Journal of the Geological Society, London, Vol. 143, 1986, pp. 185-191, 3 figs. Printed in Northern Ireland

Oceans, island arcs and olistostromes: the use of fossils in distinguishing sutures, terranes and environments around the Iapetus Ocean

W. S. McKERROW & L. R.M. COCKS’ Department of Earth Sciences, Parks Road, Oxford OX1 3PR, UK l Department of Palaeontology, British Museum (Natural History), Cromwell Road, London SW7 5BD, UK

Abstract: Theshallow-water benthos of cratonicareas distinguish different continents, and the closure of the Iapetus Ocean during the and can bemonitored by the faucas whichcrossed the ocean at differentgeological times. Brachiopod and trilobitedistributions carry differentpalaeobiological evidence than, forexample, contemporary conodont provinces, which probably reflected a combination of temperature differences and water depths. The Celtic ‘Province’ is reviewed, and it is concluded that each of the island faunas should be treated separately (since many have links with adjacent continents), rather than together as a province. In Newfoundland, faunas help to locate the site of the main Iapetus closure at the Reach Fault, although a substantial island arc in the Notre Dame Bay area accreted on to America in early Caradoc times. In the late Ordovician and early Silurian of Newfoundland fossils assist in the dating of quite local fault-controlled marginal basins,some of whichinclude large olistostromes; undisturbed assemblages of brachiopods in life position give indications of the depth of water in some basins.

The purpose of this contribution is to show that fossils can migratenorthward between the Pliensbachian and Batho- serve in many more ways than mere dating of rocks. nian, and then southward again in the late (Hallam In many cases after careful analysis, faunas may be assigned 1975, p. 200). to their continent of origin, and thus exotic terranes can be The ability to migrate, either across oceans or across identified,and the sites of old sutureslocated after climatic belts, vanes with different animals, so that different integration with stratigraphicaland structural studies: provinces (or realms) have been recognized on the basis of sedimentaryenvironments can also be illuminated by different animals. This may be confusing but it is logical: it is palaeontology. As examples of these principles, we take the easy to believe today thatthere are moreearthworm margins of the Ordovician and Silurian Iapetus Ocean, and provinkes than seagull provinces; so, in the past, we cannot discuss in turn wider aspects of faunal migration, between expect distributions to have been similar for different types continents and also more local biostratigraphy, with of animals. particular examples from our work in Newfoundland. Clearly, pelagic faunas include taxa thatare more In any one region, it is necessary to analyse the faunal widespread than benthic faunas;but even within agroup associations andto assign them toappropriate animal such as the graptolites there are distinctions between genera communities before consideration of any larger categories. that were truly oceanicand those that lived aroundthe Fauna1 provinces (or realms) are recognized in terms of the continental margins. Cocks & Fortey (1982) have shown total community composition of each region. There are two that some early Ordoviciangraptolites can help to distinctive factors controlling provinces and realms: barriers distinguish three continents: Laurentia, Baltica and to migration and climatic differences.We have two terms Southern Europe (including Englandand Wales); the last available, so perhaps we should use ‘Province’ for regions was close to Gondwana. These three provinces are shown separated by physical barriers to migration and ‘Realms’ for more distinctly by the common shelf benthos (brachiopods climatically controlledfaunas and floras. Then provinces and trilobites). would be separated by barriers like oceans and continents During the middle Ordovician, the benthicfaunas of (which are usually recognizable in the geological record), England and Wales show increasing affinities with Baltica, while boundariesbetween realms, like the faunalchanges and by late Caradoc (Actonian) times most species are the associated with latitude seen today onboth margins of same. It would appearthat the late Precambrianarc of NorthAmerica, would be less obvious and,perhaps, Avalonia (which contains similar and early gradational. The boundaries of provinces would not migrate OrdovicianGondwana faunas from England to Mas- through time, for example, the line of the Iapetus Ocean sachusetts) rifted off from the margins of Gondwana separatingAmerican andEuropean faunasremains the (perhaps starting as early as the Cambrian) to forma samethroughout the earlyPalaeozoic (except, of course, separate microcontinentand initiating the RheicOcean when small terranes may havetraversed theocean). By (Fig. 1). contrast, the boundariesbetween faunal realms associated Estimates of the duration of pelagic larvae in modern with climate may migrate with timedepending on world benthos(McKerrow & Cocks 1976; Scheltema 1977; temperaturesor continentalmovements, for example the Jablonski & Lutz 1980) show that it may range from 2 to 6 boundary between the Jurassic Boreal and Tethyan realms weeks in temperate zones andup to 1 yr in the tropics. 185

