Parallel geological development in the Dunnage Zone of Newfoundland and the Lower Palaeozoic...

Transactiors of the Royal Society of Edinburgh: Earth Sciences, 83,571-594,1992

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Parallel geological development in the Dunnage Zoneof Newfoundland and the Lower Palaeo zotc terranes ofsouthern Scotland: an assessment

S. P. Colman-Sadd, P. Stone, H. S. Swinden and R. P. Barnes

ABSTRACT: The Notre Dame and Exploits subzones of Newfoundland's Dunnage Zone arecorrelated with the Midland Valley and Southern Uplands of Scotland, using detailedcomparisons of two key Lower Palaeozoic successions which record similar histories of extensionand compression. It follows that the Baie Verte Line, Red Indian Line and Dover Fault areequivalent to the Highland Boundary Fault, Southern Upland Fault and Solway Line,respectively.

The Betts Cove Complex and overlying Snooks Arm Group of the Notre Dame Subzone areanalogous to the Ballantrae Complex of the Midland Valley, both recording the Arenigevolution and subsequent obduction of an arc and back-arc system. The Early Ordovician toSilurian sequence unconformably overlying the Ballantrae Complex is poorly represented in theNotre Dame Subzone but important similarities can still be detected suggesting correspondinghistories of continental margin subsidence and marine transgression.

In the Exploits Subzone, Early Ordovician back-arc volcanic rocks are overlain by Llandeilomudstones and Late Ordovician to Early Silurian turbidites. A similar stratigraphy occurs in theNorthern and Central Belts of the Southern Uplands and both areas have matching transpressivestructural histories. Deeper erosion in the Exploits Subzone reveals Cambrian and EarlyOrdovician volcano-sedimentary sequences structurally emplaced on the Gander Zone, and suchrocks are probably present beneath the Southern Uplands. Combined data from the Notre DameSubzone and Midland Valley suggest an Arenig southeast-dipping subduction zone. EarlyOrdovician volcanic rocks in the Exploits Subzone ahd Southern Uplands have back-arc basingeochemistry and support the model of the Southern Uplands as a transition from back-arc toforeland basin. Preferential emergence of the Dunnage Zone ar,d contrasts between ExploitsSubzone and Southern Uplands turbidite basins are attributed to collision of Newfoundland witha Lhurentian promontory and Scotland with a re-entrant. This hypothesis also explains thetranspressive structural regime common to both areas.

KEY WORDS: comparison, Notre Dame Subzone, Exploits Subzone, Midland Valley,Southern Uplands, ophiolites, back-arc, transpression, promontory, re-entrant, delamination

Prior to the Mesozoic opening of the Atlantic Ocean,Newfoundland was contiguous with Ireland and Britainwithin a 2000 km segment of the Appalachian-Caledonianorogenic belt (Fig. 1). This sinuous orogen was formed byclosure of the Late Precambrian to Early Palaeozoic IapetusOcean with the consequent emplacement of oceanic terranesand continental margin successions upon opposing marginsof Laurentia and the Acado-Baltic continent (H. Williams1964; Wilson 1966; Bird & Dewey 1970; H. Williams er a/.1974; Pickering et al. 1988 and references therein). Suspectterranes are recognised in Newfoundland (H. Williams &Hatcher 1983) as the tectonostratigraphic zones lying to thesoutheast of the Laurentian continental margin (HumberZone) (H. Williams 1979). Vestiges of Iapetus are preservedin the composite Dunnage Zone (H. Williams 1979; H.Williams et al. 1988) and a continental margin that lay in theeastern part of Iapetus may be represented by the Ganderand Avalon Zones (Fig. 1) although the status of the formeris ambiguous (Kennedy 1976). The correspondence ofNeMoundland's Humber Zone with Scotland north of theHighland Boundary Fault has long been recognised (e.g.Swett & Smit 1972; Kennedy et al. 1972), and H. Williams

(1978) proposed a regional correlation of the other zoneswhich has since been supported by geophysical evidence(Haworth & Jacobi 1983). The Dunnage Zone inNewfoundland is generally correlated with the MidlandValley and Southern Uplands of Scotland but no attempthas been made to develop this regional correlation by a

detailed comparison of the stratigraphy of the apparentlyrelated areas.

Over the last decade, Cambrian to Silurian strata incentral Newfoundland and southern Scotland have been thetarget of systematic mapping by the NewfoundlandGeological Survey Branch and the British GeologicalSurvey, respectively; this work has accompanied majoradvances in interpretation based on detailed and thematicstudies by geologists from a wide range of institutions. Thegreatly expanded database has provided the framework for a

more detailed comparison of geological relationships onopposite sides of the Atlantic Ocean than has hitherto beenpossible. This paper presents the initial results of a

cooperative project between the Newfoundland GeologicalSurvey Branch and the British Geological Survey, aimed atbetter defining the geological links between the northern

S. P. COLMAN.SADD, P. STONE, H. S. SWINDEN AND R. P. BARNES572

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DUNNAGE ZONE NEWFOUNDLAND AND SOUTHERN SCOTLAND

Cambrian snd oarly Ordovician ophiolito complgxes:Ad Advocate, An Annieopsquotch. BC Betts Cove,Bol Bay of lslands, GR Gander River Complex,LB Lushs Bight. MCi HH Mansfield Cove/Hall Hill,PP Pipestone Pond Complex

Cambrian .nd garly Ordoviclan non-ophioliticvolcanic/6piclastic soqu6nc6s:Bu Buchans Group, C Cutwell Group, Er Exploits Group,RA Robens Arm Group, SA Snooks Arm Group,Sp Sleepy Cove Group, Su Summerford Group,VLG Victoria Lake Group, WA Western Arm Group,WB Wild Bight Group

573

geology in the major subdivisions is summarised for centralNewfoundland and southern Scotland, flrst for the units thatwe view as geologically analogous, and then for thestratigraphic successions above and below these startingpoints. This provides the necessary information to identifyboth similarities and differences in geological development.We then suggest tectonic processes that can account formuch of the observed variation, as well as pointing outproblems that remain to be addressed by future work.

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Appalachians and the Scottish Caledonides. It is focussed onthe vestiges of Iapetus in central Newfoundland andsouthern Scotland; areas in which we have personal researchexpertise and which we have jointly studied in the field.

We recognise similar geological units of similar ages in thetwo areas, in each case recording speciflc phases in thegeological development of the orogen, and use these as

starting points from which to investigate similarities anddiffereuces in lithostratigraphic development. The known

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the Dunnage Zone in Newfoundland,

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L. Regional geological framework

Newfoundland's Dunnage Zone (Fig. 2) has beensubdivided into the Notre Dame and Exploits Subzones,based on regional geological, geochemical and geophysicalconsiderations (H. Williams et al. 1988). The two subzonesrecord geological histories that ate broadly contem-poraneous but that contrast in the detailed timing andnature of events. They have been interpreted as

representing arc-back-arc sequences formed in disparateparts of the Iapetus Ocean (Swinder, et al. 1988). We believethat the tectonostratigraphic equivalents of the Notre Dameand Exploits Subzones can be found in the Midland Valleyand Southern Uplands of Scotland respectively (Fig. 3),implying analogous structural positions for the SouthernUpland Fault and Red Indian Line. We recognise specificcoeval stratigraphic intervals in both the Notre DameSubzone-Midland Valley and the Exploits Subzone-Southern Uplands for which the similarities are particularlystriking, and use these as starting points to develop thecomparison into overlying and underlying geological unitswhere the similarities are less obvious.

The starting points are: (i) the Betts Cove (ophiolite)Complex and Snooks Arm Group in the Notre DameSubzone and the Ballantrae Complex in the Midland Valley;(ii) the Ordovician and Early Silurian volcanic, black shaleand greywacke sequence of Notre Dame Bay (ExploitsSubzone) and the Northern and Central belts of theSouthern Uplands. The significance of geological similaritiesand contrasts between the Dunnage Zorte and southernScotland can be evaluated with some confidence because ofthe clear analogies between the starting points. Differencesin preservation and exposure in the two areas allow us toarrive at a much more complete picture of the LowerPalaeozoic geology of this part of the Appalachian-Caledonian Orogen than has hitherto been possible.

The extension of this comparison into adjacent parts ofthe orogen is beyond the scope of this paper, but we wouldemphasise the clear and widely discussed similaritiesbetween the Scottish and Irish paratectonic Caledonidesparticularly with respect to the Southern Uplands and theIrish Longford-Down terrane (e.g. Leggett et al. t979;Morris 1987; D. M. Williams 1990). We would expect ourgeneral conclusions to be as applicable to the Longford-Down terrane of Ireland as they are to the SouthernUplands of Scotland. Possible correlations between theMidland Valley terrane in Ireland and the Notre DameSubzone have also been discussed by Hutton et al. (1985)and Winchester et al. (1992).

The oldest rocks exposed in both the Notre DameSubzone and the Midland Valley are ophiolites and volcanicand volcano-sedimentary sequences. These are bestdisplayed in Newfoundland where the composite NotreDame Subzone contains at least six structurally distinct LateCambrian and Early Ordovician ophiolitic andvolcanic/epiclastic terrane fragments which formed in aseries of island arcs and back-arc basins. Church and Gayer(1973) recognised that the ophiolitic part of one of thesefragments, the Early Ordovician Betts Cove-Snooks Armsequence, has detailed similarities to the BallantraeComplex which is exposed in the largest Lower Palaeozoicinlier in the Midland Valley. The Ballantrae Complex isoverlain by a nearly continuous Llanvirn to Early Siluriancover sequence of marine sediments, the equivalent ofwhich in Newfoundland is fragmentary.

In the Exploits Subzone and the Southern Uplands, thebasis of comparison is a condensed mid-Ordovician black

S. P. COLMAN-SADD, P. STONE, H. S. SWINDEN AND R. P. BARNES

mudstone sequence. [n Newfoundland, this mudstoneoverlies extensive Cambrian and Early Ordovician volcanicand volcano-sedimentary sequences of island arc andback-arc origin, of which only very locally exposedfragments are preserved in Scotland. In both areas, themudstone is succeeded by turbidite-dominated sedimentarysuccessions and in the Southern Uplands these form thebulk of the exposure.

Throughout the Dunnage Zone, the Ordovician and EarlySilurian marine strata are overlain by late Llandovery andWenlock subaerial volcanic and sedimentary rocks, and inthe Midland Valley of Scotland the Girvan sequenceunconformably overlying the Ballantrae Complex alsopasses up into subaerial sediments of about the same age.The Southern Uplands sequence, however, records con-tinuous marine turbidite sedimentation until the middleWenlock, although there is evidence for redeposition ofsubaerial detritus from the late Llandovery onwards.

