Provenance and sedimentary processes controlling

Sedimentary Geology 375 (2018) 203–217

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Special Issue Contribution: ANALYSIS OF SEDIMENT PROPERTIES AND PROVENANCE

Provenance and sedimentary processes controlling the formation of lower Cambrian quartz arenite along the southwestern margin of Baltica Sanne Lorentzen a,⁎, Carita Augustsson a, Johan P. Nystuen b, Jasper Berndt c, Jens Jahren b, Niels H. Schovsbo d a

Department of Petroleum Engineering, University of Stavanger, 4036 Stavanger, Norway Department of Geoscience, University of Oslo, P.O. Box 1047, Blindern, 0317 Oslo, Norway Institut für Mineralogie, Westfälische Wilhelms-Universität, Correnstrasse 24, 48149 Münster, Germany d Geological Survey of Denmark and Greenland, Øster Voldsgade 10, DK-1350 København K, Denmark b c

a r t i c l e

i n f o

Article history: Received 30 May 2017 Received in revised form 19 August 2017 Accepted 30 August 2017 Available online 5 September 2017 Keywords: Detrital zircon U-Pb geochronology Cambrian Baltica Provenance Quartz arenite

a b s t r a c t Lower Cambrian shallow marine quartz arenite records a transgressive regime related to a global basal Cambrian eustatic sea-level rise. Six sections from southern Norway, southern Sweden, and Denmark are investigated to explore the genesis and sourcing of these mineralogically mature deposits and the early Cambrian tectonosedimentary history of Baltica. U-Pb ages of detrital zircon grains are dominantly 0.9–1.8 Ga, in accordance with transport from the Transscandinavian Igneous Belt (TIB) and domains related to the Sveconorwegian and Gothian orogenies. These zircon grains have a hydrodynamic relation with the quartz grains, suggesting a common provenance for the zircon grains and the main clastic material. Similar provenance for Norway and southern Sweden favors a setting with the present day Sveconorwegian Orogenic Belt originally extending further southeast into the present Skagerrak and Kattegat area and northwest in westernmost Norway. Furthermore, marked provenance differences with other earliest Cambrian deposits on Baltica indicate a catchment divide between the Sveconorwegian Orogenic Belt and the TIB-Gothian domains. First sediment-cycle origin for the studied sandstone is proposed based on: 1) a general low age diversity and 2) a lack of late Palaeoproterozoic and Archaean zircon U-Pb ages, which are typical for Mesoproterozoic quartzite. The high sandstone maturity may instead be the result of prolonged exposure to weathering and reworking processes made possible by the low gradient of the Sub-Cambrian Peneplain. © 2017 Elsevier B.V. All rights reserved.

1. Introduction Thick quartz-arenitic successions are commonly assumed to be formed through recycling processes in high-energy marine and aeolian realms (Pettijohn et al., 1987; Johnsson et al., 1991; Cox and Lowe, 1995; Prothero and Schwab, 1996; Dott, 2003). In these environments with repeated and long-lasting reworking of sand grains, mechanical and chemical breakdown of feldspar and other silicate minerals results in a relative increase in quartz and other mechanically and chemically stable minerals (Blatt and Christie, 1963; Suttner et al., 1981). Recycling of clastic particles derived from break-down of sedimentary rocks is estimated to account for 80% or more of the sedimentary system (Cox and Lowe, 1995, and references therein). Nevertheless, world-wide, several Precambrian and Cambro-Ordovician quartz arenite occurrences have been interpreted as first-cycle deposits. Some of these deposits were produced from intense weathering and erosion in tropical humid climate (Soegaard and Eriksson, 1989; Dott, 2003; Avigad et al., 2005) ⁎ Corresponding author. E-mail addresses: [email protected] (S. Lorentzen), [email protected] (C. Augustsson), [email protected] (J.P. Nystuen), [email protected] (J. Berndt), [email protected] (J. Jahren), [email protected] (N.H. Schovsbo).

https://doi.org/10.1016/j.sedgeo.2017.08.008 0037-0738/© 2017 Elsevier B.V. All rights reserved.

and subsequent transportation from a low-relief source area with low sedimentation rates (Pettijohn et al., 1972; Suttner et al., 1981; Basu, 1985; Chandler, 1988), in addition to aeolian processes and marine reworking. Quartz arenite has also been interpreted to originate from the dissolution of chemically unstable clastic minerals during burial diagenesis (Chandler, 1988; Went, 2013). Lower Cambrian quartz arenite occurs on many continents, such as Baltica, Laurentia, Amazonia and Siberia (Goodwin and Anderson, 1974; Lindsey and Gaylord, 1992; McKie, 1993; Wahab, 1998; Sears and Price, 2003). Quartz arenite deposits are also part of CambroOrdovician occurrences on Gondwana (Avigad et al., 2005; Bassis et al., 2016). These continents formed as separate plates during the Neoproterozoic-Cambrian break-up of the supercontinent Rodinia (Li et al., 2008; Torsvik and Cocks, 2016). Active sea-floor spreading gave rise to early Cambrian eustatic sea-level changes with sea-level high stand and transgression (Haq et al., 1988), resulting in a lower Cambrian quartz arenite drape on denudated continents. These quartz-arenitic formations display many similarities in stratigraphy and petrography, although deposited on different continents (e.g. Swett et al., 1971; Lindsey and Gaylord, 1992; Avigad et al., 2005). The Precambrian basement of Baltica includes Fennoscandia in the north and the East European Platform in the south. Baltica was

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surrounded by passive margins with the exception of the accretionary, latest Neoproterozoic Timanian orogen in the north (Pease et al., 2008; Kuznetsov et al., 2014). It was separated from Siberia, Gondwana and Laurentia by the Ægir, Tornquist and Iapetus oceans, respectively (Hartz and Torsvik, 2002; Cocks and Torsvik, 2005; Torsvik and Cocks, 2005). Furthermore, a rift succession developed along the western margin of the continent during Neoproterozoic time, during which the Sub-Cambrian Peneplain also formed as a low-relief denudation surface on Baltica (Lidmar-Bergström, 1993; Gabrielsen et al., 2015). Transgressive, shallow-marine quartz-arenitic sandstone is found directly on the peneplain (Nystuen, 1982; Kumpulainen and Nystuen, 1985; Moczydłowska and Vidal, 1986; Nystuen, 1987; Nielsen and Schovsbo, 2011). For Baltica, the cause of high mineralogical maturity has been little discussed in previous studies: is the maturity a function of multicyclic reworking? Or first-cycle sand formed during processes of weathering and denudation of basement rocks? Baltica included felsic magmatic and metamorphic rocks as well as Palaeoproterozoic-Neoproterozoic quartz arenite (Singh, 1969; Lidmar-Bergström, 1993; de Haas et al., 1999; Bingen et al., 2001; Laajoki et al., 2002; Andersen et al., 2004; Bingen et al., 2008a). Therefore, the provenance is of significance when discussing the formation of the lower Cambrian quartz arenite on Baltica. Provenance studies of Neoproterozoic-Cambrian sandstone from the Scandinavian part of Baltica are scarce. The studies are mainly restricted to U-Pb age determinations of detrital zircon grains from a CryogenianEdiacaran succession along the northwestern Baltica margin, that was included in lower and middle Caledonian nappe units during Silurian time (e.g. Bingen et al., 2005; Be'eri-Shlevin et al., 2011; Bingen et al., 2011; Lamminen et al., 2015). Detrital zircon ages are also available for Ediacaran-Cambrian sandstone or quartzite of the autochthonous to parautochtonous part of central and northern Norway (Andresen et al., 2014; Zhang et al., 2015), as well as for Cambro-Ordovician shale from southwestern Norway (Slama and Pedersen, 2015; Slama, 2016) and quartzite from Denmark (Olivarius et al., 2015). The method of using U-Pb ages in provenance studies have been criticised for being too low in precision to distinguish source rocks of various continents, and for being insufficiently specific in relating protosources to identifiable rocks in a crystalline basement (Andersen, 2014; Kristoffersen et al., 2014). It has also been emphasised that detrital zircons can only be used to deduce the opposite direction of sediment transport, i.e. from ‘sink’ to ‘source’ (Andersen et al., 2017). In accordance with this statement, we apply U-Pb ages of detrital zircons to identify protosource provinces of the clastic detritus, in combination with analysis of sediment distribution as function of structural framework and sedimentary processes in the present study. The objective of this study is to obtain better knowledge and understanding of the genesis of the high maturity in lower Cambrian quartz arenite on the southwestern margin of Baltica. Emphasis is given to potential source rock areas and the effect of weathering and transportation processes to sediment dispersal. Furthermore, the aim is to improve the tectonosedimentary history of Baltica, by identifying differences in provenance and clastic sediment routing for Norway, Sweden and Denmark. 2. Geological setting Between 570 and 550 Ma (late Ediacaran) Baltica formed an independent continent when it separated from Laurentia as part of the break-up of Rodinia (Cocks and Torsvik, 2005; Torsvik and Cocks, 2016). This break-up resulted in the formation of the Iapetus Ocean, and caused several local basins to form along the northwestern edge of Baltica, as well as intercontinental basins (Siedlecka et al., 2004; Nystuen et al., 2008; Pease et al., 2008). One of these basins, the Hedmark Basin (Fig. 1), was formed as a rift basin facing the Iapetus Ocean on the northwestern margin of Fennoscandia (Kumpulainen and Nystuen, 1985).

