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Palaeogeography, Palaeoclimatology, Palaeoecology
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The Early Cretaceous Barents Sea Sill Complex: Distribution, 40Ar/39Ar geochronology, and implications for carbon gas formation
Stéphane Polteau a,s*, Bart W.H. Hendriks b,\ Sverre Planke a,c, Morgan Ganerad b, Fernando Corfuc,d, Jan Inge Faleidec,d, Ivar Midtkandald, Henrik S. Svensenc, Reidun Myklebuste
a Volcanic Basin Petroleum Research, Oslo Innovation Center, Gaustadalléen21,0349 Oslo, Norway b Geological Survey of Norway, Postboks 6315 Sluppen, 7491 Trondheim, Norway
c The Center for Earth Evolution and Dynamics, University of Oslo, PO Box 1028 Blindern, 0315 Oslo, Norway d Department ofGeosciences, University of Oslo, PO Box 1047, Blindern, 0316 Oslo, Norway e TGS, Lensmannslia 4, Asker, Norway
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ARTICLE INFO
ABSTRACT
Article history: Received 3 March 2015 Received in revised form 3 July 2015 Accepted 8 July 2015 Available online 7 August 2015
Keywords: HALIP
Large igneous provinces Cretaceous paleoclimate Barents Sea 40Ar/39Ar chronology Anoxic event
Mafic igneous rocks of Cretaceous age (80-130 Ma) scattered around the Arctic Ocean are commonly referred to as the High Arctic Large Igneous Province (HALIP). We have mapped out the distribution of HALIP igneous rocks in the Barents Sea region over the past decade based on integrated seismic-gravity-magnetic interpretation, field work, review of publications, and analyses of new and vintage borehole and field samples. The mapping reveals abundant igneous rocks in the northern and eastern Barents Sea covering an area of-900,000 km2 with a conservative volume estimate of 100,000 to 200,000 km3 of intrusions. The igneous province is dominated by sheet intrusions injected into Triassic and Permian sedimentary rocks. Hydrothermal vent complexes are rare, and only two potential vent complexes have been identified on seismic data in the eastern Barents Sea. We have further done extensive radiometric dating of the igneous samples in the Barents Sea region. New
40Ar/39Ar dating of thirteen samples from Svalbard reveal ages of crystallization and alteration. The large age span (60-140 Ma for the raw ages) is likely due to partial or complete overprint of the K/Ar system in plagioclase, and the age of the magma emplacement is better represented by U/Pb TIMS ages. Only one of our 40Ar/39Ar analyses of plagioclase yielded a statistically valid age that is in line with the recently published U/Pb TIMS ages of 122-125 Ma. The new data clearly document that relying on published data from the K/Ar system can lead to erroneous conclusions on the age of crystallization in this province without a careful use of additional 40Ar/39Ar degassing data (i.e., K/Ca). We propose that the magmatism on Svalbard and Franz Josef Land represents a distinct magmatic event near the Barremian/Aptian boundary (125 Ma) in the Barents Sea. This Early Cretaceous Barents Sea magmatism resulted in the formation of the BSSC (Barents Sea Sill Complex). BSSC age rocks are also present in Arctic Canada (Sverdrup Basin) and on Bennett Island (New Siberia Islands). The massive injection of hot magma into potentially organic-rich sediments in the eastern and northern Barents Basin caused rapid organic matter maturation and formation of thermogenic gas and oil in contact aureoles. We estimate that up to 20,000 Gt of carbon were potentially mobilized, corresponding to 175 trillion barrels of oil equivalent. The production rates and fate of the carbon gases are uncertain. However, we speculate that rapid release of aureole greenhouse gases (methane) may have triggered the Oceanic Anoxic Event 1a (OAE1a) and the associated negative 813C excursion in the Early Aptian. Some of the methane may also be trapped in the vast hydrocarbon gas accumulations found in the east Barents Basin.
© 2015 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license
(http://creativecommons.org/licenses/by/4.0/).
1. Introduction
The Cretaceous is a remarkable period in the Phanerozoic with generally warm climates, high sea-level associated with large shallow
* Corresponding author.
E-mail address: stephane@vbpr.no (S. Polteau). 1 Present address: Statoil Forskningssenter, Arkitekt Ebbells veg 10, 7053 Ranheim, Norway.
epicontinental seas, supercontinental fragmentation, and several episodes of ocean anoxia (Fluteau et al., 2007; Hay, 2008; He et al., 2008; Keller, 2008). Several large igneous provinces (LIPs) were also emplaced during this period, e.g., Parana-Etendeka and the Ontong Java oceanic plateau in the southern hemisphere, and the High Arctic LIP (HALIP) in northern hemisphere (Buchan and Ernst, 2006; Coffin and Eldholm, 1994; Coffin and Eldhom, 1992; Ernst, 2014; Janasi et al., 2011).
When Tarduno et al. (1998) first coined the term HALIP, they considered the Late Cretaceous volcanic event on Axel Heiberg Island in the
http://dx.doi.org/mi 016/j.palaeo.2015.07.007
0031-0182/© 2015 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).
Sverdrup Basin, Arctic Canada. Later, the HALIP included lava flows, sills and dykes identified in Svalbard, northern Greenland, and the Sverdrup Basin (see Fig. 1, Maher, 2001). Senger etal. (2014b) recently published a comprehensive review of the HALIP magmatism in Svalbard. The recognition of HALIP-related rocks is more uncertain in offshore areas (Grogan et al., 1998), but the biggest challenge similar to other well-known LIPs is the determination of the accurate age and duration of the magmatic event. Radiometric dating of HALIP rocks indicates ages between 130 and 80 Ma (40Ar/39Ar) or 192-34 Ma (K/Ar) (see Table 1, Buchan and Ernst, 2006; Estrada and Henjes-Kunst, 2013; Evenchick et al., 2015; Nejbert et al., 2011). These geochronology results suggest an extended period of magmatism punctuated by several volcanic pulses (Fig. 1 for distribution and Table 1 for age summary; Buchan and Ernst, 2006). However, the robustness of the results can be questioned when considering published K/Ar ages, which give a 50 Ma difference between samples from the same sill intrusion (Birkenmajer et al., 2010; Corfu et al., 2013; Nejbert et al., 2011).
