J. geol. Soc. London. Vol. 137, 1980, pp, 65-75, 3 figs., 2 tables. Printed in .

The geology andpalynology of the Clays, Northern Ireland

G. C. Wilkinson, R. A. B. Bazley & M. C. Boulter

SUMMARY: Results from research on the Tertiary deposits in the Lough Neagh region of Northern Ireland are presented. The fossil pollen and spores show that the Lough Neagh Clays are of Chattian (late Oligocene) age. The deposits were formed in shallow temporary lakes around river deltas. Comoarisons are made with the Oligocene deposits in the western part of the British Isles.

The Lough Neagh Clays extend over about 500 km’, material from the Lough Neagh Clays. Brief results of of which 300 km2 underlie the lough; their outcrop is the palynological work on the Lough Neagh Clays are almost entirely sub-drift to thick Quaternary deposits, given, together with chemical analyses of the sediments mostly in the form of glacial boulder clay which forms in the new cores. the major surface feature of the area. The sequence consists mainly of pale-coloured clays and silts which reacha maximum knownthickness of 363 m; plant Historical review remainsare common but other fossils arerare. In places the plants are sufficiently concentrated to form The first scientificdescription of the LoughNeagh lignites similar in consistency to German brown coals. Clays was made by Hardman (1875),who had ob- The formation outcropsin the SW region of the lough, served sections exposed in workings for clay. wherethe clayswere formerly extensively dug from He reasoned that the sedimentswere post-basaltic and pits and mines to make coarse pottery and tiles. Com- were laid down during a period of greater extension of parable deposits in the Bovey and Petrockstow basins thelake. His very tentative Pliocene dating was re- in SW are a major commercial source of ball vised by Wright(1924), who postulated an early clays used for pottery, refractories and other manufac- Tertiary age on palaeontological evidence. Present day turingprocesses. The possibility of clays of similar interpretation of this data,both fromthe literature, quality,perhaps includinglignite, led the Geological museumspecimens and recentlycollected material, Survey of NorthernIreland to investigate thesedi- suggests that the only macrofossils that can be reliably ments by a borehole programme in 1964. Since then identified are specimens of fossilwood described by 28 shallow boreholes up to c. 60 m deep have been Gardner (1885) and Johnson & Gilmour (1922) and drilled,15 by the GeologicalSurvey of Northern referred by them to Taxodioxylon, the dicotyledon leaf Ireland and the remainder by English Clays Lovering described by Johnson & Gilmour(1921) as Dewal- Pochin & Co. Ltd. Most were concentrated to the E of quea, and the freshwater molluscs Viuiparus and Unio the lough where lignitereserves of 10-20,000,000 upon which Wrightbased his evidence for anearly tonnes have been proved. Only a single borehole was Tertiarydating. The plant macrofossilsfrom the drilled to the SW of the lough, but this is the area of WashingBay Borehole wereidentified by Johnson older bores (drilled in search of an extension of the (1941), but his conclusions cannot be confirmed since nearbyCarboniferous coalfields) from which some most of the specimens have subsequently been lost samples have been preserved. All these borehole sam- and the few that remain in the National Museum of ples presented an opportunityto re-study the sedi- Irelandare verybadly preserved. Cole et al. (1912) ments and to review the structure and origin of the wereaware of the differences betweenthe inter- deposits.The core samples becameavailable atthe basaltic sequence and the clays but were unable to be time of our first detailedcomparative palynological precise aboutthe position of thelatter within the study of Tertiarysediments in the westernBritish Tertiary. Indeed, none of these early authors gave a Isles, and this forms the basis of our new conclusions. confidentdating tothe LoughNeagh Clay deposits As well assamples fromthe recent drilling around and it would be wrong to imply that they had come to Lough Neagh (Fig. 1) this study involves palynological anyparticular conclusion: they had a limited know- work on material from the Bovey basin (), the ledge of Tertiaryfaunas and floras and had limited off-shoreStanley Bank basin (Bristol Channel) techniques at their disposal. (Hetcher 1975), the sand pits atSt. Agnes (Cornwall), Since the middle of the last century exposures of the theMochras borehole (N Wales),the clay pitsat clays have beenpoor, and mostinformation of the Ballymacadam (Co. Tipperary) and the older borehole deposits since Hardman’s day has come from borehole

