Authors:

Valeri A. Basov

Jrg Ebbing

Laurent Gernigon

Marianna V. Korchinskaya

Tatyana Koren

Natalia V. Kosteva

Galina V. Kotljar

Geir Birger Larssen

Tamara Litvinova

Oleg. B. Negrov

Odleiv Olesen

Christophe Pascal

Tatyana M. Pchelina

Oleg V. Petrov

Yugene O. Petrov

Hans-Ivar Sjulstad

Morten Smelror

Nikolay V. Sobolev

Victor Vasiliev

Stephanie C. Werner

ISBN 978-82-7385-137-6

ATLA

S Geological H

istory of th

e Baren

ts Sea

Morten Smelror, Oleg V. Petrov, Geir Birger Larssen & Stephanie C. Werner (editors)

ATLAS

Geological History of the Barents Sea

Geological History of the Barents Sea

Svanem

erket trykksak fra Skipnes K

omm

unikasjon. Lisensnr. 241 731

Morten Smelror, Oleg V. Petrov, Geir Birger Larssen & Stephanie Werner (editors)

ATLAS

Geological History of the Barents Sea

Trondheim, Norway, Juni 2009

ISBN 978-82-7385-137-6

Geological Survey of Norway

Editors:

Morten Smelror, Oleg Petrov, Geir Birger Larssen & Stephanie Werner

Authors:

Valeri A. Basov, VNIIOkeangeolgia, Jrg Ebbing, NGU, Laurent Gernigon,

NGU, Marianna V. Korchinskaya, VNIIOkeangeologia, Tatyana Koren,

VSEGEI, Natalia V. Kosteva, PMGRE, Galina V. Kotljar, VSEGEI,

Geir Birger Larssen, StatoilHydro, Tamara Litvinova, VSEGEI,

Oleg. B. Negrov, VSEGEI, Odleiv Olesen, NGU, Christophe Pascal, NGU,

Tatyana M. Pchelina, VNIIOkeangeologia, Oleg V. Petrov, VSEGEI,

Yugene O. Petrov, VSEGEI, Hans-Ivar Sjulstad, NPD, Morten Smelror, NGU,

Nikolay N. Sobolev, VNIIOkeangeologia, Victor Vasiliev, VSEGEI,

Stephanie C. Werner, NGU

Design and layout: Bjrg Svendgrd, NGU

Front cover design: Bjrg Svendgrd

Front cover photo: Odd Harald Hansen, NGU

The photo is from Nordvestbukta, Bjrnya.

Printed in Norway by Skipnes AS

Bound in Norway by Gjvik Bokbinderi AS

Paper: Pro matt 130 gr

Font: Celeste

Publisher:

Norges geologiske underskelse (Geological Survey of Norway)

Tel.: +47 73 90 40 00 Fax: +47 73 92 16 20

e-mail: ngu@ngu.no

www.ngu.no

Mys Sakhanina, Novaya Zemlya. Photo: Odd Harald Hansen

7Contents

Chapter 1 INTRODUCTION EXPLORATION OF THE BARENTS SEA 9

Chapter 2 IMAGING DEEP STRUCTURES BENEATH THE SURFACE 15

Gravity 16

Magnetics 20

Derivates of the potential eld and structural interpretations 23

Geological structures seen on gravity and magnetic maps 24

Heat ow of the Barents Sea 30

Chapter 3 FROM RIFT - TO MEGA-BASINS 33

Top basement 34

Crustal thickness 35

Isostasy: compensation of sedimentary in ll 36

Isostatic Residual Map 37

Chapter 4 CONTINENTS IN MOTION - THE BARENTS SEA IN A PLATE TECTONIC FRAMEWORK 39

Timanian and Caledonian orogenies and Late Devonian-Early Carboniferous times 42

Uralian Orogeny and subsequent Mesozoic times 44

Stable platforms and pre-breakup basins 46

North Atlantic break-up 52

Chapter 5 LOCHKOVIAN Caledonian mountains in the west, and lowlands and shallow marine basins in the east 55

Chapter 6 FRASNIAN Active rifting, and expansion of the marine basin in the east 59

Chapter 7 VISEAN Extensive alluvial plains in the west and marine carbonate shelves and deep basins in the east 63

Chapter 8 MOSCOVIAN Rising sea level and dryer climate 67

Chapter 9 ASSELIAN Shallow carbonate shelves and deep basins 71

Chapter 10 WORDIAN Temperate climate and extensive marine shelf 75

Chapter 11 INDUAN Uralian uplift in the east and progradation into the shallow-water clastic shelf 79

Chapter 12 ANISIAN Enclosed, restricted basins in the west, uctuating shorelines in the east 83

Chapter 13 CARNIAN Orogen and uplift in the east, extensive westward coastal progradation 87

Chapter 14 HETTANGIAN Wide continental lowlands 93

Chapter 15 TOARCIAN Extensive coastal plains transgressed from east and west 97

Chapter 16 BAJOCIAN Central uplift, maximum regression and prograding coastlines in the west and east 101

Chapter 17 TITHONIAN Maximum transgression on an extensive shelf 105

Chapter 18 VALANGINIAN Open marine shelf 109

Chapter 19 BARREMIAN Tectonic uplift and prograding deltas in the north 113

Chapter 20 ALBIAN Uplift in the northeast, deeply subsiding basins in the west 117

Chapter 21 EOCENE Expanded hinterlands and shrinked basins 121

Chapter 22 LATE NEOGENE UPLIFT AND GLACIATIONS 125

Acknowledgements 128

Literature - References 129

Ch

apte

r 1

Figtextvvklklbkvlbkglbklbkvblnkbvln l nl

IntroductionExploration of the Barents Sea

Guba Sakhanina, Novaya Zemlya. Photo: Odd Harald Hansen

10 Introduction

In the Middle Ages, the Barents Sea was known

as Murmanskoye Morye, the Murman Sea, and

this name can be found on many sixteenth-cen-

tury maps, including Gerard Mercators Map

of the Arctic published in 1595. The Barents

Sea aquired its present name after the Dutch

navigator and explorer Willem Barents. At the

end of the sixteenth century, Willem Barents

led early expeditions to the far north in search

of the North-East Passage to Asia, south of the

Arctic Ocean. He discovered Svalbard and vis-

ited the Novaya Zemlya archipelago. His accu-

rate charting and valuable meteorological data

made him one of the most important of the

early Arctic explorers.

In the following decades many generations

of explorers were attracted to the Arctic, with

a strong desire to discover new land areas and

with the mission to try to nd sea routes con-

necting the European, American and Asian

continents. Some of the expeditions were also

planned to carry out various research tasks.

Several important Russian expeditions

into the Arctic were mounted in the early 19th

century. Fyodor Litke undertook four voyages

to Novaya Zemlya (18211824), and Pyotr Pa-

khtusov travelled there twice, in 183233 and

1834-35, both times involving overwintering.

These expeditions enriched the geographical

sciences with reliable maps of the coastlines of

the entire South Island and part of the North

Island; in the west up to Nassau Cape and as

far as Dalny Cape in the east.

One of the more famous advances in the

history of the Arctic was the expedition led

by the Norwegian explorer Fridtjof Nansen in

189396, with the mission to reach the North

Pole, drifting with the vessel Fram in the ice

and continuing on foot towards the pole. The

mission failed, but Nansen and his crew man-

aged to add a large amount of new informa-

tion concerning the Arctic Ocean. During their

return Nansen and his assistant Hjalmar Jo-

hansen overwintered on Franz Josef Land, and

Nansen brought with him collections of fossils

and rocks from Northbrook Island back to the

museum in Oslo. In 1904 Nansen was the rst

to suggest that the southwestern Barents Sea

had experienced Late Tertiary uplift (approxi-

mately 500 m) and deep erosion. This rst

model of uplift and erosion was based simply

on the shallow bathymetry of the Barents Sea

and the existing geological information about

the surrounding land areas.