The aims of this publication are (1) to document the distribution, volume, and age of Cretaceous igneous rocks in the Barents Sea and (2) to briefly assess the impact of the Cretaceous magmatism on the Barents Sea basins, petroleum system, and paleoclimate. We have used a variety of industry and published geological and geophysical data to map the distribution of the igneous rocks. In addition, new samples were obtained from outcrops and wells on Svalbard and in Russia to date the rocks using modern 40Ar/39Ar (this contribution) and U/Pb
methods (Corfu et al., 2013) on samples sometimes from the same sill. The data reveal an Early Cretaceous sill complex in the Barents Sea, referred to as BSSC (Barents Sea Sill Complex), and distinct in age from the Late Cretaceous volcanics dominantly found in Arctic Canada and Northern Greenland. We suggest that this igneous event had a major impact on the Barents Sea basin development, on the evolution of petroleum systems, and possibly triggering the global paleoenvironmental changes in the Early Cretaceous.
2. Data and methods
The main data available for this study are summarized below and shown in Fig. 2. A very dense grid of industry 2D-seismic data was available in the southwest Barents Sea, whereas an open grid of vintage and recent seismic data was interpreted in the eastern Barents Sea. Only potential field data were available in the formerly disputed zone between Norway and Russia. Dolerite samples were collected during Svalbard field work in 2004 and 2010. Additional samples were obtained from existing collections and boreholes.
2.1. Geophysical data and interpretation method
2.1.1. Seismic, gravity, and magnetic data
The multi-channel seismic database consists of more than 300,000 km of line data in the western Barents Sea and about
Fig. 1. Regional setting of the Barents Sea within the North Atlantic-Arctic region. The Barents Sea is a shallow platform area surrounded by deep ocean basins in the NE Atlantic and the Arctic. Ages of major Cretaceous igneous complexes from Table 1. GR: Gakkel Ridge; MJR: Morris Jessup Rise; NSI: New Siberian Islands; SZ: Severnaya Zemlya; YP: Yermak Plateau. Bathymetry from IBCAO (International Bathymetric Chart of the Arctic Ocean).
Summary of published ages and distribution of HALIP rocks (see also Fig. 1). (1) Midtkandal et al. (2008), (2) Dallmann (1999), (3) Grogan et al. (1998), (4) Harland (1997), (5) Parker (1967), (6) Dibner (1998), (7) Koryakin andShipilov (2009), (8) Filatova (2007), (9) Shipilov et al. (2009), (10) Fedorovetal. (2005), (11) Drashev and Saunders (2003), (12) Kos'koand Korago (2009), (13) Villeneuve and Williamson (2006), (14)Tarduno etal. (1998), (15) Kowallis etal. (1995), (16) Trettin and Parrish (1987), (17) Williamson (1988), (18)Jokat (2003), (19) Buchan and Ernst (2006), (20) Corfu et al. (2010), (21) Corfu et al. (2013).
Province
Region
Ages (Ma)
Igneous Rocks
Sediments enclosing lava/tuff/bentonite
References
Barents Sea
Svalbard U/Pb: 124.5-124.7 on two onshore sills
Sills offshore not above Late Jurassic Kong Karls Land Sills offshore not above Late Jurassic Franz Josef Land U/Pb. 122.2 ± 1.1 Ma on sill
Ar/Ar. 125.2 ±5.5 Ma on dyke K/Ar: 34-175 Ma on sills K/Ar: 94-192 Ma on dykes Sills offshore not above Late Jurassic
U/Pb: 123.3 ±0.2 Ma on bentonite in the Barremian Helvetiafjellet Fm.
Basalts in Barremian Kong Karls Land Fm. Basalts in Barremian Tikhaya Bay Fm.
New Siberian Islands
Bennett Island K/Ar: 106-124 Ma on basalts
Basalts in Barremian unnamed formation
[1-3,21]
[3-5] [3,6,7,21]
[8-12]
Arctic Canada Sverdrup Basin
Axel Heiberg Island Ar/Ar: 95.3 ± 2.0 on basalts
Ellesmere Island
Queen Elizabeth Islands
Ar/Ar: 126-129 Ma on sills
U/Pb: 92.0 ± 1.0 Ma on gabbroic intrusion
Ar/Ar: 94-97.2 Ma on two sills
Ar/Ar: 92.0 ±2.0 on basalts in Hassel Fm.
Ar/Ar: 127-129 Ma on the Lightfoot River Dyke
Ar/Ar: 80.7-96.1 Ma on basalts
Basalts in the Albian-Cenomanian Strand Fjord Fm. Basalts underlying the Turonian Kanguk Fm. Basalts in the Albian-Cenomanian Hassel Fm.
Arctic Ocean
Alpha Ridge
Ar/Ar: 82.0 ±1.0 Ma on a single altered dredged basalt sample
[13-16] [13,16,17]
Northern Greenland
Peary Land
Rb/Sr and Ar/Ar: 82-103 Ma
Dykes cross-cut Early Cretaceous strata
Novaya Zemlya
Timanides
U/Pb: 704-716 on sill
Devonian sediments re-interpreted as Precambrian azoic succession
25,000 km in the eastern Barents Sea (Fig. 2). Ship-track gravity data were available for most of the seismic profiles in the western Barents Sea (287,000 km). Bouguer corrected 200-km high-pass filtered satellite-derived gravity data were available in the entire study area (Sandwell and Smith, 2009). The aeromagnetic compilation is based on 252,000 km of line and public-domain gridded magnetic data covering the entire study area (Verhoef et al., 1996).