5 0016-7649/80/0100-0065$02.00Geological Society@ The 1980

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I /Crurnlin/GlenavvArea -

36/2984).77 + Bellbrook Borehole + + /

++++

4,Imre nuux Are< . ," ~, -8. +++ t++++ t+ C'y+ + +

INDEX

+++ Lough Neagh Clay: I I++y+t+Portadown Tertiary basalts

FIG. 1. Geological map of the Lough Neagh region of Northern Ireland showing location of boreholes

cores.With the exception of the1976 cores little A second borehole was drilled in 1925 at Aughrim- material has been retained from the earlier boreholes derg to a depth of 502m (Hartley1948). NO fossils and critical appraisal of the flora is not possible. The havebeen described from thissection andthe core results of work on most of these boreholes are sum- samples were not retained. A few odd samples of core marized in theGeological Surveymemoirs on the survive from the boreholes at Mire House (1954) and LoughNeagh region (Fowler & Robbie1961; Man- Dernagh (No. 1, 1949; No. 2, 1954) and the descrip- ning et al. 1970). The best known borehole is the first tion of the 3 sections by Fowler & Robbie (1961) is all and deepest to have been drilled, 600 m at Washing thathas been published. These 5 boreholeswere Bay in 1919 (Wright 1924). This proved a thickness of drilled in the region to the SWof the lough 350 m of LoughNeagh Clays lying on theUpper aspart of theexploration for Coal Measures. No Basalts of the Antrim Lava Group. Wright described3 complete scientific examination of the cores was made, units within the clays, theUpper Clays andSands and now that so little of the material remains we must (245m), the Middle Shales (38 m) andthe Lower awaitnew exploration in the region for complete Clays and Sands (58 m) (Fig. 3). It was the lower unit analyses to be made. Although thepublished results of which containedthe thickest bands of lignite, up to Wright,Hartley, and Fowler & Robbieare mainly about 2 m thick in parts, though black layers of plant concerned with brief details of the borehole logs and derived material were presentin the other two unitsas discussions of the stratigraphy of the sections, some well. conclusions on the origin and age of the deposits have

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been made as a result of other studies on the material In the Crumlin-Glenavy sector of the lough (Fig. l), from these boreholes. This work was summarized by drillinghas added considerably to our knowledge of Watts (1970), who undertook a preliminaryanalysis of the lignites, though since none of theboreholes ex- the fossil plants. From the limited amount of material ceeded 60 m in depth the complete sequencewas only available to him fromthe WashingBay and Mire penetrated near the edge of the basin. In this area the House boreholes and from surface exposures at Ver- sediments are dominantly silty clays, light to medium ner’s Bridge(Co. Tyrone) and Thistleborough (Co. grey in colour,though plant debris is commonand Antrim) (a glacial erratic), Watts concluded that ‘the causesa darkening of thesediment. Lignites upto later or Oligocene is the most probable date 22 m in thickness are developed locally. Correlation of for the Lough Neagh Clays, as was suggested earlier the lignite seams between the 20 boreholes drilled has (Watts 1962)’. proved difficult, due presumably to the very localized, On the basis of Wright’schemical analysis of the more or less autochthonous deposition. But thereis no clays from the Washing Bay Borehole, as well as from evidence thatthe lignitewas deposited in lenses or contemporary field observations,Fowler & Robbie thatthe sequence is much brokenup by interbasin suggested that the main source of material forming the faults. A more likely factor is that the thicker marginal Lough Neagh Clays was the Millstone Grit, with other seams split up towards the centre of the basin, espe- possiblesources being the Newry andMourne gra- cially as much of the plant material is of local origin, nites, basalts and rhyolites of the Antrim Lava Group, as indicated by the presence in many of the clay beds the Chalk, and the Limestone. of plantroots and more rarely of stems in growth positions. The plants forming the lignite clearly grew New details of the sediments around the basin, probably in small, shallow, swampy lakes. It is possible that the sediments sag towards the Fullresults of the GeologicalSurvey of Northern centre of the basin owing to greater compaction of the Ireland’sdrilling programme are published in their thicker sediments. Open File Reports 55 and 58; Table 1 gives some of In the Crumlin-Glenavysector of the basin there the new chemical data. aresubordinate thin sandy horizons but no pebble

TABLE1: Chemical analyses of the clays from Lough Neagh Clay Formation.

