| Computer Code: |
GARD |
Preferred Map Code: |
notEntered |
| Status Code: |
Full |
| Age range: |
Tournaisian Age (CT)
— Visean Age (CV) |
| Lithological Description: |
Over wide areas of the East Irish Sea the Garwood Group comprises thick-bedded limestone. However, in a central E-W axial belt, much thicker sequences are thought to comprise thinly interbedded fine-grained bituminous limestones and calcareous mudstones which are closely comparable with similar lithologies along the depositional strike in the onshore Craven Basin (Ribblesdale Fold Belt; Aitkenhead et al., 1992; Riley, 1990; Gawthorpe, 1987). This axial basinal facies has not been penetrated offshore to date. Adjacent to the Ramsey-Whitehaven Ridge, a shallow marine Yoredale facies is predicted in the highest strata along the depositional strike from a similar facies in west Cumbria. |
| Definition of Lower Boundary: |
The lower boundary of the Garwood Group has not been proved offshore, but regional evidence demonstrates that the group rests unconformably everywhere on Caledonian basement rocks (BGS, 1994; Jackson et al., 1995), comprising red Devonian sandstones or mudstones (e.g. offshore from Anglesey), or Lower Palaeozoic greywackes and cleaved mudstones, or Precambrian metasediments and igneous rocks (BGS, 1994). |
| Definition of Upper Boundary: |
Over most of the area, Pendleian strata are believed to overlie the Garwood Group conformably, and the top of the group is placed at the base of the thin high gamma mudstone of the Cravenoceras leion Marine Band or a lateral equivalent (e.g. the First (Great) Limestone of Cumbria). |
| Thickness: |
In available offshore wells, drilled thicknesses vary from 111 m in 112/25a-1 to 168.5 m in 113/27-2, but in both cases penetrate only the uppermost beds of the Garwood Group. From seismic data, the Garwood Group is estimated to range up to 2050 m in thickness (Jackson et al., 1995), but may exceed this in axial regions. |
| Geographical Limits: |
The Garwood Group is believed to extend uninterruptedly throughout the East Irish Sea. |
| 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 112/25a- 1: ?2669.5-2780 m TD driller; [to 2777.5 m TD logger] (8758-9120.5 ft driller; [to 9112.5 ft logger] below KB (Jackson and Johnson, 1996). |
| Reference Section |
Irish Sea well 113/27- 2: 2491-2659.5 m (8172-8726 ft TD) (Jackson and Johnson, 1996). More precisely dated type and reference offshore sections with a greater stratigraphical range will be chosen when further drilling results become available. |
| 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. |
| Adams, A E, Horbury, A D, and Abdel Aziz, A A. 1990. Controls on Dinantian sedimentation in South Cumbria and surrounding areas of north west England. Proceedings of the Geologists' Association, Vol. 101/1, 19-30. |
| 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) |
| British Geological Survey. 1994. East Irish Sea (Special Sheet Edition, Solid Geology). 1:250 000, British Geological Survey, Edinburgh. |
| Davies, J R, Riley, N J, and Wilson, D. 1989. The distribution of Chadian and earliest Arundian strata in North Wales: implications for Dinantian (Carboniferous) lithostratigraphy and palaeogeography. Geological Journal, Vol. 24/1, 31-47. |
| Gawthorpe, R L. 1987. Tectono-sedimentary evolution of the Bowland Basin, N. England, during the Dinantian. Journal of the Geological Society of London, Vol. 144, 44-59. |
| Jackson, D I, Jackson, A A, Evans, D, Wingfield, R T R, Barnes, R P, and Arthur, M J. 1995. United Kingdom offshore regional report: the geology of the Irish Sea. HMSO, London, for the British Geological Survey. |
| Mitchell, M, Taylor, B J and Ramsbottom, W H C, 1978. Carboniferous, 168-188 in The geology of the Lake District. Moseley, F (editor). Yorkshire Geological Society Special Publication. No.3. |
| Ramsbottom, W H C. 1973. Transgressions and regressions in the Dinantian: A new synthesis of British Dinantian stratigraphy. Proceedings of the Yorkshire Geological Society, Vol.41, 261-291. |
| Riley N J. 1990. Stratigraphy of the Worston Shale Group (Dinantian) Craven Basin, north-west England. Proceedings of the Yorkshire Geological Society, Vol. 48, 163-187. |
| Somerville, I D, Strank, A R E and Welsh, A, 1989. Chadian faunas and floras from Dyserth: depositional environment and palaeogeographic setting of Visean strata in Northeast Wales. Geological Journal, Vol.24, 49-66. |
| Gawthorpe, R L, Gutteridge, P, and Leeder, M R. 1989. Late Devonian and Dinantian basin evolution in northern England and North Wales. 1-23 in The role of tectonics in Devonian and Carboniferous sedimentation in the British Isles. Arthurton, R S, Gutteridge, P, and Nolan, S C (editors). Yorkshire Geological Society, Occasional Publication, No. 6. |
| 1:50K maps on which the lithostratigraphical unit is found, and map code used: |
| none recorded or not applicable |