2.1.2. Geophysical interpretation method
The seismic interpretation of igneous complexes follows the methods of seismic volcanostratigraphy for extrusive rocks (Planke et al., 2014; Planke et al., 2000) and Planke et al. (2005) for intrusive complexes. The depth to magnetic sources were calculated based on Peters' half-slope method (Peters, 1949). Sorting of the source depths while simultaneously checking geological constraints was done during this process (Grogan et al., 1998; Myklebust, 1994). Furthermore, the ability to correct for strike direction of the magnetic body is important during the processing. The integrated interpretation of the seismic, gravity, and magnetic data was mainly done using the Kingdom Suite seismic interpretation system. High-pass filtered gravity and magnetic data were imported as pseudo-horizons and represent density and magnetic attribute maps. The SGM increases the efficiency and quality of seismic interpretation by facilitating the identification of sedimentary basins, basement highs, geological trends, and lithological discrimination, including increased confidence of the interpretation of igneous rocks.
22. 40Ar/39Ar geochronology 2.2.1. Sampling
Radiometric dating of dolerites is commonly done using the 40Ar/39Ar method on whole rock, groundmass, or plagioclase. All collected samples on Svalbard are from the Diabasodden Suite (Dallmann, 1999). We selected seven medium-grained samples from Diabasodden
(DB5 and DB9), Hatten (DB47 and DB51), Gasoyane (DB31), and Gipsvika (DB38) from the paleomagnetic samples of E. Halvorsen (see http://toposvalbard.npolar.no/ for a detailed topography map of Svalbard). The Diabasodden and Hatten localities are 3 km apart and are from the same sill outcropping 15 km northeast of Longyearbyen, whereas the Gasoyane and Gipsvika localities are 10 km north on the opposite side of Isfjorden. Previously, five sill localities were visited in Svalbard during a field expedition in April 2010 (samples 2A, 4A, 5C, 6D, 7B). Sample SV04-04 was collected in 2004 from the Dunerbukta area. Sample DH4 is a dolerite from 951.2 m depth in the Longyearbyen CO2 Lab borehole DH4 drilled 4.5 km southeast of Longyearbyen. The field expeditions targeted coarse-grained and pegmatitic dolerites primarily for zircon U/Pb dating and 40Ar/39Ar dating for comparing method results. All collected samples were mainly medium grained. The coordinates of the sampling sites are shown in Table 2 and locations in Fig. 2. The results of the U/Pb TIMS dating are reported in Corfu et al. (2013).
222. Analytical protocol
The samples were crushed and sieved to isolate grains of 180-250 |am. A portion was set aside for whole rock analysis. Magnetic separation using a Frantz isodynamic separator was followed by heavy liquid separation to concentrate feldspars. Visual inspection using a binocular microscope showed moderate degree of alteration in most samples, with a mix of altered and fresh plagioclase. The groundmass had an altered character. Fresh inclusion-free mineral grains were handpicked under the binocular microscope. The samples were washed in 5 M HCl for five minutes at room temperature in an ultrasonic bath to remove carbonate and light surface alteration, and subsequently washed ultrasonically several times in distilled water.
The transformation 39 K(n, p)39Ar was performed during irradiation in three separate batches: samples 2A-DH4 (2010, IFE in Norway), samples DB5-DB51 (2009, IFE), and sample SV04-04 (2005, McMaster in
Fig. 2. Barents Sea bathymetry map showing onshore and offshore distribution of Cretaceous igneous rocks, selected offshore hydrocarbon fields and wells, and interpreted seismic profiles in the Eastern Barents Sea.
Canada). Samples irradiated in 2009 and 2010 have been calculated against the Taylor Creek Rhyolite standard (28.619 ± 0.034 Ma), which is calibrated directly to U/Pb (Renne et al., 2010). The Tinto bio-tite (410.3 Ma, but not directly calibrated to U/Pb) was used as a fluence monitor standard for SV04-04. The 40Ar/39Ar analytical protocol followed by the NGU lab is thoroughly presented in Ganer0d et al. (2011).
A plateau is defined according to the following requirements: at least three consecutive steps, each within 95% confidence level, comprising at least 50% of total 39Ar and mean square of weighted deviates (MSWD) less than the two tailed student T critical value. We calculated a weighted mean plateau age (WMPA), weighting by the inverse of the variance. The weighted York-2 method was used to calculate the
Table 2
Sample locations and main 40Ar/39Ar results. The superscripts P or WR in the sample column denotes Plagioclase separates or whole rock. 39Ar% is the percentage of the total 39Ar used in the age calculation. Uncertainties on the age are reported as analytical and external errors. MSWD is the mean square weighted deviation for the steps included. TGA denotes Total Gas Age. K/Ca is calculated from 39Ark/37ArCa. Intercept is the trapped 40Ar/36Ar ratio from inverse isochron analysis. The bold sample names highlights the results where ages have a high confidence for statistically valid plateau and isochron ages.
