Synthesis of the tectonic and sedimentological evolution of the late Proterozoic-early Cambrian Hedmark Basin, the Caledonian Thrust Belt, southern Norway

JOHAN PETIER NYSTUEN

Nystuen, J. P. : Synthesis of the tectonic and sedimentological evolution of the late Proteroroic-early Cambrian Hedmark Basin, the Caledonian Thrust Belt, southem Norway. Norsk Geologisk Tidsskrift,

Vol. 67, pp. 395-418, Oslo 1987. ISSN 0029-196X.

The Hedmark Basin was formed by rifting c. 750-590 Ma ago in an area about 130-230 km NW of the present position of the basinal sequence in the Caledonian nappe region. A pre-rift sequence may be represented by fine-grained ftuvial sandstones. The rift episode is divided into three thermo-mechanical phases which individually are characterized by one or several spans of structural-sedimentary evolution. A western graben with marine turbidite sandstones and black shales was formed during the phase of initial crustal stretching (A). The main phase of crustal stretching (B) included basin widening by an eastward moving fault system, renewed deep fracturing with basalt extrusion, block faulting causing sub­basins and local structural inversions, and possible strike-slip movements along the Imsdalen Fault (IMF ). Sedimentary environments included alluvial fans and plains, are as with shallow-marine sand and carbonate mud, fan deltas, sub-marine fans and basin lows with organic-rich mud. The phase of thermal cooling (C) was characterized by waning fault activity, decreasing deposition in sub-basins and a long period of

slow regional subsidence. The Varangerian glaciation (c. 650 Ma) took place during this phase and was succeeded mainly by ftuvial and shallow-marine siliciclastic sedimentation. The Baltoscandian rift episode

was terminated by the Early Cambrian transgression.

J. P. Nystuen, Saga Petroleum a.s., Postboks 9, N-1322 Høvik, Norway.

Major problems concerning the interpretation of ancient sedimentary basins are related to how one distinguishes between sedimentary responses to local and regional tectonic movements from the effects of (a) isostatic rise and subsidence, (b) sediment compaction, (c) changes in climate and energy of the transporting and depositional systems, (d) changes in source rocks and (e) Iocal and regional changes of base leve!. Synthesis of the depositional history presupposes furthermore some knowledge about the size, con figuration and palaeostructural position and framework of the basin during the time of sedimentation. Geo­metric restorations of the basin- fill sequence are thus usually needed in basin analysis.

Problems of the kind outlined above are dealt with in this review of the Hedmark Basin. The basin is named after its major lithostratigraphical unit, the late Riphean to lower Cambrian Hed­mark Group, which is at !east 3000-4000 m thick ( Bjørlykke et al. 1967) (Figs. l, 2 & 3). The basin was formed by extensional tectonics along the western margin of the Baltoscandian craton prior

to the opening of the Iapetus Ocean in the early Cambrian (Gale & Roberts 197 4; Gee 1975; Bjør­Iykke et al. 1976). The basinal rocks were tec­

tonically emplaced and deformed during the Scandian stage of the Caledonian Orogeny in late Silurian time ( Bockelie & Nystuen 1985).

The terms Riphean and Vendian are used here in the sense of Vida! (1979, 1981a).

Palinspastic restoration of the Hedmark Basin The amount of Caledonian displacement of the Hedmark Group (the 'sparagmite sequence') has been debated for more than one hundred years. The main arguments were reviewed and appraised by Nystuen (1981). In this chapter only a brief account is given of the previous hypothesis, and the attention is paid to current methods and the results of palinspastic restoration.

Schiøtz (190 2) introduced the concept of the 'Sparagmite basin' by suggesting that the Hed-

396 J. P. Nystuen

+ + + + + + + +

Sparagmite Southern

NORSK GEOLOGISK TIDSSKRIFr 67 (1987)

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NORSK GEOLOGISK TIDSSKRIFT 67 (1987) TSGS Symposium 1986 397

LEGEND TO THE MAP 'SPARAGMITE REGION, SOUTHERN NORWAY

l C!:] Permian rocks a. 5 • Cambro-Silurian rocks ITJ Post-Caledonian fault

. Basalt

[]] Atna Quartzite

CJ Rendalen Fm.

W Storskarven Fm.

D Brøttum Fm.

----------"T� � Trondheim and associated a. � nappe complexes

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[i::!] Basement rocks � Blskopåsen, l msdalen, l � Osdalen and other Cgls.

15] Osen.:..Røa Thrust (ORT)

Wind6ws, from the SW to the NE: Sn = Snødøla, At = Atnsjøen, Sp = Spekedalen, ø = Øversjødalen, Tu = Tufsingdalen, St

· = Steinfjeilet, Rø = Røa

Syn-sedimentary fault, reactivated during Caledonian deformation: IMF = lmsdalen Fault Post-Caledonlan faults: RF = Rendalen Fault, OF = Osen Fauit, EF = Engerdalen Fautt

KEY MAP: [�/(i'2!:iJ Sparagmite Region

� Erosional nappe front

f'VYl U.Carboniferous-Pennian L.:LJ igneous rocks

• Cambro-Silurian rocks

Fig. l. Geological map of the Sparagmite Region, southern Norway. Modified from Nystuen (1982).

mark Group mainly crops out within the frame­work of the original depositional basin. Schiøtz (190 2) thought that only parts of the uppermost sandstone unit, the Vangsås Formation, had been thrust 30-40 km towards the south out of this basin (Figs. l, 2 & 3). This view, founded on the occurrence of outliers of the Vangsås Formation in overthrust position and the structural relation between the basin- fill sequence and the regional normal faults of the area (Fig. 1), was recently advocated and repeated by Bjørlykke (1983). However, the Engerdalen Fault, Osen Fault, Rendalen Fault and other faults which, according to Schiøtz (190 2), formed the structural margins of the late Proterozoic basin are all post-Cale­donian, cutting Caledonian thrust sheets (Nystuen 1981, 1983; Bockelie & Nystuen 1985; Siedlecka et al. 1987). The 'Sparagmite basin' is thus a graben structure into which thick piles of sedimentary rocks of the Osen- Røa Nappe Complex and higher thrust sheets have subsided, probably in the early Permian when the Oslo Graben was formed in the Oslo Region further to the south (Fig. 1).

Along the sutured, eroded nappe front west and east of Lake Mjøsa (Fig. 1), the peneplained

crystalline basement is overlain by a thin auto­chthonous sedimentary cover. This cover includes formations from the middle Vendian Moelv Tillite up to the lower Ordovician. The sedimentary cover rocks also crop out beneath the Osen- Røa Thrust ( O R T ) on the basement windows in the north em part of the Sparagmite Region ( Bockelie & Nystuen 1985; Siedlecka & Hebekk 198 2; Sied­

lecka et al. 1987). ( The basement rocks of the window structures may also be allochthonous, underlain by the regional Caledonian sole thrust, but this is still very uncertain.) In the lake Mjøsa area, successively younger formations of the thrust Hedmark Group pinch out towards the south due to front ramping of the O R T. Because of a higher erosional leve! on the eastern side of the lake than on its western side, the Cambro­Silurian rocks on top of the Vangsås Formation are here preserved in continuation with the Cam­bro-Silurian sequence of the Oslo Region to the south. The southernmost outcrops of the Hed­mark Group rocks in this pile thus de fine the southern thrust margin of the Hedmark Basin (Fig. 1).

Reusch (1907) demonstrated that the folded Cambro-Silurian sequence in the Oslo Region

398 J. P. Nystuen

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NORSK GEOLOGISK TIDSSKRIFT 67 (1987)

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� Limestone

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ø Sandstone of uncertain orig in .

Fig. 2. Stratigraphy of the Hedmark Group in the area west of the Imsdalen Fault. Data from Løberg (1970), Englund (1972,

1973), Bjørlykke et al. (1976) and Nystuen (unpublished).

