The conglomerates of the Sel Vaga area, Central Norway: an terrane-linking succession.

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The conglomerates of the Sel Vaga area, Central Norway: an terrane-linking succession.

REIDU LVBOE,BRIAN A. STURT&DONALD M.RAMSAY

Group, Otta- example of a

Boe,R., Sturt,BA &Ramsay,D.M.1993: The conglomeratesof theSelGroup,Otta-Vaqaarea,Central Norway:anexampleofa terrane-linkinqsuccess ion,Nor.geol.unders.Bull.425,1-23.

The conglomeratesatornearthebase of the SelGroupin theOtta-Vaqaarea haveaspecialsignificance in the interpretation of the palaeogeographicaland geotectonicdevelopme ntof the ScandinavianCaledo- nides.They overlie a stratigraphicunconformity of Arenig-L1anvirnage,whichseparatesthem fromasub- strateofalreadyobducted,forward-thrustedand folded ophiolitic rocks,whichreposeonabasement- covercoupletpossibly ofFennoscandianShield type. The conglomeratesvary fromthe wellknown near- monomictserpentineconglomerate(Ottaconglomerate facias)throughpolymict conglomeratescontai- ning clasts oftheunderlying substrate(Toconglomeratefacies) to matureconglomeratescontaining clastsofessentiallyveinquartz andquartzinaquartziticmatrix (Skardshoiconglomeratefacies).

The conglomerate-bearingsuccessions showconsiderablelateralandverticalvariation throughtheregi- on,andalthoughcertainareasare dominatedby aparticularclast populationimplying aspecialsource- depositional system,the various conglomeratic andnon-conglomeratic lithologies show interdigitating relationships.Theconglomeratepetrologyshows that thebasalpartof the Sel Groupisaterrane-Iinking succession,indicatingthatthe Vagamo ophiolitecomplex hadalready beenemplaced,priorto theAre- nig/L1anvirn,ontorocksofcontinentalaffinity.Theselatter possibly represented a western(relativetothe present)extensionof theBaltoscandiancontinentalmarginmiogeo cline,orpart of amicrocontinent. It alsoshowsthat the Vagamo ophiolitecomplexandits substrate had beenupliftedanddeeply eroded beforethedepositionofamajor clastic wedgewhich includesfan delta,littoralanddeepermarinesedi- ments.TheSelGroupwasapparently deposited inanextensionalbasindevelopedas aresponseto back-arc spreadinginMiddleOrdoviciantimes.

ReidulvB0e,BrianA.Sturt,Norges geologiskeunderseketse,Postboks 3006-Lade,7002Trondheim, Norway.

DonaldM.Ramsay,GeologyDepartment,University of Glasgow,GL82800Glasgow,Scotland.

Introduction

The Otta Nappe, as defined by Strand (1951),comprises four groups,the Rudihoi Complexat the base succeededby theHei- dal (Gjelsvik 1946), Greenston e and Sel Groups. Recent revisionsof this nappesuc- cession (Sturt et al. 1991) have demonstrated that the volcanic Greenstone Group is really a fragment of ophiolite, the Vagamo ophiolite complex (Fig. 1). Although disrup- ted by pre-Arenig/L1anvirndeformation,typical members of an opholite pseudostrati- graphy arestillrecognised.Thebase ofthe nappeis asurfaceof thrusting marked by a prominentmylonite .Thiszoneofhighstrain extends into the underlying Heidal Group, so that competent quartzites close to the thrust are actually quartz mylonites. The mylonitisation is superimposed on an alrea- dy complexstructural-metamorphic fabric in the rocks of the Heidal Group.

The contact between the Sel Group and the ophioliteand its substrate is a major unconformity, representing a period of uplift and erosion.This major break partitions the tec- tonothermal history of the region into two discreteorogenies (Sturt et al. 1991).

The lower horizons of the Sel Group are characterised by the development of various conglomerates (Fig.1). Originally these were identified as three discrete members (Strand 1951), the Greenstone, Otta and Skardsho Conglomerates, each at a different stratigraphic level. The Greenstone Conglomerate was placed at the top of the Greenstone Group (laterincorporated into a new Svartkampen Group (Strand 1964)), the Otta Conglomerate marked the base of the Sel Group and the Skardsho member was viewed as a younger member within the Sel Group.In a later revision, however, Strand (1964) tentatively correlated the

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LEGEND

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RUDIHOI GNEISS OTTA THRUST

PSAMM ITE(L.ALLOCHTHON) BASEMENT GNEISS

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NGU .BULL 425,1993 ReidulvBoe,BrianA.Sturt&DonaldM.Ramsay 3

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^SK^OTTA^{AAOSH OI}^CONGLOME RAT E FACIE $^CO^NGLO^{ME RATE} ^FAC^IES TO CONGLOME RAT E FACIES

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Fig.2.Generalised map showingthedistributionof the various conglome rate facies in thelower most partoftheSelGroup. The boundaries oftheVAgAmo ophiolitecomplexareshownbydashed lines.Numbers1-29referto keylocalities summaris ed inTable1 anddescribedinthetext,Numbe r30isKleiverudtjern,31 isToFarm,32 is Adalen,33is Skarvanqszetrin.V:VAgAmo;0:Otta.

BASEMENT AND BASAL SEL GROUP LITHOLOGIES

Table 1.Basement and basal Sel Group Iitholgies at keyloca- Iities inthe Otto-Vaqaarea.Asterikes showwherethe contact is exposed.The localitiesare shownFig.2.

Skardsho and Greenstone Conglomerates and renamed them the To Conglomerate, still within the Svartkampen Group.