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...... arc, with trench T accretionary prismwith trench

BHBronson Hill T Tetagouche MV MidlandValley SU SouthernUplands GGF GreatGlen Fault HBF HighlandBoundary Fault s'u I AP AvalonPeninsula A Ardennes -

RH ElC OCEANRHElC

Fig. 1. A possible distribution of the Avalon terrane (Avalonia) relative to Laurentia, Baltica and Gondwana during the Caradoc. It is possible that the SW part of Avalonia was not attached to England, Wales and southern Ireland at this time; the Caradocchangeover from Gondwana to Baltic faunas is only recorded in southern Britain.

During this phase of itslife, an animal may drift for history of the NW (American) margin is complicated by a distances up to 3000 km. However, the majority of larvae series of orogenies ranging fromearly Ordovician in may travel for 1000 to 2000 km. So perhaps the Rheic Ocean Scotland (Grampian Orogeny) to late Caradocin New had a width in this range before fauna1 distinctions become England (Taconic Orogeny). These orogenies have been observable in the middleOrdovician. Similarly, the interpreted as the result of collisions between island arcs and Tornquist Sea, separating England and Norway, may have North America (Doolan et al. 1982; Hall & Robinson 1982; still been over 1000 km wide when the benthicfaunas Hatch 1982; Mitchell 1984). These collisions have no became similar in the late Caradoc. Stratigraphic evidence bearing on the final date of Iapetus closure, when Baltica fromboth Britain (Williams et al. 1972) and Norway and Avalonia collided with Laurentia in late Silurian or (Roberts 1980, p. 57) suggests possible early Ashgill folding early . in both countries; this might be the time of collision. A second complicating factor along the NW margin of After England had collided with Baltica, Avalonia and Iapetushas been strike-slipfaulting. Evidence from the Baltica moved together towards Laurentia as Iapetus closed Midland Valley of Scotland (Bluck 1984; Curry et al. 1984) (Fig. 1). Although the closure of Iapetus is progressive, the shows that many Ordovician and Silurian conglomerates