NORSK GEOLOGISK TIDSSKRIFr 67 (1987)

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TSGS Symposium 1986 399

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Fig. 3. Stratigraphy of the Hedmark Group in the area east of the Imsdalen Fault. Data from Sæther & Nystuen (1981), Nystuen (1982), Nystuen & Hebekk (1981).

400 J. P. Nystuen

was thrust as a nappe along a detachment at the base of the sequence. Oftedahl (19 43) showed that this detachment was the continuation to the south of the O R T as a flat within the Cambrian Alum Shale. Folding and thrusting appear to die ou! at Langesundsfjorden in the southwestern part of the Oslo Region (Fig. 1). Oftedahl (19 43) pinned the Cambro-Silurian sequence here, restored the beds above the O R T and concluded that the Palaeozoic rocks at Mjøsa had been displaced 150 km towards the SSE. Morley (1986a), using the balanced cross-section method,

attained a similar value of 135 km. This implies a shortening by at !east 50% for the Palaeozoic sequence at Lake Mjøsa. Furthermore, the sou­thern margin of the Hedmark Basin must have been located at !east 135-150 km NNW from its present thrust position in the Mjøsa area. Hossack et al. (1985), also using the balanced cross-section method, found a shortening of 11 4% for duplexes in the Osen- Røa Nappe Complex west of Lake Mjøsa and a corresponding minimum thrust dis­tance of 50 km for the frontal parts of the thrust basinal sequence.

Another approach to the palinspastic problem of the Hedmark Basin is to match lines of cor­responding hangingwall and footwall cuttoffs. The line connecting all pinchouts of the Vangsås Formation in the eastern Lake Mjøsa area marks the position where the Osen- Røa Thrust as a hangingwall ramp cuts up section through the Vangsås Formation and continues southwards into the Oslo Region as the Alum Shale flat (Fig. l) (Morley 1986a, his Figs. 3 & 4). Morley (1986a)

matched this hangingwall cutoff with a footwall cutoff at the top of the Vangsås Formation, which rests on crystalline basement rocks on the north­em side of the Atnsjøen window (Fig. 1). This correlation gives a displacement of 1 30 km for the southern margin of the Hedmark Basin. Nystuen (198 2) and Kumpulainen & Nystuen (1985), however, restored the hangingwall ramp in the Vangsås Formation at Mjøsa to a position north of Trollheimen (Fig. 1), where a correlative unit of the Vangsås Formation rests on crystalline basement (Gee 1980). This implies a displacement of 230 km for the southern margin of the Hedmark Basin.

Fig. 4 illustrates a restored position of the Hed­mark Basin, assuming a Caledonian translation of 230 km for the southern basin margin. Together with other restored late Proterozoic basins, now represented by allochthonous sequences in the

NORSK GEOLOGISK TIDSSKRIIT 67 (1987)

Fig. 4. Palaeogeographic reconstruction showing late Riphean­Vendian sedirnentary basins in restored positions on the Balto­scandian margin. The Valdres, Hedmark, Risback, Engerdalen and Tossåsfjiillet Basins form the western Baltoscandian basins. These basinal sequences are now presented in the Lower and

Middle Allochthon of the Caledonian Thrust Belt. Heavy dots:

graben basins, light dots: coastal plain and shelf basins. Modified from Kumpulainen & Nystuen (1985).

Caledonides, the Hedmark Basin is here located on the western margin of the Baltoscandian craton. All of these western Baltoscandian basins originated as the result of continental rifting ( Kumpulainen & Nystuen 1985). In this respect these basins occurred in a structural framework very similar to that of the Mesozoic extensional basins off present-day western Norway (e.g. Zieg­ler 198 2; Gage & Dore 1986).

The displaced basinal sequence de fines a root­less palaeobasin (Nystuen 1986) of complex inter­nal thrust geometry (Nystuen 1983). Vertical and lateral variation in deformation and shortening, due to translations along bedding-parallel detach­ment horizons (Morley 1986b), complicate recon­structions of the original spatia ! interrelationships of depositional facies. Another constraint on the interpretation of the basin's history is the scarcity of correlative chronostratigraphic horizons.

NORSK GEOLOGISK TIDSSKRIFT 67 (1987)

Review of stratigraphy and sedimentary history Stratigraphical and sedimentological studies by Løberg (1970), Englund (197 2, 1973), Bjørlykke et al. (1976), Nystuen (1976, 1981, 198 2), Nystuen & Hebekk (1981) and Sæther & Nystuen (1981)

form the basis for the stratigraphy presented in Figs. 2 and 3 and the review of the sedimentary history given below. The type section of the Hed­mark Group is in the Mjøsa area (Fig. 2, Loe. 3).

The Brøttum Formation consists of sandstones deposited by turbidity currents and intercalated grey and black phyllitic hemipelagic shales. Acri­tarchs of early Vendian age in the upper part of the formation indicate deposition in a marine environment (Vida ! 198lb ). The stratigraphical lower boundary of the Brøttum Formation is unknown. The fluvial Elstad Formation which locally underlies the Brøttum Formation in Gud­brandsdalen has an uncertain stratigraphic and tectonic position (Englund 1973). The regional overthrust position of the Brøttum Formation is clearly demonstrated from the tectonostrati­graphy at the southern margins of the Atnsjøen and Snødøla windows (Siedlecka et al. 1987).

In the north and northeast, the fluvial Rendalen Formation and the shallow-marine Atna Quartzite are lateral equivalents to the Brøttum Formation or parts of it (Figs. l, 2 & 3). The arkosic sand­stones and conglomerates of the Rendalen For­mation are preserved with their depositional contact on granitic basement within large thrust sheets at Lake Femunden and in Hlirjedalen in Sweden. Alluvial fan conglomerates also occur here in contact with basement rocks (Nystuen 198 2). Locally, to the north, the Storskarven For­mation predates the Rendalen Formation as a fine-grained fluvial sandstone (Nystuen & Hebekk 1981; Nystuen 198 2) (Fig. 3, Loe. 1 4).

In the Mjøsa type section, the Brøttum For­mation is succeeded by shales of the marine Biri Formation or coarse-clastic beds of the Bisko­påsen Conglomerate. The Biri Formation consists of carbonate rocks of various facies and shales, and it has a wide lateral extent in the Sparagmite Region. The unit occurs transgressively above the Atna Quartzite in the north and the Rendalen Formation in the east. The Hede Formation in Hlirjedalen (Stålhos 1956) is an equivalent to the

Biri Formation in a similar transgressive strati­graphic position above the continental Rendalen Formation.

TSGS Symposium 1986 401

The Biskopåsen Conglomerate occurs as sev­eral fan-shaped bodies along the southern and western margin of the basin (Figs. l & 2). These deposits are interpreted as formed by subaqueous gravity mass flows in front of fan deltas ( Otter, in Nystuen 198 2). Similar thick conglomerate bodies also occur along the Imsdalen Fault ( Imsdalen Conglomerate), and in Øvre Rendal ( Håkenstad Conglomerate), wedging into Brøttum sand­stones and Biri shales, respectively (Figs. 2 & 3) (Sæther & Nystuen 1981). A fluvial counterpart to

these conglomerates is the Osdalen Conglomerate which overlies the transgressive Biri Formation and the Rendalen Formation with an erosional contact in the east (Figs. l & 3) (Nystuen 198 2).

In the Mjøsa type area and elsewhere in the southern and western marginal parts of the pal­aeobasin, Biri shales and limestones cover the Biskopåsen conglomerate bodies and are in turn overlain by the Ring Formation (Figs. l & 2). This unit occurs in several coarse-clastic pro­grading fan delta bodies (Bjørlykke et al. 1976).

The Moelv Ti/lite consists of lithi fied till and glaciomarine laminated mudstones with ice­dropped stones formed during the Varanger lee Age about 650 Ma ago in middle Vendian time ( Bjørlykke & Nystuen 1981). The glacial unit also occurs on the crystalline basement and east of Lake Storsjøen and in the windows to the north (Nystuen 1976, 198 2; Siedlecka & Hebekk 198 2).