BASEMENT LITHOLOGY 1 Raggehaugen Gabbro/ullramafics 2MegArdsAsen Gabbrowithdykes 3Skytningen Gabbro withdykes 4 Tnsteinen Heidal quartzite 5Amhaugen Serpentiniteandgabbro 6Strcndsseterhot Gabbrowithdykes 7Kleppdalen Gabbro 8Eastern Sseterhoi Gabbro 9SvarthamarbeksenGabbro with dykes 10 Bukkehaugen Gabbrowithdykes '1Finna Greenstcne 12 Dalen Heidalquertzue/calcsaicete 13 Delen Greenstone 14Gronlii Heidelquartzite 15Hovca Uttremefics 16Svarthovda HeidaJ quartzue/calcsaicate 17Svarthovdasouth HeidaJquartzitefcalcsilicate 18Raudbergshoi Heidal garbenschiefer 19Fjelltithoi Heidalquartzite/ca!csilicate 20 BoIstad Serpenlinite 21svertuern HeidaJquartzite/schist 22 AsAren Serpentinite 23 Geithomet Heidalquartzite 24Sore JostenseterGabbro 25 Gauklihaugen Greenstone

26 Dale Greenstone

27Myroms.:etri Greenstone 28 Bondaseetn Gabbro

29 Rusli Gabbro

BASAL SElGROUpLITHOLOGY

OttaConglomerate OnaConglomerate OnaConglomerate One Conglomerate OrtaConglomerate Sel schist Gabbro/Greenstone Congl One Conglomerate Sel schist

Selscbist /Otta Conglomerate Selgarbenschiefer To Conglomerate oneConglomerate OnaConglomerate OrtaConglomerate To Conglomerate Skardshoi Conglomerate Skardshoi Conglomerate Skardshoi Conglomerate OttaConglomerate Ona ConglomeratelToCong!.

Ot taConglomerate To Conglomera:e ToConglomera:e ToConglomera:e/Sel phylllte Sel phyllite

To Conglomera:e/Sel phylllte Selccoeomerencsandstone Sel phylhte

Sturt et al. (1991) demonstrated that the conglomerates are all facies of a major development within the Sel Group, which pass laterally and vertically into one another.The tectonicfabricswithin pebbles of the polymict To conglomerate facies,together with itsinterbedded relations withinthe Sel Group,indicate that it cannot belongto the pre-Selsuccession.

The basalSel Group,in placesconglomera- tic, locally oversteps the ophiolite on to the subjacent Heidal Group (Sturt et al. 1991) (Fig.1). The lowstrain state in conglomerates whenjuxtaposed against quartzmyloni- tes in the upper Heidal Group provides an ageconstraintonthe thrustingofthe ophiolite and confirms the primary nature of the Sel overstep. This indicates that the Otta Nappeisa composite structure,comprising a pre-Arenig/L1anvirnnappe ofophiolitewith- in a younger Otta Nappe. This scenario is confirmedby the variedcomposition of the To conglomerate facies which reflects an ophiolitic and metasedimentaryprovenance .

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4 ReidulvBoe,BrianA.Sturt&DonaldM.Ramsay GU ·BULL425.1993

The presentaccount furtherdevelopsprevi- ousfindings andfocuses on thesedimento- logy of the Selconglomerates.Thework is based to large extent on the complete remapping (by the authors) of 1:50,000 map-sheet Vaga, substantial parts of 1:50,000 map-sheet Otta and neighbouring parts of adjacent map-sheets.Fig. 1repre- sents asimplified compilationof the results ofthismapping.

Co nglomerates of the Sel Group

The various conglomerates occurring atthe baseof the SelGroup have previouslybeen mapped, described and interpreted by a number of authors (Bjorlykke 1905, Vogt 1915, 194 5, 1947,Goldschmidt 1916, Car- stens 1928, Strand 1951, 1964, Oftedahl 1969, Bruton & Harper 1981, Siedlecka et al. 1987) .Until recently, lack of knowledge concerning the general statigraphy has hampered detailed correlation of the conglomerates(Sturtetal.1991).

The lower part of the SelGroup comprises polymict and monomict metaconglomera- tes, separated by intervening garbenschiefer phyllites, slates, mica schists, meta- sandstones, metasiltstones and thin meta- morphosedlimestonehorizons. In the following, the sedimentary terms conglomerate, sandstone, siltstone and limestone will be employedforrocksin the SelGroup, instead ofthe metamorphic termsmetaconglomera- te,metasandstone,metasiltstoneandmeta- limestone.

The thickness of thelowermost,conglomerate-bearing part of the Sel Group varies from almost zero to several hundred metres. The conglomerates may locally

represe n t the base oftheSe lGro u p,though

in other instances first appear at higher levels,whilesandstonesandvarious phyllites occur atthe base.There areconsidera- ble variations in textu re, internal structures and composition from one areato another,

and different names have been applied to the different conglomerate lithologies. The- se included Otta Serpentine Conglomerate

(Vogt 1945), Serpentine Conglomerate

(Bjorlykke1905,Strand1951),OttaConglo- merate (Bruton & Harper 1981), Gabbro Conglomerate (Goldschmidt 1916), Green- stone Conglomerate (Strand 1951) , Horn- blende Conglomerate (Bjorlykke 1905), Skardsho Conglomerate (Strand 1951)and ToConglomerate (Strand1964).

In the following we will continue the use of the names Otta,Skardshoi (Skardshoi is a modern form of Skardsho) and To. How- ever,theconglomeratesare treated as faci- estypesdescribingcomposition:Ottacong- lomerate facies (monomict to near mono- mictconglomerate with serpentineclastsin a talc-chlorite-magnesitematrix),Skardshoi conglomerate facies (conglomerate dominated by c1asts of quartzite and/or vein quartz inamatrix of micaceousor massive quartzite) and To conglomerate facies (polymictconglomeratewithvaryingpropor- tions of ophiolitic rocks, quartzite, calc-silicate, granite and trondhjemite clasts in a matrixofquartz and greenstone materials).

