ARTICLE IN PRESS Marine and Petroleum Geology 25 (2008) 235–253 www.elsevier.com/locate/marpetgeo Nearshore Mesozoic basins off Nordland, Norway: Structure, age and sedimentary environment Reidulv Bøea,, Morten Smelrora, Børre Davidsena, Olav Walderhaugb a Geological Survey of Norway, 7491 Trondheim, Norway b Statoil ASA, 4035 Stavanger, Norway Received 13 March 2007; received in revised form 6 June 2007; accepted 6 July 2007 Abstract Seismic data acquisition along the Nordland coast (Norway) has resulted in the discovery of two downfaulted sedimentary basins of Mesozoic age in an area of Precambrian–Lower Palaeozoic metamorphic rocks. The Stabbfjorden Basin (informal name) west of Meløya has a sedimentary rock succession that is up to 800 m thick, while the Lyngværfjorden Basin (informal name) northeast of Træna is at least 350 m deep. Microfossil dating of ice-transported erratic blocks thought to be eroded from these basins and found on nearby skerries range in age from Barremian to Triassic. The majority of the samples are of Middle-Late Jurassic age. The sedimentary blocks comprise conglomerates, sandstones, siltstones and mudstones, and frequently contain shells and coal fragments. Lithofacies analyses show that they reflect shallow marine deposition, but it is possible that the oldest samples were deposited in a continental environment. The lithofacies reflect the well-known tectonic phases and sea-level changes that have been reported for the Middle Jurassic–Early Cretaceous in the Nordland offshore region. Vitrinite reflectance measurements show that organic material in the samples can be classified as immature, and the newly discovered basins have no hydrocarbon exploration potential. r 2007 Elsevier Ltd. All rights reserved. Keywords: Norwegian Sea; Mesozoic; Nearshore; Fault basin; Erratic block 1. Introduction Over the past 30 years or so, Mesozoic rocks and fault basins have been extensively mapped in fjords and the coastal zone of southern and central Norway, and in parts of northern Norway (e.g. Oftedahl, 1975; Dalland, 1981; Holtedahl, 1988; Bøe and Bjerkli, 1989; Bøe, 1991; Bøe et al., 1992; Fossen et al., 1997; Bøe and Skilbrei, 1998; Davidsen et al., 2001; Sommaruga and Bøe, 2002). Age estimates of the Mesozoic rock successions are mainly based on dating of glacially eroded erratic blocks found on nearby islands and skerries. On land, Mesozoic rocks are only found on Andøya in northern Norway. The area of Norway that has gained least attention is the Nordland coast, between Vikna and Vestfjorden (Fig. 1). This area has not been opened for hydrocarbon exploration, and large open sea areas have never before been Corresponding author. Tel.: +47 73904000; fax: +47 73921620. E-mail address: reidulv.boe@ngu.no (R. Bøe). 0264-8172/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.marpetgeo.2007.07.004 mapped by reflection seismic methods, e.g. between Vikna and Vega. The landward boundary of Mesozoic rocks is commonly uncertain, and it has not been known if Mesozoic basins occur between islands and skerries landward of this boundary. The region between Træna and Bodø is of special interest as it is at a junction between the Trøndelag Platform to the south, the Vestfjorden Basin to the north and the Norwegian mainland to the east. From previous work (Gustavson and Gjelle, 1991; Gustavson and Blystad, 1995), it was known that the geology of this area is complicated, and the age and structure of the sedimentary rock successions have not been properly established. Bugge et al. (2002) documented a thick Permo-Triassic succession in boreholes south of Træna. It was thus considered possible that sedimentary rocks could occur also in Trænfjorden, immediately east of Træna. In this contribution, we present results of work undertaken in the period 1998–2005 in the Træna and Meløya areas (Fig. 1). Results of seismic profiling and erratic block ARTICLE IN PRESS 236 R. Bøe et al. / Marine and Petroleum Geology 25 (2008) 235–253 Fig. 1. Overview map showing the Norwegian shelf between Trondheim and Andøya. The locations of Figs. 2, 3 and 5 are shown. sampling have been used to outline structural and stratigraphic evolution. 2. Methods Seismic surveys were carried out with NGU’s reasearch vessel FF Seisma in 1998, 2002 and 2003 (Bøe et al., 2003, 2005). Altogether, 89 single channel seismic lines with a total length of 960 km were acquired from Vikna in the south (65 1N) to Gildeskål in the north (671100 N) (Figs. 1 and 2). The majority of the lines were planned perpendicular to the assumed strike direction of sedimentary strata. A 40 in3 sleevegun fired every 3.5 s (7–9 m horizontal separation between each shot point at 4.5 knots) was used as acoustic source. This was deployed together with a Geopulse boomer, and the two signal sources were run sequentially. The reflected signals were recorded by a Benthos streamer. Marine magnetic data were acquired along seismic profiles with a GSM 19 Overhauser proton magnetometer. The seismic data exhibit a maximum penetration of 0.2–0.3 s TWT (seconds two-way travel time) into the Mesozoic sedimentary successions. Assuming a seismic velocity of 3500 m/s in these rocks, that corresponds to a penetration of 350–525 m. Calculation of bedding orientation of the Mesozoic strata (at the crossing points of seismic lines) was also done assuming a sound velocity of 3500 m/s. Searches for erratic sedimentary blocks were conducted on islands and skerries seaward of the fault basins in Ternholmfjorden–Stabbfjorden and Lyngværfjorden– Trænfjorden according to the assumed late glacial icetransport direction (e.g. Olsen, 2002). A total of 87 