mesoproterozoic frost action at the base of the svinsaga ...depth where the rate of cooling is...

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Introduction

Paleoweathering can provide valuable information about the formation of the different rock units and the paleoclimatic conditions operative during their deposition. Thus, the frost weathering is a fundamental geomorphic process operating in periglacial environments. The term periglacial refers to the non-glacial processes, conditions, and landforms associated with cold climates (Ballantyne & Harris, 1994; French, 2007). In a periglacial environment, frost action has been used as a collective term to describe distinct processes that result from alternate freezing and thawing cycles. Frost weathering is usually most active at some finite depth where the rate of cooling is sufficiently slow for significant moisture to migrate to microfractures as the freezing front advances (Hallet et al., 1991), and the rock fails when the shattering force exceeds the tensile strength of the rock. According to Hall (1988), Matsuoka (1991) and Ballantyne & Harris (1994), the breakdown mechanism depends on the thermal regime (amplitude of freezing cycles and rate of freezing), moisture conditions (e.g., degree of rock saturation), and lithology (texture and chemistry). Frost weathering is most intense where ground temperatures oscillate around 0o C, promoting numerous freeze-thaw cycles, disintegrating of bedrock

and releasing rock debris, which affects transports and deposition systems in the sedimentary basins (see Matsuoka & Murton, 2008).

Descriptions of Mesoproterozoic (ca. 1.5 – 1.0 Ga) deposits ascribed to periglacial frost action or frost shattering are rare in the geological literature. Williams & Tonkin (1985) and Williams (1986) described the late Precambrian Cattle Grid breccia in South Australia, within which frost shattering of quartzite bedrock produced an in situ breccia horizon interpreted to have formed through frost action under seasonal freeze-thaw cycles during the Marinoan Glaciation, about 635 – 600 Ma. The interpretation of periglacial in situ brecciation of the quartzite is supported by the existence of associated sand-wedge polygon and sand-wedge structures, although these features are not necessarily unambiguous paleoenvironmental indicators (Murton et al., 2000). Mustard & Donaldson (1987) suggested that freeze-thaw action may have played an important role in separating blocks in the upper part of a breccia in the Paleoproterozoic Gowganda Formation in Canada. Young and Gostin (1988) interpreted the thin Paleoproterozoic basal breccias at the base of the Sturtian rocks in the North Flinders Basin, Australia, as periglacial. Laajoki (2002a) interpreted that the basal

NORWEGIAN JOURNAL OF GEOLOGY Mesoproterozoic frost action at the base of the Svinsaga Formation

Juha Köykkä & Kauko Laajoki

Köykkä, J. & Laajoki, K. 2009: Mesoproterozoic frost action ath the base of the Svinsaga Formation, central telemark, South Norway. Norwegain Journal of Geology, vol 89, pp 291-303. Trondheim 2009, ISSN 029-196X.

The Mesoproterozoic Svinsaga Formation (SF) in central Telemark, South Norway, was unconformably deposited after ca. 1.347 Ga on the quartz arenite of the upper member of the Brattefjell Formation (UB), which is part of the Vindeggen Group. The unconformity is marked by a fracture zone a few meters thick developed in the UB quartz arenite. The lower part of the fracture zone, which contains sparse and closed fractures, and microfractures, grades upwards into a system of wider and mostly random oriented fractures and fracture patterns. The fractures are filled with mudstone. The fracture zone is overlain by several meters of in situ SF breccia, in which the fragments consist solely of the UB quartz arenite. The in situ breccia gradually grades into a basal breccia and clast- or matrix-supported conglomerates. The SF conglomerate beds contain solitary quartz arenite fragments and blocks up to 9.0 m3 that were derived from the UB. The fractures, fracture patterns, and microfractures in the UB quartz arenite and shattered quartz arenite fragments in the in situ SF breccia are ascribed to in situ fracturing, brecciation, and the accumulation of slightly moved blocks in a cold climate. The characteristics of the fractures and fracture patterns indicate rapid freezing and volumetric expansion. Because the paleotopography is not known, the origin of the breccia zone can-not be reliably established, but it most likely represents an ancient debris-mantled slope or blockslope accumulation. Some of the fragments in the breccia zone are slightly rotated and tilted, probably due to periglacial frost-heaving and mass-wasting processes. Chemical solution features around quartz arenite clasts are attributed to cold-climate chemical weathering. Observations suggest that the contact zone between the UB and the SF was affected by cryogenic weathering in a periglacial environment during the Mesoproterozoic. The block accumulation affected sedimentation pat-terns of the overlying SF, which was deposited by later alluvial processes. This study supports the idea that the unconformities and associated depo-sits record valuable information about the paleoclimate and paleosedimentology of the time.