Theseconglomeratefacieswillbe characte- risedby referenceto a number of keylocalities.

The Otta conglomerate facies is most pro- minently developed in the western and southwestern part of the investigated area (Fig. 2, Table 1). It predominates in the ESE-WNW trending synformalstructuresof the Flatningen area to the south of Vaga (localities 1-6), in the Bukkehaugen area (localities8-11),atGronlii-0yagarden(locality 14) and at Hovda (locality 15),but also occurs atSvarttjern (locality21)and Asaren (locality22)(Fig.2,Table1).TheSkardshoi conglomerate facies predominates in the northernpart oftheinvestigatedarea,to the east of Svarthovda (localities 16-19), but it also occurs on Gauklihaugen (locality 25).

Th e To co n gl om e ra te facies occurs in the

northern andeastern partsofthe area,from Svarthovda (locality 16) to Otta,and in the Kleppdalen synform (locality 7) in the west (Fig.2).

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NGU- BULL 425,1993 ReidulvBee.BrianA.Sturt&DonaldM.Ramsay 5

Locality Thickness of Degree of Conglome rate S, n"htoncl Maximum (cong lo merate eonglome rate defo rmation b<. ^.ill5l:^0ne ^{(aven ge)}

('Cles) bearing lhkknc» bed clu~

"",,cc"ion thKknc~ diameter

Bukkchlu,en(IO) IQ-lOOm Moocralc. 5·200cm 1·100cm 20 (1)cm

(on.) increuin, l-Wcm

upw.rd,

Ragg ch.ugcn(I) Cl.200m Weak10 lo-200c m 1-60cm SO(l2)c m

(Ot u ) moderat e

Grainsupponl Scdimo:nlary I'o u nd nc... of stru(IURa cong lofne r:ll c inthe

dn u coog1omen lu

M.Cl ErosionalSCOUR,pl' lll!parallel Subroundcd10 5I.ratification.nd trou gh.:ron· wellrounded "'ratificationIn:CUmmon.

M.Cl Con glomeratebed•• malguTUlled or Subrou nded10 sep.ratedby erol iona l Koun.

.."c11rounded Up....·.rdfiningi.(0fIUT'I0lI.Cl..~

M1ppon edbed s<ImIhKlmay be ungra ded or invenc-lo-normaUy gr.d.:d.Imb ric.tionoccurs.

Scdi~nlary stn.oC lu n:ain the interbedd edsandstone.

andIill.Jl:oncs

S.ndstonn oftenshowupward.fininglrcrllb.pl.ne pa rallel stratification.ndtroughcro"·Mratific' lion;,iltS'lonca show pllllC parallel lamination,troughem..-lamin euo nand flaser bedding.

Plane parallel~rali ricalion.ndtrough(l'OSI-Jlratification.

Cro u·Jlratified acl. up1040cmIhid..Trou gh(l"O$S- stratiflCd illle....I.usually occurontopor^plane^parallel^llralified^inle...^ala•

indic.ting p.l . cocum: ntl lO\>.'ard .!hoeoeUI.1-2(m L'l icl.red.

pl.nep.rallel laminated .ndIrou Sh cmu-I. mi nated,iltllo ne may occuron toporsanJ "'onebed•.

Tristcinen(4) (on.)

e•.SOm Moderate 10-200cm 1·)0 cm 40(10) (m Cl Poo rlysorted,gene rallymauive Some51ructur.:1cn.co ngfomereticJoIIl<h.lonebed.occu r.They Subrounded10 .nd.malg.mated.matriJl-suppo rted wed, eoutlate rally due toeroWonillrunc . tion.

....ellrounded kll5C.occu r.

Sv. nhove4(16) (To )

u.400 m Mo&craloe 100IlX)(hm C.(!>.{) Goene rally 51ruclurck...

See deta il.inmaintext.

S«doetail . in mainteX'l.

Fjclltithoi(19) (To)

c•.400m Moderate 1(}.IOOcm 1·50cm 50 (7)cm M! Relatively wellsorte d.Sizefree - Subrou nded to lionswithinh.:dslypk .l1)'1-5 . well rounded S-IO .nd.'1-1.'1cm.

f'y. ,. rden-G ru nlii(14) c•.SOm Weak (on.)

5-100 (m 1·50(m 10 (2)cm M.(C)/ M. y grade upinao ..nd"'onc. S.ndslone..ndcong lomc rati<:..nd.tonc.UlOWpl.ne p. ral lel.nd Rou nde d10 Imb~ .lionOo;(un. troughcrou-5lrI Iifi;lI ion.Undulatina.nderoliveJoo,a.·er well rounded Trou ghcroMo- JltlIl if"::lI ion. Conla.:b.Erosional ..:00 ....arc(ommon.

Palacoc um mls tow.rds!he eaol.

Sv.ntjem(21) c•.150m Mode rale 5·1.'10(m 1·20cm 25(8)cm

(To )

G.ulliha ug en{251 c•.ISOm Mod e rare 5-I(Xhm 1-IO (m 70 (1)(m

(To)

M. (Cl! Usu allynonna ll)' gradedor Subrou nde d to I.\rU,tUr.:!.:ss.

... ell ruu nde d

MI Late rally di5Contimw:ouabeds. PebblyN1nd51OTlC.shov.'pl.ne p.rallelstratifl>.tio n andCfOI$- Sub.nf\ll.r10 Freq ue ntup"...rd·fini n~from ~rati lk.lion.S.nd~OOClon loporconJlomcralcsmay gradeup wellrounded boulder/cobble10pebble'lI.e. intoIhin§chi~s.