erratic sedimentary blocks were collected of which 40 were dated/ attempted dated using acid-resistant microfossils (palynomorphs). No samples were found on visited islands in the Vikna–Vega area (Fig. 1). Petrographic thin-sections were made from five blocks of sedimentary rock collected on the islands Rorstabben and Grønna (Fig. 2). The thin-sections were examined with a petrographic microscope and point-counted with three hundred points per thin-section. Detailed multibeam bathymetry was used for studying ice movement directions and source areas for erratic blocks transported by glaciers. In addition, 50 m swath bathymetry grids were combined with a 50 m grid covering ARTICLE IN PRESS R. Bøe et al. / Marine and Petroleum Geology 25 (2008) 235–253 237 Fig. 2. Seismic lines and sampling localities in the Træna–Meløya area. Yellow dots: localities with finds of erratic sedimentary blocks of Mesozoic age. Red dots: localities without finds of erratic sedimentary blocks of Mesozoic age. See Fig. 1 for location. Norway’s onshore and offshore to study regional structures and erosional/depositional features. 3. Stratigraphy 3.1. Stabbfjorden Basin In Ternholmfjorden and Stabbfjorden northwest of Meløya, sedimentary rocks occur over a 28 km long stretch, from the small island Moholmen, in the southwest, to north of Støttvær, in the northeast (Fig. 3). A 3–10 km wide area of basement rocks separates the sedimentary rocks, informally named the Stabbfjorden Basin, from the sedimentary rocks in the Vestfjorden Basin. The Stabbfjorden Basin reaches a maximum width of ca. 6 km between Flatværet/Varkgård and Kallsholmen (southernmost of the Ternholman islands). In the southwest, the Stabbfjorden Basin terminates in two wedge-shaped structures. A small area of sedimentary rocks occurs between the wedges, and a similar occurrence is found east of Kallsholmen. In addition to a detailed mapping of the Stabbfjorden Basin, the nature and position of the southeastern boundary of the Vestfjorden Basin is revised (Fig. 3). The remapping has shown that between Valvær and Grønna there are depositional contacts with slightly undulating trends. It is possible that sedimentary strata extend eastwards between Ternholman and Grønna. It is also possible that such rocks may occur west of Kallsholmen, between the Stabbfjorden Basin and the Vestfjorden Basin, but the seismic data do not allow certain identification. The thickness of Quaternary deposits on top of the Stabbfjorden Basin varies from zero to more than 100 m. The greatest thicknesses occur along the margins of Vestfjorden, and locally in the deeper parts of Ternholmfjorden, Stabbfjorden and Valværfjorden (Figs. 2 and 3). In Stabbfjorden, the character of the seismic signals locally allows subdivision of the sedimentary succession into several units. The lowermost unit, which occurs with a primary, depositional unconformity on top of the basement (Fig. 4a), is well stratified with strong, low amplitude reflectors that are laterally persistent. In the upper part of this unit, a prograding interval with reflectors showing downlap towards the northwest occurs (Figs. 4a and b). The next unit is relatively homogeneous, with weak, highamplitude, laterally persistent reflectors. In its lower part, 238 ARTICLE IN PRESS R. Bøe et al. / Marine and Petroleum Geology 25 (2008) 235–253 Fig. 3. Geological interpretations, seismic lines and 50 m bathymetry grid in the Stabbfjorden–Ternholmfjorden area. Note the NE–SW ice-marginal delta extending from NE of Grønna to Valvær (approximateley the boundary between yellow and green colors, 120–130 m depth). See Fig. 1 for location of the survey area. ARTICLE IN PRESS R. Bøe et al. / Marine and Petroleum Geology 25 (2008) 235–253 239 Fig. 4. (a) Part of seismic line NGU0207010 from Stabbfjorden. (b) Part of seismic line NGU0207007 from Ternholmfjorden/Stabbfjorden. Note the prograding interval, between the orange lines, with reflectors downlapping (red arrows) towards the northwest. Also note that the downlapping reflectors terminate beneath reflectors subparallel to the orange lines towards the SE. The prograding interval was probably deposited in a shallow-marine environment during progradation of a coastline or a delta. Also note the homogeneous interval, without clear reflectors, between the uppermost orange refelector and the blue reflector. These sediments were probably deposited after a transgression towards the southeast. See Fig. 3 for location of the seismic lines. reflectors onlap the underlying wedge in a southeasterly direction. The uppermost unit again has strong, low amplitude reflectors, but they are less laterally persistent and parallel than in the lowermost unit. The sedimentary succession in Stabbfjorden can be traced into Ternholmfjorden, where it is cut by several faults (see below), which makes it hard to follow units laterally. The succession reaches a thickness of more than 400 m in Ternholmfjorden and as much as 800 m in Stabbfjorden. It is possible, however, that sedimentary rocks also occur at deeper levels in Ternholmfjorden. Commercial seismic data would be needed to determine this. 3.2. Lyngværfjorden Basin and Træna area The NNE-trending Lyngværfjorden Basin is about 12 km long and 3 km wide (Fig. 5). The southernmost termination of the basin is poorly defined by the seismic data. The sedimentary succession reaches a thickness of at least 350 m in the northern part of the basin, while in the south it is less than 200 m. However, one seismic line indicates that there are inclined strata also at deeper levels, below an angular unconformity. If this interpretation is correct, several