lithostratigraphy of the mesoproterozoic telemark ... · e-mail: [email protected] (j. köykkä...

49

Introduction

The Mesoproterozoic volcanic-sedimentary Telemark supracrustal rocks in southern Norway represent a suc-cession of sedimentary and volcanic rocks that have been well-preserved, despite diverse tectonic and low-grade metamorphic processes. These supracrustal rocks are an important feature of the Fennoscandian Shield. The clas-sical lithostratigraphic framework of the Telemark supra-crustals was established by Wyckoff (1934) and Dons (1960a, 1960b), who subdivided the entire package into the Rjukan (oldest), Seljord and Bandak Groups, separated by major unconformities. Traditionally, the oldest Rjukan Group was subdivided into the lower Tuddal Formation and the upper Vemork Formation. This was later modified by Dahlgren et al. (1990a, 1990b), who suggested that the contact between the Vemork Formation and the Seljord Group wasis transitional, and therefore the former should be included in the latter. Laajoki et al. (2002) recom-mended abandoning the term Seljord Group, and. They divided this sequence into the older Vindeggen Group and athe younger Lifjell Group. The overlying Vindeggen Group is separated from the Rjukan Group by the sub-

Heddersvatnet unconformity (Laajoki, 2005). The Vin-deggen Group starts with the Heddersvatnet Formation, but the contacts between the Tuddal, Vemork and Hed-dersvatnet Formations are considered problematic (see Werenskiold, 1910; Wyckoff, 1934; Dons, 1960a & 1960b; Falkum & Petersen, 1980; Brewer & Field, 1985; Dahlgren, 1990a, 1990b; Starmer, 1993; Richards, 1994; Menuge & Brewer, 1996; Brewer & Menuge, 1998; Sigmond, 1998; Laajoki, 2005; Laajoki & Corfu, 2007). The present study addresses this controversy with new field-based lithostrati-graphic, sedimentological and geochemical evidence.

The main contributions of this study are (i) the revision of the sub-Heddersvatnet unconformity, (ii) the corre-lation between the Vemork and Heddersvatnet Forma-tions, and (iii) the paleotectonic reconstruction of newly-discovered basaltic lavas in the Heddersvatnet Forma-tion. This study concludes that the present lithostrati-graphic subdivision and nomenclature of the Telemark supracrustals will require some adjustment.

Köykkä, J: Lithostratigraphy of the Mesoproterozoic Telemark supracrustal rocks, South Norway: revision of the sub-Heddersvatnet unconformity and geochemistry of basalts in the Heddersvatnet Formation. Norwegian Journal of Geology, Vol 90, pp. 49-64. Trondheim 2010, ISSN 029-196X.

The Mesoproterozoic Rjukan Group represents the oldest part of the Telemark supracrustal rocks in southern Norway. Under the traditional nomenclature, the Rjukan Group contains the felsic volcanic Tuddal Formation (ca. 1.512 Ga) and the volcanic-sedimentary Vemork Formation (ca. ≤1.495 Ga), and is overlain by the Vindeggen Group. The Rjukan and Vindeggen Groups are separated by the sub-Heddersvatnet unconformity. The lowest component of the Vindeggen Group is the Heddersvatnet Formation, which is deposited directly above the Tuddal Formation. The contact regions between the Tuddal and Heddersvatnet Formations, and the stratigraphic relationship between the two formations, are controversial and revisited here. New evidence from the sub-Heddersvatnet unconformity and basaltic lava interbeds in the northern-flank of the Gaustatoppen suggests that the Heddersvatnet Formation interfingers laterally with the Vemork Formation. The deposition of the Heddersvatnet Formation started at a late stage of the Vemork Formation sedimentation. The geochemistry, sedimentology and lithostratigraphy indicate that the Vemork Formation represents the axial depocenter, whereas the Heddersvatnet Formation was deposited near uplifted flank(s) of the rift basin. The sub-Heddersvatnet unconformity continues under the Vemork Formation. Observations seem inconsistent with the traditional lithostratigraphic division of the Telemark supracrustals. This study argues that the term (Rjukan Group) should be abandoned, and that the Vemork Formation should be included as part of the Vindeggen Group. The geochemistry of the basaltic lavas of the Heddersvatnet Formation basaltic lavas suggests deep intraplate melting combined with crustal contamination in a within-plate paleotectonic setting. The geochemical signature is similar to that of some continental flood basalts and the Vemork Formation basalt.

Juha Köykkä, Department of Geosciences, University of Oulu, P.O. BOX 3000, 90014 Oulu, Finland. Tel.: +358 8 5531439; Fax: +358 8 5531484. E-mail: [email protected] (J. Köykkä

Juha Köykkä

NORWEGIAN JOURNAL OF GEOLOGY Lithostratigraphy of the Mesoproterozoic Telemark supracrustal rocks, South Norway

Lithostratigraphy of the Mesoproterozoic Telemark supracrustal rocks, South Norway: revision of the sub-Heddersvatnet unconformity and geochemistry of basalts in the Heddersvatnet Formation

50 J. Köykkä NORWEGIAN JOURNAL OF GEOLOGY

tions (Brewer & Atkin, 1987; Atkin & Brewer, 1990).

The sub-Svinsaga unconformity is the most important unconformity within the Telemark supracrustal rocks, and separates the sequence into two major packages. The Vestfjorddalen Supergroup, deposited in the interval between ca. 1.512 and 1.347 Ga, and an upper succession of Sveconorwegian units, deposited between aboutca. 1.169 and 1.019 Ga. The Vestfjorddalen Supergroup con-sists of two groups: (1) the older Rjukan Group, compris-ing continental felsic volcanics of the Tuddal Formation (ca. 1.512 Ga, Bingen et al., 2005), and the 2 km thick sequence of the volcanic-sedimentary Vemork Forma-tion (ca. ≤1.495 Ga, Laajoki & Corfu, 2007), character-ized mainly by mafic volcanism; and (2) the 5 km thick

Geological settingThe Mesoproterozoic (ca. 1.5 – 1.0 Ga) volcanic-sedi-mentary Telemark supracrustal rocks or sequences (Sigmond et al., 1997) occupy the northern part of the Sveconorwegian Telemark sector (Bingen et al., 2005), or Telemark block (Andersen, 2005), of the Southwest Scandinavian Domain (Gaál & Gorbatschev, 1987) in the Fennoscandian (Baltic) Shield (Fig. 1). In contrast to most of the metamorphic Precambrian crust in South Norway, the Telemark supracrustal units are relatively well-preserved (Dons, 1960a; 1960b; Laajoki et al., 2002; Bingen et al., 2003, 2005). They form a supracrustal sequence approximately 10 km thick, metamorphosed under low-grade greenschist-amphibolite facies condi-

Fig. 1. Simplified lithostrati-graphical and structural map of the southern part of the Telemark supracrustals. The bedrock is subdivided into diverse structural domains by several faults (thick lines). The study area is marked and the Fig. 2 is framed. SHU = sub-Heddersvatnet unconfor-mity (thick dash line). GDF = Gausdalen fault.

51NORWEGIAN JOURNAL OF GEOLOGY Lithostratigraphy of the Mesoproterozoic Telemark supracrustal rocks, South Norway

cance of the lithostratigraphic contacts between the Rju-kan and Vindeggen Groups, and between the Tuddal, Vemork and Heddersvatnet Formations (for a review, see Laajoki, 2005 and his Table 1). Wyckoff (1934) and Dons (1960a, 1960b) stated that a regionally large angu-lar unconformity exists between the Rjukan and the Vindeggen (previously Seljord) Groups, but this conclu-sion was subsequently called into question by Starmer (1993), Menuge & Brewer (1996) and Brewer & Menuge (1998). Dahlgren et al. (1990a) stated that the Vemork Formation represents initial volcanism and sedimenta-tion of the Vindeggen Group, whereas Brewer & Menuge (1998) considered the Rjukan and Vindeggen Groups as one package, within which the discordance of individual units is a function of the mode of deposition in the con-text of important tectonic changes in the evolution of the Rjukan Rift Basin (see Brewer & Menuge, 1998; Laajoki & Corfu, 2007).