The postglacial Ekre Formation (or Ekre Shale) is a laminated or homogeneous sla ty siltstone with thin sandstone laminae and interbeds. It has a wide lateral extent within the thrust basinal sequence.

The youngest unit in the Hedmark Group is the late Vendian--early Cambrian Vangsås For­mation. It commences with the Vardal Sandstone Member, which in most areas consists of deltaic to braided stream sandstones and conglomerates. However, in the Osen-Jordet area, at Øvre Rendal and east of Lake Femunden, turbidite sandstones, mass- flow conglomerates and, in part, dark shales also occur within this member (Figs. l & 3). The Vardal Sandstone Member is conformably overlain by and partly grades into shallow-marine quartzites and quartz conglom­erates of the Ringsaker Quartzite Member. In the Mjøsa type section the Ringsaker quartzite beds reveal pipe-rock facies with Skolithos and Diplo­craterion in the uppermost part (Skjeseth 1963). A minor unconformity separates the Ringsaker Quartzite Member from the overlying trans-

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NORSK GEOLOGISK TIDSSKRIFr 67 (1987)

gressive sandstone, siltstone and shale beds con­taining a 'Holmia-stage' fossil fauna. These beds introduce a several hundred meters thick Cam­bro-Silurian sequence that includes foreland sedi­ments and synorogenic foredeep facies ( Bockelie & Nystuen 1985).

The Vangsås Formation onlaps the basement beyond the original structural boundaries of the Hedmark Basin (Fig. 3). The westernmost known outcrop of this onlap sequence occurs in Troll­heimen (Gee 1980), and the basal beds young towards the southeast. The Ringsaker quartzite beds thin out within the pinned autochthonous cover east of the Rendalen Fault. Farther south along the erosional nappe front and in the Oslo Region, basal beds of the Lower Cambrian 'Holmia-stage' and the Middle Cambrian Alum

Shale successively onlap the basement ( Bockelie & Nystuen 1985).

The Hedmark Basin was affected by volcanic effusions in pre-Varangerian time. Tholeiitic basalt occurs in the 5-30 m thick Svarttjørn­kampen Basalt above the Atna Quartzite in the Imsdalen- Atna area and in smaller erosional rem­nants beneath the Biri Formation and within the Atna Quartzite in the area between the Rendalen and Osen Faults (Fig. 2) (Sæther & Nystuen 1981; Nystuen 198 2; Furnes et al. 1983). Fragments of similar basalt type occur in the Biskopåsen Conglomerate at Mjøsa, in the lmsdalen Con­glomerate in Imsdalen and the Håkenstad Con­glomerate at Øvre Rendal. This indicates that tholeiitic basalt ftows had a wider extent within the basin and/or in adjacent areas in the time prior to the formation of these conglomerates.

The history of the Hedmark Basin can be div­ided into the following structural-sedimentary spans of evolution: (1) Pre-rift sedimentation and initial formation of the main graben; (2) Rifting and basin expansion; (3) Rifting, volcanism and basin submergence, (5) Late rifting and glaci­ation; (6) Late rifting, waning sub-basin activity and regional subsidence; and (7) Post-rifting and regional subsidence. It is suggested that these evolutionary spans formed during three, major thermal-mechanical phases of one rift episode (Fig. 5). The terms 'span', 'phase' and 'episode' are used as diachronous terms in the sense of the North American Corrimission on Stratigraphic

Fig. 6. Example of sequence recorded from the Brøttum For­mation, Lillehammer. The thick-bedded turbidite sandstones of the inferred lobe sequences represent stacked channel-fill units.

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TSGS Symposium 1986 40 3

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40 4 J. P. Nystuen

Nomenclature (1983) and the Norwegian Com­rnittee on Stratigraphy (Nystuen 1986).

Span l: Pre-rift sedimentation and initial formation of the main graben The deepest stratigraphical level of the Hedmark Group is generally considered to be present in Gudbrandsdalen (Fig. 1). According to depths to magnetic (crystalline) basement (Åm, in Nystuen 1981), there appears to be a tectonic thickness of 4000-5000 m with sedimentary rocks above the Caledonian sole thrust in this district. Most of this rock pile probably belongs to the Brøttum Formation, which has a stratigraphic thickness here of at !east 2000-3000 m. The earliest graben formation is thought to be reflected by the lower part of the Brøttum Formation in this area.

The Brøttum Formation consists of massive and graded sandstones interbedded with black phyllitic shale beds in the Mjøsa-Gudbrandsdalen area. The shale units in total constitute about 25% of the total rock volume in the lower part of the forma ti on and attain maximum thicknesses of some tens of meters. The sandstones are fine- to coarse-grained, thick- to thin-bedded, forming coarsening- and fining-upwards sequences inter­changed with sequences of no distinct vertical trend of changes in grain size or thickness (Fig. 6).

LATE RIPHEAN

LEGEND TO BA SIN DIAGRAMS

- Black mud

Q - Mud

� Carbonate mud

� Sand, f/c

Gravel & !oJ:O.ot equivalent rocks

T Basalt & feeder

D Ba sement 0 1km

50 km lmsdalen

NORSK GEOLOGISK TIDSSKRIFf 67 (1987)

Palaeocurrent structures and mineralogy indicate that sediment was supplied mainly axially from the south and laterally from the west. The Brøt­tum sandstones were deposited in sub-marine lobes and fans by turbidity currents (Englund 1972, 1973; Bjørlykke et al. 1976; Nystuen 198 2).

The western margin of the graben structure is thought to be indicated by the westernmost position of the most coarse-grained facies in the basin- fill sequence. These subaqueous clastic wedges probably originated along the relatively fixed western master fault of the Hedmark Basin (Fig. 5). The eastern boundary of the western main gra ben is thought to coincide with the Imsda­len Fault ( I MF ) (Figs. l & 5). The E-W dimen­sion of the graben might have been 40-50 km. In the NNW-SSE direction the basin must have been at !east 150 km, as estimated from the present outcrop of the lower Brøttum beds, and by adding an average 50% to compensate for tectonic shortening.

Nothing is known about the very onset of the rifting. We can only speculate that it cor­responded with the sudden subsidence of an elon­gate graben along steeply dipping initial faults. The sea entered the fault basin, and during milli­ons of years, tectonic subsidence, compaction and isostatic adjustments must have been balanced by the rate of sedimentation. This is demonstrated by the repeated cycles of monotonous turbidite

Crystalline ba sement

IMF Fault SPAN 1: INITIAL FORMATION OF MAIN GRABEN

Fig. 7. Hedmark Basin. Schematic diagram showing initial formation of the marine main gra ben (Span l) during the Phase of initial crustal stretching. The graben is filled with turbidite sands and organic-rich hemipelagic mud (lower part of the

Brøttum Formation). Br= Brøt­tum Formation.

NORSK GEOLOGISK TIDSSKRIFr 67 (1987)

l 1111'1

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m 202 4 6 .,.� Muå�\ · ; j

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TSGS Symposium 1986 405

facies associations in the lower Brøttum sequence, which is at !east 1500 m thick. A conceptual dia­gram of the initial gra ben and initial graben fill is shown in Fig. 7.

A pre-rift history may be reflected by the Stor­skarven Formation of which 2000 m is preserved at the eastern apex of the Atnsjøen window (Figs. l & 3). It occurs beneath the conglomeratic and immature arkoses of the Rendalen Forma­tion, and its stratigraphic position may correspond to the lowermost parts of the Brøttum Formation, or even lower (Fig. 5). It is suggested that the fine-grained and well-sorted sandstones of alluv­ial plain origin (Nystuen & Ilebekk 1981) were form ed prior to the fault activity that triggered the deposition of the overlying Rendalen Formation. The Storskarven sandstones must have been deposited in an exterior position relative to the main graben segment, northwest of the area shown in Fig. 7.

