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Silurian Microfossils from West of the Leinster Granite

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Silurian Microfossils from West of the Leinster Granite Silurian microfossils from west of the Leinster Granite PETER M. BR~CK & CHARLES DOWNIE SUMMARY The upper part of the Lower Palaeozoic younger. These Silurian rocks are part of a sequence west of the Leinster Granite in eastern turbidite succession which extends down to Ireland is shown, by means of chitinozoa, to the lower Ordovician west of the granite and be Llandovery in age, with some of it perhaps down to the lower Cambrian on the east. TI-IE LOWER PALAEOZOIC sequence west of the Leinster Granite (Fig. I) is a thick group of greywackes that conformably overlie, along the edge of the granite, a narrow belt of more slaty rocks. The greywackes are differentiated into five formations and the slaty rocks into two formations (Brtick I97O , Briick et al. 1974). The succession and the approximate thicknesses in metres are as follows: (7) Carrighill Formation 76o (6) Tipperkevin Formation I IO (5) Glen Ding Formation 43 ° (4) Slate Quarries Formation 4 ° (3) Pollaphuca Formation 175 ° (2) Aghfarrell Formation 57 ° (I) Butter Mountain Formation 675 The two lowest formations are part of the Ribband Group. The lithologies are described in Brfick (1972) and Brfick et al. (1974). In some earlier work that dates and correlates the rocks immediately flanking both sides of the Leinster Granite (Briick et al. i974) microfossils were obtained from the Butter Mountain, Aghfarrell and Pollaphuca Formations indicating that these formations are lower Ordovician. The Slate Quarries Formation has yielded less diagnostic microfossils that merely hint at a mid-Ordovician to lower Silurian age (Briick 1971 ). The Glen Ding, Tipperkevin and Carrighill Formations resemble in lithology Silurian strata occurring in other parts of Ireland, and have been classed on this basis as Silurian (Briick i97o ). This note presents microfossil evidence indicating a definite Silurian age for the three youngest formations. I. Microfossils Seventeen samples were processed and examined. These include one from the Pollaphuca Formation, two from the Slate Quarries Formation, five from the Glen Ding Formation, four from the Tipperkevin Formation and five from the Carrighill Formation. The localities are shown in Fig. I. The samples from the Pollaphuca, Slate Quarries and Carrighill Formations yielded only unidentifiable Jl geol. Soc. Lond. vol. x3o, I974, PP. 383-386. I Fig. Printed in Northern Ireland. Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/130/4/383/4884738/gsjgs.130.4.0383.pdf by guest on 24 September 2021 384 Peter M. Briick & Charles Downie ~oo N~C STRATIGRAPHICAL SUCCESSION AND LEGEND ~ Lower Carboniferous Carrlghilt ~ Llandoveryor younger Formation Fo, mot~on Glen Drag Lk:mdover y Formation ....... F.... t,o. i I to k Silurian Format.on ~o. ~ Go Agh~a,,ell For~atio. Lr. Ordoviclan :ili iiiiiii i!i!; ; ::iiii!ili Butter Movntoln Format,on 1-17 Microfossil localities listed in Table I ~/~ M~crofos~illocalities previously described (see text) Geolog,cal boundaries ! ~ ~ Fauffs [] Towns 0 t ~ ,i 5 :~:: i i!i""+"::":':':':!:i:':':':':':'":':':'"'"'"'"" i!ii::iiiiiiiiii: :: .... ..... rocks 3. La,gely Precombrlon F .......... ." ***÷÷ ° **+ - ~o 4. te.nstef G,on;le '::?:i:i:!:i:i:!:i:i:i:i:!:i:i:i:!:!:! NIO ~oo Fzo. I. Geologicalmap of the Lower Palaeozoic rocks west of the Leinster Granite, showing microfossil localities. chitinozoa that provide no indication of age. However, the samples from the Glen Ding and Tipperkevin Formations yielded diagnostic assemblages of chitinozoa and also scolecodonts. 2. Stratigraphical implications The known stratigraphical ranges of individual species are shown in Table I. The same species have been recorded from elsewhere by Eisenack (1968), Taugourdeau et al. (1967) and Taugourdeau & Jekhowsky (z96o). The overall stratigraphy indicated by the assemblages are a Silurian age for the Glen Ding Formation and a Llandovery age for the Tipperkevin Formation. The Glen Ding Formation, however, underlies the Tipperkevin Formation and must therefore also be Llandovery. Sample 73/0506 (Table 1) was taken only 4 m below the top of the Tipperkevin Formation, and is clearly Llandovery. Therefore the lower part of the Carrighill Formation, which conformably overlies the Tipperkevin Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/130/4/383/4884738/gsjgs.130.4.0383.pdf by guest on 24 September 2021 Silurian microfossils from west of the Leinster Granite 385 T A ~ L ~ I " Microfossils from west of the Leinster Granite Locality Known Specimen number Grid stratigraphical Age of Formation number (Fig. 1 ) Reference Microfossils ranges of species assemblage 73/o5o7 (6202) 13 N 882o76 unidentifiable chitinozoa 73/o5o9 (6204) 11 N 892079 unidentifiable chitinozoa Carrighill 73/o5Io (6205) 12 N 893078 unidentifiable chitinozoa Formation 73/o5I I (6206) x7 N 805055 unidentifiable chitinozoa 73[o512 (62o7) I6 N 868o36 unidentifiable chitinozoa 73/o5o6 (62oi) 14 N883o75 Ancyrochitina sp. Ancyrochitina ancyrea ? Llandovery/Lr. Dev., rare in AshgiU. Rhabdochitina canna ? Wenlock Conochitina cf. parvidecipiens Silurian Conochitina proboscifera ? Llandovery/Wenlock Conochitina edjelensis Llandovery Tipper- Conochitina armillata Llandovery/Wenlock Llandovery kevin Gonochitina turris ? Caradoc/Silurian Formation Conochitina brevis Llanvirn/Llandovery Clavachitina dactylus U. Ordovician/ Wenlock Desmochitina minor Caradoc unidentifiable scolecodants 73/o5o8 (6203) IO N 888o8I chitinozoa, all identified 73/o513 (6208) I5 N 873038 species included in 73/o517 (6212) 8 N 932138 specimen 73/o5o6 73/o5oI (6196) I 0 OI32I 9 poorly preserved chitinozoa including Gonochitina spp. 73/o5o4 (6199) 4 N 991187 Rhabdochitina canna ? Wenlock Cyathochitina conica ? Caradoc Conochitina armillata Llandovery/Wenlock Gonochitina proboscifera Llandovery/Wenlock Silurian Gonochitina latifrons Ludlow Glen Ding Conochitina turris Caradoc/Silurian Formation Bursachitina lagenomorpha Llanvirn]Ludlow 73[o514 (6209) 7 N 963154 poorly preserved chitinozoa including Ancyrochitina sp. 73/o515 (621o) 6 N 958 x58 poorly preserved chitinozoa including Conochitina probosdfera Llandovery/Wenlock and Conochitina spp. 73[o516 (621 x) 9 N 94II32 poorly preserved chitinozoa including Gonochitina spp. Slate 73/o5o2 (6197) 2 N oi4219 unidentifiable Quarries 73/o5o5 (6200) 5 N 992186 chitinozoa Formation Pollaphuca 73/o5o3 (6198) 3 N oi6218 fragments, possibly of Formation chitinozoa University of Sheffield numbers are in parentheses; the specimens are housed in the Geology Department, University of Sheffield. Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/130/4/383/4884738/gsjgs.130.4.0383.pdf by guest on 24 September 2021 386 Peter M. Briick & Charles Downie Formation with a 2 m thick lithological transition, may also be Llandovery while the remainder may be Llandovery or younger. Two species recorded are inconsistent with a Silurian age. These are Desmo- chitina minor in the Tipperkevin Formation and Cyathochitina conica ? in the Glen Ding Formation. The latter is a doubtful identification of the species and need not be inconsistent. The former is anomalous and cannot be readily explained. Perhaps it ranges into the Silurian. The preservation of the material is not good and unbroken specimens are rare. Since the certain identification of chitinozoan species often requires a reasonably large sample, several of the identifications are queries, and it would be desirable to make a more exhaustive study to consolidate the findings. 3. Conclusions The stratigraphical succession shown in Fig. I summarises the stratigraphy and deduced ages. A succession from lower Ordovician to at least Llandovery is present, but it may not be complete. ACKNOWLEDGEMENTS. We thank Professor C. H. Holland and Dr M. D. Max for reading the manuscript. The paper is published with the permission of the Director of the Geological Survey of Ireland. ft. References BROCK, P. M. 197o. Stratigraphy, petrology and structure of the greywacke formations of the Blessington area. Bull. geol. Surv. lreland x, 31-45 . 1971. Fossil content and age of the greywacke formations west of the Leinster Granite in Counties Dublin, Kildare and Wicklow, Ireland. Geol. Mag. xo8, 3o3-3IO. 1972. Stratigraphy and sedimentology of the Lower Palaeozoic greywacke formations in Counties Kildare and West Wicklow. Proc. R. It. Acad. 7211, 25-53. POTTER, T. L. & DOWNIE, C. 1974. The Lower Palaeozoic stratigraphy of the northern part of the Leinster Massif. Proc. R. Ir. Acad. 74B, 75-84. EISENACK, A. 1968. Uber chitinozoen des baltischen Gebietes. Palaeontographica A. x3x , I37-I98. TAUGOURDEAU, P. & JEKHOWSKY, B. I96O. Repartition et description des chitinozoaires siluro- devoniens de quelques sondages de la C.R.E.P.S., de la C.F.P.A. et de la S.N. Repal au Sahara. Rev. Inst.fr. Pdtrole 15, 1 I99-126o. BOUCH% P., COm3AZ, A., MAOLOIR% L. & MaLLEemD, P. 1967. Les chitinozoaires. Edit. C.N.R.oe. x, 1-97. Received 28 November i973; revised typescript received 22 December I973. PETER Mac~. BROCK, Ph.D., F.G.S., Geological Survey of Ireland, x4 Hume Street, Dublin 2. CHArU.ES DOWr~E, Ph.D., F.G.S., Department of Geology, The University, Sheffield $I 3JD. Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/130/4/383/4884738/gsjgs.130.4.0383.pdf by guest on 24 September 2021.
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