| Computer Code: |
BISA |
Preferred Map Code: |
notEntered |
| Status Code: |
Full |
| Age range: |
Namurian Stage (CN)
— Namurian Stage (CN) |
| Lithological Description: |
In common with other NW European Namurian sequences, within which some 60 eustatically-controlled marine bands can be recognized (Ramsbottom, 1969a; Holdsworth and Collinson, 1988), the Bisat Group displays a lithological cyclicity at a variety of scales. It contains a much higher proportion of mudstone than the equivalent 'Millstone Grit' successions of the Central Province, than the modified Yoredale cyclothems of Northern England, or than successions containing orthoquartzitic Cefn-y-Fedw sandstones in North Wales (Ramsbottom, 1969a, 1974, 1978; Ramsbottom et al., 1978). Within the group the chief vertical and lateral facies variations are noted as follows:
1. Sandstones occur in greater proportion in the upper part of the succession.
2. A modified Yoredale facies, including sandstones and limestones, of H| age is developed over the crest and on the flanks of the Ramsey-Whitehaven Ridge.
3. A thick sequence of thinly interbedded basinal limestone and calcareous mudstone is present in an axial facies belt (e.g. E,-?H, in 110/7b-6). |
| Definition of Lower Boundary: |
The base of the Bisat Group is placed, where possible, at the base of the Cravenoceras leion Marine Band. However, the diagnostic goniatite fauna of the marine band has not been reported offshore. |
| Definition of Upper Boundary: |
The top of the Bisat Group is placed, where possible, at the base of the Subcrenatum Marine Band, which in Lancashire, Cumbria and North Wales commonly overlies a thin coal and/or mudstone seatearth. However, the diagnostic macrofauna have not been identified from the marine band in available offshore wells. |
| Thickness: |
Within the East Irish Sea area, the thickness of the Bisat Group broadly increases from both the north and south towards the E-W trending basin axis, and the group shows a greater degree of depositional thinning towards the contemporary Lower Palaeozoic highs of the Isle of Man, the Lake District, the Ramsey-Whitehaven Ridge and North Wales than either the Garwood or Kidston groups. Jackson et al. (1987) originally estimated a thickness of about 1450 m for the Namurian of the East Irish Sea Basin. Offshore, the Bisat Group has been proved to a maximum drilled thickness of 809.5 m in the E2-G1 succession of 112/30-1, though seismic data suggest that thicknesses of at least 2900 m may occur in the Quadrant 109 Syncline (Jackson and Mulholland, 1993). In 113/27-2, the apparently complete 588.5 m thick succession is likely to contain disconformities and to be internally faulted, since the E1 succession in the Roosecote borehole along strike to the east in south Cumbria exceeds 455 m (Rose and Dunham, 1977). In adjoining onshore areas, thicknesses range from an aggregate maximum of 2160 m along the depocentral axis in the Clitheroe area (Earp et al., 1961), through an estimated c. 365 m maximum in the south near the Point of Ayr (Ramsbottom, 1974), to a maximum preserved c. 60 m over the Ramsey-Whitehaven Ridge (Taylor, 1961; Ramsbottom, 1978). Onshore in basinal settings, beds of E1 and E2 age locally comprise 50% of the entire Namurian succession (Earp et al., 1961). |
| Geographical Limits: |
In the North-Irish Sea, rocks of Namurian age accumulated in an elongate E-W depositional area extending westwards from the Central Province, through the East Irish Sea Basin-Quadrant 109 Syncline, to the East Leinster Basin (Ramsbottom, 1969a, b; Ramsbottom et al., 1978; Sevastopulo, 1981; Cope et al., 1992). The Bisat Group and lateral equivalents are now preserved throughout most of the East Irish Sea Basin. Offshore the group is largely concealed beneath Permo-Triassic deposits (Smith, 1985), but it subcrops surficial deposits around much of the basin perimeter and in parts of the Ogham Inlier. In the east, the group was removed by pre-Permo-Triassic and/or intra-Carboniferous erosion from offshore (BGS, 1994) and onshore south and west Cumbria, including the Sellafield area (Trotter et al., 1937; Barclay et al., 1994). Onshore to the north, E2b-Gla beds were not deposited over the Ramsey-Whitehaven Ridge (Taylor, 1961; Ramsbottom, 1978), and offshore, the entire Bisat Group may never have been deposited over parts of the ridge crest (Smith, 1985). |
