Tidally influenced clastic epeiric sea at a Mesoproterozoic continental margin, Rjukan Rift Basin, southern Norway

Precambrian Research 185 (2011) 164–182

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Tidally influenced clastic epeiric sea at a Mesoproterozoic continental margin, Rjukan Rift Basin, southern Norway Juha Köykkä a,∗ , Jarkko Lamminen b a b

Department of Geosciences, University of Oulu, P.O. Box 3000, FIN-90014 Oulu, Finland Department of Geosciences, University of Oslo, P.O. Box 1047, N-0316 Oslo, Norway

a r t i c l e

i n f o

Article history: Received 21 September 2010 Received in revised form 29 November 2010 Accepted 2 December 2010 Available online 14 December 2010 Keywords: Clastic Epeiric sea Lithofacies Mesoproterozoic Nearshore Southern Norway Tidal

a b s t r a c t Precambrian epeiric sea paleoenvironments record information about ancient tidal conditions and nearshore sedimentation patterns. The Mesoproterozoic Brattefjell Formation, part of the Rjukan Rift Basin in southern Norway, presents diagnostic criteria of an ancient tidal influence in a shallow clastic epeiric sea environment during the post-rift stage of the sedimentary basin. Eight different sedimentary lithofacies/subfacies and two genetically related lithofacies associations were recognized from the formation. The conglomerate lithofacies records a transgressive ravinement surface due to wave scouring. The sandstone lithofacies consists of six subfacies: ripple-marked, planar cross-bedded, trough crossbedded, low-angle cross-bedded, and horizontal laminated sandstones. The sandstone lithofacies records sedimentation by shallow current flows or oscillatory waves of lateral migrating straight- or slightly sinuous-crested sandbars and channel-fill deposits. The associated bi-polar cross-bedding, slump-folds, ladder-back type ripples, shrinkage cracks, double-mud drapes, iron ooids, load casts, and flame structures indicate variation of the flood-ebb cycles in tidal sandbars, rapid sedimentation, and sediment compaction. Tidal bundle measurements suggest that 16–18 bundles were formed during the neap-spring cycle, indicating mixed semidiurnal tides. A mudstone lithofacies (two subfacies) records waning current strength deposits, channel abandonment, and either vertical aggradation of the channels or coastal plain deposits. The lithofacies associations reflect fluctuating energy conditions in a shallow clastic shoreline influenced by tides and waves. At least 23 shallowing upward cycles caused by autocyclic tidal water level changes were recognized from the lower part of the formation. Based on the lithofacies assemblages, the Brattefjell Formation can be subdivided into sandy subtidal, lagoon-estuary, and shallow marine shoreface-offshore environments deposited above the fairwater wave base. The Brattefjell Formation records millions of years of fluctuating water levels with an overall transgressive trend, indicating allocyclic patterns caused by regional subsidence. The formation is interpreted to represent a tidally influenced epeiric sea in which shallow water levels extended over part of a continent and it was associated with marine transgression. The shoreface was fed sediment by alongshore and coastal current drifts during shoreface transgressive erosion, in a low shoreline gradient with reduced sediment input and lack of major braided-fluvial channel(s). © 2010 Elsevier B.V. All rights reserved.

1. Introduction Tidal environments are usually associated with shallow nearshore sediments, which rim around continents and inland seas within continental areas. In these wave or tide dominated environments, sediment is supplied from continental drainage systems, and the accommodation space is sensitive to fluctuations of base level changes. Wave dominated transgressive nearshore settings are typically characterized by ravinement surfaces that record wave scouring, which is an attempt to maintain balance between base level rise and shoreface profile. Stratigraphic patterns of tidal

∗ Corresponding author. Tel.: +358 8 5531439; fax: +358 8 5531484. E-mail address: juha.koykka@oulu.fi (J. Köykkä). 0301-9268/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.precamres.2010.12.005

environments usually consist of sequences of cyclic sedimentation with channel fills and tidal point bars. Cyclicities in siliciclastic nearshore deposits and their associated diagnostic structures are unique criteria for recognizing a tidal influence in ancient shallow marine clastic environments (Nio and Yang, 1991). These cyclicities usually include lateral thickness variations of bundling sandy foresets bounded by mud drapes, reflecting semi-lunar cycles of neap-spring tides and reactivation surfaces, indicating flood-ebb cycles. Ancient tidal and nearshore sediments provide unequivocal evidence of epeiric sea conditions in otherwise non-fossiliferous sediments of Earth’s longest eon, thus forming an important part of the Precambrian sedimentary record. Studies of ancient tidal systems have been used to calculate Earth–Moon dynamics, anomalistic months and tidal effects on basin sedimentation

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patterns (e.g. de Vries Klein, 1970; von Brunn and Hobday, 1976; Adkins and Eriksson, 1981; Song and Gao, 1985; Chan et al., 1994; Eriksson and Simpson, 2000; Mazumber and Arima, 2005). Most of the inferred Precambrian tidal deposits are subtidal sandwave deposits with tidal sandstone channel fills that usually lack intertidal zones, barrier islands and associated tidal inlets and deltas (e.g. Tirsgaard, 1993; Eriksson et al., 1998). Uniform sedimentation suites with thick, well-sorted sandstone successions, a lack of clear vertical trends, and a lack of mudstone characterize the associated shallow nearshore settings (Soegaard and Eriksson, 1985; Harris and Eriksson, 1990; Walker and Plint, 1992; Eriksson et al., 1998). Tidally influenced epeiric sea settings were common during the Precambrian, which were characterized by greater tidal conditions, higher energy, shallow water levels, and usually braided-fluvial influences compared to the younger successions, reflecting reduced topography with aggressive weathering regimes, rapid erosion, and lack of vegetation (Eriksson et al., 1998; Corcoran and Mueller, 2004; Eriksson and Simpson, 2004; Eriksson et al., 2002, 2004, 2005, 2008). Thus, the recognition of various tidal signatures and cycles from the stratigraphic record has been a major contribution to the interpretation of ancient tidally influenced epeiric nearshore environments and to the understanding of rates of sediment accumulation and preservation. This study concentrates on the Mesoproterozoic Brattefjell Formation in the Rjukan Rift Basin, southern Norway, which is a part of the Telemark supracrustal rocks. The Brattefjell Formation is composed of sandy subtidal and lagoon-estuarine sandstone