Sample Lat. Long. Spectrum analysis MSWD TGA K/Ca± 1.96a Inverse isochron analysis
39Ar % Steps (N) Age ± 1.96a Age ± 1.96a MSWD Intercept ± 1.96a
2AP 78.35 16.26 83.48 2-4 (3) 75.44 1.47/1.52 1.24 76.85 ± 1.59 0.181 ± 0.010 81.5 ± 6.7 0.292 219.7 ± 87.3
4AP 78.14 18.78 74.3 1-7(7) 120.22 1.79/1.91 0.873 121.23 ±2.11 0.023 ± 0.001 121.0 ± 2.0 0.610 288.5 ± 13.0
5CP 78.16 18.93 61.4 1-7(7) 109.34 5.48/5.5 1.984 116 ± 3.82 0.027 ± 0.001 110.9 ±6.4 2.145 289.7 ± 22.7
6DP 78.22 17.88 97.43 3-8 (6) 82.64 0.63/0.76 0.937 82.3 ± 0.82 2.770 ± 0.1100 82.9 ± 1.0 1.067 290.4 ± 23.5
7BP 78.03 13.88 87.64 3-6 (4) 77.72 3.76/3.78 12.62 80.2 ± 1.46 0.430 ± 0.020 83.2 ± 3.0 2.873 205.1 ± 42.5
7BP 78.03 13.88 7.7 8-16 (9) 115.1 14.21/14.22 3.715 " 0.067 ± 0.003 113.6 ± 14.6 1.307 395.3 ± 140.0
DH4P 78.20 15.83 69.11 1-3 (3) 60.68 2.72/2.74 5.228 73.87 ± 1.57 0.150 ± 0.008 62.6 ± 1.8 1.363 262.6 ± 24.5
DH4P 78.20 15.83 10.32 5-8 (4) 110.96 6.67/6.7 1.066 " 0.029 ± 0.002 113.3 ± 7.8 1.410 222.8 ± 217.5
DB31WR 78.46 16.28 33.76 3-6 (4) 119.42 2.24/2.53 1.787 105.45 ± 1.54 0.520 ± 0.033 119.4 ±5.1 2.625 299.7 ± 44.1
DB38WR 78.45 16.40 32.38 2-6 (5) 118.07 4.31/4.49 5.311 102.0 ± 1.9 0.550 ± 0.032 112.8 ± 10.6 5.058 329.5 ± 55.8
DB38WR 78.45 16.40 17.67 2-4 (3) 122.88 4.67/4.82 1.979 " 0.520 ± 0.040 125.6 ± 17.1 3.547 288.30 ± 63.2
DB5WR 78.36 16.13 100 1-11 (11) 101.05 ± 1.55 0.440 ± 0.033
DB9wr 78.36 16.13 50.35 3-7 (5) 131.11 3.26/3.55 1.78 122.16 ±2.31 0.510 ± 0.030 136.3 ± 11.9 1.922 277.4 ± 47.7
DB47WR 78.36 16.99 78.36 4-9 (6) 124.09 3.58/3.81 10.30 125.11 ± 1.82 0.470 ± 0.025 129.3 ± 5.1 6.031 216.3 ± 63.1
DB51WR 78.36 16.99 36.99 1-5(5) 123.8 5.20/5.36 3.61 107.64 ±2.12 0.260 ± 0.017 128.4 ± 5.7 1.950 275.7 ± 21.2
SV04-04P 78.23 16.28 78.49 4,5,7,8 (4) 128.39 2.44/2.81 0.348 120.13 ± 6.0 0.030 ± 0.0010 127.9 ± 3.7 0.437 300.9 ± 10.9
inverse isochron results, where a valid isochron has an MSWD value less than the two tailed F-test critical value.
3. Geological framework
The Barents Sea is a more than 1.5 million km2 large epicontinental sea, including the Svalbard and Franz Josef Land archipelagos in the north and Bear Island (Bjornoya) in the west. The average water depth is about 250 m, with wide glacial channels up to 450 m deep. The Barents Sea consists of a complex system of rift basins and basement highs in the west, which are separated by the Central Barents Monocline from an ultra-deep north-trending sag basin in the east.
The geological history was recently summarized by Smelror et al. (2009) and Henriksen et al. (2011b). The basin history started near the end of the Caledonian orogeny with formation of Devonian and Carboniferous clastic rift basins. This period was followed by the formation of predominantly shallow water evaporitic basins in the middle Carboniferous to the middle Permian, including the deposition of thick carbonate and salt successions. This Paleozoic interval forms a karstified carbonate reservoir saturated in oil in the "Gotha" discovery in the southern part of the Loppa Ridge south of the Hoop Area. Middle Permian uplift of the Uralian mountains to the east led to the deposition of prograding siliciclastic sediments in the Southeast Barents Basin and the formation of an unconformity in the west. Renewed uplift in the east at end-Permian times, possibly associated with the Siberian Traps, led to the formation of a major clastic delta prograding across the platform from southeast to northwest during the Triassic. Marine clastic mudstones were subsequently deposited in a shallow sea during the Jurassic and earliest Cretaceous, whereas deep Late Jurassic-Early Cretaceous rift basins were formed in the southwest. This period was followed by massive Arctic volcanism in the Early Cretaceous with the formation of the Diabasodden Suite (e.g., Dallmann, 1999; Senger et al., 2014b), causing uplift of the northern margin and formation of southward-prograding delta sequences. Most of the Barents Sea was high standing in the Late Cretaceous and Cenozoic, whereas continental breakup to the north and west formed Cenozoic passive continental margins along the Eurasia and Norway basins. The dominantly Plio-Pleistocene glaciations led to episodic large-scale uplift and erosion of the Barents Sea (Henriksen et al., 2011a), with the deposition of major glacial fans along the northern and western margins. The main petroleum source rock is the Upper Jurassic (Hekkingen Formation), but organic-rich rocks are also locally present in Paleogene, Cretaceous,
Triassic, Permian, Carboniferous, and Devonian sequences (Ohm et al., 2008).
4. Results
4.1. Extent of igneous roclks
The areal distribution of Mesozoic igneous rocks in the Barents Sea is shown in Fig. 3. The map is compiled from new seismic, gravity, and magnetic interpretation, borehole data, field data, and adjusted using previous published local studies (Grogan et al., 1998; Kos'ko and Korago, 2009; Maher, 2001; Minakov et al., 2012; Myklebust, 1994; Senger et al., 2014b; Werner et al., 2011). The confidence of the interpretation is high onshore and in the Southeast Barents Basin where seismic imaging is very good. The confidence is moderate north of 74° in the Norwegian segment of the Barents Sea toward Svalbard and poor north of 76° in the east Barents Basin where seismic data are less abundant or of lower quality.