2. Notre Dame Subzone and MidlandVaIley

2.1. Lower Ordovician-Ophiolite complexesNotre Dame Subzone. The Betts Cove Complex,

outcropping on the eastern side of the Baie Verte Peninsula(Fig. 2), displays a complete ophiolite stratigraphy(Upadhyay 1973). The ophiolitic pillow lavas includeMORB-like tholeiitic basalts and refractory magnesianandesites of boninitic affinity, as well as various types of arctholeiites (Coish et al. 1982; Swinden et al. 1989; G. A.Jenner, pers. comm., 1990). The presence of the boniniticrocks has led to the conclusion that the Betts Cove Complexwas formed in a supra-subduction zone (SSZ) environment(e.g. Sun & Nesbitt 1978; Coish et al. 1982). The ophioliticrocks pass conformably upward into the Snooks Arm GroupGig. a), a thick sequence of red and green volcaniclasticsedimentary rocks and cherts containing two mafic pillowlava units, which comprise light rare earth element(LREE)-enriched tholeiites of oceanic island basalt affinity(Jenner & Fryer 1980). The tectonic history of the BettsCove-Snooks Arm sequence has been interpreted, usingregional geological, geochemical and Nd isotopic data, interms of island arc rifting (Betts Cove Complex), followedby sedimentation and volcanism in the resulting back-arcbasin (Snooks Arm Group) (Coish et al. 1982; Swinden er a/.1989). The early to middle Arenig age of the arc to back-arctransition is based on graptolites in the lower part of theSnooks Arm Group and a U/Pb age of 489 +31-2Ma onpegmatitic gabbro from the Betts Cove Complex (Dunning& Krogh 198s) (Fig. a).

The Lushs Bight and Western Arm groups, which outcropon the Springdale Peninsula immediately southeast of theBetts Cove Complex (Fig. 2), have been interpreted bysome workers as the structurally displaced extensions of theBetts Cove Complex and the Snooks Arm Grouprespectively (Upadhyay et al. I97l; Marten 1971) (Fig. a).The Lushs Bight Group lacks the plutonic ophioliteelements, but includes pillow lavas which are dominated bytypical arc tholeiites, moderately refractory tholeiitic rocksand strongly refractory rocks of boninitic affinity (Jenner efal. t988; Swinden et al. 1989) (Fig. 7). The conformablyoverlying Western Arm Group comprises a basal sedimen-tary and tuffaceous unit, broadly similar to epiclastic rocksin the Snooks Arm Group, and, high in the sequence, pillowlavas of oceanic island tholeiite affrnity that are geochemi-cally similar to those in the Snooks Arm Group. Jenner e,

DUNNAGE ZONE NEWFOUNDLAND AND SOUTHERN SCOTLAND 575

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4/. (1988) interpreted the Lushs Bight Group as havingformed in a SSZ environment and suggested that the LushsBight-Western Arm succession recorded the rifting of anisland arc and the establishment of a back-arc basin, a

similar tectonic progression to that deduced for the BettsCove-Snooks Arm sequence.

The Bay of Islands Complex is an early Arenig ophioliteoutlier of the Notre Dame Subzone that structurally overliesthe Humber Zone (Figs 2 and 4). Recent geochemical(Elthon 1991; Jenner et al. l99l) and petrological (B6dard1991) studies have shown that, contrary to previousinterpretations (e.9. Suen e/ al. t979) this ophiolite was

probably generated in a transitional SSZ to back-arc setting.It is unconformably overlain by the Llanvirn Crabb BrookGroup (Casey & Kidd 1981) and the hiatus is coeval withinitial obduction of Notre Dame Subzone ophiolitic rocksonto the Laurentian continental margin (Fig. 5). This eventis dated stratigraphically as late Arenig by the appearance ofophiolitic detritus in fossiliferous Humber Zone forelandflysch deposits (Stevens 1970; Botsford 1987; S. H. Williams& Stevens 1988). A on At f 'o Ar age of 469 + I -5 Ma(recalculated) has been determined on hornblende from themetamorphic sole of the Bay of Islands Complex(Dallmeyer & Williams t975) (Figs 2 and 5) andcorresponds within error to the stratigraphic age forobduction.

An extensive belt of MORB-like ophiolites in the easternpart of the Notre Dame Subzone is slightly younger than theBetts Cove-Ballantrae association (Figs 2 and 5). Theophiolitic rocks include mafic, ultramaflc and trondhjemiticrocks assigned to the Mansfield Cove/Hall Hill complexes(Bostock et al. 1979; Dunning et al. 1987) and gabbro,sheeted dykes and pillow lavas assigned to the Annieop-squotch Complex (Dunning 1987). The Mansfield CoveComplex has yielded a U/Pb date of 479 + /-3 Ma(Dunning et al. 1987), which overlaps ages of 481 + 4l-2Maand 478+31-zMa for the Annieopsquotch Complex(Dunning & Krogh 1985). These are resolvably youngerthan the SSZ ophiolites in the western Notre Dame Subzoneand Swinden (1990) has suggested that they may recordmagmatism at a mature back-arc spreading centre, theinitiation of which was recorded by the Betts Cove, LushsBight and Bay of Islands ophiolites.

Dominantly calc-alkaline island arc volcanic sequences arein structural contact with the ophiolites of the eastern partof the Notre Dame Subzone. They are everywhere adjacentto Red Indian Line and form the eastern boundary of theNotre Dame Subzone. The volcanic rocks includecalc-alkaline and some tholeiitic basalt, with significantamounts of andesite and rhyolite, and they exhibit clear arcgeochemical signatures (Swinden 1990 and unpublisheddata). They are assigned principally to the Buchans,Roberts Arm, and Cutwell groups (Figs 2 and 5).Conodonts from the Buchans and Cutwell groups indicatean age range from late Arenig to late Llanvirn (Nowlan &Thurlow 1984; O'Brien & Szybinski 1989), consistent withU/Pb dates of 473+31-2, 473+ l-2 and 469 +5/-3Mafrom the Buchans, Roberts Arm, and Cutwell groups,respectively (Dunning et al. t987; Dunning & Krogh 1988).

It is uncertain how these volcanic rocks relate to theslightly older ophiolites immediately to the west. Bostock e/al. (1979) suggested that dykes cutting the Mansfleld CoveComplex feed volcanic rocks in the Roberts Arm Group,implying a basement-cover relationship. This proposalremains to be tested by geochemistry. The Cutwell Groupshows a history of volcanism extending from the late Arenigto the late Llanvirn without any known unconformities. This

577

contrasts with the hiatus at the Arenig-Llanvirn boundary,and the absence of post-Arenig volcanism, in the Bay ofIslands Complex; this critical time span is not welldocumented in the Betts Cove-Snooks Arm sequence.

Midland Yaltey. Structurally disrupted ophiolitic se-

quences outcrop at the faulted margins of the MidlandValley, adjacent to the Highland Boundary Fault in thenorth and the Southern Upland Fault in the south (Fig. 1).

The Highland Border Complex, in the north, is fragmentaryand the disrupted sequences in each of the small,tectonically isolated slivers are difficult to correlate.However, generalised lithological groupings have beenrecognised and biostratigraphically controlled by Ctxry et al.(1984). Serpentinite and associated ophiolitic rocks areoverlain by early Arenig limestone and conglomeratewhereas a structurally separate sequence of maflc lava, tuff,chert and black shale may be about Llanvirn-Llandeilo inage. The geochemical characteristics of the lava and theterrigenous character of some associated sedimentarylithologies, suggested to Robertson and Henderson (1984)

that eruption took place in a small marginal basin. Thesimilarities with the Clew Bay supercomplex which occupies

an analogous structural position in the west of Ireland havebeen discussed by Harper et al. (7989).

The Ballantrae Complex, adjacent to the SouthernUpland Fault (Figs 1 and 3), is a disrupted ophiolite(Church & Gayer 1973; Bluck et al. l98O; Stone & Rushton1983), in which serpentinised ultramaflc rocks, arc-tholeiitelavas and lava breccias are structurally imbricated with a

within-plate volcanic-epiclastic sequence. Geochemical datafrom whole rocks and chrome spinels (Stone & Smellie1990) suggest that the ultramafic assemblage originated in a

SSZ environment. The volcanic rocks, dominantly maflcpillow lavas, are locally interbedded with volcaniclasticsedimentary rocks and chert (Stone & Rushton 1983 andreferences therein). Geochemical studies by numerousresearchers over a number of years (Stone & Smellie 1990

and references therein) have identifled a polygeneticsequence; within-plate lavas are juxtaposed with at least twotypes of arc tholeiite (Mains Hill and Games Loup varieties;Thirlwall & Bluck 1984), one of which (GL) is interbeddedwith magnesian rocks of boninitic affinity (Smellie & Stone,L992). The tholeiites with oceanic island affinities (e.g.Bennane Head and Pinbain sequences) are geochemicallyfairly uniform and contain a far higher proportion ofinterbedded clastic sediment than the arc sequences. AtBennane Head and Pinbain Bridge the clastic interbedsyield early to middle Arenig graptolites (Stone & Rushton1983; Rushton et a\.1986). The different volcanic rock unitsare separated by faults and their relative stratigraphicpositions are not well constrained. However, severalsandstone beds at the base of the Pinbain oceanic islandsequence contain clasts geochemically similar to island arctholeiites and thus seem likely to have an arc provenance(Stone & Smellie 1990). Arc material was thereforeprobably available for erosion before the eruption of theoceanic island tholeiites. Smellie and Stone (1992) deduceda history of arc rifting and back-arc basin development fromthis apparent sequence of arc tholeiites and boninitessucceeded by ocean island tholeiites (Fig. 4).

The Ballantrae Complex is mainly early to middle Arenigin age, based on graptolites (Stone & Rushton 1983;

Rushton et al. 1986). A fauna from an adjacent, butstructurally separate, fine-grained sedimentary sequenceoverlying a sedimentary ultramafic breccia is late Arenig(Stone & Rushton, 1983) and may record erosion duringobduction (Smellie & Stone 1992). Trondhjemite in the


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plutonic part of the complex has yielded a U/Pb date of483 + l-4Ma (Bhck et al. 1980), consistent with lessprecise Sm/NO isochron ages on the arc tholeiites at GamesLoup and Mains Hill (Thirlwall & Bluck 1984). Theyoungest palaeontological dates from the BallantraeComplex allow for structural imbrication, probably duringobduction onto the Laurentian continental margin, at aboutthe Arenig-Llanvirn boundary. This event is marked by thehiatus beneath the overlying cover sequence, the oldest partof which is probably late Llanvirn in age. Obduction may bedated radiometrically by a K/Ar age of 478+/-8Ma(Bluck et al. 1980) on amphibolite, originally described aspart of the sole of the complex (Spray & Williams 1980) butin fact preserved as an intra-ophiolite zone separatingultramafic rock from tholeiitic lavas.

