Marine Geology, 26 (1978) 199--230 © Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
THE GEOLOGICAL STRUCTURE OF THE WESTERN BARENTS SEA K. HINZ and H.-U. SCHLUTER
Bundesanstalt far Geowissenschaften und Rohstoffe (BG R), Hannover (F.R.G.) (Received October 26, 1976; revised and accepted March 17, 1977)
ABSTRACT
Hinz, K. and Schliiter, H.-U., 1978. The geological structure of the western Barents Sea. Mar. Geol., 26: 199--230. A large-scale geophysical reconnaissance survey has been carried out within the framework of the BGR project "Geoscientific research in the North Atlantic". From these geophysical data and results obtained from other investigations, the western Barents Sea can be divided from west to east into the following geological units: (1) A prograded sedimentary wedge. (2) The N--S-running Senja Ridge, a structural high built-up of folded and faulted sediments. (3) The Spitsbergen Platform, a flat-lying rock complex with high seismic velocities. (4) The Troms~b Basin, a NNE-running fault-bounded basin with salt diapirs. (5) The Bear Island Basin. (6) The Transitional Unit, a zone subdivided in a western monoclinal structure and an eastern flat-lying sub-unit. (7) The North Cape Basin, an ENE-running fault-bounded trough with salt diapirs. (8) The Murmansk Basin, a basin adjoining the North Cape Basin in the East. During the Tertiary and up to the Quarternary the western Barents Sea was above sea level and acted as the source area for the Cenozoic sedimentary wedge situated west of 17 ° E. The often-observed refractor with a seismic velocity of about 5.5 km/sec interpreted as Caledonian basement by previous investigators represents Paleozoic sediments in large parts of the western Barents Sea.
INTRODUCTION
The geophysical investigation of the Barents Sea, one of the largest epicontinental seas in the world, was first started in 1969 with cruises 27 and 28 of the R/V "Vema" (Eldholm and Ewing, 1971) from the Lamont-Doherty Geological Observatory and with an aeromagnetic survey (/~m, 1973, 1975) of the Geological Survey of Norway. They were followed by reflection and refraction seismic surveys in the western parts of the Barents Sea from the Seismological Observatory, Bergen (Holtedahl and Sellevoll, 1970; Sundvor and Sellevoll, 1971; Sundvor 1974). In 1970 CNEXO (Renard and Malod,
200 1974) carried out seismic, gravimetric, and magnetic surveys in the "Nestlante II" cruise program. Only a small amount of data was available from the eastern Barents Sea upto 1973 (Litvinenko, 1968; Emelyanov et al., 1970) when " V e m a " cruise 30 was carried out. Recently Eldholm and Talwani (1976) have given a geological analysis of the Barents Sea. In 1974 a reconnaissance survey was carried out b y BGR in the western Barents Sea and the adjacent areas of the Norwegian Continental Shelf from the Norwegian vessel M/V "Longva" (Hinz and Weber, 1975). On behalf of the Norwegian Petroleum Directorate, comprehensive commercial reflection seismic surveys have been carried out north of 72°N (Report No. 81 to the Storting 1974--1975); however, the data have not been released. In this report, results of the BGR investigations gained in the program "Geoscientific research in the Northern Atlantic" are presented. Using mainly refraction seismic data from " V e m a " cruises 27--30, the "Nestlante II" cruise and the published data of the Seismological Observatory, Bergen (see Fig.1.) an attempt is made to define an outline of the geological structure of the western Barents Sea. BATHYMETRY AND GEOLOGICAL SETTING The western Barents Sea (Fig.2) is physiographically divided into a north and south province b y the Barents Sea Valley (Malod and Mascle, 1975) with water depths between 400 and 500 m in places. North of the Barents Sea Valley lies the Spitsbergen Bank with an average water depth of 100 m and the islands Hopen and Bear. The Spitsbergen Bank is separated from West Spitsbergen b y the Storfjord. The area south of the Barents Sea Valley is divided into numerous depressions and rises (e.g., Fugley Bank, Troms¢ Bank, North Cape Bank, Murmansk Rise). The relatively narrow shelf west of Spitsbergen is crossed by several channels, the Isfjord, Bellsund and Hornsund. Adjoining the continental slope west of Spitsbergen is the Atka Graben (Malod and Mascle, 1975) and the Knipovich Ridge, which consists of small depressions and submarine ridges. From the distribution of Precambrian metamorphic rocks (Fennoscandian Shield), Eocambrian sediments (in Finnmark and on Varanger) and the folded Caledonian Hecla--Hoek complex (Spitsbergen and Bear Island), Siedlecka (1975) concluded that the Barents Sea is underlain by both cratonic basement (Barents Sea Craton in the north, Fennoscandian and Petschora Craton in the south) and Caledonian basement (Caledonides in the Norwegian Barents Sea, Spitsbergen--Caledonides, Timanian Baikalides). On Spitsbergen, which can be regarded as an uplifted part of the northern Barents Sea Shelf (Nysaether and Saeb~e, 1976), the Hecla--Hoek complex is covered by a nearly complete series of Devonian to Tertiary marine and non-marine sediments (Orvin, 1940; Freebold, 1951). A north-south trending block faulting occurred in the Late Devonian/Early Carboniferous (Spitsbergen orogeny) and NW-running horst structures originated in the Late Permian (Freebold, 1951). During that time on central West Spitsbergen
201
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Seismological ODservatory, Bergen
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Fig.1. Track chart of BGR lines (1974) in the Barents Sea with locations of seismic refraction profiles (1974, 1975). The thick lines indicate the locations of Figs.12--20.
