Ordovician stratigraphy in the western Helgeland

TOM HELDAL ^{NGU- BULL438, 2001- PAGE47}

Ordovician stratigraphy in the western Helgeland

Nappe Complex in the Brenneysund area, North-central Norway

TOMHELDAL

Heldal,T.2001:Ordovicianstrat igraphy in the weste rnHelgelandNappe Complexin the Bronnoysun darea,Nort h- cent ralNorway.Norgesgeologiskeundersekelse Bulletin 438,47-61.

In thewestern part of the Helgeland Nappe Complex, asmall ophiolitefragment (t he BolveerComplex)anda metasedimentarysuccession(the Bronnoysund Group)are exposedon severalislands to thewest of Bronnoysund . The BolvzerComp lexwas emplaced,upliftedand subjectto subaerialweat hering anderosion priorto the deposi- tio nof theBron noysundGrou p. The unconformity defi nesan irregulartopogr aphic surfacewhichwas gradually filled withsedimentsduringaregionaltransgression,prob ably influencedby act ive tect onics.The sedime ntat ionof the Bron noysund Groupinitially st artedwit h thebuild-up of a coastalsetting,deliveringcoarseand finec1asticsinto a shallow-marineenvironment (Risoy Formation).This formationis st rat igraphicallyoverlain by calcareous tur- bidit es (TorgnesFormat ion),pelites(AspoyFormation)and carbon aterocks(ToftsundFormati on).The Bronnoysu nd Grou p is correlatedwithseveralcoverseq uencesabove ophiol ites in the Helgeland NappeComplex,and deposition inEarly to MiddleOrdovician time is suggested.The c1astpopulatio nsin conglomeratesand the compositionof finerclasticsedime ntsshowanexotic,ensialicsource forthe detritus,inadditio nto theophiolite.Thisensialicsource consisted essent ially of psammit es and schists,defor med and metamorphosed prior tothe deposition of the BronnoysundGroup.

TomHeldal,Norgesgeologiskeundersekelse,7491Trondheim, Norway.

Introduction

During the 1980s and early 1990s,several investigations focused onthe complextect onostrat igraphy in the western partoftheHelgeland NappeComplex(HNC).Import ant dis- coverieswere made,such asthe recognition of a number of ophiolite fragments and a primary, angular unconformity between the ophiolites and metasedimentary cover sequences.The ophiolite fragments were correlated with other Late Cambrian-Early Ordovician ophiolites in the Caledonides,including the LekaOphioli teComplex(Fig.1).

Severalofthese ophio lites haveyieldedages of 500-485 Ma (Dunning &Pedersen 1988).Other metasedimentary rocks in the area include migmatite (mica) gneiss,cale-silicate gneissand marble interpreted to represent remnantsof a Neoproterozoiccontinental margin sequence.

Both sedimentary successions have been affected by pervasive deformation and medium-grademetamorphism, and primary featuresare, inmostareas, totallyobliterated.

However, on somesmallislands to the west of Bronneysund, the contact relations between an ophiolite fragment and the coversequence,aswell as sedimentary st ruct ures in the latt er, are excellentlypreserved.Unlikeat mostother places wit hin the western part of the HNC,a detailedstudyof the stratigraphyof the cover seq uence and even an interpreta- tion of the palaeoenvironment during itsdeposition was possible(HeldaI1987).lnthe author's opinion,this particular area may beofsignificance forabett erunderstand ingofthe

struct ureand strat igraphyof thepileofnappes andthrust sheetsforming the west ern part of the HNC, andact as a typearea forcorrelations wit hinand betweenthem.Inaddi- tion,the area has one of the bestpreserved andexposed unconformitie sin the NorwegianCaledonides.

Regional geology

The Helgeland NappeCompl ex,whichformsthedominan t part of the Uppermost Allochthon in the Cent ral Scandinavian Caledonides(Gee et al. 1985,Stephens &Gee 1985,Stephensetal. 1985), comprisesa seriesof nappe s and thrust sheets,predominantly composed of medium-gr ade metamorphosedsedimentaryrocks.These areintr udedby plutonicrocks,rangi ngin composition from olivinegabbro to leucog ranite, the Bindal Bath olith (Nordgulen &

Schouenborg 1990,Barnes et al.1992,Nordgul en &Sundvoll 1992,Nordgulenet al. 1993,Birkeland etal. 1993).Mafic and ultramafic rocks of ophiolitic affinity occur at several tectonostratig raphic levels,predomina ntl yinweste rnareas (Fig.1).In theeast,the HNC restswithathrustcontactupon theRodinqsfjalletNappe in the northeast,and theGjersvik Nappein the southeast. In the southwest,theHNC issepa- rated from basement gneisses and inferred Seve Nappe supracrustal rocks by the Devonian KolIstraum en DetachmentZone(Braathenetal. 2000).

Inthe western,coastalpart of theHNC(Fig.1),it isfairly wellest ablished byfield relat ionships(Gustavson 1975,Riis

NG U-BULL 43 8,2001- PAGE48 TOM HELDAL

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l+_+_+_T_+-+

rvVvV~V~VVVv

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Strontiumisotopes consisten twith

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or ~Ordov ician

seawater(Trennes 1994)

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_ ^Gran ~ iII!t1 ^{itic gneiss} (E Ordov ician? ) ~

Granitoids, undifferentiated (L Ordovicia n) Gab bro, diorite , monzodiorite (L Ordovician . Metasedimentary rocks (E-M Ordovician) Ophiolite fragments (E Ordovician) Metasup racrustal rocks (Neoproterozoic) Gje rsvik Nappe (Ktil i)

Proterozoic basemen t gneiss Thru st / inferred thrust

Fault

Way up (sedimentary structures)

Fig.1.Regionalgeologicalmap of thewest ern partof the Helg eland Nappe Com plex;modifiedfrom Nordgulenet al.(1993).

TOM HELDAL ^{NGU-BU L L438,2001- PAGE 49}

_-.,I"

./ AxialtraceofF2antifo rrns

~^{AxialtraceofF Isyncline}

Fig.2.Thegeolog y of the Bronnoysund - Bolveer-Torghatt en area, showingthedistri- butionofthe BolveerComplex,thefour for- mationswit hintheBronnoysundGroupand the Torghatten Granite. In addition, fold interferencepattern sbetweenF2 and F3in the vicinity of theTorghatten Granite are shown in the inset (right). From Heldal &

Hj elmeland1987.

Ikm

>---<

of syn-D2 granite s (granite gneisses, see Fig.1) in the area (Hje lme land 1987), earlier interpreted as base- ment window s,would pushthe min i- mum ageeven further back.Thus,the obduction of the ophiolitic rocks, uplift, erosion, depo sition of the cover sequenc e and the maj or episodesof deformation allseem to haveoccurred wit hi n the Ordo vician period.