Comparison. The geological resemblance between theNotre Dame Subzone and the Midland Valley is moststriking in Arenig rocks (Figs 4 and 5). The pre-Areniggeology of the Notre Dame Subzone is represented by twoisland-arc volcanic sequences, the Little Port Complex in theHumber Arm Allochthon, and the Sleepy Cove Group ineastern Notre Dame Bay. Both are intruded by datedCambrian trondhjemite (505 +31-2Ma, Dunning & Krogh1986; and 507 +31-ZMa, Elliott et al. 1991, respectively).This magmatic episode is not represented in the MidlandValley, although there is a hint of older components withinthe Ballantrae Complex in a Sm/Nd age of 576 + l-32Mafor a garnet clinopyroxenite (Hamilton et al. 1984). Oldermaterial has also been reported from the Highland BorderComplex where a K/Ar age of 537 + l-ZMa (and animprecise Sm/Nd age of 546 + l-42 Ma) for an amphibolitesuggested to Dempster and Bluck (1991) an early ophioliteobduction event. The younger, post-Arenig volcanics of theBuchans and Cutwell groups may possibly have analogues inthe Llanvirn-Llandeilo lavas of the Highland BorderComplex.

Of the various ophiolitic assemblages, the BettsCove-Snooks Arm sequence is most similar to theBallantrae Complex in lithology and stratigraphy, age,volcanic geochemistry, and interpreted palaeotectonichistory. Harzburgite and layered ultramaflc rocks form thebasal units of both, minor remnants of layered gabbro,leucogabbro and trondhjemite are locally preserved, andextensive pillow lava and lava breccia occurs at higherstratigraphic levels. In each area, mafic volcanic rocksdominate the lower parts of the volcanic unit, whereas redand green, volcanically-derived clastic sediments are mostabundant in the upper parts. In particular the BennaneHead and Pinbain areas at Ballantrae show a remarkablelithological correspondence with the Snooks Arm Group atBetts Cove.

The two sequences are the same age (Figs 4 and 5). TheU/Pb date on gabbro of the Betts Cove Complex overlaps,within error, with that from trondhjemite of the BallantraeComplex. Graptolites have been recovered from the uppervolcano-sedimentary parts of each succession and the earlyto middle Arenig range at Ballantrae includes the morerestricted range so far determined for the Snooks ArmGroup.

Perhaps the most striking resemblance between theScottish and Newfoundland sequences is in the geochemistryof the volcanic rocks and the tectonic interpretations thatthey yield. Each sequence is polygenetic and although notall of the volcanic rock types occur in each area, there is aclear correspondence between their compositions. The BettsCove Complex contains strongly refractory volcanic rocks

with concave-upward REE patterns characteristic ofboninites (Fig. 6a). Similar high field strength element(HFSE)-depleted rocks have been identified in theBallantrae Complex (Stone & Smellie 1990), althoughdetailed comparison with Betts Cove boninites awaits REEanalysis now in progress. Tholeiites in the Games Loup areaare moderately depleted in HFSE and the LREE andalthough similar rocks have not been identified rn the BettsCove Complex, there are similar rocks in the Lushs BightGroup, immediately to the southeast (Fig. 6b); both havebeen interpreted as the results of partial melting ofmoderately refractory sources (Thirlwall & Bluck 1984;Jenner et al. 1988; Swinden et al. 1989; Smellie & Stone1992). Arc tholeiites in the Betts Cove ophiolite vary fromslightly LREE-depleted to slightly LREE-enriched and aresimilar in REE pattern, although relatively depleted inoverall REE abundance, to arc tholeiites from the MainsHill section of the Ballantrae Complex (Fig. 6c).LREE-enriched lavas in the Pinbain and Bennane Headareas of the Ballantrae Complex, interpreted to be ofoceanic island affinity by Thirlwall and Bluck (1984) andconsidered to overlie sandstone derived from the arctholeiites by Stone and Smellie (1990), are similar tovolcanic rocks of the Snooks Arm Group (Fig. 6d) whichconformably overlie the Betts Cove Complex inNewfoundland.

Stratigraphy in both western Newfoundland and theMidland Valley, as well as radiometric dating of themetamorphic soles beneath ophiolites in each area, suggestinitial obduction onto Laurentian crust at about the sametime, in the late Arenig to early Llanvirn. In both cases,tectonic, geochemical and isotopic evidence supportspalaeotectonic models involving southeasterly subduction(Strong et al. 1974; Stone & Smellie 1990) although forBallantrae the case for northwesterly subduction has alsobeen argued (Bluck et al. 1980). In the Betts Cove Complex,where there are relatively good controls on stratigraphy, thegenerally preferred model involves tectonic evolution fromSSZ ophiolite (boninitic and arc tholeiitic magmatism) tomature back-arc (oceanic island tholeiitic volcanism) (Coishet al. 1982; Swinden et al. 1989; Crawford et al. 1989). Asimilar model has been proposed for the area just to thesouth, where arc rifting, recorded by the Lushs Bight Grouplavas, was succeeded by back-arc volcanism, forming theupper part of the Western Arm Group (Jenner et al. 1988).Recent models for Ballantrae also relate refractory lavas tothe initiation of subduction in an oceanic environment, andlater basalts of oceanic island affinity to eruption in the nearback-arc (Stone & Smellie 1990; Smellie & Stone 1992).This model differs from those developed in Newfoundlandmainly in the spatial and stratigraphic associations betweenisland arc and oceanic island environments, although bothare based upon very similar data sets.

2.2. Middle Ordovician to Lower Silurian-marinecoYer sequences

Mdland Valley. At the northern margin of the MidlandValley, the Highland Border Complex is overlain by poorlypreserved Caradoc-Ashgill conglomerate and sandstone(Cvry et al. 1984). A much more extensive successionoutcrops farther south in the Girvan area where theBallantrae Complex is unconformably overlain by a lateLlanvirn to earliest Wenlock marine cover sequence (Fig.5), in turn overlain by Wenlock subaerial sedimentary rocks.The Ordovician part of this sequence is divided into theLlanvirn and Llandeilo Barr Group and the conformably

580

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overlying Ardmillan Group of Caradoc and Ashgili age(Ingham, 1978). Detailed work by A. Williams (1962)showed that the Llanvirn to Caradoc rocks are markedlytransgressive northwards across a series of syndepositionalfaults downthrown to the south (Fig. 5).

The Barr Group comprises two prominent conglomerateunits, the lower Kirkland and the upper Benan conglomer-ates, separated by finer-grained sedimentary rocks and theStinchar Limestone (Fig. 5). These rocks record initialdeposition in a constructive fan environment, followed bydeeper water sedimentation as the fan was temporarilyabandoned, and then renewed conglomerate deposition as

fan progradation was re-established (A. Williams 1962; Ince1984). Clasts in the Barr Group conglomerates arecomposed largely of Ballantrae Complex lithologies, butinclude granite and syenite which have yielded Rb/Sr ages

ranging from 648 + l-32Ma to 459 + /-10 Ma (Longman e/

al. 1979) and Tremadocian limestone (Rushton & Tripp1979). These ages are consistent with the oldest precisepalaeontological age for the group (Bergstrom 1971), whichplaces the Llanvirn-Llandeilo boundary within the StincharLimestone (Fig. 5). The Benan Conglomerate is associatedwith mudstones that contain Llandeilo-Caradoc graptolites(Peach & Home 1899).

The prevalent fan-delta style of deposition in the BarrGroup persisted into the lower Ardmillan Group, where theCaradocian Kilranny Conglomerate (Fig. 5) has a similardebris flow character and clast provenance to the BenanConglomerate (Ingham 1978). However, the Caradocmarked the flnal submergence of the Ballantrae Complex bycontinued northwest transgression, and the burial of theirregular basement topography that characterised Llanvirnand Llandeilo time. Thus, in the Craighead inlier (Fig. 3),pillow lavas, forming the most northerly exposures of theBallantrae Complex, are unconformably overlain by lateCaradoc reefoid limestone (Fig. 5), which correlates withthe lower part of the Ardmillan Group, a mainly turbiditicsequence (Ince 1984). The upper part of the ArdmillanGroup overlapped northwestwards across the shallow waterfacies and was deposited over the full width of the basin; itsyoungest beds are of latest Ashgill age.

The Silurian part of the cover sequence above theBallantrae Complex was for the most part deposited in a

deep-marine turbidite environment (Cocks & Toghill 1973),but near the top the succession passes into shallow marineand red subaerial rocks of basal Wenlock age. Thistransition is also recorded in other Silurian inliers of theMidland Valley (Bluck 1983) where some sections continueupwards into the Ludlow and merge with the Siluro-Devonian Old Red Sandstone lithofacies.

Notre Dame Subzone. The Llanvirn to Early Silurianrecord in the Notre Dame Subzone is poorly preserved andfragmentary. The only marine unit preserved in associationwith Betts Cove-type ophiolites and presumed to be of thisage is the Flatwater Pond Group (Fig. 5), which outcropsclose to the strongly tectonised Baie Verte Line (Fig. 2).The group includes the matrix-supported, polymict KidneyPond congfomerate (Kidd L974, 1977; Hibbard 1983) and is

in fault contact with the Advocate Complex, a probablecorrelative of the Betts Cove Complex. The conglomerate,which contains deformed and altered ophiolitic fragments, is

inferred to be Llanvirn or younger because some other clasts

are lithologically similar to the nearby, radiometrically-dated Burlington Granodiorite (ca. 460Ma, Hibbard 1983).Probable Silurian subaerial volcanic rocks overlie theFlatwater Pond Group (Fig. 5) and place a younger limit onits age.

Yb Lu

Figure 6 Geochemical comparison between volcanic rocks in theBetts Cove and Lushs Bight sequences, Newfoundland, and theBallantrae Complex, Scotland. Extended REE patterns for theBetts Cove Complex from G. A. Jenner (unpublished data) andSwinden et al. (1988), and for the Lushs Bight Group from Jenneret al. (1988) and Jenner et al. (1991). Data for Ballantrae Complexfrom Thirlwall and Bluck (1984). Primitive mantle normalisingvalues as used by Swinden et al. (1990).

'...,' ,,,,.:,,' . Lushs Bight retractory arc tholeiites

-

Games Loup volcanics, Ballantrae

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Bennane Head, Pinbain volcanics, Ballantrae


Two other Ordovician sedimentary units in westernNewfoundland may be analogous to parts of the Ballantraecover sequence. Sedimentary rocks that unconformablyoverlie the allochthonous ophiolite of the Bay of IslandsComplex, the Crabb Brook Group (Figs 4 and 5), includevery coarse conglomeratic olistostromal units and areoverlain by probable shallow marine or subaerial purplish-red sandstones. The age of the Crabb Brook Group hasbeen determined as Llanvirn from acritarchs derived frommudstones (Casey & Kidd 1981). The Long Point Group(Bergstrrim et al. 1974) overlies rocks of the Humber Zone(Fig. 2), west of the western-most outlier of the Notre DameSubzone. It consists of a lower limestone formation, whichcontains small coral and bryozoa reefs and an early Caradocage conodont fauna (Fihraus 1973; Bergstrcim et al. 1974),and an upper formation of thin-bedded shales, sandstonesand limestones. Nearby turbidites, that are also earlyCaradoc, contain ophiolitic detritus (Stevens 1970) andindicate the proximity of obducted rocks of the Notre DameSubzone at this time. The Long Point Group overlies theleading edge of the allochthon, but it is presently uncertainwhether the contact is an unconformity (Rodgers 1965) or athrust (Stockmal & Waldron 1990).