In 1899 the rst icebreaker entered the Arc-

tic seas. Under the ag of Admiral Makarov,

the Yermak reached Spitsbergen. Two years

later the ship made its way to Novaya Zemlya

and Franz Josef Land. Yermak also made a

successful pioneer icebreaker voyage through

the Northeast Passage. The same year, the

Norwegian polar explorer Roald Amundsen

on the ship Gja, carried out oceanographic

observations in the Barents Sea, between No-

vaya Zemlya and the Greenland Sea. In the

following years, there were several Norwegian

expeditions to the Barents Sea, Svalbard and

Novaya Zemlya, including the research expedi-

tion to Novaya Zemlya in 1921 led by the geolo-

gist Olaf Holtedahl.

The early explorers

Partcipants of the 11th International

Geological Congress on excursion to

Spitsbergen in 1910.

Photo: Oscar Halldin,

Geological Survey of Norway, NGU

11Introduction

Sea-bed mapping

Equally noteworthy are the activities in the

Arctic in the early 20th century of the team of

military hydrographers led by Andrei Vilkit-

sky, A. Varnek and N. Morozov, who made a

large contribution to the Russian geographical

sciences. Seabed mapping in the Barents Sea

was completed in 1933, with the rst full map

produced by Russian marine geologist Maria

Klenova. Throughout the Soviet era, the Arc-

tic was explored on a regular and systematic

basis, so that by the early 1940s there were no

more blank spots left on the map of the Rus-

sian Arctic.

Today, high-resolution sea-bed mapping by

use of the modern multibeam echosounder is

being carried out in the southwestern parts of

Norwegian Barents Sea within the Mareano-

program. MAREANO maps depth and topog-

raphy, sediment composition, biodiversity,

habitats and biotopes as well as pollution on

the sea-bed in Norwegian coastal and o shore

regions. The program was initiated to ll gaps

in our knowledge of sea-bed conditions and bi-

odiversity as de ned in The Integrated Man-

agement Plan for the Marine Environment of

the Barents Sea and the sea areas o the Lofo-

ten Islands.

600'0"E500'0"E400'0"E300'0"E

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KilometersBathymetry

Topography

Metre

-5 000-4 000-3 000-2 000-1 500-1 000

-750-500-250-100-50

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BARENTS SEA

KARA SEA

NO

RW

EGIA

N S

EA

S V A L

B A R D

N O

V A

Y A

Z

E M

L Y

A

Franz Josef Land

Norway

Russia

Pay Khoy

Kolguyev

Bjrnya

The Barents Sea (Norwegian: Barentshavet, Russian: , lies to the north of Norway and Russia, covers an area of 1.4 million km, and forms a part of the Arctic Ocean. It is a moderately deep shelf, bordered along the shelf edge towards the Norwegian-Greenland Sea in the west, the Svalbard archipelago in the northwest, and the Russian islands of Franz Josef Land and Novaya Zemlya

in the northeast and east. Novaya Zemlya separates the Barents Sea from the Kara Sea to the east.

12 Introduction

In a recent hydrocarbon assessment by the

USGS, it has been estimated that about 30 % of

the worlds undiscovered gas and 13 % og the

worlds undiscovered oil may be found in the

Arctic. The Timan-Pechora region is one of the

worlds most proli c hydrocarbon provinces.

The adjacent Barents and Kara Seas also have

a proven, signi cant, petroleum potential with

numerous giant discoveries. Although there

are large uncertainties regarding the Russian

estimates, there is nevertheless little doubt that

the potential is very substantial.

In the Barents Sea, hydrocarbon explo-

ration began in the 1970s. Prior to the 1980s,

exploration activity in the Norwegian sector

included only seismic surveys and early NGU

aeromagnetic surveys, as drilling north of the

62nd parallel had not been authorised. Subse-

quently, discoveries were made on both the

Russian and the Norwegian sides. The rst

major producing eld is Snhvit in the Norwe-

gian sector. The largest discovery to date is the

Shtokmanovskoye eld in the Russian sector,

which is largest o shore gas eld in the world.

Even though exploration activities have

been going on for almost 40 years, knowledge

of the petroleum potential of the Barents Sea

is still limited. In the Norwegian sector, fewer

than 70 exploration and appraisal wells have

been drilled to date, and exploration in this

vast region is still regarded as being in its early

stage. NPD estimates the total undiscovered re-

sources in the Barents Sea at 6.2 Bboe, with an

uncertainty range between 2.8 and 10.7 Bboe.

Oil in place is put at 1.25 Bboe. The Russian

sector of the Barents Sea is estimated to con-

tain recoverable (P+P) resources of 430 MMbo,

180 MMbc and 96 Tcfg. The Shtokmanovskoye

eld, discovered in 1988, contains gas reserves

(ABC1 category) of some 90 Tcf and condensate

reserves of 150 MMb in several Jurassic reser-

voirs. It is expected to be on-stream in 2010.

During the last 25 years of exploration in

the Barents Sea, substantial geological and geo-

physical data have been accumulated. Most of

our knowledge is based on industrial seismic

data, potential eld data, and data from explo-

ration wells. In addition, there exists detailed

information from continuously cored shallow

boreholes on the Norwegian Barents shelf,

and from several onshore studies on Svalbard,

Franz Josef Land and Novaya Zemlya.

The main hydrocarbon source rocks are

present in the Upper Devonian, Upper Per-

mian, Middle Triassic and Upper Jurassic

successions, while the most signi cant res-

ervoirs are proven in Devonian, Carbonifer-

ous and Permian carbonates, and in Silurian,

Devonian, Carboniferous, Permian, Triassic,

Jurassic and Cretaceous sandstones. Major hy-

drocarbon plays in the Barents Sea have been

proven by the huge gas accumulations in Mid-

dle-Upper Jurassic sandstones in the Russian

Shtokmanovskoye eld, and in Lower-Middle

Jurassic sandstones in the Norwegian Snhvit

eld. In the Timan-Pechora Basin oil discover-

Exploration for hydrocarbon resources

The observatory on Heisa Island, Franz Josef Land. Photo: VSEGEI

Polar bear swimming ashore on Franz Josef Land. Photo: VSEGEI

Memoral stone of William Barentz on the northeastern side

of Novaya Zemlya. Photo: Geir Birger Larssen

13Introduction

ies in the Upper Palaeozoic dominate. The few

wells in the Kara Sea have so far shown a good

potential for hydrocarbons in the Mesozoic.

Studies of outcrops in the Palaeozoic,

Mesozoic and Tertiary successions on Sval-

bard and from Novaya Zemlya and Franz Josef

Land have helped us to unravel the geological

history of the Norwegian and Western Rus-

sian Arctic basins. By the introduction of 3D

seismic surveys, a far better understanding of

the internal morphology of the sedimentary

sequences and spatial facies distribution has

been possible, as illustrated from recent stud-

ies of the Upper Palaeozoic carbonate buildups

on the eastern Finnmark Platform and the Tri-

assic, siliciclastic, shelf deposits in the south-

western Barents Sea.

The expectations for future discoveries are

high. However, there are still many gaps in our

knowledge of the geological history and ba-

sin evolution in this geographically very large

area. Major parts of the region are still to be

considered as exploration frontiers, and the

tectonostratigraphic models linking the east-

ern and western Barents Sea are far from being

thoroughly understood.

In order to address this problem, the Geo-

logical Survey of Norway and the Russian Geo-

logical Research Institute (VSEGEI) agreed to

carry out a joint project on the Geological his-

tory of the Barents and Kara seas the Geo-

BaSe project. The main is to produce re ned

palaeogeographic models for the Barents Sea,

northern Pechora region and Kara Sea hydro-

carbon provinces. The project was joined by

StatoilHydro as contributing and nancial

partner, and by the Norwegian Petroleum Di-

rectorate (NPD) as a contributing partner. The

GeoBaSe project was further nancially sup-

ported by the Norwegian Research Councils

Petromaks-programme.

