| Computer Code: |
BAMU |
Preferred Map Code: |
notEntered |
| Status Code: |
Full |
| Age range: |
Lopingian Epoch (PUL)
— Lopingian Epoch (PUL) |
| Lithological Description: |
The Barrowmouth Mudstone Formation is composed of mudstone and anhydride mudstone, interbedded with, and passing laterally to, siltstone; coarser-grained intervals commonly comprise upward-fining cycles. Minor anhydrite, sandstone and dolomite are also present. |
| Definition of Lower Boundary: |
The base of the Barrowmouth Mudstone Formation is taken in undisturbed sections at the sharp downward change from siltstone or mudstone of the Barrowmouth Mudstone Formation to evaporite of the St Bees Evaporites Formation. The uppermost evaporite is commonly clean halite (e.g. 112/25a-l, 110/3b-4), but locally a thin, (c. 1 m) argillaceous anhydrite directly underlies the Barrowmouth Mudstone Formation (e.g. 113/27-3, 110/2b-9). On wireline logs, the boundary lies at or directly above a pronounced downward decrease in gamma values. Where the Barrowmouth Mudstone Formation is directly underlain by anhydrite, the contact coincides with a small downward increase in velocity (e.g. 113/27-3, 110/2b-9), but where directly underlain by halite there is a downward decrease in velocity (e.g. 112/25a-l), which in some cases (e.g. 110/3b-4) may be large.
The boundary is probably widely affected by halite dissolution of the underlying St Bees Evaporites Formation (e.g. 110/2b-9; 113/26-1) or by a fault detachment on top of the halite (cf. Jackson and Mulholland, 1993). These anomalous sections may also show the overall downward increase in velocity at the base of the Barrowmouth Mudstone Formation (e.g. 113/26-1; 110/2b-10). |
| Definition of Upper Boundary: |
The top of the Barrowmouth Mudstone Formation is taken at the sharp downward change from sandstone (with numerous thin mudstone partings) of the St Bees Sandstone Formation (Rottington Sandstone Member) to mudstone or siltstone. Atypically in the north of the basin, in 110/3b-4, a thin development of the Freshfield Sandstone Member lies directly beneath the Rottington Sandstone Member and is included as a member within the Barrowmouth Mudstone Formation.
On wireline logs, the boundary is generally marked by the abrupt downward increase in gamma values (e.g. 113/27-3, 110/2b-9). However, in 110/3b-4 there is a downward decrease in gamma values into the Freshfield Sandstone Member. The change on sonic logs is slight and varied, and is partly dependent on depth of burial as well as the nature of the juxtaposed lithologies. A slight downward decrease in velocity is recorded in 110/2b-9, but more commonly there is little downward change at the boundary (e.g. 113/27-3; 112/25a-l; 110/3b-4). It is likely that in shallow buried sections there is either no change in velocity or a downward decrease (cf. Barnes et al., 1994, fig. 2). |
| Thickness: |
The Barrowmouth Mudstone Formation varies in drilled thickness from a minimum of 83 m (113/27-1) to a maximum of 135 m (110/3b-4). |
| Geographical Limits: |
The Barrowmouth Mudstone Formation occurs throughout the northern and central parts of the East Irish Sea, and shows slight thickening towards a depocentre north of the Deemster Platform, though there may be local thickening adjacent to the Lake District Boundary Fault. Locally, the tripartite gamma profile typical of the Barrowmouth Mudstone Formation can be recognized within equivalent strata in the upper part of the Manchester Marls Formation (e.g. 110/6b-l). |
| Parent Unit: |
Cumbrian Coast Group (CCO)
|
| Previous Name(s): |
none recorded or not applicable
|
| Alternative Name(s): |
none recorded or not applicable
|
| Stratotypes: |
| Type Section |
Irish Sea well 113/27- 3: 2598.5-2700 m (8526-8858 ft) below KB (Jackson and Johnson, 1996). |
| Reference Section |
Irish Sea well 110/02b- 9: 2328.5-2435 m (7640-7989 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, and Hemingway, J E. 1972. The St Bees Evaporites - a carbonate evaporite formation of Upper Permian age in west Cumberland, England. Proceedings of the Yorkshire Geological Society, Vol. 38, 565-591. |
| Arthurton, R S, Burgess, I C, and Holliday, D W. 1978. Permian and Triassic. 189-206 in Moseley, F (editor) The geology of the Lake District. Yorkshire Geological Society Occasional Publication, Vol. 3. (Leeds: W.S. Maney and Son Ltd.) |
| Barnes, R P, Ambrose, K, Holliday, D W, and Jones N S. 1994. Lithostratigraphical subdivision of the Triassic Sherwood Sandstone Group in west Cumbria. Proceedings of the Yorkshire Geological Society, Vol. 50, 51-60. |
| Jackson, D I, and Mulholland, P. 1993. Tectonic and stratigraphic aspects of the East Irish Sea Basin and adjacent areas: contrasts in their post-Carboniferous structural styles. In: Parker, J R (ed.) Petroleum Geology of Northwest Europe: Proceedings of the 4th Conference, 791-808. The Geological Society, London. |
| Jackson, D I, Mulholland, P, Jones, S M and Warrington, G, 1987. The geological framework of the East Irish Sea Basin. 191-203 in Brooks J and Glennie, K (editors), Petroleum Geology of North West Europe. [Graham and Trotman.] |
| Macchi, L. 1991. A Field Guide to the continental Permo-Triassic rocks of Cumbria and northwest Cheshire. Liverpool Geological Society. |
| Smith, D B, Brunstrom, R G W, Manning, P I, Simpson, S, and Shotton, F W. 1974. A correlation of Permian rocks in the British Isles. Geological Society of London, Special Report No. 5. |
| Burgess, I C, and Holliday, D W. 1974. The Permo-Triassic rocks of the Hilton Borehole, Westmorland. Bulletin of the Geological Survey of Great Britain, 46, 1-34. |
| Holliday, D W. 1993. Geophysical log signatures in the Eden Shales (Permo-Triassic) of Cumbria and their regional significance. Proceedings of the Yorkshire Geological Society, 49, 345-354. |
| Jackson, D I. 1994. Discussion of Geophysical log signatures in the Eden Shales (Permo-Triassic) of Cumbria and their regional significance. Proceedings of the Yorkshire Geological Society, 50, 175-184. |
| Smith, D B. 1992. Permian. In: Duff, P McL D, and Smith, A J (eds.) Geology of England and Wales, 275-301, The Geological Society, London. |
| 1:50K maps on which the lithostratigraphical unit is found, and map code used: |
| none recorded or not applicable |