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successions with associated mudstone interbeds and shallow nearshore sediments. The main contributions of this study are twofold: (i) providing the sedimentology of a Mesoproterozoic tidal sedimentation system and a reconstruction of its depositional history; and (ii) illustrating how tidal and nearshore sedimentation patterns correspond with fluctuating base level changes during the post-rift stage of a rift basin. Sedimentary basins in or along the Precambrian continents are generally reworked in younger orogenic events and have therefore lost most of sedimentological information. However, the Telemark supracrustal rocks display well preserved primary sedimentary structures, recording ca. 400 Ma years of sedimentation processes and history in evolving tectonics during the Mesoproterozoic. 2. Geological setting The Southwest Scandinavian Domain (Gaál and Gorbatschev, 1987) is the youngest part and southwestern-most part of the Fennoscandian Shield, characterized by Sveconorwegian deformation and metamorphism between ca. 1.14 and 0.90 Ga (Bingen et al., 2008a). It is divided along Proterozoic shear zones and Phanerozoic faults into a number of crustal domains, which are variously classified as terranes, sectors, segments or blocks (Andersen, 2005; Åhäll and Connelly, 2008; Bingen et al., 2001, 2005, 2008a,b) (Fig. 1). The main domains are, from east to west, the Eastern Segment, the Idefjorden terrane, the Bamble–Kongsberg sectors/blocks/terranes, the Telemark sector/block and the Hardangervidda–Rogaland

Fig. 1. Simplified geological map of the Sveconorwegian belt and domains affected by ductile deformation after 0.97 Ga (modified from Bingen et al., 2006, 2008b). Study area marked. HR = Hardangervidda–Rogaland block, T = Telemark block, B = Bamble block, K = Kongsberg block, I = Idefjorden block, ES = Eastern segment, MU = Mandal–Ustaoset fault and shear zone.

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block. The Eastern Segment consists of reworked Paleoproterozoic felsic orthogneisses related to the Transcandinavian Igneous Belt. The Idefjorden terrane exposes the typical “‘Gothian” rocks, reflecting a volcanic arc setting between 1.64 and 1.52 Ga (Åhäll and Connelly, 2008). The Bamble–Kongsberg sectors include metasediments and orthogneisses formed after 1.57 Ga. The Telemark block is characterized by a thick supracrustal sequence, called the Telemark supracrustal rocks, deposited at and after 1.51 Ga. The Hardangervidda–Rogaland block is exposed to the west of the Mandal–Ustaoset fault and shear zone. This block consists of ca. 1.5 Ga and younger gneisses increasing metamorphic grade toward the southwest, and it hosts the Rogaland anorthosite–mangerite–charnockite complex. 2.1. Telemark supracrustal rocks The Mesoproterozoic volcanic-sedimentary Telemark supracrustal rocks form a ca. 10-km-thick succession that has been metamorphosed under low-grade greenschist-amphibolite facies (Dons, 1960a,b; Brewer and Atkin, 1987; Atkin and Brewer, 1990; Sigmond et al., 1997; Laajoki et al., 2002; Bingen et al., 2001, 2005). The Telemark supracrustal rocks can be divided into two main successions separated by a first-order unconformity. The lower succession, referred to as the “Vestfjorddalen Supergroup” was deposited in the Rjukan Rift Basin between ca. 1.512 and 1.347 Ga (Fig. 2). The overlying succession was deposited between ca. 1.17 and 1.019 Ga and is separated by the sub-Svinsaga unconformity, which records a ca. 150-Ma hiatus in the stratigraphic record. The Rjukan Group forms the base of the Vestfjorddalen Supergroup (Fig. 2). No depositional basement to the Rjukan Group has been identified, as the base of the group progressively grades northward into a gneiss complex. The Rjukan Group consists of the Tuddal Formation, which is made up of ca. 1.51-Ga continental felsic volcanic rocks (S. Dahlgren et al., 1990; S.H. Dahlgren et al., 1990; Bingen et al., 2005) overlain by ca. 2-km-thick succession of the volcanic-sedimentary Vemork Formation (ca. ≤1.495 ±2 Ga, Laajoki and Corfu, 2007), which is mainly characterized by mafic volcanism (Fig. 2). The lithostratigraphic position of the Vemork Formation has been a matter of debate (S. Dahlgren et al., 1990; S.H. Dahlgren et al., 1990; Laajoki and Corfu, 2007; Köykkä, 2010). According to Köykkä (2010), the volcanic-sedimentary Vemork Formation represents the axial part of the Rjukan Rift Basin, where the extrusion of abundant mafic lavas reflects a change in the magmatic regime in the rift basin that was possibly coeval with a syn-rift tectonic phase. The Rjukan Group is unconformably overlain by the ca. 5-kmthick sedimentary-dominated Vindeggen Group (Fig. 2), which contains several sandstone and mudstone bearing formations deposited in different environments and is faulted into several internally folded blocks. The sedimentation of the Vindeggen Group is bracketed between ca. 1.495 and 1.347 Ga (Laajoki and Corfu, 2007; Corfu and Laajoki, 2008). The Brattefjell Formation represents the topmost part of the Vindeggen Group, which is the focus of this study (Fig. 2). This formation is composed of several sandstone and interbedded mudstone successions with diverse sedimentary structures. The Brattefjell Formation overlies the Vindsjå Formation, where the contact is considered transitional (Fig. 2). The Vindeggen and succeeding Oftefjell groups are separated by the sub-Svinsaga angular unconformity. Köykkä and Laajoki (2009) concluded that cryogenic weathering played an important role in the genesis of the pre-Oftefjell Group fracture system, which is associated with the sub-Svinsaga unconformity. The Oftefjell Group and overlying supracrustal rocks deposited after 1.17 Ga and largely overlap in time with development of the Sveconorwegian orogeny. They consist mainly of coarse sedimentary rocks interbedded with

Fig. 2. Regional stratigraphic summary column of the Rjukan and Vindeggen groups. HF: Heddersvatnet Formation; SHU: Sub-Heddersvatnet unconformity; GDF: Gausdalen fault zone; SSU: Sub-Svinsaga unconformity; Brattfjell Formation = LB: lower member; MB: middle member; UB: upper member.