Carboniferous to Permian carbonate and sandstone strata were deposited alternating with evaporites and shales (Gipsdalen Group: Nysaether and SaebCe, 1976). The younger sedimentary rocks are found predominantly in the southern part of West Spitsbergen where, during the Tertiary, a strong faulting and folding took place (Orvin, 1940; Freebold, 1951), reactivating older fault structures. Sediments of Devonian to Triassic age overlie the Hecla,Hoek complex on Bear Island. The horst--graben structures of the Spitsbergen orogeny seem to
202
i Spils
bergen
-
-~
-_ ]2
I '\
-
Hopen
I
JSpilsbergen Bank
Triassic, Jurass c, Crelaceous Carboniferous, Permian
>
Cambrian, Ordovician ( Hecla Hoek) I
Caledonian basic and uit rabaslc rocks
Cambro-Silurian
Eocambrian
~
Basal complex Piutonic rocks of Lofoten -VesterAlen
<
~
/
,
'
/
/
Fig.2. Bathymetric map after Renard and Malod (1974) and uncorrected BGR soundings in meters and main geological features after Brooks (1970) and Kornfeld (1965). be younger (Carboniferous/Permian) on Bear Island than those on Spitsbergen (Worsley, pers. comm.). There are sediments of Carboniferous to Jurassic age on Hopen Island, including thick deltaic sediments of Triassic age partly with exposed erosional features (channels). Studies of the coarse fractions of sea-bottom samples on the Spitsbergen Bank (Edwards, 1 9 7 5 ) and geological sampling by several Norwegian institutes in the Barents Sea indicate that it is underlain by flatAying
203 Mesozoic sediments of the Spitsbergen type. From the onshore geology bordering the Barents Sea it can be assumed that the Barents Sea area was an epicontinental platform which developed after the Caledonian orogeny and upon which thick sediments were deposited, predominantly during subsidence and transgressive phases. Based on paleontological studies of the arctic marine Carboniferous to Permian fauna, an isolation of the Arctic Seas during the Early Permian is assumed (Dutro and Saldukas, 1973), thus effecting carbonatic to saline sedimentation in the basins and on the shelf areas. During the Trias thick deltaic sandstones were deposited in the Barents Sea area covered by Jurassic shales. For the Barents Sea four main hiatuses are assumed by extrapolating geological data from known onshore areas and on the basis of seismic recordings; i.e., a Lower to Middle Carboniferous hiatus, an Upper Permian to Lower Triassic hiatus, a Lower to Middle Jurassic hiatus and a Lower to Upper Cretaceous hiatus. GEOPHYSICAL INVESTIGATIONS BGR measured a total of 6920 km of reflection seismic lines (48 traces, DFS IV, 31 1 air-gun array) and 16 sonobuoy refraction profiles in 1974-1975. Fig.1 shows the position of the 1974 reflection lines and the refraction profiles of 1974 and of 1975, the results of which have been integrated in our analysis.
