NGU · BULL 427.1 995 B. A. Stutt, R. ee« D. M. Ramsay & T. Bjerkg~ rd 25

Stratigraphy of the -Vaqa tract and regional stratigraphic implications

8RIAN A. STURT. REIDULV 80E, DONALD M. RAMSAY & TERJE BJERKGARD

Brian A. Sturt & Reidulv Bee, Norges geologiske unaersokelse, P.O.Box 3006 · Lade, 7002 Trondheim, . Donald M. Ramsay, Geology Departmen t, University of Glasgow, GLB2 BOO Glasgow, Scotland. Terje Bjerkg~rd, Institutt for geologi, Universitetet i Oslo, P.O.Box 1047, Blindern, 0316 Oslo, Norway.

Sturt et al. (1991) and Bee et al. (1993) have ve the unconform ity, and the logs are presented described a first-order stratigraphic unconformity as Fig. 2. beneath the Group in the Otta-Vaga map The log from the stream Verkesae (Fig. 2) areas (1:50,000 sheets 1618 I, 1718 I). This shows the basal sediments to be typical schists unconformity separates the Sel Group from a and phyllites of the Sel Group reposing on polyp­ substrate of the Vagamo Ophiolite which was hasally folded semi-psammites of the already thrust in, on the Ottadalen Thrust, onto a Group, replete with garnets up to 1.5 cm in dia­ basement-cover couplet of the Rudiho Gneiss meter. The Sel schists and phyllites have dis­ Complex (Baltic continental crust) and the tinctly preserved, delicate bedding structures unconformably overlying cover succession of the and pass up into well-bedded sandstones which Heidal Group. The Heidal Group had undergone preserve sporadic examples of cross-bedding a complex tectonometamorphic evolution , and showing downward facing. The Sel sediments the ophiolite was folded, prior to uplift, deep ero­ show an initial coarsening-upward succession sion and deposition of the Sel clastic wedge culminating in a massive coarse conglomerate which includes continental, fan-deltaic and mari­ some 150 m thick. This is a poorly sorted, clast­ ne sediments. The sub-Sel unconformity and supported conglomerate of the Skardshoi type conglomerate composition indicate that the Sel (Bee et al. 1993) containing sub-angula r clasts Group is a terrane-Iinking succession , showing with a maximum c1ast diameter around 20 cm. that the Vagamo Ophiolite had already been The c1asts comprise mainly quartzite, psammite, obducted onto a former westward extension of semi-psammite (sometimes garnet-bearing), the Fennoscandi an Shield prior to the Scandian calc-silicate gneiss, vein quartz, brown limesto­ thrusting of the Otta Nappe. During fieldwork in ne/dolomite and banded amphibolite. These the summer of 1994, a perfect example of the c1asts preserve abundant evidence of pre-pebble unconformity between the Heidal Group and ser­ deformation and strongly reflect the lithologies of pentine conglomerate (Otta conglomerate facies) the Heidal Group. In addition there are c1asts of was discovered at Gronlii (fig. 2 in Bee et al. basic and acidic volcanic rocks which show no 1993). The conglomerates rest on an irregular signs of pre-pebble deformation as do a number surface cut into a substrate of polyphasally of c1asts of what we interpret as metamorphose d deformed Heidal calc-silicate gneisses and ferrosaprolite.The overall characteristics of the psammites , though at this locality no indication of log are of a fine-graine d succession (fine-grained preserved regolith is present. sandstones , schists and phyllites) making up to The northeastward extension of the mapping 50% of the section, limestone 5% and conglome­ into map-sheets Hjerkinn (1 519 Ill) and Folldal rate 40%, producing an almost bimodal (1519 11) has allowed the unconform ity to be tra­ fine/coarse succession. At various levels in the ced into these areas, and provides additional evi­ succession , well-preserved sedimentary structu­ dence as to the nature of this unconformity. The res (cross-bedding, channels , grading, etc.) can unconformity can, in fact, be traced from the be observed, all indicating a consistent down­ Vaga-Otta tract across the river Lagen and map­ ward-facing stratigraphic polarity and indicating ped semi-continuously along the northside of at best only limited repetitions by folding. Grimsdalen until it is exposed in the streams The log from the stream Buae shows many Verkesae and Buae (Fig. 1). The mapping shows similar facies, although here schists and phyllites that the unconform ity is stratigraphically down­ represent some 55% of the section, conglomera ­ section from the Folldal volcanite (Fundsje te 15% and limestones (dolomites) 10%. A major Group), although the rocks are structurally inver­ difference in this log is that lavas, tuffs and volca­ ted. The streams Buae and Verkesae were stra­ niclastic sandstones , with interbedded schists, tigraphically logged, each for some 1.25 km abo- phyllites and thin limestones/dolomites , occupy a 26 B. A. Sturt, R. Boe, D. M. Ramsay & T. Bj erkg i!J rd GV - BULL 427. 1995