The ophiolit e fragments wit hin the western HNC occur at two tectonostratigrap hic levels,defi ni ng an easternbelt from TerraktoVelfj ord (Nordgulen&Husmo 1988,Thorsnes

& t.eseth 1991) and a weste rn belt from Leka to Red oy (Prest vik 1980, Bang 1985), includ ing the Bolv zer Complex describ ed in the present paper.In addition,relicsof a possibl e middle belt can be seen in the Bindalsfjord area(Fig. 1).All majo rcont act s are dipping eastward,and sedimentary st ruct ures defin ing way- upin the cover sequence are facing the same directi on .The unconformity betweenthe ophiolitic rocks and thecover off metasedimentary rocksisexposedatanumber of localit ies in this area,and in several placesit can be shown that the cover sequence on laps onto a highly irregula rpalaeotopog- raphy formed by subaerialerosion and weat heringof the ophioliticrocks.

Stratigraphy and tectonometamorphic development in the Brenneysund area

The western,coastal belt of ophiolitic rocks and itscover sequence can be observed west of Brenneysund.The rocks can be dividedinto three main units (Fig.2).ln the weste rn- mostareas.aroundtheislandof Bolveer,thereisasmall ophi- olitecomplexcomposed of peridotites,layeredmetagabbro and amphibolites,named theBolveerCamp/ex(Helda l 1987).

Lying unconformably upon the ophiolit e isasuccession of metasedimentaryrocks,occupying most of the studied area - the Brenneysund Group.The third unit isthe Torghatten Granite,forming a dome centred at theTorghatten moun- tain, enveloped by the upper part of the Brenn eysund Group. Hjelmeland (1987) suggeste d that the granite intruded the metasedimentary rocks simultaneously with the D2 deformation episo de.

Plutonicrocks

S888TorghanenGranite

HmflI

o

~TorgnesFormation

~RisoyFormation Botvrc r Com p lex

_ Serpentinite.gabbro,amphibol itc

.« Strike/dipof foliationintheTorghattenGranite / Strike/dip of 52 foliationinrnetascdiments

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& Ramberg 1981, Bang 1985, Heldal 1987, Husmo &

Nordgulen 1988,Thorsnes&Leseth 1991),radiometric dat- ing (Nissen 1986) and dating by C and Sr isotopic data from marbles (Trennes 1994,Tronnes & Sundvoll 1995) that the metasedimentary rocks belong to at least two differentsue- cessions.An inferred Neoproterozoic succession comprises migmatites/micaceous gneiss, cale-silicate gneiss and mar- ble.Basedon correlations with the Leka Ophiolite (Prestvik &

Roaldseth 1978,Sturt et . al 1984,Furnes et al. 1985),which has a minimumage of 497 +/- 2 Ma (Dunning & Pedersen 1988),a Late Cambrian to Early Ordovician age for the ophi- olite fragments has been suggested, separating the Neoproterozoicsuccessionfromyoungermetasedimentary rocks (metasandst ones, conglomerates, schists and mar- bles), such as the Bronneysund Group (described in the pre- sent paper).The latter rocks were deposited unconformably on top of the ophiolites,and can be correlated with the sequencelyil1g above the Leka Ophiolite - the Skei Group (St urt & Ramsay 1994). Rocks similar to the inferred Neoproterozoic-aqe metasedimentary succession,in addi- tion to the ophiolitic rocks,formed the source for the clastic sediments in the cover sequence.A minimum age for the younger sequence is given by the444+/-11 Ma dating of the Heilhornetpluton (Nordgulen&Schouenborg 1990)and 447 Ma dykes in the Andalshatten Pluton (Nordgulen et al.

1993) in theBindal Batholith,both postdating D2st ruct ures inthe metasedimentary succession.The possible existence

NGU-BULL 438,2001 - PAGE50

The Bronnoysund Group isdivide d intofourformations.

The Risoy Formation makes up the lower part of the group, comprising metasandston es, conglomerat es and schists onlapp ing theunconform it y(Fig. 3).This is overlain by cal- careous met asandstonesof theTorgnesForm ation ,whichis followedby mica schists (t heAspoy Formation)and marbles wit hschistsand cale-silicate rocks(t he Tofts undForm ation).

The latter continues eastw ard to the eastern part of Brenn eysund,and isst ruct urally overl ainbymica schi stsand psam m ites.It isuncertain whet her these schists lie st rati- graph ically above theToftsund Formation or if they repre- sent a structural repetit ion of the lower formations.

Furt herm ore,the easte rnlimi tof the Bronnoysu nd Group is not know n.However, marb les and mica gnei ssesin the Velfj ord area somew hat fart her to the east belong to the Neop rot erozoic metased im ent ary succession menti oned above.

The Brenneysund Group has experienced polyp hasal defor matio n,and stru ct ures relati ng to four deformati on episode s havebeen recognised.Upp er am phiboli te-facies metamorphism was attainedduring thesecond (0 2) event , which,as menti oned above, probab ly took place prio r to emplacemen t of the Heilh ornet Granit e and dykes in the Andalshatten Pluton.Two later episodes (0 3 and 04) of defo rmat ion most likely represent Scandian (or even younger?) deform ation related to the final amalgamati on and thrusting of the HelgelandNappeCompl ex.

The Bolveer Complex

The Bolvee r Complex comprises serpe ntrrute, layered gab bro, c1inop yroxenit e wit hserpenti nitelayers and amphi- bolite (Fig 3).The complex shows evidence of polyp hasal defo rmat ion pre-datin g the unconformi ty that was proba- bly related to an early oceanic crust stage and to a later stage whenthe ophioli tewas em placedonto a continental marg in (Heldal 1987). The serpenti nite contain s relics of olivine and orthopyroxen e,reflectinga prim aryharzburgitic com posit ion of the perid ot it e.Indicat ions of non-orogeni c, ocean-crust deformat ion are seenin several generations of pyroxenite veinsthatwere progressivelydeformedsimulta- neouslywit htheinje ct ion of new veins.Toget her wit h the lith ological compositi on of the BolvzerComplex,this pro- vides evid enc e for a correlat ion wit h the dated Leka Oph iolit eComplexfarth er sout h.The layeredgab bro occu- pies most of the outcrop area of the Bolveer Com p lex.

Evidentl y, large-scale, monoclinal foldstruct uresanda min- erallineatio n pre-datethe unconfor m it y.

Asmall body of amphiboliteoccursto the south of the serpent in ite. The contact is subparal lel to theearlyharzbu r- gitefoliatio n and to apenetrative,pre-un conform it y folia- tionin the amphi boli te.Since the amphibolitedoes not fit clearly into an ophiolite pseudo st rat igrap hy,and shows stro ng pre-unconformitydeformation,it issug gested thatit may represent a metamorphi c aureole to the obduc ting ophiolit e, with the cont act totheserp ent in it e representing

TOM HELDAL

..