The Ordovician rocks of the Notre Dame Subzone areunconformably overlain by extensive Silurian subaerialvolcanic sequences, capped by terrestrial and fluviatile redsandstones and conglomerates (Fig. 5). The Silurianvolcanic rocks are calc-alkaline, and have been interpretedas the eruptive products of a series of collapse calderas in astructural regime dominated by strike-slip faulting (Coyle &Strong 1987). Szybinski et al. (1990) have suggested that atleast one of the sequences, the Springdale Group, may berelated to the formation of pull-apart basins in response tosinistral transpression. The Springdale Group is unconfor-mable on the Roberts Arm Group (Kalliokoski 1955; Coyle& Strong 1987) and its correlative, the Cape St John Group,is unconformable on the Betts Cove Complex (Neale et a/.1975). Similar rocks of the Micmac Lake Group lie withslight angular unconformity on the Flatwater Pond Group(Kidd 1974). The Silurian subaerial rocks are notfossiliferous, but U/Pb dates ranging from 432+21-l to425+ l-3Ma (Chandler et al. 1987; Whalen et al. 1987;Coyle 1990) indicate a Llandovery or Wenlock age for thelower, volcanic sections.

Comparison. In the Midland Valley, the BallantraeComplex is overlain by a cover sequence that records almostcontinuous marine sedimentation from the Llanvirn to theWenlock. Although there are only a few isolatedoccurrences of Ordovician rocks representing this timeinterval in the western part of the Notre Dame Subzone,they are comparable, in age and lithofacies, to parts of theBallantrae cover sequence.

The Betts Cove Complex is unconformably overlain bySilurian subaerial volcanic rocks, so a marine coversequence similar to that at Ballantrae was either neverdeposited in the western part of Notre Dame Bay or waseroded before the Wenlock. A few kilometres to the west,however, the undated Kidney Pond conglomerate, contain-ing deformed ophiolitic detritus and overlain unconformablyby subaerial volcanic rocks, is a possible candidate for partof the missing cover sequence. In particular it includes aprominent matrix-supported, pebble to boulder conglomer-ate that is similar in clast composition and generalsedimentary character to the submarine conglomerates ofthe Barr Group. Clasts thought to be derived fromgranodiorite dated by a variety of methods at about 460 Ma(Kidd 1974; Hibbard 1983), suggest possible equivalence to

either the Benan or the Kilranny conglomerates. The BenanConglomerate contains granite clasts that have yielded ages

as young as 459+ l-l0Ma (Longman et al. 1979).

However, we would urge caution in attaching significance tothese relatively ill-deflned ages, all of which need to beconflrmed by more precise geochronological studies.

Hibbard (1983) pointed out similarities between theKidney Pond conglomerate and the Crabb Brook Group,which lies unconformably on the Bay of Islands Complex.The same comparison can be made with the Ballantraecover sequence. Common features include the clastcompositions in the Crabb Brook Group, the evidence fromsubmarine screes of syndepositional faulting, the matrix-supported olistostromal deposits, and the presence ofshallow marine or subaerial rocks. The Llanvirn age of theCrabb Brook Group makes it approximately coeval with theKirkland Conglomerate and the lower part of the StincharLimestone.

The most northerly exposures of the Ballantrae coversequence are in the Craighead inlier where onlap during thelate Caradoc caused the deposition of reef limestone onpillow lavas of the Ballantrae Complex (Fig. 5). InNewfoundland, reef-bearing limestone in the lower part ofthe Long Point Group was evidently deposited west of thelimit of ophiolite obduction and is slightly older (earlyCaradoc). The upper part of the Long Point Group,however, is quite likely the same age as the Craighead reefand is interpreted to represent the progradation of a deltainto a shallow marine environment (Fflhraus 1973). Thelithostratigraphic differences between the two units mayrepresent deposition in different parts of the samesedimentary system, the well-bedded Long Point Grouprepresenting a sheltered lagoonal environment continent-ward of the allochthon, and the Craighead Limestoneforming as an isolated reef-mound assemblage on top of theobducted ophiolite.

The Arenig-Llanvirn ophiolites and calc-alkalic island arcvolcanic sequences of the eastern part of the Notre DameSubzone do not have exposed counterparts in the MidlandValley. Some comparison may be justifiable with theLlanvirn-Llandeilo mafic lavas of the Highland BorderComplex although these are not arc volcanics and wererelated by Robertson and Henderson (1984) to eruptions ina small marginal basin. Arc volcanism that postdates theBallantrae Complex has been deduced from calc-alkalinegranitoid boulders, dated at about 470 Ma and derived fromthe north, in Barr Group conglomerate (Lortgman et al.r979).

Terrestrial red beds at the top of the Ballantrae coversequence are early Wenlock based on biostratigraphy(Cocks & Toghill 1973). Although there is no radiometriccontrol for the age of this sequence, lavas interbedded withthe Old Red Sandstone facies red beds elsewhere inScotland give ooArf'nAr dates as old as 421 + l-4Ma(Thirlwall 1988). Age control from Scotland thus overlaps,within error, U/Pb age determinations of about 429Maforfelsic lavas in the Springdale Group (Chandler et al. 1987;Whalen et al. 1987) which are interbedded with redsandstone and conglomerate (Fig. 5).

3. Southern Uplands and Exploits Subzone

The Exploits Subzone forms the southeastern portion of theDunnage Znne in Newfoundland and is structurallyseparated from the Notre Dame Subzone by Red IndianLine (H. Williams er a/. 1988) (Fig. 2). The Scottish


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Southern Uplands occupy an analogous position and areseparated from the Midland Valley and the BallantraeComplex by the Southern Upland Fault (Fig. 3). A keytripartite stratigraphic succession, recognised in both theExploits Subzone and the Northern and Central Belts of theSouthern Uplands (Fig. 7), consists of maflc volcanic rocksof Arenig to Llandeilo age, Llandeilo to Ashgill black shaleand Caradoc to Early Silurian greywacke. This successionprovides the starting point for geological comparisonbetween the two areas.

The deeper level of erosion in Newfoundland has exposeda fragmented Arenig and older succession, including theGander Zone and the older rocks of the Exploits Subzone,that predates the oldest known rocks in the SouthernUplands. Conversely Late Ordovician and Silurian strata aremore extensive in the Southern Uplands, where structuraland stratigraphic relationships have been described in muchgreater detail than in Newfoundland. In the ExploitsSubzone, the Ordovician and Silurian greywackes areoverlain by Silurian terrestrial and shallow marinesediments. This transition is apparently contemporaneouswith a change in sedimentological character in the upperpart of the Southern Uplands greywacke sequence, markedby an influx of reddened detritus which continues into themid-Silurian.

3.1. Southern UplandsThe Southern Uplands of Scotland (and the geologicallycontiguous Longford-Down area of Ireland) are principallycomposed of Llandeilo to late Wenlock turbidite sequences.The outcrop pattern is controlled by strike-parallel(NE-SW) faults, initiated as early thrusts, which divide thearea into structural tracts (Figs 3 and 7). Within each tractthe bedding is dominantly steeply dipping or vertical andoverall younging is generally to the NW; in contrast thetracts themselves become sequentially younger to the SE(Leggett et al. 1979 and references therein). This has beeninterpreted as the result of forearc accretionary prismdevelopment above a north-dipping subduction zone, activeat the Laurentian margin from mid-Ordovician to lateSilurian times (Leggett et al. 1979). An alternativesuggestion is of late Llandeilo to mid-Llandovery turbiditefacies progradation southwards, ahead of a southwards-migrating thrust stack initiated in a back arc basin (Stone etal. 1987). In both models the greywacke facies stratatransgress a pelagic sequence of black shale, bentonite andchert, which locally rests on pillow basalts, basalt brecciasand/or hyaloclastites. The main thrust decollement occurredwithin the fissile black shales and the basal volcanicsequence in the Southern Uplands is only preserved as thinselvages caught up as fault slices within and at the base ofthe shale sequence (Barnes et al. 1989 and referencestherein). The pelagic sequence and the underlying volcanicsare included together in the Moffat Shale Group.

The age of the basal volcanic sequence is constrained by asingle Sm/Nd date from a basalt lava of 490+ l-74Ma(Thirlwall in McKerrow et al. 1985) and by thebiostratigraphy of the overlying or structurally associatedchert or black shales. Although the reported occurrence ofTetragraptus fruiticosus from mudstone overlying the datedlava (Peach & Horne 1899, p. 288) cannot be substantiated(A. W. A. Rushton, pers. comm.), associated cherts docontain upper Arenig conodonts (Lamont & Lindstrom1957; Armstrotg et al. 1990). Elsewhere Llanvirn andLlandeilo conodont faunas have been described from cherts(Lamont & Lindstrom 1957; Armstrong et al. t990), b:ut

black shales directly overlying the basalt are invariablyLlandeilo or early Caradoc. The basal volcanic rocks havebeen described as a mixture of alkali basalt and probableocean-floor tholeiites which, by assuming a decrease in agefrom north to south, have been interpreted as the productsof rifting and subsequent sea-floor spreading (Lambert et al.1e81).

Volcanic rocks are also interbedded in a few places withthe lower part of the greywacke sequences which overlie theMoffat Shale Group. These assemblages are signiflcantlyyounger than the basal Moffat Shale Group lavas andcommonly have within-plate geochemical characteristics. AtBail Hill, mildly alkaline lavas and tuffs of within-plate origin(McMurtry 1980) overtie black shales of the gracilis Zone;biotite from a lava has given a K/Ar date of 453 + /- 10 Ma(Harris et al. 1965). In West Nithsdale, lavas of unknownaffinity are interbedded with probable gracilis Zonegreywackes (Floyd 1982) whereas, at Wrae Hill, peralkalinerhyolites of clingani age are of likely oceanic island origin(Thirlwall 1981). At the northern edge of the SouthernUplands, separated from the Ballantrae Complex by theStinchar Valley Fault, the lavas of the Downan Point area(Fig. 3) have within-plate characteristics, and have given aSm/Nd mineral age of 468 + l-22 (Thirlwall & Bluck 1984);they are interpreted to be interbedded, at the base of theformation, with gracilis Zone shales and cherts (Lewis &Bloxam 1977).