Basal5COIlrscommon.Bimod al size..!istribution~omm<)n

PilatAdren(221 ?u.100m Weak (0 11')

2·).'1 cm 1·15 cm )(I) cm Cl Upwardfining .Undulating.ero sive Well-...med.Overlie~onglomc ratClwitherosio nal or~arp Rou nded10 lowerSlIrfacca.Wellsorted. gradlliOfllI(onta~ta.P1.ne parallelI.mination,troughere ....

...ell rounded lamination.ndtrough(fQU· atral ir..::. tton (0.5-3(mIhi~k

rore acl•.

Table2,Characteristicsof theSel conglomerateatvarious keylocalities intheOtta-vaqa area,Thelocalities are showninFig.2,

Description and interpretation of key localities

Bukkehaugen area

Description

At Bukkehaugen (Fig. 2, locality 10), the basal part of the SelGroup is a succession of garbenschiefer phyllites (2-20 m thick), succeeded by ultramafic conglomerates and sandstones of the Otta conglomerate facies (Table 2). The Otta conglomerate facies overlies greenstones of the Vagamo ophiolite complex (Table 1) in a series of NE-SW-trending synforms and antiforms.

This is followed by slates and phyllites alter- natingwith thinsandstone beds.

A representative section of the Otta conglomerate facies, on the northeastern side of Bukkehauqs n (UTM 998617), is summari- sed in Table 2 and shown in Fig. 3. The clasts in the conglomerates are of serpentine,soapstone and greenstone,whereasthe coarse- to very coarse-g rained sandstone matrix has a similar composition. Between the levelsof conglomeratethere are bedsof siltstone and very fine- to very coarse-gra i- ned serpentinitic sandstone (Table 2). A

well-preserved, coarsely ribbed brachiopod was recovered fromone of the plane parallel-laminated siltstones .

A similar sequenceoccurson the southwestern flank of the Bukkehaugen antiformal core. Along the strike, towards the northwest, there is a change to coarser-grained deposits, with serpentinitic conglome rates and sandstones in approximately equalpro- portions . At UTM 989618 this is a well- bedded sequence of erosively based and normally-graded conglomerates and conglomeratic,very coarse-grained sandstones, in beds up to 1m thick.These are usually massive, but trough cross-stratification and clast imbrication sometimes occurs,indicating sediment transport in an easterly direc- tion . The conglomerates are overlain by very fine-grained, plane parallel-laminated and trough cross -laminatedsandstone beds up to10 cm thick.

Towards the northw est (UTM 986622) the succession is dominated by medium- to very coarse-grainedsandstones,conglomeratic sandstones and conglomerates with pebbles generally not exceed ing 5 cm in

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6

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ReidulvBoe.BrianA.Sturt& DonaldM.Ramsay

Raggeha uge n(1)

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LEGEND

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<:> Len ticul a r tarmn atio n

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~~ ^C^{ros s-}^{s trat}^{if icat}^ion

a> Fossil

Fig.3.Stratigraph ic sections throughrepresentative lithologiesat Raggehaugen.Bukkehaqen andGauklihaugen.SeeFig.2 for location of thesections.

diameter.Sandstoneis the dominantlitholo- gy, and occurs in laterally continuous and amalgamatedbeds up toapproximately1 m in thickness. Individual sandstone beds have planarorslightly undulatinglower con- tacts and are normally-graded,with conglomeratic lags at theirbases.Thesandstones commonly appear to be structureless, but several beds show plane parallelstratificati- on and trough cross-stratification in their upper parts. Beneath this sandstone interval there are poorly exposed massive greenstone conglomerates, which overlie gar- benschieferphyllites,on top ofthe ophiolite.

Above the sandstone interval there is a rapid transition to phyllites with thin sandstone beds.

Well-bedded ultramafic sandstones and phyllites of the Sel Group occur to the

southwest of Bukkehaugen,along the stream Svartharnarbekken (Fig. 2, locality 9).

Individual beds of sandstone and phyllite can be50 cm thick,but the sandstones are usually thinner than the phyllites. Individual sandstone beds can be traced for 100-150 m along the strike, and show a very con- stant thickness (Fig. 4). The sandstone bedstendto show an upward- finingtrend, but otherwise appear to be structureless.

These are separated from the ophiolite by 20-30 m of garbenschiefer phyllites, phyllites and thin,pebbly, quartzose sandstones containingclasticquartzgrains.

Interpretation

A possible interpretation of the Bukkehau- gen succession could be that it represents submarine fan or sheet system deposits

(e.g. Ethridge & Wescott 1984, McDougall

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NGU•BULL425.1993 Reidulv Boe,Brian A. Sturt&DonaldM.Ramsay 7

Fig. 4. Well beddedsands tonesandphyllites of the Sel Group at SvarthAmArbekken.Hammer is30cm long.SeeFig.2for location.

et al.1987,Pickeringet al.1989).withsedi- ment input from a coastal or subaerially exposed source area composed of mafic and ultramaf icrocks.Well-roundedc1astsin theconglomerates indicatethoroughrework- ing priorto deposition,probablyon a shore- line orin a fluvialenvironment. Aninterpre- tation of thesediments as submarine (possibly proximal)gravity flow deposits is supported by the predominance of massive beds and matrix-su pported texture in the conglomerates. The well preserved brachiopod found in serpentinitic sandstone could be either a marine orabrackishwater spe- cies,transported intodeeper waterbymass flow.