- '

.a;xm:r,,-!'..Y/ FaultIshear-zone

fStroke/dIpof beddIng .' Stroke/dIp of S2 fol"'t"'"

Fig.3.Geolog ical mapofthe Bol veerarea.

the act ualplane of obduction.Unfortunately,no relictmeta- morphic parageneses pre-dating the amphibolit e-facies metamorphismin the cover sequence have yet beenfound, leavingthequest ion still open.

The unconformity - palaeoweathering, morphology and basal breccias

The unconformity between the ophiolitic rocks and the overlying metasedimentary succession isexposed at a num- ber oflocalit ies in theHNC(Bang 1985,Husmo&Nordgulen 1988, Heldal 1987,Thorsnes & Loseth 199 1),and in several places it has been shownthat the coversequence onlaps an irregu larpalaeotopog raphy formed by erosion and weat h- ering of the ophiolitic rocks. A strikingfeature of the Bolveer area,however,isthe excellent state of preservation of the uncon formity, leaving it virtually undefo rmed over a large area,even though it is situated in the core of a F1 syncline, modified by a series of F2 step-foldswit h NE-SWtrend ing axial planes (Fig.2).The unconformity is expos ed on the islands and islets around Bolveer overa 1000 metre section (Fig.3).Thebest place toview the unconformity andregoli th is at Esjeoyene(Fig.4).

At the timeofdepositi on of the Bronnoysund Group,the unconformity probably climbed towa rd s south-south east, with a vertical change in altitude of at least 100 metr es

Fig.4.Geological map ofEsj eoyene.

TO M HELDAL

within the distan ce of the outcrop area.Thi s isshown by the onlapp ing deposit ion of thedifferentmembers ofthe Risoy Format ion onto theunderlying BolveerComplex.Theirregu- lartop ography isalsoseenat out cropscale(Fig. 5),where the ang ulardiffe rencebetweenthe sedi menta ry layeringin theRiseyForm ation and theunconformity surface canvary from 0 to 60 degrees.

Basalbrecciasarelocallydevelopedalong theunconfor- mity,reflect ingthe characterofthe rocksbeneath thecon- tact.Thick deposits of

short-transported breccios

OnHatey and Risey,the breccias have an asymm etri c lent ic- ular shape wit h their southern flanks steeper than their northern, and are fining upwards and northwards, away from steep slopes in the palaeote rrain.In north ern Hatey and on Bolveer, the mostproximaland coarsebrecciasare very poorly sorted and c1ast-supported, and these were probably deposited as talus beneath a scarp (Fig. 6a).

Ot herw ise, the breccias occur as matrix-suppo rted beds ranging from 0.5to 6 metresin thickness,wit hapositive cor-

LEGEND BR0 NN0 YS UNDGROUP

TorgnesFonn ation

i'i'iCalcareous sandstone RisoyFonnation

IiIIISericit eschist

;·!mf~Polymict conglomerate

,~:j.~Basal breccias

BOLWERCOMPL EX

BIBRegolith _ Layeredgabbro

NGU-BUL L438,2001- PAGE51

_,,_Fault andshearzone

! Strike/dipof bedding

o

Fig.5.Theunconformity.A) Irregularerosionalsurface(white dotte dline) onLilleEsjeoy.Gabbro (lower part)in contactwithconglomeratesof the Risoy Fo rmat ion(upper part).B)Erosional surfaceinregolit h(rig ht)onEsjeoy, filledbytheupper partof the RisoyFor mation.ClDetail ofareain B) (white square).Note large. roundedbouldersof gabbroalongthe contact.

NGU-B UL L 438,2001 - PAG E52

Fig.6.Basalbreccias immediatelyabovetheunconfo rmity.A)Bou lders of amphibolitein basalbreccia atBolveer,showingapre-unconf ormity folded and transposedfoliation.B)Viscou smass-movementdepositon Lille Esjeoy;largebould ersof gabbro(left part)havecracked during transport.

relation betweenmaximumboulder size and bed thickness.

Commonl y, the largest bould ers are concentrated in the cent ral part s of the beds (Fig.6b).Besid esthe bed thick- ness/bould er size correlation, bould ers fractured durin g transport,sheared bases and lack of reworkin g by waves indicates that these brecciasrepresent highly viscous,sub- aerial mass-movements(Nemec &Steel 1984).On Hatey,the scarp thatdefin esthe southernlimitof the basal brecciasis seencont inuingasa faultbelowtheuncon formity, suggest- ing that syn-sedimenta ry fault s playedanimp ortant rolein the shaping of the palaeotopography.

Insituweathering breccia,or regolith,is present in the southern part ofthe area, e.g.in the higher levelsof the palaeot op ogr aphy.On Esj eey, such brecciation of the lay- ered metagabbro exte ndsdown to10metresbeneath the unconformity(Fig. 4 and 5),and is characterisedby rounded gabbro fragments surrounded by anastomosing chlorite veins.The only feature show ingthe in situcharacter ofthe brecciati on istheconsistent modal layering of the gabbro.

Accomp anyin gthe brecciationisanenrichmentof iron-car- bonate intheuppe r partof the fossilweatheri ngprofi le,as irregular patches,crack fillings and individualgrains replac- ing mafic minerals in the gabbro.Such carbonate enrich- mentin aweat hering profileis characteristi cofweathering

TOM HELDAL

in hot and arid- to semi-arid climat es;the higher up the calichedevelop s,the more arid is the climate (Blatt et al.

1980).In addit ion to the carbon ate enrichme nt, the fossil weat hering profileis characterisedby anupwardincreasing contentof biotit e,chlorit e and even kyanite,a decreasing contentof amphibolesand plagioclase and abreakdown of the ophitic texture of the gabbro.Thechangein mineralog y refl ect s the chemical alteration of the deeply weathere d gabbro,such as growt hofkyanit e during lateramphibo lit e- facies metamorphism in theAI-enriched,upper part of the regolith.Furthermo re,biot it e woul d be expected togrowat the expenseof clayminerals,and leachingof Cafrom plagio- c1ase contributedto the carbonateenrichment.

The occurrence of a deep, fossil weathering profile mainlyrestricted to thehigher levelsofthe palaeotopogra- phy suggeststhat these partswere exposed to subaerial weatheringfor asig nificantl y longer periodof timethanthe lowermost levels.Thisis supported by thecommon occur- rence of clastic grains from the weathering profil e in the lowe rmembers of theRis0YFormation.