The upper part of the Moffat Shale Group is generallypreserved as discontinuous slivers of shale exposed at thebase (the southeast margin) of each thrust-deflned tract inthe Northern and Central belts of the Southern Uplands.The onset of turbidite sedimentation, and hence the age ofthe uppermost shale horizons, becomes progressivelyyounger in successive tracts southward, but the oldestmaterial remains of Llandeilo/Caradoc (gracilis Zone) ageso that progressively longer time intervals are represented insuccessive outcrops (Fig. 7). In some exposures, the MoffatShale Group ranges from late Llandeilo to mid-Llandoveryand is about 100 m thick. At Dob's Linn (Lapworth 1878; S.H. Williams 1988b), it includes tp to 20Vo thin (less than0.5 m) layers of bentonitic clay, which are derived fromglassy silicic volcanic ash and are interpreted to recordintermediate to felsic volcanism in an ensialic arc andback-arc environment (Merriman & Roberts 1990).

The age of the oldest greywacke in a given tract is usuallydeduced from the youngest graptolite zone present in theunderlying Moffat Shale Group, but is also indicated bysparse graptolite faunas in pelagic mudstones interbeddedwith the greywacke. The palaeontological evidence dem-onstrates that each of the thick greywacke tracts rarely spansmore than one graptolite zone, in contrast to the wide agerange present in the relatively thin outcrops of the MoffatShale Group.

The oldest of these greyracke units (the MarchburnFormation) (Figs 3 and 7) contains significant detritus ofophiolitic provenance. flowever, the dominant, backgroundmaterial in the greywackes is quartzo-feldspathic withgranitic and deformed meta-sedimentary lithic detritusderived from a northerly cratonic provenance (e.g. Kelling1962). Superimposed on this, and locally dominant, isvolcaniclastic material transported from the south orsouthwest during mid-Caradoc to early Llandovery times(Stone er al. L987). Greywackes derived from the twocontrasting terranes are interbedded at various scales fromindividual bed to formation. The southerly-derived grey-wackes contain volcanic lithic detritus, fresh pyroxene andamphibole, indicative of a maflc to intermediate ensialic arc

584 S. P. COLMAN-SADD, P. STONE, H.

terrane source (Styles et al. 1989), and a variable proportionof quartzo-feldspathic detritus. Rare interbedded con-glomerates, derived from the northwest (Kelling et al.1987), are dominated by granite and acid and maflc volcanicclasts, but chert, quartzite and gabbro are also present. Thegranite clasts have been compared to various potentialsource rocks as far west as Newfoundland (Elders L987;McKerrow & Elders 1989).

Most of the Ordovician clastic sediments were deliveredaxially into a NE-SW, elongate sedimentary basin (Evans e/al. l99l). Sediment sourcing from the Laurentiancontinental margin to the north is generally assumed (e.g.Leggett et al. 1979) but more controversial are suggestionsof an active volcanic arc provenance to the south. Thecomposition of detrital pyroxenes derived therefrom showthe volcanic provenance to have been calc-alkaline andensialic (Styles et al. 1989). Sedimentological and strat-igraphic evidence has been advanced in support of thesoutherly derivation of the volcanic-rich greywackes whichwould thus have been deposited in a back-arc basin (Morris,1987; Stone et al. L987), rather than fore-arc trench (Leggettet al. 1979). More or less continuous tectonic developmentwould have occurred in the proposed accretionary prism(Leggett et al. 1979) but in a back-arc setting thrustdeformation during basin closure, culminated in theover-riding of the southern margin by the thrust stack.Thereafter sedimentation continued until the mid-Wenlockin a foreland basin setting (Stone e/ al. 1987). An alternativeto the back-arc model, involving the arrival at the trench ofan island-arc remnant (probably with a large measure ofstrike-slip movement) has been proposed by Kellirrg et al.(1987). Ilowever, such a solution is difficult to reconcile withthe proven availability of the provenance area from the lateLlandeilo to the early Llandovery (at least) and theorthogonal nature of pre-Wenlock deformation prior to theonset of sinistral strike-slip (Barnes et al. 1989). Whichevermodel is preferred there were, during the middle and lateLlandovery, gradual changes in the facies and compositionof the sediments. The sedimentologically variable GalaGroup greywackes gave way to very uniform sequences offine to medium-grained, thin to medium-bedded greywackeand interbedded siltstone and red mudstone of the HawickGroup (Figs 3 and 7). Detritus in the Hawick Group isquartzo-feldspathic, markedly calcareous, and includesmaflc and felsic volcanic fragments and haematite-coatedmuscovite flakes. A distinctive laminated and carbonaceoushemipelagic lithology appears in early Wenlock beds and inyounger parts of the group the red mudstone is absent. Amajor strike fault juxtaposes the relatively intenselydeformed Hawick Group against the much less deformed,more variable turbidite facies of the Riccarton Group to thesouth. The latter contains interbedded laminated hemipela-gite and ranges up to late Wenlock in age.

Deformation of the Southern Uplands sedimentarysequence was diachronous and contemporaneous withdeposition of younger parts of that sequence (Barnes er a/.

1989). Initial deformation was by thrusting, but sinistralshear became important in the late Llandovery (Anderson1987) or early Wenlock (Barnes et al. 1989). There is somelocal evidence for dextral shear superimposed on the earlierand dominant sinistral phase.

3.2. Exploits SubzoneOceanic volcanic and epiclastic rocks of the ExploitsSubzone in Newfoundland overlie dominantly quartzosemetasediments and pelites of the Gander Zone. Therelationship is structural, and the Gander Zone is exposed in

S. SWINDEN AND R. P. BARNES

a number of tectonic windows through the oceanic rocks(Colman-Sadd & Swinden 1984; Colman-Sadd, 1985, 1988)(Fig. 2). The minimum age of emplacement of the ExploitsSubzone is late Arenig, based on an overlap sequence thatcontains ophiolitic detritus and late Arenig-early Llanvirnfossils (Wonderley & Neuman 1984; H. Williams & Piasecki1990; O'Neill 1991) and on the 474+61-3Ma age of astitching pluton (Colman-Sadd et al. 1992).

The Exploits Subzone records an extensive Cambrian toEarly Ordovician geological history that is not seen in theSouthern Uplands. The oldest stratifled rocks are bimodalvolcanic and epiclastic rocks of island arc derivationassigned to the Tally Pond volcanics in the southeastern partof the Victoria Lake Group; felsic volcanics of thisassemblage have returned U/Pb dates of 513 + l-2Ma(Dunning et al. l99l). Slightly younger arc rocks elsewherein the Victoria Lake group (Tulks Hill volcanics,498+61-4Ma, Evans et al. 1990) may record either acontinuation of this early arc episode or a younger andunrelated episode of arc volcanism.

A SSZ environment is also inferred for Exploits Subzoneophiolites (Swinden 1988; Jenner & Swinden 1989), one ofwhich, the Pipestone Pond Complex, has yielded a U/Pbage of 494+31-2Ma (Dunning & Krogh 1985). Theophiolites are overlain by basalts of MORB and oceanicisland tholeiite affinity and have been interpreted, like theirwestern Dunnage Zone counterparts, to record arc riftingfollowed by magmatism in a back-arc basin (Jenner &Swinden 1989). Clastic sediments that overlie the ophiolitescontain late Arenig fossils (S. H. Williams et al. l99t).Other volcano-sedimentary units in the Exploits Subzoneinclude the thick sequences of volcanic and epiclastic rocksassigned to the Wild Bight, Exploits and parts of theSummerford groups. Palaeontological control on thevolcanic rocks of the Summerford Group shows them torange from Tremadoc to Llanvirn (Kay 1967; Bergstrcim e/al. t974; Neuman 1976) and recently discovered graptolitesin the Exploits Group place the upper part of it in theLlanvirn (O'Brien 1991), consistent with the gracilis Zoneage of overlying black shale and chert (Dean 1978). Thelower parts of the correlative Wild Bight and Exploitsgroups record a primitive ensimatic atc environmentdominated by arc tholeiites (Swinden et al. t990; Dec et al.1992). Higher in the stratigraphy, these are joined byrefractory mafic volcanic rocks and rhyolites. Theuppermost parts of these groups are dominated by oceanfloor tholeiites, oceanic island tholeiites and mildly alkalirrebasalts (Wasowski & Jacobi 1984; Swinden et al. L990; Decet al. 1992). Similar within-plate tholeiites occur in theSummerford Group (Jacobi & Wasowski 1985). Swinden e/al. (1990), using geological, petrological, geoctremical andisotopic evidence, attributeci the Wild Bight sequence torifting of an island arc and the formation of a back-arcbasin.

The cessation of submarine volcanism was followed by thewidespread deposition of black shale, beginning in theLlandeilo and extending into the Caradoc or Ashgill (Fig.7). Locally, volcanic rocks directly underlie the black shalesbut elsewhere epiclastic turbidites occupy this stratigraphicposition. The black shale interval shows a consistentlithologic sequence wherever its internal stratigraphy hasbeen determined (Dean 1978). The basal parts aredominated by chert and siliceous argillite and have beeninterpreted to be in conformable and gradational contactwith underlying volcanic and volcaniclastic rocks (Helwig1967'Dear & Meyer 1982; S. H. Williams 1988a).

The chert is manganiferous, locally rich in radiolaria, and


has well-developed laminations that are commonly bioturb-ated; in many places it contains light coloured, cleavedinterbeds of tuff (Dean 1978; Dean & Meyer 1982). At a

number of localities, the siliceous lower part of the sequencepasses conformably upwards into fissile black shale which,wherever it is fossiliferous, contains a fauna belonging to thegracilis Zore (e.g. Dean & Meyer L982; S. H. Williams,1988a). The start of black shale deposition appears,therefore, to have been synchronous across the ExploitsSubzone.

The transition to the overlying greywackes has beenprecisely located in the clingani or linearis zones at severallocalities (Fig. 7). The major exception to this pattern is theHamilton Sound sequence where Llandovery fossils (Berry& Boucot 1970) occur near the base of greywackes thatoverlie black shale. The contact was interpreted to beconformable by Karlstrom (1982), but a fault is favoured byH. Williams (1992); hence the doubt expressed in Figure 7.The greywackes are principally assigned to the Sansom andPoint Leamington formations and sporadic age controlindicates a range from Caradoc (clingani Zone) to Ashgilland early Llandovery (Fig. 7). The succession is dominatedby an outer or mid-fan turbidite facies, in whichthick-bedded sequences and conglomerate lenses reflectdeposition in a channelised environment and thin-beddedsequences are attributed to unconfined lobe and interchan-nel deposition (Watson 1981; Arnott 1983).

In most places the greywacke is overlain by the main partof the Goldson Conglomerate, which is principally ofAshgill to late Llandovery age and probably represents atransition to an inner fan or even fan-delta environment(Watson 1981; Arnott et al. 1985). Olistostromes occur atseveral levels in the Late Ordovician and Silurian succession(McKerrow & Cocks 1978; Nelson 1981) and reflect thetectonically unstable nature of the basin, particularly at itsnorthwest margin along Red Indian Line. This instability isalso the probable cause of widespread slump folding (Nelson1981; Arnott et al.1985; Pickering 1987).

