Polarforschung 68,25 - 34,1998 (erschienen 2000)

Cretaceous Magmatism South and East of Svalbard:Evidence from Seismic Reflection and Magnetic Data

By Paul Grogan', Kristin Nyberg-, Bente Fotland', Reidun Myklebust', Sven Dahlgren' and Fridtjof Riis'.

THEME 1: Magmatic Provinces around the Eurasian Basin:Interplay with Tectonism

Summary: In the Svalbard archipelago, late Mesozoic magmatic rocks in theform of basaltic sills and dykes are recorded from the land areas between centraland eastern Spitsbergen and the Hinlopen Strait. In addition, tholeiitic f100dbasalts are identified on Kong Karls Land, where they typically form the sum­mit plateaus ofthe main islands, and in Pranz Josefs Land in the Russian Arctic.Seismic mapping, combined with the forward modelling of magnetic anorna­lies, has revealed the cxtcnt of the magmatic province in the offshore area to thesouth and east of Svalbard. Sills and dykes are identified within sedirnentarysequences of Penno-Carboniferous, Triassie, Jurassie. and Cretaceous age. Theevolution of the basalt province was related to the initial brcak-up of theEurasian-Laurentian supercontinent. This event involveel extension anelvolcanism associated with the opening of the Canada Basin and subsequentformation of the basaltic Alpha Ridge, presently situateel in the Arctic Ocean.The mapping of this province has important implications for the thermalevolution of petroleum source rocks, and the evaluation of the hydrocarbonpotential of the northern Barents Sea.

REGIONAL GEOLOGY

The purpose of this paper is to present the results of the NorwegianPetroleum Directorate's (NPD) preliminary mapping of thedistribution of magmatic rocks associated with early Cretaceousvolcanism in the Norwegian sector of the northern Barents Sea(Fig. 1). The mapping of magmatic rocks has been made possibleby seismic interpretation and constrained by magnetic data, fieldstudies on the Svalbard archipelaga, and shallow stratigraphicboreholes on the continental shelf GROGAN et al. (1999).

The Barents Sea shelf is an intra-cratonic basin bounded by thepassive North Atlantic and Arctic margins in the west andnorthwest respectively, and by the Eurasian basin in the north(GUDLAUGSSON et al. 1998). The platform areas situated south andeast of Svalbard comprise an underlying rift system containingDevonian and Early to Middle Carboniferous sedimentary rocksoverlain by relatively flat-lying or gently folded sequences ofLate Palaeozoic (Late Carboniferous and Permian), andMesozoic (Triassic, Jurassie and Cretaceous) age. Teetonicactivity was characterised by regional extension in the Late Earlyto Middle Carboniferous, and locally throughout the Permian

I Norwegian Petroleum Directorate, P.O Box 600, 4001 N-Stavanger, Norway.Present Address: Amoco Norway Oil Co. EO. Box 8088, N-4003 Stavanger, Norway.

} Present Address; Statoil, P.O. Box 40, N-9400 Harstad, Norway., TGS-Nopec Geophysical Company, Baarsrudveien 2, N-3478 Nrersnes, Norway., Present Address; Fylkeshuset, Svend Foynsgt. 9,N-3110 Tonsberg. Norway.

Manuscript received 10 May 1999, accepted 03 November 1999

(N0TTvEDTet al. 1992), Early Triassic, and also locally in the LateJurassie and Early Cretaceous (FALElDE et al. 1993). In the LateJurassic, the Kong Karls Land Platform and Sentralbanken High(Fig. 1) underwent compression resulting in the inversion ofPalaeozoic rift basins, and the gentle folding of overlyingMesozoic sequences. Cycles of Cenozoic burial and uplift relatedto sea floor spreading in the west, and glacio-isostatic reboundin the Quaternary, have combined to remave any late Lower andUpper Cretaceous and Tertiary sediments that may have beendeposited. As a result, Lower Cretaceous strata of Aptian toAlbian age are probably the youngest subcropping sequencesbelow the thin Quaternary cover (Fig, 1).

