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The ¢Aravarri Formation of the Kautokeino Greenstone Belt, Finnmark, North Norway; a Palaeoproterozoic Foreland Basin Succession

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The ¢Aravarri Formation of the Kautokeino Greenstone Belt, Finnmark, North Norway; a Palaeoproterozoic Foreland Basin Succession NGUBull442_INNMAT 23.11.04 09:54 Side 5 TORE TORSKE & STEFFEN G.BERGH NGU-BULL 442, 2004 - PAGE 5 The ¢aravarri Formation of the Kautokeino Greenstone Belt, Finnmark, North Norway; a Palaeoproterozoic foreland basin succession TORE TORSKE & STEFFEN G. BERGH Torske, Tore & Bergh, Steffen G. 2004: The ¢aravarri Formation of the Kautokeino Greenstone Belt, Finnmark, North Norway; a Palaeoproterozoic foreland basin succession. Norges geologiske undersøkelse Bulletin,442,5–22. The ¢aravarri Formation is the youngest supracrustal unit in the Palaeoproterozoic Kautokeino Greenstone Belt of western Finnmark.This greenstone belt and correlative units at Alta farther north were formerly ascribed to a conti- nental rift setting near the northern margin of the Fennoscandian Shield.The ¢aravarri Formation is metamorpho- sed at very low grade and made up of c. 4000 m-thick, upward-coarsening, siliciclastic sandstones and conglomera- tes deposited in a presently NNW-SSE-trending basin. It rests with depositional contact on the underlying argillitic Bik’ka∏åk’ka Formation. The eastern boundary is an east-dipping thrust, with a hangingwall block of older, more highly deformed and metamorphosed metasupracrustals. The ¢aravarri Formation is subdivided into three lateral segments, from south to north, the Njargavarri, Gæsvarri and ¢aravarri segments, separated by inferred faults.The facies associations reveal: (i) an extensive lower conglome- ratic member (unit 1), present in all segments and interpreted as subaqueous debris-flow deposits, (ii) pebbly sand- stones and conglomerates interpreted as shallow-marine turbidites, debris flows or fan-delta deposits with locally derived carbonate clasts, which dominate in the Njargavarri segment and lower part of the Gæsvarri and ¢aravarri segments, and (iii) conglomerates and channelled and cross-bedded sandstones interpreted as prograding alluvial fan and fluvial trunk-river deposits that characterise the northern, ¢aravarri segment. Uniform transport directions from an easterly source can be demonstrated for the fluvial and alluvial successions. Distinct differences in clast maturity and various proportions of quartzite, jasper/chert, and carbonate clasts, suggest a younger age of clast deposition when moving northwards. The ¢aravarri Formation is reinterpreted as a molasse-type succession, deposited in a foreland basin that reflects changing provenance and depocenter migration through time. In the south, the basin was underfilled, with subsi- dence-induced, relatively high-gradient topography generating fan deltas with repeated mass flows and turbidity currents. In the central and northern segment, the basin became overfilled, i.e., fluvial trunk-rivers were overridden by large, westward-prograding alluvial fans with clasts derived from a more distal source, probably the uplifted, Svecokarelian thrust belt to the east. The lateral segmentation of the ¢aravarri basin may have been controlled by frontal transverse faults of the thrust belt. The ¢aravarri Formation is tentatively correlated with the Kumpu Group (<1.88 Ga) of Central Finnish Lapland,which overlies the Palaeoproterozoic Kittilä Greenstone Complex.The Kumpu Group is now interpreted as a foreland basin, molasse-type unit deposited after the Svecokarelian orogeny. Such an interpretation and tectonic setting support derivation of the ¢aravarri Formation from the Svecokarelian collisional orogen of eastern Finnmark, during which time the Lapland granulite belt and related allochthonous units were thrust westward onto the Karelian craton. Tore Torske & Steffen G. Bergh: Department of Geology, University of Tromsø, N-9037 Tromsø, Norway. Tel. 77644464, e- mail: [email protected] Introduction and geological setting cover, the Skuvvanvarri and Masi Formations (Fig. 1; The ¢aravarri Formation is the youngest supracrustal unit in Siedlecka et al. 1985). the Kautokeino Greenstone Belt of western Finnmark, North The rocks of the ¢aravarri Formation compose a 3-6 km Norway (Fig. 1; Holmsen et al.1957, Olsen & Nilsen 1985, wide,NNW-SSE-trending,linear mountain ridge in the north- Siedlecka et al. 1985, Solli 1983,1990, Olesen & Sandstad western part of the Finnmarksvidda plateau.It rises to 300 m 1993). Together with the Karasjok Greenstone Belt, farther above the generally more subdued and heavily covered east, the Kautokeino belt forms the bulk of the areas on both sides. The ridge is well exposed for about 25 Palaeoproterozoic cover rocks; major units of tholeiitic km along strike, from lake ¢arajav’ri in the north to the hill metabasalts, tuffaceous greenstones and amphibolites Lulli