At ca. 540 Ma, Baltica was situated 35° to 60° south (Torsvik and Cocks, 2013, 2016), and experienced a warm temperate to moderately humid climate (Willdén, 1980; Dreyer, 1988). By then, the northwestern part of Baltica, the Baltoscandian craton, had been subjected to severe erosion subsequent to the ca. 0.90–1.20 Ga Sveconorwegian-Grenvillian orogeny (e.g., Bingen et al., 2008b). The denudation of the mountain chain resulted in the Sub-Cambrian Peneplain and gave rise to huge amounts of clastic detritus filling the rift basins and shelf areas at the Baltoscandian margins. In southern Scandinavia, the Sub-Cambrian Peneplain was of low relief to extremely flat, with a maximum relief in the order of 40 m (Gabrielsen et al., 2015). The low-relief configuration allowed lower Cambrian transgressive quartz sand to fill shallow elongated depressions, likely drowned incised valleys, on the craton (Gabrielsen et al., 2015). During Terreneuvian time in the earliest Cambrian (Lontonvan Stage, Table 2) the subsequent stepwise transgression caused flooding of the southernmost and westernmost parts of Baltica (Nielsen and Schovsbo, 2011). This transition is reflected in the late Ediacaran to early Cambrian siliciclastic depositional environments in Scandinavia, changing from coastal plain and fluvial to shallow marine, eventually resulting in deep anoxic shelf mud being deposited during middle Cambrian to Early Ordovician time (Nielsen and Schovsbo, 2015). The transgressive deposits in southern Norway, the upper part of the Hedmark Group, was deposited in the allochthonous Hedmark Basin and onto the adjacent cratonic basement of Baltica, in autochthonous position (Gabrielsen et al., 2015). In south-central and southern Sweden, and on Bornholm in Denmark, corresponding Neoproterozoic to lower Cambrian deposits are found directly on Precambrian basement (Fig. 1). 2.1. Structural setting The Baltoscandian craton consists of four main Precambrian domains: 2.6–3.7 Ga Archaean rocks, the 1.77–1.92 Ga Svecofennian Orogenic Belt, the 1.67–1.86 Ga Transscandinavian Igneous Belt (TIB) and the 1.50–1.75 Ga and 0.90–1.14 Ga Gothian and Sveconorwegian orogens and crustal blocks, respectively (Fig. 1). The Archaean rocks, encompassing the Kola-Karelia block, are located in the northeast of the craton and are bounded to the west and southwest by the Palaeoproterozoic Svecofennian Orogenic Belt (Bingen et al., 2008a; Johansson, 2016). The Gothian and Sveconorwegian orogens and crustal blocks occupy the southwestern domain of Fennoscandia, bounded by the TIB in the east. In this paper, emphasis is given to TIB and younger basement rocks. The TIB, being dominated by granitic and rhyolitic volcanic rocks, is subdivided in 1.83–1.86 Ga (TIB 0), 1.76–1.81 Ga (TIB 1) and 1.67– 1.71 Ga (TIB 2–3) magmatic phases (Larson and Berglund, 1992; Andersson et al., 2004; Gorbatschev, 2004; Lamminen et al., 2015). The Sveconorwegian orogenic belt corresponds to the western portion of Fennoscandia that was crustally remobilised during the 0.90–1.14 Ga Sveconorwegian orogeny (Bingen et al., 2008b). It is bounded by the Sveconorwegian Front in the east (Fig. 1a), and composed of increasingly younger Palaeoproterozoic-Mesoproterozoic crust westwards. The Sveconorwegian Front is a deformation and magmatic zone that divided the eastern Blekinge-Bornholm block from the influence of the Sveconorwegian orogeny on the western block (Johansson et al., 2016). In the exposed part of the shield in southern Norway and southern Sweden, the Sveconorwegian orogen can be divided into five main lithotectonic domains with distinct geochronology: the Eastern Segment, Idefjorden, Bamble, Kongsberg and Telemarkia terranes (Bingen et al., 2008b) (Fig. 1a). The Eastern Segment includes deformed granitoid rocks of the TIB dated mainly at ca. 1.64–1.80 Ga. The Idefjorden, Bamble and Kongsberg terranes were formed during the Gothian accretionary event between 1.52 and 1.65 Ga. The 1.48–1.52 Ga Telemarkian accretionary event formed the Telemarkia Terrane in the west. Since early Palaeozoic time, the part of Baltica southwest of the Sorgenfrei-Tornquist Fault Zone has been displaced 15–20 km towards

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b)

a)

b) c)

c) d)

d)

Fig. 1. a) Simplified map of Scandinavia subdivided into Precambrian domains, the Caledonides and the Oslo Rift. The Eastern Segment is here regarded as part of the Transscandinavian Igneous Belt. The main structural elements comprise the Sorgenfrei-Tornquist Fault Zone (STFZ), Mylonite zone (MZ) and the Sveconorwegian Front (SF). The study areas are marked with black rectangles (b–d). Palaeogeographical location of the shoreline and inner shelf is shown as a light-yellow band and the palaeo-location of the Hedmark Basin (HB) is show in grey. b) Mjøsa in Southern Norway, c) Scania in Southern Sweden and d) the southeastern part of Bornholm, Denmark. Locality numbers correspond to numbers in Table 1. The maps are modified from Nystuen (1981), Ahlberg (2003), Möller et al. (2007), Bingen et al. (2008b), Graversen (2009), Nielsen and Schovsbo (2011), Bergh et al. (2015) and Lamminen et al. (2015).

northwest along the fault zone relative to the Fennoscandian shelf (Mogensen, 1994; Olaussen et al., 1994). The volume of Mesoproterozoic pre-Sveconorwegian (1.15–1.35 Ga) and Sveconorwegian magmatic rocks (0.90–1.06 Ga) increases westwards in the Sveconorwegian orogen. This includes a suite of 1.20– 1.22 Ga syenite plutons along the Sveconorwegian Front (Söderlund and Ask, 2006), and 1.14–1.18 Ga volcanic and plutonic rocks in the Telemarkia Terrane (Laajoki et al., 2002). The Sveconorwegian orogen

probably also constitutes the basement of the Norwegian-Danish basin, as inferred from geophysical data and few deep well logs (Thybo, 2001; Olesen et al., 2004; Lassen and Thybo, 2012; Olivarius et al., 2015). The Sveconorwegian orogen thus may extend to the southern boundary of the Baltica plate, beyond the present Sorgenfrei-Tornquist Fault Zone (Fig. 1a). The western Baltoscandian margin and a northern portion of the Sveconorwegian orogen were crustally reworked during the Silurian-