Prior to our joint project, the published pal-

aeogeographic reconstructions were generally

rather rough (i.e., each map covers relatively

long time spans involving several sedimen-

tary cycles and facies changes) and/or based

on limited datasets. The new project involves

a synthesis of existing geological and geo-

physical data and their interpretation using

an interdisciplinary approach. By combining

new data from the most recent geophysical

surveys, exploration drillings and eldwork,

and closely integrating the regional geological

and geophysical expertise held by the project

groups and collaborating partners, signi -

cantly improved and higher-resolution palaeo-

geographic models are gradually being made

available through the present work. A series

of new paleogeographic maps is based on co-

herent interpretations of the entire Barents

Sea and Kara Sea region. Such an integrated

study of the geological history is expected to

lead to a better understanding of the spatial

distribution of hydrocarbon source rocks and

reservoir rocks in this extensive Arctic region.

Ammonite collected at Northbrook Island on Franz Josef Land by the

Norwegian explorer Fridtjof Nansen. Photo: Geological Museum, Oslo.

Participants of the GeoBaSe-project and the NORGEX expedition to Svalbard in July 2006. Photo: NGU

Ch

apte

r 2

Gravity and magnetic measurements are remote-sensing methods and are made in order to study structures beneath the Earths surface by measuring the effect of different physical properties of rocks (density and magneti-sation) in the subsurface.

Imaging deep structures beneath the surface

16 Imaging deep structures at the surface

Gravity measurements are used at a wide range

of scales and for many di erent purposes. On

a global scale, understanding the details of the

gravity eld is important to determine precise-

ly the rotation and shape of the Earth, which

is needed for navigation. On a regional scale,

gravity measurements can be used to study the

extension and depth of basins and to better un-

derstand their formation. On a local scale, the

gravity method has been used widely for both

mineral and petroleum exploration. For exam-

ple, salt structures are easy to detect because of

the large negative density contrast of salt com-

pared to the surrounding sedimentary rock, at

almost all depths.

When measuring gravity, all known non-

geological e ects on gravity (tidal variations,

Earth rotation, distance from the Earths centre

and topographic relief) are removed, thereby

correcting for the Earths normal gravity eld.

A gravity anomaly is what is left, and an anom-

aly map re ects mainly the unknown compo-

sition and structure of the outer shell of the

Earth, the lithosphere, the part that is of most

interest for geologists (see info box: Interpreta-

tion of gravity anomalies).

To compare gravity anomalies measured

at di erent elevation, the measurements must

be corrected for the gravity e ect caused by

height di erences. The gravity anomaly, which

is corrected for the height above the reference

level, is called the free-air gravity anomaly and

essentially corresponds to a measurement at

zero elevation, which is roughly the ocean sur-

face. On land, corrections are also needed for

the mass between the observation point and

the zero elevation. This gravity anomaly is

called the Bouguer gravity anomaly and is the

standard for geological interpretations. In the

Barents Sea, the interpretation of gravity data

assists in the linkage of structural information

from the eastern and western parts of the Bar-

ents Sea, across the border between Norway

and Russia, where seismic data are not avail-

able.

The gravity data presented here for the

western part of the Barents Sea are based on

land and shipborne measurements provided

by the Geological Survey of Norway (NGU), the

Norwegian Mapping Authority, the Norwegian

Petroleum Directorate, TGS-NOPEC Geophysi-

cal Company and contributions from Norwe-

gian and international universities.

For the eastern Barents Sea and Kara Sea

areas, the Karpinsky All-Russian Geological

Research Institute (VSEGEI) and VNIIOkean-

geologia provided these gravity data by partly

re-digitising contour maps. These maps were

compiled on the basis of medium-scale gravi-

metric surveys conducted over a period of 20

years. In the program World Gravimetric Sur-

vey, MAGE PGO Sevmorgeologia has carried

out systematic, shipborne, gravimetric surveys

of the Barents Sea on a network of pro les with

10-20 km line separation and 3-4 km point dis-

tance along the lines. Di erent Russian institu-

tions have worked on the homogenisation of

the data sets. For the entire territory of the

former USSR, a gravity map on the scale 1: 2

500 000 is provided by VNIIGeophysika.

To homogenise the data measured in the

Russian and Norwegian parts of the Barents

Sea, the geodetic reference systems for the Rus-

sian data set (projection reference: Pulkovo

1942; normal gravity formula: Helmert 1901)

were transferred into the International Grav-

ity Standardization Net 1971 (IGSN 71), and

the Gravity Formula of 1980 for determining

normal gravity were used for the derivation of

anomaly values for the entire map. The data

was cross-checked against satellite gravity

data. The combined data set has been inter-

polated to a square cell of ten-kilometre size

using a minimum curvature method. The nal

grid was also low-pass ltered with a cut-o

wavelength of 20 km. The resulting data set has

a good aerial coverage for the Barents Sea, an

advantage compared to seismic data which are

more focused on certain areas.

Ideally, the gravity data for Novaya Zemlya

and Svalbard need to be corrected for the per-

manent ice cover. Simple assumptions of the

ice thickness on Novaya Zemlya indicate that

the gravity e ect of the ice cover is as high as

20 mGal, and hence makes a signi cant contri-

bution to the gravity anomaly.

Gravity

600'0"E500'0"E400'0"E300'0"E

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A) The map shows the free-air gravity anomaly fi eld (EIGEN-

GL04C) as derived from observations from CHAMP and GRACE

satellite missions, based on the fl ight height (about 300 km or

more) of the satellite. Such models describe only very long-

wavelength anomalies, as gravity decreases with the distance

(1/R2) to the source.

B) The free-air gravity anomaly map derived from the Earth

gravitational model (EGM2008), that incorporates surface and

satellite measurements.

C) The Bouguer gravity anomaly map of the Barents and Kara

Seas, calculated from the free-air gravity model EGM2008. The

terrain-corrected Bouguer anomaly values are computed using

a rock density of 2670 kg/m3. The westernmost part is char-

acterised by high-amplitude positive anomalies, marking the

continental shelf edge (the abrupt transition between shallow

sea and deep oceanic units). Farther to the east, small-scale

anomalies are visible, which are associated with basin structures

and salt domes. The eastern (Russian) part of the Barents Sea

is characterised by medium-scale anomalies, which have been

partly attributed to extraordinarily deep and extensive basin

structures.

600'0"E500'0"E400'0"E300'0"E

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1

40

mGal

A

B

C

Imaging deep structures at the surface 17

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Legend

Offshore gravimetric profiles

Onshore measuring points

Gravimetric map (scale 1:6 000 000)

Gravimetric map (scale 1:2 500 000)

Gravimetric map (scale 1: 200 000)

Norwegian Gravimetric Surveys

Russian Gravimetric Surveys

Interpretation of gravity anomalies

Observed gravity anomalies are a direct indication of density variations in the subsurface that are related to dif-

ferent densities of the material. These rocks can be located close to the Earths terrestrial surface or sea fl oor, or

at depths ranging from about 10 m to more than 100 km. The gravity surveying process measures the sum of all

lateral density contrasts at all depths. Data fi ltering allows one to isolate portions of the gravity anomaly signal

that are of geological or exploration interest.

The important parameter in gravity investigations are the density differences in the subsurface. For the interpreta-

tion of gravity anomaly maps, subsurface models can be constrained by seismic and geological data. A straightfor-

ward interpretation of such maps is often not possible, because of complex subsurface situations and ambiguous

solutions for depth and shape relationships.

Gravity Anomaly

Depth 1

2

3

Gravity Anomaly

Depth

Ambiguity and superposition of potential fi eld

sources. (Left) The same gravity anomaly (here

positive) can be caused by multiple sources at

different depths. (Right) The observed gravity

anomaly changes in width and amplitude

depending on the burial depth, even though the

body has the same shape and density contrast.

Therefore, additional information from geology

and seismic data are useful to better interpret

the observed gravity anomaly.

Map of the gravity measurement sources. For

the western Barents Sea, this map shows the

station density and the ship-tracks along which

gravity was measured. For the eastern Barents

Sea, the distribution of map-sheets, which

were re-digitised for this Barents and Kara Seas

compilation, are shown.

18 Imaging deep structures at the surface

600'0"E500'0"E400'0"E300'0"E

780

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760

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700

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680

'0"N

1

40

mGal0 150 30075

Kilometers

The gravity anomaly map of the Barents and Kara Seas. On land, the combined data sets consist of terrain-corrected, Bouguer anomaly values computed using a rock density of 2670 kg/m3. For the

oceanic area the free-air anomaly is retained.