Refraction seismics The refraction seismic data (see Table I) were interpreted in terms of discrete layers with constant velocity for better correlation with earlier measurements (Sundvor and Sellevoll, 1971; Eldholm and Ewing, 1971; Renard and Malod, 1974; Sundvor, 1974; Eldholm and Sundvor, 1974; Eldholm and Talwani, 1976). Fig.3 shows the seismic velocities for all discrete velocity layers, where the heavy horizontal lines mark that velocity range which mostly is representative for the formations of the western Barents Sea {Sellevoll, 1975). The following groups of velocities can be outlined (see also Fig. 21): (1) a layer with Vp-Values of 2.2--2.8 km/sec normally corresponding to Tertiary sediments; (2) a layer with Vp-Values of 3.0--3.4 km/sec which according to the observations on And~ya (Dalland et al., 1973) are interpreted as Cretaceous-Jurassic sediments; (3) a layer with Vp-Values of 3.6--4.8 km/sec, normally indicating Mesozoic sediments; (4) a layer with Vp-Values of 5.2--5.6 km/sec, interpreted by previous investigators as Caledonian and/or Rhiphean basement (Sundvor and Sellevoll, 1971). (5) a layer with Vp-Values of 5.8--6.2 km/sec, interpreted as Precambrian metamorphics (Eldholm and Talwani, 1976).
204 TABLE I Listing o f B G R seismic r e f r a c t i o n results Sonobuoy No.
Lat. (N)
Long. (E)
Water r', d e p t h (sec) h I
v~ h:
v~ ]z~
c,, h~
t,~ h~
t,, h~
29/75
72°3,81 '
21~ 41,69 '
0.51
2.5 0.51
3.2 0.24
3.3 0.49
3.9 0.36
4_1 2.04
6.0
30/75
71038,93 '
19040,85 '
0.50
2.2 0.62
2.8 0.32
2.9 0.54
3.25 0.61
3.5
25/75
69035,56 '
11°48,51 '
2.94
1.85 0.46
2.5 0.58
2.7 1.20
3.55
26/75
70025,53 '
14037,66 '
2.45
1.8 0.25
2.1 0.22
2.3 0.32
2.45 0.72
28/75
71°17,72 '
18003,93 '
0.37
2.0 1.21
2.8 0.55
3.0 2.83
3.3
12/74
69°25,07 '
16°32,16 '
0.29
2.0 0.34
2.5 0.51
3.15 0.41
19/74
72°56,19 '
16°02,92 '
0.48
1.95 0.13
2.1 0.62
22/74
73°23,64 '
19018,69 '
0.40
2.3 0.21
23/74
72°35,05 '
24055,55 ,
0.32
24/74
72°25,71 '
30°47,13 '
25/74
73°20,85 '
26/74
u~ (kmls)
h 7 (km)
2.7 0.62
3.05 1.96
6.1
3.8 0.42
4.25 1.16
4.45
2.35 0.53
2.6 0.83
3.55 0.40
3.7 1.34
3.25 0.35
3.4 0.96
3.95 0.26
4.05 1.3
5.0
2.85 0.36
3.15 0.14
3.45 0.46
3.9 0.49
4.35 0.64
4.7
0.37
2.35 0.40
2.95 0.43
3.3 0.55
3.95 0.54
4.25 0.84
4.65 1.2
5.05
26o06,74 '
0.52
3.1 0.28
3.55 0.50
4.3 0.53
4.65 0.77
3.8 0.29
4.65 0.61
5.05
75°07,28 '
13010,68 '
1.75
1.65 0.38
2.15 0.33
2.35 0.34
2.6
27/74
74°55,24 '
21°08,01 '
0.2
3.65 0.11
4.0 0.17
4.3 0.57
4.8 1.04
5.6 1.33
6.25
28/74
76°06,52 '
30°44,49 '
0.26
3.0 0.22
3.55 0.28
4.0
29/74
77°20,73 '
26009,92 '
0.15
3.35 0.22
4.25 0.30
4.5 0.29
4.7 5.9 1.14 0.71
6.65 0.84
30/74
76°08,59 '
14°19,32 '
0.43
1.9 0.38
2.1 0.29
2.35 2.55 0.36 0 . 3 6
4.7
6.9
2.7
An example of a refraction seismic recording with an expendable Sonob u o y is g i v e n i n F i g . 2 1 w h e r e t h e s e i s m i c v e l o c i t y l a y e r s 1, 2 , 3 a n d 5 c o u l d be determined and which are typical for widespread areas of the southwestern part of the Barents Sea (Troms¢ Basin). Previous authors made the assumption that the seismic velocity changes predominantly in the vertical direction and that thereby, logically, refraction
205
T e d i clry
I (,~etoceous
I
Jur.q~_ s TrIQSS •
JO
~
[(}0 Z C I
8osement
20E
0
I
20
I
,
J
I
I
30
P
l
I
I 0
T
SO
60
ve/oc,r x
[km/sec)
Fig.3. Histogram of Seismic velocities. Data from Eldholm and Ewing (1971 ), Sundvor (1974), Renard and Malod (1974), Eldholm and Sundvor (1974), Eldholm and Talwani (1976) and BGR studies.