The most detailed provenance study (Watson 1981)concentrated on the greywacke-conglomerate sequence ofsoutheast New World Island, deposited between theCaradoc or Ashgill and the mid-Llandovery. Quartz iscommon, felsic clasts are abundant in the volcanic detritus,and significant potash feldspar indicates potassic plutonicrocks in the source area, although tonalite is the dominantplutonic rock type. Sedimentary clasts include greywacke,chert and limestone. Significantly there is no detritus frommetamorphic rocks of higher grade than the greenschistfacies. Data from other parts of the Exploits Subzone(Helwig 1969; Nelson L979; Pickering 1987) suggest thatthese results are generally applicable, but in additionchromite has been reported as a minor, but widespread,constituent by Nelson and Casey (1979). Although theremay locally be abrupt variation in clast composition frombed to bed, there is little overall variation through the wholesequence, indicating a constant geological composition forthe source area (Helwig & Sarpi 1969). Watson (1981)concluded that the main source of the flysch was a magmaticarc which was sufficiently dissected by Late Ordoviciantimes to expose plutonic rocks, but which still had abundantvolcanic and sedimentary rock available for erosion in thelate Llandovery. The presence of chromite indicates thatophiolites were also exposed at this time (Nelson & Caseyte79).

Most workers (e.9. Helwig & Sarpi 1969; Dean 1978;Nelson 1981; Watson 1981) have regarded the Notre DameSubzone as a reasonable source for much of the flysch, but

similar lithologies also occur in the Exploits Subzone andsome distinctive units in this subzone can be identified in theclast assemblage (e. g. Llanvirn-Llandeilo limestone, Nelson1981). Where current directions or the orientations ofdepositional slopes can be inferred, they all indicatederivation of sediment from the north or northwest, or axialredistribution in southwesterly or northeasterly directions(Nelson & Casey 1979; Arnott 1983; Arnott et al. 1985;Pickering 1987). They are therefore in broad agreementwith the conclusions drawn from sediment compositions.

Flysch deposits in many parts of the Exploits Subzone areoverlain by Silurian shallow marine and subaerial rocksassigned to the Botwood Group (Fig. 7). The stratigraphicbase of the Botwood Group, outcropping from HamiltonSound to the southern Bay of Exploits, is marked by felsicand maflc volcanic rocks extruded into a shallow marine orsubaerial environment. The contact with the underlyingflysch is apparently conformable, in contrast with the NotreDame Subzone, and both clasts and matrix of interbeddedred conglomerate contain Llandovery fossils that are similarto those in the flysch (Eastler 1969; H. Williams L972). Thevolcanic rocks are conformably overlain by a mainlysedimentary succession, consisting of red and grey sandstoneand siltstone with minor interbedded conglomerate (H.Williams, 1972). Mud cracks and rain prints suggest a partlyemergent environment, but other sedimentary structures, aswell as corals and shelly fossils, indicate mainly shallowmarine conditions (Wessel 1975). The fossil localitiescontain late Llandovery-early Wenlock and Wenlock faunalassemblages although one graptolite specimen may be ofLudlow age (H. Williams 1972). A sequence of subaerialvolcanics that is either part of the Botwood Group oroverlies it has been dated at 423+31-2Ma (Wenlock)(Dunning et al. 1990).

The polyphase deformation of the Late Ordovician andSilurian sequences of the Exploits Subzone involved aninitial thrust phase, a subsequent episode of major sinistraltranspression, and late dextral shearing (Karlstrom er a/.1982; Blewett & Pickering 1988; 1989). The timing ofdeformation is constrained to some extent by thepost-tectonic Loon Bay batholith and associated dykeswhich are dated at 408 + l-2 Ma and the presence of firstdeformation structures in rocks as young as Wenlock(Karlstrom et al. 1982). The pattern of sinistral followed bydextral shearing is consistent with Silurian movement at themargins of the Gander Zone (Caron & Williams 1988;Piasecki et al. t990; Holdsworth 1991), in what is consideredto have been the basement to the Ordovician and Silurianrocks of the Exploits Subzone.

3.3. ComparisonsBasal Yolcanic Suite. The volcanic assemblage beneath

the mid-Ordovician black shale in the Exploits Subzone ismuch more extensive, both aerially and stratigraphically,than that preserved at the base of the Moffat Shale Group inthe Southern Uplands. The association of Exploits Subzonevolcanic rocks with thick volcanogenic turbidites seems tocontrast with the pre-Caradoc pelagic environments of theSouthern Uplands. However, greywacke clasts in thehyaloclastic debris flow units at the base of the Moffat ShaleGroup provide evidence for pre-Caradoc greywackesequences and the youngest volcanics in the SouthernUplands (e.g. the Bail Hill volcanic complex and volcanicrocks in the Marchburn Formation) were extruded in anenvironment of rapid deposition similar to that of theExploits Subzone. The temporal correspondence betweensequences in the two areas is striking but not exact. Whereas

586 S. P. COLMAN-SADD, P. STONE,

the absence of pre-Arenig mafic volcanic rocks in theSouthern Uplands may well be a function of tectonicdecollement coupled with erosion level, whichever tectonicmodel is preferred, there is real difference between the endof mafic volcanism in the Caradoc in Scotland and in theLlanvirn-Llandeilo in Newfoundland.

Geochemical evidence from the final phase of submarinevolcanism allows a similar tectonic setting to be deduced forboth areas. In Figure 8, the geochemistry of the basalvolcanic sequences of the Southern Uplands is comparedwith the three types of back-arc basin basalts present in theupper part of the Wild Bight Group. Incompatible elementconcentrations suggest that lavas at the base of theMarchburn Formation, the most northerly tectonic slice ofLambert et al. (198I) compare closely with the transitionaloceanic island tholeiites of the Wild Bight Group. Tholeiiticrocks farther south in the Southern Uplands, beneath theKirkcolm and Portpatrick Formations (tectonic slices 2 and3 of Lambert et al. l98l), compare more closely with theMORBJike to slightly LREE-enriched tholeiites thatcharacterise the top of the Wild Bight Group immediatelybelow the black shale. REE data from late Llandeilo-Caradoc pillow lavas of the Downan Point Formation (Fig.3) show that these lavas are geochemically similar to themildly alkaline basalts and transitional tholeiites of the WildBight Group. The data are consistent with latest SouthernUplands and Exploits Subzone volcanism in a back-arc basinenvironment.

Shale sequence. In both the Exploits Subzone and theSouthern Uplands, black shales of the gracilis Zone overlieradiolarian cherts, which in turn overlie the Llandeilo andolder volcanic and epiclastic substrate. The principalcontrast between the two areas is in the duration of shaledeposition. In the Exploits Subzone there is little variability;shale deposition continued into the late Caradoc or earlyAshgill, and only in the Hamilton Sound sequence is there apossibility, as yet unproven, that it continued as late as theOrdovician-Silurian boundary (Fig. 7). In the SouthernUplands the Moffat Shale Group represents a southward-increasing time interval; restricted to the gracilis Zonebeneath the northernmost Southern Uplands formations butextending up into the late Llandovery (turriculatw Zone)beneath the Gala and Hawick groups (Fig. 7).

The principal variation in timespan of the black shale ofthe Southern Uplands is across strike, although there arealso smaller and less systematic variations along strike (e.g.Leggett 1980a; Barnes et al. 1987). All the Exploits Subzone

WBAlkalic basalts

TilZr = 110(chondritic)

WBMORB - like

rO^

WBTransitional oceanicisland tholeiites

H. S. SWINDEN AND R. P. BARNES

shale-flysch sequences, except that at Hamilton Sound, lieclose to Red Indian Line and so are broadly along strikefrom each other. The variations in length of shale depositionin these sequences is, therefore, best compared with thevariation within single tracts in the Southern Uplands, andin this respect is very similar to the gracilis to clingani rangeseen beneath part of the Kirkcolm Formation, thePortpatrick Formation and the Shinnel Formation (Fig. 7).Newfoundland lacks stratigraphy characteristic of the mostnortherly greywacke tract (Marchburn Formation) in whichgreywacke lies directly on the volcanic substrate with onlysporadic intervening black shale. The Hamilton Soundsequence, where the black shale unit may extend up to theOrdovician-Silurian boundary, is possibly more comparablewith tracts in the Southern Uplands Central Belt. If thiscomparison is justified, it may prove possible to identify ananalogue to the Orlock Bridge Fault (Anderson & Oliver1986) in Newfoundland (Fig. 7).

Light coloured tuffaceous beds are interbedded with theshales and cherts in the Exploits Subzone (Dean & Meyer1982) and may correspond to metabentonites that are widelyrecognised in the Southern Uplands (Merriman & Roberts1990). The volcanogenic beds have an apparent range inNewfoundland from Llandeilo to Ashgill, but in Scotlandthey record the continuation of arc and back-arc vulcanicityinto the late Llandovery and possibly beyond. Theirapparent post-Ashgill absence in Newfoundland may merelybe the result of difficulty in recognising them in backgroundlithologies other than shale.

Greywacke sequence. If the Exploits Subzone sequencesare considered equivalent to one or two tracts in theSouthern Uplands, a marked contrast in the stratigraphy ofthe greywackes becomes apparent. In the Exploits Subzoneon New World Island, deposition was probably continuous

Caradoc to the late Llandovery (Fig. 7),single tracts of the Southern Uplands,

greywacke sequences spanning more than one graptolitezone ate rarely preserved. The Exploits Subzone sequencesgenerally show a facies transition upwards from sandyturbidites with local conglomerate lenses, typical of outer ormid-fan deposition, to conglomerates deposited in inner fanchannels, to mdlange at the basin margin. Southern Uplandsgreywackes are dominated by sandy facies with only localdevelopment of conglomerates, and mostly lack the moreproximal inner fan and basin margin deposits of the ExploitsSubzone.

Lateral palaeocurrent patterns in the Southern Uplandsindicate that sediment of contrasting character was derivedfrom both the northwest and the south with interbedding ofthe two types on various scales. Lateral currents in theExploits Subzone are exclusively from the north andnorthwest, and sediment composition shows a remarkableconsistency from the earliest flysch in the late Caradocthrough the youngest deposits of Llandovery age (Watson1981). Sediment derived from the northwest in the SouthernUplands is generally quartzo-feldspathic and lithic clasts aredominated by acid intrusive and plutonic rock types with aminor sedimentary and metamorphic component; accessorymaflc igneous material is usually present and the oldergreywackes contain ophiolitic detritus (Leggett 1980a). Thegeneral composition of the Exploits Subzone flysch is verysimilar (Helwig & Sarpi L969; Watson 198L), including thelocal occurrence of ophiolitic detritus (Nelson & Caseyre79).