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Devonian Caledonian orogeny (Fig. 1a). Remnants of the Precambrian basement are found in windows, the largest being the Western Gneiss Region in western Norway, and in Caledonian thrust sheets (e.g., Lamminen et al., 2011). The Neoproterozoic Hedmark Group was thrusted at least 130 km towards the east-southeast within a thrust sheet of the Lower Allochthon, the Osen-Røa Nappe Complex (Nystuen, 1981; Morley, 1986; Nystuen, 1987). Thus, lower Cambrian strata can be found in autochthonous and para-autochthonous position on top of the Precambrian basement as well as in an allochthonous position in the Osen-Røa Nappe Complex (Bjørlykke et al., 1976; Nystuen, 1981). 2.2. Stratigraphic setting The emphasis of this study is the quartz arenite of the Hardeberga Formation in southern Sweden and on Bornholm, and the Ringsaker Member of the Vangsås Formation from southern Norway. 2.2.1. Southern Norway The Vangsås Formation represents the final filling stage of the Hedmark Basin, and constitutes the main rock unit in the frontal part of the Osen-Røa Nappe Complex east and west of the Oslo Graben. At

the type section Lake Mjøsa (Fig. 1b), the basal Vardal Member of the Vangsås Formation (Fig. 2) reflects a change in the depositional environment from a prograding delta to a braided river system. It consists mainly of arkosic sandstone and conglomerate, with the upper ca. 130 m comprising fluvial red and grey, trough cross-bedded sandstone and sparse layers of shale (Bjørlykke et al., 1976; Dreyer, 1988). The Proterozoic-Cambrian boundary is suggested to exist in the lower part of the member (Vidal and Nystuen, 1991a). The arkosic Vardal Member is gradually succeeded by the quartzarenitic Ringsaker Member (Bjørlykke et al., 1976; Dreyer, 1988), which is 20–120 m thick at Mjøsa. Both at Mjøsa and Glomstadfossen (Table 1), the Ringsaker Member is part of the Osen-Røa Nappe Complex, but it is autochthonous beneath the nappe on the eastern side of Rendalen. At Mjøsa, it consists of white to blue-grey, wellsorted and mainly medium-grained, low-angular cross-bedded sandstone with very little feldspar and matrix (Bjørlykke et al., 1976). The deposits represent a storm-and-wave-dominated, shallow-marine environment with minor tidal influence (Bjørlykke et al., 1976; Nystuen, 1982, 1987; Dreyer, 1988). Syn-sedimentary rifting, particularly in the eastern part of the basin, likely caused axial sediment transport before the basin was overfilled. Nonetheless, during the early Cambrian

Fig. 2. Simplified lithostratigraphic column for correlation of the upper Ediacaran to lower Cambrian successions in Scandinavia modified from Nielsen and Schovsbo (2006, 2011). Stage division is according to East European regional stage classification. The approximate sampled profile is marked by vertical black stripped lines, with blue circles marking sample positions. The white boxes with numbers refer the locality numbers in Fig. 1. Stripped separation line between units represent uncertain boundaries and crosses mark hiata. La. sk. Mbr. = Langeskanse Member. Stratigraphical position within the sample unit is uncertain for samples SK50 and SY128. The lithostratigraphy from southern Norway is for the type locality in the Mjøsa area. The dotted texture represents medium-coarse grained sandstone, whereas the stripy texture represents fine sand with silty layers. The red and yellow colours mark red-coloured and grey-coloured sandstone, respectively.

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Table 1 Sample information for lower Cambrian localities in Scandinavia. Sample namea

Unit

Age

Thickness

Depositional setting

Structural position

Locality

References

Coordinates

GF28

Ringsaker Mbr.

Stage 3

50 m

Shallow marine

Allochtonous

Vidal and Nystuen (1991a)

GF38

Ringsaker Mbr.

Stage 3

50 m

Shallow marine

Allochtonous

SY128

Vik Mbr?

Stage 2–3

90 m

Tidal shoreline

Autochthonous

SY137

Tobisvik Mbr.

Stage 2–3

90 m

Tidal shoreline

Autochthonous

BH97

Vik Mbr.

Lower Cambrian

3.75 m

Shoreface

Autochthonous

BH99

Vik Mbr.

Lower Cambrian

3.75 m

Shoreface

Autochthonous

SO10

Ringsaker Mbr.

Stage 3

115 m

Shallow marine

Allochtonous

SK50

Vardal Mbr.

Stage 2

20–30 m

Fluvial

Allochtonous

BG61

Vik Mbr.

Stage 2–3

115 m

Shoreface

Autochthonous

BG70

Tobisvik Mbr.

Stage 2–3

115 m

Shoreface

Autochthonous

Glomstadfossen, S. Norway (1) Glomstadfossen, S. Norway (1) Skrylle Quarry, S. Sweden (2) Skrylle Quarry, S. Sweden (2) Strøby Quarry, Bornholm (3) Strøby Quarry, Bornholm (3) Steinsodden, S. Norway (4) W. Rena, S. Norway (5) Borggård-1, Bornholm (6) Borggård-1, Bornholm (6)

N 61° 04′ 25.8″/ E11° 22′ 01.6″ N 61° 04′ 26.5″/ E11° 22′ 03.3″ N 55° 41′ 48.7″/ E013° 21′ 05.7″ N 55° 41′ 58.0″/ E013° 21′ 15.0″ N 55° 03′ 42.5″/ E014° 54′ 30.9″ N 55° 03′ 42.4″/ E014° 54′ 30.8″ N60° 54′ 38.3″/ E10° 41′ 34.0″ N61° 06′ 56.7″/ E11° 21′ 52.7″ N 55° 1′ 42.32″/ E015° 0′ 42.83″ N 55° 1′ 42.32″/ E015° 0′ 42.83″

a

Vidal and Nystuen (1991a) Hamberg (1991) Hamberg (1991) Clemmensen et al. (2016) Clemmensen et al. (2016) Dreyer (1988) Bingen et al. (2011) Nielsen and Schovsbo (2011) Nielsen and Schovsbo (2011)

Sample names in italics refer to samples with b30 concordant ages/sample. The locality numbers (1–6) refer to locality numbers in Fig. 1b–d.

transgression, sediment spilled over the basin margins onto the denudated Baltica craton (Nystuen et al., 2008). Acritarchs from shale in the Ringsaker Member suggest an early Cambrian age (Vidal and Nystuen, 1991a). At the Mjøsa type section, the upper part of the Ringsaker Member contains Skolithos and Monocraterion burrows (Skjeseth, 1963). The Ringsaker Formation is unconformably overlain by the late, early Cambrian Ringstrand Formation (Vidal and Nystuen, 1991b; Nielsen and Schovsbo, 2006).