Borehole Depth below %O loss on Silicon Aluminium Ironas Calcium Magnesium SodiumPotassium (G.S.N.I. No.) surface (m) ignition as % SiO, as ‘70 A1,0, % Fe,O, as %O CaO as % MgO as %O Na,O as %O K,O (a) Eastern side of Lough Neagh CrumlinIGlenauy area Bellbrook 3612984 14.35-14.8361298411.56 57.28 16.92 5.34 0-03 1.05 0.43 1.35 33.1-33.6 23.85 29.12 10.09 29.50 0.59 1.71 0.59 1.44 41-85-42.315.33 45.08 23.64 5.07 0.006 1.23 0.48 1.26 Ballinderryl Aghalee area 3613012 48.77-49.99 17.50 45-11 16.43 15.47 0.13 2.67 0.34 1.15 3613014 28.65 13-66 47.75 16.46 11.33 0.17 2.17 0.40 1.31 3613014 45.11 11.78 57.49 12.36 11.69 0.005 1.86 0.31 1.13 3613015 30.48 17.22 42.19 13.87 16.79 0.005 1.04 0.32 0.94 3613015 53.95 15.54 44.69 19-86 13.36 0.016 1.24 0.20 0.84 (b) Western side of Lough Neagh Washing304.8-306.3Bay 0.44 60.441.64 20.60 0.40 1.18 0.20 2.21 Borehole*3074-308.3 0.33 75.13 1.04 12.69 0.21 0.72 0.22 2.11

* Results from analyses by J. Keane 1920 (Fowler &L Robbie 1961, p. 236); these results are similar to those obtained for samples from the Ballynakilly Borehole. All other results from analyses by the Department of Industrial and Forensic Science, Northern Ireland. For details of analytical methods see the Open-File Reports of the Geological Survey of Northern Ireland, 55 and 58.