The Tuddal Formation was most likely deformed and deeply eroded before deposition of the Heddersvatnet Formation (Sigmond et al., 1997; Sigmond, 1998; Rich-ards, 1994, 1998). This led Laajoki (2005) to recognize the sub-Heddersvatnet unconformity. In addition, Laa-joki (2005) and Laajoki & Corfu (2007) suggested that the Vemork and Heddersvatnet Formations might inter-finger, which would indicate that the sub-Heddersvatnet unconformity continues beneath the Vemork Formation.

This study focuses on the area around Lake Heddersvat-net, on the southern flank of the Heddersfjell, Svineroe, in the northern flank of the Gaustatoppen, and the Dipla-nut, where the contacts between the Tuddal, Vemork and Heddersvatnet Formations are well -exposed (Fig. 2). In

sedimentary Vindeggen Group (≥1.35 Ga, Corfu & Laa-joki, 2008). The lowermost unit of the Vindeggen Group is called the Heddersvatnet Formation and it is domi-nated by conglomerate and sandstone. The Hedders-vatnet Formation is overlain by the quartz sandstone-dominated Gausta Formation. The Rjukan and Vindeg-gen Groups are separated by the sub-Heddersvatnet unconformity, which has recently been described by Laa-joki (2005). In the Rjukan-Bandak area, the Sveconorwe-gian unit includes the oldest Oftefjell Group, the Høy-dalsmo Group and the Eidsborg Formation.

Previous studiesThe Rjukan and Vindeggen Groups were previously mapped in the Rjukan area by Wyckoff (1934) and Dons (1960a, 1960b, 1961), and in the Åmotsdal and Frøys-taul areas by Dons et al. (2003, 2004). Previously, Dons (1960a, 1960b, 1961) included the basal conglomerates and associated sandstones, which locally overlie the Rju-kan Group, as part of the Gausta Formation, Vindeggen Group. Laajoki (2005) studied the areal distribution and the nature of the sub-Heddersvatnet unconformity and concluded that the basal conglomerates and associated sandstones form an important and clearly mappable unit and should be treated as a formation, which he named the Heddersvatnet Formation. More recently, Laajoki and Corfu (2007) subdivided the Vemork Formation into smaller lithostratigraphic units, and discussed its rela-tionship with the Tuddal Formation and the Vindeggen Group.

In most previous studies, the most controversial prob-lems have been associated with the nature and signifi-

Fig. 2. Geological map of the Vemork-Diplanut, Svine-roe and Heddersfjell areas (modified from Laajoki & Corfu, 2007). The Vemork Formation interfingers with the Heddersvatnet Forma-tion, which are separated by the Gausdalen fault zone (GDF). SHU = sub-Hedder-svatnet unconformity (thick dash line). VF = Vemork Formation. HF = Hedders-vatnet Formation. Note that basaltic lavas are missing in the area around Lake Hed-dersvatnet and Heddersfjell. The locations of the outcrop photographs are given.


the south-southwest. The Vemork Formation forms an asymmetrical syncline, the Vigfithova syncline, north-west of the Midtfjell anticline. The lithological asym-metry and the strongly sheared conglomerates adjacent to the Hortenuten dome (Fig. 1) indicate that the lower parts of the Vemork Formation have been truncated tec-tonically along the northwest margin of the exposure (Laajoki & Corfu, 2007).

The primary upper contact of the Vemork Formation is preserved only in the Diplanut area, between the Grass-fjell shear zone and the Gausdalen fault (Figs. 1 & 2). In other areas, the contact is highly sheared and represents the folded and faulted extensions of the shear zones. The bedding orientationsdirections are similar in both the Vemork and Gausta Formations (Fig. 2).

Heddersvatnet Formation

The Heddersvatnet Formation is best exposed around Lake Heddersvatnet, along the southern flank of the Heddersfjell, or Svineroe, and along the northern flank of the Gaustatoppen (Fig. 2). The formation is defined by the broad Vindsjå syncline, plunging to the north-north-east on Heddersfjell and to the southwest on Gaustaråen (Fig. 2). The Vindsjå syncline was formed during the main Sveconorwegian deformation, and was later refolded along the Jonnbu anticline (Fig. 2). The bedding directions in the Heddersvatnet Formation are the same as those in the Gausta Formation. The axial-plane cleav-age dips at about 35o – 45o to the east-southeast. The general trend of the bedding and the axial-plane cleavage is also parallel to that in the Vemork Formation, west of the Gausdalen fault zone (Fig. 2). This trend is a result of the E-W oriented folding phase preceding the Sveconor-wegian orogeny, which deformed the Rjukan and Videg-gen Groups. Some of the beds near Svineroe are over-turned due to this deformation (Fig. 2).

Sub-Heddersvatnet unconformityThe sub-Heddersvatnet unconformity separates the Tud-dal Formation of the Rjukan Group from the Vindeggen Group (Figs. 1 & 2). It is named after the lowermost for-mation of the Vindeggen Group (Laajoki, 2005), and its nature varies from one locality to another.

Contact between the Tuddal Formation and the Vemork Formation

Laajoki & Corfu (2007) placed the lower contact of the Vemork Formation against coherent Tuddal domes, made up of felsic volcanic rocks (Fig. 1). These bound-aries can be mapped regionally. These authorsy noticed, however, that the change from felsic Tuddal volcanism to basaltic Vemork volcanism is of interbedded charac-ter, with felsic lava units in the lower part of the Vemork Formation. The primary contact between the formations is, however, not exposed in the Vemork-Diplanut area

the Diplanut area, the Vemork Formation continues lat-erally east to the Gausdalen fault and interfingers with the Heddersvatnet Formation (Fig. 2). Around the Lake Heddersvatnet area and on the Heddersfjell, the Vemork Formation is absent. The Heddersvatnet Formation lies directly over the Tuddal Formation, but is separated from it by the sub-Heddersvatnet unconformity (Fig. 2).

Structural featuresAlthough all the rocks in our study area have been meta-morphosed into low-grade greenschist facies, the meta-prefix is not used in some of the lithological names. The sandstone nomenclature used here was proposed by Folk (1980) and Pettijohn et al. (1987) and is based on the relative percentages of the major components in the rel-evant sandstones. The term volcanoclastic is used for any rock with a preponderance of fragments of volcanic ori-gin and derived bythrough any process, e.g., weathering (see Pettijohn, 1975).

The structural observations around the Vemork and Heddersvatnet Formations are summarized here. Although the Tuddal Formation serves as the basement for both formations, the structural features of the Tuddal Formation are difficult to establish. The main reason is that the Tuddal Formation porphyre is massive, and the contacts withof the individual lava beds are unexposed. In addition, the commonly observed flow banding is sig-nificantly distorted due to deformation. The only reliable bedding observation in the Tuddal Formation involves the lapilli tuffs on the southern shore of Lake Hedders-vatnet (Fig. 2). These tuffs trend northeast, with variable dips to the southeast/northwest, or have been steeply folded (cf. Wyckoff, 1934; Laajoki, 2005). Around Lake Heddersvatnet, the flow banding is steeply-dipping and is limited by the sub-Heddersvatnet unconformity. Laa-joki & Corfu (2007) structurally subdivided the Tud-dal Formation into the Våerskarven, Dalsnuten, Kov-vatnet, Heidalsnuten and Hortenuten domes, which are cut by several faults (Fig.1). The domes represent pri-mary volcanic complexes folded as a resulton account of Sveconorwegian deformations. To the east of the Gaus-dalen fault, the Tuddal Formation is overlain by the Hed-dersvatnet Formation, whereas to the west of the fault, it is overlain by the Vemork Formation (Figs. 1 & 2).

Vemork Formation

The Vemork Formation is exposed along a ca. 50 km long northeast-trending area extending from Rjukan to Trovasshovet (Fig. 1). Due to strong deformation, most of the Vemork Formation contacts are sheared, unex-posed or disturbed by other faults. The lowermost units of the Vemork Formation are best preserved along the southeast limb of the Midtfjell anticline, although these are cut by the Gausdalen fault (see Laajoki & Corfu, 2007 and Fig. 2). The Midtfjell anticline plunges 60° – 80° to


ding is almost perpendicular to the bedding of the Hed-dersvatnet Formation. This observation indicates at least a 30°-40° angular discordance between the Tuddal and Heddersvatnet Formations. The contact is enriched with sericite, indicating intensive in situ weathering and/or reworking of weathered products of the Tuddal Forma-tion porphyre. The Tuddal Formation homogenous por-phyre is blastoporphyric, with angular broken quartz and feldspar phenocrysts in a sericite-rich matrix.

