| Computer Code: |
LEMU |
Preferred Map Code: |
notEntered |
| Status Code: |
Full |
| Age range: |
Anisian Age (TA)
— Anisian Age (TA) |
| Lithological Description: |
In areas where it can be formally subdivided into members, the Leyland Formation comprises thick units of dominantly mudstone with interlaminated siltstone, alternating with thick units of dominantly halite. The halite members pinch out towards the basin margins. Sandstone, anhydrite, dolomite and limestone comprise sporadic and thin individual beds. The mudstone is dominantly red-brown and structureless, but includes grey-green intervals especially in the Cleveleys Mudstone Member. It is more silty in the south. The siltstone, which shows a similar colour range, is both thinly interlaminated and more thickly interbedded with the mudstone and increases in overall proportion and individual bed thickness towards the basin margins and the south. The sandstone occurs in thin beds (generally less than 2 m) and is very fine-grained. The halite is generally clear and colourless where pure, but in the south and towards the basin margins becomes contaminated by incorporated argillaceous sediment and eventually passes into halitic mudstones. |
| Definition of Lower Boundary: |
Lithological contrasts across the basal junction of the Leyland Formation show changes across the basin from north to south. In the north and centre, the lower boundary is generally taken at the downward change from the thinly interbedded high gamma mudstones, and high velocity dolomites and anhydrites of the Stanah Member to the lower gamma fluvial sandstones (unit OS2) of the Ormskirk Sandstone Formation (e.g. 110/2-5). However, beyond the pinch-out of the Stanah Member, the boundary is marked by the downward change from the Fylde Halite Member to the Ormskirk Sandstone Formation. In the south, the boundary is coincident with the downward change from very high gamma grey mudstones of unit LI of the Leyland Formation to the low gamma aeolian sandstones of the Ormskirk Sandstone Formation (unit OS2b, cf. 110/13-1). |
| Definition of Upper Boundary: |
The top of the Leyland Formation is taken at the downward change from the low gamma, high velocity clean halite of the Preesall Halite Formation to argillaceous sediments. Over large areas of the East Irish Sea, the contact is with the higher gamma, lower velocity mudstones or the thinly interbedded mudstones, halitic mudstones, argillaceous halites and siltstones of the Cleveleys Mudstone Member (e.g. 110/7-1; 110/6b-1). In the south, where the Leyland Formation is not subdivided into members, the upper boundary is taken at the downward change from clean halite of the Preesall Halite Formation to siltstones and mudstones of unit L2; no available offshore well shows this type of contact in the East Irish Sea.
On wireline logs, the boundary is taken at the downward increase in gamma values and downward decrease in velocity from the relatively linear wireline log traces of the Preesall Halite Formation to the more ragged and serrate signature of the Cleveleys Mudstone Member (e.g. 110/2-5; 110/7-1) or to unit L2. A downward increase in velocity is observed in deeply buried sections. |
| Thickness: |
The Leyland Formation steadily thickens towards the Keys Basin depocentre. This trend is broken over the Morecambe Field because of a reduction in late Triassic subsidence rates over the northeastwards continuation of the Quadrant 109 Arch. The thickness ranges from an estimated 250 m in the southeast, against the Llyn-Rossendale Ridge, to an estimated 1100-1200 m in the Keys Basin. There is also a probable reduction in thickness north of the Keys Basin towards the Ramsey-Whitehaven Ridge, as shown by 112/25a-1. Many drilled sections of the Leyland Formation intersect the bounding fault to the Ormskirk Sandstone tilt-block target (e.g. 110/2a-8; 110/8a-5), and unfaulted well sections are scarce. The equivalent of the Leyland Formation averages 400 m in thickness along the Fylde coast. |
| Geographical Limits: |
The Leyland Formation occurs throughout the East Irish Sea, except where it has been eroded from the major intrabasinal highs of the Deemster Platform, the Ogham Inlier and surrounds, and the Ribble Estuary Inlier. The formation has also been partly removed by erosion from the outer limit of the Mercia Mudstone Group outcrop, from the perimeter of some internal sub-basins and from the crest of the larger anticlines and footwall tilt blocks (e.g. the Morecambe Field). Strata extend onshore as equivalents into south Cumbria and west Lancashire. |
| 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 110/02- 5: 408-1002 m (1339-3287 ft) below KB (Jackson and Johnson, 1996). |
| Reference Section |
Provisional: Irish Sea well 110/13- 1: c. 377.5-764.5 m (1238-2509 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.) |
| 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 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 |