Imaging deep structures at the surface 19

600'0"E500'0"E400'0"E300'0"E

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0 150 30075

Kilometers

> 600

500

400

300

200

100

50 0 - 50

-100

- 200

- 300

20 Imaging deep structures at the surface

In a similar manner to gravity maps, magnetic

anomalies re ect lateral variations in the dis-

tribution of subsurface rocks and can be used

to interpret changes in structure and rock type

at depth. In this case, density is not the im-

portant property, rather it is the magnetisation

of a rock. Magnetisation is a varying response

of materials in the Earths magnetic eld and

depends, for example, on the amount of tita-

nium and iron present in oxide minerals and

the degree of metamorphism to which a rock

has been subjected.

The Earths magnetic eld changes with

time and ultimately leads to reversals of this

same eld. Such eld reversals occur on geo-

logical time scales and are recorded on the

ocean oor. At mid-ocean spreading ridges,

the direction of the ambient magnetic eld at

the time of formation is frozen into the cool-

ing magma. As a result, a series of stripes in

the total intensity anomalies run parallel to

and symmetrically on either side of the cen-

tral ridge, and these are interpreted as alternat-

ing blocks of normal and reversely magnetised

oceanic crust. Rocks formed at a certain time

remember the actual magnetic eld at that

time, despite having moved or being subjected

to magnetic eld changes.

For the interpretation of magnetic anomaly

maps, not only the structure of the crust but

also the time of rock formation is important.

Magnetisation can vary greatly in the same

rock type, due to di erences in the external

magnetic eld when the rock was formed, and

to variations in the content of magnetic min-

erals. Such magnetisation is often associated

with remanent magnetisation, a type of mag-

netisation that would also be present in the

absence of an external magnetic eld.

To study magnetic anomalies associated

with geological structures, the e ect of the

Earths normal magnetic eld must be removed.

Magnetic eld models of the Earth are calcu-

lated from observatory measurements to give

the so-called International Geomagnetic Refer-

ence Fields (IGRF). Due to the change in the

eld with time, the IGRF is updated every ve

years. Short-term variations such as magnetic

storms, which in the Arctic regions are visible

as northern lights (aurora borealis), need to be

monitored during the survey and corrected for.

Such magnetic variations are not predictable.

NGU and TGS-NOPEC Geophysical Compa-

ny have covered large parts of the Norwegian

Barents Sea and Svalbard with aeromagnetic

measurements. An aeromagnetic survey is nor-

mally own using a magnetometer attached to

an aircraft. The total intensity of the magnetic

eld is then measured along ight lines with

varying spacing. Most of the Norwegian Bar-

ents Sea has been measured at ight altitudes

ranging from 200 m to 1500 m.

Over the eastern Barents Sea, most aero-

magnetic surveys were carried out by VNI-

IOkeangeologia (NIIGA), Polar geophysical

expedition NPO Sevmorgeo and FGUP Sev-

morgeologia between 1967 and 2000. The cov-

erage over the Russian part of the Barents Sea

is shown in upper gure on next page. Meas-

urements were carried out at ight levels of

300, 600 and 3000 m. The line separation was

typically between 5 and 10 km. The mean least

square errors of the aeromagnetic surveys are

in the order of 11 - 14 nT.

The resulting aeromagnetic anomaly map

allows us to characterise the basement underly-

ing the sedimentary basins, as well as to iden-

tify di erent domains as expressed by di er-

ent characteristics in the magnetic anomalies

(see info box).

Magnetics

NS

Simple illustration of the Earths normal magnetic fi eld, which

as a fi rst approximation is similar to the magnetic fi eld of a

bar magnet (dipole) located in the Earths centre. The present-

day, Earths magnetic fi eld axis has 11 degrees deviation from

the rotation axis of the Earth.

Imaging deep structures at the surface 21

600'0"E500'0"E400'0"E300'0"E

780

'0"N

760

'0"N

740

'0"N

720

'0"N

700

'0"N

680

'0"N 0 150 30075

Kilometers

Legend

1987

1991

1999

1998

1999

1999

1976

1962

1983

1980

1975

2000

1976

1973

1985

1967

1971

1971

1972

1982-84

SX 2

SVA 1991

SPA 1988

SEV 1989

SEV 1989

NGU 1970

NGU 1969

BSA 1987

BAMS

2001

2002

2003

2004

2005

Russian AreaSurvey (Year)

Norwegian AreaSurvey (Year)

Interpretation of magnetic anomalies

Rocks can remember the magnetic fi eld direction at the

time when they were formed, but the intensity of mag-

netisation depends on the type and amount of magnetic

minerals a rock contains. This magnetisation is called re-

manent magnetisation. The second contribution of rock

magnetisation is called induced magnetisation and ex-

ists only in the presence of an external magnetic fi eld,

such as that on Earth, which allows one to use a compass

to fi nd the north direction.

The magnetic effect of a dipole (bar magnet), whose mag-

netisation is induced and aligned with the Earths mag-

netic fi eld, is shown as total (solid) and vertical (dashed)

magnetic anomalies. Depending on latitude, these two

curves deviate the farther the observation point is situ-

ated from the pole (i = 90). For interpretations, vertical-

fi eld curves are commonly used. Total-fi eld and vertical-

fi eld curves are similar in high magnetic latitudes, such as

those for the Barents Sea, and no further corrections are

needed for an interpretation. Nevertheless, in interpret-

ing magnetic anomalies, a combination of remanent and

induced magnetisation always needs to be considered.

Overview of the aeromagnetic surveys in the Barents and Kara Seas. Various airborne surveys have been performed over the past 50 years.

4

6

2

0

-2-40 -20 0 20 40

8

I = 30

4

6

2

0

-2-40 -20 0 20 40

8

I = 0

4

6

2

0

-2-40 -20 0 20 40

8

I = 90

4

6

2

0

-2-40 -20 0 20 40

8

I = 60

22 Imaging deep structures at the surface

Imaging deep structures at the surface 23

Structural interpretations can be based on

gravity and magnetic anomaly maps and their

rst and second derivatives. Such derivative

maps make an interpretation of the potential

eld maps easier, as they enhance the gradi-

ents/changes in the potential elds that re-

ect changes in rock properties. These maps

are especially useful for mapping the struc-

tural outline of the bodies in the subsurface

that cause the observed anomalies. The val-

ues represented in tilt-derivative maps can be

used to calculate the location of potential eld

sources, or simply for structural mapping. The

gures on the next two pages show a tenta-

tive structural-morphological interpretation

of the gravity and magnetic anomaly maps,

respectively. Based on a qualitative compari-

son of the eld anomaly and its derivatives,

domains with similar anomaly characteristics

can be outlined. For the interpretation, the in-

tensity of the anomalies (positive or negative),

the direction and strike of the tilt derivative

lineaments and the change in frequency can

be used. Derivative maps can also be used to

analyse the quality of the data compilation, as

boundaries between areas with di erent line

spacing will be visible.

Based on the derivative images, six di er-

ent domains can be observed in the gravity

anomaly map. These domains correlate well

with information about the tectonic setting de-

rived from other sources. The interpretation of

the magnetic anomalies leads to the identi ca-

tion of eight domains with di erent magnetic

patterns. When comparing the interpretation

of the gravity and magnetic anomaly maps,

one can observe di erences in many details.

This is because the same rock formations do

not necessarily produce corresponding gravity

and magnetic anomalies. Therefore, it is use-

ful to rst interpret the gravity and magnetic

anomalies independently, and in a second step

to identify similar anomalies. For example, the

transition from the southwestern Barents Sea

to the Baltic Shield can be identi ed on both

interpretational maps.

Gravity-based domains: I. Western Barents Sea (blue), characterised by intensive

positive anomalies in the gravity anomaly,

weak positive signal in the rst derivatives,

Derivatives of the potential eld and structural interpretations

and negative anomalies in the second-order

derivatives. II. Baltic Shield (pink). This unit

is characterised by positive gravity anoma-

lies or an anomaly eld with broad negative

anomalies, containing locally positive anoma-

lies. III. Eastern Barents Sea basins (bright

green): This unit is characterised by a grav-

ity eld with positive and negative anomalies

and a unit with anomalies of medium intensity

against a weak negative background eld. IV.

