Vitrinite Reflectance Data from the Kinsale Harbour-Old Head of Kinsale Area, Southern Ireland, and Its Bearing on the Interpretation of the Munster Basin

Journal ofrhe Geological Society, London, Vol. 146, 1989, pp. 611-616, 3 figs, 2 tables. Printed in Northern Ireland

Vitrinite reflectance data from the Kinsale Harbour-Old Head of Kinsale area, southern Ireland, and its bearing on the interpretation of the Munster Basin

GEOFFREYCLAYTON Department of Geology, Trinity College, Dublin 2, Ireland

Abstract: Vitrinitereflectance of theUpper Devonian-Carboniferous succession in theKinsale Harbour-Old Head of Kinsale area ranges from 4.06-5.21% (R,) and 6.21-7.82% (Rmax)which is indicative of meta-anthracite coal rank. No correlation is apparent between vitrinite reflectance and stratigraphic position through c. 2 km of section, suggesting that near-maximum temperatures were maintained until after Variscan folding. Low reflectance values of 0.62% and 0.30-0.35% (R,) at the local base of the Jurassic succession in the North Celtic Sea Basin (Well 48/30-1) and onshore at Cloynerespectively, indicate a major discontinuity in thematuration profile in theregion. Com- parison with high reflectance gradients from Upper Palaeozoic successionsin the Halen and Turnhout boreholes in Belgium and the Miinsterland No. 1 borehole in West Germany suggests that a thick (5-7 km) cover of pre-Permo-Tnassic rocks must once have been present in the Munster Basin to accountfor the high maturationlevels observed. Whether the cover wasemplaced by Silesian sedimentation or by late Palaeozoic thrustingis unclear.

Thestratigraphic successionin the KinsaleHarbour-Old maximumreflectance (Rmax) and minimumreflectance Head of Kinsale area consists of morethan 2 km of (Rmi,) weremeasured by rotation of eachparticle in uppermostDevonian and Carboniferous clastic sediments. plane-polarized light. In many of the samples, however, the Five formations are recognized representing a wide range of small particle size precluded measurement of R,, and Rmi,, shallow todeep marine depositional environments. The andmean random reflectance was determinedin non- succession is dominated by sandstonesand mudrocks in polarized light. In this paper ‘mean random reflectance’ is subequal quantitites, with thin limestones present within the abbreviated to ‘R,’ rather than ‘Rran’or ‘R;. The highest Courtmacsherry Formation. A strong axial planar cleavage 10% of the R,, and lowest 10% of the Rminwere taken as is developed in the mudrocks and finer-grained sandstones. the bestapproximation thetotrue R,,, and Assumingtectonic shortening of 30%in themudrocks respectively,following Cook et al. (1972).Cubic zirconia associatedwith cleavage development,and 20%in the (R,,, 3.28%)and silicon carbide (R,= 7.54%)standards sandstones, the pre-deformation thickness of the currently were utilized. exposedsuccession may have been c. 2.7km. Numerous accounts of the sedimentology, litho- and biostratigraphy of thearea have been published, includingClayton et al. Maturation data (1974), Higgs ef al. (1988), Kuijpers (1972), Naylor (1966, Mean reflectance (l?,) varies from 4.06% to 5.21% (Table 1969), Van Gelder & Clayton (1987). E-W to ENE-WNW l), with standard deviations from 0.31 to 1.06. The mean of trending Variscan folds are the dominant tectonic structures the highest 10% of the maximum reflectance (Rmax) varies in the area (Fig. 1). from6.21% to 7.82%;the corresponding minima ranging Palynologicalinvestigations of the area (Clayton et al. from 2.15% to 3.29%. The vitrinite reflectance and spore 1974; Van Gelder & Clayton 1978; Higgs et al. 1988) reveal colour dataare totally consistent,and indicate that the highly carbonizedmiospores. These were all blackwith wholesection is of meta-anthraciterank in terms of the commonstructural damage, equivalent to ‘Thermal North American (ASTM) coal rank classification (Table 2). AlterationIndex’ 4-5 interms of Staplin’s1969/1977 Maturation level is controlled by bothtemperature and classification. The geographicallocations of selected duration of heating,but the lowerboundary of the palynologysamples are shownin Fig. 1. Further work meta-anthracite rank clearly indicates palaeotemperatures in intended to produce more precise estimates of maturation excess of 250 “C (Hkroux et al. 1978). level hasinvolved vitrinite reflectance determination from There is little variationin vitrinite reflectance in the 21 samples in the area. The geographical and stratigraphical KinsaleHarbour-Old Head of Kinsale area,and no positions of the productive samples are summarized inFigs 1 apparentcorrelation between reflectance and the strat- and 2respectively. Organic residues were obtained by igraphic position of the samples (Fig. 2); Isotherms appear standardhydrofluoric acid techniques. After drying and to be horizontal or near-horizontal and undeflected by the mounting in cold-setting resin, these were ground flat using major folds, which in the area discussed involve at least a silicon carbide paper and then polished with 1 pm, 0.25 pm stratigraphic thicknessof 2 km. and 0.04 pm grade alumina laps. Whole-rock mounts proved unsatisfactory due to the low vitrinite contentof the samples. Burial history Vitrinitereflectance was determined usinga kitz MPV-1microscope/photometer under standard conditions Offshore,within theNorth Celtic Sea Basin, maturation (British Standard 6127, 1981 part 5). Wheneverpossible, levels in theCarboniferous economic basement are 611