The Heddersvatnet Formation volcanoclastic conglom-erate along the northern shore of Lake Heddersvatnet lies directly over the Tuddal Formation flow banded lava, and forms a sharp and deeply erosional contact between the two formations. FThe flow banding in the Tuddal Formation rhyolite is a result of the mixing of different cooling layers, indicating slow internal movement of the lava.

The sub-Heddersvatnet unconformity on the northern flank of the Gaustatoppen can be traced from the Gaus-dalen fault in the west to the Svineroe in the east (Fig. 2). The Heddersvatnet Formation epiclastic sandstone is deposited directly above the homogenous, massive to foliated porphyre of the Tuddal Formation, forming a sharp erosional and slightly sheared contact between the formations (Figs. 3B & C). The Tuddal Formation has an in situ weathering crust a few meters thick, enriched with sericite. Due to the nature of the Tuddal Formation por-phyre, the angular unconformity cannot be established with certainty at these localities. The Heddersvatnet For-mation coarse-grained sandstone contains quartz phe-nocrysts, partly altered feldspar clasts, variable amounts of lithic volcanoclasts from the Tuddal Formation, and a few granitoid or coarse gneiss clasts in a sericite-rich matrix. Some of the well-rounded epiclastic volcanic pebbles are up to 5.0 cm in diameter (Fig. 3D). In other localities, the contact represents a sharp erosional surface with irregular paleorelief or in situ weathering crust.

Lithostratigraphy of the Vemork FormationThe general descriptions of the Frøystaul and Vemork-Diplanut areas to date have been primarily based on studies by Laajoki & Corfu (2007). The Frøystaul area is the only place where the lower regions of the Vemork Formation are exposed. In most areas, the Vemork-type basaltic lavas have lost all of their primary features, and the interbedded sedimentary beds cannot be traced lat-erally forto great distances. Although the volcanic-sedi-mentary beds are not well exposed, at least laterally, the Vemork-Diplanut area presents an important section for lateral stratigraphic correlation with the Heddersvatnet Formation. This is the only place where the upper con-tact of the Vemork Formation is exposed. OThe other areas exist, but are beyond the scope of this paper.

(Fig. 2). The massive conglomerate member (Maristi member) represents the first evidence offor the Vemork Formation in the Vemork-Diplanut area, but the exact contact with the Tuddal Formation porphyre cannot be ascertained established. According to Laajoki & Corfu (2007), the conglomeratice member is most likely under-lain by a thin sequence of Vemork-type basalts and sedi-ments.

In the Frøystaul area, the Vemork Formation starts with a sandstone deposited directly uponabove ahomogenous massive and foliated porphyrehomogenous of the Tud-dal Formation massive and foliated porphyre, forming a sharp and slightly sheared erosional contact. The sand-stone contains well-rounded epiclastic volcanic pebbles (<6.0 cm) from the Tuddal Formation, bluish quartz-phenocryst clasts, and partly or completely altered pla-gioclase clasts in a sericite-rich matrix. According to Laa-joki & Corfu (2007), this suggests significant erosion of the Tuddal Formation porphyre before the extrusion of the first Vemork Formation basaltic flow.

In localities where the lowermost Vemork Formation sandstone lies directly above the Tuddal Formation flow-banded rhyolite lava, it is greenish and rich in sericite, and contains plagioclase-phenocryst clasts and glomero-phyric plagioclase clasts, and sericite schist (op. cit.). The nature of the unconformity cannot be established due to the small size of individual outcrops and lack of regional-scale mapping. However, the unconformity cuts both the massive porphyre and the flow-banded rhyolite. The existence of an angular unconformity under the Vemork Formation is therefore possible. Temporally, its signifi-cance seems to be modest, because the time gap is mini-mal between the Tuddal Formation felsic lavas (ca. 1.512 Ga, Bingen et al., 2005) and the Skardfoss member rhyo-lite (ca. ≤1.495 Ga, Laajoki & Corfu, 2007) in the lower part of the Vemork Formation.

Contact between the Tuddal Formation and the Hedders-vatnet Formation

The sub-Heddersvatnet unconformity is well-exposed around Lake Heddersvatnet, along the southern flank of the Heddersfjell, and along the northern flank of the Gaustatoppen (Fig. 2). Along the southern shore of Lake Heddersvatnet, near Nutan, and onin the southern flank of the Heddersfjell (Fig. 2), the Heddersvatnet Forma-tion has been deposited directly above the in situ weath-ered and brecciated Tuddal Formation porphyre (Fig. 3A) and lithophysae. The Heddersvatnet Formation erodes the Tuddal Formation volcanoclastic conglom-erate, porphyre and associated lithophysae. The Tud-dal Formation lithophysae haveare spherulite growths formed around expanding vesicles in rhyolitic lava, with fibrous quartz-feldspar needles radiating from the nucleius. These kinds of vVesicles of this type can be formed in a flowing melt, i.e., before solidification (Cash & Wright, 1988). The Tuddal Formation lapilli tuff bed-


graphic pattern in this section is that mass-flow conglom-erates and pebbly sandstones occur in the lower part of the formation, whileand sedimentary units in the middle and upper parts consist mainly of immature, cross-bedded or parallel-laminated sandstones. The sandstone beds are relatively rich in epidote and consist of abundant rock and mineral clasts from felsic and mafic sources. Petrographi-cally, the detrital framework of the Vemork Formation sandstones classifiesy them as arkoses or lithic arkoses in all areas, with a variable prevalence of lithic volcanic clasts.

Vemork-Diplanut area

The contact between the Tuddal and Vemork Forma-tions is not exposed in the Vemork-Diplanut area, but

Frøystaul area

The volcanic-sedimentary sequence is about 2.2 km thick and consists mostly of subaerial basaltic lavas and some interbedded sandstone units. The main unit of the Vemork Formation starts with a 10 m thick pebbly sand-stone containing well-rounded felsic volcanic clasts, quartz phenocrysts and altered plagioclase grains in a ser-icite-rich matrix. This is followed by alternate basaltic lava and sandstone beds with variable thicknesses. According to Laajoki & Corfu (2007), all the basaltic lavas are monot-onous, consisting of massive greenstone or amygdaloi-dal lavas. They concluded that the ubiquitous amygdales and lack of pillow structures in the basalts indicates that the lavas probably erupted subaerially. The general strati-

Figs. 3A-D. Photographs of the outcrops. A) The in situ weathered Tuddal Formation porphyre. Hammer handle 20 cm. B) An erosional unconformity between the foliated flow-banded of the Tuddal Formation and the lithic arkose of the Hedders-vatnet Formation. Unconformity is marked with a dash line. Stick 1.60 m. C) A close-up of the erosional unconformity bet-ween the foliated Tuddal Formation and the Heddersvatnet Formation. Unconformity is marked with a dash line. Hammer handle 20 cm. D) A close-up of the Heddersvatnet Formation pebbly massive arkosic sandstone above the unconformity. The epiclastic arkose contains abundant well-rounded volcanoclastic pebbles derived from the Tuddal Formation. Pencil 14 cm.


altered feldspar clasts that have been replaced with ser-icite. The topmost trough cross-bedded sandstone bed contains dark, angular mudstone rip-ups. The size of these rip-ups varies from a few cm up to 15 cm.Contact between the Vemork Formation and the Gausta Formation

The contact between the Vemork and the Gausta For-mations is eithermostly unexposed or intensely sheared along the Grassfjell shear zone (Fig. 2). The Vemork For-mation Aamygdaloidal lava in the Vemork Formation in the northern flank of the Diplanuten is in primary contact with a quartz-feldspar wacke followed by the cross-bedded sandstone and Gausta Formation quartz sandstone, which is free of feldspar and volcanic clasts. It starts with a polymictic conglomerate consisting of well rounded orthoquartzite and felsic volcanic pebbles and minor jaspis clasts (Laajoki & Corfu, 2007).

Laajoki & Corfu (2007) considered that the sandstone and lava beds between the Grassfjell shear zone and the Gausdalen fault belong to the Heddersvatnet Formation (Fig. 2). However, our study showsconsiders that the Gausdalen fault separates and represents the boundary marker between the Vemork Formation and the Hed-dersvatnet Formation. This is because the sandstone and basaltic lava beds are disturbed by diabase (Fig. 2) and cannot be laterally traced from the northern flank of the Gaustatoppen to the Diplanut.