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bimodal volcanic/subvolcanic rocks. On a regional scale, the formations are laterally restricted and variable, which suggests that they were laid down in separate basins (Laajoki et al., 2002; Andersen et al., 2004). 3. Lithofacies characteristics This study focuses on the Mount Brattefjell, Svafjell and Mefjell areas (Fig. 3), where the Brattefjell Formation’s primary sedimentary features are well preserved. The ca. 2.5-km-thick Brattefjell Formation is subdivided into ca. 1.1-km-thick lower (LB), 0.4km-thick middle (MB), and 1.0-km-thick upper (UB) Brattefjell members based on their distinctive lithofacies assemblages (Fig. 4). The lithofacies classification used here is subdivided into conglomerate, sandstone, and mudstone lithofacies (Table 1), which are described and interpreted below. 3.1. Conglomerate lithofacies The conglomerate lithofacies marks the boundary between the MB and the UB, and it is visible in the lowermost bed of the UB (Fig. 4). The conglomerate bed is only approximately 20 cm thick and is composed of moderately- to well-rounded orthoquartzite clasts. The grain size of the conglomerate pebbles is generally less than 5 cm. The clasts are ungraded, and the matrix is a structureless medium-grained quartz arenite of the UB. On the outcrop scale, the basal contact with the underlying MB is difficult to establish, but it is likely erosional. The texture and fabric of the conglomerate beds is mostly ungraded, and the clasts do not show any significant stratification. However, clasts are deposited horizontally to the same bedding surface, laterally forming a ca. 20-km-wide occurrence. Based on the lithofacies assemblages below and above (described later), the pebble-sized conglomerate represents a transgressive lag deposit, which accumulated with a rising base level during the landward migration of the coastline. The occurrence and nature of the MB sediments below the ravinement surface, the thin nature of the lag bed, and a lack of thick mudstone deposits in the UB indicate that transgressive deposits developed above a wave ravinement surface in a low-gradient setting. Wave scouring occurred to maintain a shoreface profile balance with wave energy by reworking older and newly deposited sediments. 3.2. Sandstone lithofacies The sandstone lithofacies is subdivided into five different subfacies (Table 1). Petrographically, the sandstones in the LB and MB are medium- to coarse-grained subarkoses. The detrital framework of the samples contains 75–95% quartz in a sericite-rich matrix, and the feldspar vs. rock fragment ratio is 3:1. The rock fragments are sandstones and chert. The texture is submature to mature and moderately- to well-sorted, with subrounded to rounded clasts. The UB is composed of medium-grained quartz arenite, where the detrital framework contains >95% quartz in a sericite-rich matrix. The texture is mature to supermature and well- to very well-sorted, with angular to subrounded clasts. The color of the sandstones varies from reddish to purple, grayish and white. 3.2.1. Subfacies A Subfacies A is the most common feature in the LB, although individual units of this subfacies occur throughout the formation and are exposed in several areas. This subfacies contains ripple-marked sandstones with ripples that vary from asymmetric to symmetrical sinuous out of phase, straight crested with tuning-fork intersections, catenary out of phase (Fig. 5A and B), or ladder-back ripples (Fig. 5C). Ripple crests vary from low-relief and peaked to more

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rounded with higher relief (Fig. 5D). The thicknesses of the ripplemarked beds vary from a few cm to 50 cm, and their basal contacts are usually sharp and non-erosional. In the asymmetric ripples, the toss sides are steeply concave, and the lee sides are gently sloping convex. The ripple lengths are generally from 1.0 to 8.0 cm and have heights of 0.2–2.5 cm. The lee sides vary from 0.5 to 1.2 cm, and the stoss sides vary from 1.2 to 2.0 cm. The ladder-back ripples are mostly asymmetric, in which subordinate ripples are formed in different angles and directions compared to the first-order ripples. The height of the ladder-back ripples varies from 1.0 to 1.5 cm and the length is generally from 8.0 to 10 cm. The ripple lee sides vary from 1.8 to 3.2 cm, whereas the stoss sides vary from 5.5 to 7.0 cm. Subfacies A is interpreted to represent a lower flow regime of migrating 2-D ripples deposited during the waning stage of the current flow or by oscillatory waves in shallow water depths. The different nature and morphologies of the ripple marks indicate pulsating wave-orbital velocities at the bed, different wave periods, combined flows, and possible fluctuating water levels (Bose et al., 1997). Out-of-phase crests are typical in transitions from low- to high-energy current ripples produced by currents on noncohesive bed surfaces. The measured (n = 78) ripple index (RI) and ripple symmetry parameters (RSI) from the ripple-marked beds suggest combined oscillatory waves and unidirectional currents over sand beds (Fig. 5E). RI values for the LB vary from 1.0 to 17 and for the MB from 5.0 to 14, whereas RSI values for the LB vary from 1.0 to 3.5 and for the MB from 0.8 to 2.9 (Fig. 5E). The low relief and peaked crest ripples suggest rolling grain ripples in very shallow conditions, reflecting movement just above critical erosion (Collinson et al., 2006). The more rounded crest ripples with higher relief indicate stronger waves and deeper waters that reflect eddies as each wave passes, thus forming vortex ripples (op. cit.). The ladder-back ripples were formed where the current ripples and/or oscillatory ripples were generated at right angles to one another (Reddering, 1987) during super-imposition on existing bedforms or at the falling water-level stage. Thus, the parallelism of the ladder-back ripples indicates the same system of flow and subsequent changes in flow direction (de Vries Klein, 1970). 3.2.2. Subfacies B Subfacies B occurs in all three members and is exposed in several places. This subfacies contains planar cross-bedded sandstone beds, which are generally from 15 cm to 1.5 m thick, with sharp and partly erosional basal contacts (Fig. 5F). The foreset thickness varies from less than 1 cm up to 3.0 cm, and the foreset dips are generally steep and planar with tangential bottom sets. These bottom sets are usually bounded by a laminated mudstone that forms cyclic bundles and shows alternating thick and thinning planar cross-stratifications (Fig. 6A). The planar foreset angles-of-repose are generally from 20◦ to 30◦ , and they form stacked cosets. Some of the foresets in the LB and MB dip to opposite directions, forming a bi-polar herringbone cross-bedding with (or without) thin mudstone separating the adjoining sets (Fig. 6B). Reactivation surfaces (Fig. 6C) that are either planar angular or concave-upward are a common feature in the MB. In the MB, subfacies B is also characterized by iron ooids in the lowermost part of the MB, (Fig. 6D), by shrinkage cracks (Fig. 6E), by slump-folds (Fig. 6F), and by load casts with associated flame structures (Fig. 6G). The ubiquitous and mostly asymmetric iron ooids with quartz nuclei are concentrated in the basal contact between the LB and MB. The ooid-bearing part of the bed is ca. 10 cm thick, which can be traced laterally for hundreds of meters. The spindle-shaped shrinkage cracks with positive relief range from 1.0 to 4.0 cm, and they are 0.5–1.0 cm wide. The cross-sectioned shape or penetration cannot be ascertained. The cracks have irregular patterns, and some may crosscut one another. The individual cracks are mostly lenticular or straight, and they pinch out other

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Fig. 3. Geological map and cross section of the study area (modified from Dons, 2004). The Brattefjell Formation is subdivided into lower, middle, and upper members. The locations of the stratigraphic logs and general bedding and dip directions are marked. Thick, dashed lines represent major fault zones.