The region has been affected by several phases of tectonismsince the establishment of its basement foundation during theEarly Devonian (STEEL & WORSLEY 1984, N0TTVEDT et al. 1992).Three pre-late Devonian orogenies; the Precambrian Baikalian,the Caledonian (Ordovician to Silurian), and the Early to MiddleDevonian Innuitian events (DORE 1991, ALSGAARD 1992) haveeach produced distinct lineament patterns. The initial openingof the Canadian Basin and the formation of the predominantlybasaltic Alpha Ridge in the early Cretaceous (LANE 1997,TARDUNO 1998) led to large scale crustal updoming and exten­sive magmatic activity. These events are now manifest as lavasobserved on and adjacent to Kong Karls Land (Fig. 1), and assills and dykes emplaced into the sedimentary sequence.

THE KONG KARLS LAND BASALTS

In 1992 the NPD conducted field work in the Kong Karls Landarchipelaga (LARssEN 1992) (Fig. 2). One of the main objectiveswas to map the distribution of basaltic extrusive and intrusiverocks, and their effect on the maturation of source rocks. Thearchipelaga is situated within the The Kong Karls Land Platform(Fig. 1) which is characterised by elongate north-northeasterlytrending flexures at Mesozoic levels. These flexures developedabove Palaeozoic normal fractures reactivated as reverse faultsin the Late Mesozoic and Tertiary. The largest island of thegroup, Kongsoya (Fig. 2), is located above a fold of this type.Field relations and evidence from seismic mapping of parallelanticlines situated offshore, suggest that flexuring was probablyinitiated in one 01' more discrete phases of compression andtranspression in the Late Jurassie and Early Cretaceous. Thepresent day structural configuration is probably the result offurther compression in the Tertiary.

25

80'

78'

76'

3'

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............ .;:;:;:; ,'<-.:.- - -:-:-:-:.

.'.:-:-:;:-:-:...........;.....

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19' 27' 35' 43'

80'

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26

Subcrop below Quaternary overburden

~ Basalts at seafloor IIIIJ] Triassie • Jurassie-- - (mostlyupper Triassie)

I~<:}~ TertiaRj sediments Iyingon Te iarybasalts Lower • Middle Triassie

~ Tertiary Permo - Carboniferous

• Basalts of Early Cretaceous age Devonian

~ Cretaceous ~

Pre-Devonian basement...."" ~

Fig. 1: Geological map of the Norwcgian Sector of the northern Barents Sea.

26° 28° 30°I

KONG KARLS LANDBasalt outcrops ABEL0YA

79°-KONGS0YA ~

79°

50I

40I

30I

Km

20I

10I

oI

""-- Snespurvstranda

Kapp l/Altmann "

SVENSK0YA

78°30' -1--------------,--------------,------4-78°30'

26°Fig. 2: Basalt outcrops on theKong Karls Land Archipclago.

The most prominent exposures of the Kong Karls Land basaltsare on the mountain tops (at between 150-300 m above sealevel), and their presence has served to protect the underlyingand surrounding sediments from erosion. They occur both asextrusive lavas and as intrusives. Petrographically they aretypical plateau flood basalts, comprising a single geochemicalpopulation classified as low-Al, low-K tholeiitic to quartz­tholeiitic basalts. Biostratigraphie data constrain the volcanismonly within the period Ryazanian to Cenomanian, whereaspreliminary Ar/Ar isotope dating studies give a Barremian toAlbian age for the basalts.

The basalts are divided into three main units. The two mostdominant occur within the Lower Cretaceous (Hauterivian andyounger) Helvetiafjellet Formation, which are the youngestrocks exposed on the islands. The two units are emplaced atstratigraphically distinct levels and it is considered likely thatthey were emplaced as sills. Basalts which may have beenextruded as lavas are restricted to Retziusfjellet andHärfagrefjellet on the island of Kongseya (Fig. 2), and fieldrelations suggest that the lavas appear to have flowed out overan almost flat landscape.

A third unit, most probably an intrusive body, is also recordedlocally in the Helvetiafjellet Formation. Intrusions are alsoobserved within the Upper Jurassie Agardhfjellet Formation andin older sequences on both Kongsoya and Svenskeya islands(Fig. 2). Many intrusions appear to have been emplaced atdepths of less than 100 m into relatively unconsolidatedsediments which makes it difficult at many localities todistinguish them from lavas. At Snespurvstranda on Kongseya(Fig. 2) the lands cape exhibits a distinctive hummocky relief,where the hummocks are interpreted to represent the more mas-

sive parts of a large intrusive body. At other localities, forexample at Alkenebbet on Svenskeya (Fig. 2), intrusive magmaappears to have pushed and distorted the host rock sedimentsof the Agardhfjellet Formation.