Lithostratigraphy of the Heddersvatnet Formation

The lithostratigraphy of the Heddersvatnet Formation is described from the northern flank of the Gaustatop-pen, Svineroe and the southern flank of the Heddersfjell, where the outcrops and the contacts of different beds are exposed (Fig. 2).

Northern flank of the Gaustatoppen

Previously, the bedrock in the northern flank of the Gaustatoppen was mapped as an orthoquartzite intruded by metadiabasice sills (see Wyckoff, 1934; Dons, 1961). However, remapping proved that many of the latter were in fact bedding-parallel amygdaloidal basaltic lavas. In total, six different basaltic lava units have been identi-fied (Fig. 4B), including the one reported byin Ander-son & Laajoki (2003). The Heddersvatnet Formation in this area is ca. 400 m thick (Fig. 4B). It starts with a ca. 70 m thick epiclastic sandstone deposited directly above the Tuddal Formation in situ weathered porphyre in the Tuddal Formation (Fig. 4B). The coarse-grained epiclas-tic sandstone is massive, with a lack of visible sedimen-tary structures. It contains well-rounded Tuddal Forma-tion porphyre and silica-rich lithophysae fragments in a sericite-rich matrix (Fig. 3D). The detrital framework of the Heddersvatnet Formation sandstones classify them as arkoses or lithic arkoses in all areas, with variable dis-

the lower part of the Vemork Formation contains two important members, namely the Maristi and the Skard-foss members (Figs. 2 & 4A). The Maristi conglomerate member is a few tens of meters thick, containing up to a few meters of matrix-supported conglomerate beds capped and separated by thin, poorly-laminated sand-stone units. The sizes of the conglomerate clasts ranges from a few cm up to 1.5 m. The bed thickness and clast sizes in the conglomerate beds decrease upwards, and the poorly-laminated and cross-bedded sandstone beds are common in the upper part of the member. The Maristi conglomerate member may representindicate a mass flow deposit (Dons et al., 2004; Laajoki & Corfu, 2007).

The Maristi member passes upwards into the ca. 20-50 m thick Skardfoss member, which consists solely of flow-banded rhyolite.,It and slightly erodes the underlying sandstone-mica schist. The most important feature of this member is that it consists of two types of volcano-clastic breccia occur in its the upper contact. of the mem-ber. The first type has a jigsaw-like texture and the frag-ments are free of quenching features, which indicates an autoclastic breccia. The second type contains larger rhy-olite fragments with roundish or curviplanar margins. The groups of clasts with jigsaw textures are supported in a matrix of smaller rhyolite fragments embedded in a granoblastic orthoquartzite, without any evidence of clastic origin (Laajoki & Corfu, 2007). The second type of breccia has been interpreted as hyaloclastic (Laajoki & Corfu, 2007). The presence of autoclastic and hya-loclastic breccias indicates that the Skardfoss rhyolite was extruded under subaerial conditions, and probably flowed into water.

Thin tuffitic schist, followed by alternating basaltic lava and sedimentary beds, overlies the Skardfoss mem-ber (Fig. 4A). However, these beds are poorly -exposed, and their real thickness and lateral continuity cannot be precisely measured. The porphyric basaltic lava beds contain mostly rounded or partly elliptical amygdales <7.0 cm in diameter, infilled with silica-rich material and some individual lithic fragments derived from pre-existing volcanic rocks. The primary textures have been mostly destroyed due to metamorphism, but some sam-ples exhibit weak signs of intergranular to trachytic tex-ture. The large and angular plagioclase grains are trans-formed into carbonate, and clinopyroxene grains are either leached or altered. Dark, iron-rich opaque miner-als (magnetite) in a fine-grained groundmass are usually abundant.

The intermediate sedimentary sandstone interbeds are mostly low-angle or trough cross-bedded, medium to coarse- grained, and feature occasional well-rounded volcanic pebbles in a sericite-rich matrix. These cross-bedded units are usually capped with ca. 5-20 cm thick mudstone beds. In addition, the moderately-sorted sandstones contain some sub-rounded to angular poly- and monocrystalline quartz phenocrysts and abundant


The mineral assemblages, withoutsuch as lack of garnet, biotite and amphibole minerals, indicate that the meta-morphic grade was no higher than low-grade green-schist facies. The entire topmost unit is interpreted to represent a wvaning flow conditions with traction cur-rent deposition, followed by pebbly mass flow deposits, and calmer flowing conditions, and overlain by subaer-ial basaltic lava flows.

Svineroe

The lithostratigraphy of the Heddersvatnet Formation in the Svineroe area is slightly different from thatn in other localities. It features ripple-marked and laminated mud-stone, ripple-marked sandstone with desiccation cracks and pillow lava structures. The area displays only small, discontinuous outcrops, making lithostratigraphic corre-lation between individual outcrops difficult.

Laterally, the easternmost outcrop of the Heddersvatnet Formation in the Svineroe area comprises at least a ca. 5 m thick and ca. 80 m wide mudstone unit, which is normally -graded, laminated and ripple-marked. It was deposited directly above the Tuddal Formation porphyre, but the contact is not unexposed. This mudstone deposit probably represents an individual depositional system such as a ponded lake with a smaller feeder channel. Lithostratigraphically, the topmost bed in this area con-tains basaltic lava with pillow structures, and was which is deposited directly above the cross-bedded and ripple-marked sandstone. The size of the elliptical pillows varies from 10 to 30 cm. Although no hyaloclastite breccias can be found in this area, the nature of other basaltic lavas, subaerial debris flows, paleoflow and the associated structures of the sandstones indicate subaerial eruption and flow into water. The development of basaltic pillows does not necessarily require under-water eruption (Cas & Wright, 1987).

Southern flank of the Heddersfjell

Along the southern flank of the Heddersfjell (Fig. 2), the Heddersvatnet Formation lies directly above the in situ weathered and brecciated lithophysae of the Tuddal For-mation, which is here a few meters thick. The Hedders-vatnet Formation in this area is ca. 250 m thick and starts with a matrix-supported massive breccia a few meters thick, lying above the felsic lava of the Tuddal Forma-tion felsic lava (Fig. 4C). It is overlain by a ca. 10 m thick clast-supported breccia with an erosional contact. These poorly-sorted breccia beds shows no signs of grading or internal structure. The clast size of the sub-rounded to angular volcanoclastic breccia fragments varies from a few cm up to 50 cm. The matrix between the fragments is sericite-rich medium to coarse-grained arkosic sand-stone. These beds represent subaerial plastic and pseu-dopblastic debris flows, in which the epiclastic material is derived from the intensively weathered Tuddal Forma-tion porphyre and lithophysae.

tributions of lithic volcanic clasts. The lack of any visible structures, tohether withand the pebbles at the bottom part of the unit, probably indicate rapid deposition from a sediment -gravity flow deposit.

The epiclastic sandstone is overlain by a ca. 70 m thick, medium to coarse-grained trough cross-bedded sandstone unit with mudstone interbeds. These trough cross-bedded sandstone beds are 1.5 to 2.0 m high and 3.0 to 5.0 m wide. The thicknesses of the mudstone interbeds vary from 2.0 to 50 cm. The cross-bedded sandstone bed also con-tains occasional angular, dark, mudstone rip-ups, which are usually less than 20 cm in diameter. Paleoflow mea-surements indicate flow direction from the present east-northeast to west-southwest (Fig. 4B). The angularity of the mudstone rip-ups indicates a short transport distance, or at least no intensive re-working. This unit provides is evidence for the existence of sinuous-crested and linguoid (3-D) dunes under unidirectional flow conditions.

The trough cross-bedded sandstone is overlain by a ca. 10 m thick basaltic lava bed (Fig. 4B), containing mostly rounded or partly elliptical amygdales <2.0 cm in size. The amygdales are former vesicles and are completely infilled with silica or talc-rich material, representing the flow tops. The elliptical amygdales and the mineral assemblage with heteroblastic intergrowths indicate even later regional metamorphism. The basaltic lava also con-tain a few angular lithic fragments that are <6.0 cm in diameter, derived from pre-existing volcanic rocks. Some of the larger, unrounded fragments may also represent previous vesicles that were deformed during flowage and later infilled with silica-rich material. This basaltic lava bed is again overlain by similar trough cross-bedded sandstones, followed by interbedded basaltic lavas and sandstones of variable thickness. The common feature of the basaltic lavas is that the primary structures and tex-tures are almost completely destroyed due to metamor-phism, but they show no signs of notable epidotization.

