Intra-Iapetus brachiopods from the Ordovician of ...

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Depafiment of Geology, The Universiv of Western Ontario, hndon, Ont., Canada N6A 5B7 ... Iapetus suture along the Reach Fault on the basis of their iden-.

DISCUSSIONS

Intra-Iapetus brachiopods from the Ordovician of eastern Ireland: implications for Caledonide corre1ation:l Discussion W. R. CHURCH

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Depafiment of Geology, The Universiv of Western Ontario, h n d o n , Ont., Canada N6A 5B7

Received November 25, 1991 Revision accepted February 12, 1992 Can. J. Earth

Sci. 29, 830-832

(1992)

In an important paper dealing with the Hilltown brachiopod fauna of eastern Ireland and its implication for Caledonide correlation, the authors reiterate the view that the Iapetus suture of Ireland correlates with the junction between the Dunnage and Gander terranes of Newfoundland, implying thereby that the Dunnage Terrane forms part of the Laurentian oceanic margin. As a consequence they are led to propose that the rocks containing the Indian Bay "Celtic" fauna of the Gander Terrane (Wonderley and Neuman 1984) lie either within a klippe thrust across the suture zone or within a zone gradational with the rocks of the Gander Group. Furthermore, in allowing that the Bellewstown Terrane has affinities with part of the Dunnage, it would seem necessary that in Ireland the Early Ordovician suture be placed between the Bellewstown and Leinster terranes. It is the purpose of this discussion to suggest that the junction of the Dunnage and Gander terranes (the Reach Fault boundary of McKerrow and Cocks (1976, 1977, 1986), McKerrow (1988), and Stillman (1988)), does not represent the Iapetus suture in Newfoundland. It is argued that the faunal evidence allows location of the Iapetus cryptic suture to the north of the Exploits volcanic zone, along a Lukes Arm - Red Indian line (cf. Williams et al. 1988). It is further suggested that the Exploits and Grangegeeth terranes of Newfoundland and Ireland, respectively, were located in a more oceanward position within the Avalonian margin than was the Bellewstown Terrane, from which they were separated perhaps by a backarc basin. Consequently, the Indian Bay rocks, even if thrust, need not be exotic, nor is it necessary to locate the Iapetus suture to the southeast of the Bellewstown Terrane. The assumption that the Dunnage rocks formed to the north of the Iapetus suture (e.g., Arnott et al. 1985; Stillman 1988) is largely based on the conclusions of McKerrow and Cocks (1977) concerning the paleogeographic significance of the shelly faunas of central Newfoundland. Counter to earlier interpretations placing Iapetus along a Solway - Lukes Arm line (Church and Gayer 1973), McKerrow and Cocks (1977) located the Iapetus suture along the Reach Fault on the basis of their identification of a Caradocian "Scoto-Appalachian" fauna in the Cobbs Arm Limestone of New World Island. The Early Ordovician faunas of New World Island that were obviously not Scoto-Appalachianwere explained as having developed endemically (McKerrow and Cocks 1977, p. 493). However, the Early Ordovician faunas of New World Island and Anglesey were subsequently shown by Neuman and Bates (1978) to belong to a common Celtic biogeographic province. Further'Paper by D. A. T. Harper, M. A. Parkes, A. N. Hoey, and F. C. Murphy. 1990. Canadian Journal of Earth Sciences, 27: 1757- 1761. Printed in Canada i IrnprirnL au Canada