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/137/1/65/4886585/gsjgs.137.1.0065.pdf by guest on 30 September 2021 68 G.C. Wilkinson, R. A. B. Bazley &M.C. Boulter beds,suggesting generally low energy conditions. includemica andfeldspar. Siderite appears to be Whilst no major river system seems to have flowed in absent in the samples tested. from this direction, the presenceof sand suggests some element of deltaic conditions. A small river may have Palynology flowed in from the NE and possibly its floodplain cut across the plant rich beds causing some of the appar- These investigations of the pollen andspores are ent lateral discontinuity of the lignites. Such drainage basedon core samplesfrom the Mire Houseand would be expected to have existed around the basin, Washing Bay boreholes as well as the recent boreholes muchas today. Anotherfeature of interestin this atBallynakilly and Bellbrook (No. 1) (Fig.1). The sector is the iron-rich nature of the clays. The associa- work was done as part of a detailed comparative study tion of lignite andsiderite clays is notuncommon of the pollen and spores from Tertiary deposits in the elsewhere in the world. In many parts of the sequence western part of the British Isles and will be presented nodularclay-ironstone bands are developed, but indetail in aspecialist palynological journal. This everywhere the clay has a relatively high iron content section summarizes the results from Lough Neagh in (Table 1). This high iron content makes the clays in established palynological terms in order to support the this sector virtually useless for ceramic purposes. stratigraphic, palaeoecological and palaeogeographical Further S, boreholes6 were drilledin the conclusions. Microscope slide preparations of the pol- -Aghalee area (Fig. l), and proved an un- len and spores aswell as samples of the sediment from usually thick sequence of Quaternary deposits (Bazley theboreholes are deposited in theUlster Museum, 1978).The Lough NeaghClays, wherepenetrated, BotanicGardens, Belfast BT95AB (nos.K4, were almost entirely of clay and silty clay with only 401-K4, 600) and in the I.G.S., 20, College Gardens, thin lignites (less than 2 m total thickness in 5 seams). Belfast BT9 6BS (nos. MPA 7192-7391). Plant roots and stems in growth positions were com- 56form-genera of fossilspores and pollenhave mon and the iron (expressed as percentage Fe,O,) was been identified (Table 2). These taxa are represented generally over 10% (Table 1).The virtual lack of sand by several hundreds of specimens in our preparations, or coarser grade material indicates no river source in which show ranges of morphological character which this area. enablefurther sub-division of thesegroups of Theremainder of theTertiary sediments around palynomorphsinto a total of 95 kinds. Experience LoughNeagh haverarely been seen except in the shows that there is littlevalue in making subgeneric older boreholes to the SW. The thick pebble beds at identificationfor this Palaeogene pollen(Krausel the base of many of these boreholes suggest a major 1961, p. 238; Boulter, 1979). influx of material from a westerly direction. A single The histogram (Fig. 2) gives details of the quantita- new borehole (G.S.N.I. no. 31/1091) was drilled to a tive results for those form-genera which have a per- depth of 31 m at Ballynakilly near Coalisland. It was centage frequency greater than 10% within the sam- sited near the former brick pits, which are now filled, ples examined; all other form-genera listed in Table 2 to evaluatethe quality of the clay. Thesequence, occur with a frequency of less than 10% in any one beneath c. 10 mof Quaternary deposits, was mostly of sample. Trends such as the higher count of Pityospo- light grey silty clay. In parts the clay contained plant rites below 200 m in Mire House and WashingBay remains which caused a darkening of the colour. The boreholes are not possible to interpret here due to the excavation of clay in this area may have beenre- uncertaincircumstances of depositionand the un- stricted due to water inflow from a 50 cm thick bed of knownposition of theupland vegetationwhich is sand at a depth of 20 m. The iron content of the clay thought to have produced the fossils of that particular here is generally low, but large ironstone nodules were form-genus. But together with the other forms pres- reported in the claypits nearby. The clay hasbeen ent, the quantitative results give a clear indication of analysed andtested by the Mineralogy Unit of the thetype of vegetation which existed in the Lough Institute of GeologicalSciences (G.S.N.I. Open File Neagh region at the time of deposition. Report 58, Bain et al. 1976). It was concluded that the The annotations to the form-generic names (Fig. 2) claywas a finely divided plastic clay containingthe give some indication of the kind of palaeoecological triplemineral assemblage of kaolinite-mica-quartz deductions that can be made from the Lough Neagh usually associated with English ball clays. Kaolinite is assemblage.Such reconstructions of environmental themajor clay mineralpresent, usually comprising featuresare traditionally made by referenceto the just under SO%, with a second clay phase that may be presentday ecology of what are assumed to be the broadlydescribed as asomewhat chloritized low- most closely related modern equivalents of the fossil charge vermiculite. In some samples this approaches a plants. They should be treated with some reservation, regularly interstratified 1 : 1 chlorite :vermiculite. In however, since little is known either of the effects of othersamples the vermiculite appearsto contain a different quantities of pollen production from the par- certainamount of micainterlayering hydrobiotite. ent plants, or of the evolutionary adaptations of the Quartz makes up 10-25'/0 of the samples, which also plants to the environments that have existed since the

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TABLE2: The occurrence of form-genera of pollen and spores in the 4 Lough Neagh sections and in the comparable deposits at Mochras and Bovey