Timan-Pechora (dark brown), which is char-

acterised by a weak positive gravity anomaly

eld with positive and with negative anomaly

zones with intensive local positive anomalies.

V. Novaya Zemlya fold system (orange). This

domain is characterised by negative anomalies

and a gravity eld with intensive positive and

large negative anomalies. VI. Franz Josef Land

domain (dark green) is characterised by posi-

tive and locally strongly positive anomalies

and regional anomalies with medium intensity.

Magnetic-based domains: I. Western Barents Sea (blue) characterised by intensive

positive magnetic anomalies. II. Baltic Shield

(pink). This unit is characterised by high-inten-

sity, negative and positive anomalies against

a background of broad positive anomalies.

III. Eastern Barents Sea basins (green). This

unit is characterised by anomalies of low in-

tensity against a generally reduced magnetic

background. IV. Timan-Pechora (light brown)

characterised by strong positive and nega-

tive anomalies. V. Novaya Zemlya fold system

(dark brown), which is characterised by areas

of negative magnetic anomalies of intermedi-

ate intensity, and strong positive anomalies.VI.

Franz Josef Land area (yellow) characterised

by intermediate negative anomalies against

a background of high positive intensity. VII.

Transitional area between Novaya Zemlya and

Kara Sea (orange), which can only be seen in

the magnetic anomaly eld. This unit is char-

acterised by areas of positive and negative

anomalies of low intensity and intermediate

negative anomalies. VIII. Pay-Khoy fold sys-

tem (light green), characterised by a magnetic

eld with dominating intermediate negative

and middle to high-intensity, positive magnetic

anomalies.

The Northern lights. Photo: Bjrn Jrgensen

24 Imaging deep structures at the surface

Geological structures seen on gravity and magnetic maps

The sea oor of the Barents Sea is generally at

with a depth less than 500 metres. Only a few

features are visible: relics of the latest phase

of the ice age, during which large glaciers and

meltwater carved their path in the sediments,

but also deposited new sediments on top. Grav-

ity and magnetic maps therefore are useful for

studying the geological units below the sedi-

mentary overburden.

While the gravity map is useful for under-

standing density variations, the magnetic map

represents variations in the magnetisation.

Generally, sediments have very low magnetisa-

tion, but the underlying rocks are magnetised,

implying that the sediments seem to be trans-

parent. As discussed in the previous section

such maps can be used to de ne domains, but

smaller features can also be interpreted. For

example, near the Billefjord Fault Zone, which

crosses Svalbard from north to south, a promi-

nent positive magnetic anomaly is visible and

represents most likely an upthrusted slice of

Hecla Hoek basement. On the other hand, the

strike-slip Trollfjorden-Komagelva Fault Zone,

located onshore Norway does not have an ex-

pression in the magnetic map at this resolution.

Many of the positive magnetic anomalies in

the Barents Sea area are associated with base-

ment highs, e.g. the Ludlov Saddle, Loppa High

and Stappen high. The latter two anomalies

represent a continuation of the regional mag-

netic anomalies in Troms and Nordland and are

interpreted to re ect the northward continua-

tion of the c. 1.8 Ga Transcandinavian Igneous

Belt extending all the way to southern Sweden.

The local o shore maxima are partly related

to basement highs that are con rmed by coin-

ciding positive gravity anomalies, e.g. for the

Loppa and Stappen Highs, and also observed

on seismic data. The coinciding magnetic and

gravity anomalies may represent metamorphic

core complexes formed during exhumation of

lower crustal rocks along lowangle detach-

ments zones resembling the tectonic situation

along the Lofoten-Vesterlen margin further to

the south. Other highs are de ned by positive

gravity anomalies, but no clear correlation with

either positive or negative magnetic anomalies

is observed (e.g. Senja Ridge and Veslemy

High). One obvious example of a positive grav-

ity anomaly, (positive) magnetic anomaly and a

correlation with exposed magmatic rock types

is found around Srya, Seiland, Stjernya,

and the nearby ksfjord peninsula. Such a re-

lationship between magmatic rocks and posi-

tive magnetic anomalies can also be found o -

shore between Svalbard and Franz-Josef Land,

although the two magmatic provinces are not

related. These latter magmatic rocks possibly

continue towards Franz-Josef Land, but the

resolution of the magnetic map there is too

low. Some of these magmatic-related anoma-

lies could also be associated with gravity highs,

but again the resolution here is limited and a

clear correlation is not possible.

In the westernmost part of the Barents Sea

and also in the far north (not shown on the

map), the transition between shallow conti-

nental shelf and deep ocean is marked by a

strong positive gravity anomaly. These gravity

anomalies represent partly the approximately 2

km thick sedimentary wedges deposited along

the Barents Sea continental margins during

the Plio-Pleistocene glaciations. The weight of

these wedges cause present day subsidence and

seismicity on the continental margin and adja-

cent oceanic crust. Farther onto the shelf, many

small-scale negative anomalies are associated

with graben structures, other basins lled with

sediments and salt diapirism. Sediments and

salt have lower densities than the surrounding

material and are, therefore, discernible as nega-

tive anomalies. Such basins include the Nord-

kapp, Maud and Harstad basins.

A peculiar feature in the magnetic anomaly

map, that is not visible on the gravity map, is a

series of small-scale anomalies east of Svalbard.

These anomalies close to the transition from

continental shelf to oceanic area are related to

magnetic intrusions (sills) into the continental

shelf. Such sill intrusions are a typical feature

at the border of most passive margins and can

be observed all along the edges of the Norwe-

gian shelf. Such sills would also be expected

further to the east, and have been imaged by

seismic and high-resolution magnetic data. The

resolution of the magnetic anomaly map pre-

sented here does not allow identifying these

expected anomalies. Also in the deep Eastern

Barents Sea the presence of deep-seated sills

is known, which are also not re ected in the

magnetic anomaly map, as they are emplaced

in depths greater than 5 km and have typically

a thickness of less than 500 m.

A high magnetic anomaly which does not

correlate directly with the known geological

structure of the Barents Sea, but which may

hold the key to the understanding of its tecton-

ic history and more speci c, the understanding

of the Eastern Barents Sea basins, is located

in the central Barents Sea, directly adjacent to

the wester margin of the Eastern Barents Sea

basins. This anomaly is located directly on the

transition between the Eastern and Western

Barents Sea, where apparently a change in the

style of basin formation occurs.

The relief of the Barents Sea. Structural and tectonic features

are superposed for comparison. The gravity and magnetic

anomaly maps include prominent features such as basins and

basement highs. These features are not necessarily recognised

in the relief, but can be detected on such maps.

Imaging deep structures at the surface 25

600'0"E500'0"E400'0"E300'0"E

780

'0"N

760

'0"N

740

'0"N

720

'0"N

700

'0"N

680

'0"N 0 150 30075

Kilometers

-5 000-4 000-3 000-2 000-1 500-1 000-750-500-250-100-50025501002505007501 0001 500