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] .

Mesoproterozoic frost action at the base of the Svinsaga Formation, central Telemark, South Norway

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brecciation at the Neoproterozoic unconformity around Varangerfjorden, northern Norway, to have formed in a periglacial environment by physical weathering. Laajoki (2002b) also described the Mesoproterozoic sub-Heddal unconformity in Telemark, South Norway, and argued that the irregular geometry of individual fractures, the fracture framework in general, and associated in situ mosaic breccias indicate that break-up of the quartz arenite substratum was caused by surficial processes such as freeze-thaw. A glacigenic interpretation was also suggested for the fragmentation of the basement gneiss of a Neoproterozoic roche moutonnée feature at Karlebotn, Finnmark (Laajoki, 2003).

The rarity of descriptions and reports of Mesoproterozoic periglacial frost action deposits is not surprising, since deposits of that age have usually been affected by diverse tectonic and metamorphic processes and later weathering and erosion. This can make certain recognition of any typical periglacial or cold climate indicators like sand wedges, frost-shattered blocks, or frost fractures difficult or even impossible. Only in some rare cases, have these

indicators survived and remained recognizable. However, it is probable that Mesoproterozoic periglacial and glacigenic deposits were produced by processes similar to those observed in their younger, better preserved counterparts (cf. Eriksson et al., 1998; Eriksson et al., 2004; Donaldson et al., 2004).

This study describes and interprets the unique, slightly metamorphosed and deformed basal fracture and breccia zones at the base of the Mesoproterozoic SF, central Telemark, South Norway. The base of the SF is defined by the Sub-Svinsaga unconformity (SSU), which was first recognized by Dons (1960a, 1960b), but has only recently been defined formally (Laajoki et al., 2002).

Geological settingThe study area belongs to the Mesoproterozoic (ca. 1.5 – 1.0 Ga) sedimentary-volcanic Telemark supracrustal belt or sequence (Sigmond et al., 1997), which occupies the northern part of the Sveconorwegian Telemark

J. Köykkä & K. Laajoki NORWEGIAN JOURNAL OF GEOLOGY

Fig. 1. Simplified lithostratigraphical and structural map of the southern part of the Telemark supracrustals, where the bedrock is subdivided into diverse structural domains by seve-ral faults (thick lines). The study area is framed. SSU = Sub-Svinsaga unconformity. MUFS = Mandal – Ustaos shear and fault zone. MF = Marigrønutan fault zone.

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In Svafjell and Meien, the SF defines the western flank and the NE hinge of the steeply plunging Meien syncline, and is cut by the Marigrønutan fault (Figs. 2, 3A & B). In Sjånuten, the SF defines a north-west trending syncline cut by a fault near Lake Meisetjørn (Fig. 3B).

In the following, we first briefly describe the lithostratigraphy and then provide detailed descriptions and interpretations of the fracture zone developed in the UB and the overlying breccia zone of the lowermost SF. Although all rocks in the study area have been metamorphosed in low-grade greenschist- to lower-amphibolite-facies conditions, for simplicity the meta-prefix is not used in lithological names. The sandstone nomenclature used here is based on relative percentages of the major detrital components (Folk, 1980; Pettijohn et al., 1987).

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-amphi-bolite facies conditions (Brewer & Atkin, 1987; Atkin & Brewer, 1990).

The SSU is the most important unconformity within the Telemark supracrustal rocks, and separates the sequence into two major packages: (1) the Vestfjorddalen Supergroup (Rjukan and Vindeggen Groups), deposited between ca. 1.51 – 1.347 Ga, and (2) the upper succession of Sveconorwegian units, deposited between ca. 1.169 – 1.019 Ga. The Vestfjorddalen Supergroup starts with the Rjukan Group, which contains continental felsic volcanics of the Tuddal Formation (ca. 1.51 Ga) and the volcanic-sedimentary Vemork Formation (ca. 1.495 Ga, Laajoki & Corfu, 2007; Corfu & Laajoki, 2008). The Rjukan Group is overlain by the Vindeggen Group, which is dominated by sedimentary rocks deposited between ca. 1.495 – 1.347 Ga (Corfu & Laajoki, 2008) in a continental rift setting (Lamminen & Köykkä, submitted and references therein). The Rjukan and Vindeggen Groups were folded before deposition of the volcanic-sedimentary Oftefjell Group, which in the study area consists only of the SF and a small body of the Ofte Formation porphyre that has been separated tectonically from the main body to the south (Fig. 1). The Sveconorwegian units are laterally restricted and variable, suggesting deposition in separate basins (Laajoki et al., 2002, Andersen et al., 2004), possibly in a transtensional tectonic regime (Lamminen et al., 2009b).