hundred meters of older sedimentary rocks may occur below the uppermost succession. The character of the seismic signals does not allow subdivision of the upper sedimentary succession in the Lyngværfjorden Basin. This can partly be related to a steep dip of the layering and complex structuring (see below), but may also reflect coarse-grained and poorly layered sediments, giving rise to discontinuous reflectors. A welllayered succession is, however, displayed in some seismic sections (Fig. 6). Triassic rocks occur in an up to 6 km-wide zone along the coast northwest of Lyngværfjorden (Rokoengen et al., 1988). Lower-Middle Jurassic rocks occur northwest of this and Triassic rocks are locally exposed between the Jurassic ARTICLE IN PRESS 240 R. Bøe et al. / Marine and Petroleum Geology 25 (2008) 235–253 Fig. 5. Geological interpretations, seismic lines and 50 m bathymetry grid in the Træna and Lyngværfjorden areas. See Fig. 1 for location of the survey area. ARTICLE IN PRESS R. Bøe et al. / Marine and Petroleum Geology 25 (2008) 235–253 241 Fig. 6. Part of seismic line NGU0304014 from Lyngværfjorden. Note dipping reflectors showing presence of sedimentary rocks below seabed. See Fig. 5 for location of the seismic line. deposits due to faulting along NE–SW trends. A predominant NE–SW fault direction in the Triassic succession offshore is also apparent in our seismic data. It is possible that the Triassic rocks can be traced into Lyngværfjorden northeast of Træna, but this interpretation is uncertain. Almost 100 m of Quaternary deposits occur offshore and in the southwestern parts of Lyngværfjorden. The seismic resolution is poor below the Quaternary. Southern Trænfjorden and the area southwest of Træna were mapped in 1992 (IKU, 1995; Bugge et al., 2002), and a 750 m-thick, Upper Permian–Lower Triassic fully marine succession of sandstones, coarse-grained turbidites, shales and reworked sabkha sediments was cored 12 km southwest of Træna (Fig. 5). IKU’s seismic interpretation shows Triassic sediments also in southern Trænfjorden. Our mapping has confirmed the presence of a sedimentary basin trending north-northeastwards in southern Trænfjorden (Fig. 5). The basin is fault-bounded on both sides, but its termination towards the north–northeast, 3 km southeast of Austholmen, is probably a depositional contact. Sedimentary rocks farther northeast in Trænfjorden were not observed, but the coverage of the presently available data set is too sparse to be conclusive. 3.3. Age, lithofacies and source areas of erratic sedimentary blocks Dating of erratic blocks was done by studying their content of palynomorphs, i.e. pollen, spores and dinoflagellate cysts. Cores from the continental shelf are commonly dated by this method because palynomorphs are usually numerous, they are degradation-resistent, and their stratigraphic distribution is well documented (Vigran and Thusu, 1975; Birkelund et al., 1978; Riding and Thomas, 1992; Hardenbol et al., 1998). Detailed results of the microfossil analyses are given in Bøe et al. (2005), while interpreted ages are presented in Table 1. The dated samples range in age from Barremian to Triassic although the majority are of Middle-Late Jurassic age. The erratic sedimentary blocks comprise conglomerates, sandstones, siltstones and mudstones (Table 1). Sandstones and mudstones predominate. The samples are generally rich in carbonate, and many contain coal fragments and shells/shell fragments. Based on color, texture, structure and age, the erratic blocks are divided into 13 lithofacies (Table 1, Fig. 7). In the Stabbfjorden area, the upper part of the Quaternary succession is locally fluted by moving glaciers, and a late glacial sediment transport direction towards the northwest is evident. This trend is especially clear north of Varkgård in Stabbfjorden, and west of Kallsholmen in Ternholmfjorden (Figs. 3 and 8). Southeast of Rorstabben, lineations indicate a more northerly movement. The direction of glacial lineations is consistent with transport of the erratic sedimentary blocks on Kallsholmen from Ternholmfjorden (Stabbfjorden Basin), and with derivation of the erratic blocks on Rorstabben from the Stabbfjorden Basin southeast of Rorstabben. The beach ridge at Grønna, at the margin of Vestfjorden, comprises reworked morainal material. From multibeam bathymetry it is evident that it occurs at the outermost part of a major, glacial debris lobe derived from the Svartisen glacier in the southeast (Figs. 2 and 3). Bathymetric features such as glacial lineations show that the lobe postdates the SW-moving Vestfjorden ice stream (Ottesen et al., 2005). The Mesozoic erratic blocks at Grønna are thus most likely derived from the Stabbfjorden Basin, to the southeast, and not from the Vestfjorden Basin. In the Lyngværfjorden area, glacial lineations and flutes point to a glacier movement direction towards the west–northwest (Fig. 8). Erratic sedimentary blocks were not found on the small skerries north–northwest of the Lyngværfjorden Basin. On Selvær and Torvvær (Fig. 2), however, blocks of Middle-Late Jurassic and Triassic age ARTICLE IN PRESS 242 R. Bøe et al. / Marine and Petroleum Geology 25 (2008) 235–253 Table 1 Lithofacies and age (interpreted from microfossils) of erratic sedimentary blocks Facies 1 2 3 4 5 6 7 8 9 10 11 12 13 Description Age Gray, stratified, poorly sorted conglomerate with shells and coal fragments Dark gray, massive to vaguely bedded, well-sorted medium sandstone Light gray, massive, well-sorted mudstone Gray–very dark gray, massive, well-sorted mudstone and siltstone with shells and coal fragments Dark gray, massive, well-sorted very