Theweat hering profileintheBolveer area showsstriking similaritiesto thosedescribed fromother EarlyOrdovician ophioli tesinthe Scandi navianCaledo nides.Boththe high palaeoreliefanda carbonate-enriched regolit h are fou nd in the ophiolites at Otta (B0e et al. 1993),on Leka (St urt &

Ramsay 1994)and on R0d0Y(Bang 1985).An examp leof a younger unconformit y wit hin the Norwegian Caledonides, where field relations are quite similar to those described here,has beenreporte dby B0e(1989).

Lithofacies of the Brenneysund Group

The Ris0Y Formation

The Risey Formation comprises a variety of sandston es, schistsand conglomerates,fillingand onlappingtheirregu- lar topography formed by erosion of the BolveerComplex.

The thickness varies from5 to atleast100metres, reflecting the irregular surfacebeneath.Althoug hthe metasedimen- tary rockshaveundergone middl e amphibolit e-facies meta- morphi sm,with stauroli te,kyanite and sillimanite as index minerals,sedimentary structures are remarkably well pre- served in part s of the studied area.The reason for thisis thought to be decreasing st rain in the cover sequence towards theunconformitydue to ashadowing effect bythe Bolveer Complexunderthe pervasiveD2 deformation,allow - ing int erpretation of the depositional environment wit hin partsof theRis0YFormation.Furthermo re,the F1syncl ine describedabove providesan addit ionalcontribution to the preservation of primary struct ures.The outcrop pattern of the Ris0YFormation is,however,complex,dueto bot hthe rapid lateral changes in lithology and thickness and to a complexinterferencepattern betweenF1andF2 folds.

The Ris0Y For mation shows a south-southeastward onlap to theBolveerComplex,andis subdividedinto4 mem- bers: greenmetasandstone,chlorite schist,sericiteschistand

TOM HELDAL

Fig.7.The green metasandstone (Risoy Formation)on Hatskjeer,A) Cross-beddingandahigh -angle scour.B) Conglomerate layers,single- pebbl elayers andscatte red pebbles (peb bles are allof layered gabbro).

polymict conglomerate.The green metasandston es,occur- ring predominantly in the northern part of the area, are entirely derived from the ophiolitic basement.The chlorite schistsand theconglomeratesoccurin the centralpart of the formation,and reflecta mixed source of ophiolitic and quartzofeldspathicdetritus,while the sericite schist member (w hich occurs in the upper part of theformation )is essen- tiallyderivedfrom aquartzofeld spath ic source.

The typearea for thelower partof the Risey Formationis at thenorthern tip of Hatey,whiletheupper part is excel- lently exposed on Esjeoyand Lille Esjeoy (Fig.4).

Green metasandstone

The lowest member of the Risey Formation occurs predomi- nantly in the lowermostpart of the paleotopog raphy,e.g.in nort hern Flate y and thesmallislets to the north.The thick- ness variesfrom 0 to 20 metres,thinningtowards the scarp in northern Hatey described above.In the southern part it overlies the basal breccias, while farther north it rests directlyon the weat hered gabbro.Thecontacttowardsthe breccias is characterised by a narrow transition zone where the brecciasare reworked and filledwith sand.Upwards,the green sandst one gradua llychange sin characterand colour, into the greyishgreen,planar bedded silt stone and sand- stone of the overlying member.

The mineralogy of the green sandsto nemirrors that of

NGU-BULL 438, 2001 - PA GE 53

thegabbro beneath,wit hact inolit icamphibole and plagio- c1ase,and alteration products such as c1inozoisite andepi- dote,as themaj or constituents.Furthermore,pebbl esofthe gabb roarecommon in conglomeraticbeds.

The green metasandstonedisplays avariet yofsedimen- taryst ruct ures.In the lower part of themember,low-angle cross-strata and troug h cross-strata arethemostcommon sedimentar y st ruct ures, while hor izont al lamination and symmetrical ripplesdominat ein the upper part.Low-angle, undulatory lamination,remi niscent of hummockystrata,are found in the middle part.Conglo merates and layers with scatteredpebblesarecommon .In thesouthernpart of the member (nort hern Hatey ),channel-shaped,upward-fining grits are followed by afew metresof thick,poorly sorted cong lomeraticchanneldeposit s,mat rix- to c1ast-supported.

Farther west (Flatskjeer), conglomerates occur as several matrix-supported beds «60 cm)with the largest pebbles concentratedinthe middl eof the beds.Bet weenthese con- glomerates,singl e,continuou s,pebb lelayers and isolated pebbles in the metasandstoneoccurquitecommonly,indi- cating strong reworking of the conglomerates.

Chlorite schist

The upwardfining of the greensandstones marksthetransi- tion to the overlying member.Generally,thisconsistsofpla- narand thin-bedded, fine-grainedsandstone,siltsto neand schist,characterised by a generallyhigh contentof Mg-chlo- rite.In the central part of thearea,the member restsdirectly upon the Bolveer Complex or basal breccias, while it is absent in the sout hern part ,e.g.on thepalaeotopographic highs.Its occurrencein the core of the Flatey syncline,com- bined with F2 refolding and lateral variation hinders an exactcalculation of thethickness,but the memb er reachesa minimumthicknessofat least60 metres.Upwards, it grades into pelitesand semipelit es of the sericite schist member.

Severalconglomerate lenses occurin the chlorite schi st,and these are treated separatelybelow.

Sedimentarystructures arewell preserved in thelower part of themember,but upwardsandaway from theuncon- formity the bedding is increasingly transposedby thepene- trative52 foliation.The lowe rpartof themember mineralo g- ically reflects the rock compositio nsof the BolveerComplex, with plagioclase and amphiboleas commonclast ic grains, and Mg-chloriteand biotite as met amorphic minerals.Asthe clay layers are totally recrystall isedto coarse-grained biotit e and chlorite, a metamorphic inversionof thegrain-sizehas occurred,demanding carein the interpret ati on of the sedi- mentary st ruct ures.However,thegeneralappearanceof the member is that of awell-bedded deposit, wit h sandstone and siltstone beds of 5 to 50 cm in thickness (Fig.8a). The beds are generally massive,normally graded in the lower parts and planar or convolutedin the upper part wit h int er- calations of clay material. Thus,theA,B, C and E subdivisions of classical Bouma sequences are recognised.Load casts (Fig.8b)andf1ame-,ball- and-pillowandot herescape-stru c-

NGU-BUL L438 , 2001- PA G E 54

Fig. 8.Chlorit eschist member(Risoy Format ion)on Flato y.A)Typical appearanceof awell-preserved partof thechloriteschist memb er.Way up towardsthe left of the photograph.B)Load castsand planarlami- natedbeds.The small roundspots are garnetporp hyrobla sts.ClPlanar laminatedsilt stone andsandstoneanderoded escapest ruct ures.

tures indicating rapiddepositi onandburialare common.In several places, erosional bases are seen, cutting throug h escape-st ruct ures(Fig.8c).