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White Strand Frn. mLispatrlck Frn. Courtmacsherry Fm.

...... Pigs Cove Mbr......

Narrow Cove Mbr.

Castle Slate Mbr.

Old Head Sandstone ...

Anticline

+Synclln. -- - Fault Reflectance Sarrple

1 t Miospore Sample

...;;;fl......

Fig. 1. Location of vitrinite reflectance samples, and selected spore colour samples. Generalized geology of the Kinsale Harbour-Old Head of Kinsale area after Naylor (1966), Naylor er al. (1985) and Van Gelder & Clayton, (1978).

considerablyhigher than in the Mesozoiccover. For correlatives of the Carboniferous Narrow Cove Member of example, inEsso-Marathon well 48/30-1(see Fig. 1 for the Kinsale Formation. R, values from the latter unit range location), R, increases throughthe Lower Cretaceous- from 4.52% to 4.93% (Haughey & Claytonunpublished Jurassic section at a rate of 0.12% per km, to 0.62% at the data). base of the Jurassicsequence. Approximately 400 mof At Cloyne, 35 km northeast of Kinsale (Fig. l), a very barrenTriassic redbeds separate the Jurassic fromtime thin Lower-Middle Jurassic lacustrine deposit known as the

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Table 1. Summary of vitrinite reflectance determinations completed

ReflectanceStandardNumber TopNumberLowest10% 10% sample R,,, (%) deviation of grains R,,, (%) Rmi, (%) of grains

1 4.41 0.90 100 - - - 2 4.50 0.67 100 0.67 4.50 2 - - - 3 4.22 0.70 49 - - - 4 4.30 4 1.06 100 7.54 2.15 100 4.73 0.77 78 0.775 4.73 - - - 4.86 0.80 100 6.21 3.29 6.21 100 0.806 4.86 100 7 4.51 0.63 100 0.63 4.51 7 - - - 8 4.82 0.60 100 - - - 9 5.213.31 0.447.82 100 100 4.83 0.67 100 0.6710 4.83 - - - 4.93 0.89 100 7.11 100 0.8911 4.93 3.11 100 12 4.14 0.61 100 0.61 4.14 12 - - - 4.23 0.51 100 0.5113 4.23 - - - 140.48 4.47 100 - - - 15 4.65 0.49 100 - - - 16 5.05 0.36 100 6.72 3.20 100 17 4.61 0.31 100 0.31 4.61 17 - - - 18 4.12 0.66 100 - - - 19 4.25 0.73 4.25 19 64 - - - 20 4.06 0.46 100 - - - 21 4.22 0.65 4.22 21 55 1002.89 7.39

Table 2. Correlation of vitrinite reflectance, spore colour, coal rank ColbondClay is preservedin small depressions on the and palaeotemperature (T) (assuming 2230 Mu duration), after CarboniferousLimestone surface (Bishopp & McCluskey Hhow et al. (1978), Stach et al. (1982) and Staplin (1969, 1977) 1948, Higgs & Beese1986). Mean reflectance (R,) determinationsfrom two samples of theseclays of COAL RANK Rmax Rm SPORE T KINSALE 0.30-0.35% suggest only very limited post Middle Jurassic (ASTM) (X) (W) :oLouI; (‘C) RANGE burial of this area(Clayton, Haughey & Thompson - - unpublished information). Organic maturation in the Munster Basin-North Celtic Peat SeaBasin clearly post-dates the youngestsediments - 0.3 affected, which are of Lower Namurian age, but pre-dates Lignite deposition of the oldestnon-affected rocks in the region

a, (LowerJurassic). The uniformity of maturationlevel c 65 a a throughout the Kinsale Harbour-Old Head of Kinsale area, sub-Bituminous regardless of stratigraphicposition ortectonic setting suggests that maximum temperatureswere maintained at - 0.5 least until the termination of Variscan folding. The timing of Variscan folding in southern Ireland and in the North Celtic Sea Basin is poorly constrained stratigraphically due to the Bituminous 120 absence of upper Silesian and Permian strata in the region. c However, K-Ar ages of clay concentrates from post-folding fissure veins at several localities in southwest Ireland suggest I2 2.2 - 2.0 m ’ 200 mineralization at approximately 290 Ma(Halliday & ;emi-Anthracite Mitchell 1983), which is consistent with folding during latest Carboniferous (Stephanian) or early Permian times. 2.8 - 2.5 . 250 There is no positive record of Permian deposition either