fine to fine sandstone with shells and coal fragments Light (brownish) gray, massive and (cross-) laminated, well-sorted siltstone with shells Dark gray, poorly sorted, massive and bedded/cross-bedded, pebbly, medium to very coarse sandstone with shells and coal fragments Light (yellowish) gray to gray, massive to weakly stratified, moderately to wellsorted, medium to very coarse sandstone with shells and coal fragments Light gray, stratified, bioclastic mediumgrained sandstone Light gray–gray, massive and (cross-) laminated very fine–fine sandstone with shells and coal fragments Yellowish gray, moderately sorted, bioclastic medium-grained sandstone with intraformational silt clasts and shells Yellow, poorly sorted, bioturbated (roots), medium-coarse sandstone with shells, coal fragments, terrestrial palynomorphs and fungal remains Light yellowish brown, massive, wellsorted, fine-medium sandstone Early Cretaceous Early Cretaceous Early Cretaceous Late Jurassic Middle and Late Jurassic Undefined MiddleLate Jurassic Undefined MiddleLate Jurassic Middle and Late Jurassic Undefined MiddleLate Jurassic Middle Jurassic Triassic Not determined Not determined occur. This could indicate that such rocks are present in central Trænfjorden although we have not been able to identify them during our seismic investigations (see above). It is possible that local glacier tongues may have been directed towards the southwest, and that erratics on the Træna islands may have been eroded from the Lyngværfjorden Basin. 4. Thin section petrography Three of the examined thin-sections are from Bajocian– Callovian blocks of pervasively calcite-cemented sandstone (Fig. 9, Table 2). Grain sizes are fine, medium and very coarse respectively, and all samples are poorly sorted. The sandstones are quartz-rich with moderate to high Kfeldspar contents, and also contain minor volumes of plagioclase. Mica content is highly variable with one sample containing a maximum of almost 30% mica, mostly brown biotite. A few plant fragments, brachiopod frag- ments and glauconitic grains are also present together with trace amounts of heavy minerals. The heavy mineral suite comprises zircon, tourmaline, rutile, garnet, staurolite, kyanite, sphene, opaque iron–titanium oxides, and in one case possibly epidote. Diagenetic cements other than calcite are restricted to traces of siderite, possibly traces of dolomite, variable amounts of pyrite, minor K-feldspar overgrowths in one sample, and traces of iron oxide from recent weathering. Porosities are practically zero because of the extensive calcite cementation. No evidence for grain contact dissolution is found. One thin-section was made from a boulder of Volgian sandstone found on Rorstabben. The Volgian sandstone sample is poorly sorted, coarse-grained, pervasively calcitecemented and quartz-rich (Fig. 10, Table 2). K-feldspar content is moderate, and minor to trace amounts of plagioclase, muscovite, brown biotite and heavy minerals are present. Observed heavy minerals are restricted to zircon, tourmaline and sphene. A few percent of brachiopod fragments are also present. Other than calcite, diagenetic cements are limited to a few K-feldspar overgrowths, traces of pyrite, and some iron oxide staining due to recent weathering. No porosity is present due to the calcite cementation. One thin-section was prepared from a Barremian erratic from Grønna. This sample is poorly sorted, very coarse to conglomeratic and pervasively calcite-cemented sandstone (Fig. 11). Quartz is the dominant detrital mineral, although K-feldspar content is very high (Table 2). Minor volumes of plagioclase, muscovite, brown biotite and schistose to phyllitic rock fragments are also present. A substantial part of the K-feldspar occurs within granitic rock fragments together with quartz and in some cases minor plagioclase, muscovite or biotite. Additional grains include trace amounts of brachiopod fragments, plant fragments, chlorite, glauconitic clasts and the heavy minerals zircon, rutile, garnet and staurolite. Diagenetic minerals other than calcite are limited to traces of kaolinite cement. Porosity is practically zero because of the extensive calcite cementation. 5. Stratigraphic correlation and interpretation 5.1. Stabbfjorden Basin The oldest dated samples in the Stabbfjorden area are of Middle Jurassic age. These comprise sandstone and siltstone (lithofacies 5–10), with numerous marine microand macrofossils and coal fragments. We interpret the lowermost, stratified unit in the Stabbfjorden Basin to represent marine sediments deposited on a shallow shelf in Middle Jurassic times. The sedimentation may have been interrupted by pulses of coastal and/or continental deposition. Proximity to such an environment is suggested by the high number of coal fragments in many of the shallow marine sediment samples. The northwestward prograding sequence in the upper part of the unit probably reflects ARTICLE IN PRESS R. Bøe et al. / Marine and Petroleum Geology 25 (2008) 235–253 243 Fig. 7. Photographs of erratic blocks showing examples of lithofacies 1–13. See Table 2 for description of lithofacies and ages. The pictures are approximately 4  8 and 4  4 cm of the samples. building out of a coastline or a delta system during a relative sea-level fall. The prograding sequence is transgressed by a middle unit, which we interpret to be deposited in a marine environment following a relative sea-level rise. We interpret this homogeneous unit, with weak, continuous, high- amplitude reflectors to represent mainly fine-grained sediments (lithofacies 4–9) deposited in late Middle Jurassic–Early Late Jurassic times. The