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were derived neither from the GrampianHighlands to the N are all of late Precambrian age (Peat 1984, and pers. comm.) norfrom the Southern Uplands to the S. It is for these 3. The Arenig faunas of S Wales (R. A. Fortey & M. G. reasons that we show several large strike-slip faults in the Bassett pers. comm.), and the Llanvirn faunas near Builth BritishIsles (Fig. 1) with horizontalmovements of 100 to (Lockley 1984) which werecertainly integral parts of 200 km (see also McKerrow, 1986). The Southern Uplands Avalonia, are now known to have many species in common accretionary prism shows that subduction of the ocean with the Celtic faunas of Anglesey (including Rhynchorthis). occurred onthe Scottish (i.e., American) side fromearly Apart from Anglesey, many of the occurrences assigned Caradoc until the Wenlock. By the late Ashgill, the ocean to the Celtic ‘Province’ occur on volcanic sequences which was narrowenough for trilobite and brachiopodlarvae to may have formed oceanic islands (McKerrow 1983; Neuman cross,but the benthicostracodes andthe fresh-water fish 1984). These islands appear to have formed the centre from remained distinct through most of the Silurian (McKerrow which someCeltic ‘Province’ animals radiated, but the & Cocks 1976). evidence from Wales suggests that, like the conodont- The timing of collision is not clear, but thrusting started in bearing animals, some of these forms were able to cross the N Greenland during the latestLlandovery (Hurst & Iapetus on occasions (see also Neuman 1984, table 1). McKerrow 1981) and in Norway slightly later in the Silurian, There is now some evidence of island arcs that contain whether this thrusting was a collisior, between the two main sedimentsbearing brachiopods assigned tothe Celtic continents, or between smaller masses is uncertain. Collision ‘Province’ subsequently collided with the continents is confirmed by the presence of similar late Silurian bordering Iapetus. In Norway, the Koli Nappe included arc fresh-water fish in Scotland and Norway (Turner & Turner material with Arenig shelly faunas (Bruton & Harper 1981) 1974), though the benthicostracodes remain distinct until which were emplaced on the western edge of Baltica during the Devonian. In the latter case, perhaps climate had some the Silurian. In Newfoundland, early Ordovician arc rocks control on the ostracode distributions; to get across would include some Celtic ‘Province’ brachiopods (see Figs 3b-g; require some northward migration. Neuman 1976; Arnott et al. 1985). In some sections, these The conodont-bearinganimals also had distinctive arc rocks are followed by Llandeilo sediments with typical distributions in the Ordovician. Many species are present on shallow-water American Province benthos, which have been both sides of the Iapetus Ocean, indicating that some, at interpreted, on the basis of the regional geology, to have least, were very widespread. These faunas which bordered been deposited just before and after the arc collided with the Iapetus Ocean are distinct from taxa in the centre of the North America in the Llandeilo/early Caradoc (N. gracilis North American craton (Bergstrom 1971, 1973; Barnes & zone). Comparable occurrences of North American faunas Fahraeus 1975). The boundarybetween these two lying stratigraphically above Celtic ‘Province’ faunas have distributions is clearly neither an ocean nor a land barrier; also been reported in New Brunswick and Maine (Neuman the differences are probably due to temperature, depth of 1984). water or some other environmentalfactors (Fortey & Of the 41 brachiopodgenera listed as occurring in the Barnes 1977). In the terms employed in this paper, we thus Celtic Province (Neuman 1984, p. 1195), only 14 are interpret them as conodont realmsand not conodont endemic to early Ordovician islands, and of these ten are provinces. only known eachfrom a single locality; of the remaining Some specific problems will now be discussed as genera,ten are also known from Laurentia, eight from examples of the discriminative use of fossils. Baltica,and seven fromGondwana (including Avalonia); the remaining two are cosmopolitan. This indicates that the spat of somebrachiopods, like some of the conodont- bearinganimals, could cross theIapetus in the early The Celtic ‘Province’: island arcs and exotic terranes Ordovician, at a time when the ocean was wide enough to prevent migration by the bulk of the remaining benthos. It is Distinctive brachiopodfaunas of Arenig age from SE probable that the pelagic stages of some brachiopods had a Ireland and Anglesey caused Williams (1973) to distinguish longer duration than others; we think it may be this factor a Celtic ‘Province’ from faunas of the same age elsewhere, (together with pure chance) which allowed a few brachiopod although in fact only one genus Rhynchorthis was listed as species to migrate across oceanicbarriers earlier than the endemic; the other generaof this province were also present rest. inWales and Baltica. The concept of this province was The two distinct concepts of fauna1 provinces (separated extended by Neuman (summarized in Neuman 1984), who by physical barriers to migration) and realms (with a chiefly includedArenig-Llanvirn faunasfrom Maine, New climatically-related fauna) are not easy to apply to the Celtic Brunswick,Newfoundland and Norway as well as those ‘Province’. While theLaurentian, Baltic andGondwana from SE Ireland and Anglesey, and who interpreted these (including the early Ordovician Avalonia) are provinces of occurrences as representingoceanic mid-Iapetus islands the first type, the Celtic ‘Province’ was not. However, the distinct fromthe main continental masses (Laurentia, Celtic ‘Province’ is not clearly related to climate either. It Baltica and Avalonia). However, there is great difficulty in may owe its origin to distinctive environments(perhaps envisaging any great separation of Anglesey from the rest of related to volcanic activity) which werepresent both on Wales for the following reasons: oceanic islands and along certainborders of the Iapetus 1. Bates (1968) describednormal Anglo-Welsh faunas Ocean. of Llanvirn to Caradoc ages in the samesedimentary sequence as the ‘Celtic’ Arenig forms: 2. The Lower Cambrian sediments of N Wales reflect a The Iapetus suture in Newfoundland provenance area identical to Anglesey (Woodland 1938), Traditionally, Newfoundland hasbeen divided intothree where we now believe the Gwna Group and its equivalents regions: W Newfoundland, which has always been a part of

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Fig. 2. Ordovician and Silurian brachiopods from Newfoundland: a, a life position group of Sfricklandia lens progressa Williams, BB 72223 X 1.5, from GreenCove Fm. (Llandovery: Aeronian), New World Island, Grid. Ref XF 749002: b to g, ‘Celtic Province’ orthids, b-f, aff. Eostrophomena sp., b, c BB 71329, d BB 71309, e, f BB 71314, g. Hesperorthis sp., all X 1.5 from Davidsville Group (Arenig-Llanvirn), N shore of Gander Lake, Grid Ref.XE 653231: h, Bilobia sp., from Caradoc age matrix of an olistostrome, BB 69300 X 4, W of Village Cove, New World Island, Grid Ref XE 574834; i, j, typical North American forms of Llandeilo age from west of the Reach Fault, i,Anopfambonifes aff. grayae (Davidson), BB 71433 X 2, j, Murinella aff. muralis Cooper, BB 71398 X 1, Cobbs Arm Limestone, New World Island, Grid Ref. XE 712963: k, Skenidioides sp., BB 69314, X 4, from Ashgill age matrix of an olistostrome, showing how this brachial valve snapped into two pieces during the slumping and the two halves became separated during dewatering of the sediment, Intricate Harbour, Grid. Ref. XE 534846: 1, Leptosfrophia sp, BB 72227, X 1.5 (Llandovery: Aeronian), Milliners Arm Formation, New World Island, Grid. Ref. XE773976.