Y onshore in southern Ireland, or offshore in the Celtic Sea Anthracite - 3.0 0 W Basins, though the basal part of the undated ‘Triassic’ red h5 6.0 - 5.0 bedsuccession offshore could conceivably be Permian in age.Although there is no directevidence of maturation ?eta-anthracite level within the (pre-Rhaetian) Trias, it is unlikely that this 10-18 wouldvary significantly fromthat of the conformably- overlying Jurassic section. Graphite Major problems exist in attempting to explain the high rank of theCarboniferous rocks by simpleburial

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REFLECTANCE (%l OUTLINE STRATIGRAPHY 1 2 3 4 5 8789

.l1 I 0 1. .I Member 4 14 0' I

Member 3 b 1 ur' Member 2 l? r' Member 1 I ...... l ...... 1

I l I .I

-8 ReflectanceSample

c Rmax IHlghest 10%) Fig. 2. Stratigraphic distributionof reflectance data. Outline stratigraphyof the Kinsale Harbour-Old Head of [,,,m Kinsale area after Naylor(1966), Clayton et al. (1974) and Naylor et al. (1985).

metamorphism. The (R,) reflectance of 0.12% per km from the latter borehole (which is probably less meaningful observed in the Mesozoicsection of well48130-1 would at suchhigh rank) suggests c. 5.4 km of cover (Fig. 3). imply burial of the Carboniferous section beneath c. 10 km Buntebarth et al. (1981) have suggested that Carboniferous of post-Dinantian-pre-Jurassic cover (Fig. 3); a figure far in geothermalgradients in 'Variscanforedeep' areas were excess of the knownmaximum thickness of sediment much higher than at present. They attributed this to crustal representing this interval in the North Celtic Sea Basin or thinning,and calculated that inthe Ruhr Basin, for adjacent areas. example, the crust was only 20 km thick. Extrapolation of the reflectance (Rmax)profiles from the Matthews et al. (1983) interpretedthe thick Upper Halenand Turnhout boreholes in theCampine-Brabant Devonian-Lower Carboniferous succession in the Munster Basin of Belgium (Muchez et al. 1987) indicates c. 6.2 km of Basin as being attributable to mantle delamination caused cover (Fig. 3),whereas the R,,, profile fromthe by the continent-continent collision which occurred in early Miinsterland No. 1 borehole in West Germany impliesa Devonian times, on closure of the Iapetus Ocean. Two lines cover thickness of 7 km. (Stach et al. 1982). The R, profile of evidence were used by Matthews et al. to support their

