Silurian (Llandovery) graptolites from the Bay of Exploits, north-central Newfoundland, and their geological significance

Abstract

Only one specimen of a Silurian graptolite has ever been recorded from Newfoundland, and it was not identifiable below generic level. The graptolite assemblage discussed and figured here, from a sequence of turbidites on Upper Black Island, northcentral Newfoundland, includes the first positively identified Silurian taxa from the province, and provides the first unequivocal evidence of Silurian oceanic sedimentation in the Dunnage Zone. The graptolite taxa include Rastrites peregrinus (Barrande), Coronograptus? sp. cf. C. gregarius (Lapworth), Monograptus austerus sequens Hutt?, Orthograptus insectiformis (Nicholson), Monograptus spp. indet., "Climacograptus"? sp., and Glyptograptus? sp. This assemblage demonstrates that the strata, previously assigned to the Middle Ordovician Lawrence Harbour Formation, are actually Aeronian (middle Llandovery) in age and that the turbidites considerably postdate both the Lawrence Harbour and Point Leamington formations of the Exploits Subzone. Furthermore, the graptolite fauna is similar to that found in coeval sediments in the United Kingdom and Scandinavia but unlike any assemblages known from elsewhere in North America. This suggests the presence of open oceanic conditions, or deep marginal basins during the Llandovery hosting graptolites with European affinities, and raises the possibility that at least part of the Iapetus Ocean was still open in central Newfoundland during Early Silurian times. Jusqu'i prksent, un seul spCcimen de graptolite silurien a Ct C enregistrC i Terre-Neuve, et il n'a pu Ctre identifie au deli du I niveau gknerique. L'association de graptolites discutte et illustrCe ici, provenant d'une sCquence de turbidites sur I'ile Upper Black, au centre-nord de Terre-Neuve, inclut pour la premiltre fois un taxon silurien clairement identifik dans cette province, 1 et dCmontre aussi pour la premibre fois, et sans Cquivoque, la prCsence de stdiments ocCaniques siluriens dans la Zone de I Dunnage. Le taxon de graptolites inclut Rastrites peregrinus (Barrande), Coronograptus? sp. cf. C. gregarius (Lapworth), I I Monograptus austerus sequens Hutt?, Orthograptus insectifomis (Nicholson), Monograptus spp. indCt., << Climacogratus D? sp. et Glyptograptus? sp. Cette association dCmontre que les strates, assignkes anttrieurement i la Formation de Lawrence 1 Harbour d'bge ordovicien moyen, sont vkritablement d'bge aCronien (LlandovCrien moyen), et que les turbidites postdatent considCrablement les formations de Lawrence Harbour et de Point Leamington de la Sous-zone d'Exploits.En plus, cette faune de graptolites est semblable B celle trouvCe dans les sCdiments accumulCs simultanCment dans le Royaume-Uni et la Scandinavie, mais elle se distingue de toutes associations connues ailleurs en AmCrique du Nord. Ce qui suggbre la prtsence de conditions ocCaniques ouvertes, ou de bassins marginaux profonds habites par des graptolites d'affinitCs europCennes durant le Llando-

Early Silurian paleolatitude for central Newfoundland from paleomagnetism of the Wigwam Formation

“…For example, they pointed out that Early to Middle Ordovician brachiopod faunas found east of the Red Indian Line have oceanic island affinities in contrast with the North American affinities of a coeval Ordovician conodont fauna in the Notre Dame Subzone. Williams and O'Brien (1991) reported Middle Llandovery graptolites of European rather than North American affinity southeast of the Red Indian Line (at the locality marked by a star in Fig. I), supporting the view that a n ocean within the Dunnage Zone remained open in the Early Silurian.…”

Palaeomagnetism of theca.440 Ma Cape St Mary’s sills of the Avalon Peninsula of Newfoundland: implications for Iapetus Ocean closure

“…6, widening the Iapetus Ocean. A wider Iapetus is supported by graptolites that retain non‐Laurentian affinities at least to middle Llandovery time in central Newfoundland (Williams & O’Brien 1991) and to middle Wenlock time in central Ireland (Lenz & Vaughan 1994). Both narrower and wider Iapetus Ocean models than that of Fig.…”

Paleogeography, accretionary history, and tectonic scenario: A working hypothesis for the Ordovician and Silurian evolution of the northern Appalachians

“…The generally shallowly dipping Botwood Group overlies a sequence of steeply dipping graywacke turbidites with some interbedded conglomerates. These marine deposits are correlatives of clastic deposits on eastern New World Island and areas to the west that yielded Ashgillian-Llandoverian ages (Dean, 1978;Kean and others, 1981;Arnott and others, 1985;van der Pluijm and others, 1987;Williams and O'Brien, 1991). Isolated occurrences of corals in graywackes on western Change Islands indicate a Llandoverian age for this unit (Eastler, 1971).…”