Outcrops of Mesozoic igneous rocks from the Diabasodden Suite are widespread on Svalbard and Franz Josef Land (Figs. 2 and 3). Extrusive basaltic rocks are abundant on Franz Josef Land (Dibner, 1998; Evdokimov and Stolbov, 2004), whereas Mesozoic basaltic lavas are only found in Kong Karls Land on Svalbard (Bailey and Rasmussen, 1997; Grogan et al., 1998; Harland, 1997). Basaltic sills are common both on Svalbard (Dallmann, 1999; Maher, 2001; Nejbert et al., 2011; Shipilov et al., 2009) and Franz Josef Land (Dibner, 1998). Numerous sills have been intersected by three boreholes on Franz Josef Land (Dibner, 1998) and five wells in Svalbard (Senger et al., 2014a; Senger et al., 2014b; Skola et al., 1980). In outcrop, the sills are normally less than 50 m thick and mainly intrude Triassic clastic sequences. A northwest trending dike swarm has been mapped on Franz Josef Land (Buchan and Ernst, 2006; Dossing et al., 2013), whereas dikes are rare on Svalbard (Fig. 3).
The areal extent of the igneous rocks is also difficult to map in the regions around Svalbard and Franz Josef Land. The distribution of igneous intrusive and extrusive rocks offshore Svalbard has been interpreted by Grogan et al. (1998) and Minakov et al. (2012) based on seismic reflection and seismic wide-angle data combined with magnetic data. Well-defined sill reflections are identified in Permian to Jurassic age sequences, locally terminating or showing a hummocky relief at the BCU. Erosional igneous remnants are locally identified on the seafloor, commonly preventing deeper seismic imaging. Finally, volcanic flows are identified on and around Kong Karls Land, and magnetic data clearly
Fig. 3. The 200-km high-pass filtered Bouguer anomaly map combined with the distribution of magnetic anomalies grouped by depths from Myklebust (1994). The region with interpreted Early Cretaceous BSSC is shown as a dark grey transparent polygon, but also contains some flows and dykes. High confidence area for the presence of volcanic rocks correspond to Svalbard (130,000 km2), Franz Josef Land (150,000 km2), and SE Barents Sea (180,000 km2). Satellite gravity data from Wessel and Smith (1991).
reveal a north-trending dike swarm (D0ssing et al., 2013). In the Northeast Barents Basin, the extent of the igneous complex is delineated by the potential field data calibrated by published seismic profiles of Khlebnikov et al. (2011).
Seismic mapping revealed a very extensive sill complex of more than 180,000 km2 in the east Barents Basin, underlying giant gas discoveries
such as the Stockman field. Sill intrusions are interpreted from seismic data as high-amplitude, crosscutting and sometimes saucer-shaped reflections (Figs. 4-6). The sills are mainly present in the central part of the basin and are rarely identified in structural highs or inverted basin segments along the basin flanks. In addition, sills are interpreted not only in the Triassic sequence but also in the Upper Permian stratigraphic
Fig. 4. Seismic profile across the deep Southeast Barents Basin showing an extensive igneous sill complex. The sill intrusions are absent from the structural highs on the flanks of the basin. Prominent prograding reflections onlap sequences just above the base Cretaceous unconformity (BCU) and represent evidence of uplift, erosion, and transport of sediments during the Early Cretaceous times. P: Permian, T: Triassic, J: Jurassic, C: Cretaceous. Sequence boundaries modified from Henriksen et al. (2011b). See Figs. 2 and 3 for location. Data courtesy ofTGS.
intervals. Up to 7-8 levels of sills are imaged, and two different sill facies have been mapped on a regional seismic grid (Fig. 4): the deep, domi-nantly layer-parallel, and shallower transgressive and saucer-shaped reflections. The extent of the two facies is similar, with the deep sills extending only a few kilometers outside the shallow sills along the western and eastern flanks of the basin. However, the deep sills extend for up to 100 km south of the shallow sills in the Southeast Barents Basin (Fig. 3). At least one offshore borehole in the east Barents Basin has penetrated two dolerites sills (Ludlow well; Corfu et al., 2013; Komarnitskii and Shipilov, 1991).
Very few seismic profiles were available in the Northeast Barents Basin in the area east and south of Franz Josef Land. The available profiles are difficult to interpret, with an almost opaque top volcanic reflection, which could alternatively represent shallow sills or volcanic flows. The magnetic data reveal distinct northwest-southeast trending directions in this region, interpreted as representing the signature of a possible dike swarm (Fig. 3).
The vertical distribution of the igneous rocks was determined by correlating the magnetic data with regional seismic profiles, effectively allowing the separation of magnetic source depths into three strati-graphic levels (Fig. 3). (1) High-frequency magnetic anomalies related to intrusive and extrusive rocks are closely correlated with Mesozoic subcrop patterns. The distribution of this magnetic signature has been
traced east beyond Franz Josef Land. (2) Intra-sedimentary magnetic sources were found extensively in Paleozoic rocks in the western Barents Sea. In the eastern Barents Sea, intra-sedimentary magnetic sources are typically found in Triassic sequences. (3) Several high-amplitude magnetic anomalies, earlier interpreted as basement, have been re-defined as intra-sedimentary boundaries in Paleozoic sequences.