2.2.2. Southern Sweden and Bornholm The basal Cambrian sedimentary succession in southern Sweden and on the Danish island of Bornholm is composed of the red, arkosic, fluvial to aeolian Nexø Formation, which was deposited directly on ca. 1.40–1.78 Ga old basement (Clemmensen and Dam, 1993). The Nexø Formation is conformably overlain by the quartz-arenitic to subarkosic Hardeberga Formation. The Hardeberga Formation is divided into the Hadeborg, Lunkaberg, Vik, Brantevik and Tobisvik members, whereby the Hadeborg Member is found only on Bornholm (Fig. 2). The Hadeborg and Brantevik members comprise mudstone and siltstone sections, whereas the remaining members are composed of well-sorted, white to grey quartz arenite (Nielsen and Schovsbo, 2006). On Bornholm, the Hardeberga Formation is ca. 110 m thick, and dominated by fine- to medium-grained sandstone with very low matrix and feldspar contents. In southern Sweden, the Hardeberga Formation is similar, but with a stronger degree of bioturbation. Similar to southern Norway, these deposits represent the early Cambrian transgression. The Hardeberga Formation represents deep marine to outer shelf conditions transitioning upwards into a more tidally and storm-influenced shore-face environment as a result of the transgression (Hamberg, 1991). The tidal and paralic signature is seen in the cross-bedded and bioturbated Vik Member and the upper part of Tobisvik Member. The upper Lunkaberg and Brantevik members reflect a storm-dominated inner shelf environment (Hamberg, 1991), upward shifting into a barrier island coast (Clemmensen et al., 2016). The microfossil content in the Hadeborg Member implies that the age is Stage 2 within the Terreneuvian Series (Lontovian; Nielsen and Schovsbo, 2011) (Fig. 2). The younger Brantevik and Tobisvik members are suggested to be deposited during the latest Stage 2 (Lontovian-Dominopolian; Vidal, 1981; Nielsen and Schovsbo, 2006, 2011). The silty to sandy Læså Formation follows the Hardeberga Formation above a sharp unconformity.

3. Sampling and methodology Six sections were logged in the Cambrian Vangsås Formation in Norway and the Hardeberga Formation in Sweden and Bornholm (Fig. 2). Forty-two sandstone samples were selected and processed for heavy mineral separation. Detrital zircon grains were analysed for U-Pb geochronology in ten of the samples by LA-ICP-MS (Table 1). Six of these samples yielded abundant zircon grains and a statistically representative dataset of 138–292 U-Pb spot analyses of N 108 zircon grains per sample. Four samples gave b50 zircon grains, for which only an indicative dataset of 14–29 analyses could be secured (Table 1). 3.1. Sample description In southern Norway, the Ringsaker Member of the Vangsås Formation was logged and sampled at Glomstadfossen and Steinsodden (Table 1). The sampled stratigraphic section at Glomstadfossen is ca. 50 m, with GF28 collected ca. 5 m from the logged base in medium-grained sandstone, and sample GF38 at the very top of the section in pebbly conglomerate, at the contact to the overlying Redalen Member of the Ringstrand Formation. The medium-grained sandstone of sample GF28 is well to moderately sorted, and the grains are rounded to sub-rounded. In sample GF38, the sand size is coarse to very coarse, with individual pebbles up to 6 cm long. The sand is also moderately sorted, with sub-rounded to rounded grains. At Steinsodden, SO10 was obtained in well-sorted, medium-coarse-grained cross-bedded sandstone with rounded grains, approximately 70 m from the logged base. The Vardal Member, SK50, was collected at the base of a ca. 20–30 m section in western Rena, and is composed of well to moderately well sorted, coarse- to very coarsegrained sandstone, with rounded to subrounded grains. The Borggård-1 core from Bornholm comprises the Hardeberga Formation (Table 1), at ca. 218–323 m depth. BG61 was obtained from well to moderately sorted, coarse to medium-grained, rippled sandstone at 198.6 m depth, containing rounded to sub-rounded grains. BG70 was collected at 151.15 m depth in well sorted, medium-finegrained sandstone, with rounded to well-rounded grains. The section from Strøby Quarry on Bornholm comprises 3.75 m of the Vik Member. BH97 is from medium-grained sandstone with wave-rippled surface and load casts at 0.55 m from the section base. BH99 was taken 3.40 m from the base level. The medium to coarse-grained sandstone is well sorted and contains rounded grains.

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The ca. 90-m-thick log section from Skrylle Quarry in southern Sweden consists of the upper part of the Hardeberga Formation. At ca. 7 m from the logged base, interpreted as the Vik Member, SY128 was collected in cross-bedded, bioturbated sandstone. SY137 was collected at the very top of the Hardeberga Formation, in the Tobisvik Member around 90 m from the logged base. SY128 is moderately to well sorted, medium-grained sandstone with rounded grains and laminae comprising b100 μm-sized grains. In sample SY137, the medium- to coarse-grained sandstone is well to very well sorted and the grains are well rounded. 3.2. U-Pb methods The 10 analysed rock samples were crushed manually in a metal mortar. The crushed material was sieved, resulting in grain sizes b250 μm. The rock powder was placed in sodium polytungstate liquid with a density of 2.9 g/cm3 to separate the heavy minerals. Zircon grains were randomly handpicked from the heavy-mineral concentrate under a binocular microscope, independent of shape, colour and size, and mounted on 1-inch epoxy discs. Individual zircon grains were imaged with backscatter electron (BSE) and cathodoluminescence (CL) with a Zeiss Supra 35VP FE-Scanning Electron Microscope (SEM) instrument at the University of Stavanger. SEM micrographs were used to determine areas of zircon grains suitable for laser ablation spot analysis. Primarily, rims and areas free of inclusions were selected. Zircon grains displaying oscillatory and sector growth zoning on CL images were considered magmatic, whereas irregular and homogenous zoning was regarded to reflect metamorphic origin in accordance to Corfu et al. (2003). Characterization of zircon grains was done based on the roundness scale of Powers (1953), shape, number of growth phases and whether the analysis was done in the rim or the core of the grains (Appendix A). U-Pb analysis was carried out at the University of Münster in Germany by using a Thermo-Finnigan Element 2 sector field ICP-MS. An Analyte G2 Photon Machines Excimer laser system was coupled with the instrument to perform laser ablation. The measurements were used to determine the isotopic ratios of 207Pb/206Pb,206Pb/238U and 207Pb/235U, in addition to 204Pb (common Pb). Common lead correction (Stacey and Kramers, 1975), was applied only if common 206Pb of the total 206Pb exceeded 1%, reaching analytical uncertainty of the Pb-isotope ratio measurement. The ICP-MS and laser operation conditions are given in the supplementary data (Appendix A, Table S1). Depending on the size of the zircon grain and the area of interest, spot sizes were of 20, 25, 30 or 50 μm. The background-, ablation- and washout times were set to 14, 36 and 15 s, respectively (Appendix A, Table S1). Groups of ten unknowns were bracketed with three analyses of the GJ-1 reference zircon (608.5 ± 0.4 Ma (Jackson et al., 2004) for external calibration. Measurements (n = 66) of the 91,500 standard zircon (Wiedenbeck

et al., 1995) as unknowns were done during the analytical session to estimate precision and accuracy of the analyses. Yielded values are 0.1806 (±2.8%, 2σ) for 206Pb/238U, 1.862 (±3.5%, 2σ) for 207Pb/235U, and 0.0748 (±2.0%, 2σ) for 207Pb/206Pb ratios, which match published values (Wiedenbeck et al., 1995) within error. The results encompass U-Pb ages of 838 detrital zircon grains, listed in online Supplementary data (Appendix A, Table S2) of which ca. 15% were corrected for common lead. Two or more analyses were performed on many grains in order to obtain potential age variations for different growth phases. An in-house Excel® spread sheet (Kooijman et al., 2012) was used for data processing. Age estimates within a −10% or a +5% 206Pb/238U vs. 207Pb/206Pb discordance were included in the results. 207Pb/206Pb ages were used for interpretation at all times because 99% of the concordant ages are N0.90 Ga. The uncertainties are always given at 2σ level. We evaluated the data with Isoplot 4.15® (Ludwig, 2012). Unless stated otherwise, the U-Pb ages are concordant, and individual ages are stated without error. When categorizing ages according to zircon-forming events, only 207Pb/206Pb ages restricted within the given tectonic time intervals were included, without the 2σ errors. Duplicate analyses of the same grain are only used when determining age differences between core and rim. The correlation coefficient to determine the relationship between U-Pb ages and grain length was calculated for all samples, using the formula P Þ ðx−xÞðy−y ffi; CorrelðX; Y Þ ¼ qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi P 2P Þ2 ðy−y ðx−xÞ  are sample means. where x and y 4. Results 4.1. Zircon characteristics Oscillatory growth zoning exists in N70% of the grains, indicating a magmatic origin (Table 2). N60% of the grains are sub-rounded to subangular. They mostly are either oval-shaped (40–50%) or spherical (30–50%), and only 10–30% are elongated. Primarily, the grains have one growth phase. However, the Bornholm samples and GF38 from southern Norway contain ca. 40% multi-phased grains (Table 2). The mean grain length is 90–180 μm. No significant correlations between grain morphology or growth zoning and the grain size were detected. SY128 and SY137 from southern Sweden contain ca. 30% zircon grains with irregular and homogenous zoning, compared to ca. 20% for the remaining samples, indicating a stronger metamorphic influence. Furthermore, the Swedish samples contain less euhedral grains