The SSU and the basal parts of the SF above the UB quartz arenites, which form the topmost unit of the Vindeggen Group, are best exposed on the eastern flank of Svafjell Mountain and in the smaller hills of Meien and Sjånuten (Fig. 2). Deep erosion, which started after ca. 1.347 Ga (Corfu & Laajoki, 2008), produced the SSU on the folded Vindeggen Group. The isotopic dating indicates that at least a 200 Ma hiatus is associated with this unconformity (Lamminen et al., 2009, Lamminen & Köykkä, submitted).

In the type area of the SF, about 20 km south of the study area, the SSU forms a clearly mappable angular unconformity (Dons, 1960a, 1960b, 2003), but in the study area, bedding positions in the UB and SF are so similar that these formations appear to be conformable. This paper describes a major unconformity that occurs between the UB and the SF. It should be noted, however, that the three-dimensional geometry of the unconformity itself cannot be defined precisely, because it does not form any larger surface and is visible only sporadically.


Fig. 2. Simplified geological map of the study area. Areas of Figs. 3A & 3B are framed. SSU = Sub-Svinsaga unconformity.

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Svafjell-Meien area

In the Svafjell area, the first sign of the SSU between the UB and SF is a 1 – 3 m thick zone of mostly random oriented, closed or mudstone-filled fractures developed in the uppermost part of the UB quartz arenite (Fig. 4A). The fracture zone grades upward into an in situ breccia, where the blocks, matrixed by scanty mudstone, are solely of the UB quartz arenite. The fractures and in situ breccia zone represent a weathering crust developed on the UB quartz arenite, and define the SSU. The in situ breccia grades upward into a basal breccia, where the fragments of cobble and block size have been moved or rotated slightly.

On the western flank of the Meien syncline, the basal breccia is overlain by a matrix- and clast-supported SF conglomerate containing distinct fragments and blocks up to four meters long that have been transported from

LithostratigraphyBased on distinctive facies assemblages, the Brattefjell Formation is subdivided into lower, middle, and upper (UB) members (Köykkä & Lamminen, in prep.). The horizontal- and parallel-laminated UB quartz arenite forms the depositional basement for the SF (Fig. 2). Dons (2003) mapped the major southern part of the SF starting with the Oftefjell Group in Fig. 1, whereas Köykkä (2006) described the lithologies and sedimentology of the SF in the Svafjell – Meien and Sjånuten areas, which are separated tectonically from the southern part of the SF. This paper concentrates on the northern area. In this paper, the term “fragment” refers to a fragment of a sedimentary breccia, the size being less than that of a boulder (256 mm).


Figs. 3A & B. Geological maps and cross sections of the Svafjell, Meien and Sjånuten study areas (modified from Köykkä, 2006). Figure and outcrop numbers are given. SSU = Sub-Svinsaga unconformity. A) The SF is seemingly conformable with the bedding of the underly-ing UB. The SF defines the western flank of the Meien syncline. B) The SF in Meien defines the hinge and eastern flank of the Mein syncline cut by the Marigrønuten fault. In the Sjånuten area it occupies a north-west directed syncline or minor synclinorium bounded by a fault near the Lake Meisetjørn.

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organized in texture. The quartz arenite clasts in the diamictite unit are well-rounded to angular. The overlying horizontally laminated subarkose with occasional thin (1 – 5 cm) mudstone interbeds forms a unit about 80 – 90 m thick (Fig. 4A). The uppermost part of the SF (ca. 70 m) consists of large-scale trough cross-bedded subarkose with channel lag and some polymictic conglomerate beds (Fig. 4A). Paleoflow measurements of the trough cross-bedded sets and imbricated conglomerates indicate a unimodal southwest transport direction (Fig. 4A).

the UB quartz arenite (Fig. 4A). Upwards, well-rounded quartz arenite clasts derived from more distal sources become increasingly dominant, although solitary blocks of the UB up to half a meter long may occur among them. In the Meien area, the basal breccias and the SF subarkose are overlain by a muddy diamictite unit about 30 m thick, interpreted to have been deposited by subaerial debris flows and hyperconcentrated flow processes (Köykkä, 2006) (Fig. 4A). The diamictite beds with erosional bases are muddy, poorly sorted, and chaotic or only slightly


Figs. 4A & B. A) Stra-tigraphic summary column of the SF in the Svafjell-Meien area. See framed box for the facies codes (modified Miall, 2006) and legend. The UB quartz arenite frag-ments in the SF conglo-merate and subarkose beds are marked by “UB”. Paleoflow measu-rements from the trough cross-bedded sets and imbricated conglomera-tes indicate a southwest transport direction. B) Stratigraphic summary column of the SF in the Sjånuten area. See framed box for the facies codes (modified after Miall, 2006) and legend.