The relatively thick, laterally continuous, well sorted, erosively based and normally graded sandstone and conglomerate beds occurring on the southwestern partof Buk-

kehaugen were probably deposited as

high- density turbidites (Lowe 1982). Shanmu-

gam & Moiola (1991) and Mutti& Normark

(1987) havedescribed similar deposits and interpreted them as channel fill and proximal fan lobe deposits. Plane parallel and trough cross-stratification occurs in the upper part of many of the sandstones.This intervalmay be overlainby laminated,very fine-grained sandstone,and several exam- ples of almost complete Bouma sequences occur. The well-bedded serpentinitic sandstones and phyllites at Svartharnarbekken were probably deposited as high- to low- density turbidites, possibly in a mid-fan to distalfan setting oronasheetsystem.

An alternative interpretation of the sediments in the Bukkehaugen area could be that they were deposited in a fan delta environment; however, the complete lack of structuresindicatingwave reworkingmakes such an interpretation uncertain. The sedimentary processes depositingsedimentson fan deltasandsubmarine fansmay bevery

Fig.5. Otta conglomerate faciesatRaggehaugen.a)Conglo- merate withbeds of conglomeratic sandstone.Lens capin lower rightcorneris6cm in diameter. b)Conglomeratewith beds of plane parallel stratified andthrough cross-stratified sandstone.Compassis 10 cm long.SeeFig.2 for location.

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8 ReioutvBoe,BrienA.Sturt&OonaldM.Ramsay

CONGLOMERATECLAST LITHOLOGIES

NGU .BULL425,1993

LITHOLOGY/

MINERAL

SVARTBOVDA

2 3

SKARVANG5ErRIN TO GAOKLI - SVART- TRI- A- FJi.LLTIT B"I 1 2 3 _1_ _2 IlAOGEN TJ£RN ST£lNEN DALEN _1_ _2

GABBRO 10 6 7 1, 3 2 8 17 s 3 3 s 5

GREENSTONE 15 40 27 29 I 42 35 5 2 20 5

TRONDHJEMI TE 2 1 16 1< 9

DIABASE 6 5 9 16

GREI::: NSCHI ST 5 3 6

ULTRA."1A.FICS 1 10 95

EP I DOTE I 1

QUARTZ IT E/ 30 29 38 27 28 81 64 66 11 18 12 63 60 15 40

PSA!-'.MITE

MI CASCHISTI 1 2 4

QUA RT Z HI CA SCHI ST

MYLONIT I C I

QUARTZ I TE

CALCSILICATE 45 8 10 4 13 21 32 1 2 3 25

LIMESTONE 2 1 <5 10

AMP Hl BOL IT E 2

VEIN QUARTZ 3 7 6 72 2 9 2 18 7 6 17 40 15 15

GRANITE I 1 2 5 37

GNEIS S 1 2

PEGMATITE 1 4

FELDSPAR

RHYOLI TE 2

Table3.Conglomerate c1astlithologies (percentages).Asteriks denoteless than 1percentof a lithology,Svarthovda analysis 1 is in thebasalbreccia,10m aboveHeidalbase ment;Svarthovda analysis2is intheTo conglomeratefacies.25m aboveHeidal basement;Svarthovdaanalysis 3 isin thetalc-magnesitematrix.mass-flowconglomerate;Svarthovda analysis4isin the To conglome- ratefacies20mabovethemass-flow;Svarthovda analysis5isinthe Skardshoi conglomera tefacies150m aboveHeidalbasement; Skarvangsretrin analyses1- 3areintheTo conglomerate facies;Toanalyses1-2arein theToconglomerate facies;Gauklihaugen isin theToconglomerate facies;Svarttjernis in theTo conglomerate facies;Tristeinenisin theOtta conglomeratefacies ;Adalenis intheSkardsho i conglomerate faciesnorth of Svarthovda :Fjelltithoi localities1-2arein theToconglomerate facies.SeeFig.2for location ofthedifferentsites,

similar (Nemec 1990, Prior & Bornhold 1990).

Raggehaugen and Tristeinen

Descrip tion

The Otta conglomeratefacies iswell exposed on the small hill Raggehaugen (UTM 052508) (Fig. 2, locality 1),where it occurs above metagabbros and serpentinitic ultra- mafics of the ophiolite complex (Tables 1 and 2). The conglomerate has a monomict composition of serpentine c1asts, variably altered totalc and magne site,in a matrix of coarse- to very coarse-grained serpentine/talc sandstone. Conglomerates usually show an upwardfiningtrendintoconglome- ratic serpentinitic sandstones (Figs. 3 and

5a) intheiruppermost parts (Table 2).The sandstonebeds typically wedge outover5- 30 m, and frequently show plane parallel- stratification and trough cross-stratification (Fig.5b) (Table2).

OnTristeinen(UTM 980534)(Fig.2,locality 4)the Otta conglomeratefacies(Table 2) is a near monomict serpentine conglomerate with sporadic gabbro andgreenstonec1asts (Fig. 6,Table 3).The conglomeratedirectly overliesmyloniticquartziteandpsammiteof theHeidalGroup(Table 1).

Interpretation

The conglomerates at Raggehaugen and Tristeinen arethought to be lateral equiva-

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NGU-BULL425,1993 ReidulvBee,BrianA.Sturt&Donald M.Ramsay 9

Fig.6.Massive Ottaconglomerate faciesatTristeinen.Vertical side of photograph is 0.6m.SeeFig.2forlocation.

lents of one another, deposited on top of previously eroded ophiolitic rocks .~n d

psammites, respectively. The composition of the conglomerates indicates derivation from a nearby source area with abundant ultramaficrocks in the basement. The coar- seness ofthe detritus suggests a relatively short distance of transport from the source area, although the rounding of the c1asts reflects a thorough reworking. This may have beenachievedon ashoreline orinan alluvialenvironment,prior to deposition.