Table 2 Zircon characteristics. Sample

N

n

GF28a GF38 SY128 SY137 BH97 BH99 SO10 SK50 BG61 BG70

138 226 179 282 199 292 15 21 61 22

105 135 102 172 110 154 8 13 29 9

Growth zoning %

Roundness %

Shape %

Growth phases %

Magmatic

Metamorphic

Euhedral

Sa-sr

Rounded

Elongated

Oval

Round

1

2

3

80 89 76 68 95 86 – – 86 –

21 11 25 32 5 14 – – 14 –

14 25 3 8 16 17 – – 17 –

70 64 61 67 66 77 – – 77 –

16 10 36 25 18 6 – – 6 –

18 15 24 25 27 13 – – 30 –

48 44 39 38 42 43 – – 43 –

34 41 36 37 31 45 – – 45 –

84 62 74 79 61 58 – – 58 –

16 36 26 18 34 38 – – 38 –

– 2 – 3 5 4 – – – –

Mean grain length (μm)

Locality

110 145 180 125 145 140 105 110 90 45

1 1 2 2 3 3 4 5 6 6

N = number of analysis, n = number of concordant zircon ages. Growth zoning: magmatic = oscillatory and homogenous, metamorphic = irregular and sector. Sr-sa = sub-rounded to sub-angular. a In sample GF28, 74 zircon grains have not been visualized in CL. For samples BG70, SK50 and SO10, the zircon characteristics are not shown because the number of concordant zircon grains was too low to get statistically significant information. Mean grain length is calculated based on non-broken grains. Locality numbers refer to localities shown in Fig. 1b–d.

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(b10%) and more rounded grains (20–40%) than the other samples (Table 2). In GF28 from southern Norway, as many as 25% of the zircon grains are euhedral. Zircon mean grain length and degree of roundness (Table 2) generally correlate with the same characteristics of the quartz grains in the corresponding samples. Hence, the samples with the largest zircon grain size also comprise the largest quartz grain size, apart from BG61. SY128 has exceptionally large zircon grains (mean of 180 μm), which is explained by gradation with b100 μm to 4 cm-sized quartz grains. Notably, the samples with b30 zircon grains have the lowest average zircon grain size. The average zircon grain size is 50–110 μm for these samples, despite a medium to very coarse sand grain size. Zircon grains b100 μm have a similar diversity in age as the larger zircon grains. 4.2. Detrital zircon U-Pb ages All concordant zircon ages are Proterozoic (Fig. 3). The majority of them are 1.0–1.4 Ga in the Mesoproterozoic era and 1.6–1.8 Ga in the Palaeoproterozoic era. Grains with multiple growth zones reveal that the age difference between cores and rims mostly is within or very close to the 2σ error. BH99 from Bornholm is an exception with some ca. 1.24 Ga cores, and ca. 1.04 Ga rims (Fig. 4). This was not observed for BH97, which is stratigraphically very close. Zircon grain length and U-Pb age do not correlate significantly, with correlation coefficients between −0.2 and +0.3. Neither of the textural characteristics (Table 2) correlate with the U-Pb ages. Additionally, no textural sorting according to age of tectonic phases is demonstrated (Table 3). 4.2.1. U-Pb ages in southern Norway and southern Sweden The age distribution in the samples from southern Norway and southern Sweden are strikingly similar (Fig. 3). Over 70% and 80% of the ages, respectively, are 0.90–1.14 Ga (including a ± 40 Ma error; Table 3) and therefore coincide with the timing of the Sveconorwegian orogeny. Among these ages, 1.14–1.15 Ga dominates. Subordinate populations occur in a ca. 0.95–1.05 Ga interval. The oldest components (comprising at least 3 U-Pb ages) are ca. 1.65 Ga in SY137, 1.55 Ga in SY128, and 1.50 Ga in GF28. These ages are not represented in GF38. Other pre-Sveconorwegian ages are a 1.14–1.22 Ga age interval present in all samples. The most apparent difference between the localities in Sweden and Norway is that the Swedish samples have an older zircon population and are more restricted to the 0.90–1.14 Ga interval. PostSveconorwegian ages occur only in SY137 from southern Sweden, with the youngest component at 885 Ma. The youngest individual zircon ages are 606 ± 25 Ma and 645 ± 37 Ma in Norway and Sweden, respectively. SK50 from the Vardal Member of the Vangsås Formation in southern Norway has a large age span, mainly of 0.9–1.8 Ga, as well as an accumulation of Palaeoproterozoic ages. Combination of SK50 ages and previously published data on the Vardal Member (sample 108 in Bingen et al., 2011) indicates that our 13 concordant ages are representative for that unit (Fig. 5). 4.2.2. U-Pb ages on Bornholm The samples from the Danish island of Bornholm contain older and broader age populations than those from Norway and Sweden (Fig. 3). A bimodal age distribution exists with a dominating age component at ca. 1.7 Ga, and a less prominent 1.20–1.25 Ga interval (only present in BH99 and BH97). Sample BH99 has an older, smaller, age component at ca. 1.8 Ga, which may overlap with TIB 1. Furthermore, the zirconcore ages in this sample have a bimodal distribution, with 1.23 Ga being the dominating component (Table 4). A small age component at 1.46–1.47 Ga occurs in all samples from Bornholm. Additionally, BG61 has a 1.54 Ga age component, which is much less prominent in BH99 and BH97. In contrast to the samples

