Polarforschung 69, 85 - 94. 1999 (erschienen 2001)

The Reconstruction of the Early Paleozoic Structureof the Barents Sea Sedimentary Basin

Inferred from Geophysical Surveys along Profile I-AR

By Mark L.Verba l and Tamara S. Sakoulina'

THEME 5: The Barents Shelf and the East Greenland Margin:A Comparison

Summary: The tectonic zoning of the basement and the geological structureof the deeply buried base of the Barents Basin sedimentary cover have longbeen subject of much controversy due to the deficiency of adequate geophy­sical information and lack of perception of the magnitude of lateral displace­ments that occurred in the course of post-mid-Paleozoic rift-related crustalextension. Application of palinspastic reconstructions for geological interpre­tation of new geophysical evidence obtained under federal-supported programof deep crustal investigations enabled better understanding of latest Precam­brian to early Paleozoic basin history which apparently was also stronglyaffected by rifting of ancient crystalline basement. A major early PaleozoicWest Kola Paleo-Trough was recognized for the first time and interpreted asepicontinental connection between Iapetus and Ripheus paleo-oceans. Nofeatures suggesting Caledonian collisional tectonism were detected in eitherearly Paleozoic fill of this trough 01' in underlying essentially undisturbedRiphean sequences resting on crystalline infrastructure; however, as the resultof deep burial beneath younger cover strata in central Barents depression andimpact of strike-slip tension, the units within this stratigraphic interval experi­enced significant compaction and acquired geophysical properties transitionalbetween those typical of high-velocity crust and middle-upper cover deposits.

INTRODUCTION

The Riphean and lower Paleozoic sequences forming the baseof the sedimentary cover of the Barents Sea shelf represent theleast studied portion of the basin fill section; as the result, theircontribution to oil and gas generation potential of this regionremains largely unknown. This paper concentrates on earlyPaleozoic history of the Barents Sea area, whereas availableevidence on composition and hydrocarbon prospects of theNorth European Riphean successions has been summarizedelsewhere (SIEDLECKA et al. 1995, ROBERTS 1995, SIMONOV etal. 1998. OLOVJANISHNIKOV et al. 1997).

Lower Paleozoic sequences on the North European margin areknown on Svalbard, in Scandinavia, in Pechora lowlands andon the northern island of Novaya Zemlya (Fig. 1). All theseoccurrences are not only widely separated around the vast Ba­rents Sea shelf containing record of post-early Paleozoic rift­ing and lateral crustal displacements (VERBA 1977, 1985,1992, GRAMBERG 1988), but also represented by differentlithologies with variable degree of Caledonian deformation.

In Scandinavia classic Caledonian thrust assemblages arecomposed of predominantly out-of-shelf deposits (GAYER

I GNPP SEVMORGEO. Rozenshtein ul. 36, 198095 St.Petersburg, Russia;<verba@sevmorgeo.com>

Manuscript received 29 December 1998, accepted 24 November 2000

1989). On western Spitsbergen the lower Paleozoic faciessuggests active shelf environment. Their Caledonian penetra­tive deformation postulated by HARLAND & GAYER (1972) isstrongly obscured by Cenozoic tectonic imprint and has beenchallenged in later publications (DALLMANN et al. 1988,MANBY 1988, MORRIS 1988, BERGH et al. 1997). An apparentconformity of structural lineations in late Riphean to earlyPaleozoic and younger (up to early Tertiary) sequencessuggests the Cenozoic age of major deformational event,whereas Caledonian tectonism was probably marked mainlyby local intense blue-schist metamorphism associated withfault zones (OHTA 1979) and caused by transpressional stress(HARLAND 1998). On Novaya Zemlya the entire exposed sec­tion, including lower Paleozoic upper slope facies, is affectedby Mesozoic folding (KoRAGO et al. 1992). In the PechoraBasin the lower Paleozoic sequence consists of variable, mild­ly deformed lithologies incorporated in a syneclise platformcover. Clearly, correlation and paleotectonic interpretation ofearly Paleozoic assemblages in all these locations along theperiphery of Barents shelf could be greatly facilitated by betterknowledge of their stratigraphic equivalents in the centralBarents Sea which are likely to provide the criticallinks.