The lithostratigraphy is disturbed by a ca. 400 m thick medium to coarse-grained diabase, which is paral-lel to the planar structure of the surrounding rock, and formsing a massive sill in the northern flank of the Gaustatoppen (Fig. 2). The topmost mappable unit in this area is a basaltic lava bed at least 5.0-10 m thick (Fig. 4B), which can be laterally correlated with the basaltic pillow lavas in the Svineroe area (Fig. 2). The primary features of this basaltic lava bed are trachytic textures resulting from magmatic flow, which have been well preserved in many cases. The original glassy groundmass material between the feldspars has been replaced by chlorite. Weakly-developed alignment of the feldspars due to magma flow indicates a gradational texture between the intergranular and the trachytic tex-tures. It should be noted that the term trachytic texture is not restricted to rocks of trachyte chemical composi-tion (Mackenzie et al., 1982). It can also exist in basaltic rocks, which tend to have a more intergranular texture.


arkosic sandstone. The contacts of the beds are mostly sharp, erosional, and unchannelized, although some-more channelized contacts also exist. The bottom parts of the beds indicate subaerial pseudopblastic debris flow with an inertial bedload, or a more fluidal and turbulent sediment flow. The top parts indicate a waning current and traction in the context of a stream flow.

These beds are overlain by a matrix-supported volcano-clastic conglomerate unit several meters thick (<20 m), and interbedded, medium-coarse grained sandstone

The clast-supported volcanoclastic conglomerate beds overlie the breccia beds (Fig. 4C). The bed thickness var-ies from 2.0 to 10 m. The common feature among all the clast-supported conglomerate beds is that the topmost part of the beds contains stratified cross-bedded sand-stone or siltstone with volcanic pebbles. The texture and fabric of the beds vary from ungraded and disorganized to crudely imbricated and normally-graded. The clast size of the rounded-angular epiclastic fragments varies from a few cm up to 50 cm. The matrix between the frag-ments varies from silt-size material to medium-grained

Figs. 4A-C. Lithostrati-graphy of the study area. See the framed box for the facies codes and legend (modified from Miall, 2006). VF = Vemork Formation. HF = Heddersvatnet Formation. A) Lithostratigraphic sum-mary column of the Vemork Formation in the Vemork-Diplanut area. The true thickness of the different basaltic lava beds is hard to establish and those probably contain sedimentary inter-beds. The contact between the Tuddal and the Vemork Formations is unexposed. B&C) Lithostratigraphic summary columns of the Heddersvatnet Formation in the northern flank of the Gaustatoppen (B) and Heddersfjell (C) areas. The paleoflow measurements indicate flow directions from the present NE to the pre-sent S-SW. The locations of the outcrop photographs are given. In the Heddersfjell area, the Heddersvatnet For-mation is dominated by the volcanoclastic conglomera-tes and arkosic sandstone. The basaltic lavas are com-pletely missing.


volcanic pebbles and some slate fragments, but these are totally missing from the subarkose – quartz arenite of the Gausta Formation.

Paleoenvironmental depositional model

The paleoflow directions and genetically related lithofa-cies assemblages indicate that the Heddersvatnet For-mation represents alluvial fan sedimentation during the syn-rift phase of the sedimentary basin (Fig. 5 & Köykkä, submitted). Subaerial debris flows, stream-flows and sheet flood deposits mostly characterize the proximal part of the Heddersvatnet Formation, whereas the distal part contains shallow water channel-fill dunes and conti-nental flood basalts (Fig 5). The laterally quite extensive mudstone bed in the Svineroe area suggests a sub-basin development and intra-rift faulting inside the Hedders-vatnet Formation.

The volcanic-sedimentary Vemork Formation is the main component that gradually filled the rift basin, whereas the Heddersvatnet Formation was deposited near uplifted flank(s) i.e. rift shoulder(s) of the basin. The uplifted flank was dominated by intensive erosion of the Tuddal Formation porphyre, leading to the accumu-lation of thick volcanoclastic debris flow deposits in the lower and proximal part of the Heddersvatnet Formation (Fig. 4C) and followed by alluvial stream sedimentation in the distal parts (Fig. 5).

with trough cross-bedding and minor pebbles (Fig. 4C). The clast size of the conglomeratese beds varies from a few cm up to 20 cm. The matrix-supported conglomera-tese beds represent a subaerial plastic debris flows with a high viscous strength. The interbedded sandstone may suggest scour-fills or antidunes in a channelized stream flow.

Lithostratigraphically, the topmost part of the Hedders-fjell area comprises a medium-grained trough cross-bed-ded sandstone unit at least 80 m thick, with mudstone interbeds and minor volcanic pebbles (Fig. 4C). The thickness of the mudstone interbeds varies from 15 cm up to 50 cm and these occur in the upper part of the bed, i.e., before the transition to the Gausta Formation. The paleoflow measurements indicate flow directions from present day northeast to south-southwest. This topmost bed represents sinuous-crested linguoid (3-D) dunes.

Contact between the Heddersvatnet Formation and the Gausta Formation

The primary upper contact between the Heddersvatnet Formation and the Gausta Formation is best exposed in the Heddersfjell area (Fig. 2), where the trough cross-bedded sandstone becomes more mature and free of feld-spar and volcanic material. The contact is transitional, with an arkosic sandstone texture in the contact rock, which contains up to 15 % sericite-rich pseudomatrix between the grains. This contact rock still contains a few

Fig. 5. A schematic paleose-dimentological figure of the Heddersvatnet Formation depositional environment (modified from Köykkä, sub-mitted). The rift setting was possibly combined with the development of the intra-rift faulting. Figure not to scale.


Heddersvatnet and Vemork Formation samples are char-acterized by relatively low Zr/TiO2 and Nb/Y ratios, and also classify as basalts (Fig. 6B).

The multi-element spider diagram shows that the Heddersvatnet Formation basaltic lavas are generally enriched in light rare earth elements (LREE) and high field strength elements (HFSE), relative to primitive mantle and N-MORB, except for Sr (Fig. 6C). The clus-tered data for Nb, P, Zr and Ti suggest that HFSE were relatively immobile during the regional metamorphism (Fig. 6C). The scatter for Rb, Ba, Th, U, Sr, Hf and Dy may suggest that these elements were mobile during metamorphism (Fig. 6C). The minor variations in U and Zr may reflect analytical biasing, and those in Sr and Ba may indicate fractional crystallization of plagioclase, and variable plagioclase phenocryst abundance in the sam-ples (Menuge & Brewer, 1996). In general, the Hedders-vatnet Formation basalts express a geochemical signature similar to that of some continental flood basalts and to the Vemork Formation basalts. Low Ni (<49 ppm) and Cr (<42 ppm) values suggest substantial crystal fraction-ation prior to eruption, which is also a general feature of continental flood basalts. The high concentrations of some of the LILE elements (e.g., Rb, Ba and K), which is also a characteristic feature of continental flood basalts, suggest crustal contamination. However, these elements may have been disturbed by metasomatism and/or regional metamorphism.

The K2O vs. Na2O values and the low FeO/MgO ratios indicate a high-K alkaline series, a conclusion that is also supported by the relatively high Al2O3 content asso-ciation. The negative Nb anomaly, together with enrich-ment in Nb and Zr compared to primitive mantle and N-MORB, may suggest a mantle source for the Hedders-vatnet Formation basalts (Fig. 6C). However, supplemen-tary isotopic geochemical data may be needed for reliable interpretation. The negative Nb anomaly relative to Th and Ce is a general feature of volcanic arc basalts (Pearce, 1996), but the enrichment in Nb, Zr, Ti and Y com-pared to N-MORB argues against this origin (Fig. 6C). In addition, the relatively high Ti/Y ratios suggest deep intraplate melting, which is a characteristic feature of within-plate basalts (op. cit.). Melting at plate boundaries (MORB or VAB) is generally shallow and results in lower Ti/Y ratios. Compared to the middle and upper con-tinental crusts, there is a slight enrichment in P, Ti and Y, and a depletion of Th and Sr (Fig. 6C). However, the general trend suggests at least some contamination of the Heddersvatnet Formation basaltic lavas by continent al crust (Fig. 6C).

