Structural Development of the Continental Shelf Offshore Lofoten–Vesterålen, Northern Norway 19

NORWEGIAN JOURNAL OF GEOLOGY Structural development of the continental shelf offshore Lofoten–Vesterålen, northern Norway 19 Structural development of the continental shelf offshore Lofoten–Vesterålen, northern Norway Roald B. Færseth Færseth, R.B.: Structural development of the continental shelf offshore Lofoten–Vesterålen, northern Norway. Norwegian Journal of Geology, vol. 92., pp 19-40, Trondheim 2012. ISSN 029-196X. A striking feature of the continental shelf along the Lofoten–Vesterålen margin is the relatively thin sequence of Jurassic-Triassic sediments not only on structural highs, but also in parts of some of the sub-basins. This reflects limited sedimentation as well as periods of uplift and erosion, factors that are crucial for the hydrocarbon potential of the area. Structures in the shelf area off Lofoten-Vesterålen result primarily from major rift episodes in the Middle-Late Jurassic and Late Cretaceous- Paleocene, whereas this area appears unaffected by large faults resulting from the Late Permian-Early Triassic rift episode. This is in strong contrast to near shore and well-explored areas to the south, where large fault-bounded basins filled with thick Permo-Triassic sediments developed during and following this rift episode. At this stage of development, the Lofoten-Vesterålen archipelago and the shelf area are interpreted as parts of a huge NE-SW oriented basement high characterized by a peneplained basement relief, later progressively covered by thin Triassic to Middle Jurassic sediments. Jurassic faults in the Lofoten–Vesterålen margin segment are basement-involved, and both their orientation and listric geometry demonstrate the strong basement-governing control. A major along-strike change in structural pattern takes place across an accommodation zone at a high angle to the NNE-SSW trend of the Jurassic rift structures, and this zone acted as a rift propagation barrier during Jurassic crustal stretching. The accommodation zone reflects an E-W basement grain, and the change in dip direction of the Jurassic faults and the tilt of major fault-blocks across this zone took place without evidence of strike-slip motion. The Jurassic rift topography became overprinted by NE-SW to ENE-WSW striking faults resulting from a Campanian-Paleocene rift episode and by the overlying and NE-SW oriented Late Cretaceous Ribban Basin. A major uplift with NE-SW elongation and a maximum width of c. 70 km is located in the shelf area. The uplift is interpreted to result from tectonic compression related to the continent break-up to the NW, the main growth occurring during Oligocene-Miocene times. Roald B. Færseth, Ryenbergveien 75A, 0677 Oslo, Norway (e-mail: [email protected]) Introduction the areal distribution of Jurassic as well as older sediments throughout the study area. In most of the area a thin interval The Lofoten–Vesterålen continental margin, situated is mapped between top basement and the base Cretaceous between the Bivrost Lineament and the Senja Fracture Zone reflector (Fig. 2), and workers have treated this interval as an represents the transition between the passive and much unspecified sequence to which they have assigned a Juras- wider continental Vøring margin and the sheared West sic to Late Paleozoic age (Jørgensen & Navrestad 1981; Bøen Barents Sea margin (Fig. 1). The Lofoten–Vesterålen margin et al. 1984; Mokhtari & Pegrum 1992; Blystad et al. 1995; segment is approximately 400 km long, typified by a narrow Løseth & Tveten 1996; Mjelde et al. 1996; Tsikalas et al. 2001; continental shelf with a steep slope. It is in contrast to adja- Tsikalas et al. 2005; Bergh et al. 2007; Faleide et al. 2008). cent margin segments in terms of crustal structure, break- up magmatism and sediment thickness (e.g. Mjelde et al. The area under consideration is characterized by basement 1993; Eldholm & Grue 1994; Blystad et al. 1995; Tsikalas et highs with an overall NE-SW orientation and large al. 2001; Steltenpohl et al. 2004). Cretaceous basins (Fig. 1), but becomes structurally more complex beneath the basins (e.g. Mokhtari & Pegrum 1992; The Lofoten-Vesterålen shelf has not been opened to Blystad et al. 1995; Bergh et al. 2007). Structural elements in petroleum exploration and accordingly no commercial the offshore area are bounded by normal faults with orienta- wells have been drilled. Shallow wells drilled in the area tions similar to the basement structural grain that has been have however, proven potential Jurassic reservoir and source studied onshore Lofoten–Vesterålen (Gabrielsen & Ramberg rocks (Hansen et al. 1992), and it has been speculated that 1979; Gabrielsen et al. 2002; Bergh et al. 2007). Authors have reservoirs of this