Bell- Mire Washing Bally- brook House Bay MochrasnakillyBovey Echinosporis + + + + Microfoveolatosporis + + Monolites + + ++ Polypodiaceaesporites ++ ++ + ++ + ++ Polypodiidites + + + + + + Polypodiisporonites + + + + + + Baculatisporites + + + + + Camarozonosporites + + + + + ++ Cicatricosisporites + + + + + ++ Deltoidospora + + + + + Echinatisporis + + + + Gleicheniidites + + Microfoveolatisporis + + Lycopodiumsporites + + + + + Osmundacidites + + + ++ Polypodiaceoisporites + + + + Rugulatisporites + + Stereisporites + + + Toroisporis + + ++ Trilites + + + + ++ Triplanosporites + + + + + Verrucingulatisporites + + Abiespollenites + + + Cedripites + + + ++ Corrusporis + + + + Cycadopites + + + + t + Ephedripites + Inaperturopollenites ++ ++ ++ ++ + + Piceapollenites + + F’ityosporites ++ ++ ++ + ++ ++ Arecipites + + + + + ++ Monocolpopollenites + + + + + ++ Graminidites + + + + + + Dicolpopollis + + + ++ Ilexpollenites + + + + + + Nyssapollenites + + + ++ ++ ++ Tricolpopollenites ++ ++ + + ++ ++ Tricolporopollenites ++ ++ + ++ ++ ++ Mediocolpopollis + + + Salixipollenites + + ++ + ++ + Engelhardtioipollenites + + + + + + Momipites + + + + + ++ Myricipites + + + + + ++ Subtriporopollenites + Corsinopollenites + + Boehlensipollis + + + + Gothanipollis + + Porocolpopollenites + + + + + + Tiliaepollenites + + + ++ + ++ Tetracolporopollenites + + Periporopollenites + + + Polyatriopollenites + + + + + + Polyvestibulopollenites ++ ++ + + ++ ++ Erecipites + + + + Anacolosidites + Pompeckjoidaepollenites +

+ present in quantities up to 5% relative pollen frequency, ++ present in quantities over 10%.

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/137/1/65/4886585/gsjgs.137.1.0065.pdf by guest on 30 September 2021 Polypodiaceaesporites 250'0 fernspores, usually characterising damp or humid conditions - !5 '/o Pityosporites coniferpollen, perhaps dispersed from a rela- tively long distance upland forest

Inaperturopollenites smaller conifer pollen, similar to that produced 25OlO frommodern Taxodium (swamp cypress) and deposited near to the place of growth - Arecipites palm pollen from tropical or sub-tropical plants

Monocolpopollenites - pollen from tropical or sub-tropical plants Tricolporopollenites - pollen from trees or herbs with broad affinity Tricolpopollenites pollen from trees or herbs with broad affinity

7

5v0 Polyoestibulopollenites pollen similar to modern Alnus (alder)

FIG.2. Annotated histogram of the occurrence of ecologically important palynomorphs from 4 of the boreholes.

