The Clare shale consists of black, organic rich, euxinic, goniatite-bearing shale. This formation forms the lower 300 m thick portion of the 1600 m Namurian where it fills and overlies the Iapetus Suture of Co Clare, a major structural lineament that trends southwest-northeast through Ireland and into northern England and southern Scotland (Pyles in press). The thickest exposure of the Clare shale occurs in the sea cliffs at Kilcondy Point Cliff just north of Ballybunnion and south of Leck Point. Here the lower portions of the Clare shale, as at other localities, has a very condensed character.
These shales and the rest of the Namurian are subdivided by the Homoceras and Lower Reticuloceras goniatite condensed sections or marine beds of Hodson (1953). Measuring vertical sections Hodson (1953) identified seventy-three fossiliferous bands that belonged to 10 goniatite horizons. Using these goniatite zones it is possible to demonstrate that the deepwater black Namurian shales of Clare shale Formation are thickest over the axis of the Shannon basin and that thinning occurs, not only the Clare shale, but the rest of the Namurian strata, in all directions away from the Loop Head area (Pyles, in press). The Clare shales are succeeded and progressively onlapped by the parallel bedded turbidititic sandstones of the Ross Formation that were transported from the SW to be confined within the Shannon Trough.
Outcrops in north Co Clare show that the Clare shale lies above the Carboniferous limestone, and onlaps onto it. Where the contact is visible, the Upper Viséan limestones are truncated and unconformably overlain by Serpukhovian siliciclastic rocks at the base of the Clare shale (Gallagher et al 2006). At Lisdoonvarna, Fisherstreet Co. Clare, this contact is expressed by 10 m of black Clare shale that overlies a phosphatised disconformity with everything below the Homoceras Beyrichianum zone of the Namurian missing (Hodson, 1954a;& 1954b & Hodson and Lewarne 1959). Strogen et al., (1996) show that the boundary between shallow and deepwater Viséan carbonate matches the margin of the Shannon basin, and that the Clare shale is thickest over deepwater Viséan carbonates and thins over the shallow water Visean carbonates. The episode of marine deepening and deposition of the Clare shale is driven by platform-wide subsidence. The glacioeustatic cyclic character of the Viséan in Ireland is superimposed on this, though at the end of the Brigantian localized rapid shallowing and emergence occurred. Strogen et al. (1996) show that the Clare shale is largely absent elsewhere in Ireland, having been truncated during the deposition of the Serpukhovian deltaic siliciclastics.
Apocryphally Professor Daniel Gill of TCD when confronted with previously unrecognized oily seeps from the Clare shale believed they had hydrocarbon potential onshore. He enthusiastically encouraged the drilling of a well nearby before, much to his and everyone else's chagrin, it was revealed that an old U Boat diesel fuel dump was the source!
Since this tragic anti climax, studies have been made of the organic matter from the near surface Clare shale of Co Clare and these show that it is over-mature. However the TOC values reach 15%. This suggests that the Clare shale could be major source rock in the less mature offshore margins of the Shannon Trough to the south and west (Goodhue & clayton, 1999). Vitrinite reflectance data from two inland cored boreholes suggest high maturation levels throughout the onshore part of the Irish Clare Basin and the erosion of 2 to 4 km of late Carboniferous cover (Goodhue and clayton, 1999). The recorded maturation values and elevated palaeogeothermal gradients in the Carboniferous section match the hypothesis that in the late Carboniferous/Permian there was superplume' beneath Pangaea while local vertical reversals in gradients support the idea of a complex thermal regime probably involving advective heating (Goodhue and clayton, 1999).
Summary - Key features of Clare shale
- The shales were deposited in a tectonically relatively quiescent basin, with the basin geometry largely inherited from an earlier stretching event.
- The big picture is one of overall shallowing through time, such that deep basinal shales are overlain by turbidites.
- There were probaly changes in relative sea level and sediment flux that occurred throughout deposition.
- The Clare shales onlap the basin margins and thin spectacularly onto the hangingwall dip slope.