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Fig. 4. Stratigraphic summary column and paleoflow measurements from the Brattefjell Formation. The Paleoshoreline is roughly N–S oriented, which coincides with the orientation of the Rjukan Rift Basin (see Fig. 1).

cracks in the bedding surface. The shrinkage cracks are associated with load casts and flame structures, which occur in the lower surfaces of the sandstone beds and are interbedded with the mudstone. The load casts are mostly moderately rounded with irregular lobes,

and their diameter varies from a few cm up to 15 cm, covering the whole bedding surface. The load casts are associated with upwards pointing flame-like wedges, which are composed of mudstone. The flame structures are generally a few centimeters high and thick. The

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Table 1 Brattefjell Formation lithofacies classification, associated structures, and depositional process interpretation. Facies name

Lithofacies

Sedimentary structures

Depositional process/interpretation

Pebble-sized conglomerate

Ungraded without any significant stratification

Trangressive ravinement lag; Wave scouring

Sandstone lithofacies Subfacies A

Medium-grained sandstone

Lower flow regime migrating 2-D ripples deposited during a waning stage of the current flow or by oscillatory waves

Subfacies B

Medium-grained sandstone

Ripple marks varying from asymmetric to symmetrical sinuous out of phase, straight crested with tuning-fork intersections, or catenary out of phase, or ladder-back type Planar cross-bedding which may contain cyclic bundles, iron ooids, shrinkage cracks, bi-polar cross-bedding, load casts with associated flame structures, re-activation surfaces, and slump-folds

Subfacies C Subfacies D

Medium-to coarse-grained sandstone Medium-grained sandstone

Subfacies E

Medium-grained sandstone

Mudstone lithofacies Subfacies F

Mudstone (silt and clay size)

Subfacies G

Mudstone (silt and clay size)

Conglomerate lithofacies

Trough cross-bedding Low-angle cross-bedding with graded laminas Horizontal-lamination with graded laminas

Lamination and individual double-mud drapes Lamination without any regular grouping

associated slump-fold structures in the sandstone beds are generally less than 1 m thick and are bounded by undisturbed sediments from above and below. Subfacies B is interpreted to represent the lateral migration of low-flow regime sandbars that are straight- or slightly sinuouscrested (cf. Visser, 1980; Terwindt, 1981; Richards, 1994; Houthuys and Gullentops, 1988; Mueller et al., 2002; Simpson et al., 2002). Planar migrating sand wave foresets were formed when sand was carried up the seaward slope of the ridge by swash and deposited by avalanching down the steep slope into the standing wave of the rumen. Sharp reactivation surfaces in the MB reflect fluctuations in flow velocities that were produced by the action of a subordinate current, eroding the lee side of the dominant current ripple. The concave shape reactivation surfaces suggest vortex erosion in front of a cross-bed (Houthuys and Gullentops, 1988). The bi-directional herringbone structures are the result of a separation of the floodand ebb-dominated zones or the superposition of a unidirectional current on a tidal current, which were formed due to dune migration by traction from reversing tidal currents. Alternating thick and thinning planar cross-stratifications favor cyclicities of tidal origin (Visser, 1980; Bose et al., 1997). The origin of the iron ooids in the lower part of the MB is problematic. Ooids are generally formed by the chemical precipitation of cryptocrystalline iron oxyhydroxides on grains on the seafloor. Here, the seawater is enriched with Fe, Al, and Si, and the enrichment is the result of hydrothermal fluids, volcanic ash falls into a shallow basin, or rapid weathering of previous volcanic rocks (Sturesson et al., 2000). However, the source of the iron in this case is unclear. The irregular and discontinuous shapes of the spindle-shaped cracks and their associated structures indicate that these synaeresis cracks were formed subaqueously at the water sediment interface due to salinity changes (Plummer and Gostin, 1981; Kidder, 1990) or during sediment compaction induced by overlying sediments. The occurrence of load casts in the same sandstone subfacies supports the latter interpretation. However, Pratt (1998, 2001) argued that salinity and burial are not enough to form subaqueous shrinkage structures, and he suggested earthquake-induced

Lateral migrating of straight- or slightly sinuous-crested sandbars. Fluctuating flow velocities; tidal cyclicities and currents; Compaction and salinity changes, or biological mat-destruction features, or earth-quake-induced ground motion; Rapid sedimentation Channel deposits of 3-D dunes Lateral migration of straight- or slightly sinuous-crested sandbars Lower flow regime or high- velocity oscillatory flow straight- or slightly sinuous-crested sandbars Waning current strength deposits Abandonment and vertical aggradation of the channel or low-energy suspension coastal plain deposits

ground motion and dewatering as the origin of subaqueous shrinkages in argillaceous sediments. The load casts were formed due to a temporary loss of strength after rapid sedimentation. The associated flame structures in the mudstone were caused by the sliding down of the mud drape during the following dominant current stage together with part of the newly deposited sand. The angle of repose in the sand was too steep for the mud, which caused plastic flow dewatering and loading by the superimposed sand. This process eventually led to the pre-pressure buildup and upward flow of the mud, thus forming flame structures. The slump structures formed in a rapidly deposited unconsolidated sediment, possibly in a relatively steep slope between a fair weather and storm wave base. The presence of undisturbed beds and layers above and below the slump indicates a soft-sediment deformation rather than a tectonic origin. The associated slump and load structures in the same sandstone lithofacies could suggest the rapid sedimentation and expulsion of pore water during compaction.

3.2.3. Subfacies C Subfacies C is only visible in the LB and is mostly exposed in the Svafjell and Brattefjell areas (Fig. 3). This facies consists of a trough cross-bedded sandstone, where the sets are generally from 0.30 to 1.0 m thick, and the troughs are less than 2.0 m wide with convex-up shape. The sets are commonly grouped into cosets. The basal bed contacts are sharp and erosional where the boundaries are parallel and troughs cut into the underlying set. The foresets are concave upward with tangential bottom contacts. Reactivation surfaces were not observed in this lithofacies. Basal erosion, unidirectional cross-stratification and a lack of wave influence suggest channel deposits of 3-D dunes (e.g. Sultan and Plink-Björklund, 2006). Thus, subfacies C is interpreted as representing channel fills (e.g. Jensen and Pedersen, 2010). The channels were probably shallow because the trough cross-bedded structures did not have any visible macroforms. The cosets are the result of migrating ripples and the net accumulation of sediment on the bed.

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Fig. 5. (A–F): (A) Asymmetric sinuous out-of-phase straight crested ripple marks in the MB. Hammer is 34 cm long. UTM X = 466,067/Y = 6,621,102. (B) Similar asymmetric sinuous out-of-phase straight crested ripple marks in the MB showing tuning-fork intersections. Hammer is 63.5 cm long. UTM X = 465,973/Y = 6,620,906. (C) Ladder-back type ripples in the MB. Hammer in lower right corner is 34 cm long. UTM X = 465,441/Y = 6,620,965. (D) Small-scale asymmetric ripple marks with rounded and peaked crests in the LB. Pencil is 14 cm long. UTM X = 464,974/Y = 6,620,399. (E) Ripple index (RI) and ripple symmetry index (RSI) measurements from the LB and MB. Ripple fields from Tanner (1967) and Collinson et al. (2006). (F) Stacked planar cross-bedded set in the LB. Plate is 16 cm long. UTM X = 465,319/Y = 6,620,371.

