Journal ofthe Geological Society, London, Vol. 148, 1991, p. 1144. Printed in Northern Ireland

Discussion on sequence stratigraphy of the Welsh Basin Journal, Vol. 147, 1990, pp. 537-547.

M. Parkes & A. Vaughan write: Therecent paper on have already pointed out the difficulty of drawing a single sequencestratigraphy of the Palaeozoic Welsh Basin suture in the complex fossil ocean system, but suggested that (Woodcock 1990) illustrates how new developments can be a line between Grangegeethand Bellewstown is the most applied to extensively studied areas.Our discussion, by appropriate for a single suture trace. contrast, concerns eastern Ireland where accumulated data In relation to Woodcock’s (1990) comparison of Welsh are fewer, but where recent work has clarified some details Basin sequencestratigraphy to adjacentparts of eastern of structure andfaunal palaeogeography. Woodcock’s , we consider the inclusion of Grangegeeth to be (1990: fig. 7) inclusion of Grangegeeth in between the Lake inappropriate,and its excision from fig. 7 improves the District and SE Ireland as part of the Leinster Basin is a boundary correlation of Mega sequences I1 and 111. misconception that has been promoted in several papers of 29 May 1991. the last few years involving models of Iapetus development N. Woodcock replies: I am grateful to Parkes & Vaughan andclosure, e.g. McKerrow & Soper (1989), Hutton & for clarifying recent conclusions onthe easternIreland Murphy (1987). terranes, which became available after my paper went to Ourintention, therefore, is not to criticize the press. I agreethat the Grangegeeth area should now be interpretation of Woodcock (1990), but to point outthat assigned to rather than to Avalonia and am pertinent informationhas been overlooked, and also that pleased to note that this assignment removes some awkward new work concerning the Grangegeeth terrane is in press. diachroneity in the proposed megasequence boundaries Harper & Parkes (1989) summarized the palaeontologi- across Eastern Avalonia. The correlation of these cal constraints on the definition and development of Irish boundaries across the Anglian Basin and into Belgium has Caledonide terranes. Concerning Grangegeeth,the old recently been refined (Woodcock 1991). provincial assignment to Baltica was dismissed, since 3 June 1991. Productorthis, the most diagnostic genus of Williams (1956) is known from contemporaneous Laurentian rocks. Overall, References in the light of improvedinformation on brachiopod HARPER, D. A. T.& MURPHY,F. C. 1989. The Iapetus suture in the British distributions, theGrangegeeth assemblages show a strong Isles--comment on its position in eastern Ireland. Geological Magazine, Scoto-Appalachian affinity, as do the trilobites (Roman0 et 126, 723-724. al. 1990 & pers.comm.). This is a major contrast to the -. & PARKES,M. A. 1989. ShortPaper: Palaeontological constraints on Caradoc brachiopodsignature from the Leinster terrane the definition and development of Irish Caledonide terranes. Journal of the Geological Society London, 146, 413-415. which is part of an Eastern Avalonia centred Anglo-Welsh --, , HOEY, A.N. & MURPHY,F. C. 1990. Intra-Iapetus brachiopods faunal province (Parkes 1990, 1991). The Bellewstown from the of easternIreland: implications for Caledonide terrane, also in thesuture zone, has an Anglo-Welsh correlation. Canadian Journal of Earth Sciences, 21, (12), 1757-1761. signature in theCaradoc, rather than affinities with the HUTTON, D. H. W. & MURPHYF. C. 1987. The of theSouthern Uplands as a successor basin to an end-Ordovician closure of Iapetus. DunnageZone of Newfoundland as seen in the Llanvirn Journal of the Geological Society, London, 144, 765-772. (Harper et al. 1990). It was probably amalgamated with the KEMP, A. E. S. 1987. Tectonicdevelopment of theSouthern Belt of the Leinster terrane by orat thistime (Murphy 1987). Southern uplands accretionarycomplex. Journal of the Geological Society Grangegeeth, however,has clear faunal affinity with London, 144,827-838. MCKERROW,W. S. & SOPER,N. J. 1989. The Iapetus suture in the British Laurentia. Isles. Geological Magazine, 126, 1-8. Furthermore, compelling new structuraldata from MURPHY,F. C. 1987. Evidencefor late Ordovician amalgamation of eastern Ireland(Vaughan & Johnston 1992) show volcanogenic terranes in theIapetus suture zone, eastern Ireland. Grangegeeth to be part of theLaurentian margin. It was Transactions of the Royal Society of Edinburgh: Earth Sciences, 78, 153-167. accretedas ocean island or oceanic arc basement tothe PARKES, M. A. 1990. The Palaeontology of the Duncannon Group Rathkenny tract in Late Wenlock times prior tohard (middle-upper Ordouician) of southeast Ireland. PhD Thesis, National collision of Eastern Avaloniaduring the Ludlow. The University of Ireland. Rathkenny tract (Vaughan & Johnston 1992) is equivalent - 1992. Caradocbrachiopods from the Leinster terrane(southeast to tract 1Oc (Kemp 1987) of the Scottish Southern Uplands. Irelandta lost piece of the Iapetus puzzle? Terra Noun in press. ROMANO, M.,OWN, A. W. & HARPER,D. A. T. 1990. The biogeographical Grangegeeth and the overlying Rathkenny tract have similar affinities of early Caradoc shelly faunas from the Grangegeeth terrane, structure tothe Southern Uplands of Scotland. The easternIreland. Palaeontological Association AnnualConference, combinedGrangegeeth-Rathkenny tract lies inboard,on Durham, Abstracts, 23. the Laurentian margin, of the Clogherhead tract (Vaughan VAUGHAN,A. P. M. & JOHNSTON,D. J. 1992. Structuralconstraints on closure geometry across the Iapetus suture in eastern Ireland. Journal of & Johnston 1992), which hasstructural similarities to the the Geological Society London, in press. Lake District and is considered to belong toEastern WILLIAMS, A.1Y56. Productorthis in Ireland. Proceedings of the Royal Irish Avalonia. The structural evidence places the inferred trace Academy, 57(b), 179-183. of thesuture between Grangegeeth and the Clogherhead WOODCOCK,N. H. 1990. Sequence stratigraphy of the Welsh Basin. Journal tract, and includes the Grangegeeth tract in the Laurentian of the Geological Society London, 141, 537-547. - 1991. The Welsh, Anglian and Belgian Caledonides compared. Annales margin. It should be noted that Harper & Murphy (1989) de la Sociktt! gkologique de Belgique, in press.

MATTHEWA. PARK= & ALAN VAUGHAN, Departmentof Geology, Trinity College Dublin, Dublin 2, Ireland, NIGELWOODCOCK, Department of Earth Sciences, The University, Cambridge CB2 3EQ, UK. 1144

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/148/6/1144/4891966/gsjgs.148.6.1144.pdf by guest on 25 September 2021