- The basin fill is an overall deepening to shallowing-upwards succession from shallow and deep-water carbonates through a source-rock quality deep basinal shale succession.
- Above is a 460m thick turbidite accumulation (the Ross Formation).
- A further 550m thick overall fine-grained, very complex slope succession overleis this (the Gull Island Formation).
- A thick succession of deltaic cyclothems occurs at the top of this Namurian succession.
Click on highlighted Deepwater Gallery for access to a complete index photographs of the geology of Co Clare including the Clare shale.
Useful References
Collinson, J.D., Martinsen, O. Bakken, B. and Kloster, A. (1991), Early fill of the western Irish Namurian Basin: a complexship between turbidites and deltas. Basin Research, 3, 223-242.
Gallagher,S.J., C.V. MacDermot, I.D. Somerville, M. Pracht , A.G. Sleeman, 2006, biostratigraphy, microfacies and depositional environments of upper Viséan limestones from the Burren region, County Clare, Ireland. Geol. J. 41: 61–91
Gill, W.D. (1979), Syndepositional sliding and slumping in the West Clare Namurian Basin, Ireland. Geological Survey of Ireland Special Paper 4, 31pp.
Goodhue R. and clayton G. 1999, "Organic maturation levels, thermal history and hydrocarbon source rock potential of the Namurian rocks of the Clare Basin, Ireland" Marine and petroleum geology (Mar. pet. geol.) vol. 16, no7, pp. 667-675
Hodson, F., 1954a, The beds above the Carboniferous limestone in north-west County Clare, Eire: Quarterly Journal of the Geological Society of London, v. 109, p. 259-283.
Hodson, F., 1954b, The Carboniferous rocks of Foynes Island, County Limerick: Geological Magazine, no. 2, p. 153-160.
Hodson, F. & Lewarne, G.C. (1961) A mid-Carboniferous (Namurian) Basin in parts of the counties of Limerick and Clare, Ireland. Quart. Geol. Soc. Lond., 117, 307-333.
Strogen, P., I. D. Somerville, N. A. H. Pickard, G. Jones, and M. Fleming, 1996, Controls on ramp, platform and basinal sedimentation in the Dinantian of the Dublin Basin> and Shannon trough, Ireland, in P. Strogen, I. D. Somerville, and G. Jones, eds., Recent advances in Lower Carboniferous Geology: Geological Society of London Special Publication no. 107, p. 263-279.
Martinsen, O.J. (1989), Styles of soft sediment deformation on a Namurian (Carboniferous) delta slope, western Ireland Namurian Basin, Ireland. In Whatley, M.K.G. & Pickering, K.T. (eds) Deltas: sites and traps for fossil fuels, Geological Society Special Publication, 41, 167-177.
Martinsen, O.J. & Bakken, B. (1990), Extensional and compressional zones in slumps and slides in the Namurian of County Clare, Ireland. Journal of the Geological Society, 147, 153-164.
Pyles, D.R. (in press), Stratigraphic architecture of a structually confined submarine fan, Carboniferous Ross Sandstone, Western Ireland, Bulletin of the American Association of Petroleum Geologists.
Rider, M.H. (1969), Sedimentological studies in the West Clare Namurian Basin, Ireland and the Mississippi Delta. Unpublished PhD thesis, Imperial College London.
Rider, M.H. (1974), The Namurian of West County Clare. Proceedings of the Royal Irish Academy, 74B, 125-142.
Rider, M.H. (1978), Growth faults in the Carboniferous of western Ireland. Bulletin of the American Association of Petroleum Geologists, 62, 2191-2213.
Sevastopulo, G.D. (1981), Upper Carboniferous. In Holland, C.H. (ed.) A geology of Ireland. Scottish Academic Press, 173-199.
Wignall, P.B. and Best, J.L. (2000), The Western Irish Namurian Basin reassessed. Basin Research 12, 59-78.
Wignall, P.B. and Best, J.L. (2002), The Western Irish Namurian Basin> reassessed – a discussion. Basin Research 14, 523-542.