Inthe upper part,thecontent of quartz,garnet,staurolite andsericite increases,reflecting anincreasinginputof detr i- tusfrom an ensialicsource.Sediment arystruct ures are com- pletely or partlyobscured by52, but generallya vague pla- narlayeringwith int erchanging sandy and clayey layers is seen.Isolatedgabbroicclasts are sporadi callypresent,and in north east Flatoy achaot ic congl omerate lens,surrou nded by slump folded siltstone,ispresent.Several smaller con- glomerate lensesassociated with slumping in the surround-

TOM HELDAL

ingschist also occur, indicating down-slopesliding of poorly consolidated conglomerate bodies.

Sericite schist

This member is a semipelitic schistlocally wit h sandy facies, especially close to the unconformityand conglomerates.In the southern part of the area it rest s directly upon the unconformity,while incent raland northernareasit showsa transit ion to the underlying chlorite schistmember.Thecon- tact is defined where sericite replaces chlorite as the main flaky mineral. The member is about 10 m thick on Lille Esj eoy.Towards the westand southwest its thicknessproba- bly increases considerably,but a lack of continuous expo - sures makes this difficult to assess.Sedimentarystructures are only common close to the unconformity and/orcon- glomerates; elsewhere, SO is completely transposed by 51/52.

In the more pelit ic lithologies of the member, fine- grained quartz and sericite compose the matrix of the schists,whilegarnet,staurolite,plagioclaseandkyanite are common porphyroblasts.In some samples,sillimanite isin equilibrium wit h kyanite,reflecting the syn-D2 metamor- phicpeak in middle to upper amphibolitefacies.Where sed- imentary structures are preserved,the Iithofacies in the lower part resemblesthat of the underlying member, with ryt hm ic,planarbedded sandstone interbeddedwit h shale as thedominant type.Farther up,towardsthecontact with theTorgnesFormat ion,the member turns more pelit ic in character.Closeto the conglomerates,cross-bedded sand- stone wit h grit layers are present,grading into foliated schistsawayfrom the contact.

Conglomerates within the Risey Formation Thecong lomerateswit hinthe Risoy Formation arefound in the upper part of the chlorite schist member and in the sericite schist member.Locally,they rest direct ly on the BolveerComplex,as on Esjeey,LilleEsj eey and the west ern part of Ris0Y.They are generallypolymictwit h a mixedclast population reflectingboth the BolveerComplexand a quart- zofeld spathic source.Apure ultramaficconglomerateoccurs on Risey.Inthe polymictconglomerates,the matrix is essen- tially composed of quartzofeldspathic sandstone with biot it e commonly defining the 52 foliation.Clasts are gener- ally deformed and rotated into the same foliation,making inte rpretation ofsedimentaryc1ast morphology and imbri- cat ion difficult.However,in spiteof the D2 deformation,sed- imentarystructures are stillquite well preserved,makingan interpretation of the depositional faciespossib le(Table 1, Fig.9).

As shown in Table 1,four facies of conglomera tes have been distinguished- interpreted as braided stream,subaer- ial mass-movement, subaqueous mass-move ment, and beach gravels and conglomerates .The distribution of the different conglomerate facies varies within the area.On Ris0Y,braided stream conglome ratesaremost common.On

TOM HELDAL ^{NGU-BULL43 8 ,}2001 - PAGE 55

Fig. 9.ConglomeratesintheRis0YFo rm at io n. A)Type 1(w ay uptowardstheright); B)Type 3;C)Type 4 (Table1).Notethat c1asts arerotate dintothe52 foliation (N-5 in thephotogr aph ).

Table 1.Cong lo mera te facieswit hinthe Ris0Y Formation.

FACIES Typ e1 con gl om erat es

Typ e 2 conglomerat es

Type3 conglo me rates

Typ e4 conglome rates

DESCRIPTION

Clast-s upported,packed,essentiallymediumtowellso rte d,upward-fin ing seq uencestowa rd sintercalat ed sandandgrit layers.Upw ard-coarsening bedsareseen locally.Cong lo mera ticchannels. Thesandy andgrittysu bfacies are massive,crudely beddedorlamin at ed,thelatterintheupperparts of the sand layers.Unidi rect ion alc1ast imb ricationsoccur atthe tops of somebeds.

One 6-m etr ethick,lenticul arbed whic herod es underlyin gtype1 conglo me rates.Clast to matrix-supported,poorly sor te d,showsno obvio us gradingandconta insisolated,larg e blocksup to 60 cm indiame te rsurrounde d byco bbl esandpebblesofvaryi ngsizes.

Tab u larcong lo merate beds «1m)inte rcalate dwit hsandand/orschist. Clast- to matrix-supported,poorl yso rt ed, butcommonlywithincreasing matri x conte ntfrom bottom totop.Bothupwardfinin g andcoarseningisseen (t helatterismost com m on),and c1asts areessentiallyang ular.

Gritty co nglo me rate,minorpebblybed s.Layersare wellso rte d

(alt houg h singlepeb blesoflarg ersize can occu r),c1ast sup por ted,packed and lat erally persiste nt. Pebbles are essent iallywellrounde d.Intercalat ed sand and peliticlayers are usually thin ner or ofthe same thicknessasconglomerate layers,commo nlywith c1asts.No gradingis seen. Upwards,thisfaciesgrade sinto cong lo mera teswithcarbo nate matrix(nointerpretationofsedimenta ry st ruct u res hasbeenpossib le)orsericiteschists.

INTERPRETATION

Braided st reamcong lo mera tes

High lyviscou sdep o sit,pro babl y of subaerial mass-m ovem ent origin (Nemec& 5tee I1984).

5ubaq ueousmass-m ovem ent

Beachconglomerat es ,

(basedon criteria describedby Clifton(19 73) andKleinspehnet.al.(1984))

NGU- BU LL 438,2001 - PAG E56 TOM HELDA L

Table 2.Pebblepopulat ionin polymict conglomeratesat differentlocalities in the Bolveer area.X= present,XX = abundant,XXX= freque nt.

•Ultramafics includeactinolite-ri chrocks.

SOURCE ROCKTYPE NORTHFLAT0Y SOUTH FLAT0Y RIS0Y L1LLE ESJE0Y ESJE0Y STOR0Y

TheBolvzer Ultr amafics' XX XXX XXX

Complex and Gabbro XX XX X

associated Amphibolite X XX

rocks Chert X X X

Weat heringprofile Magnesiticcarbo nate XX X X X

Meta- Psammit e XXX XXX XXX X XX XX

sediments Quart zite XXX XXX XXX XXX XX XX

Calc-silicate schist X

Other Vein quartz XX XX XX XX XX XXX

Lille Esj eoy thisfacies and possibly subaerial mass-move- ment conglom erates occur, while the third type is most commononEsj eey.The beach-t ypeconglomerate s are seen onStorey,where theygradelaterallyinto coarser conglom- erates oftheformer typ es.