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Sjånuten areaThe SF in the Sjånuten area (Fig. 3B) starts with an in situ breccia developed on the UB quartz arenite (Fig. 4B). The fragments and boulders in the breccia are derived solely from the UB quartz arenite, with a matrix of scanty mudstone. This breccia zone is a couple of meters thick and is overlain by about 18 m thick massive, clast-supported, polymict conglomerates that contain solitary blocks of the UB quartz arenite up to four meters long (Fig. 4B). The volume of the fracture blocks varies from 1.0 m3 to 9.0 m3 or more. In other places, the breccia is missing and the clast-supported boulder conglomerate, made up solely of UB blocks, lies directly on the UB (Fig. 5A).

The facies associations, coarseness of the basal conglomerates and the presence of the large UB blocks indicate that the SF in the Sjånuten area represents the most proximal area of the SF. This is supported by the southwest paleoflow direction in the SF (Fig. 4A).

Fracture zone

Description

The following description of the fracture zone on the eastern flank of Svafjell represents a vertical traverse of the original section and is given from the base of the zone upwards. The UB quartz arenite immediately under the unconformity does not have any features indicating significant deformation before the deposition of the SF. The only structural feature in the UB quartz arenite and the Sub-Svinsaga paleosurface is a widely spaced fracture framework perpendicular to the bedding (Fig. 5B). Some of the fractures are a few millimeters wide and are filled with mudstone. This zone, which is a few meters thick, represents the deepest part of the surficial fracture system generated in the UB quartz arenite when it was uplifted to the erosion level. It grades upwards into a zone where the mudstone-filled fractures are wider (Figs. 5B – D). The bottom part of the fracture zone begins with fractures ranging from a few millimeters up to five centimeters wide, and from a few centimeters up to one meter long. They were originally filled by clay and/or silt, which is now metamorphosed into purple sericite-rich mudstone. The contact margins between the UB quartz arenite and the SF mudstone fill are partly corroded, and the mudstone may replace the silica cement of the quartz arenite over a distance of a few millimeters. These mudstone-filled fractures are the first distinctive hint of the SSU (Fig. 3A). Some of the fractures are subparallel to bedding in the UB, but most of them are randomly oriented (Figs. 5C & 5E). Many of the lowermost fractures are rectilinear with sharp margins, possibly representing previous fractures or other pre-existing structural weaknesses in the quartz arenite, but curved fractures are also common (Fig. 5C). Microfracturing is also a common feature in the bottom part of the fracture

zone (Fig. 6A) Upwards, the fractures become wider and their margins are usually smooth or partly corroded.

The only outcrop where a larger surface parallel to the bedding plane of the topmost part of the UB can be seen is in Meien. There, the fractures are up to 15 cm wide and are filled by mudstone with occasional quartz arenite pebbles (Fig. 6B). This view represents a unique section viewed across the top part of the fracture zone and parallel to the SSU. The fracture zone grades upwards into the basal breccia zone, where a fracture network and in situ breccia are developed oblique to the bedding in the UB quartz arenite. Interpretation

Fractures and fracture patterns can be produced in bedrock by several different mechanisms, including tectonic, hydrothermal, unloading, and different weathering processes (solution, insolation, and frost action). It is important to consider all these potential mechanisms when interpreting ancient fracture systems. The possible effects of these different mechanisms on the UB quartz arenite are considered below.

A tectonic fracture system can be produced in bedrock by compressive, tensile, or shearing stress and is rarely randomly oriented. Thus, one or several dominant orientations typically develop and basic fracture sets can usually be identified by statistical methods (Scheidegger, 2001). Using a parametric statistic, Kohlbeck and Scheidegger (1977) determined that 21 measurements of fracture orientations in an outcrop should be sufficient to determine the stress field (if any) that produced the fractures. Sampling of fracture orientations can be biased by the geometry or scale of the sampling domain, which means that fractures that are subparallel to bedding are less likely to be sampled than those at higher angles. However, this kind of bias error can be corrected using the method described by Terzaghi (1965), which is based on a trigonometric correction factor and an assumption that the fracture spacing in different sets is fairly equal. The fracture measurements carried out using the method of Kohlbeck and Scheidegger (1977) from the fracture zone in the UB quartz arenite included 70 directional and dip measurements (Fig. 5E). These indicate that the major fracture system in the UB quartz arenite is mostly random oriented (Fig. 5E), and that it is unlikely that it was produced by a tectonic process. Some of the fractures, for instance those that are vertical or parallel to bedding, may be pre-existing cracks or fractures, or may be related to some inherent structural weakness.