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m Intrusions m :;ar?$;,;nd clastlclater m ODhlOllteSotherand OCeanlC rocks Gander Group m F’m3~;;ynIly Dost-CaradocPre-Caradoc Island volcanlcsarc andvolcanlclastlcs Davldsvllleand 0olwood Groups 0 10 20 30 40 50km kid a I ,

Fig. 3. A simplified geological map of the Notre Dame Bay area, Newfoundland (modified from Dean 1978). Although Celtic ‘Province’ benthic faunas of Early Ordovician age occur on both sides of the Reach Fault, the sequences above the pre-Caradoc island arc to the west of the Reach Fault contain normal American Llandeilo and Caradoc faunas. The Reach Fault is considered to be the suture where the Iapetus finally closed because the later Ordovician faunas to the east are not diagnostic of America, and detritus from the schists of the Gander Group is confined to the east of this fault; it is also present on the Avalon Peninsula further east (seeinset).

North America;theCentral Mobile Belt;and E The rocks in theNotre Dame Bay area of the Central Newfoundland (the AvalonPeninsula) which has similar Mobile Belt, consist of an early Ordovician island arc resting Cambrian and earlyOrdovician faunas to England, Wales on older oceanic crust and its volcanic-rich detritus (Fig. 3). and other parts of ‘Avalonia’ (Williams 1964, 1979). In the late Llandeilo much of this area was submerged and Since the development of ideas about an early Palaeozoic siltstones are widespread. Then in the late Ordovician and oceanbetween Europe and America (Wilson 1966), there early Silurian, several fault-bounded basins show independ- hasbeen some debate as to where, in Newfoundland, the ent development of thick sequences of turbides, conglomer- ocean finally closed.Close study of the Newfoundland ates and olistostromes. Prior to the Llandeilo, the sediments faunas (Fig. 2) has been crucial in unravelling this problem. in this region are entirely volcaniclastics and carbonates, Inthe westernparts of theCentral Mobile Belt, several consistent with an oceanic island setting (Nelson & Casey ophiolite zones show that oceanic crust, probably all of early 1979), butin the lateOrdovician and early Silurian the Ordovician age, was present in places. Some of these zones American continent contributed to the provenance region. have been considered to be the suture where Iapetus finally This switch in sedimentprovenance corresponds to the closed, but, in the Ordovician, undoubted American faunas switch in faunasfrom the Celtic ‘Province’ in the extend as far east as the Cobb’s Arm Limestone of New Arenig-Llanvirn to the American Province in the Llandeilo World Island (see Fig. 2i, j; McKerrow & Cocks 1977). We and later times (Arnott et al. 1985). believe thatthe Baie Verteand Betts Coveophiolites The Notre Dame Bay sequences include Llandeilo faunas represent oceanic or back-arc-basin crust which was of American aspect in New WorldIsland (Figs 2i, j) but obducted in the early Ordovician prior to or contemporarily these do not extend to the Reach Fault. The schists of the with the collision of an island arc. These ophiolites have Gander Group to the east are unconformably overlain by rocks with similar American faunas on both sides (east and the Davidsville Group, which yield Arenig-Llanvirn Celtic west);they thuscannot represent a wide oceanafter the ‘Province’ brachiopodsat GanderLake (McKerrow & Llandeilo.Some Newfoundland geologists believe that Cocks 1977) and Indian Bay (Wonderley & Neuman 1984) Iapetus closed in the Ordovician,but the evidence from and NorthAtlantic Province conodonts of Llanvirn- Norway, Britain and New England indicates firmly that, to Llandeilo age at Weirs Pond (Stouge 1980). By themselves the east of the island arcs, an ocean persisted through most, these faunas do not prove that the Reach Fault is the suture or all, of the Silurian Period. where Iapetus finally closed in the Silurian or Devonian, but