more, faunas with supposed Celtic affinities were subsequently found south of the Reach Fault in Arenig sediments (Jenness 1963; McKerrow and Cocks 1977; Neuman 1984) overlying GRUB line ophiolites (high-Cr low-Ti primitive arc type material) obducted onto rocks of the Gander Group, in rocks of the Baie d'Espoir Group (cf. Orthambonites; cf. Productorthis; Colman-Sadd 1976) of southeast Newfoundland, in tuff associated with pillow lava of the Indian Bay Formation within the Gander zone (Wonderley and Neuman 1984), and more recently (Colman-Sadd and Swinden 1984; Dec and ColmanSadd 1990) in ophiolite debris-bearing limestone conglomerates above the Spruce Brook Formation (Mount Cormack window of Gander Terrane rocks?). Boyce (1988) has also argued on the basis of trilobite faunal distributions that the Reach Fault of central Newfoundland is not the Iapetus suture. A rare Arenig cyclopygid trilobite recorded previously only from England and South Wales has recently been found (Williams et al. 1991, p. A132) in sediments of the Coy Pond ophiolite complex of the Exploits zone, "suggesting that south central Newfoundland lay on the northern oceanic margin of Avalonia in a peri-Gondwanan position' ' . Stratigraphically upwards, Dunnage faunas develop ScotoAppalachian affinities. The Scoto-Appalachian brachiopod Valcourea present in the Cobbs Arm Limestone is also found in association with the Celtic Mount Cormack Group fauna, and Boyce (1988) states that by late Llanvirn - early Llandeilo times trilobite faunas on both sides of the Reach Fault display strong North American affinities. Since the authors mention that by the Caradocian the Bellewstown fauna are typically Anglo-Welsh, this may indicate that the volcanic islands of New World Island, and perhaps the Grangegeeth Terrane of eastern Ireland, represent the most northerly of the terranes located off Avalonia, and that by Llanvirn times ScotoAppalachian faunas had begun to bridge Iapetus, migrating via volcanic islands and following north-to-south oceanic currents. The Bellewstown Terrane would not be the lateral Irish equivalent of the Dunnage (Exploits) Terrane, but would have been located closer to the Avalonian margin (Table I), whereas the Grangegeeth Terrane would have lain farther outboard within the Avalonian margin. The paleomagnetic data of the Van der Pluijm et al. (1990) also indicate that the volcanic rocks of central Newfoundland (Wild Bight volcanics) south of the Luke's Arm fault were formed at 30-40' relative to 12's for the volcanics north of the fault. Faults north of the Wild Bight volcanic zone that might mark the location of the Iapetus suture (the Lobster Cove, Mansfield Cove, Crescent Lake, Tommy's Arm, or Sops Head faults) were ruled out of contention by Arnott et al. (1985) on the sup-

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DISCUSSIONS

TABLE1. Ordovician faunas of Ireland and Newfoundland (after Neuman 1984; Harper and Parkes 1989) Ireland

Mayo

Tyrone

Newfoundland

Southern Uplands northern belt

Grangegeeth

New World Island

1

Hirn. Fol. NEF S-A

Ashgillian

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Fol. S-A

Llandeilo Upper Llanvirn

I I

Toquima

Lower Llanvirn

Pac grap

Arenig

Pac grap

Pac

grap

I 1I

Bellewstown

I I

Leinster

Him. NEF

I

A-W

"*,Prduc. S-A S-A +Val

I

Mid-Llanvirn

Ireland

Indian Bay

2

I I I Fol.

II

Caradocian

Mount Cormack

AU grap

Celtic

I I

1I I

Gondwana

A-W + S-A in d-w carb + Baltic A-W + S-A in d-w carb

Celtic Celtic At1 grap

+Val

Celtic

NOTES:Pac grap, Pacific graptolites; At1 grap, Atlantic graptolites; NA, North American; NEF, northern European fauna; S-A, Scoto-Appalachian; Produc., Productorthis; Val., Valcourea; Hirn., Hirnantia; Fol., Foliomena; A-W, Anglo-Welsh; d-w carb, deep water carbonates; Transp, transported fauna; I, possible location of Iapetus. The vertical broken line marked "1" denotes the Navan - Lukes Arm fault; the verticle broken line marked "2" denotes the Slane Reach fault (GRUB back-arc suture).