seismic horizons can be used for the construction of isopach maps and for the development of geological models. According to our data, this assumption is n o t generally valid for the western Barents Sea. The lower part of Fig.4 shows the main reflecting horizons (heavy lines) with the interval velocities derived from the stacking velocities. In the upper part of the figure the results of s o n o b u o y refraction seismic studies of the same structure are indicated. In both cases, the velocity lines do n o t follow the actual layering conditions. This demonstrates that lateral velocity changes must be expected in the sediments in the western Barents Sea. Furthermore, we observed that the top of the sediments containing evaporites of the western Barents Sea may have seismic velocities of 5.0--5.4 km/sec which are usually defined as Caledonian basement. Only Eldholm and Talwani (1976) extensively discuss the velocityage relations of the western Barents Sea. For our a t t e m p t to define the main structural units and the distribution of sediments in the western Barents Sea, we therefore contoured the refraction seismic velocities in discrete depths, e.g., in 1000, 2000, 3000, 4000 and 5000 m below sea b o t t o m (Figs.5--10). Fig.5 shows the velocity distribution at 1000 m below sea b o t t o m . On the Spitsbergen Bank and in its vicinity -- in the following called the Spitsbergen Platform (Eldholm and Talwani, 1976) -- rocks with Vp-Values of > 4 km/sec axe found. Velocity values of less than 3 km/sec are observed only in the western part of the Baxents Sea and in the continental slope area. In the Norwegian Sea this low-velocity rock complex overlies oceanic crust of
206 26
27
A
-o I, 1
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3.16-/,,16 I~m/5
3 35 km/s
L?
km/s
_
=z:i211
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location
Fig.4. Line drawing showing lateral change of seismic velocities. The velocity lines do not follow the actual layering.
Tertiary age (anomalies 20--24). This proves that the age of these "lowvelocity sediments" is Tertiary. Fig.6 shows the velocity distribution at a depth of 2000 m below sea b o t t o m . North of 74 ° N in the Spitsbergen Platform, "high-velocity rocks" with Vp-Values of > 5 km/sec dominate at a depth of 2000 m, with the exception of a narrow N--S zone. South of 74°N, there is a zone with Vp-values of 3.0--4.5 km/sec, which is subdivided by NE--SW striking "highvelocity rock complexes". At a depth of 3000 m below sea b o t t o m (Fig.7), there is within the Spitsbergen Platform a "high-velocity rock c o m p l e x " with Vp-values of > 5.0 km/sec. In the area surrounding Hopen Island, seismic velocities of 6 km/sec were observed. South of the Spitsbergen Platform is a narrow E--W striking zone with relatively low seismic velocities (4--4.8 km/sec) which extends eastward of 35°E. The Tertiary low-velocity rocks are limited to the continental slope area. The velocity distributions at 4000 m (Fig.8) and 5000 m (Fig.9) below sea b o t t o m are similar. There is a "high,velocity rock c o m p l e x " within the Spitsbergen Platform, which is defined as crystalline basement from velocities of Vp> 6 km/sec. South and east of the Spitsbergen Platform
207 Vp-Values of > 5 km/sec predominate, which may represent Caledonian basement as well as Paleozoic carbonate and/or evaporitic sediments. The distribution of high-seismic-velocity rocks with Vp-Values > 5.0 km/sec at depths of 2000, 3000, 4000 and 5000 m below sea b o t t o m is summarized in Fig.10. I
]1
4
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-
r
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j42ol
~30
1~.261 30(
249
12.~s)
3.3. 3.00 3j? " ~.3 3.~s a.o
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z.~ 2:sl 3.02
2 r,s. i s 2.15
2.2 / 2~5 2.25. " 2.9?
I° ' ° r
I
11°"
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I
i ~°°
I
I=°"
I
Fig.5. Refraction seismic velocities (kin/see) at a depth of 1000 m below sea bottom.