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time when the deposit was formed. The largest pollen 1975;Gruas-Cavagnetto 1976; Roche & Schuler countsare of the coniferpollen Inaperturopollenites, 1976),thus confirmingearlier work in Germany familiarin theGerman browncoaldeposits, which (Krutzsch 1967a,1969). reliablyconfirms a warm and swampy environment 2. Assemblage types are comparable to those from (Teichmiiller & Teichmiiller 1968). Other members of the Bohemian Chattian (Pacltovi 1960), the Slovakian the assemblages,such as Polyuestibulopollenites, Chattian(Pacltov5 1966),the Oligocene of the Tricolpopollenites, and Engelhardtioipollenites are com- Grande Limagne (Gorin 1975), the Belgian Tongrian monly associated with such an environment. Although (Roche & Schuler1976), the PolishChattian there is some debate aboutits precise botanical affinity (Grabowska1965; Doktorowicz-Hrebnicka 1957; (Sivak 1976), the commonly occurring conifer pollen Ziembinski-Tworzydlo 1974), the Bouches-du-RhBne Pityosporites is generally interpreted as comingfrom of France (Chateauneuf 1972) and the East German moreupland regions, though inthis areathese may Oligocene (Krutzsch 1967a). have beenhummocks in the swamps. The simple 3. The form-genera Monocolpopollenites, Arecipites monocolpate morphology of Arecipites and and Dicolpopollis, althoughnot restricted tothe Monocolpopollenites is thought to be characteristic of Oligocene, occur together with such regularity only in palmpollen, which, together with other tropical or NW European Oligocene deposits. sub-tropicalforms suggest, from comparison with 4. A number of forms described in the literature are modern distribution maps, that the climate at the time known to be restricted to the Chattian, whilst others of deposition was completely free from frost. A similar are generally well dispersed in, and hence considered assemblage is found within the other Tertiary deposits betocharacteristic of, the middle andupper in the western British Isles, though the proportions of Oligocene of Europe. Severaltaxa recorded in this theabundant plants varyconsiderably. Thegeneral investigation display a close but untested similarity to picture of a low-lying warm frost-free tree-shrub vege- some of thesepreviously recordedcharacteristic tation with no significant botanical variation through- forms. Verrucingulatisporites treplinensis Krutzsch out the timeof deposition of the deposits is confirmed. 1961, Cicahicosisporiteschattensis Krutzsch 1961, Since thepioneering work of Thomson & Mug Comsporis granotuberculatusKrutzsch 1967, Coms- (1953), at least 12 important papers have been pub- pons Megabaculus Krutzsch 1967 andMediocolpopollis lished describing Oligocene pollen and spore assemb- compactusellenhausensis Krutzsch 1959are lages in Europe. There is a strong similarity between forms restricted to the Chattian of Europe. Tricolporo- the assemblage lists, which show little floristic varia- pollenites ipilensis Pacltovi1966 has beendescribed tion throughoutthe greater part of the Oligocene, from the Neogene of Europe (Boulter 1971) as well as making the use of pollen andspores asa means of the Palaeogene (Pacltovi 1966, 1978) and is thought determining preciseagevery difficult. Krutzsch to be particularly significant for indicating low temper- (1967b) gave evidence which suggested that many ature zones in the upper Oligocene, where it is espe- plant migrations were taking place in NW Europe at cially abundant. Forms which are generally well dis- about the beginning and end of the Oligocene. How- persedin, and characteristic of,Oligocene strata in ever,the evidence from pollen andspores suggests Europe (Krutzsch 1967a) include Polypodiisporonites that the rapid cooling which took place throughout the alienus PotoniC 1931, Tiliaepollenitesinsculptus (Mai Northern Hemisphere at the end of the Eocene or in 1961)and T. instructus Potonie1931, Tricolporo- the early Oligocene (Buchardt 1978; Wolfe 1978) was pollenites spinusKrutzsch 1961, Porocolpopollenites followed by a period of climatic stability with slightly calauensis Krutzsch 1961 and Polyatriopollenites stel- warmer phases occurring during the deposition of the latus PotoniC 1931 ex Pflug 1953. Lough NeaghClays. Theearlier view of Barghoorn 5.The sediments examined donot containany (1951) that the climatic fluctuations that began in the characteristic Eocene or Miocene forms. Neogene in Europewere preceded by asubstantial Palynologicalanalysis of all sediments revealsa period of more or less constant mean annual tempera- climatic andvegetational consistency throughout the tures, is therefore confirmed. Oligocene, with anysignificant quantitative variation Dating of the Lough Neagh Clays was undertaken predominantlydue to localinfluences. Comparisons by comparison with the assemblages fromMochras, with European Oligocene assemblages show that the Stanley Bank(Fletcher 1975; Boulter & Craig,in Lough Neagh Clays are of a Chattian age. Lithological press), and Bovey. After detailed comparisons of the considerationsalone indicate that the sequence of pollenand spores, 5 major lines of evidencehave lignite bands in the Mire House borehole at a depth of resulted in the view that the Lough Neagh Clays are 120-181 m are comparable with those from the Wash- Chattian in age: ing Bay borehole at a depthof 204-265 m. Palynolog- 1. The occurrence in all 4 boreholes of the charac- ical analyses are in agreementat levels 213m and teristicpollen Boehlensipollis; work on FrenchTer- 277 m respectively in these sections. tiarypalynology shows that this form-genus is re- Morphographicalfeatures of the dispersedspore strictedto the Oligocene (Gorin 1975; Sittler et al. Cicatricosisporites show the lower half of the Mochras