3.2.4. Subfacies D Subfacies D is the most common feature in the MB and UB, although some individual beds in the LB may occur. This subfacies is exposed in several areas, and it consists of low-angle cross-bedded sandstones. The bed thickness varies from 20 cm to 1.5 m, with sharp or partly erosional basal contacts. This subfacies has fea-

tures similar to subfacies B, but the foresets are generally shallower, dipping <15◦ . The low-angle cross-beds are also characterized by moderately developed graded laminae in some of the beds in the UB. The low-angle features of the sandstone foresets indicate a flow regime transition from dune to upper plane bed stability conditions

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Fig. 6. (A–G): (A) Cyclic cross-stratification in the MB. Arrows indicate thinner and thicker bundles. Pencil is 14 cm long. UTM X = 465,956/Y = 6,620,935. (B) Bi-polar crossbedding in the MB. Arrows mark the paleocurrent directions. Compass is 12 cm long. UTM X = 465,678/Y = 6,621,727. (C) Re-activation surface in the LB. Scale is 80 cm long. UTM X = 465,028/Y = 6,620,891. (D) Electron microscope photograph of quartz-cored iron ooid in the lower part of the MB. (E) Underside of a bed showing casts of shrinkage cracks in the MB. Pencil is 14 cm long. UTM X = 465,979/Y = 6,621,249. (F) Slump-fold above undisturbed bedding in the MB formed in unconsolidated sediment. Compass is 12 cm long. UTM X = 465,063/Y = 6,620,095. (G) Load casts and flame structures in the MB. Plate is 16 cm long. UTM X = 465,659/Y = 6,621,417.

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or vice versa (e.g. Fielding et al., 2006). Thus, this subfacies is interpreted to represent a lateral migration of low-relief bedforms and, possibly, straight- or slightly sinuous crested sandbars similar to subfacies B. 3.2.5. Subfacies E Subfacies E is the most common in the MB and UB and is exposed in several areas, but some individual beds may occur in the LB. This facies consists of horizontal-laminated sandstones with bed thicknesses varying from 10 cm to 1.5 m. The basal bed contacts are sharp and erosional with the underlying beds. Due to grain-size differences, the laminae in the horizontal-laminated sandstone beds are less than 1 cm thick. The graded horizontal-laminated sandstone beds usually occur at the top of the planar cross-bedded sandstone beds, but the position may vary. Some of the beds are also defined by moderately developed graded laminae, similar to those in subfacies D. Horizontal-laminated sandstone beds are interpreted to represent sandbar deposits as in the subfacies B and D. The moderately developed graded laminae in some of the beds indicate that the plane beds were deposited in the lower regime by the accretion of low amplitude sand waves over sandbar tops. However, some of the subfacies E beds may indicate the deposition products of unidirectional upper flow regime or high-velocity oscillatory flow processes (cf. Richards, 1994). 3.3. Mudstone lithofacies The mudstone lithofacies is subdivided into two different subfacies (Table 1). The mudstone comprises both clay- and silt-sized detritus. The color of the mudstone varies from brownish to purple. 3.3.1. Subfacies F Subfacies F occurs only as sandstone interbeds in the LB and MB. The laminated mudstone beds are only a few centimeters thick and are characterized by distinct, alternating layers of silt- and claysized detritus. Double-mud drapes are also a common feature in the MB, occurring in the cross-bedded foresets (Fig. 7A). The basal bed contacts with the underlying sandstone beds generally vary from sharp to transitional. The siltstone and mudstone layers are a few millimeters thick, and they have gradational boundaries without any regular groups. In some of the beds, the first mud drape and a small part of the underlying bundle are eroded. In other associated beds, the mudstone drapes are thicker and have a sharp boundary to the sandstone foresets. Subfacies F is interpreted to represent a waning current strength, which was weak enough to deposit mud and silt-sized detritus from the flow (e.g. Bridge and Gabel, 1992). The sharpbased layers in the mudstone suggest relatively sudden events superimposed upon a background of constant sedimentation of fine-grained detritus, whereas the graded bases suggest a waning of the suspended load followed by more active episodes (Collinson et al., 2006). The occurrence of the double mud drapes favors a subtidal setting (e.g. Mellere and Steel, 1995; Sultan and PlinkBjörklund, 2006). The erosion of the first mud drape and a small part of the underlying bundle indicate at least moderate to strong asymmetrical tides deposited by the accretion of low amplitude sand waves over sandbar tops. The thicker mudstone drapes and the sharp boundary between the sandstone foresets and mudstone drapes could reflect spring tide erosion and a more suspended physiochemical flocculation. 3.3.2. Subfacies G Subfacies G is only visible in the middle and upper parts of the LB (Fig. 4). In the Brattefjell area, this subfacies forms two distinct

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units of laminated mudstone. These units are characterized by relatively weak, but distinct and alternating, laminae of silt (minimum amounts of very fine sand) and clay-sized detritus. The lateral continuum of the distinct layers cannot be established with certainty. The mudstone beds are approximately 2–3 m thick, and the basal and upper contacts of the beds are not exposed. A darker, clay-sized detritus dominates the mudstone layers, whereas the lighter and coarser layers tend to be thinner. The layers are generally a few millimeters thick, and they have weak gradational boundaries. The layers are not arranged into any regular grouping, and they do not show any sign of ripple marks or other sedimentary structures. Based on sedimentary structures and lithofacies assemblages below and above, subfacies G is interpreted to represent the possible abandonment and vertical aggradation of the channel or coastal plain, suggesting low-energy suspension deposits. The lack of ripple marks, climbing ripples or convoluted bedding in the pale silty laminae suggests that the mudstone deposits do not represent a flood basin, crevasse splay, or natural levee. The gradational contacts of the laminae probably suggest gradually increasing and waning high-discharge episodes in the tidal channel abandonment. 3.4. Paleoflow and bundle measurements Paleoflow measurements were obtained from the wave ripple crests (sandstone subfacies A) and the planar cross-bedded beds (sandstone subfacies B). Orientation of the paleoshoreline was estimated by measuring the strike of the wave-ripple crest-lines (e.g. Leckie and Krystinik, 1989; Cheel and Leckie, 1992). All the field measurements were corrected for tectonic folding and tilting. A total of 77 measurements were obtained from the LB, 48 from the MB, and 55 from the UB, which was considered statistically adequate (Fig. 4). The crest-line measurements show a distinctive change from a N–S trending shoreline orientation in the LB to a NE–SW orientation in the UB. The paleolandward direction is estimated from the drifting of asymmetric wave ripples, and based on assumption than it is more likely for the wind to be stronger from sea to land than vice versa. The paleoflow measurements from the tabular cross-bedded beds indicate that the planar angle-of-repose is unimodally seaward directed (Fig. 4). The shore-oblique paleoflow reflects deposition in longshore bars produced by an oblique wave approach or tidal flow (Simpson et al., 2002). The measured foresets from the LB and MB reveal slightly trimodal paleocurrent patterns (Fig. 4). The prominent eastward direction in the LB and MB is interpreted as the dominant ebb phase. The bimodal part of the rose diagram of the LB is oriented roughly perpendicular to the inferred strandline orientation. The measured ripple-marks from the LB and UB generally show a tendency for seaward-directed currents (Fig. 4). Paleocurrent reversal and cyclic changes can reveal the repeated waxing and waning of the tidal water flow (Bose et al., 1997). Thus, the thickness of the planar cross-strata and the mud layer couplet records individual ebb-flood and slackwater events, reflecting tidal neap-spring cycles. The measured foresets from the planar cross-bedded sets indicate that at least 16–18 bundles were formed during the neap-spring cycle (Fig. 7B), which is suggestive of mixed semidiurnal tides assuming no major erosion modification after deposition (cf. Yang et al., 2005). The thicker bundles were formed in a strong dominant current during the spring tide, whereas the thinner bundles formed in a weak dominant current during the neap tide. The mudstone couplet indicates the presence of two slack-water stages during a flood-ebb cycle, which is a characteristic feature in subtidal zones (cf. Visser, 1980). The nature of the foreset dips, the well-preserved mudstone drapes and the subcurrent (cross-laminae) structures described earlier indicate weak or moderate asymmetric tides.