uppermost unit in the Stabbfjorden Basin was probably deposited in a shallow-marine, near-coast environment in the Late Jurassic-Early Cretaceous, following a ARTICLE IN PRESS 244 R. Bøe et al. / Marine and Petroleum Geology 25 (2008) 235–253 Fig. 8. Ice transport directions superimposed on regional bathymetry in the Træna–Stabbfjorden area. Blue arrows: directions derived from swath bathymetry; white arrows: directions derived from regional bathymetry. Fig. 9. Micrograph of poorly sorted, medium-grained subarkosic to arkosic sandstone of Bathonian to Callovian age. White grains are quartz; white grains with abundant small brown specks are K-feldspar; the elongate brown grain to the left is a biotite flake. All pores are filled by calcite cement. Sample Rorstabben 12. relative sea-level fall. This unit is probably represented by sandstones of Volgian–Ryazanian (lithofacies 8) and Early Cretaceous age (lithofacies 2). A mudstone sample of Valanginian–Hauterivan age from Grønna (lithofacies 3) is probably also derived from the Stabbfjorden Basin (uppermost part of the succession). The conglomerates of lithofacies 1 possibly reflect faulting and rejuvenation of the hinterland in the Barremian. ARTICLE IN PRESS 245 R. Bøe et al. / Marine and Petroleum Geology 25 (2008) 235–253 Table 2 Modal analyses of thin sections of erratic sedimentary blocks Sample Age Rorstabben 3 ?Bajoc.–?Callov. Rorstabben 5 Volgian Rorstabben 12 Bathon.–Callov. Grønna 1 Barremian Grønna 18 (upper) Bathon. Quartz clasts K-feldspar clasts Plagioclase clasts Muscovite clasts Biotite clasts Chlorite clasts Heavy minerals Plant fragments Clay clasts Glauconitic clasts Schist/phyllite clasts Brachiopod fragments Detrital clay matrix K-feldspar overgrowths Authigenic kaolin Siderite cement Calcite cement Pyrite cement Hematite cement Porosity, primary Porosity, dissolution Grain size (mm) Sorting (mm) 19.3 2.3 0.3 2.3 25.7 – 0.3 0.7 0.3 trace – 1.3 trace – – – 47.0 0.3 trace – – 0.210 0.132 46.7 10.7 0.3 0.7 1.3 – trace – 0.7 – – 2.0 – 0.3 – – 37.0 0.3 trace – – 0.756 0.771 40.0 17.3 0.3 3.7 2.7 trace 0.3 – 0.3 trace – – – 1.7 – trace 32.7 trace 0.7 0.3 – 0.333 0.127 38.0 19.0 3.0 2.3 1.3 trace 0.3 trace – trace 1.3 trace – – 0.7 – 33.7 – – – 0.3 1.733 1.492 25.0 7.7 2.7 1.3 2.0 – 0.3 3.0 0.3 3.7 – trace 1.3 – – 2.3 43.0 7.3 – – – 1.037 1.103 Grain size is the average long axis measurement for 30 grains per thin-section. Sorting is the standard deviation of these measurements. Fig. 10. Micrograph of coarse-grained, poorly sorted subarkosic sandstone of Volgian age. White grains are quartz and in some cases K-feldspar, a brownish brachiopod fragment is present in the upper right corner. All pores are filled by calcite cement. Sample Rorstabben 5. Palaeogeographical maps for the Middle Jurassic show the Stabbfjorden area located close to the boundary between coastal/shallow-marine conditions, with deposi- tion of predominantly sandstones in the north, and shallow-marine conditions, with deposition of more finegrained sediments in the Helgeland Basin, in the south ARTICLE IN PRESS 246 R. Bøe et al. / Marine and Petroleum Geology 25 (2008) 235–253 Fig. 11. Micrograph of very coarse-grained, poorly sorted arkosic sandstone of Barremian age. White grains are quartz and feldspar, K-feldspar typically contains brownish specks, and a plagioclase grain to the upper right contains parallel cleavage traces. All pores are filled by calcite cement. Sample Grønna 1. (Fig. 12). Brekke et al. (2001) interpreted the source areas for these sediments to be uplifted domes in southern Norway, in the present Møre and Vøring Basins, and between northern Norway (Troms/Finnmark) and Greenland. This is a speculative model as the palaeogeography depends very much on how tightly one closes the Norwegian-Greenland Sea. In principle, one can place Norway very close to Greenland without violating existing reconstruction constraints, removing the need for a landmass in the middle. However, domes may have been an early expression of the volcanic activity and rifting to come later in the Jurassic, Cretaceous and Tertiary, and the connection between the ocean areas in the Norwegian Sea and the ocean areas farther north may have been closed due to doming, in the Middle Jurassic. This has also been suggested by other authors, e.g. Dalland (1981), Larsen (1987), Doré (1992), Brekke et al. (1999) and Brekke (2000). The interpretation is supported by data from Andøya, which show that in the Bajocian–Bathonian, a delta was supplied with sediments from the west, north and east (Dalland, 1981). Further southwest, in Vesterålen, shallow marine deposits from the Bathonian–Callovian occur in the Sortlandsundet Basin (Davidsen et al., 2001). These can be correlated both in age and depositional environment with the deposits in the Stabbfjorden Basin. Shallow-marine deposition possibly also prevailed in the Vestfjorden area, between Sortlandsundet and Stabbfjorden. In 1960, belemnites of Jurassic–Cretaceous age were found on land close to the Svartisen glacier (Grønlie, 1973). Fig. 12. Palaeogeographic reconstruction for the Middle Jurassic (Bajocian). According to this map, the Træna area was located in a more basinal position than Stabbfjorden, in the Middle Jurassic. Modified from Brekke et al. (2001) and Johannesen and Nøttvedt (2006). ARTICLE IN PRESS R. Bøe et al. / Marine and Petroleum Geology 25 (2008) 235–253 247 Fig. 13. Generalized time and lithostratigraphic section from the Halten Terrace to the Trænabanken area (modified from Blystad et al. (1995). The interpreted stratigraphic distribution of samples from the Stabbfjorden and Lyngværfjorden