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CLARE SHALES - UPPER CARBONIFEROUS DEEPWATER SEDIMENTS 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).
Useful References Collinson, J.D., Martinsen,
O. Bakken, B. and Kloster, A. (1991), Early fill of the
western Irish Namurian Basin: a complex relationship between turbidites
and deltas. Basin Research, 3, 223-242. Gill, W.D. (1979), Syndepositional
sliding and slumping in the West Clare Namurian Basin, Ireland.
Geological Survey of Ireland Special Paper 4, 31pp. 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. 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. |
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