The contoured depth to magnetic sources is shown on top of the sill distribution interpreted from the seismic data in Fig. 3. In general, the distribution of intra-sedimentary magnetic sources (<4 km) correlates well with the outline of the sill facies in the Southeast Barents Basin. However, some shallow magnetic anomalies are also present outside the sill domain, in particular, northwest of Murmansk and along the western flank of Novaya Zemlya. These magnetic anomalies may have a different origin than intrusive igneous rocks or be related to igneous rocks not identified/interpreted on the seismic data (e.g., near vertical dikes) due to limited seismic coverage. Finally, the trends of anomalies in the 200-km Bouguer data correlate very well with the distribution of the sill complexes. Two broad, low-amplitude, and north-trending positive gravity anomalies correspond to the mapped extent of the sill complexes for more than 600 km (Fig. 3).
Hydrothermal vent complexes are common in many volcanic basins, e.g., the V0ring Basin of mid-Norway (Planke et al., 2005), onshore in
Fig. 5. Seismic profile in the east-central Barents Sea showing an extensive sill complex and an interpreted crater structure at the Base Aptian level. See Figs. 2 and 3 for location. Data courtesy of TGS.
the Karoo Basin (Svensen et al., 2008), and Tunguska Basin in Siberia (Svensen et al., 2009). One potential hydrothermal vent complex has been identified in the east Barents Basin with its characteristic crater-shaped structure near the interpreted Base Aptian level (Fig. 5). The crater is present above faulted Jurassic sedimentary rocks and the termination of transgressive sill segments. Another crater-shaped structure has been interpreted at a similar stratigraphic level above the termination of a deep transgressive sill further south (Fig. 6). However, the reflections beneath both craters are not discontinuous but parallel to the base crater reflection. Such good reflection continuity is uncommon for typical hydrothermal vent complexes mapped elsewhere. Senger et al. (2013) identified another hydrothermal vent complex in central Isfjorden based on magnetic, seismic, and multibeam bathymetry data.
4.2. 40Ar/39Ar geochronology
The 40Ar/39Ar results are summarized in Table 2. The raw data tables (corrected for blanks and baselines), spectrum, and inverse isochron plots are presented in Appendices A and B.
Samples 2A, 4A, and 6D yield statistically valid plateau and isochron ages for more than 50% of the cumulative 39Ar released. In all other samples, the K/Ar system clearly has been disturbed and these data are therefore included as background documentation in the Appendices and not discussed in detail.
Sample 4A (Fig. 8B) defines a weighted mean plateau age of 120.2 ± 1.9 Ma (2ct). The K/Ca ratio varies around 0.02 throughout the spectrum, suggesting the age is representative of the crystallization age of plagio-clase. The 40Ar/39Ar age for this sample is comparable with the U/Pb ages from Svalbard documented by Corfu et al. (2013).
Two of our plagioclase samples (2A and 6D, Figs. 8B and 8C) produced statistically valid plateau ages (Table 2), varying from 75.4 ± 1.5 to 82.6 ± 0.8 Ma. However, the K/Ca ratio (0.18-2.8) is higher than would be expected from unaltered plagioclase in both of these samples (2A and 2D). Such high K/Ca ratio indicates that a higher K and/or lower Ca mineral is degassing in addition to plagio-clase. Sericite [KAl3Si3Oi0(OH,F)2] is known to replace plagioclase by mineralogical substitution and/or fills microfractures within pla-gioclase crystals. This substitution process may occur at low temperature conditions (<300 °C), for example, during hydrothermal alteration (Verati and Jourdan, 2014). Given the high K2O content of sericite (~10 wt%), pervasive sericite replacement will override the low K (<0.1 %) signal from the plagioclase, and the obtained age may represent a sericitization event. We therefore conclude that the ages of 75.4 ± 1.5 to 82.6 ± 0.8 Ma are most likely representative of alteration, rather than of the original emplacement age of our samples.
At the high-temperature end of both spectra, K/Ca ratio drops are associated with climbing step ages. At the high-temperature end of the spectrum (7.7% of the cumulative 39Ar), the K/Ca ratio varies around 0.067 for sample 7B (Appendix A). These steps produce a weighted mean age
Fig. 6. Seismic profile across the southeast Barents Sea showing an extensive sill complex and an example of a possible hydrothermal vent complex. The vent is rooted near the tip of a deep sill intrusion, and the crater terminates at the interpreted Base Aptian horizon representing the paleosurface during BSSC magmatism. See Figs. 2 and 3 for location. Data courtesy of TGS.
of 113.6 ± 14.6 Ma (2a), i.e., overlapping with the 40Ar/39Ar age of plagio-clase sample 4A. The high-temperature step ages for samples 2A, 6D, and 7B may relate to the age of crystallization, but due to low gas volume left in the sample, statistically valid age cannot be calculated from these steps.
5. Discussion
5.1. BSSC distribution and volume
The mapped areal extent of Mesozoic sill intrusions in the Barents Sea is 900,000 km2 (transparent dark grey polygon in Fig. 3) and is
Table 3
Estimated volumes of BSSC. Minimum estimates correspond to the minimum sill layers, and maximum volumes for maximum number of sill layers.
Province Area, Interpretational Number of Min. volume, Max. volume,
km2 confidence sill layers a km3 km3
1 180,000 High 4-8 36,000 72,000
2 310,000 Moderate 3-6 46,500 93,000
3 130,000 Moderate 1-2 6,500 13,000
4 280,000 Low 1-2 14,000 28,000
Total 900,000 103,000 206,000
a Average sill thickness of 50 m.
based on the integrated interpretation of seismic, gravity, and magnetic data. Accurate volume estimates are much more difficult to determine as both the thickness and number of sill intrusions are commonly very difficult to interpret from seismic data.
The volume of intrusive rocks is calculated separately for four different provinces: (1) the Southeast Barents Basin to about 77° north, (2) the Northeast Barents Basin and Franz Josef Land, (3) the eastern Svalbard and the surrounding offshore shelf, and (4) the remaining platform and formerly disputed areas. The average number of sills in each province is estimated from seismic, outcrop, and well data, but the uncertainty is fairly high due to limited data, imaging problems, and scarce outcrops. Well-defined high-amplitude sill reflections typically originate from units >50 m thick (Planke et al., 2005), and we have used 50 m as a conservative estimate of the thickness of one sill layer in the BSSC. The estimated volume of intrusive igneous rocks is in the range from 103,000 to 206,000 km3 (Table 3).