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4* 1 Matthews et al. (1983) dismissed as being a local and very I .. late accumulation of sediment, although Naylor et al. (1978) -.-. . considered it to be part of a geographically more extensive E ... section.Some 400 m of LowerNamurian strata are also ...l Mean Y W .‘ Kinsal e preserved in a restricted area on Seven Heads, County Cork L .... Q) 7.13% (Naylor et al. 1988). Further support may be found in the a 0 thick Namurian succession of Castleisland to the north, and U in the lack of any evidence of proximity to major positive areas in the Westphalian A ‘coal swamp’ facies of Kanturk I I andcoalfields furthernorth. The maximumcomposite 0.2 1 .o 10 thickness of Namurianand Westphalian strata in Ireland Rrnax (%l and western Britain is only c. 5 km (1860 m of Namurian in the Coalisland area, 1142 m of WestphalianA-B in Lancashire, and 2000 m of Westphalian C-D in the Bristol area).It is difficult, therefore,to envisageSilesiana -6 succession in south Munster which exceeds this thickness. E Y . Mean An analogoussituation may exist in the Pennsylvania W .. L .. Kinsale AnthraciteRegion of theeastern United States, where 01) . a .. 4.53% Levine (1986) has interpretedthe high rank of the 0 . U 2 \ MU Pennsylvanian strata as beingcaused by burial under 6-9 km I of subsequently erodedlate Palaeozoic sediment,either rapidly deposited, or emplaced tectonically as thrust sheets. Q 1Levineconcluded that in easternPennsylvania, maximum 0.2 burial and maximumheating were roughly contem- poraneouswith folding during the AlleghanianOrogency (c. 285-270 Ma). However, in many cases thrusting clearly Fig. 3. Approximate reconstruction of eroded cover in the Kinsale post-dates organic maturation (Ogunyomi et al. 1980). Harbour-Old Head of Kinsale area basedon mean R,,, and R, Several authors commenting on the seismic evidence from values from the 21 samples investigated, and reflectance gradients the North Celtic Sea Basin have noted southerly dipping, from the Halen, Turnhout and MunsterlandNo. 1 boreholes (see low-anglereflectors in the pre-Permo-Triassicrocks text for sourcesof data). Solid lines indicate the actual reflectance flooring the basin. Pinet et al. (1987) considered that these ranges determined in theboreholes. structures constitute a discontinuous ‘Variscan Front’. There is thereforesome possibility thatthe Upper Palaeozoic section in the region may have been thickened by thrusting, theory of inversion of thesouth Munster area during as described in Pennsylvania. Onshore, Cooper et al. (1984, VisCan-Silesian times. Firstly, they cited thermal modelling 1986) have mapped numerous ENE-WSW trending thrusts by Bird (1979) which predicted high heatflow in 30 km crust within the Munster Basin, although displacement on these peaking after approximately 10 million years, then decaying appears to be too small to tectonically thicken the basin fill asymptotically.Secondly, they described thecondensed to the required extent. The structure of the southern Irish nature of the upper Dinantian in the area, which contrasts Variscides can,however, also be explained by alternative stronglywith the verythick UpperDevonian and lower modelswhich do notnecessitate thrust stacking (e.g. Dinantiansequence. They further suggested that the Sanderson1984). Based mainly on sedimentation rates, presence of mantle-derived tuffisite intrusions in the Bantry Sandersonsuggested thatthe evolution of theMunster area supported the concept of mantle delamination. Basin is consistent with the McKenzie (1978) model which The present maturation data are not consistent with the would imply thinning of the crust to around half its original mantle delamination model discussed above in terms of the thickness (assumed to be 30 km) and an 8 km rise in the timescale proposed by Matthews et al. (1983). Even allowing Moho. Consideration of the present position of the Munster forhigher palaeogeothermal gradients than those which Basin, interpretedas beingregionina with slightly affected theUpper Palaeozoic basins of Belgium and thickened crust, led Sanderson to suggest crustal thickening Germany, a substantial thickness of strata would have had from c. 22 km to c. 37 km duringVariscan transpression, to be present overlying the currently exposed succession in with some of the thickening possibly achieved by thrusting. order to produce the observed maturation levels. Asdiscussed earlier,the high maturation levelin the UpperPalaeozoic cannot be attributedto burial by the Conclusions Jurassic and younger succession in the region, noris it likely Vitrinite reflectance values from the Upper Palaeozoic rocks on the basis of offshore evidence, that great thicknesses of in the KinsaleHarbour-Old Head of Kinsale arearange Permo-Triassicsediments would have been deposited and from6.21%-7.28% R,, andfrom 4.06%-5.21% Rmi,,. theneroded. The thickcover must, therefore,have been Theseare indicative of meta-anthraciterank and suggest depositedmainly during Silesian time, and have been palaeotemperatures in excess of 250 “C. The lack of erodedduring or immediatelyfollowing the Variscan dependence of maturation level onstratigraphic position deformation to a relatively low structural level. stronglysuggests thatelevated temperatures were main- Evidencefor continuing post-Dinantian subsidence in tained until termination of Variscanfolding which, on south Munster is provided by the presence of a thick lower regionalevidence, was during late Carboniferous or early Namurianturbidite sequence on WhiddyIsland, which Permian times.