SCW

600'0"E500'0"E400'0"E300'0"E

780

'0"N

760

'0"N

740

'0"N

720

'0"N

700

'0"N

680

'0"N

Cenozoic

Mesozoic

Paleozoic

Proterozoic

General

Tectonic Structures

BARENTS SEA

KARA SEA

NO

RD

IC S

EA

S V A L

B A R D

N O

V A

Y A

Z

E M

L Y

A

Franz Josef Land

Norway

Russia

Pay Khoy

Kolguyev

Bjrnya

Bjarmeland Platform

Olga Basin

Loppa

High

North Barents Basin

South Barents Basin

Ludlov Saddle

Sentralb

anken Hi

gh

Stapp

en Hi

gh

Nordka

pp Basin

Pechora Basin

Adm

irality

High

s

GardarbankenHigh

Finnmark Platform

Hammerfest Basin

Harsta

d Basi

nTroms

Bas

in

Bjrn

ya Ba

sin

Kong Karl Platform

Senja

Ridg

e

Veslem

y

High

Srv

estn

aget

Bas

in

Vest

bakk

en

Srkapp Basin

Varanger Basin

Central Barents

High

Trollfjord-Komagelv Fault Zone

Storba

nken

High

Perseus High

Svalbard Platform

Norsel

High

Mercu

rius H

igh

600'0"E500'0"E400'0"E300'0"E

780

'0"N

760

'0"N

740

'0"N

720

'0"N

700

'0"N

680

'0"N

0 150 30075

Kilometers

1

40

600'0"E500'0"E400'0"E300'0"E

780

'0"N

760

'0"N

740

'0"N

720

'0"N

700

'0"N

680

'0"N

> 600

500

400

300

200

100

50 0 - 50

-100

- 200

- 300

26 Imaging deep structures at the surface

600'0"E500'0"E400'0"E300'0"E

780

'0"N

760

'0"N

740

'0"N

720

'0"N

700

'0"N

680

'0"N 0 150 30075

Kilometers

18

78

74

70

Lines of correlation disturbance

positive: a - 1st order, b - 2nd order**negative: a - 1st order, b - 2nd order**

IVa b c

Va b c

VIb c

Area of the Pechora Plate, characterised by differentiated weak positive (b) GF with positive (a) and negative (c) ano-malous zones and intensive local positive anomalies

Area of the Novaya Zemlya fold system, characterised bydifferentiated negative (b) GF with intensive positive (a) and negative (c) large anomalies

Area of Franz Josef Land, characterised by differentiated positive (c) GF with intensive positive (b) anomalies and local anomalies of middle intensity

Ia b c

IIa b

IIIa b c

Area of Spitsbergen anticline, characterised by differentiated weak positive (b) GF with intensive positive (a) and negative (c) anomalies

Area of the Baltic Shield, characterised by differentiated posi-tive (a) GF with large negative (b) anomaly and local positiveanomalies

Area of the Barents Sea basins, characterised by differentiated GF with positive (a) and negative (c) anomalies of middle intensity against a weak negative background (b)

* GF gravity field

Axes of anomalies

Local anomalies: a - positive, b - negativea b

main anomaly zones local uplifts

Areas

Boundaries

Structural Lines

Other Symbols

** Anomaly axes of the 2nd order are revealed after the anomalies of gravity from 30 to 30mGal; anomaly axes of the 1st order are revealed after field

of gravity field below 30 and above 30 mGalthe anomalies

A

Magnetic map.

Imaging deep structures at the surface 27

600'0"E

I

V

III

II

VI

IV

787266

60

60

54

54

48

48

42

42

36

36

30

30

24

24

181260

74

70

B C

Structural interpretations of the gravity anomaly map and its derivatives. Interpretation T. Litvinova, VSEGEI

Horizontal tilt derivative. Vertical tilt derivative.

28 Imaging deep structures at the surface

IIII

V

IV

VII

II

VIII

787266

60

60

54

54

48

48

42

42

36

36

30

30

24

24

18

181260

78

74

70

600'0"E500'0"E400'0"E300'0"E

780

'0"N

760

'0"N

740

'0"N

720

'0"N

700

'0"N

680

'0"N 0 150 30075

Kilometers

BA

Structural interpretations of the magnetic anomaly map and its derivatives. Interpretation T. Litvinova, VSEGEI

Gravity map. Horizontal tilt derivative.

Imaging deep structures at the surface 29

VIII

74

70

Boundaries

Axes of anomalies

positive: a - 1st order, b - 2nd order**

ab

negative: a - 1st orderb - 2nd order**

ab

Lines of correlation disturbance

Area of reduced low-intensive AMF*

*AMF- anomalous magnetic field

I

II

III

IV

Area of Spitsbergen anticline, characterised by the prevalence of positive intensive magnetic anomalies

Area of the Baltic Shield, characterised against a back-ground of a large positive intensive magnetic anomaly by differentiated intensive negative and positive magneticanomalies

Area of the Barents Sea Basin, characterised by anomalies of low intensity against a general reduced AMF background

Area of the Pechora Plate, characterised by large inten-sive positive and negative magnetic anomalies

Areas

V

VI

VII

VIII

Area of the Novaya Zemlya fold system, characterised by areas of negative magnetic anomalies of middle intensity and positive anomalies of high intensity

Area of Franz Josef Land, characterised by negative ano-malies of middle intensity against a background of AMF of high intensity

Transitional area distinguished after AMF, characterised by areas positive and negative anomalies of low intensity and negative anomalies of middle intensity

Area of Pay Khoy fold system, characterised by differentiated AMF with prevalence of middle-intensive negative and middle- and high-intensive positive anomalies

main other of blocks

**Anomaly axes of the 2nd order are revealed after the anomalies of magnetic field from 100 to 100nT; anomaly axes of the 1st order are revealed after the anomalies of magnetic field below

above 100 nT100 and

Structural Lines

Other Symbols

C

Vertical tilt derivative.

30 Imaging deep structures at the surface

Heat ow of the Barents Sea

The thermal state of the Barents Sea shelf ap-

pears to be variable and dominated by a NW-SE

trend. Maximum heat- ow values a ect mainly

the Svalbard archipelago region where recent

volcanism and present-day geothermal activity

are observed. Heat- ow values in the SW Bar-

ents Sea are in the range between ~50 and ~70

mW/m2 and can be considered as normal for

a Phanerozoic sedimentary basin. In agreement

with long-wavelength gravity anomalies (i.e.

suggesting a gradual deepening of the base of the

lithosphere), heat- ow values seem to decrease

towards the east (i.e. ~50 mW/m2) and reach typi-

cal cratonic values on the Kola Peninsula.

Understanding heat- ow variation in sedi-

mentary basins is of importance for the suc-

cess of petroleum exploration as petroleum

reservoirs occur mainly within the so-called

Golden Zone, which is limited to the tempera-

ture interval 60 -120o C. Approximately half of

the heat ow in thermally relaxed sedimentary

basins (i.e. older than 60 Myr) originates in

the crystalline basement, while the other half

comes from the mantle. The age and thickness

of the lithosphere also a ects surface heat ow.

Geothermal state of the western Barents Sea

Available public data documenting the present-

day geothermal state of the western Barents Sea

consists mainly of (1) marine heat- ow data

collected by means of gravity-probe measure-

ments at sea-bottom, (2) temperatures meas-

ured in deep exploration wells now released by

the Norwegian Petroleum Directorate, and (3)

four heat- ow measurements in shallow wells

made by Sintef Petroleum Research (i.e. former

IKU) in the 1980s.

Marine heat- ow studies focused on the

continental-ocean transition and the oceanic

crust of the NE Atlantic, where the sea-bottom

is deep enough in to neglect disturbances

caused by short-term variations in seawater

temperatures. The marine data show the ex-

pected increase in heat ow from the Barents

Shelf towards the Knipovich and Mohns ridges.

Because the Knipovich Ridge comes close to the

continent at the Svalbard Margin, the variation

in heat ow from the continent to the ocean is

dramatically sharper there than farther to the

south. As a consequence, large amounts of heat

are expected to be transferred from the Knipov-

ich Ridge to the adjacent Svalbard Margin. This,

in turn, suggests that the Moho heat- ow in

the region of the Svalbard archipelago is much

higher than elsewhere in the Barents Sea region.

The occurrence of Miocene to Pleistocene basal-

tic volcanism and present-day hot springs in

northern Spitsbergen adds geological support

to this hypothesis.

Farther to the south, the SW Barents Sea

meets older oceanic crust and heat- ow values

are generally lower than those measured at

higher latitudes. Local heat- ow maxima (up

to 1000 mW/m2!) are not representative of sta-

ble geothermal conditions but are caused by

gas and uid seepage. Most of the determined

heat- ow values in the ocean remain in the

range between 50 and 70 mW/m2 and agree

reasonably well with the age of the underlying

basement (i.e. 33 to 43 Ma). In the SW Barents

Sea, similar heat- ow values were determined

from shallow drilling projects carried out by

IKU, suggesting that this region of the Barents

Shelf is characterised by normal continental

heat- ow values of ~60 10 mW/m2 (the highest

IKU value being 74 mW/m2). The compilation

of available Bottom Hole Temperatures (BHT)

and Drilling Stem Test (DST) data also shows

that the geothermal state of the SW Barents Sea

does not present, at rst glance, any peculiar-

ity with respect to other sedimentary basins

worldwide. Indeed, the estimated geothermal

gradients from BHT and DST data are ~31 C/

km and ~38 C/km, respectively.