position that the volcano-sedimentary successions on either side of the Lukes Arm - Lobster CoveICrescent LakeISops Head fault zone could be correlated. However, the condodont and graptolitic faunas described from shales interlayered with volcanics of Long Island (O'Brien and Szybinski 1989; Williams 1989) are Arenig to Llanvirn - earliest Llandeilo(?) in age, and the shales do not therefore correlate with the widespread Caradocian shale of central Newfoundland (Williams 1989). According to O'Brien and Szybinski (1989) the conodonts are similar to faunas described from North America by Stouge (1980, 1984), whereas Williams (1989) describes the graptolites as being of open-ocean affinity but with similarities to other North American faunas. The Lobster Cove fault separates the Cutwell arc volcanics to the north from the Roberts Arm arc volcanics to the south, whereas the Crescent Lake fault separates the Roberts Arm from a titanium-rich Oceanic Island Suite to the south of the fault (Bostock 1978). If the Roberts Arm and Cutwell groups form part of the same subduction zone system, as suggested by Nowlan and Thurlow (1984), and if the Oceanic Island Suite is comparable to other large-ion lithophile-enriched volcanic suites commonly found in the 'One (e'g'9 lacobi and Wasowski 198'), the Lake may represent the Iapetus suture. Since the ca. 461-410 Ma (Hibbard 1983) Burlington Granodiorite intrudes the obducted Betts Cove ophiolite and the overlying late Arenig Snooks Arm Group (Epstein 1983, it is also conceivable that following obduction of the Internal Zone ophiolites the region north of the Lukes Arm fault was during Llandeilo-Caradoc times the locus of calc-alkaline magmatism related to northeasterly subduction of oceanic crust to the southeast of the fault (cf. Church and Gayer 1973). In this case only the region north of the Lukes Arm fault is required to have been in collision with the eastern margin of North America during Arenig -Llanvirn time. The timing of final closure of Iapetus south of the Lukes Arm fault is as debatable as the location of the suture. The Burlington Peninsula was exposed to erosion from Late Ordovician, or even earlier, to Late Silurian time, and therefore

could have been the source for the younger westerly derived flysch sequences (Crescent Lake, Gull Island, Port Leamington, Sansom formations) of the Exploits volcanic zone (Horne 1969). The apparent absence of debris derived from the Burlington arc to the west of the arc might suggest that the Harnpden Fault marks the toe of a major Middle Ordovician thrust that has brought the rocks of the Burlington Peninsula over a thinned easterly extension of the western Newfoundland continental crust. Furthermore, the discovery by Williams et al. (1991) of mid-Llandovery graptolites in shales overlying volcanic rocks of the northern Exploits zone might suggest that the Dunnage basin separating the Exploits arc from the obducted GRUB belt arc may have persisted as a starved basin beyond the point in time when Iapetus was being flooded with westerly derived turbiditic debris.

Acknowledgments I am grateful to Dr. Steve Colman-Sadd for bringing to my attention recent work in the southern Notre Dame Bay and Mount Cormack areas. Amon, R. J., McKcrrow, W. S., and Cocks, L. R. M. 1985. The tectonics and depositional history of the Ordovician and Silurian rocks of Notre Dame Bay, Newfoundland. Canadian Journal of Earth Sciences, 22: 607-618. BOS~OC H.~ ,H. 1978. Geology and petrochemistry of the Roberts Arm Group, Notre Dame Bav, Newfoundland. Geological Survey of ~ a n a d a ;Bulletin 369. Boyce, W. D. 1988. The Reach Fault of central Newfoundland is not the Iapetus Ocean suture. 5th International Symposium on the Ordovician System, Memorial University, St. John's, Nfld., Program and Abstracts, p. 12. Church, W. R., and Gayer, R. A. 1973. The Ballantrae ophiolite. Geological Magazine, 110: 497 -510. Colman-Sadd, S. P. 1976. Geology of the St. Alban's map-area, Newfoundland (1Ml13). Newfoundland Department of Mines and Energy, Mineral Development Division, Report 76-4. Colman- Sadd, S. P., and Swinden, H. S. 1984. A tectonic window in central Newfoundland? Geological evidence that the Appalachian