Evidence indicating that the lavas were extruded over a rela­tively flat landscape constrains the volcanism to after the initialphases of formation of the Kongscya anticline when theresulting relief was infilled by the fluvial sediments of the lowerpart of the Helvetiafjellet Formation. This is confirmed byunconformities on seismic data in the offshore areas whichindicate that neighbouring anticlines were initiated in the LateJurassie during deposition of the Agardhfjellet Formation.

THE OFFSHORE BASALT PROVINCE

The majority of The NPD's seismic data on the Kong KarlsLand Platform were acquired in 1990, 1994 and 1996, but linesacquired during the 1980's have also been utilised ininterpretation work. High resolution seismic has also beenacquired, both as single channel, and as multifold data recordedwith a 500 m long shallow streamer. These data have supportedthe correlation with land geology and in some areas haveenabled the identification of detailed features associated with themorphology of the basalts. The resolution of the UpperPalaeozoic and Mesozoic sequences is generally good in thisarea, but poor data quality resulting from limited penetration ofthe seismic signal is common and often diagnostic for thepresence of basalts.

The seismic expression of the basalts in the offshore (Figs. 3,4) may be classified into five types; a) sills and lavas within the

27

®Kong Karls Land

IFive major sequences ofKong Karls Landcorrelated withseismic

InvertedPalaeozoie basin

®

• Lower Cretaeeous

• Upper Jurassie

Triassie, Lower-Middle Jurassie

Permian

..Cretaeeous extrusives/ intrusives

Fig. 3: Sehernarie profile showing the seismic expression of basalts (not to scale).

Lower Cretaceous expressed as bathymetric highs, b) sills withinthe Upper Jurassic, c) sills within the Triassic, d) sills within theUpper Palaeozoic, and e) vertical or steeply dipping intrusions.

Sills and lavas within the Lower Cretaceous expressed asbathymetric highs.

The sea floor over an area of approximately 2000 km2 to thesouth and east of Kongseya and Abeleya (Fig. 2) is characterisedby tabular bathymetric highs (Fig. 5), which are interpreted tobe erosional remnants of basalts emplaced within the LowerCretaceous Helvetiafjellet Formation. The impedance contrastat the sea floor across these features is high and only multiplesare observed beneath them. The poor data quality is probablyexacerbated by the presence of intrusions within Upper Jurassiesequences immediately below the Cretaceous. It is not possibleto determine whether these are extrusive lavas or sills, but beingthe southernmost evidence on seismic for basalts within theLower Cretaceous, they also represent the only candidates forextrusive lavas in the offshore areas.

Sills within the Upper Jurassie

Basalts are apparently absent from the Lower Cretaceoussequences in the other synclines of the Kong Karls LandPlatform (Fig. 1). However, these synclines are characterised bya discontinuous hummocky topography developed within thesequence immediately below the Base Cretaceous unconformity,often deforming the unconformity itself (Fig. 6a). Thisphenomenon is interpreted to result from sills emplaced into theUpper Jurassie Agardhfjellet Formation. The hummockytopography and deformation of the Base Cretaceous are

28

reminiscent of field relations previously described on bothKongsoya and Svenskeya.

The seismic character of these intrusions is variable. Althoughmost have hummocky and discontinuous tops (Fig. 6b), othersare apparently flat and smooth over considerable distances. Theyalso lie at different intervals within the unit. The intrusionsappear to be restricted to the synclines where the AgardhfjelletFormation is thickest. They become thinner and die out towardsthe syncline flanks and are nowhere observed to be eroded atthe sea floor. This confirms observations in the field onKongsoya, where intrusions are not observed on the mainanticline, but are well developed on the flanks (e.g at Kapp Alt­mann and Snespurvstranda; Fig. 2). It appears that the basaltshave followed a path of least resistance and are preferentiallyemplaced into the thicker Agardhfjellet Formation whichinvariably includes black shales. These sills cover an area ofbetween 15 000 - 20 000 km2 (Fig. 4) south and east of KongKarls Land.