In commonwit h the fine-grained clastic sediments,the conglomerates arederived from several sources.A studyof thepebblepop ulations issummarised in Table2.

Not surp risingly,theBolveerComplexiscomm onl ypre- sent in the pebble population.Additionally,clastsof'cot ic- ule'chertcontainingveryfine-grained,pink spessartine are quite abundant. These rocksarenotobservedin the Bolvzer area,butsimilar rocksare found on themain land fartherto the east, occurring as thin band sin amphi bol it es(Heldal &

Hj elmeland 1987),possibly represent ing an up-seque nce partoftheop hiolitepseudostrati gr aphy.

Pebb les of the metasedimentary rocks include psam- mite, biot it e psam mite,quartzit e and calc-silicate schists.

Evident ly,theserocks were deformed andmetamorphosed prior to deposition ofthe con glom erates, as shown bythe occurrence of a pre-existing foliation and folde d quartz veins in unde formed pebbles.This old baseme nt of conti- nenta l-derivedmetasedimentaryrocks isnot exposedin the area, but an obvi ou s candidat e would be the inferred Neopr ot erozoic, metased iment ary succession in the Helgeland Nappe Complex descri bed above.The nearest locationof such roc ks is in the Velfjord areato the eastof Brenneysund. Fumes et al. (1988) stated that the Leka Ophioliteisrestingonabasement of psammite,marble and schist,observedonthe islands Solserneyene.

The Ris0Y Formation: source and depositional environment

TheRis0YFormation graduallyfilled the irregular topogra- phyin the eroded BclveerComplex.Areconst ructed fence- diagram ofthe palaeot op ograph y andthe Ris0YForm ation is presented in Fig.10,based on stratigraphic logsat five localities.The figure show sthat sedimentary rocks at the baseof theformati onwere deposited in topographic lows whereas highswereexposedto weat hering. The differen t members oftheformation show rapid lateral and vert ical chang esfrom fine-clasti csedimen tssuchas sandstone, silt- sto neandshale,to coarse conglo meratesand breccias.

The lack of fossils, metamorphic recrystallisation and absenceof continuou s, vertical sections make itdifficult to presen t a detailed interpre tat ion of the sedimen tary envi- ronment. How ever,there are several indicati ons that the Ris0Y Formation repr esents partly continent al and partly shallo w-marinedepositsin acoastalenvi ronm ent:

In the green metasandsto ne member, sedimentary st ructures typi cal of shoreface associations (Burgois &

Leithold 1984, Walker 1984, Johannesen et al. 1995, Fje llanger et al. 1996) are foun d,including low-angle cross-strata,small-scale trough cross-strata,high-angle scours and low-angle undulatory lami nati on.

Fur thermore,interchan ging wave rip ples and herring- bone str ata could indicate interacti on sbetween wave- and tide-d om inated environments (Hayes 1980).The conglomerates in the lower part of the member could represent channel-mouth deposit s, varying from upw ard-fin ing, c1ast-supporte d channels to matrix-sup- port ed, tab ular conglom erate s, probably subaqueous mass-movements (Kleinspehn et. al. 1984).These con- glomerates are,in part,strongly reworked,result ing in layerswit hisolated boulde rs and pebbles.

The chlorite schists im medi ately above the green metasand stonecould be inte rprete das predominantly shallow -water turbidites.This is suppo rted bythe occur- renceof slumped conglomerate bodies and boulders within them.

The upper part oftheRis0YFormat ion is characterised by pelit ic sericite schist representing more finely lami- natedclayand siltstone,reflecti ng a complete drown ing of thepalaeot opog rap hy.5t rat igrap hicallyabove these follow carbonate deposits of the Torgnes Format ion, whichareclearly of marine origin.

The polymicconglomerates reflect a variety of facies - interpreted as braid ed st ream, subaerial mass-move- ment,subaqueousmass-mo vem ent and beach,reflect- ing an interaction betwe en fluvial, alluv ial and sub- aquatic processesAccording toHouseknecht&Ethridge (1978)and Ethridge&Wescott (1984),these areimpor- tant crit eria for the recogni tionof fan-deltasin coastal environments.The vertical transit ion from subaerial deposit s to the carbo nate rocks of the Torgnes Formatio n, combinedwit hthecoarse,cyclicconglomer-

TOMHELDAL NGU-BUL L43 8,20 01 - PAGE57

Fig. 10. Reconstruct ion of the palaeotopog raphy and the Ris0Y Formation in the Bolveerarea,based on log sfrom 5 localities(seeFig. 3).