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Fig. 7. (A and B): (A) Double mud-drape in the MB tidal bundle. Arrow is 8 cm long. UTM X = 465,549/Y = 6,620,923. (B) Tidal bundle measurements from the MB showing cyclic and sinusoidal pattern. Measurements show that 16–18 bundles were formed during the neap-spring cycle, indicating mixed semidiurnal tides. UTM X = 465,955/Y = 6,620,931.

4. Lithofacies associations Two major genetically related lithofacies associations can be identified from the Brattefjell Formation (Table 2), which are described and interpreted here.

currents. Strand (2005) suggested that planar and trough crossbedded sandstones with associated laminated mudstones indicate cyclic channeling dunes, whereas Rahman et al. (2009) posited that they are suggestive of long-term changes in the hydrodynamic conditions within the sedimentary basin, which are most effective in meso- to macrotidal environments.

4.1. Lithofacies association A 4.2. Lithofacies association B Lithofacies association A is a typical feature in the LB and MB (Figs. 8 and 9), where beds that are ripple-marked (sandstone subfacies A), planar cross-bedded (sandstone subfacies B), and trough cross-bedded (sandstone subfacies C) become more fine-grained upwards, passing into laminated mudstone beds that are a few mm up to 5.0 cm thick. These associations are generally a few tens of centimeters to 2.0 m thick, forming overall fining-upward successions with generally transitional contacts. The occurrence of the mudstone interbeds is more abundant in the MB, where they form heterolithic successions with common double-mud drapes. The size, variety and thickness variations of the different types of cross-beds and associated mudstone interbeds reflects fluctuating energy conditions in a shallow siliciclastic shoreline influenced by tides and waves (cf. Dalrymple, 1992; Orton and Reading, 1993; Reading and Collinson, 1996; Mueller et al., 2002). Thus, lithofacies association A probably represents sheltered subtidal shallow marine platform deposits similarly described by, for example, de Raaf et al. (1977). Relatively thin mudstone cappings in some of the beds indicate either the dominant continuous movement of upper flow-regime currents formed during a high-wave surge and maximal tidal current activity or the erosive processes of a subordinate current (Dalrymple, 1992; Mueller et al., 2002). A coherent mudstone deposition favors mud deposition during subsequent

Lithofacies association B is the most typical feature in the MB and UB (Figs. 10 and 11), although some individual successions may occur in the upper part of the LB. Lithofacies association B consists of planar cross-bedded sandstone beds (sandstone subfacies B), which pass upward into either low-angle cross-bedded (sandstone subfacies D) or horizontal-laminated (sandstone subfacies E) sandstone beds. These associations are generally from about 30 cm to a few meters thick, and they form several different wedge shaped successions, which cannot be traced laterally very far. This association is more broadly upward fining and texturally mature in the UB than in the LB or MB. The upper contacts of the planar crossbedded foresets are eroded by the overlying horizontal-laminated or low-angle cross-bedded bed. The close occurrence of the horizontal-laminated sandstone beds over the planar cross-bedded sandstone is interpreted to represent migrating sandbars or possible washover fans transporting sand landward (cf. Quin, 2008). The landward dipping foresets (see Fig. 4 and the paleoflow measurements) are suggestive of the latter interpretation. The low-angle cross-bedding defined by graded laminae suggests that associations with the planar cross-bedded sandstone beds represent backwash deposits in the foreshore (cf. Cudzil and Driese, 1987), and possibly migrating coastal dunes or

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Fig. 8. (A–C): Stratigraphic logs from the lower part of the LB. Sections A–C are marked in Fig. 3. Lithofacies association A is a typical feature in the LB. At least 23 parasequences can be recognized from the lower part of the LB (arrows). UTM (log A) X = 464,613/Y = 6,621,907, (log B) X = 465,248/Y = 6,620,411, (log C) X = 465,297/Y = 6,620,389.

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Fig. 9. (A–F): Stratigraphic logs from the upper part of the LB. Sections A–F are marked in Fig. 3. The upper part of the LB is characterized by similar sandstone successions, as in Fig. 9, but it shows two individual mudstone lithofacies (subfacies G). Dashed lines indicate the lateral correlation lines of each log. At least six parasequences can be distinguished from the upper part of the LB. UTM (log A) X = 464,914/Y = 6,621,340, (log B) X = 465,008/Y = 6,621,207, (logs C–F) X = 465,110/Y = 6,621,012.

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Table 2 Brattefjell Formation lithofacies associations, occurrence in member(s) and depositional process interpretation. Facies associations

Lithofacies

Occurrence in member(s)

Depositional process/interpretation

Lithofacies association A

Ripple-marked, planar cross-bedded, and through cross-bedded sandstone – laminated mudstone Planar cross-bedded sandstone – Low-angle cross-bedded or horizontal-laminated sandstone

LB, MB

Subtidal shallow siliciclastic marine deposits with fluctuating energy conditions Migrating sandbars and coastal dunes or washover-fans/backwash deposits

Lithofacies association B

sandbars. Alternatively, the association of tabular and low-angle cross-bedded beds could indicate that the dominant subordinate current differed in strength (e.g. Sultan and Plink-Björklund, 2006) or that there were wave induced currents during fair weather periods (Strand, 2005). The cosets of the graded horizontal laminae indicate that the plane beds were deposited in the lower flow regime by the accretion of low amplitude sand waves over sandbar tops. The flows had a normal oscillatory motion on the crests and seaward slopes of long shore bars (e.g. Davidson-Arnott and Greenwood, 1976; Hill and Hunter, 1976). The laterally restricted nature of the successions likely suggests bar top surfaces. 5. Stacking sedimentation patterns and discussion At the early stage of the LB sedimentation, shallow marine conditions cover wider areas in the basin, likely giving rise to an early stage of transgression and a minor landward migration of the shoreline, forming shallowing upward stacking pattern cycles (Figs. 8, 9 and 12A). It is considered that sandstone subfacies C (trough cross-bedded sandstone) represents the shallowest water level, followed by sandstone subfacies A (ripple-marked sandstone). Sandstone subfacies B (planar cross-bedded sandstone), D (low-angle cross-bedded sandstone) and E (horizontal-laminated sandstone) represent the highest water level, bounded by parase-