On the basis of depths of the high-seismic-velocity rocks (> 5.0 km/sec) and on their regional distribution (Fig.10) the western Barents Sea can be subdivided into the following units:
208
(1) a flat-lying "high-velocity rock system" in the Spitsbergen Platform north of 74°N; (2) a zone with "high-velocity rocks" at varying depths running from 19°E to 27°E; (3) a flat-lying "high-velocity rock complex" north of the Varanger Peninsula (Varanger Peninsula is noted as V.R. on Fig. 10); (4) a wide zone more than 3000 m thick of rocks with Vp-values of 3km/sec to 5 km/sec east of 30°E.
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F i g . 6 . R e f r a c t i o n s e i s m i c v e l o c i t i e s ( k m / s e c ) at a d e p t h o f 2 0 0 0 m b e l o w s e a b o t t o m
209
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!
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~
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7¸
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502
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33
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Fig.7. Refraction seismic velocities ( k m / s e c ) at a depth o f 3 0 0 0 m b e l o w sea b o t t o m .
(5) The westernmost part of the Barents Sea is characterized by a north-south running zone of thick "low-velocity rocks" (< 3.0 km/sec). Velocity values of Vp> 5 km/sec, as already pointed out, may represent Caledonian basement, as well as evaporite-bearing sediments of Upper Paleozoic age. As the youngest evaporitic sediments in the Barents Sea area are of Upper Carboniferous to Permian age ("Oberer und Unterer Gipshorizont" of Isfjord, Spitsbergen: Freebold, 1951), Fig.10 also gives a rough outline of the regional distribution of the Cenozoic and the Mesozoic sequence, if the 5 km/sec velocity isoline represents the top of the evaporites.
210
31 _ ~
3::'
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_~
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_
1 .......
!- '~
5 c)
(5 5}
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/ (50( 359
.[55)
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q5"31
.
(55)
Fig.8. Refraction seismic velocities (km/sec) at a depth of 4000 m below sea bottom.
Reflection seismics The results of earlier reflection seismic surveys (Renard and Malod, 1974; Eldholm and Talwani, 1976) using 1- to 6-trace systems were unsatisfactory due to extremely strong sea-bottom multiples. In our investigations, using a 48-trace digital seismic recording system and a 31 l airgun array as energy source, generally :good results were obtained south of 74.5°N. Because of the strong multiples, our results were less satisfactory in the Spitsbergen
211
Bank region north of 74.5°N, even after the use of special processing (Hinz and Weber, 1975). The previously discussed refraction seismic data were combined with the reflection seismic data to define a regional structural map as shown in Fig.11. Accordingly, the western Barents Sea can be divided into the following geological units:
1505)/
,1611/ (5.95)
{67)
~
~
.c6g~
j
[60)
7L~ 13
(5 r'~} ~
j
{5.86)
147h..
10
g
(~,5)
~,.k..4~.
(6I) j691
Fig.9. Refraction seismic velocities (km/sec) at a depth o f 5000 m below sea b o t t o m .
Unit I." Atka Graben--Knipovieh Ridge A zone r u n n i n g N--S o f f the c o n t i n e n t a l slope of Spitsbergen w i t h a very
rough sea-bottom relief and a marked rift valley (see Fig.2). The thickness of
212
the Tertiary sediments overlying the oceanic basement varies greatly. With the exception of local depressions, they rarely exceed 1000 m. The Atka Graben is regarded by Malod and Mascle (1975) as an active oceanic ridge. I
T
i
bergen
...,-'"" ,,," ..,,"
b
~." -
i/
~ 2000 m depth
~
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~'~
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3000m de~h
~'~'~
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5000 m depth J
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130-
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Fig.10. Distribution o f high velocity rocks (> 5 km/sec) at 2000, 3000, 4000 and 5000 m below sea bottom.
Sedimentary wedge. In the western part of the Barents Sea and on the continental slope a progmded sedimentary wedge (Fig.12) of up to 4-sec reflection time exists, This sedimentary wedge overlies oceanic crust (anomalies 5--24) of Tertiary age in the deep Norwegian Sea. Accordingly,
213
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Spitzbergen Ptat~orm with sub-units
[|] Senja
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IV b
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o~--I,'-':'.~'~:'::"::~'~ ~".:..'..':...'..l::...."~.,.~.:.:..: ,.