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/137/1/65/4886585/gsjgs.137.1.0065.pdf by guest on 30 September 2021 72 G. C. Wilkinson, R. A. B. Bazley & M. C. Boulter beds to be comparable with the Bovey Formation and The Palaeogene environment at the upper half with the lower beds of the Mire House Lough Neagh borehole. The distinctive tricolporate pollen Mediocol- popollis occurs in 2 distinct phases, the first revealing itself in theMochras borehole at 80 mand in the The LoughNeagh Clays contain theyoungest Washing Bay borehole at 273 m, and the secondin the Oligocene sediments in the western British Isles and Mochras borehole at 408 m and in the Bovey basin at were deposited in a lake much larger than that indi- 140 m. The characteristic form-genera Anacolosidites cated by thepresent daypost-glacial Lough Neagh. and Pompeckjoidaepollenites arefound invery small Following pre-middle Oligocene deformation and ero- quantities at the base of the drilled upper part (297 m sion,sedimentation occurred in agently subsiding depth) of the Bovey Formation,thus suggestingthe basinwith overlap onto the Mesozoic, and probably probability of Eocenesediments at greater depths. Palaeozoic rocks to the W. A major westerly source of The presence of derived Lower Palaeozoic acritarchs detritus has been postulated (Hardman 1875; Fowler such as Vulcanisphaeru in the Lough Neagh and Tre- & Robbie1961; George 1967), with the possible madoc Bay (N Wales) basins indicates the importance growth of a delta into the lake basin. of Palaeozoic rocks as suppliers of sediment for these The old Tertiary lake certainly extended beyond the Tertiary basins. The palynologicalresults support presentlimits of Lough Neagh, butthere is no evi- these views and the stratigraphic relationship between denceto show that itslimits ever went beyond the the basins (Fig. 3). presentconfines of Ulster.The tectonicbasin was

/

MIRE WASHING MOCHRAS BOVEY HOUSE BAY

0- LATE OLIGOCENE

UpperClays andSands

Middle Sholes 7 tower Clays LigniteandClaystower Clay Unit Ballymacadam

Series------___ 141

BasalBeds Petrockstow EARLY OLIGOCENE 301 __---? ------602 EOCENE unexamined BoveyFm. Boreholedepths in metres 0Drift deposits 80 O- O- Km.

FIG. 3. Palynological correlation between the Oligocene sediments from 4 boreholes links 3 of the major Tertiary basins in the western British Isles (M, Mire House; W. Washing Bay). The correlation is based on the occurrence of Cicatricosisporites and Mediocolpopollis (see text).4 geographical locations are referred to (P, Poyntzpass; N, Newry; B. River Bann; MO, Mourne Mountains).