Norway

AtlanticOcean

Barents Sea

Svalbard

Locations of heat-fl ow measurements in the western Barents Sea. Solid circles: marine heat-fl ow measurements (Sources:

Crane et al. 1982, 1988, Sundvor 1986, Sundvor et al. 1989, Eldholm et al. 1999); inverted triangles: released IKU shallow drill-

ing measurements (Sources: Zielinski et al. 1986, Sttem 1988, Lseth et al. 1992); open circles: locations of exploration wells

for which temperature data are available (www.npd.no). KR = Knipovich Ridge, MR = Mohns Ridge.

Imaging deep structures at the surface 31

It is well known that even corrected BHT

are, in general, biased towards lower values, so

that the ~38 C/km value is our best estimate. In

the absence of more detailed data on the ther-

mal conductivity of the sediments encountered

in the wells, only a crude estimate of the heat

ow can be made. However, considering that

the sedimentary pile of the SW Barents Sea

contains mostly shale-dominated deposits, we

can infer that the bulk thermal conductivity is

generally low. Assuming a bulk conductivity in

the reasonable range from 1.4 to 1.8 W/mK and

an average thermal gradient of ~38 C/km, our

heat- ow estimation derived from DST data

ranges from ~53 to ~68 mW/m2 and remains in

good agreement with IKU determinations.

Geothermal state of the Eastern Barents Sea

Geothermal studies on the Eastern Barents Sea

shelf started in the 1970s. At rst, probe meas-

urements were carried out with the maximum

sounding borer penetrating the sediments

down to a depth of 2 metres. Results of these

measurements were not adequate, since near-

bottom temperature variations had a distort-

ing e ect. In the 1980s, heat- ow values were

determined in 67 boreholes; each borehole was

located in the vicinity of a Deep Seismic Sound-

ing (DSS) pro le.

In 1976, geothermal gradients (GG) were

measured along the central Barents Sea

transect, and the thermal conductivity of sedi-

ments was studied on board the research vessel

Academician Kurchatov. The pro le location

is in line with the Kola superdeep borehole

SG-3 located in the Pechenga Trough.

The main task of the thermal eld investiga-

tion along the geotraverse was to investigate

the heat ow on the shelf. The objectives were

further to measure the nature of the heat ow

at shallow depth conditions, and to estimate

the background heat ow with and without the

in uence of deep sea-bottom currents.

In the presence of near-bottom currents, de-

termination of heat ow using standard equip-

ment is almost impossible. Heat- ow values

close to a deep (background) value are record-

ed only near Franz Josef Land. The mean heat-

ow value, according to 4 observation stations

along the pro le, amounts to 54 mW/m2 and re-

ects the Palaeozoic age of crust consolidation

in the given region. Estimated heat- ow data

are constrained by heat- ow measurements in

the Grumantskaya well on Spitsbergen. This

well has been drilled to about 3200 m depth,

and shows heat- ow values in the order of 52

8 mW/m2.

In the south, in the area of the SG-3 well, the

heat ow amounts to 40 4 mW/m2. Although

scattered, the determined heat- ow values pro-

vide a preliminary picture of the possible back-

ground heat- ow values of the Barents Sea.

In the Eastern Barents Sea, the interval of

possible oil generation and oil accumulation

can be found at depths between 4 and 6.5 km.

The temperature during oil formation at these

depths reached 110 -140 C and has remained

almost unchanged up to the present day.

Themal conductivity

Thermal conductivity for shallow sediments in

the Barents Sea varies on average from 1.04 to

1.55 W/mK, and correlates well with bathym-

etry. For example, in the Murmansk and Ry-

bachi Banks, thermal conductivity is increased

and reaches 1.04-1.42 W/mK. The zone of max-

imum thermal conductivity occurs on the Mur-

mansk Bank (the rst metre of sediments).

Thermal conductivity gradually smoothes

out at depth and averages 1.17 W/mK. The

Samoilov Trench is characterised by a ther-

mal conductivity reduction to 0.88 W/mK.

Thermal conductivity of the sediments at this

station increases gradually with depth and

reaches 1.26 W/mK at a depth of 3 m. There

is a clear correlation between thermal conduc-

tivity and sea-bottom topography, where a low

topographic relief results in a reduction of the

thermal conductivity.

In general, the distribution of thermal con-

ductivity on the shelf is more complex than in

the oceanic domain. There is also a strong corre-

lation between thermal conductivity and lithol-

ogy. The mean thermal conductivity, 1.05-1.09

W/mK, along the Rybachi Peninsula Franz

Josef Land section is higher than the recorded

value in the deep ocean, 0.84-1.04 W/mK.

Generally, the uncertainty of thermal con-

ductivity measurements is 3-5%.

BHT and DST data from the western Barents Sea (www.npd.no). Well

locations are given in the map.

Heat fl ow along the central Barents Sea transect.

Ch

apte

r 3

Photo: stock.Xchng

From rift basins to mega-basins

34 From rift basins to mega-basins

The term top basement describes the horizon

at which the sedimentary load is separated

from the crystalline bedrock, and it therefore

also represents the base of the basins. Typically,

sedimentary rocks have weak magnetisation,

whereas the underlying bedrock commonly has

a stronger magnetisation. Due to this contrast,

the transition from sediments to basement

causes a distinctive set of magnetic anomalies

that can be used to estimate the depth to the

basement. In the Barents Sea, estimates of the

top basement depth are based mainly on the

interpretation of aeromagnetic maps and com-

bined with interpretation of re ectors found

in shallow- and deep-seismic lines. These pre-

existing studies focus on either the western or

the eastern Barents Sea or have only limited

resolution along the transition between the two

areas, partly due to the disputed political border

between Russia and Norway.

For the southwestern part of the Barents

Sea, this horizon is interpreted at a high reso-

lution (5 km x 5 km) from aeromagnetic depth-

to-source estimates combined with a variety of

industrial shallow- and deep-seismic lines. The

accuracy of the depth-to-basement estimates

from the aeromagnetic data is of the order of

+/- 1 km for the deepest parts of the basins, but

can be better where seismic data are used as

a constraint. Depending on the methods and

databases used, di erent studies may result in

di erent estimates of the basement depth.

In the Barents Sea region as a whole, the

top basement usually lies deeper than 10 km

and large di erences can be observed between

the western and eastern parts. In the western

Barents Sea, the top basement has a depth of

up to 14 km and re ects a series of narrow ba-

sins, whereas in the eastern Barents Sea, the

top basement occurat up to 20 km depth and

re ects the presence of two, broad, mega-scale

basins, the North and South Barents basins.

The basement map combines two of the recent

compilations and is best constrained along

available, wide-angle, seismic lines.

Considering a water depth of about 400

m all over the Barents Sea, the basement map

also indicates the sedimentary thickness, an

important parameter for petroleum resources.

In the western Barents Sea, the thickest sedi-

mentary sequences are found in basins, such

as the Nordkapp Basin, where sediments have

accumulated since the Devonian and crystal-

line basement is found at a depth of about 8

km. Generally, the thickness of the successions

is about 6 km in the platform units and less over

basement highs where the youngest sedimen-

tary layers are often missing. In the eastern Bar-

ents Sea, basins were lled during three main

periods, each of which is represented by the

deposited sedimentary rocks. The lowermost

sedimentary rocks correspond to the Caledoni-

an stage of regional development and can be up

to 500 Myr old. These deformed rocks are fol-

lowed by Devonian deposits that are overlain

by Carboniferous-Lower Permian strata, prob-

ably belonging to the Early Hercynian stage of

development. Constant seismic velocities (5.2-

5.5 km/s) show the homogeneous composition

and thickness of this second layer along almost

the entire pro le crossing the Eastern Barents

Sea basins. The third and uppermost sedimen-

tary succession formed during the Late Permi-

an, Triassic, Jurassic and Early Cretaceous time.