Sills within the Triassie

High amplitude reflectors interpreted as sills are observed atdifferent levels within Triassie sequences on the Kong KarlsLand Platform (Fig. 4). Those in the middle and upper part ofthe sequence occur closest to the archipelago (Fig. 7a, b). Theform and lateral extent of sills are often well demonstrated onseismic lines in this area and their masking effect is minimal,suggesting that they may be relatively thin (less than 30 m thick).Those further south are more continuous and apparently thicker(50 m or more), and occur more commonly in Lower Triassiesequences, oftenjust above the near top Permian reflector. Theycommonly mask the underlying Palaeozoic reflectors and gene-

Vertical intrusions

At various locations on the Kong Karls Land Platform, reflectorcontinuity within Mesozoicsequences is disturbed by verticaldiscontinuities which cannot be explained by seismic artefacts.Their close association with bedding-parallel phenomenadescribed above makes them candidates for subvertical, dyke­like intrusions, and this can normally be confirmed bycorrelation with high amplitude magnetic anomalies. Some, butnot all, of these features are associated with reactivatedPalaeozoic faults.

Sills within the Upper Palaeozoic

The aeromagnetic data used in this study comprise over 250 000line kilometres acquired by diverse institutions since the late1960's and reprocessed and compiled in 1993. The mapping andmodelling of magnetic anomalies utilises the magneticproperties of basaltic rocks and, combined with the constraintsprovided by seismic interpretation, provides the basis for theidentification of sills and dykes. The modelling processcompares residual potential field anomalies with the responseof an input model based on seismic interpretation. High-passfiltering is applied in an attempt to remove components derivedfrom deep crustal structures and the relief at the top of the Moho.The same filter is applied to both the observed data and themodel response. Iterative adjustment of the model and finetuning of filters makes it possible to focus on a particular depthinterval at any one time, and for the depth to magnetic sourcesto be estimated. Depth estimates to individual magnetic sourcesare determined by the "half-slope" method described by PETERS(1949). The identification of sources at different depths enablesthem to be classified as near-surface, intra-sedimentary 01'

basement-related.

The presence of sills at shallower Mesozoie levels has resultedin poor imaging of the Palaeozoic sequences over large areasof the northern part of the Kong Karls Land Platform. Close toKong Karls Land however, high amplitide, low frequencyreflectors within the Upper Permian can be correlated withmagnetic anomalies and are interpreted as bedding-parallelintrusions (Fig. 8). Sills within the Permian are more easilyidentified southeast of Hopen where there are no apparentshallower intrusions. In this area the form and lateral extent ofthe sills are weIl defined on seismic data.

On the Kong Karls Land Platform, seismic observations may becorrelated with magnetic anomalies which have sources in theupper 4-6 km of the sedimentary sequence. The ability toachieve accurate correlation is invariably dependant on theimaging resolution of the seismic. In areas where seismic dataare poor it may not be possible to identify the sources of theobserved magnetic anomalies. High frequency and highamplitude magnetic anomalies are commonly observed north of77° 'N. The anomalies have variable wavelength and correlatewith seismic observations at different depths and stratigraphie

35'

35'

'-------H 78'

-r""""'H----t--t79'

30'

30'

o km 80~_-__00äi

, Lavas and sills atLower Cretaceous level

.. Sills atUpper Jurassie level

~ Sills atTriassie level

Sills atPermian level

25'

78'j- "IIl

----~\+d,L-----_J_j7r

Fig. 4: Distribution of Cretaceous basalts identified on seismic on the KongKarls Land Platforrn. Numbers refer to locations of seismic lines of Figures 5to 8.

77'

rate significant multiple energy. There is insufficient dataregarding the detailed lithostratigraphy of the Triassie in thisarea to confirm whether these intrusions are preferentiallyemplaced into shales, although field evidence from Svalbardwould suggest that this is likely. Although Triassie sedimentscontinue to subcrop the sea floor north to approximately 79°0 50'N, there are no significant amplitude anomalies observed northof79°' 25' N.

29

w E

S (twt)

Fig. 5: Seismic seetion illustrating tabular bathymetric highs resulting frorn basalts cmplaced as either Javas or sills within the Lower Cretaeeous. Por loeation seeFigure 4. Seale bar is 1 km.

w

N

E

s

0.0

S (twt)

r.o

0.3

S (twt)

Fig. 6: (a and b) Seismie seetions illustrating the diseontinuous and hummocky morphology resulting frorn basalts emplaeed as sills within the Upper Jurassie.For location see Figure 4. Seale bar is 1km.