basins are shown in gray. Dots indicate sand and conglomerate. The fossils were found in moraine deposits in front of Østerdalsisen (Fig. 2), but it has been claimed that they were also found in rock outcrops. The exact locality of the finds was never reported. Grønlie (1973) concluded that if Østerdalsisen eroded the fossils, there is a possibility that rocks of Jurassic–Cretaceous age may occur beneath larger parts of Svartisen. In addition, Grønlie (1973) referred to the late commissioner of mines K.L. Bøckman in Nordland, who claimed that Mesozoic belemnites and ammonites were found during construction works in Glomfjord, east of the Stabbfjorden Basin, in 1909. If these reports are correct, it implies that Jurassic and possibly Cretaceous sediments were deposited far eastwards in Nordland and possibly into Sweden. Also Brekke et al. (2001) suggested that there was a connection between the Norwegian Sea and the Barents Sea via an open seaway eastwards and northwards through Sweden and Finland, in the Middle Jurassic (Fig. 12). The Middle Jurassic deposits in the Stabbfjorden Basin can be correlated in age and depositional environment with similar shallow marine and coastal deposits farther south along the coasts of Nordland and Mid-Norway (Bugge et al., 1984; Bøe and Bjerkli, 1989; Bøe, 1991; Smelror et al., 1994; Gustavson and Bugge, 1995; Bøe and Skilbrei, 1998; Sommaruga and Bøe, 2002). Erratic blocks and drill cores show that these sediments comprise alternating shallow-marine, coastal and continental deposits, and that they are generally coarse grained. Several of the authors have correlated the Middle Jurassic deposits along the coast with the Garn Formation (Bajocian–Bathonian) in the Haltenbanken area (Fig. 13). This formation was deposited by prograding braided delta lobes, and deltafront and delta-top facies with active fluvial and waveinfluenced processes are recognized (Dalland et al., 1988). The composition of the three Middle Jurassic samples from Rorstabben and Grønna are quite similar to the composition of the Middle Jurassic Garn and Ile Formations in the Halten and Dønna Terraces (Figs. 1 and 13), although K-feldspar contents are somewhat higher than what is normal in the Garn and Ile Formations. The heavy mineral suites in the Garn and Ile Formations also typically include monazite and some spinels and apatite, although the limited number of thin-sections available from the Stabbfjorden Basin makes comparison of the heavy mineral suites difficult. It is possible that the prograding interval in the upper part of unit 1 in the Stabbfjorden Basin (and possibly also the underlying succession) can be correlated with the Garn Formation, whereas the sediments in the lower part of unit 2 can be correlated with the fine-grained Melke Formation (Bajocian–Oxfordian) in the Norwegian Sea (Fig. 13). It should be noted, however, that the Garn Formation is absent in the Trænabanken area (Fig. 8) (Dalland et al., 1988), and that it may be discontinuously developed along the palaeo-coastline. In the Late Jurassic, areas subjected to rifting subsided. At the same time there was a sea-level rise. This caused deposition of steadily more fine-grained sediments in shallow-marine and deep-marine environments farther offshore. Organic-rich clays (Spekk Formation) (Fig. 13) were deposited over large parts of the mid-Norwegian margin, but have not been found in the Stabbfjorden area. ARTICLE IN PRESS 248 R. Bøe et al. / Marine and Petroleum Geology 25 (2008) 235–253 Fig. 14. Palaeogeographic reconstruction for the Late Jurassic (Oxfordian–Kimmeridgian). According to this map, the Træna area was located slightly farther offshore than Stabbfjorden in the Late Jurassic. Modified from Brekke et al. (2001) and Nøttvedt and Johannesen (2006). The occurrence of very dark grey siltstones and mudstones of Early Oxfordian to Early Volgian age (lithofacies 4) suggests that Stabbfjorden was a deposition area for shallow marine, fine-grained, clay-rich and carbonaceous sediments in the Late Jurassic, as also suggested by Brekke et al. (2001) (Fig. 14). The coarse clastics of Volgian to Early Ryazanian age (lithofacies 8) may reflect a sea-level fall in the Volgian (Dalland et al., 1988). This, along with crustal movements, caused coarse-grained sediments to be deposited on submarine fans along fault blocks, and coastal progradation with deposition of coarse-grained material in shallow-marine and deltaic settings. The Ternholmfjorden/Stabbfjorden area may have experienced rapid lateral shifts in depositional environment. Brekke et al. (2001) indicated a zone of coastal/ shallow-marine deposition east of the Stabbfjorden Basin, along the border of Fennoscandia (Fig. 14). To the southwest, the northeastern part of the Nordland Ridge experienced subaerial erosion, and crustal movements probably caused changing depositional environments along the ridge. One example is the shallow marine sanddominated Rogn Formation, which has a restricted distribution both in space and time (Fig. 13). The Ryazanian low-stand was followed by a renewed sea-level rise to an intermediate maximum in the Barremian (Brekke et al., 2001). The Early Neocomian was still dominated by emergent structural highs and platform areas, and the Ryazanian erosional unconformity on top of the carbonaceous marine shales is widespread (Vollset and Doré, 1984; Dalland et al., 1988; Surlyk, 1990; Smelror et al., 1998, 2001). In Ryazanian through Hauterivan times, deep basinal areas continued to develop by subsidence along the rift axis, e.g. the Vøring Basin. Shallow basins within the platform areas, such as the Helgeland Basin (Fig. 1), accumulated lime-rich open marine mudstones and shales (Brekke et al., 2001). A carbonaceous mudstone of Valanginian–Hauterivan age from Grønna (Table 1, Fig. 7) may be representative for the sediments deposited during this time interval, but we are not able to identify seismic units of Cretaceous age in the seismic data. The increasing sea-level that led to widespread shale and marl deposition in platform areas and in starved distal deeps was interrupted by a sea-level drop in the Barremian (Brekke et al., 2001). This gave time to renewed delta progradation from the transgressed land areas, e.g. the Wealden paralics in southern England (Doré, 1991), the Nordelva Member on Andøya (Dalland, 1981) and the Helvetiafjellet Formation on Svalbard (Nemec et al., 1988) before a new pulse of regional transgression continued into the Aptian. Two conglomerate samples of Barremian age from Grønna and Rorstabben (Table 1, Fig. 7) are interpreted to reflect the sea-level drop. The investigated Barremian sample from Grønna (Table 2) is similar in texture and composition to the Lower to Upper Cretaceous Lange Formation in well 6610/3-1R located 100 km west of the Stabbfjorden Basin although the Lange Formation tends to have a higher ratio of plagioclase to K-feldspar. 5.2. Lyngværfjorden Basin and Træna area Samples from Træna range in age from Early Oxfordian to Triassic. The sedimentary facies of the Middle-Upper Jurassic samples suggest a depositional environment similar to that in the Stabbfjorden Basin (marginalmarine/shallow-marine conditions). A Middle Permian tectonic event led to establishment of a seaway southward between Greenland and Norway from the Barents Sea (Doré, 1991). Following this event, there was a change from continental to marine conditions off Mid-Norway. A 750 m-thick succession of Upper Permian–Lower Triassic (Ufimian–Griesbachian) marine clastic sediments has been drilled south of Træna (Bugge et al., 2002). Another marine transgression from the north occurred in the Ladinian–Carnian (Jacobsen and van Veen, 1984). Triassic bioclastic sandstone found on Selvær (Table 1, lithofacies 11) was probably deposited during one of these transgressions. The sample probably derives from the seabed of Trænfjorden or Lyngværfjorden. Two other samples from Selvær may be Triassic as well, but their age could not be determined from microfossils. One sample did not contain identifiable palynomorphs, whereas the other contained abundant fungal remains of terrestrial origin (Table 1, lithofacies 12 and 13). Provided that these two ARTICLE IN PRESS R. Bøe et al. / Marine and Petroleum Geology 25 (2008) 235–253 samples are Triassic, the eastern part of Trænfjorden/ southern Lyngværfjorden may have been in a continental setting during their deposition. The source for the Permian–Triassic sediments in the Træna area was probably Upper Devonian–Lower Permian sandstones to the east (Bugge et al., 2002). 6. Structural development The sedimentary strata in Stabbfjorden/Ternholmfjorden occur in a downfaulted basin with boundaries defined by normal faults and primary depositional contacts (Fig. 3). In the southwest, the basin is cut by several faults and is structurally complex. The northeastern part is a simple half graben. Two major faults define the southeastern boundary of the Stabbfjorden Basin. The southwesternmost fault is ca. 8 km long, and passes Moholmen at a distance of 600 m. Northwest of Varkgard, there is a right-lateral, en-echelon offset of ca. 1 km, and another fault continues northeastwards for more than 20 km (Fig. 3). The layering of the sedimentary succession dips into the fault planes (towards the southeast) at angles up to 211. The sedimentary succession displays no signs of growth faulting. At the offset between the two faults there is a depositional contact where the layering of the succession dips up to 261 in northerly and northwesterly directions. South of Ternholman, the northwestern boundary of the basin is defined by a ca. 9 km long, curved fault with downthrow towards the south. The fault passes Kallsholmen at a distance of only 100–200 m. The layering in the sedimentary succession is almost horizontal along the fault, but a weakly developed syncline can be traced for 2–3 km in E-W-direction south of Kallsholmen. The western boundary of the basin is a primary unconformity. The basement appears to be relatively shallow in central Ternholmfjorden, where an ENEtrending anticline with normal faults along its limbs has been defined. East of Ternholman, the northwestern boundary of the Stabbfjorden Basin is a primary unconformity. The contact between sedimentary rocks and basement rocks is well expressed in the seismic data; at Rorstabben the sedimentary rocks occur at a distance of only 200 m from the skerries. Along the contact, the sedimentary strata locally dip up to 371 towards the southeast. West of Ternholman, the southeastern boundary of the Vestfjorden Basin is offset by several normal faults, and synclines and anticlines occur in the sedimentary succession (Fig. 3). A 2.5 km wide area of basement rocks separates the Stabbfjorden Basin from the Vestfjorden Basin. Between Grønna and Ternholman, sedimentary rocks occur in an east-west-trending syncline, and a possible continuation of the sedimentary rock succession is indicated towards the northeast (Fig. 3). The structure of the Lyngværfjorden Basin is complex, with sudden lateral changes of strike and dip and many 249 small faults displacing the layering. The southeastern and northeastern boundaries of the basin are primary depositional contacts, where the bedding generally dips into the basin