Only HFSE (e.g. Ti, Zr, Y, Nb, V) were used for the paleo-tectonic reconstruction. These elements are known to be comparatively immobile and stable under low-grade metamorphic conditions. The Ti vs. Zr and Zr/Y vs. Zr tectonic discrimination diagrams by Pearce (1982) and Pearce & Norry (1979) were used to separate the within-

Geochemistry of the basaltic lavasAccording to Brewer & Atkin (1989) and Menuge & Brewer (1996), basaltic lavas of the Vemork Forma-tion may have formed by partial melting of depleted mantle. These feature within-plate chemical signatures, with trace element patterns similar to Phanerozoic con-tinental flood basalts. Brewer & Atkin (1989) argued that low-grade metamorphic events may have caused disturbances to the entire rock geochemical systems, a situation which may result in misleading tectonic recon-structions. They suggested that the Vemork Formation basalts have passed through (i) early burial-type meta-morphism, (ii) regional metamorphism and (iii) thermal metamorphism. According to these researchers, burial-type metamorphism resulted in the formation of abun-dant granular epidotes and CaO values of greater than 10.0 wt %. Regional metamorphism caused abundant biotites associated with high K2O (>1.0 wt %). Thermal metamorphism resulted in the development and occur-rence of hornblende, biotite, scapolite and opaque por-phyroblasts, together with the following characteristic geochemical signatures: TiO2 >2.5 wt. %, P2O5 >0.40 wt. %, Zr >200 ppm, and Cr <80 ppm.

Fourteen basaltic lava samples were collected from the Heddersvatnet Formation, and were crushed and milled for geochemical analysis to characterize their geochemi-cal signatures. Major and trace elements were analyzed at the University of Oulu‘s Institute of Electron Optics, using a Siemens SRS 303AS X-ray fluorescence spec-trometer with pressed powders (Table I). All samples that exhibited petrographic evidence of effective burial metamorphism or complete metamorphic recrystalliza-tion were excluded. The petrographic and geochemical evidence related to the Heddersvatnet Formation basaltic lavas suggest that none of the selected samples showed metamorphic disturbance imprints that were similar to those described by Brewer & Atkin (1989). Accordingly, the selected samples represent a reliable data source for inferring the paleotectonic setting. Geochemical data on mafic volcanic rocks in the Heddersvatnet Formation are presented in Figs. 6A-F and are compared to published data on probable correlative rocks of the Vemork Forma-tion (Menuge & Brewer, 1996).

The Heddersvatnet Formation basaltic lavas are charac-terized by relatively high Al2O3 (16 – 18 wt. %) and K2O contents (1.73 – 3.44 wt. %), a low MgO/FeO ratio (<1.0), high Ti/Y ratio (>285), and very low Cr and Ni contents (<50 ppm). Analyzed samples generally have 45 – 50 wt. % SiO2 and ~4.0 wt % total alkali content, and can con-sequently be classified as basalts (Fig. 6A). The scatter of the Vemork Formation samples suggests that mobility of SiO2 and alkali elements took place during the regional metamorphism (Menuge & Brewer, 1996). By eliminat-ing these elements using the Zr/TiO2 vs. Nb/Y discrimi-nation diagram, a more clustered plot can be obtained for the Vemork Formation (Fig. 6B). In this diagram, the


Tabl

e 1.

Geo

chem

ical

who

le-r

ock

anal

yses

of t

he H

edde

rsva

tnet

For

mat

ion

basa

ltic

lava

s.Sample

HF-01

HF-02

HF-03

HF-04

HF-05

HF-06

HF-07

HF-08

HF-09

HF-10

HF-11

HF-12

HF-13

HF-14

SiO

248

.54

48.9

749

.04

48.6

448

.50

48.1

548

.46

48.3

048

.20

48.4

047

.02

46.4

747

.47

47.7

9Ti

O2

2.40

2.33

2.23

2.46

2.39

2.25

2.39

2.31

2.36

2.37

2.59

2.80

2.69

2.62

Al 2O

317

.14

17.6

617

.22

17.6

317

.82

17.3

917

.79

17.4

717

.64

17.6

417

.41

16.0

117

.22

17.3

5Fe

O12

.07

11.9

711

.57

12.1

312

.00

11.5

812

.01

11.7

411

.89

11.9

013

.41

14.4

713

.59

13.2

8M

nO0.

120.

140.

140.

120.

140.

140.

130.

140.

130.

130.

190.

210.

170.

17M

gO11

.22

12.0

411

.88

11.4

511

.71

11.6

911

.85

11.6

711

.68

11.6

510

.46

9.57

10.4

710

.55

CaO

1.57

1.65

2.53

1.56

1.64

2.54

1.59

2.23

2.00

1.95

3.89

4.76

3.35

3.20

Na 2O

0.71

0.84

0.84

0.69

0.80

0.79

0.73

0.80

0.73

0.76

1.62

2.23

1.55

1.38

K2O

3.44

3.27

3.12

3.42

3.26

3.13

3.34

3.18

3.30

3.27

2.45

1.73

2.52

2.70

P 2O5

0.70

0.69

0.66

0.67

0.66

0.64

0.67

0.65

0.66

0.66

0.75

0.78

0.75

0.74

LOI

5.99

6.20

5.80

5.79

5.85

6.34

5.73

6.17

5.98

6.01

5.09

5.03

5.16

5.22

Tota

l98

.14

99.7

699

.42

98.9

699

.11

98.4

999

.17

98.7

098

.80

98.9

399

.98

99.2

199

.96

99.9

8

Cl

3637

3122

1318

4532

3717

1927

3837

Sc38

3334

3533

3238

3531

3137

3437

37V

287

293

274

301

288

275

294

285

287

279

287

270

282

281

Cr

3333

2935

3234

3229

3431

3642

3532

Ni

4242

4241

4043

4039

3941

4549

4545

Zn85

8885

8585

8388

8786

8411

914

312

111

1G

a32

3230

3132

3130

2933

3029

2628

27G

e25

1217

1313

2118

1623

1112

2125

16Rb

9289

8694

9086

8987

8988

6342

6469

Sr28

2940

3030

4129

3734

3313

514

210

510

2Y

4447

4747

5050

4850

4850

4742

4346

Zr24

523

823

724

924

423

924

424

024

424

417

715

719

219

7N

b6

66

76

56

77

76

67

6Ba

936

878

845

931

875

810

896

852

871

904

731

500

710

790

La31

3635

2823

3434

3338

1834

2931

33C

e66

7356

5873

5774

5560

6859

7052

62Pr

66

86

86

88

76

75

76

Nd

2838

3733

3337

3638

3240

4341

4037

Dy

69

128

64

412

37

1710

1417

Er9

58

103

115

4nd

18nd

6nd

12H

f4

5nd

nd7

7nd

nd7

ndnd

ndnd

5Th

23

46

62

33

26

ndnd

ndnd

U4

4nd

21

ndnd

nd3

43

26

2M

ajor

ele

men

ts w

t.%; T

race

ele

men

ts p

pm.;

nd =

Bel

ow d

etec

tion

limit.


Figs. 6A-F. Binary discrimination diagrams (A-B and D-F) for the analyzed samples from the Heddersvatnet (circle plots) and Vemork (line plots) Formations and a spider diagram (C) for the Heddersvatnet Formation samples. The Vemork Formation data are from Menuge & Brewer (1996). A) Total alkali vs. SiO2 plot (TAS). Fields after Le Maitre et al. (1989) B = basalt, O1 = basaltic andesite, S1 = trachybasalt, and U1 = basanite or tephrite. B) Zr/Ti vs. Nb/Y plot. The fields are from Pearce (1996). C) The mantle primitive normali-zed spider diagram for the Heddersvatnet Formation. Normalization after Sun & MacDonough (1989). The middle and the upper crust reference curve after Rudnick & Gao (2003). The N-MORB reference curve after Sun & McDonough (1982). D) Ti vs. Zr plot. Fields after Pearce & Cann (1973). E) Zr/Y vs. Zr plot. Fields after Pearce & Norry (1979). F) V vs. Ti/100 plot. Fields compiled after Shervais (1982). A = alkali basalt + OIB, B = continental flood basalt, C = MORB + BAB, D = calc-alkaline basalt, and E = arc tholeiite.