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North Cape Basin with sub-units
VII
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c
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Transitional Zone with sub-units
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distribution of Cr~tar.~ous sedimems
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F i g . l l . Regional structural map o f the western Barents Sea.
a Tertiary age for these sediments, with vint values of 1.7--2.8 km/sec, can safely be assumed. We interpret the base of the prograding sediments as the base of the Tertiary. Accordingly, the extent of Tertiary sediments is limited to about 17°E in the western part of the Barents Sea (see F i g . l l ) . Our data confirms the observations of Renard and Malod (1974} and the suggestion of Eldholm and Ewing (1971} that the Barents Sea was above sea level during the Tertiary.
lOkm |
Fig.12. Reflection seismic record (part of BGR-line. 14) from the western Barents Sea continental margin showing a thick prograded sedimentary wedge overlying oceanic crust of Tertiary age (anomalies 5--24). Distance between horizontal lines 0.2 sec.
!
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215 Unit II: Spitsbergen Platform The Spitsbergen Platform is characterized by flat-lying rocks with high seismic velocities (Eldholm and Talwani, 1976). A 1000-m thick layer with Vp-values of 5.6 km/sec and 5.9 km/sec, is identifiable in the recordings of refraction lines 27/74 and 29/74, respectively. This layer lies under a sedimentary layer of approximately 2000 m with Vp-values of 3--4.8 km/sec. Below this complex, there are rocks with a vp-value of > 6 km/sec. This three-layer sequence appears to be typical of the Spitsbergen Platform. In the reflection seismic records from the Spitsbergen Platform (the quality is poor, for reasons not yet understood), a flat-lying reflection horizon consisting of several reflections is often observed, which coincides approximately with the top of the 6 km/sec layer. Under this reflection horizon, which appears only locally, numerous diffractions have been observed. We attribute this to an even deeper, disturbed stratum. From the poor data available to us we presume that this rock complex, which has Vp-values > 6 km/sec and is characterized by diffractions, corresponds to the folded Caledonian Hecla-Hoek complex of Spitsbergen. We consider the overlying layer with Vp-values of 5 km/sec to be Paleozoic sediments. The uppermost, approximately 2-km thick layer, is interpreted as Jurassic and Triassic sediments. Thick Cretaceous sediments are probably missing in the central part of the Spitsbergen Platform. In sub-unit IIb the Paleozoic and Mesozoic sediments appear to be somewhat thicker than in units IIa and IIc (Fig.ll). Unit III: Senja Ridge The Senja Ridge (Roennevik, 1975) is a structural high running approx. N--S and according to the observed seismic velocities (see Figs.5--10) it is built up of thick Mesozoic, and presumably also Paleozoic, sediments. The sediments axe strongly folded and faulted and this has affected the quality of the reflection seismic recordings (Fig.13). The Senja Ridge is bounded in the west by faults, which run N--S (see Fig.13 left). These faults may belong to the Senja Fracture Zone (Talwani and Eldholm, 1974) which coincides with a distinct free-air anomaly. The subsided parts of the Senja Ridge are overlain by prograded Tertiary sediments in the west. It seems that the fault-bounded Senja Ridge is bordered by oceanic crust of the Norwegian Sea. Our observations are not in contradiction to the interpretation of Talwani and Eldholm (1974) who explain the Senja Fracture Zone by a sliding of Greenland against Spitsbergen during the first phase (40--60 million years) of the opening of the Norwegian Sea and a separation of Greenland during the second phase. Unit IVa: Tromsd Basin The TromsO Basin is a trough running NNE--SSW with large diapirs, which are considered to be salt structures (see Roennevik et al., 1975). The Troms¢ Basin is bounded by faults in the east and west, which are associated with
?ig.13. R e f l e c t i o n seismic record (part o f BGR-line 1 4 ) from the Senja Ridge. H o r i z o n t a l ine distance: 0.2 sec.