Span 2: Rifting and basin expans10n As mentioned above, it is suggested that the eastern margin of the initial marine rift basin coincided approximately with the Imsdalen Fault. This curvilinear fault runs NW-SE and is pro babl y cut by the post-Caledonian Rendalen Fault to the east. The fault is covered by the Kvitvola Nappe in the southeast (Fig. 1). East and northeast of the Imsdalen Fault, the oldest preserved record of the depositional history is characterized by fluvial sedimentary rocks.

The Rendalen Formation, being at !east 25 00 m thick at its maximum, has a wide regional extent in the eastern part of the Sparagmite Region (Fig. 1). The presence of polymict conglomerate and feldspar-rich gravelly sandstone of this unit above the Storskarven Formation suggests a sud­den change in relief and tectonic setting. Depo­sitional contacts with granitic basement rocks west and east of Lake Femunden and in Hlirjedalen in Sweden indicate that the Rendalen arkosic sand and grave! were deposited in a terrain of substanti­al relief. This is well demonstrated by up to 370 m

Fig. 8. Example of sequence recorded from the ftuvial Rendalen Formation, Rendalsølen (c. 20 km east of Øvre Rendal). From Nystuen (1982). One coarsening-upwards sequence in the lower part and one in the upper part of the section may represent alluvial fan complexes that have prograded into the basin from active fault escarpments.

406 J. P. Nystuen

thick local conglomerates that were deposited along basement escarpments as several small coalescing alluvial fans (Nystuen 198 2).

The Rendalen Formation consists of braided stream and sheet flood deposits laid down on a wide alluvial plain from ephemeral streams flow­ing westwards (Nystuen 198 2). The formation of large-scale coarsening- and fining-upwards sequences (Fig. 8) is probably tectonically con­trolled, either by movements along marginal faults or by variations in tectonic subsidence and rate of accommodation within the basin.

Within a 3-4 km zone across the Imsdalen Fault, the Rendalen fluvial sandstones pass into coarse-grained turbidites and other subaqueous gravity-flow facies of the Brøttum Formation (Sæther & Nystuen 1981; Nystuen 198 2). Coarse­grained and conglomeratiC thick-bedded tur­bidites of this eastern provenance form a thick clastic wedge that has prograded westwards over the more shale-rich and thin-bedded lower part of the Brøttum Formation (Bjørlykke et al. 1976; Nystuen 1981, 198 2). The fault zone appears to have coincided with a narrow belt of sediment by­passing between an alluvial plain in the east and a marine turbidite basin in the west (Sæther & Nystuen 1981; Nystuen 198 2). The Imsdalen Fault is discussed further below.

Deposition of the coarse-grained arkoses and conglomerates of the Rendalen Formation is

LATE RIPHEAN TO EARL Y VENDIAN c.700 Ma

01km

50 km

NORSK GEOLOGISK TIDSSKRIFT 67 (1987)

interpreted as having taken place within several smaller basins formed by an eastward propagation of a marginal fault system (Fig. 9). This rifting and structural widening of the Hedmark Basin gave rise to an extensive alluvial province in the areas east and northeast of the Imsdalen Fault. The alluvial region must have extended for at !east 200 km in a NE- SW direction. The extent in the NW- SE direction of this alluvial region has probably been in the order of 200-300 km. The western, distal part of this region was an unbroken braidplain. Towards the east, the plain was dis­sected by basement ridges and separated into several interconnected sub-basins. Most of the basement highs may have been covered by alluvial sediments towards the end of the depositional period of the Rendalen Formation.

Span 3: Rifting, volcanism and basin submergence The fluvial, coarse-grained arkoses of the Rend­alen Formation in the area west of Lake Femun­den are overlain by a white or light grey, fine­to medium-grained and well-sorted feldspathic quartzite. The latter, the Atna Quartzite, is up to 100 m thick and was deposited in a shallow-marine environment of deposition (Sæther & Nystuen 1981; Nystuen & Ilebekk 1981; Nystuen 198 2).

SPAN 2: RIFTING AND BASIN EXPANSION

Fig. 9. Hedmark Basin. Schematic diagram showing eastward basin expansion (Span 2) during the Main phase of crustal stretching. Legend, see Fig. 7. Alluvial fan and all u via l plain sedimentation (Rendalen For­mation = Re) dominate east of the IMF, fan-delta deposition along IMF and marine turbidite sedi­mentation west of IMF (Brøttum Formation = Br).

NORSK GEOLOGISK TIDSSKRIFT 67 (1987)

Flows of tholeiitic basalt occur within and above the Atna Quartzite . Mafic dykes in the Stor­skarven Formation are thought to be remnants of feeders to local fissure volcanoes (Nystuen 1982) . The Atna Quartzite in several areas is overlain by a dolomite belonging to the Biri For­mation (Figs . l, 3 & 10) (Nystuen 1982) .

The quartzite-dolomite succession demon­strates that submergence accompanied by a mar­ine transgression took place across the former alluvial plain in the east . This so-called 'Biri­transgression' has also been recorded from the marginal parts of the palaeobasin in the south and west (Løberg 1970; Englund 1972, 1973; Bjør­lykke et al . 1976) . Transgressive carbonate units at a similar pre-Varangerian stratigraphical posi­tion occur in other upper Proterozoic basin fill sequences of the western Baltoscandia and also elsewhere in the North Atlantic region and in other parts of the world (e.g . Kumpulainen & Nystuen 1985; Hambrey & Harland 1981; Win­chester in press) .

The transgression and deposition of the Atna sands and early Biri carbonates in the Hedmark Basin probably resulted from a eustatic sea leve! rise. An alternative or additional component to the submergence may have been regional thermal subsidence after a period of crustal extension and basin expansion by fault tectonics . This is not,

EARL Y VENDIAN

c.690 Ma

O 1km 50 km

TSGS Symposium 1986 407

however, supported by the fact that fissure vol­canism took place during the early stage of the transgression . The tholeiitic volcanism heralded a phase of extension that involved block faulting, local structural inversions and formation of stag­naut sub-basins (Fig . 5) .

Span 4: Rifting, sub-basin formation and emergence

The early 'Biri transgression' in most parts of the eastern basinal province was interrupted by emergence and erosion . This also happened in marginal areas rimming the western marine trough (Fig . 11) .

In the eastern province, the Osdalen Con­glomerate originated in at !east four alluvial fans aligned NNW-SSE over a distance of 60 km. The depositional strike is suggested to have been struc­turally controlled. The central part of the con­glomerate formation is bounded to the east by a fault which appears to predate Caledonian deformation (Nystuen 1975) . At the base, these conglomerates are enriched in shale and car­bonate clasts derived by erosion from the trans­gressive Biri Formation . Quartzite clasts very similar to the Atna Quartzite types are common, besides extrabasinal clasts derived from base ment

Fig. JO. Hedmark Basin. Schematic diagram showing regional basin submergence (Span 3) during the Main phase of crustal stretching. Legend, see Fig. 7. Quartz-rich sands (Atna Formation =At) and carbonate mud (Biri Formation = Bi) are deposited in shallow-marine transgressed areas and as organic­rich muds (Biri Formation) in deeper parts of the basin. Local effusions of tholeiitic basalt fiows from fissure volcanoes. SPAN 3: RIFTING, VOLCANISM AND BASIN SUBMERGENCE

408 J. P. Nystuen

rocks . The conglomerate unit (up to 350 m thick) was deposited by braided fiood streams flowing from east to west . Red mudstones interbedded with conglomerate beds in the western, distal area of the fan bodies may have formed in ephemeral bkes (Nystuen 1982) .

Nystuen & Sæther (1979) discussed a glacio­fiuvial origin for the Osdalen Conglomerate . However, although this hypothesis cannot be discounted, there is no evidence of either glacial processes or glaciolacustrine or glaciomarine sedi­mentation at this stratigraphical level within the Hedmark Group. The coarse-clastic alluvial fans may have originated along a N-S running flexural zone and/or fault where the basinal area to the east was locally elevated and degraded . Such a structural lineament would have been nearly par­alle! to the Imsdalen Fault further to the west (Fig . 1) .