Lik’∏a in the south (Fig. 2). The formation has a sharp, alternating with metasedimentary rocks. This pile of conformable contact with underlying units to the west, but supracrustals rests upon crystalline basement within the is thrust-bounded to the east (Fig. 2). The ridge is also seg- Karelian craton near the northwestern margin of the mented by transverse faults. Lithologically, the formation Fennoscandian Shield.These greenstone belts are separated consists of well preserved, very low-grade metamorphic, by a structural basement dome of Archaean to siliciclastic sedimentary rocks, with a stratigraphic thick- Palaeoproterozoic tonalitic gneisses and granites:the Jer’gul ness of up to about 4000 m; its bedding dips consistently Gneiss Complex and its par-autochthonous sedimentary eastward (Fig. 2). NGUBull442_INNMAT 23.11.04 09:54 Side 6 NGU-BULL 442, 2004 - PAGE 6 TORE TORSKE & STEFFEN G.BERGH Fig. 1. Simplified geological map of western Finnmarksvidda and the Alta-Kvænangen tectonic window. Location of the ¢aravarri Formation is shown, in the rectangle (from Siedlecka et al. 1985). The units flanking the ¢aravarri Formation all belong to deformed mafic intrusive rocks and a lens-shaped lherzolite the Kautokeino Greenstone Belt (Fig. 1); they include: (i) to body are also present in this zone (Fig.2). the west, the conformably underlying, steeply east-dipping, The ¢aravarri Formation can be correlated with the 1000–1500 m thick, argillite-siltstone dominated, Skoadduvarri Sandstone Formation at Alta (Holmsen et al. Bik’ka∏åk’ka Formation (Siedlecka et al.1985).This formation 1957, Barth & Reitan 1963, Zwaan & Gautier 1980, Torske & is in turn underlain by mafic metavolcanic rocks of the Bergh 1984a,b, Bergh & Torske 1986), the youngest unit of Cas’kejas Formation, including the Stuorajav’ri and the Raipas Supergroup in the Alta-Kvænangen tectonic win- Suoluvuobmi Formations (Siedlecka et al. 1985); (ii) to the dow within the Caledonian nappe area (Fig. 1). east, the ¢aravarri Formation is bounded by a moderately to steeply east-dipping thrust system (Fig. 2) with hangingwall rocks of the Lik’∏a Formation, comprising greenschist-facies Previous work and models metabasalts similar to those of the ¢as’kejas Formation, and The rocks of the ¢aravarri Formation had not been reported a few metasandstones. No thrusts are recorded west of the in the geological literature until Holmsen et al. (1957) intro- ¢aravarri Formation. duced them under the designation ‘¢aravarri Grit’.Oftedahl Within a distance of about 500 m from the east-bound- (1981) included the ’Grit’ unit of Holmsen et al. (1957) and ing fault zone, the ¢aravarri sandstones are variably altered the Bik’ka∏åk’ka Formation in ‘the ¢aravarri Group’, while to light red and grey quartzite (Fig. 2).The fault zone itself is Siedlecka et al. (1985) gave the grit-unit the formal name, c. 1 km wide and made up of a heterogeneous, highly dis- ‘¢aravarri Formation’. rupted subzone of semi-ductilely deformed phyllitic and The depositional character and tectonic significance of graphite-bearing schists interpreted as a thrust detachment. the ¢aravarri Formation, relative to other supracrustal units Locally, sheared and folded dolomite-marble lenses are of the Kautokeino and Karasjok Greenstone Belts, have not observed which represent tectonised rocks of the hanging- yet been addressed. However, Torske (1977) and Bergh & wall, the Lik’∏a Formation (Siedlecka et al. 1985). Sheets of Torske (1986,1988) ascribed, tentatively, the ¢aravarri and ¢as’kejas Formations and the correlative Raipas Supergroup NGUBull442_INNMAT 23.11.04 09:54 Side 7 TORE TORSKE & STEFFEN G.BERGH NGU-BULL 442, 2004 - PAGE 7 ¢arajav'ri ¢arajav'ri Fig. 2. Geological map, with cross-sections, of the ¢aravarri Formation and adjoining units bet- ween lake ¢arajav’ri and Lulli Lik’∏a. of Alta, including the Kvenvik and Skoadduvarri Formations, greenstone belt has been questioned by us because no syn- to a continental rift setting near the northern margin of the sedimentary, rift-marginal, master normal faults have been Fennoscandian Shield. In this scenario, the Skoadduvarri demonstrated (see Fig. 2). Instead, the ¢aravarri Formation Formation at Alta was interpreted as fan-delta deposits from has been proposed as a foreland basin deposit of molasse rivers running along the postulated rift basin, from a source type (Torske 1997), preserved in front of remnants of a of prograding, trough-filling, alluvial fans in the ¢aravarri Palaeoproterozoic (Svecokarelian) fold and thrust belt (e.g., Formation to the south (Bergh & Torske 1986). A possible Krill 1985) to the east. strike-slip rift origin for the ¢aravarri Formation was sug- gested by Torske & Bergh (1984 a, b) on the basis of differ- ences in clast types and tentative, but elusive, source areas Purpose and methods along strike. of the present study Recently, a rift basin setting for the Alta-Kautokeino
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