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from southern Norway and southern Sweden, only 2–8% of the ages fit into the 0.90–1.14 Ga interval (Table 3). 5. Discussion 5.1. Source rocks for lower Cambrian sandstone The bimodal age distribution in the samples from southern Norway and southern Sweden indicates provenance from different primary magmatic exposures from the Sveconorwegian Orogenic Belt and older Mesoproterozoic and late Palaeoproterozoic magmatic sources (Fig. 1). The 1.14–1.15 Ga primary age component is coeval to 1.14– 1.17 Ga bimodal magmatism (Corfu and Laajoki, 2008; Bingen and Solli, 2009), and the slightly older ages may correspond to 1.13– 1.28 Ga pre-orogenic intrusive magmatic suites and sedimentary rocks in the Telemarkia Terrane (Bingen et al., 2008b). The younger ages coincide with 1.00–1.03 Ga syn-collisional granitoid and 0.93–0.97 Ga post-collisional monzodioritic to granitic plutons (Bingen et al., 2008b). However, only GF28 from southern Norway illustrates the documented 1.06–1.13 Ga age gap in the magmatic rock record for the entire Sveconorwegian Belt and the Sveconorwegian Front (Bingen and Solli, 2009). The compiled ages of the Vardal Member (Fig. 5; Bingen et al., 2011; this study) coincide with magmatism in the Telemarkia Terrane (Bingen et al., 2008b, and references therein), where the dominating age component at 0.94 Ga fits post-collisional granitoid rocks. Both members of the Vangsås Formation have ages that fit the timing of a voluminous, preorogenic 1.48–1.52 Ga Telemarkia magmatic rock suite, but also plutons from the central Fennoscandian Shield (Bingen et al., 2011) and the ca. 1.48 Ga basement slices in the Middle Allochthon (Lamminen et al., 2011). In the Hardeberga Formation in southern Sweden and the Vardal Member in Norway, the relatively small contribution of late Palaeoproterozoic to early Mesoproterozoic ages are coeval with the TIB and Gothian magmatism preserved within the Eastern Segment and the Idefjorden Terrane of the Sveconorwegian Orogenic Belt, respectively (Bingen et al., 2008b; Lamminen et al., 2015). The 1.69 Ga component in the Vardal Member and the 0.95 Ga components in the lower section of the Ringsaker Member agree with 0.96 Ga and 1.66– 1.68 Ga granitic clasts, representing the Neoproterozoic basement on the western margin of Baltica (Lamminen et al., 2015). The detrital age distribution from Bornholm is strikingly similar to the accumulated magmatic ages of the Eastern Segment located in the Sveconorwegian Orogenic Belt (Bingen et al., 2008b). The main detrital age components can be traced predominantly to magmatism of the 1.66–1.85 Ga TIB and secondarily to 1.20–1.22 Ga bimodal magmatism in the Sveconorwegian Front and 1.20–1.25 Ga pre-orogenic granitic plutons (Söderlund and Ask, 2006; Bingen et al., 2008b). The welldefined 1.69–1.71 Ga primary age components from Bornholm also fit the 1.69–1.72 Ga voluminous granitic magmatism of TIB 2 from Central Sweden (Åhäll and Larson, 2000). These ages can be found in the Småland-Värmland Belt in the south-southwestern part of the TIB (Andersson and Wikström, 2004) (Fig. 1). The 1.0 Ga zircon-rim ages in BH99 from Bornholm can be tied to the 0.98–1.05 Ga syn-collisional Agder phase of the Sveconorwegian orogeny. This suggests syn-orogenic, metamorphic overprint of these zircon grains. The ages around 1.46–1.47 Ga match ca. 1.45–1.46 Ga granitoid rocks and gneiss constituting the basement on Bornholm that formed during the Hallandian-Danopolonian orogeny (Waight et al., 2012; Johansson et al., 2016). Hence, we can demonstrate transport from local sources into the Hardeberga Formation on Bornholm. The less rounded quartz grains and moderately sorted sandstone in the upper section of the Ringsaker Member point to provenance from multicycle sources, such as Precambrian quartz arenite and quartzite. Recycling of clastic grains from Mesoproterozoic sedimentary rocks is suggested from quartz-arenitic clasts in conglomerate throughout the Hedmark Group (Lamminen et al., 2015). Mesoproterozoic quartzite

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Fig. 4. Comparison of age (Ma) difference according to growth phase in samples from Bornholm. Core/inner rims and rims follow separate age trends in BH99. In sample BH97, this is not evident. CL micrographs of zircon grains for each sample show zoning and growth phases, with red circles marking the position of analyses. The names in the horizontal axis and in the micrographs refer to sample spots in Appendix A, Table S2.

from the Telemarkia Terrane and the conglomerate commonly contain late Palaeoproterozoic and Archaean detritus (Bingen et al., 2001; Andersen et al., 2004; Lamminen et al., 2015). The lack of similarly old detritus in Neoproterozoic to lower Cambrian successions of the Hedmark Group (this study; Bingen et al., 2011; Lamminen et al., 2015), indicates that major input from these rocks are less likely. Provenance from other metasedimentary sources, such as the Eidsborg Formation quartzite, is considered unlikely because the wide distribution of detrital zircon ages is in conflict with the narrow age interval in both the Hardeberga Formation in southern Sweden and the Ringsaker Member in southern Norway (cf. Spencer et al., 2014). Recycling of clastic material from underlying Neoproterozoic strata is also not considered as the primary source because of significant dissimilarities in pre-Sveconorwegian age intervals (e.g. Bingen et al., 2011; Lamminen et al., 2015). Instead, if originally present, the recycled signature may have been diluted by the younger, magmatic detritus. The long time gap from the zircon age component at 885 Ma to the time of deposition during early Cambrian reflects a lack of production of large volumes of young granitoid igneous material prior to the break-up of Rodinia and formation of Baltica. This is a typical situation for passive-margin conditions (Cawood et al., 2012). The nearly absence of Cryogenian and Ediacaran zircon ages may be related to limited magmatic activity along the northwestern part of the Baltoscandian margin at that time. Ages in agreement with the 606 Ma and 645 Ma individual grains in the Ringsaker Member and the Hardeberga Formation, respectively, are also found in the Middle and Lower Allochthon in southern Norway (Bingen et al., 2005; Lamminen et al., 2011). They may reflect far-travelled zircon grains from the Timanian orogen on the opposing part of the Baltica margin (Lamminen et al., 2015; Slama and Pedersen, 2015), because no likely source rocks are known from

northwestern Fennoscandia (Bingen et al., 2005; Nystuen and Lamminen, 2011). Lamminen et al. (2015) also pointed out that these late Cryogenian-Ediacaran clastic zircons might have a southerly source in peri-Gondwanan terrains. 5.2. Provenance from an extended Sveconorwegian domain The strong Sveconorwegian signature of the Vangsås Formation in southern Norway and the Hardeberga Formation in southern Sweden may point towards an extension of the Sveconorwegian domain towards the northwestern and towards the southeastern part of Baltica during late Neoproterozoic to early Cambrian time (Fig. 6). The Telemarkia Terrane, or its continuation towards the EdiacaranCambrian margin of Baltica in the north, as the primary source of the Vangsås Formation is supported in general by northward-directed palaeocurrents of the fluvial Vardal Member (Dreyer, 1988) and a reconstructed local southeast to northwest palaeo-slope in southern Norway (Nielsen and Schovsbo, 2011). Additionally, provenance from southern sources is in agreement with the sequence stratigraphic framework (Nielsen and Schovsbo, 2011), since the area north of the Hedmark Basin was an open seaway connected with the Iapetus Ocean (Kumpulainen and Nystuen, 1985; Cocks and Torsvik, 2005). Correspondingly, the prevailing signatures from the Telemarkia and Idefjorden terranes point to a north-northwestern source for the zircon grains in the Swedish section of the Hardeberga Formation. This is supported by southward palaeocurrent directions in the lower section of the Hardeberga Formation in Scania, southernmost Sweden (Hamberg, 1991). However, contrary to the situation at Bornholm, the few ages that overlap with the Eastern Segment, the Sveconorwegian Front and the adjacent TIB located north of the sampling locality in southernmost

Fig. 3. Probability density plots and histograms (bin width = 50 Ma) of detrital zircon data. Dark grey histograms show concordant ages with b1% common lead only. Light grey histograms and probability curves represent all concordant ages. Up to three dominating age components in each sample are marked with a weighted average age. n = number of concordant analyses.

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Table 3 Summary of the major age components, and classification of age intervals according to major tectonic events in the Scandinavian part of Baltica. Sample

BH97 BH99 SY128 SY137 GF28 GF38 BG61 BG70 SK50 SO10

No. of spots

No. of zircon grains

No. of conc. ages

Main age components (Ga) Primary age component

Secondary age component

Tertiary age component

SNO (0.9–1.14)a

GO (1.5–1.75)b

TIB (1.67–1.86)c

199 292 179 282 138 226 61 22 21 15

141 184 125 181 109 153 49 20 14 15

110 154 102 172 105 135 29 9 13 9

1.69 (n = 28) 1.71 (n = 17) 1.14 (n = 29) 1.14 (n = 70) 1.15 (n = 27) 1.15 (n = 46) 1.67 (n = 4) 1.71 (n = 3) – –

1.77 (n = 9) 1.24 (n = 13) 1.08 (n = 10) 1.01 (n = 22) 0.99 (n = 10) 0.98 (n = 8) 1.72 (n = 4) – – –

1.21 (n = 7) 1.81 (n = 7) 0.97 (n = 8) 1.62 (n = 6) 1.50 (n = 3) 1.04 (n = 7) 1.54 (n = 3) – – –

2% 8% 61% 58% 51% 41% 7% – 31% 89%

57% 45% 7% 6% 7% 2% 59% 67% 23% –

47% 44% 1% 1% – 1% 38% 56% 38% –

SNO = Sveconorwegian orogeny, GO = Gothian orogeny, TIB = Transscandinavian Igneous Belt. Conc. = concordant. Ages are based on at least three U-Pb ages, with 95% confidence interval and b1 mean square weighted average, within 10% concordance interval. Results in % do not include the error-interval. Note the overlap in the ages between SNO and GO. The age components are defined independent of overlapping ages, separated from age adjacent age components. a Bingen et al. (2008b). b Gaál and Gorbatschev (1987). c Gorbatschev (2004).

