Hydrogeothermal Conditions in Éire - Geological Survey of Ireland

- 6 -II-2.Geological SettingThe known warm springs all issue from Dinantian limestones. Forthe main Leinster group of 11, Fig. 2 shows that they emerge mainly fromVisean limestones, except the small spring (No. 1010) at Cash's farmissuing from the underlying Tournaisian. This applies to the warm springsin Co. Limerick and in Co. Clare. The Mallow springs and boreholes arealso from the limestone, but are associated with a major thrust zone.Fig. 2 brings together such geological data as is available fromthe main area of warm springs in Leinster. Most of the area is covered byQuaternary deposits, of varying thickness and nature; rock exposures arepoor. The main data comes from MacDermot & Sevastopulo (1972), andSheridan (1972), while Turner (1952), Nevill (1957), Bruck (1971) andWilliams & McArdle (1978) have also been studied and relevant data extracted.Data on ~rtiary sills/dykes comes from Sheridan (1972), Selwyn Turner et al(1972) and Byrne et al (1971).The region of the warm springs lies between the granite, metamorphicsand Silurian of the Leinster Caledonian granite massif on its south-east, andthe Ordovician-Silurian Balbriggan and Louth blocks on its north. Here,conglomerates, sandstones and siltstones of desertic and fluvio-esturineenvironments were laid down in Devonian and into Lower Carboniferous times,as the Tournaisianseas gradually transgressed from the south and south-west,as Clayton & Higgs (1979). It could have been well into Lower Tournaisiantime before marine limestones began to interdigitate with coastal sandstonesunder the area of Fig. 2. By the Upper Tournaisian, shelf limestones withisolated Waulsortian reef mounds were forming under the main portion ofFig. 2. The Waulsortian mounds were more frequent and much thicker to thewest and away to the southwest, to the areas of warm springs Nos. 2003 and2004; this is shown clearly in Figs. 1 and 2 of MacDermot & Sevastopulo(1972) . There was a break at the end of the Tournaisian, with some upliftand faulting, but marked also by increased marine transgression, as onto theedges of the Leinster granite massif. In the region of the Leinster warmsprings, there was a sharp change to deposition of argillaceous limestonesof basinal facies; the 800 or so metres of the Rathmolyon Shale Formation(with pyrite) cut in the Trim NO. 1 boreholes is dated as "Cl or younger,ranging up to C2Sl" (Sheridan, 1972, p. 330), and so of Lower Visean age.These shales were followed by limestones, in part with reefs of Viseantype and towards the top of the Visean with bedded limestones, argillaceous,with some chert and silica. These limestones are succeeded by Namurianstrata, often found occurring as perched synclines, as at Summerhill (Nevill1957) . The total thickness of the Tournaisian + Visean is of the order of2,220 metres (Sheridan, 1972) plus more than 760 m of overlying Namurian(Nevill, 1957). The thickness of the underlying Devonian is unknown for thearea of Fig. 2.The detailed information obtained from the Trim No.1 borehole isvery relevant to the study of the rocks from which the warm springs issue.The lithology/thicknesses are summarised in Table 2. The fact that theboreholes intersected some 4.5 metres of an olivine dolerite dyke or sillat from 740.7 to 745.2 metres below surface is also of importance whenconsidering local sources of heat within the sedimentary and underlyingsequences./ ...