30

N s w E

"ClS (twt)

Fig. 7: (a and b) Seismic seetions illustrating sills cmplaced in the Triassie. For location see Figure 4. Scalc bar is J km.

levels; lavas and sills at or near the sea floor (in Cretaceoussediments), intrusives in Upper Jurassie sediments and sills anddykes in Triassie and Late Palaeozoic sediments. Intra-

w

sedimentary magnetic anomalies may be usefully classified intotwo groups which also ret1ect the relative reliablity of correlationto seismic data with depth.

E0.0

/'0

S (twt)

Fig. 8: Seismic section illustrating a sillwirhin the Permian, emplaced beJow acomplex of sills within the UpperJurassic. For location see Figure 4. Scalebar is I km.

31

ANOMALIES WITH SOURCE DEPTHS IN THE RANGE0-1 KM

pattern continues in Svalbard and is correlated with doleritesobserved at different stratigraphie levels on land.

On the Kong Karls Platform there is excellent correlationbetween the distribution of shallow magnetic sourees andpossible lavas and sills within Lower Cretaceous and UpperJurassie sequences (Fig. 9a). These sources give rise to highfrequency magnetic anomalies with variable, but generally low,amplitudes. The intensity of these anomalies decreases south ofabout 78° 40' N, although shallow intrusions are still identifiedon seismic as far south as 77° 40' N. Anomalies sourced atshallow depths die out north of approximately 79° 10' N, whichcorrelates weIl with seismic observations. In areas of intensivemagmatism, such as the northern part of the Kong Karls LandPlatform, many anomalies result from sources at differentdepths, which are effectively interfering with each other.

West of 26"E, towards the Edgeoya Platform, anomalies fromshallow sources are related to intrusives emplaced withinTriassie sequences. Individual anomalies within and south ofStorfjorden exhibiting relatively low amplitudes (in the range5-10 nT), are correlated with the offshore extension of theTusenoya skerries which are situated south of Edgeeya. This

ANOMALIES WITH SOURCE DEPTHS IN THE RANGE 1­4 KM.

Jurassie and Cretaceous rocks are only rarely encountereddeeper than 1000 m in the northern Barents Sea, and anomaliesfrom greater than these depths probably arise from intrusionswithin Triassie and older sequences (Fig. 9b). In general, it ismuch more difficult to establish reliable correlations betweensources from these depths and seismic observations. Often thesesources are poorly imaged on seismic data, particularly wherethey are masked by shallower sills as in the northern Kong KarlsLand Platform. In areas where sediment cover is relatively thin(less than 4 km), these sources will be associated with the topof magnetic basement.

The occurrence of linear, high amplitude anomalies indicates thepresence of vertical intrusions within Triassie sequences, andthese frequently correlate with mapped Palaeozoic faults.Magnetic sources of this type on the Kong Karls Land Platform

19" 27' 35' 19' 27' 35'

76' 76'

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\01/'\.' \,

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I,\\I

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1,,

76'

74'

78'

80"-

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\\

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}(I Hopen

76'

74'

Basalts inSvalbard

(b) Distribution of intra-sedimentary magnetic sources(mostlyintrusives atTriassie, Permian anddeeper levels)

(a) Distribution ofSHALLOW magnetic sources

l2TI 0 - 1 km depth 1- 2 km Q] 2-4km o Trends identified onseismic data

Fig. 9: Maps showing the distribution of magnetic sourees.

32

exhibit a distinct NE-SW trend, extending from northwest of theisland of Hopen towards the eastern part of the Kong Karls Landarchipelago. North of 77° N, low amplitude magnetic anoma­lies reveal the presence of extensive, bedding-parallel intrusionsat various levels within the Triassie sequence which correlateweil with seismic observations. These sills are commonlyobserved on seismic, but exhibit only subtle magnetic signaturesat their edges. The correlation of sills in Palaeozoic rocks is moreproblematic. In some cases apparent sills on seismic produceonly poorly defined 01' discontinuous magnetic signatures,whereas in other areas apparent magnetic sources are notidentified on seismic. The latter can probably be explained bythe relatively low impedance contrasts between magmatic rocksand carbonate host rocks.