Black pyritic shales that can be correlated with the Biri Formation rest directly upon fiuvial sand­stones of the Rendalen Formation or shallow­marine sandstones of the Atna Formation in the Øvre Rendal-Bjørånes area (Figs . l & 3) . Wedges of subaqueous gravity-flow conglom­erates (Håkenstad Conglomerate) occur at the base of the black shale unit . Clasts of limestone, quartzite, calcareous sandstone and basalt dem­onstrate that erosion of older basin-fill rocks had taken place. Clasts of crystalline basement rocks

NORSK GEOLOGISK TIDSSKRIFr 67 (1987)

could have been brought into the basin from basement highs, or they may represent reworked older intrabasinal grave! material . A turbidite sandstone within the black shale unit higher up in the sequence may have formed in response of renewed tectonic activity . Basin shallowing and reduced infiux of clastic material gave rise to a laminated micritic limestone at the top of this sequence (Fig . 3, Loe . 16) .

The Øvre Rendal-Bjørånes basin must have been formed by sudden subsidence of the basin fioor . The sub-basin was probably a fault­bounded, narrow and elongate depression con­nected with the major marine graben in the south and west (Fig . 1 1) . Remnants of similar sub-basin sequences that contain Biri carbonates and shales are recorded in the Jordet-Osen area, west of Atna and east of Lake Femunden (Nystuen 1982) .

Tectonic activity along the margin of the west­ern marine graben, and possibly also in adjacent source rock areas, is reflected in the basin fill by a series of coarse-clastic fan bodies within the Biskopåsen, Imsdalen and Ring Formations (Figs. l, 2 & 3) . Intrabasinal clasts of carbonate rocks, phosphorite, mudstone, sandstone and tholeiitic basalt in the Biskopåsen and Imsdalen Conglomerates are evidence of erosion of the early Biri sediments and basalt fiows (Spjeldnæs 1967; Englund 1972, 1973; Bjørlykke et al . 1976; Nystuen 1982) . Extrabasinal clasts refiect

SPAN 4: SUB-BASIN FORMATION AND EMERGENCE

Fig. 11. Hedmark Basin. Schematic diagram showing formation of Jocal sub-basins and structural inversions (Span 4) during the Main phase of crustal stretching. Legend, see Fig. 7. Sub-basins and main gra ben are filled with organic-rich black mud (Biri and Bjørånes Formations = Bi), and coarse-clastic wedges (Biskopåsen = Bis, Håkenstad = Hå, Imsdalen = Ims) are formed along active faults in fan deltas and sub-marine fans. The Ring For­mation was formed in a similar way at a later event along the western main graben. Alluvial fans (Osda­len Conglomerate) are formed in a local terrestric to lacustrine basin (in the centre).

NORSK GEOLOGISK TIDSSKRIFT 67 (1987)

m Ql+42024-t,luds'-· Sii\S\·

sandst0�'0 ?>'-0

conglo��'e�

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Q) U) "' ro

o o "' c 'iii "' ro

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� Wavy bedding � Parallel bedding O Massive bedding � Flute and load east

TSGS Symposium 1986 409

basement lithology, potassium-rich granite and porphyries in the east, south and southwest and sodium-rich source rocks of granite, gneisses and anorthosite in the west,. whereas metaquartzites are common in all of the conglomerates (Løberg 1970; Englund 1973; Bjørlykke et al . 1976; Nystuen 1982).

The Biskopåsen and Imsdalen Conglomerates form prograding fan bodies composed of sub­aqueous gravity-ftow deposits . The former unit wedges into dark shales and sandstones of the Biri and Brøttum Formations, respectively (Fig. 5). Biri shales and limestones also drape the Biskopåsen fans and thus reveal a stage of local transgression in the marginal areas (Fig. 2) . The Imsdalen Conglomerate is up to 700-800 m thick and grades into coarse-grained Brøttum turbidite sandstones . This demonstrates a long period of coarse-clastic sedimentation along the Imsdalen Fault (Figs. 5 & 11).

It has been suggested that the Biskopåsen Con­glomerate was deposited during a global glacio­eustatic sea-levet drop (Bjørlykke et al . 1976; Bjørlykke 1985) . This hypothesis is founded on a correlation between this conglomerate formation and the lower of the two Varangerian glacial units in Finnmark, northern Norway. Biostratigraph­ical correlations between the upper Riphean­Vendian sequence in Finnmark and the Hedmark Group do not support this interpretation; the Biskopåsen and Biri Formations appear to be of early Vendian age and the Varangerian glacial beds in Finnmark of middle Vendian age (Vida! 1981b:41; Vida! & Siedlecka 1983) . Very large volumes of continental ice are needed glo ball y for a widespread regressive event to be recorded in the stratigraphical column; coastal onlap curves recorded from continental marginal basins are primarily in response to local thermotectonic sub­sidence (Summerhayes 1986) .

The Ring Formation represents a new cycle of coilfSe-clastic fan delta bodies prograding into the western marine graben. It occurs along the southeastern, southwestern and western basin margins (Figs. l & 2). Proximal facies include subaqueous channel and lobe deposits, whereas distal or interlobe facies include thin-bedded tur-

Fig. 12. Example of sequence recorded from the Ring Forma­tien, Highway 3, 3 km south of Rena. The inferred base-of­slepe sequence consists of channel-fill and sheet sandstones of

turbidite origin and the upper gra vell y lobe of stacked channel­fill units of subaqueous gravity flow origin.

410 J. P. Nystuen

bidite sandstones and intercalated mudstones (Fig . 12) . The coarse-clastic sediments wedge out into the Biri shales . The Ring sedimentation is suggested to be related to basin margin tectonics (Bjørlykke 1978) .

The structural-sedimentary span described in this section is thought to comprise the maximum rifting event of the Hedmark Basin for the fol­lowing reasons: (l) Tholeiitic basalt extrusions (during the former span) refl.ect opening of deep fissures formed by extensive crustal stretching; (2) Subsidence of fault blocks and formation of local sub-basins; (3) Infl.ux of extrabasinal clasts from basement highs that can have formed as elevated shoulders of the rift basin; (4) Erosion of intrabasinal ridges that may have originated from the rotation of fault blocks; (5) Deposition of coarse-clastic debris along faults or fl.exures, and (6) Preponderance of mud sedimentation in stagnant marine basins which indicates that local subsidence exceeded sedimentation.

The presence of limestones stratigraphically above the Biskopåsen Conglomerates and in the Øvre Rendal sub-basinal sequence (Figs . 2 & 3) suggests that coarse-clastic sedimentation, or­ganic-rich mud accumulation and carbonate deposition have all co-existed at various sites in the Hedmark Basin during the span of sub-basin formation and emergence . Transgression and

MID-VENDIAN (VARANGERIANl

c. 6 50 Ma

� /l/

.......... \.\ \. \ " " ...... \ \ \ \

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U 1km 50 km

NORSK GEOLOGISK TIDSSKRIFT 67 (1987)

regression curves (Englund 1973; Bjørlykke 1982) refl.ect local responses to the interaction of factors such as (l) eustatic changes of sea leve!, (2) sedi­ment compaction, and (3) isostatic subsidence, overprinted by (4) fault activity and tectonic subsidence .