The conglome rates at Tristeinen and many ofthe conglomerates at Raggehaugen were probably emplaced by debris flows. Becau- se of the relatively common occurrence of normal, inverse-to-normal and inverse-graded conglomerate beds, clast imbrication, the presence of mud/silt cappings and a common upward increase in matrix content (featurestypicalfor subaqueousdebrisflow deposits (Nemec & Steel 1984)),we prefer to interpret these deposits as subaqueous, Erosional scours, planar stratification and trough cross-stratification, as observed at Raggehaugen,and c1ast-supported textur~,

as observed at Tristeinen, are common In fluvial/alluv ial fan deposits (Jolley et al.

1990). However, these features are also common on fan deltas and in submarine gravity flow deposits, e.g. on submarine fans(Nemec&Steel1984).

Thick sequences of conglomerate in shallow-marine settings are likelyto be the pro- duct of a pronounced fluvial flooding out from fan-deltaic or deltaic systems (Nemec

& Steel 1984,Prior& Bornhold1990).Typi-

cal reworked beachface orshoreface conglomerates (Nemec & Steel 1984) were not observed;however,a fan-delta (Kleinspehn et al. 1984, Nemec 1990, Prior & Bornhold 1990) interpretation of the sediments at Raggehaugen and Tristeinen is a possibil- ity. Another possible interpretation.of.the sediments at Raggehaugen and Tristelnen could be that they represent proximal submarine fan channel deposits. Ineson (1989) has described coarse-grained submarine fan and slope apron deposits similar to the conglomeratesatRaggeha ugena~?Tristei- nen,and ascribes these to deposition from debris flows and high-density turbidity currents. Because of the lack of sedimentary structures indicating wave reworking we prefer to interpret these sediments.as _s~b

marinefandeposits.The decreaseIn qrarn- size from Tristeinen eastwards towards Raggehaugen may be interp ~eted as ~

change from proximal tomore distaldeposi- tion,but the change may also be relatedto local topography and sediment dispersa l patternsatthe time of deposition.

Svarthovda area

Description

Remapping has demonstrated that the conglomerates on Svarthovda occupy the same structural and stratigraph ic level as the Otta conglomerate facies and can be correlated with it. These conglomerates are well exposed on the mountain Svarthovda and on the eastern slopes towards the river Skjerva (Fig. 2, localities 16 and 17). The best exposu res occur at UTM 982702, where a sedimentary breccia rests uncon- formably on banded calc-silicate rocks of the HeidalGroup (Table 1).This unconformity can betraced southwa rds, where it is very well exposed despite strong folding (Fig. 7).At UTM 982702 thelowermostpart of the Sel Group consists of a 10 m thick, clast-supported, polymict breccia (Table 3, Svarthovde analysis 1). Angular and subangular slabs of calc-silicate rocks, up to 1.5 mlong and identicaltothe rocks o.fthe immediately underlying substr ate,dominate inthe lowermost part ofthebreccia.

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10 ReidulvBoe,BrianA.Sturt&DanaldM.Ramsay

Fig.7.Folded unconformitybetween HeidalGroup calc-silicate rocks(upperpart of photograph)and To conglomeratefacies at Svarthovda.lens capis 6 cm in diameter.SeeFig.2for location.

The breccia issucceededby approximately 20 m of c1ast-supported, polymict conglomerate (To conglomerate facies), withgre- enstone and psammite as the dominating c1ast lithologies(Table3,Svarthovdeanaly- sis2).This has aquartz-mica schist matrix with cobbles and boulders up to 50 cm in diameter.The bouldersare commonly rounded,althoughgenerally they aresubangular to subrounded. Upwards through the conglomerate thereis anincreasing proportion of talc in the matrix.

This conglomerate is abruptly overlain by a 10 m thick, matrix-supported, polymict conglomerate (Table 3,Svarthovdeanalysis 3).Psammite and greenstone dominatethe c1ast population,andthec1asts areupto 50 cmlong.Pebbles,cobblesand boulders are evenlydistributedinatalc-magnesitematrix (Fig. 8a).There is an almost bimodal size distribution with cobbles in a very coarse-

GU • BULL 425.1993

grained sandstone matrix.This unit is succeeded abruptly by a mafic-matrix, green conglomerate with c1asts of predominantly quartzite,greenstone and gabbro (Table 3, Svarthovde analysis 4) (To conglomerate facies).The matrix contains ahigh proportion of talc, becoming more amphibolitic upwards. Clasts in the conglomerate reach 75cmin diameter,butrarelyexceed50cm.

There is a gradual transition from the To conglomerate facies into a conglomerate with matrix varying from quartz-schist to schistose quartz sandstone, and with pebbles of quartzite and vein quartz (Table 3, Svarthovde analysis5) up to 10 cm in diameter (Skardshoiconglomeratefacies) (Fig.

8b).

Eastwards, towards the Skjerva River,these two facies alternate. At Skarvanqseetrin (Fig. 2, locality 33) (UTM 992693) the To conglomeratefacies isc1ast-supported,with a quartzitic matrix and subangular to subrounded clasts up to 50 cm long (Fig. 8c).

The clastcomposition of the To conglomerate facies at Skarvanqseetrin is shown in Table 3.The c1asts at this locality dramati- cally illustrate pre-pebbletectono-metamorphic fabrics, indicating derivation from an earlier metamorphiccomplex. Thesefeatu- res include pre-pebble foliations, folds, quartz veins and metamorphic minerals cut bypebblemargins.

InKrossdalen,500 m north of Skarvanqsas- trin, the Skardshoi conglomerate facies contains beds of yellowish limestone and calcareous sandstone. The succession shows well preserved sedimentary structures, in particular channels and erosional scours, planar and trough cross-stratification and thin, intraformational mud-clast conglomerates.