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Even allowing for abnormally high geothemal gradients Transactions of the Royal Society of Edinburgh: Earth Sciences, 74, 1-14. duringthe Carboniferous, several kilometres of Silesian HEROUX,Y., CHAGNON,A. & BERTRAND,R. 1978.Compilation and correlation of major thermal maturation indicators. American Associa- cover overlying the currently exposedsuccession would have tion of Petroleum Geologirts Bulletin, 62, 2128-2144. beennecessary in orderto accountfor the observed HIGGS,& BEESE,A. P. 1986. A Jurassic microflora from the Colbond Clay of maturation levels. Thiscover could have been produced Cloyne, County Cork. Irish Journal of Earth Sciences, 7, 99-109. either by deposition of a thick succession of Silesian clastic HIGOS,K., CLA~N,G. & KEEGAN, J. B.1988 Stratigraphic and systematic sedimentscomparable with thosepreserved in outliers palynology of the Tournaisian of Ireland. Special Paper of the Geological Survey of Ireland, 7, 1-82. further north in central Ireland, or by thrusting resulting in KUIJPERS,E. P.1972.F UpperDevonian tidal depositsand associated the tectonicemplacement of UpperDevonian-Namurian sediments south and southwest of Kinsale (Southern Ireland). PhD thesis, cover.Some combination of boththe above mechanisms, University of Utrecht. involving both Silesian sedimentation and later thrusting .is LEVINE, J. R. 1986. Deep burial of coal-bearing strata, Anthracite region, Pennsylvania: Sedimentation or tectonics? Geology, 14, 577-580. also clearly possible. In any event, considerable uplift and MCKENZIE,C. P. 1978.Some remarks on the development of sedimentary erosionoccurred subsequently, though it is unclearhow basins. Earth and Planetary Science Letters, 40, 25-32. much of this attributable to basin inversion, and how much MAVIATTHEWS,S. C., NAYLOR, D. & SEVASTOPULO,G. D.1983. Palaeozoic was regional uplift as aconsequence of intense Variscan sedimentary sequence as a reflection of deep structure insouthwest Ireland. Sedimentary Geology, 34,83-95. deformation.Elucidation of the precisechronological MUCHEZ,PH., VIAENE,W., WOLF,M & BOUCKAERT,J. 1987. Sedimentology, relationshipsbetween inversion, deformation, uplift and codification pattern andpalaeogeography of the Campine-Brabant erosion is precludedfor the present by the fragmentary Basin during the Vidan. Geologie en Mijnbouw, 66, 313-326. nature of the stratigraphic record in the area. NAYLOR,D. 1966. The Upper Devonian and Carboniferous geologyof the OldHead of Kinsale, CO Cork. Scientific Proceedings of the Royal Dublin Society, 2, 229-249. Thanksare due to R. Burnett, J. R. Graham, N.Haughey, D. -1969. Facies change inUpper Devonian and Lower carboniferous rocks Naylorand G. D. Sevastopulofor their comments on the of southern Ireland. Geological Journal, 6, 307-328. manuscript,and helpful discussion. Permission from the Depart- -, HIGGS,K., REILLY,T. A. & SEVASTOPULO,G. D. 1988. Dinantian and Namurian stratigraphy of the Seven Heads Peninsula, County Cork. Irish ment of Energy (Petroleum Affairs Division) to include unpublished Journal of Earth Sciences, 9, 1-17. Celtic Sea data is gratefully acknowledged. Thanks are also due to -, JONES,P. C. & CLAYTON,G. 1978. The Namurian stratigraphy of K. Higgs (University College Cork) for providing samples including Whiddy Island, BantryBay, West Cork. Bulletin of the Geological the ‘Colbond Clay’ from Cloyne. Survey of Ireland, 3, 235-253. -, NEVILL,W. E., RAMSBOTTOM,W. H. C. & SEVASTOPULO,G. D. 1985. Upper Dinantian stratigraphy and faunas of the Old Head of Kinsale and References Galley head, southCounty Cork. Irirh Journal of Earth Sciences, 7, 47-58. BIRD,1979. Continental delamination and the Colorado Plateau. Journal of OGUNYOMI,O., HESSER, R. & HEROUX,Y. (1980). Pre-orogenic and Geophysical Research, 84, 7561-7571. synorogenicdiagenesis and anchimetamorphism in LowerPalaeozoic BISHOPP,D. W. & MCCLUSKEY,J. A. G. 1948. Sources of industrial silica in continental margin sequences of the Northern Appalachiansin and Ireland. GeologicalSurvey of Ireland, Emergency Period Pamphlet, 3, around Quebec City, Canada. Bulletin of Canadian Petroleum Geology, 1-48. 28, 559-577. BRITISHSTANDARDS INSTTIUTION 1981. Petrographic analysis of bituminous PINET,B., MONTADERT,L., MASCLE,A., CAZES,M. & BOIS,C. 1987.New coal and anthracite. Part 5. Method of determining microscopically the insights on the structure and the formation of sedimentary basins from reflectance of vitrinite. BS6127 (S), 1-7. deep seismic profiling in Western Europe. In: BROOKS, & J. GLENNIE,K. BUNTEBARTH,G., KOPPE,I. & TEICHMULLER, M. 1981. Palaeogeothermics in (eds) Pemoleum Geology of North West Europe. Graham and Trotman, the Ruhr Basin. 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Received 12 July 1988; revised typescript accepted 29 December 1988.

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