LEGEND JOTUN NAPPE COMPLEX onA NAPPE SUB-OnA NAPPES

Sel Group I conglomerate ~ f>;::::::j ~~~a:::~: :::ary cov er r.:.. j Metasedimenta ry cove r Vi\g Amo Op hiolite .. ..~ Orthogneis ses -"---L Sub-Jotun thrusts OttadaJen Thrus t Cl Helda l Gro up ~ RUdlh" Gneiss Complex ....t:...... A. ana Thr ust

Fig. 1. Geological map of the VAgA-Ona·Grimsdalen area. little over 200 m of the column. The lavas are Folldal volcanite (Fundsjo Group). Similar fin­ greenstones with scattered crude pillows. though dings were made by Bjerkqard & Bjorly ke (1994 the thin tuft horizons are bimodal basic/acidic a & b) where they describe bimodal basic/acid types. This represents the first appearance of the tufts in the Asli Formation (traditionally the upper NGU - BULL 427, 1995 B. A. Stun, R. Boe, D. M. Ramsay & T. Bjerkg/1fd 27

part of the Gula Group), beneath the main Folldal VERKESAE BuAE volcanic succession. This led Bjerkqard & Bjorlykke (op.cit) to propose that the Asli Formation is not part of the Gula Group (Heidal equivalent) and that the top of the Gula Group is 1200 probably at the contact between the Asli and Sinqsas Formations. Reconnaissance mapping by two of us (D.M.R. and BAS.) has indicated that the top of the Gula Group is, at least in part,

at a somewhat lower level than this formational 1000 =:.-; boundary. The comparison between the logs of the two streams, separated only by 3-4 km, shows consi­ 900 derable lateral facies variation, though the main conglomerate and limestone/dolomite horizons can be recognised. Such rapid facies changes 800 - have been emphasized in Boe et al. (1 993) in their analysis of the lower part of the Sel Group. The basal deposits of the Sel Group in Grimsdalen are interpreted as marine. During the mapping, a major but impersistent horizon of metamorphosed ferrosaprolite inclu­ 600 ding, near its top, local developments of meta­ morphosed lateritic rocks has been identified soo immediately beneath the basal deposits of the Sel Group. These rocks may represent relics of a formerly extensive blanket of regolith produced by chemical weathering, e.g. resembling that developed during the Tertiary in western Australia (Anangetal 1993). The regolith repre­ sents an alteration of various members of the Heidal Group. Such regoliths in their unmetamor­ phosed state are dominated by clay minerals 200 which, during the erosion of the uplifted terrane, will be rapidly re-sedimented to produce clays, 100 marls, etc. This process would provide a logical explanation for the pattern of fine-/coarse-grai­ ned sediments in the basal Sel Group; the fine­ grained sediments representing, to a large ~ ~ ~ :3 8 t:S extent, re-sedimented fines from the regolith and lL ::E O the conglomerate inputs relating to tectonic movements and fault-scarp evolution in the hin­ ~ Sc.~lsVphyih 1 e terland. The conglomerates of the Grimsdalen !==1 1::.\ I VoIcanics

