdell Mudstone Member comprises mudstone locally interlaminated with minor siltstone and thin, scarce halite. Thin, sporadic sandstones, anhydrites and dolomites also occur. |
| Definition of Lower Boundary: |
In the northern and central East Irish Sea, the base of the Ansdell Mudstone Member is taken at the sharp downward change from mudstone to the bed of relatively clean halite that marks the top of the Fylde Halite Member (e.g. 110/2-6). This particular halite bed passes southwards, and on crestal locations (e.g. 110/2-3), into dolomitic siltstone or fine grained sandstone (e.g. 110/6b-l). In these areas the base of the member is taken at a lower stratigraphic level, but again at the highest developed clean Fylde halite (e.g. 110/6b-l).
On wireline logs, the lower boundary is marked by a downward decrease in gamma values and downward increase in average velocity into clean halite of the Fylde Halite Member. Again there may be no downward increase in velocity in deeply-buried sections, or where the mudstone has been baked by Tertiary igneous intrusions (e.g. 113/27-1). |
| Definition of Upper Boundary: |
The upper boundary is taken at the downward change from the lowest clean halite or argillaceous halite of the Rossall Halite Member to siltstone or mudstone, locally with thin beds of halitic mudstone or very argillaceous halite. The boundary is most easy to identify towards the south (e.g. 110/6b-l), where the contact is clean halite on silty mudstone, and is most difficult to place in the Morecambe Field area (e.g. 110/2-6), where upward-coarsening siltstones with thin interbedded halitic mudstones comprise the highest beds of the Ansdell Mudstone Member.
On wireline logs in the south, the boundary is taken at the abrupt downward increase in gamma values and the slight downward decrease in sonic velocity. In deeply buried sections or where the mudstones have been indurated by Tertiary intrusions (e.g. 113/27-1), there is an overall downward increase in velocity. In the north and centre, the boundary is taken at the downward change from slightly argillaceous (20 API units), high velocity halite to higher gamma and lower velocity siltstones and mudstones of the Ansdell Mudstone Member. |
| Thickness: |
The Ansdell Mudstone Member thickens towards the Keys Basin depocentre. Drilled thicknesses range from 41.5 m (110/3-1) to 82 m (113/27-1). The anomalous 157 m thickness in 110/2-4A arises from the steep (70°) dip in this well (Cowan et al., 1993). The increase in thickness at 110/6b-l, compared to 110/3b-4, arises because the Ansdell Mudstone Member of the former well includes argillaceous beds laterally equivalent to the uppermost part of the Fylde Halite Member, beyond the feather edge of the highest bed of halite in the north and centre. |
| Geographical Limits: |
The beds are restricted to the north and centre of the East Irish Sea. The dominantly red mudstones of the Ansdell Mudstone Member cannot be differentiated beyond the feather edge of the Fylde Halite Member, and they pass laterally to the south and east into the dominantly grey mudstones in the upper part of unit LI of the Leyland Formation. The most southerly available well section is 110/6b-l. In 112/25a-l, a package of red-brown mudstones overlying grey mudstones, which in turn rest directly on the Stanah Member (rather than the Fylde Halite Member) have been assigned provisionally to unit LI of the Leyland Formation, rather than to the Ansdell Mudstone Member. |
| 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: 772-823.5 m (2533-2702 ft) below KB (Jackson and Johnson, 1996). |
| Reference Section |
Irish Sea well 110/06b- 1: 1215.5-1274.5 m (3988-4181 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). |
| 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 and Evans, W B. 1990 Geology of the country around Blackpool. Memoir of the British Geological Survey, Sheet 66. (England and Wales). |
| Cowan, G, Ottesen, C, and Stuart, I A. 1993. The use of dipmeter logs in the structural interpretation and palaeocurrent analysis of Morecambe Fields, East Irish Sea Basin. 867-882 in Petroleum Geology of Northwest Europe: Proceedings of the 4th Conference. Parker, J R (editor). (London: The Geological Society.) |
| 1:50K maps on which the lithostratigraphical unit is found, and map code used: |
| none recorded or not applicable |