Apparent presence of certain features common in all earlyPaleozoic assemblages around the Barents Sea was noted bymany researchers (KRASILSHCHIKOV 1973, HARLAND &DOWDESWELL 1988, SIEDLECKA 1975, R0NNEVIC et al. 1982and others) and led them to believe that Scandinavian struc­tures might extend beneath the Barents shelf and form there acontinuous ensemble linking together the above mentionedwidely separated bedrock exposures with more 01' less intenseCaledonian signature. All these concepts ignored significantlateral crustal displacements caused by late Paleozoic toTriassie rifting, and neither of them was unanimously ac­cepted. A new insight in the problem appeared possible due torecent geophysical investigations performed in 1995-1998 byRussian specialists along a regional geotransect connecting thesuper-deep Kola well with a deep stratigraphic well drilled incentral Franz Josef Land. In the southern 700 km of the geot­ransect strip marine deep seismic refraction soundings werecombined with reflection studies, ship-borne gravity andmagnetic observations, and aeromagnetic survey (MATVEEV etal. 1996, VERBA et al. 1997). As result, it appeared possible forthe first time to characterize a continuous crustal section trans­versing major structural units of the Barents shelf from thenorthern slope of the Baltic Shield and across the southernslope and the core of the Central Barents High (MITROFANOV& SHAROV 1998; Fig. 2 and Tab. 1) and to use this new know­ledge as a basis for improved interpretation of modern

85

__--.JI Cenozoic Late Paleozoic ­Triasic

Devonian

K\ \' Early Paleozoic

VA Riphean

m Archean - EarlyProterozoic

86

Fig.l: Distribution of riftogenic sedimentary sequences in relation to basement highs.

multichannel seismic data obtained under other projects(SIMONOV et a1. 1998; Fig 3), as well as of earlier seismicevidence (LITVINENKO 1968, TULINA et a1. 1988).

GEOLOGICAL INTERPRETATON OF GEOPHYSICALDATA

The sedimentary cover in the southern part of the BarentsBasin is represented by an almost continuous stratigraphieseetion incorporating sequences of Riphean to late Mesozoieage. The seetion demonstrates the absence of major uncon-

formities and only minor degree of fold deformations sug­gesting that throughout the entire Phanerozoic history theregion was not affected by collision-related tectonism. Distri­bution of supracrustal cover units is predominantly controlledby fault-bounded troughs (Fig. 1, 3) whose rift nature duringpredominantly late Paleozoic and Triassie stages of basin evo­lution was demonstrated in earlier publications (VERBA 1985,GRAMBERG 1988, VERBA 1996). In this contribution the em­phasis is made on preceding structural environments andrelated tectono-stratigraphic assemblages.

The basement of the South Barents Basin is formed by a

Distance V,IH, V,IH, V;lH, V/H, V/H, VJH6 V,/H, V /H, VjHo

30 5.7/0.3 7.3/8.450 5.0/0.2 6.1!!.2 5.9/11.2 6.3/20.5 (6.1)/26 8.1/4090 2.7/0.25 4.3/0.8 5.8/2.8 6.3/15.6 (6.1)/26.5 (6.3)/39150 2.4/0.3 6.0!!.0 5.112.8 6.8/6.2 5.5!! 3.1 6.8/22.6190 , 2.8/0.5 3.4/0.9 4.4/2.4 6.2/3.2 5.3/3.8 i 6.3/8.8 7.2!!3.8 7.0/22.6 8.3/38210 2.6/0.3 3.4/0.8 4.4/2.0 6.5/3.8 5.4/4.9 17.0/10.6 7.2!! 7.5 (6.2)/38270 2.7/0.4 3.7!! .2 4.7/2.7 605/4.0 6.4!! 0.0 16.7/10.0 5.1!!4.4 6.6!! 7.2330 2.9/0.2 3.4/0.6 4.6!! .6 5.4/3.3 7.6/25.5 (6.1)/38 1390 3.110.2 3.5/0.6 4.7/1.7 6.2/3.2 5.6/8.6 16.4/16.3420 3.4/0.2 3.8/0.8 4.8/2.0 5.2/2.9 6.017.8 6.7/20.4 8.0/34450 13.0/0.2 3.8/0.9 4.2/1.8 5.8/3.4 6.3/6.1 15.5/14.6 6.7119.6 (6.0)/40510 2.7/0.3 3.7/1.1 4.3/1.9 4.8/4.5 5.3/6.4 (6.0)/36.5550 2.4/0.2 3.3/0.5 4.4/1.8 4.8/4.4 5.2/5.9

