Bulletin of the Geological Society of Finland,Vol. 77, 2005, pp. 49–63 The Mesoproterozoic sub-Heddersvatnet unconformity, Telemark, South Norway Kauko Laajoki Department of Geosciences, University of Oulu, P. O. Box 3000, 90014 University of Oulu, Finland Abstract The 1.51 Ga Rjukan group,Telemark, S Norway, is divided into felsic volcanic rocks of the Tuddal formation and mafic rocks of the Vemork formation. It is overlain by the sedimen- tary rocks of the Vindeggen group (1.50–1.17 Ga), starting with the arkosic Heddersvatnet formation.The contact between the Rjukan and Vindeggen groups has been variably inter- preted in the literature.New field data indicate that the contact corresponds to an uncon- formity,corroborating Wyckoff’s early observations in 1934.The contact is referred to as the sub-Heddersvatnet unconformity. The nature of the contact varies. Around Lake Heddersvatnet, it most likely represents an angular unconformity with a sharp, erosional surface,whereas near Lake Skjesvatnet a thin in situ palaeoweathering crust developed on a massive Tuddal porphyry defines it. These observations indicate that the sub-Heddersvatnet unconformity represents a deeply weathered land surface cutting diverse folded Tuddal units. How big is the time gap it rep- resents and the nature of the pre-Vindeggen deformation are open questions as the sed- imentation age of the Heddersvatnet formation is unknown and the structure of the Rju- kan group has not been studied on a regional scale. Key words: metasedimentary rocks, metavolcanic rocks, lithostratigraphy, unconformities, Mesoproterozoic,Rjukan,Vindeggen,Telemark, Norway e-mail: [email protected].fi 50 K. Laajoki 1. Introduction The Telemark sector of the Sveconorwegian basement tary rocks known as the Telemark supracrustals (Sig- in southern Norway is known for the greenschist- to mond et al., 1997). epidote-amphibolite facies sequence of Mesoproter- Recent studies (Dahlgren et al., 1990a; Bingen et ozoic volcanic and sedimentary rocks known as the al., 2001; 2003; 2005, Laajoki et al., 2002; Laajoki, Telemark supracrustal belt. Tr aditionally, these rocks 2002) have shown that the Telemark stratigraphy is aredivided into the Rjukan (oldest), Seljord, and morecomplicated than Dons’(1960a, b) tripartite Bandak (youngest) groups (Dons, 1960a, b). Early subdivision into the Rjukan, Seljord, and Bandak authors (Wyckoff, 1934; Dons, 1960a, b) interpreted groups and the existence of a major angular uncon- the contacts between these units as unconformities. formity between the Rjukan and Seljord groups has The Rjukan group included two formations, the Tu d- been questioned (Starmer, 1993; Menuge & Brew- dal formation made up of felsic metavolcanic rocks er, 1996; Brewer & Menuge, 1998). For the purpose overlain by the mafic metavolcanic and arenitic meta- of this study it is enough to say that the most im- sedimentary rocks of the Vemork formation of (Dons, portant angular unconformity within the Telemark 1960a, b). The Seljord group consisted three quartz- belt is the sub-Svinsaga unconformity (Laajoki et al., ite and three schist (mudstone) formations. Dahlgren 2002), which subdivides the Telemark supracrustals et al. (1990a) suggested that the Vemork formation into the Vestfjorddalenian and Sveconorwegian se- does not belong to the Rjukan Group, but should be quences (Laajoki & Lamminen, 2005), of which the included in the Seljord group. However, as according former comprises the c. 1.51 Ga old (Dahlgren et al., to Dons (1960a, b) no unconformity can be seen be- 1990a; Sigmond, 1998; Bingen et a l., 2005) felsic tween the Tuddal and Vemork formations they both volcanites of the Tuddal formation, the mafic meta- are kept within the original Rjukan group in this pa- volcanites, and interbedded metasediments of the Ve- per.Recently the Seljordgroup has been subdivid- ed into the Vindeggen (older) and Lifjell (younger) groups (Laajoki et al., 2002). This paper addresses the unconformity between the Tu ddal formation of the Rjukan group and the Heddersvatnet formation, the lowermost unit of the Vindeggen group. 