LB, MB, UB

quences at the bottom. At least 23 typical shallowing upward cycles can be recognized from the lower part of the LB, and six can be recognized from the upper part of the LB (Figs. 8 and 9). We suggest that these cycles represent autocyclic water level changes due to tidal effects. The availability of accommodation space was the largest during the sedimentation of sandstone subfacies B and the lowest during the sedimentation of sandstone subfacies C. The unidirectional planar cross-bedded sets in the LB possibly indicate that a flow reversal left the imprints only during the abandoning phase of a tidal sandwave (Nio and Yang, 1991; Bose et al., 1997). Lithofacies assemblages similar to those in the LB (e.g., combined wave and current origin ripples) were observed in studies by Simpson and Eriksson (1991) and Mueller et al. (2002). These assemblages are suggestive of subtidal settings where tidal currents and wave actions form complex combinations. The lesser abundance of sandstone subfacies A in the upper part of the LB probably indicates deeper water levels. The sedimentation of the LB and the early stage of the MB were characterized mainly by tidal sand flats, tidal channels, sandbars, and lagoon-estuarine developments that were probably protected by sandy barrier ridges (Fig. 12A). These sedimentation patterns record the first early transgression events during the sedimentation of the Brattefjell Formation. The sedimentation of the MB was characterized by alternating deposition from suspension and traction currents. Suspension

Fig. 10. Sketch of an outcrop from the LB. Planar cross-bedded sandstone beds (sandstone subfacies B) are overlain by horizontal-laminated sandstone beds (sandstone subfacies E), forming lithofacies association B. UTM X = 464,585/Y = 6622248.

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Fig. 11. Detailed stratigraphic log from the UB. Section is marked in Fig. 3. The UB is typically characterized by tabular cross-bedding, low-angle cross-bedding, horizontal-laminated bed successions, forming lithofacies association B. UTM X = 467,257/Y = 6,619,802.

settling occurred during tidal slacks, while traction characterizes ebb and flood currents. The sedimentary structures, abundance of interfingering mudstone and sandstone beds, lack of channel sediments, presence of ooids, inferred weak tides, and wave-dominated deposits in the MB are suggestive of a lagoon-estuary environment and relatively rapid sedimentation (Fig. 12B). Horizontal-laminated successions similar to sandstone subfacies E have been reported from tidal sand flats in axial portions of modern tide-dominated estuaries, where current spreads exceed 2 m/s with water depths less than 2–3 m (e.g. Dalrymple, 1992; Sultan and Plink-Björklund, 2006). The heterolithic nature of lithofacies association B in the MB is suggestive of a beach barrier setting with fine-grained heterolithic bodies of tidal lagoon deposits or backfilling of estuaries during rising water levels (cf. Strand, 2005 and references therein). In contrast to the LB, the ripple marks in the MB are less abundant, the planar cross-bedded sets are smaller and the erosion surfaces are more abundant and of a much smaller scale. The mudstone interbed thickness and abundance are also greater compared to the LB. The occurrence of the iron ooids in the lower part of the MB is also suggestive of the first major transgressive event during the sedimentation of the Brattefjell Formation. Ooids typically occur in nearshore settings (Collin et al., 2005), so they provide valuable information about the bed(s) where they occur, assuming that no major modification took place through transportation. Typically, the water-depths are less than 2 m (Simone, 1981), and asymmetric forms are generally regarded as quiet-water (or superficial) ooids. Many studies suggest that ooids form in lagoonal settings (Bayer, 1989 and references therein; Sturesson et al., 2000). Transgression and wave scouring in the early stage and during the sedimentation of the UB led to an accumulation of the ravinement lag and, probably, barrier island drowning (Fig. 12C). In wave dominated and microtidal barrier island coasts, the transgressive erosion of the shoreface will lead to the preservation of back-barrier lagoon deposits capped by a thin, coarse ravinement lag, which records the ravinement surface (cf. Reinson, 1992 and references therein). The lithofacies assemblages and associated sedimentary structures indicate that the UB forms a sequence in which relics of shoreface-offshore sediments and, probably, a washover fan were preserved in a transgressing coast. The low preservation potential of barrier island sediments have been noted in many of the Precambrian epeiric sea deposits. This has been usually perceived either as a primary feature and paucity during the Precambrian due to influence of continental crustal growth on eustasy (e.g. Eriksson et al., 1999, 2002), or to plate-marginal tectonic collisions, or transgressive reworking (Eriksson et al., 2004, 2008). This study suggests that the shoreface was fed sediment by alongshore and coastal current drifts during shoreface transgressive erosion in a low coastal gradient. This development was probably accompanied by a relatively rapid sea-level rise that caused barrier sand ridge drowning (cf. Sanders and Kumar, 1975) and overstepping, where the shoreline retreated rapidly, preserving the older sequence. During transgression, the net erosion usually occurs in the upper shoreface and beach zones, and the preservation of barrier sand ridges in ancient rock strata is unlikely, as suggested by the ravinement surface in the lower part of the UB. The preserved lagoon-estuarine and washover fan deposits, in the MB and UB, likely indicate occurrence of the barrier island, which vanished during the transgressive wave reworking. Thus, it is evident that the lack of barrier island deposits is not primary feature in the Brattefjell Formation. The laterally wide occurrence of transgressive lag pebbles suggests that the scouring was not limited locally to certain margins of the basin. However, the amount of underlying sediments removed by wave scouring is unknown. That the sediments above the transgressive lag are completely free of feldspar grains compared to the LB and MB indicates an abrupt change to the different sedimentary environment and at least a moderate or intensive wave scouring

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Fig. 12. A schematic paleosedimentological model of the Brattefjell Formation’s depositional history during the relative base level rise in an overall transgressive setting. The possible barrier-island migration toward land is not modeled because it cannot be established from the rock strata. Figure not to scale. (A) The sedimentation of the LB and early stages of the MB was characterized by tidal sand flats and tidal channel fills protected by a littoral barrier-island. (B) During the relative base level rise, the sedimentation of the MB and early stages of the UB were characterized by estuarine-lagoon sedimentation, channel drowning, and possible washover fans. (C) In the late stages of the transgression, the wave scouring resulted in the development of a ravinement lag and the shallow clastic nearshore sedimentation of the UB followed.