Tab. 1: Selected values of boundary velocities atmajor crustal boundaries and depth of their occur­rence (V/H) from refraction data. Values in bracketsrefer to average velocities obtained from reflectiondata.

87

80 12 24 36 48 60

16

72

Fig. 2a: Location of profiles and ob­servations given in figures. Numbersrefer 10 figures and subfigures.

granite-rnetamorphic layer whose surface is represented by re­fractor F, usually observed at about 12-15 km depth; beneaththe Central Barents Rise it is found at 10 km, whereas towardsthe Kola Peninsula coast it ascends to sea bottom level. Thebasement includes Archean rocks and an overlying Paleopro­terozoic unit whose velocity parameters suggest a somewhatlower metamorphic grade. The interval between the top of the

basement (F,) and a regional seismic horizon Fa long reco­gnized as a base of undeformed Paleozoic sediments (LITVI­NENKO 1968) is interpreted as a Riphean sequence whoseupper part correlates with the terrigenous section described inthe Rybachii Peninsula coastal outcrops (SIEDLECKA et al.1995, ROBERTS 1995). The main portion of this seismicinterval has thickness up to 9 km and is characterized byuniform layer velocities on the order of 5.2-5.3 krn/s believedto correspond to the terrigenous Barents Sea Complex knownfrom the Varanguer Peninsula (GAYER 1989). The presencewithin this sequence of dense low magnetic rocks is suggestedby gravity and magnetic data. Limited, variably inclineddiffraction zones are locally observed in seismic records andbelieved to mark narrow zones of fault-related disturbancesdepicted by Russian and Norwegian magnetic surveys. This

88

agrees with the earlier notion that this area was not affected bylarge-scale collisional folding (AM 1975).

Velocity parameters of the Riphean sequence are consistentwith the interpretation of its composition as dominated by lowgrade terrigenous lithologies. Indeed, at shallow depths (0.5-

1.0 km below sea bottom) the boundary velocities at FO do notexceed 5.7-5.8 km/s but seaward these values graduallyincrease to 6.1-6.3 km/s and 6.4-6.5 krn/s at 5 km and 10 kmdepths, respectively. Such good correlation between velocityand depth probably indicates a lateral continuity of Ripheanlithologies, as well as preservation of their ability for com­paction which is in marked contrast with much stronger meta­morphosed basement complexes. Similar conclusions can bemade on the basis of analysis of organic matter which in rocksof the Rybachii Peninsula shows only relatively low degree ofmaturation corresponding to intermediate catagenic stage(SIMONOV et al. 1998). On the other hand, even the lowestvelocity values (5.7-5.8 kmJs) imply the presence of suffi­ciently dense rocks. On the whole, the data suggest thatRiphean sequences away from northern Scandinavia and theKola Peninsula become progressively less deformed and must

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therefore be considered as the base of the sedimentary cover ofthe Barents Basin.

The overlying Phanerozoic section includes three major tec­tono-stratigraphic assemblages. Of these only the two upperones were characterized by earlier MCS surveys and reliablystratified with reference to well data as "upper terrigenousBarents Sea complex" (late Permian to early Cretaceous) and"middle complex" (Devonian to early Permian, possibly in-

cludeing late Silurian) (GRAMBERG 1988). Processing of newgeophysical data aIlowed to recognize for the first time that anolder undeformed cover sequence was present between the topof the Riphean succession and the base of the "middlecomplex". Its maximum thickness reaches 2.5-3.0 km anddelineates the position of a fault-bounded West Kola Troughwhich apparently controlled the sedimentation. The conforma­ble relationship of this sequence with underlying Riphean andoverlying middle Paleozoic units implies its essentiaIly early