(Fig. 2). The deposits on the west side of the fault zone are basaltic lava flows dominated by minor arkosic sand-stone interbeds, whereas deposits on the east side of the fault zone are arkosic sandstones and conglomerates with minor basaltic lava beds (Figs. 2 & 4A-C). The thickness of the formations also varies greatly on the two sides of the Gausdalen fault zone. The Vemork Formation is aboutca. 2 km thick, whereas the Heddersvatnet Forma-tion is aboutca. 400 m thick.

The open question today is the lithostratigraphic division and nomenclature of the lowerbottom part of the Tele-mark supracrustals, i.e. whether the Vemork Formation should be included in the Rjukan Group or not (see Laa-joki et al, 2002). The sub-Heddersvatnet unconformity subdivides the Rjukan and Vindeggen Groups into two different packages. However, this study supports the idea that thisthe sub-Heddersvatnet unconformity contin-ues under the Vemork Formation. This means that the position of the Vemork Formation as part of the Rjukan Group is not logical. The Vemork Formation should be included as part of the Vindeggen Group, and the Tuddal Formation should be separate, as proposed by Dahlgren (1990a, 1990b). It is evident that more detailed geochem-ical and fieldwork studies need to be carried out to clarify the stratigraphic framework of the Telemark supracrust-als.

According to Laajoki & Corfu (2007) & Corfu & Laajoki (2008), the deposition and sedimentation of the Vemork Formation started at around 1495 ± 2 Ma, whereas the felsic magmatism age of the Tuddal Formation is ca. 1512 ± 10 Ma (Bingen et al., 2005). This age difference would not allow much time for the formation of an angu-lar unconformity between the formations. However, it is possible that the formation and the nature of the sub-Heddersvatnet unconformity may be rather explained by rotation of fault blocks inside the rift basin rather than by orogenic deformation. Fault block rotation or move-ment perhaps requires lessdoes not require much time.

The magmatic and tectonic evolution of the Hedders-vatnet Formation may be explained by continued rifting and possible development of intra-rift faulting follow-ing the late stage deposition of the Vemork Formation. This was accompanied by more evolved magma, with some crustal contamination during the deposition of the Heddersvatnet Formation. The termination of the volca-nism suggests a gradual change from the syn-rifting to the post-rifting stage, probably in the late stage of sedi-mentation, and deposition of marine-dominated sedi-ments of the Gausta Formation. The end of volcanism, and the input of extrabasinal epiclastic conglomerates, indicate tectonic evolution prior to deposition of the Gausta Formation. This is also supported by the whole-rock geochemistry and the difference in detrital frame-work between the Heddersvatnet and Gausta Formations (Köykkä, submitted)

plate tectonic setting from other settings (Figs. 6D & E). These simple diagrams are probably more accurate than, for example, the Ti/100-Zr-Yx3 ternary diagram, especially in the case of transitional tectonic settings or non-primary magmas (cf. Pearce, 1996, Fig. 9). The com-position of the Heddersvatnet and Vemork Formation basaltic lavas is compatible with a continental within-plate paleotectonic setting (Fig. 6D & E). In the V vs. Ti discrimination diagram, the Heddersvatnet Formation basaltic lavas plot within the field of continental flood basalts and alkali basalts + OIB, whereas the Vemork Formation basaltic lavas cluster near the alkali basalt + OIB field (Fig. 6F).

DiscussionThis study supports the idea that the sub-Heddersvatnet unconformity continues under the Vemork Formation, and that the Vemork Formation interfingers with the Heddersvatnet Formation. The term interfinger is used here to denote a time-equivalent stratum of volcanic-sedimentary rocks, which change laterally from one type to another within a zone that comprises two distinct types of formations. Although it is a matter of definition, the boundary between these formations can be localized to the Gausdalen fault (Figs. 1 & 2).

The key problem seems to be the contact between the Tuddal and Vemork Formations, which has recently been considered to be more or less conformable (see Laajoki & Corfu, 2007 and references therein). The same contact is an angular unconformity between the Tuddal and Hed-dersvatnet Formations. However, the contact between the Tuddal and Vemork Formations is only observable locally in areas where the Vemork Formation overlies and cuts a homogeneous Tuddal Formation porphyre. This means that the angular unconformity may exist but is may not be visible. The contact should be mapped in more detail at botha local and regional scales. At present, an angular unconformity cannot be ruled out. In addi-tion, as stated in Laajoki & Corfu (2007), the regional tectonic synthesis of the Rjukan Rift Basin should be studied properly before the nature and the causes of the different unit boundaries can be understood. The extru-sion of abundant mafic lavas of the Vemork Formation reflects a change in the magmatic regime in the rift basin, possibly coeval with a syn-rift tectonic phase. Thus, it is possible that the lower members of the Vemork Forma-tion represent post-folding felsic volcanism, i.e., the late stage and final inputs of the felsic magma chamber(s).

The Vemork Formation is mostly unexposed, and the different sedimentary units cannot be followed for any great distance laterally (see Laajoki & Corfu, 2007). It is therefore difficult to directly correlate directly smaller individual volcanic-sedimentary units between the Vemork and the Heddersvatnet Formations. The lateral correlation is also hindered by the Gausdalen fault line


southwest Scandinavia. Norwegian Journal of Geology, 85, 87-166.Brewer, T.S. & Atkin B.P. 1987: Geochemical and tectonic evolution of

the Proterozoic Telemark supracrustals, southern Norway. In: Pha-raoh, T.C., Beckinsale, R.D. and Rickard D.T. (eds.) Geochemistry and mineralization of Proterozoic volcanic suites Geological Society Special Publications, 33, 471-487.

Brewer, T.S. & Field, D. 1985: Tectonic environment and age relati-onships of the Telemark supracrustals. NATO ASI Series, Series C: Mathematical and Physical Sciences, 158, 291-307.

Brewer, T.S. & Menugue, J.F. 1998: Metamorphic overprinting of Sm-Nd isotopic systems in volcanic rocks. The Telemark Super-group, southern Norway. Chemical Geology, 45, 1-16.

Cas, R.A.F. & Wright, J.V. 1987: Volcanic successions: modern and ancient: a geological approach to processes, products and succes-sions. London : Allen & Unwin, 528 p.

Corfu, F. & Laajoki, K. 2008: An uncommon episode of mafic mag-matism at 1347 Ma in the Mesoproterozoic Telemark supracrustals, Sveconorwegian orogen – Implications for stratigraphy and tecto-nic evolution. Precambrian Research, 160, 288-307.

Dahlgren, S.H., Heaman, L. and Krogh, T.E. 1990a: Geological evolu-tion and U-Pb geochronology of the Proterozoic central Telemark area, Norway (abstract). Geonytt 17 (1), 38.

Dahlgren, S.H., Heaman, L. and Krogh T.E. 1990b: Precise U-Pb zir-con and baddeleyite age of the Hesjåbutind gabbro, central Tele-mark area, southern Norway. Geonytt 17 (1), 38.

Dahlgren, S.H., Heaman, L. & Krogh, T.E. 1990b: Precise U-Pb zircon and baddeleyite age of the Hesjåbutind gabbro, central Telemark area, southern Norway. Geonytt 17(1), 38.

Dons, J.A. 2003: Berggrunnskart ÅMOTSDAL 1514 II - M 1:50 000. Norges Geologiske Undersøkelse.

Dons, J.A., Heim, M. and Sigmond, E.M.O. 2004: Berggrunnskart FRØYSTAUL 1514 I, 1:50 000. Norges Geologiske Undersøkelse.

Dons, J.A. 1960a: Telemark supracrustals and associated rocks. In: Holtedahl O. (Ed.), Geology of Norway. Norges Geologiske Under-søkelse, 49-58.

Dons, J.A. 1960b: The stratigraphy of supracrustal rocks, granitization and tectonics in the Precambrian Telemark area, Southern Norway. Norges Geologiske Undersøkelse, 212h, 1-30.

Dons, J.A., 1961. Berggrunnskart. RJUKAN; 1.100 000. Norges geolo-giske undersøkelse. Trondheim.

Falkum, T. & Petersen, J.S. 1980: The Sveconorwegian Orogenic Belt, a case of Late-Proterozoic Plate-Collision. Geologische Rundschau, 69, 622–647.

Folk, R. L. 1980: Petrology of sedimentary rocks. Austin: Texas Hemphill’s Book Store, 190 p.