It is possible that at least some of the fractures were formed or at least widened by unloading of a hypothetical pre-Svinsaga ice overburden after the vertical compressive stress was relieved. The opening of pre-existing fractures in response to unloading is important in the case where water can reach into widened cracks and intensify the


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weathering process. However, the glacial unloading process is often masked by the patterns of existing structural weaknesses in the rock, making it difficult to recognize its effects in an ancient rock record, especially where no convincing indicators of an ice sheet exist.

Provided that a sufficient amount of moisture and water is available, dissolution weathering of a quartz arenite can lead to fracture patterns similar to those found in carbonate rocks. The dissolution removal of silica under earth-surface conditions is a significant process, even in sub-polar latitudes (Wray, 1997a, 1997b). Solution in quartz arenites begins from fractures or other easily penetrated zones within the rock and continues along crystal boundaries (Fjellanger & Nystuen, 2007), and tends to produce rounded features around the fractures. This is in contrast to the UB quartz arenite, where the fractures are mostly rectilinear and sharp and form a network with acute angles (Figs. 5C & D). Thus, we conclude that dissolution has not been a major mechanism forming the fractures and fracture patterns in the UB quartz arenite.

Frost weathering has been traditionally attributed to stresses caused by the expansion of water during freezing in fractures, joints, or bedding planes (macrogelivation) or within microcracks and pores (microgelivation). The bedrock frost-shattering process in a periglacial environment can be subdivided into three major mechanisms (McGreevy, 1981; French, 2007): (1) hydrofracturing, (2) ice segregation, and (3) thermal shock. Each distinct frost shattering mechanism is important in different periglacial conditions, and usually several mechanisms act together in breaking down the rock.

(1) Hydrofracturing is the volumetric expansion of water upon freezing, where the water penetrates rock fractures and pores and expands about 9% on freezing; the stress caused by the expansion tends to break the rock apart. This process is favored by wet conditions, rapid cooling, and frequent freeze-thaw cycles (Matsuoka, 1990). Because the fractures in the upper part of the fracture zone were open before the deposition of the SF, they provided access for water flow and hydrofracturing of


Figs. 5A – E. Photographs of the SSU and fracture zone. A) Boulder conglo-merates deposited directly on the UB quartz arenite. The SSU marked by stick (1.6 m long). B) Close-up of the open fracture zone. Scale bar is 5 cm. The white arrow points to a fracture approximately parallel to the bedding in the UB. C) A basal view of the in situ breccia fragments with irregular fracture network. Stick is 1.6 m long. D) Close-up of the open fracture zone showing a network of mostly ran-domly orientated fractures. Note that some of the wider fractures have been particularly filled by angular in situ quartz arenite detritus enclosed and infiltrated by infiltrated mudstone (arrow) E) Seventy fracture orien-tation measurements from the UB quartz arenite in Svafjell area, presen-ted in a Schmidt equal area stereonet. Contour lines and intervals represent the point densities, which indicate that some of the fracture patterns are randomly orientated, but some of them may represent pre-existing frac-tures or some other structural weak-nesses. The three different eigenvalues provide direct information about the distribution and uniformity of the data set, and indicate that distribu-tion is not entirely uniform.

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of the breccia zone. The blocks and fragments consist solely of UB quartz arenite. They show no evidence of any preferred orientation of tectonic origin. Some of the quartz arenite fragments have been broken in situ and are filled by purple mudstone (Fig. 6C). The interfragmental matrix spaces widen upwards and occasionally the matrix consists of mudstone with quartz arenite clasts (Fig. 6D). The margins of the fragments and blocks are either sharp, rounded (Fig. 6E), or partly corroded (Fig. 6F). The well-defined weathering pits typical of slow chemical dissolution weathering were not found in the UB quartz arenite fragments or blocks.

The quartz arenite in the fragments and blocks in the breccia zone is lithologically identical to that of the underlying UB, as demonstrated by the red color caused by diagenetic hematite, the mature blastoclastic arenitic texture with well-rounded quartz grains and minor silica cement, the mineralogical supermaturity, and sedimentary structures. The grain size varies between 0.125 – 0.250 mm. The characteristic feature of the quartz grains is a partly sutured texture where the grain edges are irregular due to compaction or weak tectonic action. In addition, overgrowth and relic edges due to pressure solution are observed in some of the grains. The composition of the mudstone fill between the breccia fragments and blocks varies from quartz- to sericite-rich with variable amounts of muscovite, feldspar, quartz, and iron oxide. The sericite-rich variety is more common. The mudstone matrix is slightly metamorphosed, containing occasional randomly oriented chloritoid porphyroblasts.