area are interpreted as mass-flow deposits in a I·.;..j... ..I Sandstone/schist ~ Limestone marine succession, possibly resulting from epi­ o Sandstone I ~~!~~·~ I Conglomerate sodic sediment transport from the adjacent land area (see also Boe et al. 1993). The major diffe­ ...- Cross·bedding rence in thickness of the conglomerates between Fig. 2. Simplified stratigraphical sections through the lower­ Verkesae and Buae probably indicates that the most part of the Sel Group at verkesae and BuAe in Verkesae section, at the beginning of deposition, Grimsda!en. The sections start at the boundary between the was closer to the centre of a pre-Sel palaeoval­ Heidal and the Se! Groups. See Rg. 1 for location of the secti­ ley. The limestones/dolomites with sigmoidal ons. cross-bedding in the upper part of the successi­ on are predominantly shallow-marine deposits, though water depths during deposition probably (Fundsjo Group) are, in fact, up-sequence deve­ varied due to tectonic instability and eustacy. lopments within the Sel Group is of considerable The discovery that the Folldal volcanite importance and indeed gives more precision to 28 B. A. Sturt, R. Boe, D. M. Ramsay & T. Bjerkgl1rd NGU . BULL 427, 1995 the dating of the major unconformity. Boe et al. References (1993) show the age control of the unconformity Anand, R.R., Phang. c., Smith, RE & Munday, T.J. 1993: The to have an upper limit at the Arenig/L1anvirn regolith and its exploration and economic significance. In boundary based on the Otta fauna . Recent U/Pb Williams, P.R & Haldane, JA (eds.) An international con­ ference on crusta! evolution, metallogeny and exploration zircon dating of the Folldal Trondhjemite has of the Eastern Goldfields. Excursion Guidebook. Record given an age of 488 :!: 2 Ma (Bjerkqard & 1993/53. Australian Geological Survey Organization, Bjorlykke 1994a), i.e. a date virtually at the Canberra, 75-100. Tremadoc/Arenig boundary, thus providing a Bjerkgard, T. & Bjorlykke, A. 1994a: Geology of the Folldal area, Southern Trondheim Region Caledonides, Norway. more accurate upper age constraint for the Nor. geol. unders. Bull. 426, 53-75. unconformi ty. The Vagamo Ophiolite must thus Bjerkqard, T. & Bjorlykke, A. 1994b: The stratabound sulphide be older than this boundary , though probably deposits in the Folldal Area, Southern Trondheim Region, younger than ca. 505 Ma. The latter age is Norway. Nor. Geol. Tidsskr. 74, 213-237. Bjorlykke, A.. Vokes, F.M., Birkeland, A. & Thorpe, R I. 1993: shown by Mork et al. (1988) to indicate the Lead Isotope Systemat ics of Strata-Bound Sulfide Deposits timing of eclogite-facies metamorphism in the in the Caledonides of Norway. Econ. Geol. 88, 397-417. Seve Nappe. This metamorphism is a reflection Boe, R , Sturt, BA & Ramsay, D.M. 1993: The conglomerates of the subduction of part of the Baltoscandian of the Sel Group, Otta-Vaqa area, Central Norway: an example of a terrane-Iinking succession. Nor. geol. unders. miogeoclinal assemblage during Late Cambrian / Bull. 425, 1-24. Early Ordovician times, probably resulting in Mork, M.B.E.. Kullerud, K. & Stabel, A. 1988: Srn-Nd dating of ophiolite obduct ion. Seve eclogites, Norbotlen , Sweden - evidence for early Caledonian (505 Ma) subduction . Contrib. Min. Petrol. 99 , 344-351. Discussion Sturt, BA , Ramsay, D.M. & Neuman, R.B. 1991: The Orta The major terrane-linking unconformity at the Conglomerate, the Vagamo Ophiolite - further indications base of the Sel Group can be traced through of early Ordovician Orogenesis in the Scandinavian Grimsdalen into the Folldal area. The minimum Caledonides. Nor. Geol. Tidsskr. 71,107-115 . age constraint for the unconformity is at the Tremadoc/Arenig boundary . Reconnaissance mapping northwards into the western Roros regi­ on shows that this pattern persists. The Folldal volcanite (Fundsjo Group) are demonstrated to represent volca nic effusion into the Sel sedimen­ tary basin and to have developed above conti­ nental crust. Bjorlykke et al. (1 993) have shown how the signature of the Pb-isotopes in sulph ide ores hosted by the volcanite of the Fundsjo Group, in the Folldal-Roros tract, reveal conside ­ rable contamination by continental crustal rocks of Fennoscandian Shield type, which is certain ly compatible with the results of this study. The stratigraphic results cast considerable doubt on the status of the so-called Gula Group and it is obvious that a major revision of the upper limit of the Gula Group must be made.