Sweden (Fig. 1), indicate very little input from the north-northeast in this part of the Hardeberga Formation. The proposed northwestward extension of the Sveconorwegian domain is supported by U-Pb ages in basement slices of the Caledonian thrust sheets (Lamminen et al., 2011) and granitic conglomerate clasts (Lamminen et al., 2015). Additionally, provenance from 1.14 to 1.15 Ga source rocks to both the southwestern and southern margins of Fennoscandia is evident in the upper Neoproterozoic to lower Cambrian fluvial and shallow marine sandstone of this study and others (Be'eri-Shlevin et al., 2011; Bingen et al., 2011). Basement windows and slices in the Middle Allochthon, representing a source rock to the sedimentary succession in the thrust sheet, suggest a northward extension of the Mesoproterozoic-Neoproterozoic crust presently exposed in the southern part of the Fennoscandian shield (Be'eri-Shlevin et al., 2011). Furthermore, the presence of Sveconorwegian detrital zircon ages in the Vemdal Formation quartzite in the Lower Allochthon in central Sweden, corresponding to the Vangsås Formation in Norway, suggests that this part of the Ediacaran-Cambrian passive margin of Baltica also was sourced from Sveconorwegian terranes (Gee et al., 2014). The narrow age interval in the Ringsaker Member points towards relatively short transport, and consequently supports the existence of ca. 1.0– 1.2 Ga sources in the vicinity. This fits interpretations that the Sveconorwegian Orogenic Belt may have continued northward along the margin of southwestern Baltica, and into the high Arctic (Lorenz et al., 2012; Gee et al., 2015; Zhang et al., 2015). In addition to the Sveconorwegian signature, Neoproterozoic sedimentary rocks from the Middle Allochthon in central Sweden and Norway have an age signature similar to that of the Vardal Member, with ties to the Gothian-TIB domain and the Western Gneiss Complex (Be'eri-Shlevin et al., 2011; Bingen et al., 2011). However, these rocks lack a significant contribution from the ca. 1.8 Ga age component found in the Vardal Member, suggesting a comparatively stronger influence from TIB 0 or the Sveconfennian domain in the Vardal Member. Moreover, the Hedmark Basin is suggested to have formed in a zone of crustal weakness between the TIB and the Sveconorwegian domain along NW-SE to NNW-SSE running faults that may have been linked with present fault and shear zones in southern Fennoscandia (Pease et al., 2008; Lamminen et al., 2015; Gabrielsen et al., in press). This setting is supported by the TIB-Gothian age signature in the Vardal Member, as well as several older units in the Hedmark Group (Bingen et al., 2011; Lamminen et al., 2015). The similarities in provenance for the Ringsaker Member in southern Norway and the Hardeberga Formation in southern Sweden favors a geological setting with the Sveconorwegian Orogenic Belt having extended further south-southeast than today, into the present-day

Skagerak and Kattegatt area (Fig. 6). This is in agreement with the basement ages and seismic reflection data from the Ringkøbing-Fyn High (Lassen and Thybo, 2012; Olivarius et al., 2015). On Bornholm, the dominating provenance from the Eastern Segment suggests a north-northwestern provenance, similar to southernmost Sweden. A northern provenance is supported by the southward palaeocurrent directions in fluvially-influenced facies of the Nexø and Hardeberga formations on Bornholm (Clemmensen and Dam, 1993; Nielsen and Schovsbo, 2011; Clemmensen et al., 2016). Furthermore, opaque heavy minerals from the Nexø Formation also suggest provenance mainly from southern Sweden (Jensen, 1977). Even though palaeocurrents and heavy mineral compositions favor a northern provenance, the low contribution from the Blekinge bedrock and the Småland granitoids of TIB-1 age infers that this region was not the primary catchment area. Moreover, given the proximity to the southern Sweden locality, one may have expected to find a stronger Sveconorwegian signature. Instead, the strong divergence in ages from Bornholm and southern Sweden infers that these relatively adjacent localities were fed by different sources. 5.3. Sveconorwegian dominance during early Cambrian time The increase in Sveconorwegian supply from latest EdiacaranCambrian Series 2 time is illustrated in the Vangsås Formation from the southwestern part of Baltica. Generally, the SveconorwegianGothian-TIB age signature throughout the Vangsås Formation indicates a relatively stable provenance for the Hedmark Basin, and likely also adjacent areas, during the Cambrian transgression. Despite the overall similarity in ages within the Vangsås Formation, the provenance changed from dominantly pre- and post-collisional Sveconorwegian magmatic and TIB-related sources in the Vardal Member, to a more confined catchment area related to, or just postdating, the latest phase of the Sveconorwegian orogeny for the Ringsaker Member. The trend indicates a gradual terminating provenance from the Eastern Segment or other terranes related to the TIB from Ediacaran to early Cambrian time in parts of the Hedmark Basin. In addition, the contributions are similar to zircon ages from the Dalane phase of the Sveconorwegian orogeny (Bingen et al., 2008b). This phase dominates the provenance in the Vardal Member, while being nearly absent in the upper part of the Ringsaker Member. Similar to the Cambrian sandstone, the Neoproterozoic basin-filling units of the Hedmark Group include a Sveconorwegian-Gothian-TIB detrital zircon age signature (Bingen et al., 2005; Bingen et al., 2011; Lamminen et al., 2015). This indicates that the feeding from the Sveconorwegian-Gothian-TIB domains continued at least from the

S. Lorentzen et al. / Sedimentary Geology 375 (2018) 203–217

Fig. 5. U-Pb ages (Ma) of the Vardal Member. The curves represent sample SK50 (this study), sample 108 (Bingen et al., 2011) and an accumulated curve for both samples.

initial infill history of the Hedmark Basin until Baltica was completely drowned and turned into an epicontinental sea during middle to late Cambrian time (Lamminen, 2011; Lamminen et al., 2011; Nystuen and Lamminen, 2011; Lamminen et al., 2015). No other studied unit in the Hedmark Group reveals as little U-Pb zircon age diversity and as confined age interval as the Ringsaker Member (Bingen et al., 2011; Lamminen et al., 2015), although the Neoproterozoic, coarse-grained, proximal Ring and Rendalen formations are fairly unimodal (Bingen et al., 2011; Lamminen et al., 2015). The delimited contributions from areas of considerably different ages, as well as the presence of a coarse-grained interval in the upper part of the Ringsaker Member, may signify a rather limited catchment area size. However, since the ages in the Hardeberga Formation from southern Sweden also concentrate around the timing of the Sveconorwegian orogeny, this may instead point towards an increase in the Sveconorwegian supply, effectively diluting input from more distal sources. Also, the 80 m stratigraphic separation in the samples from southern Sweden, as well as difference in grain size, have not influenced the age distribution, thus indicating a stable depositional environment and limited tectonic influence on the shelf and in the source area.