Interpretation

The SF breccia zone above the UB indicates an abrupt change in depositional system. Unfortunately, the sub-SF paleoslope is not exposed in three dimensions. The fact that the SF fragments are not rotated, or are rotated only slightly, indicates that the breccia material was not transported any long distance. This excludes solifluction, avalanching, rockfall on a talus slope, and debris flows as significant processes in explanation of its origin. Consequently, the packing of the SF fragments and blocks more likely favors block accumulation, like a blockfield, blockslope, or a debris-mantled slope where the breccia fragments at the surface are firmly embedded in the matrix grains.

In the SF breccia zone, in situ fracturing of the bedrock was the most important process, and periglacial processes (e.g., mass-wasting, frost-heaving) may have controlled the distribution of the fragments and blocks. In periglacial environments, mass-wasting processes can be subdivided into (1) the gradual downslope displacement of slope sediments due to repeated freezing and thawing, and (2) more localized rapid slope failures that occur sporadically during thaw of the active layer or underlying permafrost (Ballantyne & Harris, 1994). The former results from the combined effect of frost creep and gelifluction, for

the UB quartz arenite. The randomly oriented fracture pattern in the UB quartz arenite supports rapid freezing and volumetric expansion, which tend to produce a mostly random oriented fracture pattern (Mackay, 1999). (2) Ice segregation is attributed to freezing in a water-saturated rock. It is analogous to slow freezing in fine-grained soils. The expansion is the result of water migration to growing ice lenses and secondly to volumetric expansion (Hallet et al., 1991). In this process, frost shattering of a rock results from the progressive expansion of micro-cracks and pores. According to Murton et al. (2001), the ice segregation mechanism usually forms a dominant alignment of cracks parallel or sub-parallel to the cooling surfaces (ground surface or bedrock), because ice lenses usually form parallel to the isotherms. The mostly random orientation of the fractures in the UB quartz arenite suggests that ice segregation might not have been the dominant frost weathering process in the study area. Murton et al. (2001) noted, however, that variable orientations of cracks due to ice segregation probably reflect variable water content as a result of non-uniform wetting. The pre-existing fractures in non-porous rock are more vulnerable to volumetric expansion associated with rapid freezing than to ice segregation.

(3) Thermal shock, i.e., insolation weathering, is the fracturing of bedrock due to temperature-induced volume changes (expansion and contraction). It requires that the bedrock experiences temperature contrasts and has capillary size pores (French, 2007). The thermal shock mechanism tends to form fracture patterns that show a remarkable angularity of junctions (often at almost 90o to each other) and a relative hierarchy of cracks, where the small cracks are at right angles to the major ones (Hall & Andre, 2001; Hall et al., 2002). The fracture patterns typical of this mechanism are not found in the UB quartz arenite. Consequently, it is unlikely that the thermal shock mechanism caused the intensive granular disintegration actually observed in the fracture zone of the study area.

Breccia zone

Description

In the Svafjell and Sjånuten areas, the SF breccia zone starts with the blocks, which are either in situ or have been rotated or moved only slightly. The blocks have a scanty matrix of purple mudstone. The lower part of the SF breccia zone is tightly packed and the rock fragments are mostly large blocks, but their size and sorting decrease stratigraphically upwards. The orientation of the long axis of the blocks is more systematic in the middle part of the breccia zone, where most blocks have their long axes parallel to the bedding of the UB. However, the long-axis orientation is more variable in the upper parts

299NORWEGIAN JOURNAL OF GEOLOGY Mesoproterozoic frost action at the base of the Svinsaga Formation

The in situ or slightly displaced fragments and blocks in the SF breccia zone, like in many blockfields and blockslopes, developed during cold climate conditions where freeze and thaw processes operate (cf. Boelhouwers, 1999a, 1999b; Steijn et al., 2002). It is well known that criteria like block accumulation or clast shape alone are not sufficient as paleoenvironmental indicators. Thus, the paleoenvironmental significance of autochthonous blockfields depends on the interpretation of the origin of the fragments and blocks, which is usually attributed to in situ frost-shattering of bedrock under cold conditions (Steijn et al., 2002). Clast angularity, the absence of grussification, and a transition to underlying rock, which exist in the lower part of the SF breccia zone, imply formation by in situ frost- shattering of the bedrock (Ballantyne, 1998). The freeze-thaw depth controls the maximum dimension of the detachable rock mass, while fracture spacing determines the size distribution of the rockfall debris (Matsuoka & Sakai, 1999). Thus, the maximum size of the largest SF blocks indicates that the freeze-thaw depth in the UB paleo-weathering surface was 1 – 4 m.