| Parent Unit: |
Not Applicable (-)
|
| Previous Name(s): |
none recorded or not applicable
|
| Alternative Name(s): |
none recorded or not applicable
|
| Stratotypes: |
| Reference Section |
Irish Sea well 113/27- 2: ?1902.5-2491 m (76242-8172 ft) below KB (Jackson and Johnson, 1996). With the possible exception of 113/27-2, a complete section of the Bisat Group has not been penetrated in East Irish Sea wells available to this study. |
| Reference Section |
Irish Sea well 112/30- 1: 559 (or ??467)-1368.5 m TD (1834 [or ??1532]-4490 ft TD) (Jackson and Johnson, 1996). The identification of the Subcrenatum Marine Band in this section is especially tentative. |
| Reference(s): |
| Barclay, W J, Riley, N J, and Strong, G E. 1994. The Dinantian rocks of the Sellafield area, West Cumbria. Proceedings of the Yorkshire Geological Society, Vol. 50, 37-49. |
| British Geological Survey. 1994. East Irish Sea (Special Sheet Edition, Solid Geology). 1:250 000, British Geological Survey, Edinburgh. |
| Collinson, J D. 1988. Controls on Namurian sedimentation in the Central Province basins of northern England. 86-101 in Sedimentation in a synorogenic basin complex: the Upper Carboniferous of Northwest Europe. Besly, B M, and Kelling, G (editors). (London: Blackie and Son.) |
| Cope, J W C, Ingham, J K, and Rawson, P F. 1992. Atlas of palaeogeography and lithofacies. Geological Society Memoir, No. 13. |
| Earp, J R, Magraw, D, Poole, E G, Land, D H, and Whiteman, A J. 1961. Geology of the country around Clitheroe and Nelson. Memoir of the Geological Survey of Great Britain, Sheet.68, England and Wales, p. 346. |
| Holdsworth, B K and Collinson, J D. 1988. Millstone Grit cyclicity revisited. In: Besly, B M and Kelling, G (eds.) Sedimentation in a synorogenic basin complex: the Upper Carboniferous of Northwest Europe, 132-152. Blackie & Son, London. |
| 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.] |
| Leeder, M, Raiswell, R, Al-Biatty, H, McMahon, A and Hardman, M. 1990. Carboniferous stratigraphy, sedimentation and correlation of well 48/3-3 in the southern North Sea Basin; integrated use of palynology, natural gamma/sonic logs and carbon/sulphur geochemistry. Journal of the Geological Society, London 147, 287-300. |
| Smith, N J P (compiler). 1985. Pre-Permian geology of the United Kingdom (south sheet). British Geological Survey 150th Anniversary Publication. British Geological Survey, Keyworth. |
| Ramsbottom, W H C, Calver, M A, Eagar, R M C, Hodson, F, Holliday, D W, Stubblefield, C J and Wilson, R B. 1978. A correlation of the Silesian rocks in the British Isles. Geological Society of London, Special Report No.10. |
| Rose, W C C, and Dunham, K C. 1977. Geology and hematite deposits of South Cumbria. Economic Memoir of the Geological Survey of Great Britain, Sheet 58, part 48 (England and Wales). |
| Trotter, F M, Hollingworth, S E, Eastwood T and Rose, W C C. 1937. Gosforth District. Geological Survey Memoir, England and Wales, Sheet 37. |
| Ramsbottom, W H C. 1969a. The Namurian of Britain. Compte Rendu du Sixieme (6me) Congres International de Stratigraphie et de Geologie du Carbonifere, Sheffield, (1967), Vol. 1,219-232. |
| Ramsbottom, W H C. 1969b. Reef distribution in the British Lower Carboniferous. Nature, London, 222, 765-766. |
| Ramsbottom, W H C. 1974. The Namurian in North Wales. 161-167 in The Upper Palaeozoic and post-Palaeozoic rocks of Wales. Owen, T R (editor). (Cardiff: University of Wales Press.) |
| Ramsbottom, W H C. 1978. Namurian. 178-180 in The geology of the Lake District. Moseley, F (editor). Yorkshire Geological Society, Occasional Publication, No. 3. |
| Sevastopulo, G D. 1981. Upper Carboniferous. 173-187 in A Geology of Ireland. Holland, C H (editor). (Edinburgh: Scottish Academic Press.) |
| Taylor, B J. 1961. The stratigraphy of exploratory boreholes in the West Cumberland coalfield. Bulletin of the Geological Survey of Great Britain 17, 1-74. |
| 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 |