generation may contain major oil and considered Permo-Triassic, Permo- Jurassic, Late Jurassic gas resources similar to prolific petroleum provinces off- -Early Cretaceous, Middle Cretaceous and Late Cretaceous shore Mid Norway. In the absence of deep borehole infor- -Early Tertiary rifting as the dominant post-Caledonian mation there is still uncertainty regarding the thickness and tectonic events in the structural evolution of this margin 20 R. B. Færseth NORWEGIAN JOURNAL OF GEOLOGY Senja Figure 1. Structural elements of the Nor- FZ wegian Sea (slightly modified from Blys- tad et al. 1995). The Lofoten-Vesterålen HarstadBasin archipelago with the shelf area outlined. Late Devonian extensional shear zones onshore Norway are shown. NSZ: Nesna Bivrost FZ Shear Zone with the interpreted offshore extension at basement level (Olesen et g e d al. 2002). MTFC: Møre-Trøndelag Fault R i n s i Complex, a prominent structural feature Bivrost Lineament B a both onshore and offshore Norway. U t r ø s t b b a n R i Hel m Graben Jan Mayen FZ gh Hi n e Nyk l i Lofoten Ridge n d c r i S y n å g N n a s i a B æ n T r a r j a l l G en å s an Dønna Terrace H J n a n t e a l d H an NSZ M a y ll e H h e g i n H e L i n e a m c r a s i n a l a i n T e r B g r a M e n n e t r o a ø h F r M g i H a ø y r F sin a -b MTFC tn S ub bo i n S løre a s k B be n u l n r Basi M a agnus M gn G ra pen o m Ta S S pur NW SE Utrøst Træna Marmæla Ribban Lofoten Vestfjorden Nordland Ridge Basin Spur Basin Ridge Basin Ridge 0 1 2 3 TWT (s) 4 5 ? 6 Quaternary Jurassic - Triassic 0 10km Tertiary Jurassic - Upper Paleozoic Upper Cretaceous Basement Lower Cretaceous Færseth Figure 2. The Lofoten Ridge is a deeply eroded and narrow Precambrian basement horst that separates the oppositely tilted Ribban andFig. Vest 2- fjorden basins. The relative thin sequence of Jurassic-Triassic sediments overlying the Precambrian basement is a characteristic feature in the shelf area beneath the Cretaceous Ribban Basin. The cross section is based on seismic reflection profile LO-08-87 (see Fig. 4 for location of the cross section). NORWEGIAN JOURNAL OF GEOLOGY Structural development of the continental shelf offshore Lofoten–Vesterålen, northern Norway 21 Figure 3. Tecto- nostratigraphic frame work of the Norwegian Sea. Post- Caledonian structures result primarily from three extensional episodes (Corfield et al. 2001; Færseth & Lien 2002; Ren et al. 2003; Müller et al. 2005) followed by compression and late uplift (Gómez & Vergés 2005). The stratigraphy is repre- sentative of areas offshore Mid Nor- way where a large number of commercial wells have been drilled. Kimmeridgian Sakmarian segment (Mokhtari & Pegrum 1992; Blystad et al. 1995; Løs- documented offshore Mid Norway (Fig. 3), in the western eth & Tveten 1996; Lundin & Doré 1997; Doré et al. 1999; Barents Sea (e.g. Faleide et al. 1984; Gabrielsen et al. 1990; Tsikalas et al. 2001; Wilson et al. 2006; Bergh et al. 2007). Gudlaugsson et al. 1998) and East Greenland (e.g. Surlyk However, proposed models for the structural evolution, in 1990; Hartz et al. 2002; Hamann et al. 2005). the context of timing and importance of tectonic events, remain open to questioning. This is due to uncertain ty The present study focuses on the structural development of regarding the age and thickness distribution of the pre- the Lofoten-Vesterålen continental shelf to explore the effect Cretaceous sediments and to periods of deep erosion that and timing of latest Paleozoic to Cenozoic tectonic episodes have removed significant parts of the sediment ary succes- proven to be important in adjoining and well-explored sion over considerable areas. As a consequence, the develop- areas, and furthermore to investigate the sedimentological ment of this margin segment should be seen in a regional response to the structuring in the area under consideration. context as influenced by post-Caledonian tectonic episodes 22 R. B. Færseth NORWEGIAN JOURNAL OF GEOLOGY 10°00’ 12°00’ 14°00’ 16°00’ Offshore data Jurassic faults Tromsø Latest Cretaceous - Paleocene faults Accommodation zone The structural development presented in this paper is Location of shallow cores Harstad Basin based on the interpretation of 2D seismic sections offshore Lofoten- Vesterålen. The seismic profiles are recorded to a Andøya 69°00’ depth of 5-8 s TWT, which is well below the basement-cover Andfjorden Harstad contact. The seismic used in this study has been the found- Vesterålen ation for seismic mapping and previous structural analysis JenneggaHigh Hadseløy along the Lofoten-Vesterålen margin (Mokhtari & Pegrum Hadselfjord Austvågøy Ridge 1992; Blystad et al. 1995; Løseth & Tveten 1996; Doré et al. Utrøst Havbåen Sub-basinVestvågøy 1999; Tsikalas et al.

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