Gaál, G. & Gorbatschev, R. 1987: An outline of the Precambrian evolu-tion of the Baltic Shield. Precambrian Research, 35, 15-52.

Köykkä, J. submitted: Sedimentology and geochemistry of the Meso-proterozoic alluvial fan, Telemark, South Norway: implications for paleotectonic setting, paleoweathering, and depositional processes. Sedimentology, xx, xxx-xxx.

Laajoki, K., Corfu, F. and Andersen, T. 2002: Lithostratigraphy and U-Pb geochronology of the Telemark supracrustals in the Bandak-Sauland area, Telemark, South Norway. Norsk Geologisk Tidsskrift, 82, 119-138.

Laajoki, K. & Corfu, F. 2007: Lithostratigraphy of the Mesoproterozoic Vemork Formation, central Telemark, Norway. Bulletin of the Geo-logical Society of Finland, 79, 41-67

Laajoki, K. 2005: The Mesoproterozoic sub-Heddersvatnet unconfor-mity, Telemark, South Norway. Bulletin of the Ggeological Society of Finland, 77, 49-63.

Le Maitre, R.W., Bateman, P., Dudek, A., Keller, J., Lameyre Le Bas, M.J., Sabine, P.A., Schmidt, R., Sorensen, H., Streckeisen, A., Wool-ley, A.R. and Zanettin, B. 1997: A classification of igneous rocks and glossary of terms. Blackwell, Oxford, 193 p.

Mackenzie, W.S, Donaldson, C.H. & Guildford, C. 1982: Atlas of igne-ous rocks and their textures. Halstedt Press, 148 p.

ConclusionsNew sedimentological observations and geochemical data reported in this study show that the Heddersvatnet Formation and the Vemork Formation interfinger with each other. This suggests that the sub-Heddersvatnet unconformity continues under the Vemork Formation and represents a likely angular unconformity above the Tuddal Formation. The stratigraphic position of the Vemork Formation as part of the Rjukan Group is thus brought into question; the Vemork Formation would be better treated as part of the Vindeggen Group

The geochemistry of the Heddersvatnet Formation basal-tic lavas are characterized by a geochemical signature similar to that of some continental flood basalts and the Vemork Formation basalt. The composition is compat-ible with a continental within-plate paleotectonic setting.

The Heddersvatnet Formation is interpreted as repre-senting alluvial fan sedimentation during the syn-rift phase of the sedimentary basin (Fig. 5 & Köykkä, submit-ted). The volcanic-sedimentary Vemork Formation grad-ually filled the rift, whereas the Heddersvatnet Forma-tion was deposited in or near uplifted flank(s) of the rift basin. The magmatic and tectonic evolution of the Hed-dersvatnet Formation is characterized by more evolved magma, with some crustal contamination during deposi-tion. The termination of the volcanism and sedimentary lithofacies assemblages suggests a gradual change from the syn-rifting to the post-rifting stage, and deposition of marine-dominated sediments of the Gausta Formation.

Acknowledgements The study is based on fieldwork and geochemical laboratory work that was carried out in the years 2007-2008 and . This work was supported in part by the Finnish National Graduate School of Geology and the Faculty of Science at the University of Oulu. The aut-hor would like to thank B. Bingen and A. Solli for constructive reviews and emeritus professor K. Laajoki for guidance.

References

Andersen, T. & Laajoki, K. 2003: Provenance characteristics of Meso-proterozoic metasedimentary rocks from Telemark, South Norway: a Nd-Isotope Mass-Balance Model. Precambrian Research, 126, 95-122.

Andersen, T. 2005: Terrane analysis, regional nomenclature and crus-tal evolution in the southwest Scandinavian Domain of the Fennos-candian Shield. GFF 127, 159-168.

Atkin, B.P. & Brewer, T.S. 1990: The tectonic setting of basaltic magma-tism in the Kongsberg, Bamble and Telemark sectors, Southern Norway. In: Gower, C.F., Rivers, T. & Ryan, B. (eds.) Mid-Protero-zoic Laurentia-Baltica. Special paper – Geological Association of Canada, 38, 471-483.

Bingen, B., Birkeland, A., Nordgulen, Ø., Sigmond, E.M.O., Tucker, R.D., Mansfeld, J. and Höghdahl K. 2003: Relations between 1.19 – 1.13 Ga continental magmatism, sedimentation and meta-morphism, Sveconorwegian province, South Norway. Precambrian Research, 124, 215-241.

Bingen, B., Skår, Ø., Marke,r M., Sigmond, E.M.O., Nordgulen, Ø., Ragnhildstveit, J., Mansfeld, J., Tucker R.D. and Liegeois, J.-P. 2005: Timing of continental building in the Sveconorwegian orogen,


McPhie J., Doyle M.G. and Allen R.L. 1993: Volcanic Textures: A guide to the interpretation of textures in volcanic rocks. Hobart: CODES, University of Tasmania, 198pp.

Menuge, J.F. & Brewer, T.S. 1996: Mesoproterozoic anorogenic mag-matism in southern Norway. In: Brewer T.S. (ed.) Precambrian crustal evolution in the North Atlantic regions. Geological Society Special Publication, 112, 275-295.

Miall, A., 2006: The geology of fluvial deposits: Sedimentary facies, basin analysis, and petroleum geology. Springer (Corr. 3rd printing), 582 pp.

Pearce, J.A. 1996: A user’s guide to basalt discrimination diagrams. In: Wyman, D.A. (ed.) Trace element geochemistry of volcanic rocks: applications for massive sulfide exploration. Geological Association of Canada, Short Course Notes, 12, 79-113.

Pearce J.A. & Cann J.R. 1973: Tectonic setting of basic volcanic rocks determined using trace element analyses. Earth and Planetary Sci-encetific Letters, 19, 290-300.

Pearce, J.A. & Norry, M.J. 1979: Petrogenetic implications of Ti, Zr, Y and Nb variations in volcanic rocks. Contributions Mineralogy and Petrology, 69, 33-47.

Pettijohn, F., Potter, P. and Siever, R. 1987: Sand and sandstone. Sprin-ger-Verlag, New York, 533 pp.

Pettijohn F. 1975: Sedimentary Rocks (3rd Edition). Harper and Row, New York, 628 pp.

Richards, F.L. 1994: Sveconorwegian deformation and mega-scale fold interference in the Telemark region of South Norway. Terra Nova, Abstract Supplement 2, 16.

Richards, F.L. 1998: The structural geology of the Telemark, supracrustal suite, South Norway. Ph.D. Thesis. University College Cork. 296 p. + 12 appendix.

Rudnick, R.L. & Gao, S. 2003: The Composition of the continental crust. In: Rudnick R.L. (ed.) The Crust, Treatise on Geochemistry, Elsevier-Pergamon, Oxford, 3, 1-64.

Shervais, J.W. 1982: Ti-V plots and the petrogenesis of modern and ophiolitic lavas. Earth and Planetary Sciencetific Letters, 59, 101-118.

Sigmond, E.M.O., Gjelle, S. and Solli, A. 1997: The Rjukan Proterozoic rift basin, its basement and cover, volcanic and sedimentary infill, and associated intrusions. Norges Geologiske Undersøkelse Bulletin, 433, 6-7.

Sigmond, E.M.O. 1998: Geologisk kart over Norge, Berggrunnskart ODDA, M 1.250 000. Norges geologisk undersøkelse, Trondheim.

Starmer, I.C. 1993: The Sveconorwegian orogeny of Southern Norway, relative to deep crustal structures and events in the North Atlantic Proterozoic Supercontinent. Norsk Geologiske Tidsskrift, 73, 109-132.

Sun, S.S. & McDonough, W.F. 1989: Chemical and isotopic systematic of oceanic basalts: implications for mantle composition and proces-ses. In: Saunders A.D. & Norry M.J. (eds.) Magmatism in ocean basins. Geological Society London, Special Publication, 42, 313-345.

Werenskiold, W. 1910: Om Øst-Telemarken. Norges geologiske under-søkelse, 52, 69 p.

Wyckoff, D. 1934: Geology of the Mt. Gausta Region in Telemark, Norway. Norsk Geologisk Tidsskrift, 13, 1-72.

lithostratigraphy of the mesoproterozoic telemark ... · e-mail: [email protected] (j. köykkä...

Documents