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Dunnage Zone may be allochthonous. Canadian Journal of Earth Sciences, 21: 1349 - 1367. Dec, T., and Colman-Sadd, S. 1990. Timing of ophiolite emplacement onto the Gander Zone: evidence from provenance studies in the Mount Cormack subzone. In Current research. Newfoundland Department of Mines and Energy, Geological Survey Branch, Report 90- 1. pp. 289 - 303. Epstein, R. 1983. The eastern margin of the Burlington Granodiorite, Newfoundland. M.Sc. thesis, University of Western Ontario, London, Ont. Harper, D. A. T., and Parkes, M. A. 1989. Paleontological constraints on the definition and development of Irish Caledonide terranes. Journal of the Geological Society (London), 146: 413 -415. Hibbard, J. 1983. Geology of the Baie Verte Peninsula, Newfoundland. Newfoundland Department of Mines and Energy, Memoir 2. Horne, G. S. 1969. Early Ordovician chaotic deposits in the Central Volcanic Belt of northeastern Newfoundland. Geological Society of America Bulletin, 80: 2451 -2464. Jacobi, R. D., and Wasowski, J. J. 1985. Geochemistry and platetectonic significance of the volcanic rocks of the Summerford Group, north-central Newfoundland. Geology, 13: 126 - 130. Jenness, S. E. 1963. Terra Nova and Bonavista map areas, Newfoundland. Geological Survey of Canada, Memoir 327. McKerrow, W. S. 1988. The development of the Iapetus Ocean from the Arenig to the Wenlock. In The Caledonian-Appalachian Orogen. Edited by A. L. Harris and D. J. Fettes. Geological Society (London), Special Publication 38, pp. 405 -4 12. McKerrow, W. S., and Cocks, L. R. M. 1976. Progressive faunal migration across Iapetus Ocean. Nature (London), 263: 304 -306. McKerrow, W. S., and Cocks, L. R. M. 1977. The location of the Iapetus Ocean suture in Newfoundland. Canadian Journal of Earth Sciences, 14: 488 -495. McKerrow, W. S., and Cocks, L. R. M. 1986. Oceans, island arcs and olistostromes: the use of fossils in distinguishing sutures, terranes and environments around the Iapetus Ocean. Journal of the Geological Society (London), 143: 185 - 192. Neuman, R. B. 1984. Geology and paleobiology of islands in the Ordovician Iapetus Ocean: review and implications. Geological Society of America Bulletin, 95: 1188- 1201. Neuman, R. B., and Bates, D. E. B. 1978. Reassessment of Arenig and Llanvirn age (Early Ordovician) brachiopods from Anglesey, Northwest Wales. Palaeontology, 21: 571 -613.

Nowlan, G. S., and Thurlow, J. G. 1984. Middle Ordovician conodonts from the Buchans Group, central Newfoundland, and their significance for regional stratigraphy of the Central Volcanic Belt. Canadian Journal of Earth Sciences, 21: 284-296. O'Brien, F. H. C., and Szybinski, Z. A. 1989. Conodont faunas from the Catchers Pond and Cutwell groups, central Newfoundland. In Current research. Newfoundland Department of Mines and Energy, Geological Survey Branch, Report 89-1, pp. 121- 125. Stillman, C. J. 1988. Ordovician to Silurian volcanism in the Appalachian -Caledonian Orogen. In The Caledonian-Appalachian Orogen. Edited by A. L. Harris and D. J. Fettes. Geological Society (London), Special Publication 38, pp. 275 -290. Stouge, S. 1980. Lower and Middle Ordovician conodonts from central Newfoundland and their correlatives in western Newfoundland. In Current research. Newfoundland Department of Mines and Energy, Geological Survey Branch, Report 80-1, pp. 134- 142. Stouge, S. 1984. Conodonts of the Middle Ordovician Table Head Formation, western Newfoundland. Universitets Forlager, Oslo, Norway. Van der Pluijm, B. A., Johnson, R. J. E., and Van der Voo, R. 1990. Early Paleozoic paleogeography and accretionary history of the Newfoundland Appalachians. Geology, 18: 898 -901. Williams, H., Colman-Sadd, S. P., and Swinden, H. S. 1988. Tectonic stratigraphic subdivisions of central Newfoundland. In Current research, part B. Geological Survey of Canada, Paper 88-lB, pp. 91-98. Williams, S. H. 1989. New graptolite discoveries from the Ordovician of Central Newfoundland. In Current research. Newfoundland Department of Mines and Energy, Geological Survey Branch, Report 89-1, pp. 149- 157. Williams, S. H., Colman-Sadd, S. P., O'Brien, B. H., and Boyce, W. D. 1991. New discoveries of Ordovician (Arenig) and Silurian (Llandovery) graptolites from Central Newfoundland, and their paleogeographic implications. Geological Association of Canada, Program with Abstracts, 16: A132. Wonderley, P. F., and Neuman, R. B. 1984. The Indian Bay Formation: fossiliferous Early Ordovician volcanogenic rocks in the northern Gander Terrane, Newfoundland, and their regional significance. Canadian Journal of Earth Sciences, 21: 525-532.

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