52. Age of the BSSC
Our 13 samples ofintrusive rocks from Svalbard are located along an east-west profile from Festningen to Agarddalen (Fig. 7). The dol-erites intrude upper Paleozoic (Linnevatn; Gipshuken), Triassic
Fig. 7. Compilation of new and published radiometric ages along a west-east profile across Svalbard from Festningen to Agardhdalen, and from Franz Josef Land. The 40Ar/39Ar isochron ages are listed in Table 2. The vertical light red and yellow boxes indicate the range of ages for the crystallization and alteration events, respectively. The names in bold indicate statistically reliable 40Ar/39Ar ages. Note that Linnevatn is about 5 km south of Festningen, the Gipshuken/Gas0yane localities (DB31 and DB38) are about 10 km north of Diabasodden, and the Domen (SV04-04) dolerite was sampled about 15 km north of Agardhdalen. U/Pb TIMS ages (Corfu et al., 2013) are shown. Sample 7A yields two Concordia ages of 124.8 ± 0.4 Ma (zircon + rutile) and 123.9 ± 0.3 Ma (titanite). Sample 1AX has an age of 124.6 ± 0.3 Ma (zircon), whereas the Severnaja age on baddeleyite is 121.5 ± 0.3 Ma, which is likely 1-2 Myr too young due to Pb loss from the platy mineral. Samples DH3 and DH4 are from Longyearbyen CO2 Lab boreholes. Sample DH3 is from a thin silicic tuff in the Helvetiafjell Fm. with an age of 123.3 ± 0.3 Ma (zircon), whereas sample DH4 is from a 2.3 m thick dolerite at 950 m depth. Geological profile modified from Dallmann et al. (2011). Sample DH7 is from Midtkandal etal. (2014).
(Longyearbyen, Diabasodden, Sassendalen), and Jurassic sequences (Agardh). The westernmost samples are from strongly folded sediments, whereas thrust faults are abundant in the Longyearbyen-Diabasodden region. The 40Ar/39Ar age that reflects crystallization and the two ages reflecting alteration (Fig. 8) are plotted with the new U/Pb TIMS ages from Corfu et al. (2013) and Midtkandal et al. (2014) in Fig. 7.
There is a large spread in the raw 40Ar/39Ar ages (Table 2), spanning from 135 to 60 Ma, with two peaks of about 130-120 Ma (Early Cretaceous) and 85-80 Ma (Late Cretaceous). The large spread is in general agreement with previously published K/Ar and 40Ar/39Ar ages with a more than 50 Ma age range with peaks around 115, 100, 91, and 78 Ma (Birkenmajer et al., 2010; Nejbert et al., 2011). Locally, very large age differences exist between the 40Ar/39Ar and the U/Pb ages on the same sill, e.g., there is a >40 Ma difference at both Linnevatnet and Diabasodden (Fig. 7; Table 2). Our results clearly indicate that the large age span observed in the published K/Ar and 40Ar/39Ar data does not need to reflect a prolonged phase of magmatism but instead probably represents other geological processes that led to the alteration of plagioclase.
Only one of our 40Ar/39Ar ages, plagioclase sample 4A (Fig. 8A), can be reliably interpreted as a crystallization age based on the age statistics
and the K/Ca ratio throughout the entire spectrum. This age of 120.2 ± 1.9 Ma closely approximates the U/Pb TIMS data of Corfu et al. (2013) for Svalbard, which define a narrow range of ca. 123-125 Ma. Our best estimate for the age of the Svalbard sills, based on both 40Ar/39Ar and U/Pb data, thus spans from 120.2 ± 1.9 Ma to 124.7 ± 0.3 Ma.
Alternatively, biostratigraphy can be used to indirectly date extrusive rocks. Sedimentary rocks within and enclosing lava flows on Kong Karls Land and Franz Josef Land suggest an Early Cretaceous (Barremian) age of the volcanic rocks (Dibner, 1998; Smith etal., 1976).
Additional evidence pointing toward an Early Cretaceous age for the Barents Sea magmatism is related to the stratigraphic position of crater structures connected to potential hydrothermal vent complexes. Hydrothermal vent complexes form by catastrophic release of meta-morphic gas produced in the metamorphic aureole surrounding sill intrusions. Therefore, the position of the craters in the stratigraphy indicates the timing of sill emplacement. Even though the craters are not well defined, they lie near the Base Aptian paleosurface interpreted from seismic data (Figs. 5 and 6).
Finally, regional changes in depositional patterns of sedimentary sequences closely associated in time with BSSC can be interpreted in terms of lithospheric doming resulting from the rising mantle plume (He et al., 2003; Saunders et al., 2007). Borehole and outcrop data
A) 4A, plagioclase
y $2 0.01
0.00 300
0.023 ± 0.0005 i=i = □ □
Cumulative %^Ar released
B) 2A, plagioclase
0.00 300
250 a)200
ZD 0.181 ± 0.01
-1 0.035 ± 0.001 , , <-►
Fig. 9. Estimated range of the methane production potential, WC, in metamorphic aureoles in BSSC sedimentary basins following the method of Svensen et al. (2004). WC = FCVA, where p is the organic matter density (p = 2400 kg m-3), VA is the volume of the metamorphic contact aureole, and FC is the amount of carbon transferred to gas (estimated in the range from 0.5 to 2 wt%). The estimated volume of the metamorphic aureole is taken to be twice the volume of the sill intrusions (see Table 3). WC is mass of carbon produced in the metamorphic aureole. The production potential is also given in barrel of oil equivalent, bbl = WC/0.11, where 0.11 (t) is the mass of carbon in one barrel of crude oil (1591). An estimate of the amount of gas ultimately produced from the original organic matter during maturation and metamorphism is 50-90% depending on the kerogen type and initial basin temperature. The possible contributions to carbon gas formation from sediment decarbonation reactions, magma degassing, or petroleum reservoirs pierced by sills or hydrothermal vent complexes are not included.