Span 5: Late rifting and glaciation

The Baltoscandian craton and its rift-related basins were covered by glacial ice sheets during the Varanger lee Age about 650 Ma ago (Nystuen 1982, 1985; Kumpulainen & Nystuen 1985) . In the Hedmark Basin, the Moelv Tillite was deposited as a basal till from the sole of grounded ice sheets and as laminated mudstone with ice­dropped stones in glaciolacustrine and glacio­marine environments (Bjørlykke et al . 1976; Nystuen 1976) . The glacial formation is underlain by a regional erosional unconformity, and the basal till facies rest upon various units of the older basin fill sequence and on basement rocks outside the basin (Figs . l, 2, 3 & 5). In one locality in the central southern part of the western marine graben, a conformable contact is thought to exist between the marine Biri shales and a glaciomarine mudstone facies (Bjørlykke et al . 1976) (Fig . 2, Loe . 2) . This may indicate that the ice sheet was

SPAN 5: LATE RIFTING AND GLACIATION

Fig. 13. Hedmark Basin. Schematic diagram showing the retreating and depositional stage of the Varanger glaciation (Span 5) during the early Phase of thermal cooling. Legend, see Fig. 7. Deposition of glacial debris gave rise to the Moelv Tillite (Mo) and postglacial mud to the Ekre Formation.

NORSK GEOLOGISK TIDSSKRIFT 67 (1987)

floa ting in the deepest part of the basin (B jørlykke 1985), causing a rapid retreat of a calving ice front and moderate deposition of glacial mud during the debris-releasing waning stage of the ice sheet (Fig . 13) .

The glacial deposits are preserved with only moderate thicknesses (up to c. 5 m) in shallow depressions on the crystalline basement, whereas the thicknesses vary substantially in basinal set­tings (Figs . 2 & 3). In the latter environment, a generalizing glacial succession consists of (l) massive diamictite deposited from grounded ice on an ice-scoured surface, followed by (2) lami­nated glaciomarine mudstone which passes upwards into (3) finely laminated, postglacial mudstone of the Ekre Formation . This sequence is interpreted to reftect an evolution from glaci­ation and erosion through ice sheet retreat and subglacial deposition of till debris, ftoating ice­sheet or ice-berg sedimentation to glacio-eustatic sea leve! rise and postglacial marine or brackish­water deposition of fine mud (Bjørlykke et al . 1976; Nystuen 1976; Bjørlykke & Nystuen 1981) .

The discontinuous occurrences of tillite with moderate thicknesses on the crystalline basement are thought to have resulted from erosion brought about by glacio-isostatic recovery of the con­tinental crust . In basinal settings, thickness vari­ations are probably related to lateral variations

LATE VENDIAN

c. 630 Ma

O 1km 50km ·

TSGS Symposium 1986 411

and changes through time in physical factors such as thickness, thermal regime and till debris of the glacial ice, besides basin relief and rate of sediment accommodation. The greatest thick­nesses (c. 300 m) of the composite Moelv-Ekre sequence are recorded from sub-basins or down­ftank positions of synsedimentary ftexures or faults (Nystuen 1982) . This strongly favours the interpretation that the rate of accommodation of the glaciogenic sediments and their preservation potential have been controlled by intrabasinal tectonics prior to submergence during the post­glacial transgression (Nystuen 1985) . This, in turn, implies that crustal rifting was still con­tinuing during the glaciation, but at a reduced rate and combined with regional subsidence .

Span 6: Late rifting, waning sub­basin activity and regional subsidence

Though a glacio-eustatic rise of sea leve! caused submergence in most parts of the Hedmark Basin subsequent to the Varanger glaciation, sedi­mentation in early post-Varangerian time was still markedly inftuenced by local tectonic movements. This activity is recorded by the formation of sub­basins and structural highs (Figs . 5 & 14) .

Fig. 14. Hedmark Basin. Schematic diagram showing sub-basins of decreasing tectonic activity and the beginning of regional subsidence (Span 6) during the early Phase of thermal cooling. Legend, see Fig. 7. Fluvial to deltaic Vangsås For­mation (Vardal Member =Vs) pro­grades into the marine basin represented by the Ekre Shale ( = Ek). Turbidite sands are deposited in local sub-basins in the east.

SPAN 6: LATE RIFTING, WANING SUB-BASIN ACTIVITY

AND REGIONAL SUBSIDENCE

412 J. P. Nystuen

As a result of the glacio-eustatic rise in sea leve!, the coastline was displaced towards the periphery of the basin or into areas far outside the marginal faults . This is exhibited by the distri­bution of thickness and facies in the Ekre Forma­tion. The thickness of this formation generally

.increases from the east towards the west, and from south to north (Figs . 2, 3 & 5) . In the proxi­mal areas, the siltstone facies grade into deltaic sandstone facies belonging to the Vardal Member of the Vangsås Formation . The Vardal sandstones form a progradational sequence and cover the Ekre mudstones as deltaic, shallow-marine and tluvial braided-stream facies in most parts of the Hedmark Basin (Bjørlykke et al . 1976; Nystuen 1982).

In the Osen-Jordet area, in Øvre Rendal and in the area east of Lake Femunden, the Vardal Member of the Vangsås Formation occurs in substantially greater thicknesses than the normal 50-150 m recorded elsewhere in the basin (Figs. 2 & 3). East of Lake Femunden, the unit attains a thickness of at !east 1000 m (Fig . 3, Loe . 22) . It includes, from the base of the sequence, turbidite sandstones, dark grey marine or lacustrine shales, subaqueous debris tlow deposits, and braided­stream sandstones with intercalated siltstones of probable lacustrine origin . The sequence prob­ably represents a fan delta system that has pro­graded into a local basin, overlain by braidplain deposits .

The Vardal Member overlies unconformably the Biri Formation in the Jordet area . Here, the lower parts of the unit consist of diamicite beds of subaqueous debris-tlow origin and dark grey, partly organic-rich turbidite sandstones that inter­finger with black shales to the west . The upper part of the member is characterized by con­glomerates and coarse-grained sandstones of flu­via! origin (Fig . 3, Loe . 20) .

In the Øvre Rendal sub-basinal sequence (Fig . 3, Loe . 16), the Vardal Member consists of dark gre y, · coarse-grained sandstones of sub­aqueous gravity tlow origin, overlying shaly silt­stones of the Ekre Formation . In both the Osen­Jordet and Øvre Rendal areas, the Vardal Mem­ber of the Vangsås Formation is thought to be related to the development of fan deltas .

The early post-Varangerian sequences in these areas are suggested to be tectonically controlled by the formation of local fault-bounded sub­basins (Fig . 14) . The upward shallowing and tran­sition from black shales and subaqueous mass-

NORSK GEOLOGISK TIDSSKRIFT 67 (1987)

tlow facies into tluvial beds demonstrate an increased rate of sediment supply relative to the rate of subsidence.

Condensed sequences suggest the former exist­ence of structural highs . Such a sequence occurs at Høgberget, east of the River Trysilelva (Fig . 3, Loe. 21). The Moelv Tillite is absent here and the Ekre Formation, which consists of 5-8 m thick siltstone, rests directly on the Rendalen Forma­tion. Only 4 m of Vardal sandstone is present in the Vangsås Formation which in turn is overlain by Lower Ordovician limestone and Middle Ordovician black shale . Holtedahl (1921) inter­preted the stratigraphy in this area as the result of an earl y Ordovician orogenic rise during which about 200 m of theVangsås Formation and over­lying Cambrian beds was eroded away . This may have happened, but the absence of Varangerian glacial deposits and the strongly condensed Ekre Formation suggest that this area may also have been a structural high in early post-Varangerian time .

Span 7: Post-rifting and regional subsidence

After the waning tectonic activity and subsidence of sub-basins had ceased, the entire Hedmark basin was covered by tluvial to shallow-marine sands and gravels of the Vardal Member . These facies are also preserved lying directly on the crystalline basement or above Varangerian beds in the windows; the westernmost occurrence of the Vangsås Formation in this stratigraphical set­ting is the previously mentioned outcrop in Trollheimen (Gee 1980) . At this time the western regions of the Baltoscandian craton were low­lands drained by braided streams tlowing west­wards into an extensive alluvial plain . Regional subsidence of the rift basin area and adjacent regions of the craton resulted in the accom­modation and preservation of alluvial plain and coastal marine sediments .