South of 77° N, along the flanks of the Olga Basin andSentralbanken High (Fig. 1), magnetic anomalies exhibit an E­W trend, which reflects the underlying Palaeozoic rift systemand demonstrates the presence of intrusives along its controllingfaults. Similar patterns are observed around the Sorkapp Basinand Gardarbanken High (Fig. 1). In these areas, the highfrequency character of the anomalies becomes more sporadic,but the faults are easily identified on seismic data. This closeassociation of magnetic anomalies with fault trends is a goodindication that the faults have been utilised as conduits for risingmagma, although it is rare for these intrusives to be recognisedon seismic.

DISCUSSION

The Kong Karls Land lavas appear to form the westernextension of an extrusive magmatic province which extendseastward to Franz Josefs Land (Fig. 10; SOLHEIM et al. 1998,NTAFLOS & RICHTER 1998). Taken as a whole, the volcanicprovince inclusive of intrusive dykes and sills extends westacross Nordaustlandet, Barentsoya, Edgecya and into centralSpitsbergen. Throughout the province, the dating of basalts byK-Ar methods is inconclusive due to a combination of poor K­content and hydro thermal alteration, but a concentration of agesin the range 160-80 my has emerged from the literature (BUROVet al. 1976, CAMPSIE et al. 1988, BAILEY & RAsMussEN 1997).

The Kong Karls Land basalts form part of an extensivemagmatic province whose evolution was related to the initialbreak-up of the Eurasian-Laurentian supercontinent, resultingin widespread basaltic volcanism over large areas of the presentday Arctic Basins during the Mesozoic. This event involvedextension and volcanism associated with the opening of theCanada Basin and subsequent formation of the basaltic AlphaRidge, presently situated in the Arctic Ocean. In the SverdrupBasin in the Canadian Arctic, basaltic magmatism persistedthroughout much of the Cretaceous (EMI3RY & OSADETZ 1988,EMI3RY 1991, TARDUNO 1998). The Kong Karls Land basaltscorrelate stratigraphically most closely to the Paterson Island(Valanginian to early Barremian) and Walker Island members

30' 41)'

Fig. 10: The Cretaceous magmatic province between Svalbare! ane!Franz Josefs Lane!.

33

(Barremian to Aptian) of the Isachsen Formation. In both FranzJosefs Land and in the Canadian basins, the lava sequences aremany hundreds of metres thick and are accompanied by sills andextensive dyke swarms, suggesting that the present Kong KarlsLand archipelago was situated at the periphery of the province.

The early Cretaceous magmatic event has important implica­tions for the petroleum geology of the region, particularly forthe maturation of source rocks and the timing of hydrocarbongeneration. This not least because both field and seismicobservations indicate that the basalts tend to be emplaced withinJurassic, Triassie and possibly also Upper Palaeozoic shaleswhich represent potential hydrocarbon source rocks. Fieldobservations on Svalbard and subsequent analyses indicate thatthe thermal contact metamorphic effects of sills on maturationcan locally be very great. The intensity ofthe thermal effect withdistance from the interface with the host rock is generally pro­portional to the thickness of the sill, but will be modified byother factors such as the temperature of the magma and thetemperature difference between the magma and the host rock.Because of surface area considerations, many thin sills may havea similar effect to a single, thicker intrusion. The regional ther­mal effect is likely to have been been minimal for Upper Jurassieand Cretaceous shales lying at or near the surface, since they hada significantly lower temperature than either extrusive lavas orintrusive magma during the emplacement of the basalts. ForTriassie and older shales at greater depths, and at relativelyelevated temperatures, magma may be able to trigger "early"generation of hydrocarbons in a source rock which is alreadyheated by burial to near the oil window.

ACKNOWLEDGMENT

This paper is published with the permission of the NorwegianPetroleum Directorate. The authors wish to thank MargretheHervik for preparing the figures. The authors acknowledge thefirst regional studies of this type in the Barents Sea which werecompleted in 1994 as cooperation projects between thegeophysical consultants Amarok ASA, various oil companies,the NPD and other academic institutions. Such studies were thefirst to recognise the close correlation between high frequencymagnetic anomalies and the presence of magmatic rocks in theBarents Sea, and the first to combine seismic interpretation andmagnetic modelling in order to distinguish between anomaliesarising from intrasedimentary and basement sources.

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