Turbidites.clay/silt(ensialicsource)-sericiteschistmember Turbi dites(mafic/ult rarnafictomixe dsour ce)-chlor iteschist memb er

~~~~f{H~j Conglomerates(mixedsource)

_ Sandstone(mafic source)- green metasand stonemember

~1~1~:~ ^{Brecc ias}and congl omerates(malic/ultramafi c source) UNCONFO RMITY

_ TheBolvrer Comple x

ates inte rchangi ng with possible shallow-w ater tur- bidites,are indi cativeof a shelf-typefandelt a,according toEthridge&Wescott(1984).

Astrikingfeature ofthe Ris0YFormation howeve r,isthe extreme bimodal character of the metasedimentary rocks:

rapid transit ion from coarse conglomeratesto clay,isolated boulders in fine-grained sandstone and siltstone, and slumpedconglomeratebodiesintheturb idit es of thechl o- rite schist member. This could indicate eit her extreme storms and/or tecton ism triggerin g a sudden influx of coarse clasticsediments. Theobservation s of synsedimen - tary faultsin thefield arein favour ofthelatter.Thetransit ion from clastic sediments tot ally derived from the ophioliti c subsurface to sediments derived from a mixed and/or exotic ,ensialicsource occur sin themiddlepart of thefor- mation ,marked by the polymict conglomerates and the boundary betw een the chlorite schist member and the sericite schist memb er. At this stage, erosion had cut through the ophiolitic rocks and psammites and schists wereexposed to weathering.Theserocks then became the domin ant detrital source. The sudden character of the change insource material,in the aut hor's opinion,provides further support tosyn-sedi menta ry faulting as animportant factorcontrol ling the deposition of theRis0YFormation.Fig.

11 showsa concept ualmodel summarising the interpreta- tion of the Ris0Yformation.

-

<=

<=

COl. 700III

I)NorthFlatoy 2)Bolvrer 3) Nort hRisoy 4)SouthRisoy 5)Lille Esjeoy

2

Mixedsource fine clastic sediments

Mixedsource conglomerates

Fig.11.Interpretat ion of thepalaeoenviron- ment and the changeof sourceareaduring depo sition of theRis0YFormat ion.

NG U-BU L L 438, 2001 - PAG E58

Fig.12.Features of theTorg nes Formation.A)Fo ld ed(F2)sand sto ne layers intercalated wit h marbl e;B)Calcareoussandstone bed s with norm al grad ingandplanarlaminationin theupperpart.Wayuptowards the right. Thedarkspo ts in thelight er coloured bedsare bioti te porp hyrob lasts.

Table3.Lithofacies intheTorgnes,Aspeyand ToftsundFormat ions,and int erp retationof depo sitionalenvi ro nme nts.

TOM HELDAL

FORMATION Toftsund Formation

Aspe y Formation

Torgnes Form ati on

L1THOFACIES

Marbleconglomerate:pebbles and matr ixofcalcite marbl e Biotite schist

Calcareousmetasandstone:beds«20 cm)int ercalat edwit h calcitemarb le Calcite marbl e:banded,greyandwhite

Bioti teschistwith minorlayersand horizo nsofcalcitemarble, psam m ite,grittyconglomerateand c1ast-bear ing sch ists Garnet-micasch ist

Uppercalcitemarble:15 metre-thick,yellowto greymarbl e withcalc-sil icate layers(onlyin thenor th ern partof the area)

Calcareou s metasandsto ne:planarbedsof calc-sili cate schistand calcareou s sand st one withthin marblelayers.Generallyupwardcoarsening(increasing grain-sizeand bedthickness«1 metre)).Decreasingcarbonatecontent tow ards the contact to the overlying AspoyFormation.Individ ual bed s arenormally graded,andthe A,BandEsections of a Boumasequence areabundant.

Lowercalcite marbl e: grey, graphi tic calcitemarbl e inte rlayeredwit hgraphite schists(lower part)gradesupw ardsto yellow and red marb leintercalated wit h calc-silicateschistsand intraforma tiona l,marble conglom erat es.

INTERPRETATION

Intraforma tion al lim estonecong lomerates Marine silt-and sandsto ne

Marine, mixedclay and limy mud Carbon ate plat fo rm

Marin esilt with min or coarseclasti c sed ime nts and limestone(upper part) Pred om ina nt lymarine clay

Hemipe lagic,limymud

Calcareousturbiditesrangingfrom distal(lower)toproximal(up per) part of asub marinefan.Inte rcalat ed byhemipe lag ic,lim ymud deposit edin quiet periods

Hemip elagi c,limymud,partlydeposit ed. underhyp ersalineand reducingcond iti o ns (Bjorlykke&Olaussen 1981;start et.al 1980)

The Torgnes, Aspoy and Toftsund Formations The three remain ingformationsof the BrenneysundGroup occupythe areabetweenBolveerand Brenneysund,young- ing towardstheeast.Sedimentary st ructures aremuchless comm onthanin theRis0YForm ati on,and the rocksareiso- c1inally folded,and in some areasstrongly shearedin the long limb sof F2 folds.Therefore,only a briefdescription and an attempt to interpret the lithofacies of these formations are includ ed in thepresent paper.

Abovethe Ris0Y Formationfollows aunit dominated by well-bedded, calcareous metasandstones; the Torgn es

Formation.An averagestratigr aph icthickness of 200 metres is suggested.It contains three lithofacies:a low er calcite marb le,acalcareousmetasandstone(Fig.12)interpreted as turbidit es,and an uppe rcalcite marble(Table 3).Boththe lowerand uppercontact s are sharp, but no indications are foundofany tecton icbreak withinthe succession.Gritlayers at thebaseof theoverlying ASP0YFormation may,however, indi catea minordepositionalhiatusbetweenthetwo.

TheToftsund Format ion comprises calcite marble (Fig.

13), calcareous metasandston e, quartz-bio tite schist and minor intraforma tionalmarb le conglomerates.The last three

TO M HELD AL

litho logiesare abundantin thelower partoftheformation, marking a transition zone towards the subjacent Aspey Form at ion,whilethe centra land upperpartisdomin ated by homogeneous ,grey marble.One cannot excludethe possi- bilitythat the ToftsundFormation may represent ast ruct u ral repetition of the Torgnes Formation,displaying a lateral changefrom turbidites to moreproxima llimestones.Inthat case, the Sauren island (outcrop area of the Aspey Format ion)woulddefinean F2syncline.Based on suchcon- side rati ons,theF2 foldpattern was carefullyinvestigatedfor anyindi cation sof sucha majorsyncline,witho utaffirmat ive result s.Thus,the author regards it as most likely that the Toftsund Formation is deposited on top of the Aspey Form ati on.

The Brenneysund Group: Tectonism on a continental margin?

The unconformity at the baseof the Bronneysund Group clearly defines a highly irregular topog raphic surface exposed to subaerialweathering. This was probab lyfilled with sediments during aregionaltransg ression.The lower partof thecov ersequence showsa great variety of deposi- tional enviro nments,from subaerial to shallow marine. By thetimeofdepositi onoftheuppermost part ofthe Risey Format ion,thepalaeotopographywascompletelydrowned, andsedimentatio nwasdom inatedby mudand fine-grai ned sand andsilt. There are several indi cati ons ofthedeposit ion of theRisey Format ion being cont rolled by tectonism, e.g.

the observati on of a synsedimentaryfault and the abrupt change in source material, reflect inga sud de n,and not grad- ual,input ofensialic material.

The Torg nes Format ion marks a change in the deposi- tionalpatt ern .The format ion isinterp ret edto bemainlytur- biditic,partl yderivedfroma carbon ateplatform.During this period there was little influx ofterri genou sdet ritu s intothe basin,whichcould havebeen duetodiminishing tect oni c acti vity. TheAspey Form ati on, how ever,is essentia lly com- posed of terrigenou s material , indicati ng eit her a new