Northwest of the mountain Fjelltithoi (UTM 012686) (Fig. 2, locality 19) there is a well exposed succession of conglomerates (Table 2), with clasts in a quartz-mica schistor phylliticmatrix.The conglomerates have apolymictcomposition(Table 3)remi- niscent of the To conglomerate facies, al- thoughthec1astpopulation isusuallydomi-

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NGU-BULL 425.1993 ReidulvBoe,BrianA.Sturt&DonaldM.Ramsay ¹¹

Fig.8.a)Mass-flow conglom erate(To conglomerate facies )at Svart hovda.Visiblepartofhammeris 20 cm long.b)Skards - hoiconglomeratefadesatSvart hovda.Hammeris 30 cm long.c)To conglomerate facies at Ska rvangsretrin.Ham mer is 30 cm long.SeeFig 2.for location.

nated by quartzite and vein quartz. Some conglomerate beds have a yellowish colour, and containahigh percentage oflimestone clasts (Table 3), up to 10 cm long. The matrixof these conglomerates is generally enrichedincarbonate.

Interpretation

The lowermost part of the succession at Svarthovda is interpreted as a basal brec-

cia. The predominance of very angular c1asts of rocks of local basement affinity indicates only a short distance of transport, priorto deposition.Theoverlying clast-supported To conglomerate facies contains more rounded c1asts, interpreted as reflec- tinggreater reworkingand greater distance of transport prior to deposition.The upward increase in talc content of the matrix suggests an influx of material from a more dis- tant location and downward erosion into ultramafic rocks in the source area. The lack ofsedimentary structures and fine-grained sediments in the breccia and the conglomerate indicates a sedimentary environment dominatedby rapidmass-flowdeposi- tion. The depositional environment could have been either subaerial (alluvial fan) or subaqueous (fan-delta or submarine fan) (seebelow).

The overlying 10 m thick,matrix-supported conglomerate is interpreted as a mass-flow deposit (Sturt et al. 1991).The homogene- ous appearance suggests that thisunitwas deposited as a debris flow (e.g. Nemec 1990), although it could alternatively be classified as a mudflow deposit (Curry 1966).

Thedecrease in matrix-serpentine upwards through the succeeding To conglomerate facies may reflect a decreasing sediment supply from ultramafic source rocks, either because ultramafic rocks were no longer available forerosion or becauseofashift in thesediment dispersalpattern.Thisdecrea- se could also be a result of morethorough reworkingof the sediments,withremova lof the softer components, prior to deposition.

The latter suggestion is supported by the occurrence of mature, well sorted Skards- hei conglomerate facies, enriched in vein quartz (Table 3),on top of the To conglomerate facies. The main source rock of the Skardshoi conglomerate facies is interpreted to have been Heidal quartzite. These sedimentswere probablyextensively reworked, possibly in a shore environment, prior to deposition ina marinesetting.

It is possible that the lowermost part of the succession at Svarthovdawas depositedin

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12 ReidulvBoe,BrianA.Sturt&Donald M.Ramsay

a subae rial setting,but no marine flooding surface or transgressive lag has beeniden- tifiedto support this suggestion.Ourinabili- ty toidentify a marineflood ing surface may, however, be due to the relatively strong deform ationof these deposits.Alternatively, the lowermost part of the succession at Svart hovda may have been deposited in a fan delta environment or as channel fill on the proxima lpart of a submarine fan.Coar- se-grained fan deltasediments,very similar to the depositsatSvarthovda and deposited by debris avalanching and inertia flows, have been described by Prior & Bornhold (1990). During high-energy river floods, coarse-textured debris may plunge direct ly into the sea to be deposited on the subaqueous fan.

The gradual transition fromSkardshoicong- lomerate facies to quartzitic sandstones, calcareous sandsto nes and,insome cases, limestonesprobablyreflects a transgression and gradual transition to deeper marine conditions, dominated by turbidite deposition .Thelimestonesmay represent calcare- nite turbidites, with calcareo us material transport ed from a shallow shelf area, or calcareous oozes deposited betwee n the turbidites. Limestone c1asts in the conglomerates at Fjelltithoiwere probably eroded from a shallow shelf, where deposition of calcareous sediments occurred.

0yagarden - Gronlii

Descrip tion

The Otta conglomerate facies is well exposed inanapproximately E-W strikingsyncli- neat 0yagarden and Gronlii,alongtheriver Finna (UTM 965665) (Fig. 2, locality 14) (Table 2).At thislocalitythelocalbasement compr ises black schists and psammites of the HeidalGroup (Table1).The success ion is composed principally of conglomerates, but there are also many interv als of sandstone and conglomeraticsandsto ne (Fig.9).

The conglomerate has an almost monomict compos ition, with serpentine c1asts in a matrix of serpentine/talc sandstone.

A 2-3 m thick horizonof coarselycrystalline limestoneispresentin the lowermost part of the succession at 0yagarden. Its upper,

GU•BULL425.1993

Fig.9. WellbeddedOtta conglomerate faciesat Gronlii. ote stratificationanimbrication ofserpentine clastsindicating pala- eocurrentstowardstheleft(east).Verticalsideof photograph is 75cm.SeeFig.2forlocation.

sharp and undulating surface is well expo - sed. Numero us rounded serpentine clasts up to 10 cm in diameter are embedded in theuppermost20 cm of the limestone.The- se c1asts have a composition similarto tho- sein the overlyingconglom erate.

Interpretation

Thesedimentary rocksat 0yagarden-Gmn- lii are derived from ultramaficsource rocks.

Thewellrounded nature of c1asts show that these were subjec t to transport and reworking priorto deposition in theirpresent stratigraphic position.The relativelywell sorted natureof thedeposits,andthe presence of scours,clast imbrication,planeparallel and trough cross-stratification, indic ate deposition in an environment withstrong currents.