In late Vendian to early Cambrian time the basinal and cratonic areas became submerged and the shallow-marine Ringsaker Quartzite Member was deposited over wide areas (Fig . 15) . Similar transgressive quartzite blankets occur at a cor­responding stratigraphical position elsewhere on the Baltic Shield (e .g . Bergstrom & Gee 1985; Moczydlowska & Vida! 1986) . These and cor­responding quartzites on the western foreland

NORSK GEOLOGISK TIDSSKRIFT 67 (1987)

EARL Y CAMBRJAN

c. 590 Ma

TSGS Symposium 1986 413

. . . ..

. . . . . . . . . . . . . . . . . . . . --.:_:_�_:_�:_·��

.... · .

. · . --- :-: :::- .�-

- - �-- -_ ... ·--����-::�--· · · · · . . .. .

. . . . . · . .

· · · . . Fig. 15. Hedmark Basin. Schematic diagram showing regional submerg­ence (Span 7) during the late Phase

of thermal cooling. Legend, see Fig. 7. Quartz sands and sandy silts of the Ringsaker Quartzite Member (Ri) onlap the cratonic basement beyond the margins of the Hedmark Basin. The thickness of the Ring­saker Quartzite Member relative to the thickness of the basinal se­quence is exaggerated. SPAN 7: POST-RIFTING AND REGIONAL SUBSIDENCE

belt of the Caledonides are suggested to have formed in response to a global tectono-eustatic sea-leve! rise caused by extensive sea-floor spreading in the Iapetus Ocean (e.g. Anderton 1982; Bjørlykke 1982). The transgression con­tinued during the Early Cambrian and reached a maximum in the Middle Cambrian when organic­rich black mud (Alum Shale) was deposited in an epeiric sea which covered wide areas of the Baltoscandian craton (Bergstrom & Gee 1985).

The Imsdalen Fault: a synsedimentary strike-slip fault? The Imsdalen Fault (IMF) exhibits a complex geology with regard to stratigraphy, sedimentary facies and deformation of adjacent fault blocks, internal structural configuration of the fault zone and apparent relative movements.

The fault predates the emplacement of the Kvit­vola Nappe. The fault dips from vertical to 50° towards the SW and is seen today as a high-angle reverse fault (Sæther & Nystuen 1981). Nystuen (1983) suggested that the fault was a Caledonian lateral ramp in the Osen-Røa Nappe Complex. A further complication to the geology of this area is the presence of a post-Caledonian N-S running fracture zone that transects the IMF at a very acute angle (Fig. 1).

As pointed out by Sæther & Nystuen (1981), Nystuen (1982) and as previously mentioned in this pa per, facies transitions across the Imsdalen Fault from fluvial arkoses through fan delta beds to marine turbidites in the late Riphean to early Vendian strongly favour synsedimentary activity along the fault. Several additional features of the IMF and its footwall and hangingwall rocks can be related to a synsedimentary origin of the fault. Several of these features suggest that the IMF acted as a strike-slip fault, as judged by criteria for identifying ancient strike-slip faults (e.g. Reading 1980; Christie-Blick & Biddle 1986; Mitchell & Reading 1986): (l) There is no direct cor­respondence in rock types across the fault; for instance, the Imsdalen Conglomerate bodies have not been encountered on the eastern side of the fault; (2) there is a marked lateral facies change across the fault; (3) a very thick sequence is accumulated on the western side of the fault where the Brøttum Formation contains an out­crop thickness of at !east 2500 m; (4) up to 700 m thick conglomerate bodies west of the fault and very thick depositional units of coarse-grained sandstones in the Brøttum Formation indicate episodic rapid subsidence and sediment accumu­lation adjacent to the IMF; (5) the Imsdalen Conglomerate occurs in several separate bodies along the IMF, and all contain the same clast types including granitic and porphyritic basement

414 J. P. Nystuen

rocks; no possible basement source rocks exist in the immediate vicinity of the fault to the east within the nappe complex carrying the basinal sequence.

In addition, the fold pattern of strata adjacent to the IMF is mostly characterized by fold axes running parallel or at an angle of 30-45° to the fault; such structures are characteristic of diver­gent wrench faults (e.g. Harding et al. 1986; Christie-Blick & Biddle 1986) .

Long-lived activity of the Imsdalen Fault during latest Riphean and early Vendian times is sug­gested by the westward progradation of coarse­clastic fan delta and turbidite facies above the earl y, more thin-bedded and finer-grained Brøt­tum turbidites of westerly provenance. An asym­metric infill of the Hedmark Basin resulted from this dispersal pattern. The apparent greater width in the south of the marine, western graben (Fig. l) can be explained by assuming a left-lateral mave­ment along the curved IMF. Widening of the main gra ben in the southeast by such a pull-apart mechanism coincided with the increased infill of coarse-clastic turbidites here in the upper part of the Brøttum Formation.

Though lateral movements can be shown to have taken place along the fault, it remains to be clarified if these are synsedimentary, earl y Caledonian or of mixed origin. However, it is concluded here that the IMF is a major structural fea ture of the Hedmark palaeobasin, and that the fault has imposed a superior control on sedi­mentation. Strike-slip activity of the IMF is sug­gested in Fig. 5.

Rifting and formation of black shales

The black shales in the Hedmark Group (Fig. 5) contain up to 3 weight per cent of total organic carbon (TOC) now consisting of pyrobitumen. The original amount of kerogen may have been about three times more befare thermal destruc­tion. The major phase of thermal breakdown of the kerogen is thought to have been associated with the Caledonian greenschist metamorphism at about 200-300oC. The organic matter was pro b­a bly produced from marine eukaryotic planktonic algae and protists which radiated and flourished during the latest Riphean to early Vendian times about 700 Ma ago (Vida! & Knoll 1982) .

The organic-rich black shales in the Brøttum,

NORSK GEOLOGISK TIDSSKRIFT 67 (1987)

Biri and Vangsås Formations are all related to the development of rapidly subsiding basins, the western major graben and smaller sub-basins in the east (Fig. 5) . In periods, these basins received small amounts of terrigenous clastic material, and basin morphology must have created conditions for restricted circulation of the bottom water and anoxic conditions at the sediment-water interface. In this respect, i.e. having been the structural site for origin of organic-rich sediments, the Hedmark Basin exhibits similarities with a lot of other younger rift basins.

The thermal history of the Hedmark group during sedimentary burial is practically unknown. However, the occurrence of organic-rich turbidite sandstones that interfinger with black shales (as in the Jordet area) may indicate that fluid hydro­carbons were formed prior to the Caledonian metamorphism.

Structural model for the Hedmark Basin: discussion

Bjørlykke (1983) and Kumpulainen & Nystuen (1985) explained the formation of the Hedmark Basin by a thermal-mechanical model (McKenzie 1978) . The structural-sedimentary evolution described in the previous sections through spans l to 7 are now thought to have evolved within three major thermal-mechanical phases of gra ben formation (Fig. 5): (A) Initial stretching of the upper brittle layer of the continental crust causing steeply dipping normal faults, as in the graben model of Gabrielsen (1986); (B) A main phase of crustal stretching gi ving rise to asymmetric lateral expansion of the graben structure, volcanism, sub-basins and local structural inversions; and (C) Thermal cooling and regional subsidence.

Kumpulainen & Nystuen (1985) applied a ther­mal domal uplift model (Artyushkov 1973; Kins­man 1975; McKenzie 1978; Bott 1980) to explain the early graben structures of the western Bal­toscandian basins. Domal uplift would probably cause erosion of a pre-rift sedimentary cover sequence. In the case of crustal thinning by lateral stretching a pre-graben sequence would possess the potential necessary for preservation due to subsidence prior to crustal rupture (e.g. Harding 1984). In the Hedmark Group, the Storskarven Formation, consisting of fine- to medium-grained and well-sorted fluvial sandstone, is suggested to

NORSK GEOLOGISK TIDSSKRifT 67 (1987)

be such a pre-graben unit beneath the graben­infilled Rendalen Formation. This interpretation favours an early stretching phase and fiexural subsidence instead of domal uplift and erosion .