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217 salt structures (Fig.14). The older, presumably Triassic and Jurassic sediments are deformed b y halokinesis. The overlying younger Mesozoic sediments are, in comparison, only slightly deformed b y epeirogenic subsidence. The top of the evaporite-bearing sediments of supposed Permian to Carboniferous age probably lies deeper than 5-sec reflection time, i,e., more than 8 km assuming = 3.5 km/sec. Unit I Vb : Bear Island Basin North of the Troms~ Basin there is a nearly symmetric trough which is designated here as the Bear Island Basin (Ovreb~ and Talleraas, 1976) and which was clearly observed only on BGR Line 14 (Fig.15). It cannot be decided on the basis of our data whether this basin represents an extension of the Troms~ Basin or if it is a separate subbasin running from NE to SW. The velocity distribution (Figs.7--10) indicates that the last assumption is correct. Within this basin, the sediments, probably of Mesozoic age, reach a thickness of up to 3.4-sec reflection time, i.e. almost 6 km assuming ~ = 3.5 km/sec. The underlying, probably Paleozoic sediments are slightly deformed. This deformation could have been caused by halokinesis. Unit V: Transitional Unit This unit is divided into a western monoclinal structure, which corresponds to the L o p p a Ridge (Roennevik et al., 1975; Ovreb~ and Talleraas, 1976) (sub-unit Va Fig. 11) and an eastern flat-lying sub-unit (sub-unit Vb, Fig. 11). The monoclinal structure is separated in the west from the Troms~ Basin by NNE-running faults. The seismic recordings from the Transitional Unit show, in general, a three-layered sequence (Fig.16). Below the sea b o t t o m there is a sedimentary layer of up to 1.5-sec reflection time (layer 1) with interval velocities of 2.5-3.5 km/sec. This layer discordantly overlies the layer below. This discordancy could represent the hiatus between the Lower Jurassic and the Middle Jurassic or the hiatus between the Middle Jurassic and the Upper Jurassic which havc been found on Spitsbergen (Roennevik et al., 1975). The upper sedimentary layer is assumed to be Upper Jurassic/Cretaceous according to Freebold's assumption of a large-scale Bathonian transgression based on the surrounding onshore geology. The layer below the unconformity (layer 2i is 2.0--2.5 sec thick (4.5--5.5 km assuming ~ = 4.5 km/sec) and is often characterized by a double reflecting horizon at the top. This layer may be assumed to be of Jurassic and Triassic age. There are several incisions in the central part of the monoclinal structure filled with sediments (Fig.16) indicating a post-Middle Jurassic origin for the monoclinal structure with subsequent erosion. The Cretaceous sediments are probably missing in the central part. The deepest layer (layer 3) observed is marked by a strong top reflection and numerous reflectors, which in most cases are only correlatable over short distances. The interval velocities, derived from the stacking velocities for this layer have Vint values of 5.0--5.9 km/sec. It should be noted that in all three layers strong, horizontal velocity changes were observed.
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221 Due to the observations in the TromsO Basin and in the North Cape Basin (discussed below) this layer contains evaporites. The age of this sedimentary layer is assumed to be Permian to Upper Carboniferous. The base of the Paleozoic series is n o t clearly definable from our seismic data. There are, however, indications that the Paleozoic--Mesozoic sediments rise towards the Spitsbergen Platform and become thinner. Unit VI: North Cape Basin The North Cape Basin (Hinz and Weber, 1975), a fault-bounded ENE-WSW striking graben zone {unit V/b) which contains large salt diapirs, is east of the Transitional Unit to which the sub-unit Via ( F i g . l l ) can also be added. The faults are frequently associated with salt structures. The diapirs, separated by rim synclines, rise from great depths to near the sea b o t t o m (Fig.17 and 18). The width of the diapirs varies between 5 and 26 km. No statement can be made at present a b o u t their configuration. The sedimentary sequence is similar to that of the Transitional Unit (unit V). The top of the evaporite-bearing Paleozoic sediments in the central part of the North Cape Basin lies at a depth of more than 5 sec (at least 8 km assuming ~ = 3.5 km/sec). The top of the Triassic--Middle Jurassic sediments can be expected at a depth of 1.2--1.6 sec reflection time (e.g. 1.6--2.2 km depth assuming ~ = 2.7 km/sec). The North Cape Basin, characterized by diapirs, continues eastward into another basin (unit VII), for which the name Murmansk Basin is suggested. The North Cape Basin is b o u n d e d in the south by faults by the sub-unit VIc (see F i g . l l ) . Fig.19 shows a seismic record (eastern part of BGR-line 12) of sub-unit VIc (right side of Fig.19) and the southernmost part of the North Cape Basin, with the Paleozoic marker horizon at a b o u t 4-sec reflection time depth which suddenly disappears. The strong reflector at 2-sec reflection time, recognizable on the right side of Fig.19, is considered to be of preMiddle Jurassic age. The weaker reflector lying 0.5--0.6 sec below the strong reflector represents, from our speculative interpretation, the top of thick Eocambrian sediments. Unit VII: Murmansk Basin The North Cape Basin continues towards the east into the Murmansk Basin. Fig.20 shows a recording from this basin (eastern part of BGR-line 16), which broadens east of 35°E. The sedimentary sequence in the Murmansk Basin is similar to that in the North Cape Basin. The top of the Jurassic--Triassic sediments lies between 1.1 and 1.6 sec reflection time; the top of the evaporite-bearing Paleozoic sediments between 2.8 and 3.8 sec reflection time (Fig.20). In the part of the Murmansk Basin investigated by us, only salt pillows appear to be present (see also Fig.20). In the region investigated, there is a seismically transparent zone below the thin salt layer and it overlies rocks with high seismic velocities of u n k n o w n geological nature.