89

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90

Paleozoic age spanning most of Cambrian, Ordovician and,perhaps, lower Silurian stratigraphic intervals, whereas gravitymodeling in combination with seismic interpretations suggestsdistinctly layered, mainly moderately dense terrigenous Iitho­logies probably containing higher density carbonate facieswith boundary velocities up to 6.2-6.4 km/s in the upper partof the section. Lack of evidence of Caledonian folding incombination with graben-like configuration of the West Koladepression may indicate that early the Paleozoic evolution ofthe Barents Basin was to a large extent influenced by initialstages of epi-platform rifting.

DISCUSSION

Despite geographic proxirnity of the West Kola paleo-troughto the Scandinavian Caledonian fold belt, Riphean and earlyPaleozoic sequences accumulated in this depression are verydistinct from their Caledonian equivalents in both compositionof the main stratigraphic units and degree of their structuraldeformation. At the same time, they seem to display muchstronger affinities with more distant coeval successions onSpitsbergen and in the Pechora Basin. For example, thethickness and seismic facies of the lowermost cover sequenceson the Central Barents Rise are comparable to the Cambrian­Ordovician terrigenous Oslobreen Series and Ordovician car­bonate Kirtonryggen Series of Spitsbergen, whereas in the

Pechora Basin they can be matched with the Izhma-OmrinskyComplex (Fig. 4). Although such correlation is based on verybroad lithological characteristics accessible from seismic data,it positively shows the absence of either facial features typicalof deep-water offshore sedimentation, or strong Caledoniancollision-related tectonism. All these sections are affected byonly moderate near-fault deformations presumably associatedwith strike-slip movements. The existence of such lateral tec­tonic displacements were noted by many researches both inthe Pechora Basin and on Spitsbergen, and some of themspecifically stressed a close connection of Caledonian folds onSpitsbergen with major transcurrent faults (HARLAND &GAYER 1972, HARLAND 1998).

Field geological and geophysical data from the West Spitsber­gen zone of Alpine dislocations (MANBY 1988, BERGH et al.1977) and observations conducted by the authors in the St.Johns Fjord area show that the angular unconformity betweenthe Ordovician and late Precambrian sequences is, as a rule,not really a well pronounced one. The Bultinden Series, des­pite its stratigraphic position in place of an epi-Caledonianmolasse, has limited distribution, small thickness and mostlycarbonate composition suggesting low elevation of the sourcearea. Latest investigations have also cast doubt on a tradition alassignment to a Caledonian orogenic sequence of the Devo­nian Old Red Sandstone which occupies one of major Spits­bergen rift-related troughs. The conventional notion of an

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91

independent Caledonian structural event is further challengedby uniform involvement in Cenozoic thrusting of the entirestratigraphic section from the Riphean to Paleogene; there isno appreciable difference in intensity of deformation in oldestand youngest units 01' evidence of more than one structuraldisturbance in the Riphean beds. It may also be noted thatthrusts on Spitsbergen become successively younger to thewest, whereas in Scandinavia their age changes in the oppositedirection (Fig. 5).

The typical Caledonian geology (GAYER 1989) suggests thatclosure of the Iapetus Ocean and the formation of Scandina­vian collisional suture did not occur quite simultaneously. OnFinmark the collision was manifested by orogeny that termi­nated in Wenlockian (by 420 Ma), whereas farther to the southit started later and lasted till about 395 Ma. Judging by changein the age of trusting to southeast from 510 to 475 Ma withinthe distance of 40 km, the propagation of the collisional frontaveraged approx. 1 mm per year (Fig. 6). In West Spitsbergenthe rate of folding was similar though the direction of thrustyoungening was reverse.

It therefore appears that the lithological composition, struc­tural style and post-depositional history of early Paleozoicrocks on Spitsbergen preclude their interpretation as a conti­nuation, 01' a distal analog of the Caledonian collisional sutureof Scandinavia. We consider them as a fragment of an inde­pendent major structural zone (West Kola Trough) thatextended from Spitsbergen towards the Pechora Basin andcontrolled to a large extend the early stages of the evolution ofthe sedimentary cover in the Barents Basin.