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they do suggest that the Gander Group was not attached to Fragments of all four of these formations are now known to North America in the early Ordovician. To confirm that the occur as olistoliths in the melange on the downthrow side of Reach Fault is, in fact, the main Iapetus suture, we need to the fault. It is only by the fossil content of both the matrix use evidence of sedimentary provenance. Detritus from both and the olistoliths, that the complexities of this section has the Gander Group and the Gander River Ultramafic Belt beenunravelled. Before field studies of the fossil (which crops out to thewest of the Gander Group)occurs in occurrences were made it was thought that all the faulting the Davidsville Group, and similar detritus is also present in was subsequentto sedimentation,and the presenceand the Late Precambrian Musgravetown Group of the Avalon significance of the olistostromes was not recognized. The Peninsula(Kennedy 1976, 1979). By contrast,there is no situation only became clear when it was seen that blocks of known metamorphic detritus in the early Ordovician beds to fossiliferous Llandeilolimestone and Caradocshale were the west of the Reach Fault. suspended in a matrix with Llandovery brachiopods. Tothe north of the Boyds Island Fault, a distinct Notre Dame Bay, Newfoundland stratigraphic sequence shows that a separate fault-bounded basin developedin the late Ordovician andearly Silurian Weconsider that the late Ordovicianand early Silurian (Arnott et al. 1985). Above someturbidites, deep shelf rocks of this region were deposited on the island arc which deposits are present bdow further mass flow deposits. The collided with North America in the middle Ordovician. As shelf sediments contain Stricklandia lens progresia (Fig. 2a) statedabove, the Arenig of this region includes Celtic in position of growth (McKerrow & Cocks 1978) and show 'Province'brachiopods, while the middle Ordovicianbeds that, in this basin tothe north, quiet shelf environments have a typical American fauna. This fauna1 change is linked prevailed while the olistostrome was being deposited close with the arc-continent collision. by to the south. The fossils also indicate that the northerly One remarkable feature of the stratigraphy of the area is sequence was deposited on a shelf rather than in very deep thatthe early Caradoc sedimentsdeposited after the water. The growth position of Stricklandia indicates a depth collision are dominantly fine grained (Dean 1978; Arnott et of around 200 m (Cocks & McKerrow 1984). al. 1985). Thisquiescent interval of shaleand siltstone The presence of olistostromes and mass flow conglomer- deposition was brought to an end by several distinct pulses ates in many parts of Notre Dame Bay was first recognized of coarse clastic sedimentation, which commenced in the D. by older fossils occurring within blocks of sediments which multidens and D. clingani zones in several small were surrounded by matrix with younger fossils. The fault-bounded basins but did not spread onto the submerged stratigraphic use of these fossils also allowed recognition of intervening highs until theend of theCaradoc. The contemporarystratigraphic sequences in adjacent fault- composition of the detritus suggests that it was derived in bounded basins. And, in addition,the presence of part by erosion of a deeply dissected arc terrane similar to brachiopods in position of growth gives some firm an arc now exposed to the north of the region and in part indications of water depth.The sedimentology of these fromcontinental a terrane with acid plutons. The sequences with mass flow deposits merely indicates the stratigraphy of each small basin is distinctive, with the presence of steep gradients. It takes brachiopods assignable incoming of coarseconglomerates and olistostromes todepth-related communities to provide amore precise occurring at different times in basins only a few miles apart indication of depth. While many mass flow deposits (McKerrow & Cocks 1981). The unravelling of this complex undoubtedly occur on slopes in very deep water, we have regionhas neededthe application of palaeontology (Figs shown that some can occur on the shelf. 2 h-l) and sedimentologyin the field. The abundance of derived fossils (occurring in clasts ranging from a 1cm pebble to a 300m olistolith) meant that the sedimentology Thanks to R. A. Fortey for comments on the first draft of this paper and field relations of each fossil locality had to be studied and to NERC for funding our field work in Newfoundland. with great care. Previous field geologists (Harris 1966; Kay 1976) consistently showedfaults separating Ordovician and References Silurian beds; most of these contacts have turned out to be the margins of olistoliths in olistostromes which were ARNOTT,R. J., MCKERROW,W. S. & COCKS,L. R. M. 1985. The tectonics deposited near active faults (Arnott et al. 1985). In New and depositional history of the Ordovician and Silurian rocks of Notre Dame Bay,Newfoundland. Canadian Journal of Earth Sciences, 22, World Island, the section along the east coast (adjacent to 607-618. the Reach Fault) shows a few hundred metres of siltstones BARNES,C. R. & FAHRAEUS,L. E. 1975. 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Received 11 March 1985; revised typescript accepted 5 July 1985

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