222
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227 SUMMARY AND CONCLUSION
From an analysis of BGR seismic data and refraction seismic results available from other investigators, the western Barents Sea can be divided into the following geological units (see Fig.l 1): (1) A prograded sedimentary wedge, which is up to 4 km thick and of Tertiary age, in the western part of the Barents Sea and on the continental slope. This sedimentary wedge overlies oceanic crust of Tertiary age in the deep ocean basin off the Barents Sea and folded and faulted sediments of the Senja Ridge on the shelf. (2 } Senja Ridge, a high running N--S and built up of folded and probably strongly faulted, thick Mesozoic and Paleozoic (?) sediments. It seems that the fault-bounded Senja Ridge is bordered by oceanic crust in the west. (3) Spitsbergen Platform, a flat-lying rock complex within the Spitsbergen Bank and its surroundings, characterized by high seismic velocities. A disturbed "high-velocity rock c o m p l e x " interpreted as the Hecla-Hoek complex is overlain by approximately 3 km of sediments which are assumed to be of a Mesozoic and Paleozoic age. (4) Troms¢ Basin, a NNE-running fault-bounded basin with salt diapirs. The top of the evaporite-bearing sediments of supposed Permian to Carboniferous age is situated at a depth of more than 8 km. The Jurassic to Mesozoic sediments are deformed by halokinesis. The overlying younger Mesozoic rocks are weakly deformed mainly by epeirogenic subsidence. (5) Bear Island Basin, a nearly symmetric trough filled with sediments up to 6 km thick. With the available data one cannot decide whether this basin is an extension of the Troms# Basin or a separate NE-running subbasin as may be assumed from the compiled velocity
228
the evaporite-containing sediments is not as deep as in the North Cape Basin. Only one salt pillow was detected during our survey. The seismic data used in this analysis indicate that the western Barents Sea was already divided into basins and ridges in Paleozoic time. The Spitsbergen Platform acted as a more stable area and has subsided less since Paleozoic time than the areas in the south where subsidence and sedimentation varied in space and time. In post Middle Jurassic time this area of subsidence was further differentiated by epeirogenic uplift of the Transitional Unit followed by epeirogenic subsidence of the basins of Troms¢--Bear Island and North Cape--Murm ansk. During the Tertiary and up to the Quaternary, the western Barents Sea was above sea level and the source area for the Cenozoic sedimentary wedge situated west of 17°E. The often-observed refractor with a velociW of about 5.5 km/sec interpreted previously as Caledonian basement, represents Paleozoic sediments in large parts of the western Barents Sea. The geological structure and the seismic velocities of the western Barents Sea are rather similar to those of the Norwegian Margin (shelf and upper slope of the VOring Plateau) and eastern Greenland. It is therefore suggested that the western Barents Sea was a part of an epicontinental sea that extended from the North Sea between Greenland and Norway into the Barents Sea and into the Petchora Basin. ACKNOWLEDGEMENTS
The authors are greatly indebted to Prof. Bender, BGR, Prof. Dfirbaum, BGR, Prof. Sellevoll, Bergen, and Prakla-Seismos GmbH, Hannover, for promoting and supporting the work. We are grateful to the Norwegian Petroleum Directorate, Stavanger, for the permission to carry out this research and to our colleagues S. Gard~, G. Hildebrand, H. SchrSder, and Dr. Hiller for assistance and valuable discussions. The research was sponsored by the "Bundesministerium fiir Wirtschaft".
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230 sokkel, Lofoten-Bj~brn~bya (68 ~'N--75 ° N). J o r d s k j e l v s t a s j o n e n , U n i v e r s i t e t e t i Bergen Tek. R a p p . , 5 : 2 0 pp. Talwani, M. a n d E l d h o l m , E., J 9 7 4 . Margins of the Norwegian G r e e n l a n d Sea. In: C.A. Burk and C.L. Drake (Editors), The Geology of C o n t i n e n t a l Margins, Springer, New York, N.Y.