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of the underlying sediments. Leckie (1994) reported present day theoretical maximum of ca. 40 m sediment removal in the transgressive microtidal coastline, which can likely be much higher during the Precambrian due to lack of vegetation, more aggressive weathering, and higher energy conditions. However, this ravinement surface likely represents fluctuating water levels and a second major transgressive event during the sedimentation of the Brattefjell Formation. The rest of the UB represents a shallow shoreface-offshore environment during the highstand of the basin (cf. Davidson-Arnott and Greenwood, 1976). This shallow siliciclastic marine environment included migrating transverse and offshore bars. In addition, the lithofacies assemblages, the lack of fine-grained units and the maturity of the sandstone suggest that the UB represents a relatively high-energy nearshore environment. The mineralogical and textural maturity of the sandstones, and grain roundness could reflect tidally influenced nearshore water circulation patterns, reworking, and long transportation distances of the accumulating sediments. Similar mineralogical mature sandstones, from the ancient tidal settings, are also interpreted as a result of constantly reworking, agitation, and long distance transportation of the accumulating sand circulation patterns (e.g. Swett et al., 1971). The lack of subaerial shrinkage cracks in the different members may indicate that the Brattefjell Formation was submerged during its entire sedimentation history. Although the estimations of the water depths are difficult, abundance of ripple-marked beds, lack of storm related beds (hummocky), and lack of deep offshore-muds indicate that the Brattefjell Formation deposited above the fairweather wave base (ca. <20 m). Eriksson et al. (2008), noted the braided-fluvial influence in clastic epeiric sea successions from the ca. 3.0 to 2.8 Ga Witwatersrand Basin and from the Paleoproterozoic Transvaal Supergroup, in southern Africa. However, Sarkar et al. (2004) and Eriksson et al. (2008) described Proterozoic epeiric sea deposits from the Vindhyan Supergroup (Sea Basin), in India, with limited landderived sediment supply and lack of major braided-fluvial input. In latter case, the microbial mat growth played important role, allowing aggradation of sandy sediments and buildup of stacked highstand system tracts (Eriksson et al., 2008). The absence of major shoreline braided-fluvial deposits in the Brattefjell Formation can be explained with the following two hypotheses: (1) either associated braided-fluvial deposits are not preserved/exposed, (2) or the setting was characterized by low shoreline paleoslope with reduced sediment supply and lack of major braided-fluvial channel(s). Likely, the lack of any coarser braided-fluvial channel conglomerates or large bedforms, in the Brattefjell Formation, could indicate the latter case. However, the dominance of tidal processes ultimately controlled the evolution and facies character of the preserved Brattefjell Formation succession. Thus, the sedimentation environment is suggestive of a tidally influenced epeiric sea in which shallow water levels extended over part of a continent and it was associated with marine transgression. The estimations of mixed semidiurnal tidal cycles, from the Brattefjell Formation, indicates two high and two low tides of different size every lunar day, which are common in many present areas e.g. on the western coast of North America. This study suggests that in many ways the Precambrian tidal sedimentation signatures and patterns were similar to their modern counterparts. The whole Brattefjell Formation records millions of years of fluctuating water levels with an overall transgressive trend. These larger scale water level cycles indicate allocycles caused by regional subsidence, as there are no evidence of eustatic sea level rise and flooding during that time. The analyzed U–Pb and Lu–Hf data from the Brattefjell Formation and underlying successions suggest that the continental depositional basin (Rjukan Rift Basin) was situated closer to the

Laurentian Shield and later moved to its present location along a sinistral strike-slip fault (e.g. Lamminen and Köykkä, 2010). The syn-rift stage of the Rjukan Rift Basin comprises sedimentation and volcanism of the Tuddal, Vemork, and Heddersvatnet Formations. (Fig. 2; Köykkä, 2010, in press). Later, the cooling of the lithosphere caused the basin subsidence to slow down and the volcanism terminated, which was followed by the post-rift stage and sedimentation of the Vindeggen Group. The fluctuating base level affected the deposition of the marine, pelitic tidal, and coarser alluvial sediments (Gausta, Bondal, Lauvhovd, Skottsfjell, and Vindsjå formations) before the sedimentation of the Brattefjell Formation (Lamminen and Köykkä, 2010). During the sedimentation of the Brattefjell Formation, the transgression and associated erosion reworked the underlying sediments, and clastic detritus transported by alongshore currents in an epeiric sea setting. This resulted in sediment accumulations several kilometers thick. The whole rift basin was buried under shallow epeiric sea deposits. The lack of a deepwater sedimentation prism and oceanic crust suggest that the rifting failed to complete the break-up of the continental crust. The remaining sedimentation history of the epeiric sea remains unknown, because the Brattefjell Formation terminates in an unconformity created during the Sveconorwegian Orogeny. This boundary is characterized by a ca. 150 Ma chronological hiatus (first-order sub-Svinsaga unconformity), which records an influence of periglacial climates during the Mesoproterozoic in the Telemark (Fig. 2; Köykkä and Laajoki, 2009).

6. Conclusions The Brattefjell Formation contains diagnostic criteria of an ancient tidal influence in an epeiric sea setting. Criteria for the tidal influence are the following: (i) bi-polar sedimentary structures (herringbone); (ii) double mud drapes; (iii) sandstone foresets bounded by mudstone couplets or mudstone drapes and re-activation surfaces, reflecting flood-ebb cycles; and (iv) an alternation of successive thick and thin bundles reflecting a diurnal variation. The bundle measurements suggest that at least 16–18 bundles were formed during the neap-spring cycle, which is suggestive of mixed semidiurnal tides. The conglomerate lithofacies in the UB records a transgressive ravinement surface due to wave scouring. The sandstone and mudstone lithofacies assemblages indicate fluctuating energy conditions in a shallow clastic shoreline and a deposition of coastal dunes and sandbars or washover-fans and backwash deposits influenced by tides, waves, rapid sedimentation, and sediment compaction. Abundance of ripple-marked beds, lack of storm related beds (hummocky), and lack of deep offshore-muds indicate that the Brattefjell Formation deposited above the fairweather wave base (ca. <20 m). The Brattefjell Formation can be subdivided into sandy subtidal, lagoon-estuary, and shallow marine shoreface-offshore environments, recording millions of years of fluctuating water levels with an overall transgressive trend. Autocyclic tidal water level changes caused the formation of shallowing upward cycles at the lower part of the formation. The whole formation is interpreted to represent a tidally influenced epeiric sea affected by autocyclic tidal water level changes and allocyclic water level fluctuations due to regional subsidence. Lack of any coarser braided-fluvial channel conglomerates or large bedforms likely indicate low shoreline paleoslope with reduced sediment supply from major braided-fluvial channel(s). Although some of the recent tidal and nearshore sedimentation models may be oversimplifications of the Precambrian rock record, this study suggests that in many ways the Precambrian tidal sedimentation patterns were similar to their modern counterparts.

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