Hydrogeothermal Conditions in Éire - Geological Survey of Ireland

- 8 -The presence of sills/dykes of intrusive basalt has been reportedfrom at least three places within or bordering the region of these warmsprings. The earliest report is by Cruise (1871), an 18" wide dyke in aroad-cutting about l~ miles WNW of Duleek, Co. Meath, as noted by SelwynTurner et al (1972), when reporting on Tertiary igneous activity at Navan,Co. Meath.-- They found that a sill at the Tara mine was some 51 millionyears old by K-Ar dating, and so "not later than Lower Eocene, not earlierthan Middle Eocene". In describing the occurrences, Byrne et al (1971)note "The basalt sills transgress the Carboniferous series, independent ofstructures, and are considered to be Tertiary..• at Tara's Navan mine, thereare generally two such sills, varying in thickness from one to ten feet".The third recorded occurrence is that in the Trim No. 1 Well, where the4.5 metre thick sill or inclined dyke of olivine basalt was dated as 65(± 11) million years.It is accepted that these sills/dykes are related to the AntrimBasalts,whose K-Ar determinations indicate a Palaeocene-Eocene age some 65million years BP. According to Miller & Brown (1963) the Upper Lava Seriesof the Antrim basalts have an K-Ar age of 74 million years; later theyindicated that the presence of zeolites in basalt tend to give K-Ar ageswhich are too great (Miller & Fitch, 1964). However, on palynologicalgrounds, the Antrim basalts are considered to be of Miocene or evenPliocene age, and so much younger (Sabine & Watson, 1965, p. 506). Thematter is of importance here, as late Tertiary intrusions (say 15 to 20million years ago) might have initiated some thermal groundwatercirculations which could have persisted, after the initiating heat sourcehad diminished or ceased to act.The rocks underlying the w~rm springs of Leinster have been gentlyfolded and deformed during the Hercynian orogeny. The folds tend north-eastto south-west, rather than the prime east-west direction seen in the southof Ireland. They appear to have been reoriented by the resistant blocks intheir neighbourhood, or to follow deeper Caledonian structural trends.On Fig. 2, three anticlinal and two synclinal axial traces are indicated;the two of the north-west are based on Fig. 1 of Sheridan (1972); theother three are based on the form of the Tournaisian/Visean contact. Whiledetailed and more precise mapping of the springs and some of the structuresis required, it may be noted here that: (i) Springs 1009, 1001, 1004, 1010and 1014 lie along the synclinal structure inferred for the two Tournaisianoutcrops: and (ii) The warmest springs are not far from the axial plane ofthe Summerhill syncline (No. 1005) and from the the Rathmolyon anticline(No. 1007).The rocks were thus folded and deformed long before the intrusionof the Tertiary dyke/sill intersected 740.7 to 745.2 metres below surfacein the Trim No.1 Well. Thus, the intrusion will cross-cut the beddingwhether it be a sill or a dyke (vertical or inclined) . Such an intrusionmay be expected to swell, and form a larger body of magma, where the planeof intrusion intersects a shattered (as anticlinal crests) or open (aspalaeokarstic limestone) volume in the intruded rocks. Such large volumesof intrused lava could have provided areas favourable for heat storage andthe initiation of thermally-controlled groundwater circulation in theenclosing rocks; how long such a mechanism could continue would depend onthe size, initial temperature and rate of cooling of the intrusive body./ ...