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probably produced by a combination of synclinal fold- conditions, this is not the case with the other deposits. ing and block faulting on earlier structural lines. The The Petrockstowbeds (661 m)were probably laid volcanic outpourings of the Antrim Lava Group were down by a river system in a partially fault-controlled probably followedclosely by earthmovements and sedimentary basin which basically followed the line of crustal warping; these couldhave been accompanied the Lustleigh-Sticklepathfault (Freshney 1970). The by the intrusive episode of the Mourne Granite,whose Bovey Formation(1200 m) wasdeposited mainly age is acceptedas c. 58Ma, i.e. UpperPalaeocene under flood conditions in a basin tectonically similar to (Fitch & Hurford 1977). A period dominated by ero- Petrockstow, with recognizable alluvial fan and lacus- sion must have followed during Eocene times when a trine components (Edwards 1976). The Mochras beds humid sub-tropical climate probably prevailed. A gen- (524 m) weredeposited under conditionssimilar to eral lack of sediments from this period suggests that those at Petrockstow, with a river flowing S along the the basin had not becomeclosed. However, judging by eastern part of the basin parallel to the fault scarp; the the palynology, the basinclosed in early to middle Lignite and Clay Unit of the Mochras Tertiary beds Oligocene times and a freshwater lake was formed in and the fine-grained clays of Tremadoc Bay are consi- the gradually subsiding area. The sediments arefluvial deredto be the equivalent of the AxialFacies and in origin and were probably deposited in a relatively Shelf Faciesrespectively of the Petrockstowbeds shallowlake into whichriver deltas were extending. (O’Sullivan 1971a). The main influx of materialwas from awesterly The Palaeogene history of the western part of the source, as indicated by the pebble beds on this sideof BritishIsles is bestconsidered in terms of 3 major the basin. Much material was derived primarily from confined areas of sedimentation in which fluctuating the Carboniferous sandstones and clays to the W, with environmental conditions give rise to both lacustrine some contribution from other rocks in the surrounding and flood-plain deposits (Fig. 3). Throughout the time catchment (Fowler & Robbie 1961). The coarsening of of deposition of thesesediments, the shallow St. sediments in the Crumlin-Glenavyregion suggests George’s Channel, much less extensive than the pres- that a small river may have entered the basin from the ent day Irish Sea, would have provided an outlet for NE.In the shallow temporarylagoons around the the major river systems running through or associated deltas, the plants thrived in the warm climate, produc- with these areas of sedimentation. An outlet for the ing the carbonaceous beds. The outlet of the lake is moresoutherly Bovey Basin would have been pro- uncertain. The basalts to the N of Lough Neagh could vided by the western part of the English Channel. be imagined as thicker than at present and may have Fromlate Eocene to middleOligocene times presented a barrier. It seems likely that at some stage sedimentationspread along, and wascontrolled by, the outlet was through the remarkable straight valley the line of the Lustleigh-Sticklepath Fault, which may trendingSSE from LoughNeagh by Poyntzpass to even have extended to the coast of Dyfed, S. Wales Newry (Charlesworth1963). Even today the high (Fletcher 1975), giving rise to the upper part of the pointalong this remarkablefeature is onlya few Bovey Formation, the Petrockstow Beds, the middle metres above Lough Neagh. The present day outlet of Oligocenesediments of the off-shoreStanley Bank Lough Neagh to the N along the Lower River Bann Basin, which lies between LundyIsland andMorte seems unlikely to have operated during the Oligocene. Point in N Devon (Fletcher 1975; Boulter & Craig, in It seems much more likely that the basalts would have press) andthe sediments at Flimston,Dyfed (Dixon still formedan effectivebarrier not breached until 1921). after the Miocene earth movements. Duringearly to upper Oligocenetimes a second area of sedimentation is defined by thefault- controlled basin of Tremadoc Bay (Kent 1975) which is boundedto the E by the N-S MochrasFault. The Palaeogene sedimentsin Palynologicalevidence shows thatsediments reco- the western part of the British veredfrom a borehole at Tonfanau (SX 5600 0376) Isles (situated to the E of where the southerlyfading Moch- ras Fault is truncated by the Bala Fault System) are The Lough Neagh Clays, the Mochras beds (O’Sulli- Oligocene in age. Thesolution subsidenceoutliers van 1971b), Bovey Formation(Edwards 1976) and preserved in theCarboniferous Limestone of NE thePetrockstow beds(Freshney 1970) displaya Wales(Walsh & Brown 1971) cannot be an easterly marked sedimentological similarity. The Lough Neagh extension of thisparticular phase of sedimentation Clays were first compared with the Bovey Formation since the high-relief landmass would have acted as a by Griffith (1838); George (1967, 1974a, b) discussed barrierto suchan event.These outliers are thus the relationships of the major Tertiary basins in the considered tobe younger in age thanthe major western British Isles. Palaeogene deposits in the western part of the British Whilst theLough NeaghClays areconsidered to Isles. Further evidence of sedimentationoutside the have beendeposited under predominantly lacustrine main area is given by the solution subsidence deposit

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at Ballymacadamin Co.Tipperary (Wynne 1857; ACKNOWLEDGEMENTS.We thank English Clays Levering Watts1957; Curry et al. 1978)and at St. Agnes in Pochin & Co. Ltd. for permission to use borehole material; Cornwall (Atkinson et al. 1975; Curry et aJ 1978). for the Department of Industrial and Forensic Science chemi- A third major area of sedimentation, persisting from calanalyses and tests on materialfrom Northern Ireland middle tolatest Oligocene times, is definedin the Department of Commerceboreholes; members of the Geological Survey of Northern Ireland foruseful discussions; Lough Neagh region, giving rise to the Lough Neagh Watts,Blake, Bearne & Co.Ltd. for providing borehole Clays. Outside the main areas of this study the Ter- samples from the Bovey Formation. The palynological work tiary sediments of the Canna Basin in the. Sea of the was supported by grants from NERC and the Royal Society. Hebrides (Smythe & Kenolty 1975)have been ex- Publication is by permission of the Director of the Institute of amined palynologically (Evans et al., 1979), and are Geological Sciences. considered to bethe time equivalents of the Lough Neagh Clays.

References

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