Limestone clasts are particularly useful for makingcomparisons and determining source because of theirdistinctive lithologies and identifiable ages. Llanvirn-

from the latewhereas, in

150

100

50

oo 05 10 15 20 25 30 35 40

Tio,

a Tectonic Slice 1

SOUTHERN , _-UiLaruOd i I Tectonic Slice 2 | of Lambert e. a1, 1 981

i I Tectonic Slice 3 ,WB Wild Bight Group, Exploits Subzone

Figure E Comparative lava geochemistry: Southern Uplands andWild Bight Group, Exploits Subzone (WB).

Llandeilo clasts in the Caradoc Wrae Limestone olis-tostromal assemblage (Leggett 1980b) of the SouthernUplands have a remarkably similar (at the specific level)conodont fauna to the Cobbs Arm Limestone, at the top ofthe Summerford Group (Bergstrrim et al. 1974; Stouge1980). Furthermore, blocks of the Cobbs Arm Limestonealso commonly occur in olistostromal deposits of LateOrdovician and Llandovery age in the Exploits Subzoneflysch.

Despite the similarities in provenance, there are somenotable contrasts. The Southern Uplands greywackescontain detritus indicative of a continental cratonic source.Garnet, biotite and glaucophane schist fragments arepresent in the Ordovician greywackes (Floyd 1982) andElders (1987) has reported Rb/Sr isochrons from graniticclasts that indicate ages as old as Precambrian. In theExploits Subzone, however, both Helwig and Sarpi (1969)and Watson (1981) have remarked on the absence of anymetamorphic detritus of higher grade than greenschistfacies, and although no granite clasts have been dated, thereis little reason to suspect from their degree of deformationor associated clast types that they represent an oldcratonised source.

Sediment in the Southern Uplands with a southerlyprovenance is distinctive. It is dominated by intermediatevolcanic detritus (>25Vo), including fresh pyroxene,amphibole and lithic fragments, and has relatively lowquartz contents (about 20Vo,Floyd 1982; Evans et al. l99t).It has no obvious counterpart in the Late Ordovician andSilurian flysch of the Exploits Subzone.

The end of the Llandovery marked a substantialdivergence of Exploits Subzone and Southern Uplandssedimentary facies. The shallow marine and subaerialBotwood Group is latest Llandovery to Ludlow (H.Williams t972) ard contains substantial volcanic units. It isapparently conformable on Llandovery conglomerate andgreywacke of the Hamilton Sound sequence so, like mostsequences at the northwest margin of the Exploits Subzone,it indicates an extended period of clastic deposition in a

single basin. In the Southern Uplands, on the other hand,rocks of comparable age are turbidites and do not includeany volcanic units other than possible metabentonites.There are, however, certain characteristics of the sedimentsin each area that suggest a link. The fluviatile part of theBotwood Group contains thick units of red shale andsandstone, and its marine facies, which is relativelyhaematitic, has thin interbeds of red shale (Wessel 1975;Colman-Sadd 1989). Hawick Group turbidites of the sameage in the Southern Uplands are also characterised by redshale interbeds. In both areas, the sediments are relativelyrich in quartz and contain lithic fragments of chert and felsicvolcanic rocks with abundant haematite-stained clasticmuscovite. The latter is a common constituent of theBotwood Group (Wessel 1975), and is also a diagnosticfeature of much of the Hawick Group. It may be that theSilurian rocks of the Exploits Subzone and the SouthernUplands represent shelf and deep water facies, respectively,that derived their sediment from the same source.

Structural history. Both the Exploits Subzone and theSouthern Uplands have undergone polyphase deformation.A comparison of individual deformations from one area tothe other is not realistic, but it is possible to comparegeneral styles and timing. During the Late Ordovician andSilurian in both areas, there was an apparent progressionfrom a thrust-dominated regime to one dominated bystrike-slip movement, first sinistral, then dextral (".g.Barnes et al. 1989; Piasecki et al. 1990).

587

The start of deformation is not well constrained in eitherarea. In the Southern Uplands, it is thought to bediachronous, from latest Ordovician to Wenlock, in keepingwith the tectonic model of a southeast propagating thruststack. In the Exploits Subzone, there are few across-strikeoccurrences of Late Ordovician to Silurian sequences, so theopportunity for identifying diachronous thrust deformationis limited. However, the youngest rocks affected by firstdeformation structures are Wenlock, suggesting thatthrusting took place at about the same time as in theSouthern Uplands. Similarly, strike-slip faulting in both theExploits Subzone and the Southern Uplands appears fromstratigraphic and geochronological evidence to be Silurian,following thrusting, but for the most part completed by thetime that latest Silurian and Early Devonian granitoidintrusions were emplaced.

4. Discussion

A detailed comparison of the Lower Palaeozoic geologicaldevelopment of central Newfoundland and southernScotland reveals both fundamental similarities anddifferences. Given the independent evidence that centralNewfoundland and southern Scotland were in closeproximity during the early Palaeozoic, we feel that the closecorrespondence of the Arenig geology of the Notre DameSubzone and Midland Valley, and the middle to upperOrdovician geology of the Exploits Subzone and SouthernUplands, can be reasonably interpreted as reflectingtime-equivalent processes in conterminous tec-tonostratigraphic settings within the orogen. Contrasts in theearlier geological histories in these two areas are principallya function of difference in exposure. However, there areboth similarities and real contrasts in the younger geologicalhistories, particularly following the onset of collision of theoutboard terranes with the Laurentian margin. We feel thatthese contrasts are probably best explained as heteroge-neities in the geological environments introduced bycomplex tectonic processes at a collisional margin.

An evaluation of the significance of geological similaritiesbetween the two areas might eventually prove useful inattempting to reconstruct the geological history of each. Inthe first instance such information can be used to gain a

more complete view of the actual geology in each area,fllling gaps left by erosion or predicting the nature of rocksthat exist at depth and are not yet exposed. The argumentscan then be taken a step further, to the modelling ofgeological environments and tectonic processes. In the past,models have been proposed for each area individually,without regard to constraints imposed by the other. Theeffect of a general correlation is to increase substantially thefactual basis of models because they must be consistent witha much broader range of observations.

4.1. Principal similarities and contrastsThe following fundamental similarities in the geology ofcentral Newfoundland and southern Scotland indicate linkedgeological histories in the Early Palaeozoic:

1. The early Arenig island arc to back-arc basin transitionrecorded in the Betts Cove-Snooks Arm and Ballantraesequences, occurring respectively in the Notre DameSubzone and the Midland Valley. Both sequences containSSZ volcanic activity, including eruption of refractoryboninitic magmas, followed by epiclastic sedimentation andnon-arc volcanic activity, which is interpreted to represent awithin-plate/back-arc environment ; both are also interpreted


588 S. P. COLMAN.SADD, P. STONE,

to have been obducted northwestward on to the Laurentianmargin in the late Arenig.

2. The progression in both the Exploits Subzone and thenorthern belt of the Southern Uplands from pre-Llandeiloback-arc volcanism, through mid-Ordovician pelagic sedim-entation, to Caradoc and younger turbidite deposition.

3. A Late Ordovician and Silurian structural history inboth the Exploits Subzone and the Southern Uplands,comprising initial thrusting followed by strike-slip transpres-sion, first sinistral and then dextral.

Superimposed on these consistent features of the geologyare prominent contrasts that indicate along-strike variationwithin individual terranes:

1. The fragmentary cover sequence overlying volcanicrocks in the Notre Dame Subzone is coeval with an almostcomplete stratigraphic succession in the Ballantrae coversequence. This is not merely the result of the present daydepth of erosion; the unconformity of subaerial Silurianrocks on the Arenig arc and back-arc succession inNewfoundland indicates erosion, and perhaps non-deposition, during Late Ordovician or Silurian times.

2. The Silurian transition to shallow marine and subaerialsedimentation in the Exploits Subzone was contemporarywith continued deep marine turbidite deposition in theSouthern Uplands.

3. The Caradoc-Llandovery turbidite succession in theExploits Subzone differs in several significant ways from thatof the Southern Uplands. Continuous vertical successions inthe Exploits Subzone indicate a basin or basins that did notmigrate laterally, but were filled with sediment thatcoarsened upwards into proximal conglomeratic units. In theSouthern Uplands, each vertical succession records only a

fraction of the total time span of turbidite deposition,indicating the progressive southeastward migration of thedepocentre and little tendency for a transition into a

proximal facies. A different source pattern is also indicatedby the single provenance of the turbidites in the ExploitsSubzone, compared with the dual provenance in theSouthern Uplands.

4.2. Reconstruction of geology

The marine portion of the Llanvirn and younger coversequence to the Notre Dame Subzone has been largelydestroyed by erosion, and the few remaining fragments areinsufficient to construct a coherent geological history.However, by analogy with the Ballantrae cover sequence,we infer that the Snooks Arm Group was onceunconformably overlain by a Llanvirn and youngersedimentary succession devoid of volcanic rocks. This hasimportant implications for the geological history of thecalc-alkalic island arc sequences of the Buchans-RobertsArm volcanic belt in the eastern part of the Notre DameSubzone, which have no clear counterparts in the MidlandValley. The likelihood that a marine sedimentary coverdevoid of volcanic rocks was deposited on the BettsCove-Snooks Arm sequence suggests that the approxim-ately coeval calc-alkalic volcanism to the east does notrepresent the continuing development of the same Arenigenvironment, but instead occurred at some distance fromthe Betts Cove area. This being the case, the Buchans-Roberts Arm volcanic belt must have been accreted inLlanvirn or later times and the eastern margin of the NotreDame Subzone must be composite.

H. S. SWINDEN AND R. P. BARNES

Conversely the level of erosion is much deeper inNewfoundland than in Scotland and a more complete recordof Early Palaeozoic geology is exposed. It is thereforepossible to interpret the nature of the unexposed basementof southern Scotland by reference to Newfoundland. ThusSSZ and back-arc volcanic sequences are likely to formcomposite mosaics beneath both the Midland Valley and theSouthern Uplands and may be several kilometres thick. Inparticular, the well documented transition from arc toback-arc volcanism in the Exploits Subzone (Swinden e/ a/.

1990) provides a context for interpreting the scatteredoccurrences of volcanic rocks in the Southern Uplands, andapplication of this model supports the concept of theSouthern Uplands as a back-arc basin in the mid-Ordovician(Stone er al. 1987). The volcanic substrate of the SouthernUplands may be structurally underlain by a quartzitedominated terrane similar to Newfoundland's Gander Zone.Future interpretations of geophysical data, igneous geoche-

mistry and sedimentary provenance in southern Scotlandshould take into account the possibility of such a

stratigraphy, and should look to Newfoundland forgeological controls that are otherwise unobtainable.