which the term solifluction is often used (op. cit.). The slightly rotated fragments and blocks in the upper part of the SF breccia zone indicate frost-heaving, since larger fragment and block angles with respect to the horizontal bedding plane would represent gravitational solifluction or some other mass wasting process. In the gravitational mass wasting process (gelifluction), the longitudinal axial planes of fragments tend to be in the downslope direction, producing a slight uphill imbrication and dip parallel to the ground surface, or at an angle less than the slope angle (e.g., Hétu et al., 1995; Van Steijn et al., 1995; Perez, 1998). The rotation and tilting of the SF breccia fragment and block axes resulted from differential frost heave at the top and bottom parts of the fragments and blocks, and all particles were subjected to rotation during the heave phase. In general, the heaving is controlled by the air temperature, moisture supply, depth and rate of freezing, snow conditions, and freeze-thaw cycles. The heaving also requires either confined conditions or the presence of finer material (Dredge, 2000), suggesting that the SF breccia fragments and blocks did not have an open framework at the time of heaving.

Figs. 6A – F. Photographs of the fractures and breccia fragments. A) Tightly packed bottom part of the fracture system with minor mudstone fill and narrow fractures. Notice the microfractures (marked with arrows). Scale 15 cm. B) Wider fractures in an UB quartz arenite fil-led by pebbly mudstone. View is ver-tical to the bedding plane of the UB quartz arenite. Scale 15 cm. C) In situ fragmented quartz arenite clasts interpreted to be caused by ancient frost action and repeated freeze-thaw cycles. Mudstone fills the frac-tures (arrows). Compass base is 12 cm long. D) The interfragment space filled by mudstone with UB quartz arenite clasts .E) Pillow like breccia fragments with mudstone-filled frac-tures. The rounded, but irregular and open form of the fragments indi-cates chemical solution of the quartz or/and melt water action. Compass 12 cm long. F) Chemical solution features (arrows) around quartz are-nite clasts. Compass 12 cm long.


sedimentation, and different periglacial processes (e.g. bedrock frost-weathering, frost-heaving, mass-wasting, blockfield and/or rock slope accumulations) (Fig. 7). In the late stage (syn/post-faulting) of the basin evolution, the newly formed geomorphic features were adjusted by the sediment transport systems. The sedimentation included alluvial incision on the fractured bedrock, lacustrine and/or marine deposition, and possible volcanism (Fig. 7). A possible continuation of frost weathering at a later stage is an open question, as there is no direct evidence of this during the sedimentation. However, minor frost-weathering would have been likely during sedimentation of the SF.

DiscussionIndicators of paleoweathering and periglacial processes in the Precambrian bedrock can be difficult to recognize. The fact that these indicators are often masked and covered by other sedimentary and structural features may lead to misinterpretation of the depositional processes and even paleoclimate. The recognition of cold-climate periglacial features from Precambrian deposits should be based on an assemblage of indicators and detailed facies analysis (see Eriksson et al, 1998, Eriksson et al, 2004). Laajoki (2002b) described a fracture zone a few meters thick in the Sub-Heddal unconformity in Telemark, South Norway, with irregular fracture features, mosaic-like breccia, and irregular paleochannels excavated into the UB quartz arenite. He noticed that the fracture framework in the UB quartz arenite starts with thin fractures subparallel to bedding, and that their number, width, and curvature increase stratigraphically upwards. The lower part of the fracture zone in the Sub-Heddal unconformity contains boulder-size fragments that are partly attached to their sub-stratum, and fractures up to 10 cm wide containing pelitic material. Laajoki (op. cit.) concluded that the Sub-Heddal unconformity represents a deep paleoerosional feature, and the break-up and paleochannel carving of the UB quartz arenite may suggest surficial processes like freeze-thaw combined with solution weathering.

Although the Sub-Heddal unconformity (ca. 1.155 – 1.145 Ga) cannot be correlated definitely with the SSU or any other unconformity within the Telemark supracrustal units, when taken together with the present study, it suggests that the Telemark supracrustals may have experienced significant frost-weathering during Mesoproterozoic time.