NG U - BULL 438,2001- PAGE59

period of tectonism or a chang eto amore humid climate, increasing the sedi ment transport from themainl and.The Toft sundFormat ionagain reflect s a reduct ionin terri geno us det ritu s and/or transgr ession,leading to the development of a newcarbonateplatfo rm.

The rapid changes in the Brenn eysund Grou p devi ate fromthat expectedina transgressive sequence onastable continent al margin.The vert ical change from terrigenous sedim ents to carbo nates and back,indicates either strong climaticchangesoratect onically active margin .Giventhe clearindicat ion sof tect on ism in thelow er part ofthegroup, thelatt er seems to bethe most likely int erpretati on. Sim ilar feat ureshave beendescribedfrom cover sequencesoverly- ingtheLyngenOphiolite in northernmostNorway(Minsaas

&Sturt 1985),the Leka Ophiolite Complex (Stu rt& Ramsay 1995) and the Vagamo Ophiolit e in south-central Norway (St urt etal. 1991,Bee et.al. 1993,Stu rt et al. 1995).lnthe case of the Vagamo Ophiolite,thecoversequ ence (lower part of theSel Group)is inte rprete das a seriesoffan-li ke deposits, relat ing to act ive fault scarps,at the margin of a Lower Ordovicianback-arcbasin.

The strat igraphic problem of the HNC

For a further understand in g of the historyof the weste rn HNC,stratigraphycould playan import antrol e.Inthe pre- sent paper,it hasbeenshow n thatthe BrenneysundGroup was probablydepo sitedonatect onically active,continen tal margi n.Thearea can serveas atype locality for the Lower Ordovician met asedim entary successions wit hin the medium- to high-gradepart of the western HNC,helpingto define a stratigraph ic and tectonostrat igraphic framework for the region.The correlat ion between the Bron neysun d Group and simi larcoversequenceson Leka (St urt&Ram say 1994),at Terrak(Husmo&Nordgu len 1988),in the Velfj or d- Tosen area (Thorsnes&Leset h 1991)and on Rodey (Bang 1985) seemsobvious.Additionall y,rocks occurring on the myriad ofsmall islets in the area between the mouth of Bindalsfjord and Vega can be correlated wit h the Bolveer

Fig.13.Marble wit h folded (F3) layers of schist in theToftsund Form ation, eastern partofSauren.

NGU-BU LL 438, 2001 - PAGE60

Complexand the Bronnoysund Group(Heldal &Hjelmeland 1987), breakingthepat te rn of thin,eastwa rd -di ppi ng,thrust sheet s seen on themainland.Therelat ionship betwe enthe Bronnoysund Group and the low-grade successions to the north w est,in the Skalvee rarea(Gustavson 1975),remainsto be investigated.

The conglomerate facies wit hin theBronnoysund Group andinsimi lar ophiolitecoversequ ences in theHNCshow sa mixed source of detritus, refl ect ing bot h the fragm ented ophiolites, metamorph ic quartzo-feldspath ic sedimentary rocksand(atsome location s)crysta lline basement.Clearly, a candidate for the second group of c1asts is the inferred Neoprotero zoic succession foundfarthertothe east inthe HNC.How ever,noprimarycontactsbetwe entheold and the younger coversequences have yet been found.This mysteri- ous non-ap pearance of highly expected primary relat ions may be explained by alack ofsyste mati cresearch on this subject, and the diff iculties in locating primary contacts betwe en two ,comp arat ive ly sim ilar,sedim enta ry succes- sions in a high-strainarea.Theide ntifi cation of such relation- shi ps and the further differentiat ion bet ween the two metasedimenta ry units would be of great importance for our overallunderstand ing of thetectono strat igrap hyof the Helgeland Napp eComplex, and shouldtherefore be a main target for fut ureresearchin the regi on. Latel y,it has been shown that isotopic dating of carbon ate rocks (Tronnes&

Sundvoll 1995) canbeasuitable method ofseparat ing the 'older' and 'younge r' successions inthisregion.Interesti ng result s from such isotopic stud ies have recently been reporte dfrom areas farthe rto thenorthin the Uppermost Alloc ht ho n(Melezhik et al.2000, 2001).Combi nedwit hsedi- mentol ogicalandstru cturalwork,these stu dieshave added significant ly to our knowled ge oftheorog enicdevelopment of thispart oftheCaledon ides(e.g.Rober tsetal., inpress).

The BolvzerComplex was probably emp laced onto a cont inen tal margi n some time after the format ion of the LekaOphiolit eComplex(497+/-2Ma;Dunning &Pede rsen 1988);this was immed iately succeeded bydeposition of the Bronnoysund Group. Before empl aceme nt of the Binda l Batholith (around 450 Ma), this margin was imbricated, deformed and metamorp hosed in amp hi bolite facies.As described by several autho rs (Bang 1985, Heldal 1987, Thorsnes & Loset h 1991),this impli es thatthe most signifi- cant partofthetectonometamorp hic development of the metasedimentar y successions took place during a major, pre-Scandian event. Hj elmeland (1987) interpreted the Torghatt en Granit e as a syn-D2 intr usion in the BronnoysundGroup, and notasa basem ent wind ow,as indi- cated ongeologicalmaps.Datingof thisplutonand thesim- ilar HamnoyGranite fart her tothe north would clearlybe of great importanceforadding to ourknowl edg eofthedepo - sitionalandtect onometamorp hic evolu tion ofthisreg io n.

TOM HELDAL

Conclusion s

It has been shown thattheBolveerOphio lit eComplex was em placed,upli fted and subjected to subaerial weathering and erosion prior to the deposit ion ofthe Bronneysund Grou p.The unconformity defines an irregular topographic surface,which wasgraduallycoveredbysedimentsduringa regional transgression,probab ly influen ced by active tec- tonics.The sedimentation of the Bronnoysund Group ini- tially starte dwit hthebuild -up ofa coastalset ting,delivering coarse andfine ciastiesinto a shallow-marineenvironment.

This is st rat igraphic ally follow ed by calcareous turb idit es, micaschistsandcarbonat e rocks.

The BronnoysundGrou p is correlatedwith several cover seque ncesabove fragmented ophioli tesin the HNC,and an Early to Middle Ordovician deposit ionalage is suggested.

Theclast popu lat ion in conglomeratesandthecom position offiner clasticsediments showan exot ic,ensialicsourcefor thedetritu s inaddi t ion to theoph iolit e.This ensialicsource consistessentiallyof psammitesand schists,deformed and metamorph osed prior tothedeposi tionof the Bronneysund Group.

The extre me ly well preserved unconformity and sedi- mentary structures inthe Risey Formation make the small island s intheBolveer areaan excellen ttargetfor field excur- sions.Thiscould be ofsignificant helpin the interpretation of basement-cover relations in other,strongly deformed partsof theHNC;orjust forthepleasure of studying primary, sedimenta ryfeature sin rocksthat weretotally recrystal lised in upperamph ibolit e-facies meta morphi ccondi tions.

Acknowledgements

The aut horwishes to acknowled ge the lateProf.Brian A.Sturtfor super- visiondurin g fieldworkand forencouragement to sta rtworkingon this paper.Likew ise,Prof.DonaldRam sayisthanked for guidanceandinspi- ration durin g the fieldwo rk.Many than ks also go to Reidu lv Bee, Step hen Lip pard, 0ysteinNordgulen andDavid Roberts for reading the man uscript and foroffering construc tivecomm ents.

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