The common features recognized in widely separated earlyPaleozoic assemblages on Spitsbergen, Central Barents Riseand in the Pechora Basin suggest their former closer geogra­phical proximity subsequently disrupted by post-early Paleo­zoic tectonic displacements. The latter were most likelyrelated to late Paleozoic to early Mesozoic rifting whose ex­tensional effect must be "subtracted" from the present daystructural array in order to restore the pre-existing relations­hips.

Following such approach, pre-Devonian palinspastic recon-

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92

struction was made for the entire Barents Sea region (Fig. 7)and showed that in early Paleozoic time Spitsbergen, CentralBarents Rise and Pechora Basin were all closely associatedwithin, 01' immediately adjacent to an extended system of sub­latitudinal epi-platform depressions which probably formed anintracratonie link between the Iapetus and Uralian Oceans.Because this system was parallel to Caledonian stress produc­ing the Scandinavian collision suture, it was affected byregional strike-slip displacements which evolved in the samedirection and caused local near-fault disturbances (sometimesaccompanied by blue-schist metamorphism, for example onthe Motafiella Mountain) in the sedimentary fill of the WestKola Trough. The existence of this strike-slip displacementsalong Billefjord fault was supposed already by HARLAND(1964).

CONCLUSION

Understanding the pre-rift crustal structure of major exten­sional basins and assessing the scope of lithospheric extensionthat occurred during their evolution can only be achieved bymeans of palinspastic reconstructions. Paleotectonic sketchesfor the Arctic region based on such approach have alreadybeen published (ZONNENSHAIN & NATAPOV 1987) but only forthe Mesozoic part of its geological history. The same metho­dology applied by the authors to earlier stages of evolution ofthe Barents Basin allowed to recognize an intracratonic rift­related West Kola paleotrough believed to exist in early Paleo­zoie time between Iapetus and Uralian Oceans.

Unlike the Caledonian s.str. (collision-related) and lessseverely folded coeval complexes around the Barents Sea peri­phery, the lower Paleozoie sequence accumulated in the WestKola Trough was not appreciably affected by the Caledonianevent and became an integral part of the Barents Basin sedi­mentary cover in which it formed a transition from underlyingundeformed, strongly lithified Riphean beds composing thebase of the West Kola Trough section to more typieal coverstrata characterized by much larger lateral extent. On Figure 2the lower Paleozoic and other cover sequences appear to forma single mega-clinoform suggesting a monotonous prograda­tional evolution during almost 1.5 Ga away from the northernmargin of the Baltic Shield.

Finally, in the absence of collisional folding in the BarentsBasin throughout such a prolonged period of geologicalhistory it appears very difficult to determine a stratigraphieposition of deeply buried basement to cover boundary and,consequently, to define the age of the basement cratonizationin this part of the East European Platform (as well as in muchbetter studied Spitsbergen and the Pechora Basin where thisissue is still subject to much debate).

The great thickness of the sedimentary fill accumulated along­side ancient continental margins within an extremely largestratigraphic interval and resulting ambiguous basement/coverrelationships require implementation of seismic modeling thatwould be based on imaging geological section as a gradationalrather than discrete environment. Consequently, basin model­ing and hydrocarbon evaluations should include in considera­tion not only those sequences which occur within the "oilwindow" but also older cover sequences found at deeper struc-

tural levels. In particular, the Riphean and early Paleozoiccover units known to contain signifieant amounts of organicmatter in their early stage of catagenic transformation mustconceivably be regarded as possible hydrocarbon prospectswhose generation potential has yetnot been totally exhausted.Numerous oil shows found in these old sediments on Spits­bergen, Rybachi Peninsula and in the Pechora Basin suggestthat coeval sequences on the Central Barents High may alsoappear promising for future discoveries.

ACKNOWLEDGMENTS

Many thanks to Dr. G. Grikurov (VNIIOkeangeologia) forkindly help during all phases of present study and RFFI (Grant00-05-65277) for financial support.

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