The sediments can possibly be interpreted as submarinefan braidedchanneldeposits.

Surlyk (1984) and Ineson (1989) have described deposits very similar to the deposits at 0yagarden-Gmnlii, showing complex

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NGU-BULL425.1993 ReidulvBoe,BrianA.Sturt&DonaldM.Ramsay ¹³

multiple scouring, laterally discontinuous bedding,scarcityoffinesandlack of syste- matic internal organisation, and ascribe themtodepositioninbraided,axialdistribu- tary channels of submarine fans. Shanmu-

gam & Moiola (1991) described similar

deposits, composed of conglomerate and pebbly sandstone,and interpretedthem as braidedchanneldepositson mid-fan lobes.

The sediments at 0yagarden-Gm nlii could alternatively be interpreted as the deposits of a braided fluviatile system dominated by stream flowandstream flood (Flint&Turner 1988,80e & Sturt 1991) processes. Howe- ver, the presence of upward-fining gravel beds (interpreted as small mass-flows) and the general predominance of matrix-supported texturepointtowards subaqueous deposition.

Svarttjern

Description

On the northern face of the ridge north of Svarttjern (Fig. 2, locality 21), coarse-grained garbenschiefer with bandsof quartzite and quartz schistof theupperHeidalGroup is overlain with angular unconformity by dark green To conglomeratefacies (Tables 1,2and3) (Fig.10).The contactisexposed atonlyone locality (Fig. 10b),butelsewhere the gapin outcrop is onlycentimetric insca- le. Despitemoderate deformation in the To conglomerate facies itisnot highenough to explain the absenceof ophioliteasa conse- quenceoftectonic excision

The greenmatrixofthe conglomerateisrich in amphibolite,talc and chlorite.Immediate- ly above the unconformity, conglomerate beds are interbedded with medium-grained green sand to conglomeratic sand (Fig.

10a) (Table 2). This conglomerate has a crescentic outcrop, overlapped to the east and west by athick development of mono- mictOtta conglomeratefacies inthe coreof the Svarttjern synform (Sturt et al. 1991).

The Otta conglomerate facies is typically massive, matrix-supported and poorly sorted. Clastsare well-rounded to sub-rounded with maximum diameters up to 30 cm but averaging 10 cm.

Fig. 10.a) To conglomerate facies withbeds of laminated sandstone atSvarttjern.b)Unconformitybetween Toconglo- meratefacies (left) anHeidalGroup(right) atSvarttjern.Ham- meris 30 cm long.SeeFig.2for locatiion.

Interpretation

Although tectonically distorted, the pebble shapes of the To conglomerate facies sug- gest a well-rounded, pre-deformation sha- pe. This, together with the dominance of quartzite, suggests greater reworking and greater distance of transport priorto deposition. The general absence of sedimentary structures suggests deposition as mass- flows, possibly in a submarine fan or fan delta setting.The overlyingOtta conglomerate facies has a coarse c1ast sizesuggest- ive of a short distanceof transport.

Gauklihaugen

Description

On the ridge Gauklihaugen, to the northwest of Otta (UTM258508to 259506) (Fig.

2, locality 25),the To conglomerate facies is present above rocks of the ophiolite complex (Table1).The sedimentary succession starts with a 1-2 m thick, black phyllitic schist that can be traced southwards

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14 ReidulvBGe,BrianA.Stun&DonaldM.Ramsay _NGU_._BU_{LL 425.}₁₉₉₃

Fig. 11.a)To conglomeratefacieswithbedsoflaminated sands toneandgradedpebble-sizeconglomerateon top of granite gneiss boulders at Gauklihaugen.Right way up.Visible part of hammer is 20cm long.b)Bedsofcalcareoussandstone,limestone andphyl- liteatGauklihaugen.Noteplaneparaliellaminationand troughcross-lamination.Key is5cm long.SeeFig.2 forlocation.

towards Nedre Josten.On top of the phyllitic schist there is an approximately 60 m thick interval of talc schist, sandstone and pebble-size To conglomeratefacies.

These sediments are succeeded by appro- ximately35mof fineand coarse To conglomerate facies with minor sandstones and schists (Table 2) (Fig. 11a). The coarse conglomerates are usually poorly sorted, while the pebblyconglomerates (c1 asts typi- cally1-2 cm in diameter)may be wellsorted and have a bimoda l size distribution, with clastsembedded in coarse- to verycoarse-

grained sandsto ne.A typical cla s t co mposi-

tionis indicated inTable 3.

The intervalof fineand coarseconglomera- tes is succeeded by 8 m of conglomeratic sandstone and fine conglomerate. This is followed by approximately 30 m of fine conglomerates (Skardshoi and To conglomerate facies) which alternatewith sandsto-

nes, phyllitic schists (often with talc) and thin limestones. The sandstone beds are typically2-15cm thick (Fig.3)and wellbed- ded. The sandstones are pale grey, medium- to coarse-grainedand wellsorted,and show upward-finingmotifs,as wellas plane parallel lamination andtrough cross-lamination. Their lower surfaces are sharp and undulating. The boundaries with the overlying brown schists are usually abrupt (Fig.

11b). Individual beds in these schists are typically up to5 cmthick and laterallycont- inuous. Brown limestones are up to 15 cm thickand containthinlayers of darker brown

sc hist .

Interpretation

The compositionof the c1astsin the conglo- meratesofGauklihaugen indicatesasource forthesesediments in the ophiolite,rocksof the HeidalGroup and the graniticbasement gneisses.The fine-gra inedsedimentsat the base of the successionwere probablyorigi-

The conglomerates of the Sel Vaga area, Central Norway: an terrane-linking succession.