Top basement

-2

-16

-20

-10-12

-14-16-18

-10

-8

-10

-8-14

-10

-8

-8

-8

-8

-8

-8 -8

-8

-6

-8-8

-8

-4

-6

-4-6

-6

-4

-4

-4

-6

-4

-10

-6

-6

-10

-6-2

-4-4

-8

-6

-6

-14

-2

-6-4

Legend

Landmass

Depth to Basement (km)

Depth-to-basement in the Barents Sea compiled

after Skilbrei et al. (1991/1995) for the western part

and Gramberg et al. (2001) for the eastern part.

From rift basins to mega-basins 35

Crustal thickness

The crust is the outer solid shell of the Earth

that sits on top of the more plastic mantle. The

thickness of the crust varies depending on

whether it is of continental or oceanic origin.

The oceanic crust is much thinner than the con-

tinental crust. Nevertheless, some oceanic ar-

eas on Earth, such as the Barents Sea, are com-

posed of stretched continental crust and have

an intermediate crustal thickness. These areas

are usually found at the edges of continents and

are called continental shelves.

The horizon that de nes the boundary be-

tween the crust and mantle is called the Moho

after the Croatian seismologist Andrija Mo-

horovicic. The Moho can be de ned in di er-

ent ways: chemically, petrologically, through

seismic velocity changes, or by a density dis-

continuity. Typically, the density changes from

around 2800-3000 kg/m3 at the base of the crust

to 3200-3400 kg/m3 in the upper mantle. Thus,

the Moho is associated with a large density con-

trast and produces the main signal in regional

gravity anomalies. The gravity anomaly can be

used to calculate the crustal thickness if it is as-

sumed that the Moho indicates a density jump

and that the crust is oating on the mantle so

that the relief is compensated at depth. In the

Barents Sea, the gravity anomaly suggests only

small variations in the thickness of the crust

throughout the area. A map of the crustal thick-

ness calculated from gravity is shown below.

Detailed knowledge of the thickness and de-

formation history of the crust is a key to under-

standing the geological history of a region. The

map shows the Moho boundary as de ned in

the Barents50 model. This model has a lateral

resolution of 50 km and is mainly a seismic-

velocity model of the crust in the Barents Sea.

The velocity model is based on 2D wide-angle

re ection and refraction seismic data, passive

seismological stations and, to a limited extent,

potential eld data. The seismic Moho of the

Barents50 compilation is generally at over

large parts of the Barents Sea region. From the

continent-ocean-boundary in the west to No-

vaya Zemlya (east), the Moho depth is on aver-

age 35 km, while in the western Barents Sea the

depth (32.5-35 km) is slightly less than in the

eastern Barents Sea (35-37.5 km). The strongest

variations are related to the o shore-onshore

transition around Novaya Zemlya and the

mainland to the south where the Moho deep-

ens to more than 40 km.

From simple models of crustal extension,

the correlation between the top basement and

Moho geometry is such that deep basins are

underlain by thin crust (shallower Moho).

This is a typical observation for rift basins,

and comparisons between top basement and

Moho maps show that this observation is, in

general, true for the western Barents Sea. How-

ever, in the eastern Barents Sea such a correla-

tion is not observed. To a large extent, the total

crustal thickness appears to be una ected by

the broad, deep basins, and other mechanisms

have to be considered to explain the crus-

tal structure underlying the North and South

Barents Sea basins.

A. The isostatic Moho depth refl ects the isostatic compensation for loading by topography, bathymetry and sedimentary rocks. The reduced loading due to low-density sedimentary rocks leads to a

shallower Moho than observed on seismic profi les. B. Moho map of the Barents Sea derived from seismic data (dotted lines show regional seismic transects) by Ritzmann et al. (2007).

< - 4

0 - 3

8 - 3

6 - 3

4 - 3

2 - 3

0 - 2

8 - 2

6 - 2

4 - 2

2 >-

20

km

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! Seismic Profiles of the Barents50 Model (after Ritzmann et al. 2007)

B

36 From rift basins to mega-basins

Isostasy describes the equilibrium between

the Earths lithosphere (the upper solid shell)

and asthenosphere (weak and plastic layer be-

low the lithosphere) such that the continental

plates oat, similarly to icebergs or rafts. The

extent of the plate above and below a certain

level depends on its thickness and density.

In the simplest case, isostasy is related to the

Archimedes principle of buoyancy - when an

object is immersed, an amount of water equal

in mass to that of the object is displaced. On

a geological scale, isostasy can be observed

where the Earths strong lithosphere exerts

stress on the weaker asthenosphere, which

over geological time ows laterally such that

the load of the lithosphere is accommodated by

height adjustments. Such heights can be seen as

mountains or islands, or may also be expressed

by sedimentary basins. Isostasy is invoked to

explain how di erent topographic heights can

exist at the Earths surface. When a certain area

of lithosphere reaches the state of isostasy, it is

said to be in isostatic equilibrium.

There are three main models that are used to

explain this isostatic equilibrium. Based on these

ideal assumptions, isostatic corrections are ap-

plied to gravity data to remove the gravity e ect

of masses in the deep crust or mantle that iso-

statically compensate for surface loads. Here, we

used the Airy-Heiskanen model to calculate the

isostatic gravity anomalies.

Isostasy: compensation of sedimentary in ll

Local concepts of isostasy propose that the Earth is in hydrostatic equilibrium at depth, requiring topography to be compensated either by lateral varia-

tions in crustal thickness (Airy-Heiskanen isostasy) or crustal density (Pratt isostasy).

a) In the Airy-Heiskanen model (Airy 1855, Heiskanen 1931), the compensation is accomplished by crustal roots under the high topography that intrude

into the higher-density material of the mantle to provide buoyancy for the high elevations. Over oceans, the situation is reversed. The Airy isostatic cor-

rection assumes that the Moho is like a scaled mirror image of the smoothed topography, that the density contrast across the Moho is constant, and that

the thickness of the crust at the shoreline is a known constant. Scaling is determined by the density contrast and by the fact that the mass defi ciency at

depth must equal the mass excess of the topography for the topography to be in isostatic equilibrium.

b) Isostatic corrections can also be made for the Pratt model (Pratt 1855), in which the average densities of the crust and upper mantle vary laterally above

a fi xed compensation depth.

c) Regional isostasy after Vening Meinesz (1931): In this model, the crust acts as an elastic plate and its inherent rigidity spreads the support for topographic

loads over a broader region.

c) Vening Meinesza) Airy b) Pratt

sea level

( )nAnA hHH += 0

0 nAHnhnh

0H

MM

NH

NT

.constk =

( )= KMNT nh k

KKM >

REFERENCES:Airy, G.B. (1855) On the computation of the effect of the attraction of mountain-masses, as

disturbing the apparent astronomical latitude of stations of geodetic surveys.

Phil. Trans. R. Soc., 145, 101-104.

Heiskanen, W.A. (1931) Isostatic tables for the reduction of gravimetric observations calculated

on the basis of Airys hypothesis. Bull. Godsique, 30, 110-129.

Pratt, J.H. (1855) On the attraction of the Himalaya mountains, and of the elevated

regions beyond them, upon the plumb line in India: Phil. Trans. R. Soc., 145, 53-100.

Vening Meinesz, F.A. (1931) Une nouvelle methode pour la rduction isostatique

rgionale de lintensit de la pesanteur. Bulletin Godsique, 29, 33-51.

Isostasy

From rift basins to mega-basins 37

Isostatic Residual Map

To constrain the density distribution within

the sedimentary rocks, one can use a density-

depth relationship that represents sediment

compaction with depth. Due to this compac-

tion, the upper sedimentary layers are mostly

responsible for mass de ciency relative to

the surroundings, whereas sedimentary rocks

at greater depths have similar densities com-

pared to the surrounding bedrock. The result-

ing isostatic Moho is very di erent from the

seismic Moho. For example, the isostatic Moho

is 8 km shallower than the seismi