2. Geological setting and lithostratigraphy The study area is located in the northern part of the Sveconorwegian Telemark sector (Andersen, 2003) of the Southwest Scandinavian Domain (Gaál & Gor- batschev, 1987) of the Fennoscandian (Baltic) Shield (Fig. 1). Most of the Precambrian crust in South Nor- way has been affected by Sveconorwegian deforma- tion and metamorphism (1.2–0.9 Ga), which have obliterated primarystratigraphic relationships (e.g. Starmer, 1993). The northern part of the Telemark Fig. 1. Sketch map of the Sveconorwegian province (mod- ified from Bingen et al., 2001).The area coveredbyFig. sector forms, however, an exception, as it is under- 2isframed. Numbered sectors west of the Oslo rift: lain by rather well preserved volcanic and sedimen- (1) Bamble,(2) Kongsberg, The Mesoproterozoic sub-Heddersvatnet unconformity,Telemark, South Norway51 mork formation of the Rjukan group and the overly- tals. Unfortunately, none of the units have been es- ing quartzite-dominated Vindeggen group (see leg- tablished formally in the sense of the present Norwe- end in Fig. 2). The Sveconorwegian units will not be gian recommendations (Nystuen, 1986, 1989) and treated in this paper, as the main target is the intra- only brief descriptions are available. That is why all Vestfjorddalenian unconformity between the Rjukan the unit names used in this paper are informal. Dons and Vindeggen groups. (1960a, b) included the basal conglomerates and asso- Dons’(1960a,b) establishedthe classical lithos- ciated arkosites that locally overlie the Rjukan group tratigraphic nomenclature of the Telemark supracrus- in the lower part of his Gausta formation. However, Fig. 2. Simplified geological mapofthe southern part of the Rjukan rift basin in central Te lemark (simplified and modified from Dons & Jorde, 1978).The sub-Hed- dersvatnet unconformity (SHU) is shown by dashed line and the area of Fig. 3 and locations of the sta- tions (bold numbers) dis- cussed in the text arein- dicated. Thick lines mark faults. Form lines within the Vindeggen group indi- cate bedding trends. 52 K. Laajoki Ta ble1.Previous shortstatements of the relationships between the Rjukan group/Tuddal formation/Vemork for- mation, and Seljordgroup (Vindeggen group in present usage). Rjukan gr. /Seljord gr. con- Tuddal fm. /Vemork fm. Vemork fm. vs. Seljord gr.Tuddal fm. vs. Seljord gr. tact. contact. Werenskiold, 1910, p 11. Dons, 1960a, p. 50. Dahlgren et al., 1990a. Starmer, 1993. A likely discordance The Vemork fm. rests con- The Vemork fm. represents The Seljord gr. lies uncon- Wyckoff, 1934, p. 15. formably on the Tuddal fm. initial volcanism and sedi- formably on an irregular vol- A large unconformity. Brewer, 1985, p. 12. mentation of the Seljord gr. canic topography of the Rju- Dons, 1960a, p. 6; b, p. 6. The Vemork fm. rests un- kan gr., which has an ero- An angular unconformity. conformably on the Tud- sional surface. Brewer & Menuge, 1998. dal fm. Menuge & Brewer, 1996. The Rjukan and Seljord Brewer & Menuge, 1998. As above, but the contact groups represent one pack- The Vemork fm. lies discon- was considered as discon- age within which the discor- formably1) upon the Tu d- formable1). dance1) of individual units dal fm. Cf. Brewer & Menuge 1998 is a function of the mode of in the first column. deposition. Sigmond, 1998. The Seljord gr. lies discor- dantly1) on the Rjukan gr. Falkum & Petersen, 1980, p. 630. Major angular unconformi- ty could be interpreted as the result of fault tectonics. Richards, 1994. The Rju- kan gr. was deformed before the deposition of the Seljord group. 1) Stratified rocks being rare in the Tuddal formation, discordance and disconformable are a little confusing expressions in this connection as they refer, respectively, to lack of parallelism between adjacent strata and to formations that exhibit essen- tially parallel bedding (Jackson, 1997). as these rocks form an important mappable unit they lies on the Tuddal formation and the Vemork forma- are considered as a formation of their own named the tion in the east and in the west, respectively (see leg- Heddersvatnet formation (Laajoki et al., 2002) after end in Fig. 2). It is, however, possible, that the Hed- the lake around which they are best exposed (Fig. 3). dersvatnet formation interfingerswiththe Vemork For more detailed lithostratigraphy of the Vindeggen formation (Laajoki, 2003). andoverlying groupsit is referred toLaajoki etal. All the rocks have been metamorphosed in green- (2002), Lamminen and Laajoki (2004), and Laajoki schist facies and so meta-prefix should be used in the and Lamminen (2005). rock names, but for simplicity’s sake their protolith The Tuddal formation, the c. 1.5 Ga old floor of names are used in this paper. thestudyarea, occurs as dome-likefold structures, of which the Kovvatnet and Toreskyrkja domes are 3. Previous studies mantled by the Heddersvatnet formation, where- as the Heidalsnutan-Hortenuten and Våerskarven The Rjukan and Seljord (Vindeggen) groups around domes are rimmed by the Vemork formation (Fig. 2). the town Rjukan (Fig. 2) have been mapped in rather This means that the Vindeggen group directly over- great detail (Wyckoff, 1934; Dons, 1961) and a lot of The Mesoproterozoic sub-Heddersvatnet unconformity,Telemark, South Norway53 geochemical work is available on the metavolcanites tacts of lava flows cannot be seen due to lichen cov- of the Tuddal and Vemork formations (Brewer et al., er, metamorphism and