| Computer Code: |
ROSA |
Preferred Map Code: |
notEntered |
| Status Code: |
Full |
| Age range: |
Early Triassic Epoch (TE)
— Early Triassic Epoch (TE) |
| Lithological Description: |
The Rottington Sandstone Member consists of a thick succession of monotonous, argillaceous, very fine to medium grained (mainly fluvial), silica-cemented thinly-bedded sandstone with abundant mudstone and siltstone partings. The sandstone is relatively uniform across the basin, but scattered pebbles increase in number in the south and towards the basin margins. The member is widely and colloquially known as the 'Silicified Zone'. |
| Definition of Lower Boundary: |
In the northern part of the basin, the base of the Rottington Sandstone Member is taken at the downward change from silicified sandstone to the mudstone or siltstone of the Barrowmouth Mudstone Formation (e.g. 113/26-1; 110/2b-9; 113/27-1). In the south, the base is placed at the downward change from silicified sandstone to the non-silicified, cleaner and generally softer, sandstone of the Freshfield Sandstone Member (e.g. 110/6b-l; 110/13-1; 110/20-1). In practice, because the sandstone/sandstone contact in the south is difficult to identify precisely, then the boundary is placed at the base of the lowest developed mudstone parting within the silicified sandstone. In two wells (110/3b-4 and 110/6b-l), the basal mudstone of the Rottington Sandstone Member is unusually thick; it is possible that this mudstone should be included in the Freshfield Sandstone Member and interpreted as an ephemeral playa mudstone within dune sands laid down at the close of the Permian, rather than as an overbank mudstone precursor to the Triassic fluvial sandstones. Onshore, the equivalent mudstone bed may be that described in the Garstang area (Aitkenhead et al., 1992, p.59).
On wireline logs, the contact with the Barrowmouth Mudstone Formation is marked by an abrupt downward increase in gamma values (e.g. 113/26-1; 110/2b-9; 113/27-1); there is generally a small downward decrease on the velocity log (e.g. 110/2b-9; 113/27-1; 112/25a-l), though depending on the depth of burial, there may be no velocity shift (e.g. 113/26-1). The contact with the Freshfield Sandstone Member is marked by the downward decrease in gamma values, which is large where a mudstone parting constitutes the basal bed (e.g. 110/6b-l; 110/13-1; 110/20-1). The response on the velocity log is variable - there may be a large (110/13-1) or small (e.g. 110/12a-1; 110/11-1) downward decrease, a small downward increase (e.g. 110/6b-l; 110/20-1) or no shift at all (e.g. 110/9-1). On the northeast margin of the basin, at Sellafield, an upward transitional passage from the St Bees Shales is reported in cored onshore boreholes, though the log response is identical to that offshore above the equivalent Barrowmouth Mudstone Formation (Barnes et al., 1994, fig. 2; cf. also Jones and Ambrose 1994, fig. 2; Macchi, 1991). |
| Definition of Upper Boundary: |
The top of the Rottington Sandstone Member is taken at the downward change from slightly cleaner and generally slightly coarser, less well-silicified sandstones of the Calder Sandstone Member to finer-grained highly silicified, argillaceous sandstones.
On wireline logs, the boundary is characterized by a sharp, but slight, downward increase in velocity, and the downward change to an approximately linear velocity profile (e.g. 110/6b-l, 113/26-1). In many cases this position on slightly cleaner sandstone to a higher gamma sandstone with numerous thin high gamma mudstone spikes (e.g. 113/27-1, 110/2b-9). Towards the south and possibly in areas of more shallow burial, the coincidence of gamma and sonic shifts becomes less well marked, and in cases where the depth-dependent sonic shift is poorly developed, the boundary is taken at a break on the gamma log (cf. 110/13-1, cf. 110/20-1). In addition, the downward increase in gamma values becomes reduced to the south, possibly reflecting a trend towards the lateral transition to the Chester Pebble Beds facies. |
| Thickness: |
Flanks of the Llyn-Rossendale Ridge to ?666 m in 110/9-1, but the member is typically about 550 to 600 m thick; the maximum thickness may be adjacent to sectors of the Lake District Boundary Fault (Jackson and Mulholland, 1993). On seismic sections, the Rottington Sandstone Member locally displays substantial wedge-like thickening in the hangingwall successions of major growth faults (Jackson et al., 1987, fig. 3b). The hangingwall succession, which may exceed 1000 m, is commonly double the thickness of that in the footwall (cf. Jackson et al., 1987, fig. 3b); probably there were a host of independently subsiding half-graben rather than a single basin depocentre, though there is a slight axial increase along a NNW-SSE line through the centre of the East Irish Sea Basin. |
| Geographical Limits: |
The Rottington Sandstone Member occurs throughout the East Irish Sea; equivalents extend onshore in Cumbria, Cheshire, west Lancashire and the Wirral Peninsula. However, the member has been removed by erosion from the Ogham Inlier and the Ribble Estuary Inlier (Jackson and Mulholland, 1993), and is believed to be faulted out over large parts of the Mercia Mudstone Group conjugate graben (see distribution map; Jackson and Mulholland, 1993, fig. 2). |
| Parent Unit: |
Chester Formation (CHES)
|
| Previous Name(s): |
none recorded or not applicable
|
| Alternative Name(s): |
none recorded or not applicable
|
| Stratotypes: |
| Type Section |
Irish Sea well 110/06b- 1: 2080-2716 m (6824-8910 ft) below KB (Jackson and Johnson, 1996). |
| Reference Section |
Irish Sea well 113/26- 1: 2158.5-2852 m (7081-9357 ft) below KB (c. 2086-2645.5 m TVDRKB (c.6844-8679 ft TVDRKB)) (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. |
| Aitkenhead, N, Bridge, D M, Riley, N J, and Kimbell, S F. 1992. Geology of the country around Garstang. Memoir of the British Geological Survey, Sheet 67 (England and Wales) |
| 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. |
| Hull, E, 1869. The Triassic and Permian rocks of the Midland Counties of England. Memoir of the Geological Survey of Great Britain. |
| 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.] |
| Jones, N S, and Ambrose, K. 1994. Triassic sandy braidplain and aeolian sedimentation in the Sherwood Sandstone Group of the Sellafield area, west Cumbria. Proceedings of the Yorkshire Geological Society, Vol. 50, 61–76. |
| Jones, R C B, Tonks, L H and Wright, W B, 1938. Wigan District. Memoir of the Geological Survey of Great Britain. England and Wales Sheet 84. |
| 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. |
| Wedd, C B, Smith, B, Simmons, W C and Wray, D A, 1923. The Geology of Liverpool, with Wirral and part of the Flinthsire Coalfield. Memoir of the Geological Survey of Great Britain, Sheet 96 (England and Wales). |
| Macchi, L. 1991. A Field Guide to the continental Permo-Triassic rocks of Cumbria and northwest Cheshire. Liverpool Geological Society. |
| Meadows, N S, and Beach, A. 1993. Structural and climatic controls on facies distribution in a mixed fluvial and aeolian reservoir: the Triassic Sherwood Sandstone Group in the Irish Sea. In: North, C P, and Prosser, D J (eds.) Characterization of Fluvial and Aeolian Reservoirs, 247-264. Geological Society, London, Special Publication No. 73. |
| Turner, P. 1980. Continental Red Beds. Developments in Sedimentology, Volume 29. Elsevier, Amsterdam. |
| 1:50K maps on which the lithostratigraphical unit is found, and map code used: |
| none recorded or not applicable |