5.4. Late Ediacaran to early Cambrian catchment divide in southwestern Baltica We propose the existence of a catchment divide between the Svecofennian-TIB domain and the Sveconorwegian domain in the southern part of Baltica during earliest Cambrian time (Fig. 6). This divide could explain the scarcity of Sveconorwegian ages in the Cambrian Stage 2 Hardeberga Formation on Bornholm. Furthermore, the provenance difference in the Hardeberga Formation between localities in southern Sweden and Bornholm can be due to that these localities are Table 4 Summary of the major age components from the zircon cores. Sample

Primary age component

Secondary age component

No. of conc. core ages

BH97 BH99 BG61a GF28a SY137 SY128 GF38

1690 (n = 4) 1230 (n = 6) – – 1150 (n = 8) 1140 (n = 4) 1180 (n = 5)

– 1740 (n = 3) – – – – –

11 13 4 3 20 6 19

a Samples with b7 concordant core ages. They are therefore considered insignificant for statistical analysis. Conc. = concordant.

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situated on opposite sides of the early Cambrian catchment divide. During the late early Cambrian, the denudation of the Baltica craton may have resulted in a renewed catchment area, especially if combined with uplift or effective increase in the exposure of the Sveconorwegian domain. The proposed setting would allow more effective transport of Sveconorwegian detritus to central and eastern parts of the Baltoscandian craton, while reducing the signature from an increasingly denudated Svecofennian-TIB domain. Lower Cambrian quartz arenite from Baltica is also found in Russia, Estonia and Poland (e.g., Orlowski, 1989; Isozaki et al., 2014; Põldvere et al., 2014). Generally, these deposits differ from the southwestern Fennoscandian quartz arenite formations by the presence of late Palaeoproterozoic to Archaean zircon ages corresponding to the Kola-Karelia block in northeast Baltica and the Svecofennian Orogen (Valverde-Vaquero et al., 2000; Isozaki et al., 2014; Põldvere et al., 2014). Estonian quartz arenite is suggested to reflect deposition in an extensive palaeo-Baltic Basin, ranging from the Timanian fold belt in the north to the Tornquist margin in the south, on the eastern side of the TIB (Isozaki et al., 2014). Whereas the Ediacaran and Cambrian Stage 2 quartz arenite formations lack Sveconorwegian detrital zircon ages, the Stage 3 and Middle Devonian deposits have a consistent Sveconorwegian age signature (Isozaki et al., 2014; Põldvere et al., 2014). This supports our interpretation of an uplifted or extensively exposed Sveconorwegian domain. Furthermore, it supports the presence of a catchment divide between the Svecofennian-TIB domain and the Sveconorwegian domain, and indicates that this divide may have extended even further north towards the northwestern shoreface of Baltica (Fig. 6). Ages corresponding to the Sveconorwegian orogeny is also found in the Cambrian Series 2 Brusov Formation in the White Sea region of the East European Craton (Kuznetsov et al., 2014), suggesting very distal sources. This may point towards a geographically extensive SveconorwegianGrenvillian domain, covering a larger area of the Baltoscandian craton. Alternatively, this wide-spread distribution of Sveconorwegian ages may reflect extensive weathering from the Sveconorwegian orogen available to form sheet deposits on the denudated peneplain. Timanian provenance to the southwestern margin of Baltica during the early Cambrian Stage 3 also suggests continent-scale transportation of small zircon grains (Slama and Pedersen, 2015). 5.5. Distributary and depositional processes influencing the provenance signature The correlation between sand grain size, zircon grain size and roundness indicates that quartz and zircon grains have undergone similar transport and reworking processes. Hence, the clastic zircon population may hydrodynamically belong to the clastic quartz-grain population, suggesting that the provenance of the zircon grains represents felsic sedimentary provenance in general. The denudation of the Sveconorwegian orogen and subsequent formation of the Sub-Cambrian Peneplain (Gabrielsen et al., 2015) predate the deposition of the lower Cambrian sandstone by at least 100 million years. Thus, small zircon grains could belong to a blanket of residual sediment that covered the Sub-Cambrian Peneplain during early Cambrian time. Additionally, wind-blown zircon grains may also constitute a significant proportion of the fine-grained detritus due to a combination of transport in the westerly wind belt of the southern hemisphere, where Baltica was located (Nielsen and Schovsbo, 2011), and the low relief of the Sub-Cambrian Peneplain. However, the low age diversity, especially in the Ringsaker Member, contradicts a provenance with large-scale mixing from different domains. Furthermore, we would expect different age distributions for small and large zircon grains if the small zircon grains were detritus from residual blankets or transported by wind. The relatively large, well to moderately sorted grains and the proportionally smaller zircon grains in the samples with b 50 zircon grains, indicate a lesser hydrodynamic relation to the host quartz sand. This

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Fig. 6. Schematic palaeogeographic reconstruction of western Baltica during early Cambrian time showing the proposed catchment divide, sediment source directions and the interpreted extension of the Sveconorwegian domain. The domain here extends to the southern and northwestern boundary of Baltica. It is restricted by the Mylonite Zone (MZ) in the east and the termination of the Western Gneiss Complex in the west. The map is modified from Möller et al. (2007), Bingen et al. (2008b), Graversen (2009), Nielsen and Schovsbo (2011), Lamminen et al. (2011) and Lamminen et al. (2015).

may be due to severe shallow-marine reworking, which may have effectively removed considerable fractions of fine grains while preserving the coarse to very coarse sand. This agrees with the low amount of zircon grains in these samples. The high content of rounded zircon grains infers extensive abrasion due to reworking and relatively long continental-marine transportation distance or total transport duration, despite mostly being of first sediment-cycle origin. Extensive reworking in shallow-marine environment was likely due to the extremely low gradient of the shelf slope and high storm frequency (Nielsen and Schovsbo, 2011). Additional marine reworking by alongshore drift is in agreement with palaeocurrent directions for sand feeding southernmost Sweden (Hamberg, 1991).

6. Conclusions The provenance signature of detrital zircon grains from the quartz arenitic Hardeberga Formation in southern Sweden and Ringsaker Member in southern Norway is tied to sources from the Sveconorwegian Orogenic Belt. Given the hydrodynamic relation between the zircon and quartz grains, the U-Pb ages may also reveal provenance for the clastic material in lower Cambrian sandstone. The stratigraphical upward decrease in zircon-age diversity in the Vangsås Formation may signify that the area changed from being supplied by multiple, scattered sources to a more focused input from units corresponding mainly to the late phase in the denudation of the Sveconorwegian orogen.

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The lower Cambrian quartz arenite of southern Scandinavia is interpreted to reflect first cycle deposits, because the confined age interval in both the Ringsaker Member and the Hardeberga Formation in southern Sweden is less likely to reflect a recycled source. Furthermore, the lack of Archaean and late Palaeoproterozoic ages that are typical for Mesoproterozoic quartzite in the suggested source rock area may also point towards first sediment cycle genesis. Instead, the mature sandstone composition may be explained by reworking processes, where the low gradient of the Sub-Cambrian Peneplain, in addition to a further reduced gradient owing the transgression, allowed extensive reworking and prolonged residence time of the weathered material. The predominance of ages corresponding to the Telemarkia Terrane suggests that the domain extended further northwest to the Hedmark Basin and southeast towards the present-day Skagerak and Kattegat area, in order to supply both the southwestern and southernmost margins of Baltica. The adjacent section of the shelf comprising the Hardeberga Formation on Bornholm, on the other hand, was supplied by the Eastern Segment related to the TIB and the Gothian orogeny. Therefore, we suggest a catchment divide between the TIB-Gothian and Sveconorwegian domains during early Cambrian time. The clear provenance signature from Baltica shown in this study, without any evident Gondwana or Laurentia signals, supports the use of detrital zircon ages to identify different protosource provinces within a continent based on zircon-age distributions. However, the varied Baltica origin of similarly aged detrital samples b100 km apart illustrates regional variations that may occur along individual continental margins. Furthermore, the southeastward and northwestward extended Sveconorwegian domain also may indicate that large formerly uplifted large Baltica areas are no longer available for direct provenance comparison. Supplementary data to this article can be found online at https://doi. org/10.1016/j.sedgeo.2017.08.008.

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