Land, and the formation of extensive southward-prograding clinoforms in the southeast of the Barents Sea (Fig. 4). These sedimentary sequences have a Barremian to Early Aptian age at ca. 125 Ma that immediately pre-dates the emplacement of the BSSC (Midtkandal et al., 2014).
C) 6D, plagioclase
Cumulative %39Ar released
5 4 2 2
л ^ 2.77 ± 0.11
82.6 ± 0.71 Ma
10 12 8 6 7 8-P
0 20 40 60 80 100
Cumulative %39Ar released
Fig. 8.40Ar/39Ar ages for (A) sample 4A (crystallization age), (B) 2A (sericitization age), and (C) 6D (sericitization age).
show that uplift of the northern portion of the Barents Sea shelf resulted in the deposition of the continental Helvetiafjellet Formation on Svalbard, continental volcano-sedimentary accumulations on Franz Josef
5.3. Implication for carbon gas generation
Magmatic intrusions emplaced in carbonaceous sediments involve the sudden exposure of organic matter in contact aureoles to temperatures as high as 600 °C. The effects of contact metamorphism on organic-rich sediments has been previously investigated in terms of organic matter maturation (Aarnes et al., 2011; Golab et al., 2007; Gurba and Weber, 2001; Meyers and Simoneit, 1999), volatile generation (Cooper et al., 2007; Jamtveit et al., 2004; Suchy et al., 2004; Wycherley et al., 1999), and global environmental crisis (Aarnes et al., 2015; Aarnes et al., 2011; Storey et al., 2007; Svensen et al., 2008; Svensen et al., 2007; Svensen et al., 2010; Svensen et al., 2004; Svensen et al., 2009). The thermal effect of a thin sill emplaced in Trias-sic sediments was recently studied using core data from borehole DH4 located 4.5 km to the southeast of Longyearbyen (Senger et al., 2014a). The aureole is associated with higher maturity and decreased carbon content toward the sill intrusion and is approximately the same thickness as the dolerite both above and below the ~2 m thick intrusion.
In general, the metamorphic aureole is twice the intrusion thickness (Aarnes et al., 2015; Aarnes et al., 2010; Svensen et al., 2009), implying that the 103,000-206,000 km3 of sills thermally altered between ~200,000 and 400,000 km3 of Carboniferous-Permian-Triassic sediments during the massive BSSC magmatic event in the Barents Sea (Table 3 and Fig. 9). By using an approach similar to that of Svensen et al. (2004), the mass of carbon potentially transferred from organic matter to gas ranges between 2,400 and 19,200 Gt for a sill thickness
3 150 A
3 80 <
between 114 and 228 m over the 900,000 km2 area (Table 3 and Fig. 9). These simple calculations assume that 0.5 to 2 wt% of the carbon in the aureole is potentially converted into gas, regardless of the kerogen type and origin (marine vs. continental), initial carbon content, and maturation stage. The end member values correspond to 22 and 174 TBOE (trillion of barrels of oil equivalent) of hydrocarbon potentially produced during the magmatic event (0.11 t of carbon in one barrel of crude oil). The carbon from the aureole can be partly mobilized as thermogenic methane with a 613C value of about — 30%» (Svensen etal., 2015b).
The production rates and the fate of the aureole gases are more difficult to determine based on the current study. The release ofisotopical-ly light carbon aureole gases in LIPs may have triggered global warming a number of times during the Earth history (e.g., Svensen et al., 2015a). The timing of the BSSC is broadly similar to the Oceanic Anoxic Event 1a (OAE1a) and the release of isotopically light carbon from this event may have caused the OAE1a negative isotope excursion and associated global warming (Ando et al., 2002; Godet et al., 2006). Some of the aureole gases may also still be present in the east Barents Basin, and could form parts of the gigantic gas fields in the area (Fig. 2).
6. Conclusions
The Early Cretaceous BSSC magmatism in the High Arctic had important global paleogeographic implications that can be interpreted from the integrated seismic-gravity-magnetic data, radiochronology results, boreholes, and field data. The interaction between a mantle plume with the lithosphere caused the northern portion of the Barents Sea shelf to be uplifted, resulting in regional changes in sedimentary deposi-tional patterns during the Barremian and Early Aptian times. The massive (900,000 km2) and likely rapid injection of hot magma into organic-rich sediments in the Barents Sea caused rapid organic matter maturation and formation of thermogenic gas and oil between 120 and 125 Ma. We estimated that up to 19,200 Gt of carbon were potentially mobilized from the contact aureoles to form thermogenic gas. Finally, we have demonstrated that the large range of published ages of BSSC related rocks based on the K/Ar system is much more likely to be the result of alteration (with a dominant phase in the Early Cretaceous at ca. 80 ± 5 Ma) rather than representing an extended period of magmatism.
Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.palaeo.2015.07.007.
Acknowledgments
We gratefully acknowledge the financial support from the Norwegian Research Council with its Centers of Excellence funding scheme (project number 223272) and with Statoil through the PETROBAR project. The seismic, magnetic, and gravity data presented in this study were provided by TGS. The Norwegian Petroleum Directorate gave access to their seismic data. Erik Halvorsen (University of Oslo) supplied his paleomagnetic samples for dating, and Snorre Olaussen (University of Svalbard) coordinated the sampling of the Longyearbyen CO2 wells on Svalbard. Finally, we thank Fred Jourdan, Kim Senger, and Carol Evenchick for their constructive reviews.
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