4.3. Nature of Early Ordovician arc-continentcollision

Because of the deep level of erosion in Newfoundland, therelationship of the Notre Dame Subzone to the continentalmargin of Laurentia is well exposed and its history is

relatively well understood (e.g. Church & Stevens l97t;H.Williams 1975;H. Williams & Cawood 1989). Laurentia was

thrust under the ophiolites of the Notre Dame Subzoneduring the Taconian Orogeny, in what is most convincinglyseen as the entry of a passive continental margin into a

subduction zone. This relationship forms the foundation ofthe long standing model for an east-dipping subduction zonein the Newfoundland Appalachians (Strong et al. 1974;Colman-Sadd 1982; Stockmal et al. 1987, 1990).

A similar model of southeastward subduction has beeninvoked independently in Scotland to explain the develop-ment and emplacement of the Ballantrae Complex (Stone &Smellie 1990). Although the relationship to underlyingcontinental basement is not exposed, this process isconceptually the most reasonable way of obductingophiolitic rocks onto the ancient Laurentian margin. Thecorrelation with Newfoundland supports this view andprovides an analogue for the hidden geology of the MidlandValley.

Although it is becoming clear that the other DunnageZone marine volcanic sequences, of Cambrian as well as

Ordovician age, were also formed in SSZ or back-arc basinenvironments, the location and polarity of relatedsubduction zones is obscure. Continuing research intoDunnage Zone geology reveals an increasingly complexpicture that, in this respect, resembles the present-daygeology of the western Pacific. Such complexity precludesreconstruction of a long history of events, even inNewfoundland where exposure is good. In southernScotland, the volcanic substrate is likely to be just as

complex and the few exposures are of limited use formodelling plate interactions. One conclusion that does seemjustified is that oceanic crust from the major ocean basin ofIapetus is absent in the Dunnage Zone (Dunning &Chorlton 1985; Swinden et al. 1989; Dunning et al. l99l),and by analogy is probably also absent in southern Scotland.


4.4. Promontories, re-entrants and differentialemergenceThe time of final assembly of the terranes in the DunnageZone and southern Scotland is not well controlled. Howeverlinkage across Red Indian Line and the Southern UplandFault can be established indirectly by the apparentderivation of Caradoc and younger turbidites of theExploits-Southern Uplands sequences from a source similarto that now exposed in the Notre Dame Subzone and theBallantrae Complex. Thus the terrane boundary wasprobably established by the Late Ordovician, although it islikely that there has been considerable strike-slip movementalong it since then (Lafrance & Williams 1988; McKerrow &Elders 1989).

In developing a model that relates the post-Llanvirngeology of the Dunnage Zone and southern Scotland, it isimportant to note that the principal geological contrastsbetween the two areas date from this time. This implies thatalong-strike variation in this part of the orogen was afunction of the collision process that began with the EarlyOrdovician Taconian Orogeny and continued into theDevonian. Stockmal et al. (1987) have drawn attention tothe disruption of lateral continuity that must occur in anorogen if one of the colliding continents has an irregularmargin. The pattern of re-entrants and promontories, mostobvious in the Appalachian part of the orogen (Thomas1977), is thought to be inherited from the irregular riftedmargin of Laurentia, formed during the opening of Iapetus.The opposing continental margin was likely to have beenequally irregular. It is only at the time of collision that theoceanic terranes of the Dunnage Zone and southernScotland would have been influenced by these along-strikediscontinuities.

Newfoundland lies on the St Lawrence Promontory, thelargest promontory in the orogen, with a sharp break,presumed to be inherited from a major transform fault(Stockmal et al. 1987), separating it from the QuebecRe-entrant to the southwest (Fig. 1). Towards the northeast,the orogen trends gradually into the craton to form the nextre-entrant. Such promontory-re-entrant pairs occur thro-ughout the Appalachians and probably continue across thecontinental shelf into Scotland. Reconstruction of theAtlantic Ocean shows an offset between the Dunnage Zoneand southern Scotland and suggests that the latter waslocated in a re-entrant that was paired with a promontorynear the North Atlantic spreading axis. The contrastingsituations of Newfoundland and southern Scotland in theirrespective promontory-re-entrant pairs explain most oftheir post-Llanvirn geological contrasts.

When an irregular continental margin impinges on acurvilinear subduction zone, more continental crust issubducted at promontories than in re-entrants because thepromontories enter the subduction zone flrst (Molnar &Gray 1979). Emergence of the over-riding plate after initialcollision can be reasonably related to the buoyancy ofsubducted continental crust (Colman-Sadd 1982), so greateremergence is to be expected above promontories than abovere-entrants. Following the late Arenig arc-continentcollision at the Laurentian margin differential emergence inNewfoundland is indicated by the contrast in preservation(or deposition) of the post-Arenig cover sequences in theNotre Dame Subzone relative to the Midland Valley. It isalso indicated by the contrast in Silurian facies between theExploits Subzone and the Southern Uplands. Such a processhas previously been cited in the Appalachians to account forthe continuous Llanvirn to Silurian marine sedimentarysequence in the Quebec re-entrant, southwest of Newfound-

land, and the substantial absence of such a sequence on theNew York promontory (Doolan et al. t982; Malo 1988;Tremblay & St Julien 1990).

4.5. Residual basins and transpressionDuring continental collision, residual marine basins atpromontories are likely to be narrow, with staticdepocentres such as those in the Exploits Subzone,compared to much broader basins in re-entrants, whereconvergence continues and is reflected in longer-livedback-arc volcanism and migrating thrust stacks and turbiditefans, as in the Southern Uplands. [n effect promontories actas pivots for continuing closure in re-entrants.

A consequence of this model is that, even if convergencealong the orogen as a whole is orthogonal, convergence inany one area is oblique. In the Appalachian-Caledoniancontext, the angle of obliquity is relatively slight northeastof promontories and opens to the northeast, and the reverseis true for the sharp breaks southwest of promontories. Thesense of obliquity is such that forces along the collision zoneresolve themselves into sinistral movement northeast ofpromontories and dextral movement to the southwest. Sucha pattern agrees with the dominantly sinistral structuresobserved iq Newfoundland and Scotland (Anderson 1987;

Blewett & Pickering 1988; Barnes et al. t989;Piasecki et al.1990) and the restricted area of dextral structures thatoccurs in New Brunswick and Quebec, just southwest of theSt Lawrence Promontory (Stringer 1975; Malo & B6land1989). Thus, the Appalachian-Caledonian situation is a

mirror image of that proposed for the Alpine-Himalayanbelt by Tapponnier and Molner (1976), where promontoriesat the northwestern extremities of the Indian and Arabiancontinental blocks have created transpressive regimes ontheir flanks during collision with Asia (cf. Tapornier et al.1986).

4.6. Convergence and delaminationEven after initial collision, the underlying mechanism forclosure, on a lithospheric scale, must still be subduction ofone of the converging plates into the mantle. On a crustalscale, however, it is likely to be much more complicatedbecause of the need to accommodate the heterogeneities ofcrustal geology. Colman-Sadd (1982) and Stockmal et al.(1987, 1990) have suggested various possibilities fordetachment of the crust from underlying lithosphere, so thata simple pattern of subduction in the mantle is translatedinto complex thrust and strike-slip movements in crust thatis not subducted. Movements in the crust are merely a

response to the space problem created by continued mantlesubduction and the structural geometry produced is notnecessarily the same at the two levels.

There has been much discussion as to whether theSouthern Uplands is an accretionary wedge formed directlyby subduction (e.g. Leggett et al. t979) or whether it is athin-skinned thrust stack initiated independently in theremains of a back-arc basin (e.g. Stone e/ a/. 1987). Giventhe potential for crustal delamination, the proposal forcontinued convergence in re-entrants does not imply thatthe Southern Uplands were necessarily formed as anaccretionary prism at a subduction zone. Indeed theprovenance evidence presented by Stone et al. (1987) ar,dthe back-arc character of the youngest rocks of the volcanicsubstrate indicate a back-arc rather than fore-arc setting, a

concept reinforced by the lack of a correspondingaccretionary wedge in Newfoundland. It is most reasonableto assume that only crustal rocks were involved and thatlithospheric convergence below the level of detachment wasachieved by subduction of uncertain polarity.

590 S. P. COLMAN-SADD, P. STONE, H. S. SWINDEN AND R. P. BARNES

5. Conclusions

Detailed comparison of the geological development ofcentral Newfoundland and southern Scotland leads us toconclude that there was considerable correspondence intheir respective geological histories before collision of theoceanic terranes with the Laurentian margin, but con-siderable divergence during the progress of the Taconianand later orogenic events. The ophiolites of the Notre DameSubzone and the Midland Valley are coeval and strikinglysimilar geologically and geochemically, and we feel it isreasonable to suggest that they have a common origin,possibly in the same back.arc basin. Likewise, we feel thatEarly to Middle Ordovician volcanism and sedimentation inthe Exploits Subzone and the northern Southern Uplands issuffrciently similar that a common origin, probably also in aback-arc basin, can be supported.

Contrasts in the geological history of the two areas afterthe Early Ordovician can be explained by the position ofNewfoundland on a promontory and southern Scotland in are-entrant of the irregular Laurentian continental margin.The relatively early emergence and differential erosion ofOrdovician and Silurian rocks in central Newfoundland isinterpreted to reflect buoyancy of the subducted lithosphereon the promontory, while marine sedimentation continuedin the re-entrant until final closure much later in theSilurian.

Tectonostratigraphic comparison implies a correlation ofmajor tectonic boundaries. Red Indian Line in Newfound-land, separating the Notre Dame and Exploits subzones, isanalogous to and probably correlates with the SouthernUpland Fault, which separates the Midland Valley andSouthern Uplands in Scotland. The Solway Line, whichmarks the boundary of the Southern Uplands with theAcado-Baltic continent, corresponds to the Dover Fault,separating the Gander and Avalon Zones in NeMoundland.

6. Acknowledgements

Thanks to Peter Cawood, Paul Dean, Doug Fettes, JimFloyd, George Jenner, Baxter Kean, Byron Lintern andBrian O'Brien for stimulating discussions and guidance inthe field; to Felicity O'Brien, Adrian Rushton and HenryWilliams for advice on palaeontological matters. The manycolleagues in the British Geological Survey and theNewfoundland Department of Mines and Energy who havesubstantially contributed to many of the ideas expressedherein are too numerous to list but are neverthelesssincerely thanked. All diagrams were prepared in theDrawing Office of the British Geological Survey, MurchisonHouse, Edinburgh. The paper is published with permissionof the Executive Director, Geological Survey Branch,Newfoundland Department of Mines and Energy and theDirector, British Geological Survey (NERC).

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S. P. COLMAN-SADD and H. S. SWINDEN, Geological Survey Branch, Newfoundland Department ofMines and Energy, P.O. Box 8700, St John's, Newfoundland A1B 4J6, Canada.P. STONE and R. P. BARNES, British Geological Survey, Murchison House, West Mains Road,Edinburgh EH93LA, U.K.

MS received 6 June 1991. Accepted for publication 19 May 7992.

Parallel geological development in the Dunnage Zone of Newfoundland and the Lower Palaeozoic...

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