ConclusionsThe irregular features of the fractures, fracture patterns, microfractures, shattered UB quartz arenite clasts, and mudstone fill indicate that periglacial alteration pro-cesses played an important role in the genesis of the pre-

The solitary UB quartz arenite fragments and blocks in the upper parts of the SF subarkose (Fig. 4A) may indicate minor ploughing, avalanching, or solifluction processes. These processes require the blockslope angular gradient to be 8o – 30o in order to be active in a periglacial environment (Ballantyne & Harris, 1994; French, 2007). This suggests that the SF fragments and blocks did not accumulate on a completely flat surface. However, the assumption of the slope angle is more complex, and can vary greatly in different parts of the slope and change over time on an evolving slope. Usually, blockfields and blockslopes have complex structures comprising different slope dips, and as a consequence, varying degrees of snow and debris cover (Gruber, 2004).

The corroded quartz arenite clasts and pillow-like breccia cobbles and boulders in the SF breccia zone indicate in situ chemical solution and weathering (Figs. 6E & F). Some corroded quartz grains visible in thin sections support this interpretation, as does the geochemical CIA-index (75 – 78) from mudstone samples (Köykkä, 2006). Chemical weathering is not restricted to humid tropical areas, but can also act in periglacial environments, even to the same extent as physical weathering (Anderson et al., 1997; Wray, 1997; Allen et al., 2000; Dixon & Thorn, 2004). Johnson et al. (1970) and Robert & Reynolds (1971) also mention that low mean annual air temperatures in a periglacial environment do not necessarily limit chemical weathering processes, since melt water runoff can be chemically corrosive when it is continuously recharged with CO2. Several other authors (Sharp et al., 1995; Wadham et al., 1998; Hodson et al., 2000) have also noted chemical erosion by snow or glacial melt water. According to Sharp et al. (1995), the efficacy of meltwaters in chemical weathering can be attributed to high flushing rates, turbulent flow, high suspended sediment concentrations, and low buffering capacity of dilute melt waters. It is possible that the rounded pillow-like cobbles and boulders in the SF reflect later, pre-Pleistocene, deep chemical weathering by corrosive melt water. However, due to the lack of suitable geochronological methods, it is impossible to determine when this chemical weathering took place.

PaleosedimentologyThe SF facies associations and sedimentary structures (large-scale trough cross-bedding, alluvial conglomerates, channel lag etc) indicate that the SF represents an alluvial sedimentation system in a strike-slip (pull-apart) basin (Köykkä, 2006; Köykkä, in prep.). This suggests that the overlying SF conglomerate and subarkose beds (Fig. 4) have been deposited over the blockfield or rock slope by later alluvial processes, while part of the sedimentary material worked its way into the fractures between the fragments and the blocks. In the early stage of the basin evolution (pre/syn-faulting), the rapid subsidence was associated with lacustrine and possible marine


and nature of the breccia fragments and blocks support formation by block accumulation, in cold climate con-ditions with operative freeze-thaw processes combined with minor block ploughing and solifluction. The overly-ing SF conglomerate and subarkose beds were deposited over the block accumulation by later alluvial processes and worked their way down into the spaces between the fragments and blocks. In the early stage of basin evolu-tion (pre/syn-faulting), rapid subsidence was associated with different sedimentation processes and bedrock frost-weathering, frost- heaving, mass-wasting, and blockfield and/or rock slope accumulations. In the late stage (syn/post faulting) of the basin evolution, the newly formed geomorphic features were modified by allu-vial incision of the fractured bedrock, lacustrine and/or marine deposition, and possible volcanism.

SF fracture system associated with the SSU. This indi-cates cryogenic weathering in a periglacial environment with a “combination of mechanico-chemical processes which cause the in situ breakdown of rock under cold-climate conditions” (French, 1996; French, 2007). The characteristics of the fracture zone indicate rapid freez-ing and volumetric expansion. In the study area, the role of frost-shattering is also supported by the fracture zone passing upwards into the in situ breccia zone, and then to a debris-mantled block or blockslope accumulation with UB fragments and blocks.

Although reliable structural evidence concerning the ancient paleoslope under the SF is missing, it is evi-dent that in situ weathering was an essential process in the supply of breccia fragments and blocks. The sorting

Fig. 7. A schematic paleosedimento-logical figure of the strike-slip basin evolution and sedimentation of the SF and associated units (modified from Einsele, 2000). The pre/syn-faulting stage was characterized by different periglacial processes (frost-weathering, mass- wasting, frost-heaving etc.) and possible lacustrine and/or marine sedimentation. The extensional faulting stage was fol-lowed by alluvial incision and the SF sedimentation over block field or block slope. Figure not to scale.


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mesoproterozoic frost action at the base of the svinsaga ...depth where the rate of cooling is...

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