The main graben of the Phase of initial crustal stretching (A) probably formed when a wedge­shaped elongated block subsided along steeply dipping tensional faults. Bott (1976) calculated that this wedge subsidence mechanism could account for subsidence in the order of 2 km for sediment-filled 40-50 km wide basins . This value corresponds to the estimated thickness of the lower part of the Brøttum Formation which orig­inated during this phase (Figs . 5 & 7) .

The Main phase of crustal stretching (B) includes the spans of basin expansion (2), vol­canism and basin submergence (3) and sub-basin formation and emergence (4) (Figs . 5, 9, 10 & 1 1) . It is suggested that the Imsdalen Fault evolved into a strike-slip fault at the beginning of this phase. This could have happened through a slight change in the orientation of the principal stress axes causing a divergent strike-slip fault (transtensional, Harland 1971).

An effective mechanism for lateral spreading of an initial rift or graben structure is crustal thinning by listric or planar normal faulting (e .g . Montadert et al . 1979; Davis 1980; Bally et al . 1981; LePichon & Sibouet 1981; Wernicke 1981; Wernicke & Burchfiel 1982; Stewart 1983) . Gibbs (1984) applied this model for explaining Mesozoic graben formations in the North Sea area and suggested that successive listric extensional nor­mal faults joined at depth in a fiat-lying or slightly dipping regional sole thrust . In the Basin and Range province of the western United States such sole thrusts are thought to be located within cru­stal basement rocks as ductile shear zones at depths of 6-15 km (e .g . Proffett 1977; Bally et al . 1981; Stewart 1983) . If such zones were formed during the phase of expansion of the Hedmark Basin, then pre-Caledonian ductile shear zones might be present within thrust basement rocks in the Osen-Røa Nappe Complex .

Rotation of blocks along listric normal faults can well account for the formation of sub-basins and local structural highs (or inversions) accompanied by erosion of intrabasinal rocks. Syn- and antithetic faults in platformal and mar­ginal positions relative to the main graben in such a rift system can also give rise to subsidiary small er grabens and horsts (Gabrielsen 1986) .

The transgressive event within the Main phase

TSGS Symposium 1986 415

of crustal stretching (B) could have resulted from a eustatic sea-leve! rise . An alternative expla­nation of the submergence is that this occurred in response to incipient thermal cooling and sub­sidence of the basin . However, this view is opposed by the fact that the Hedmark Basin was affected by basaltic volcanism during deposition of the transgressive Atna sands . During this phase, fault movements along major marginal faults, structural inversions and sub-basin tec­tonics caused complex facies interrelations between conglomerates, sandstones and shales by progradation of all u via! fans and fan-delta systems into both continental and marine structural depressions .

Bjørlykke (1983) proposed, from a consider­ation of formation thicknesses in the Mjøsa type section (Fig . 2), that the Phase of thermal cooling (C) started when the Brøttum Formation had been deposited . However, and also as pointed out by Bjørlykke (1983), the volcanic episode at the 'early Biri time' still indicates a high heat fiow during this phase in the history of the basin . In the present interpretation, incipient crustal cooling and waning fault activity started in middle to early late Vendian times at about the onset of the Varangerian glaciation . During the glaciation and the early postglacial period, deposition and sediment preservation were controlled by the complex interaction of glacio-isostatic subsidence and glacio-eustatic sea-leve! drop followed by glacio-isostatic rise of sea leve!, overprinted by the effect of local, tectonic subsidence in sub­basins, regional subsidence due to thermal cooling and subsidence from compaction and isostasy (Figs. 5, 13 & 14) .

During the late Phase of thermal cooling (Figs. 5 & 15), the Hedmark Basin accumulated on ly a bo ut 200-300 m of sediments, most of them being deposited on alluvial plains and in shallow­marine settings . The basin never reached a stage of sediment starvation and water depths of hun­dreds of metres as, for instance, the North Sea grabens in late Cretaceous and Tertiary times (Ziegler 1982) . Bjørlykke (1983) explained this as the result of either a low geothermal gradient or a long cooling phase including the time span from the end of Brøttum sedimentation to the onset of 'Holmia-stage' deposition in the Early Cambrian (c. 580 Ma). From the discussion above, I prefer the first of these two alternative explanations . A low geothermal gradient can be due to moderate crustal thinning and/or high

416 J. P. Nystuen

thermal conductivity of the sandstone-dominated basinal sequence.

The Early Cambrian transgression ended the 'life' of the Hedmark Basin . This transgression is generally considered to have been caused by a te&tono-eustatic sea-leve! rise due to sea-floor spreading in the Iapetus Ocean . In the closure stage of Iapetus in the Middle Ordovician to Silurian time, the Hedmark Group was displaced by nappe translations towards the southeast, and foredeep basins evolved on top of the upper Pro­terozoic Hedmark Basin sequence (Nystuen 1981; Bjørlykke 1983; Bockelie & Nystuen 1985; Nick­elsen et al . 1985) .

The time-scale presently available for the his­tory of the Hedmark Basin is very uncertain and, for this reason, calculations of sedimentation rates are regarded to be of little significance .

Conclusions

l. The Hedmark Basin is an aborted rift basin that was initiated by crustal stretching in late Riphean to early Vendian time, probably about 750 Ma ago . Several other basins were formed during this Baltoscandian rift episode and are present as palaeobasins in the Cale­donian Thrust Belt of Scandinavia.

2 . An elongate gra ben was formed during a Phase of initial crustal stretching by subsidence along steeply dipping normal faults . Turbidite sand­stones and hemipelagic organic-rich mud were deposited in the basin . A remnant of a possible pre-rift sequence is preserved as fine-grained fluvial sandstone .

3. Further crustal stretching during latest Riphean to middle Vendian time (c . 720-650 Ma) comprises the Main p hase of crustal stretching. Widening of the rift probably occurred by listric normal faulting . Alluvial sedimentation, a marine transgression with siliciclastic and carbonate deposition, basalt volcanism, local structural inversions, sub­basin activity and basin margin tectonics gave rise to complex facies variations within this phase. The Imsdalen Fault probably evolved into a strike-slip fault during this phase.

4. The Phase of thermal cooling (c . 650-590 Ma) is characterized by waning sub-basin activity followed by regional and moderate subsidence due to a low geothermal gradient . The Varan­gerian glaciation took place at the beginning

NORSK GEOLOGISK TIDSSKRIFf 67 (1987)

of this phase, and the preservation of various glacial deposits was tectonically controlled . In the latest span of the phase, a sandstone blan­ket was forrned in alluvial plain and shallow­marine environments and onlapped the degraded Baltoscandian craton . Further sub­mergence and transgression of the craton in the Early Cambrian was in response to a eustatic sea-leve! rise brought about by sea­floor spreading in the young lapetus Ocean west of the Hedmark Basin .

6. Black shales were formed at various periods in large and small sub-basins when rates of tectonic subsidence were high and the influx of terrigenous clastic de bris was low . Some of the kerogen may have generated fluid hydro­carbons prior to thermal destruction during the Caledonian metamorphism.

7. The sequence of the Hedmark Basin was displaced by about 230 km towards the SE within the Caledonian Osen-Røa Nappe Com­plex during middle Ordovician to late Silurian time and forms today a rootless palaeobasin within the Caledonian Thrust Belt .

Acknowledgements. - The author wishes to thank Roy Gabri­elsen, Risto Kumpulainen and Snorre Olaussen for valuable

comments on an earlier draft of the manuscript, A. T. Buller

for critical comments and correcting of the English text, Aslaug

Borgan and Liv Ravdal for drawing the figures and Fride W. Bråten and Jill-Renee S. Mørk for typewriting the manuscript.

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