at angles up to 151. A major fault defines the northwestern/western boundary of the basin. Within the basin, several small faults and synclines strike NE–SW. A phase of extension, with the main movements lasting from Late Oxfordian–Early Kimmeridgian until Ryazanian–Valanginian times, was initiated in the late Middle Jurassic (Blystad et al., 1995; Færseth, 1996; Brekke et al., 2001). Tectonic movements caused a marked rejuvenation of the topography, and a complicated pattern of tectonic highs and lows at various scales developed. One of the highs is the Nordland Ridge, which reaches southwestwards from the Ternholman/Grønna area (Figs. 1 and 14). It is probable that the boundary faults of the Stabbfjorden Basin, the large faults northwest of Ternholman, and the Grønna fault north of Grønna (Olesen et al., 2002), which are all sub-parallel to the Nordland Ridge, originated during this tectonic phase. We have found no evidence for growth faulting in the successions in the Stabbfjorden and Lyngværfjorden Basins. The strata are thus considered as predominantly pre-rift, with downfaulting occurring after deposition. Træna is located south of the northeastward continuation of the Nordland Ridge (Figs. 1 and 13). Some of the faults mapped by IKU (1995) in southern Trænfjorden trend sub-parallel to the northeastward continuation of the Ylvingen Fault Zone on the Trøndelag Platform. This fault zone is of Late Jurassic to Early Cretacous age (Blystad et al., 1995). In their seismic profiles, Bugge et al. (2002) show that several of the NE-trending faults south of Træna were active during deposition of the Upper Permian–Lower Triassic succession there. 7. Hydrocarbon potential The maturity of organic material was determined by measuring vitrinite reflectance on representative samples from Rorstabben, Grønna, Selvær and Kallsholmen. The average maturity of the samples with highest confidence level is ca. 0.35% Ro. Thus, the organic material is classified as immature. Maturity is dependent on time and temperature, whereas temperature is dependent on depth and temperature gradient. Maturity is not reversible, and thus expresses maximum maturity reached by the rock sample during its temperature and depth history. Without a maturity profile of the stratigraphic section from where the samples originate and knowledge of the temperature gradient in the area, the burial history of the samples remains unknown. However, if we compare with near-by areas on the Mid-Norwegian shelf, a maturity of 0.35% Ro is reached at ca. 2000 m. Organic matter maturation in erratic blocks from Beitstadfjorden (Fig. 1) suggests a burial depth from 1.8 to 2.3 km (Weisz, 1992). Similar burial depths are ARTICLE IN PRESS 250 R. Bøe et al. / Marine and Petroleum Geology 25 (2008) 235–253 also inferred from Jurassic basins in Sortlandsundet (Davidsen et al., 2001) and in Frohavet (Gran, 1990). The average age of the samples with the most confident measurements is ca. 160 million years (ranging from 150–170 million years). From the time of maximum burial, the sedimentary rocks at the present seabed must have been through a several million-year long period of uplift and erosion, and it probably also took several million years for the rocks to reach their maximum burial depth. This implies that the effective time the rocks have been near maximum burial depth has to be much shorter than 160 million years. A diagram published by Stach et al. (1982) shows that a maximum temperature of 50 1C (1.4 km burial depth with a temperature gradient of 35 1C/km) over an effective time period of 6 million years is enough to reach a maturity of 0.35% Ro. At a maximum temperature of 40 1C, this maturity will be reached within 10–12 million years. At such low values for Ro as 0.35 there are many uncertainties in the calculations, but the vitrinite reflectance data show that the strata have never been mature enough to generate hydrocarbons. The types and volumes of diagenetic minerals present in a sandstone sample may often aid in estimating the temperatures the sample has been subjected too. However, in the present case, filling of all pores by calcite cement has prevented the precipitation of any later diagenetic phases in the studied samples, making interpretation of temperature history difficult. All the diagenetic cements present in the samples, i.e. calcite, siderite, K-feldspar, kaolinite and pyrite, are typically precipitated at shallow to moderate burial depths on the Norwegian continental shelf. Burial to depths of 1–1.5 km could therefore easily explain the occurrence of these diagenetic cements. The fact that the collected sandstone boulders are all carbonate-cemented, suggests that the sandstones were not buried deeply enough for quartz cementation to take place, and that samples without carbonate cement therefore did not survive glacial erosion. Quartz cementation starts at 70–80 1C on the Norwegian continental shelf (Ehrenberg, 1990; Walderhaug, 1994), and it therefore seems unlikely that the sediments of the Stabbfjorden Basin reached temperatures higher than this. From a correlation between offshore geology and onshore morphological elements, analysis of weathering surfaces, and apatite fission track analysis Riis (1996) suggested that the enveloping summit level of Scandinavia originated as a peneplain in the Jurassic. By Bajocian/ Bathonian times, the parts of mid Norway which were exposed to erosion in the Triassic had been eroded down to a peneplain close to sea-level, which was exposed to weathering in a warm and humid climate. These results seem to agree with fission track results (Rohrman et al., 1995; Hendriks et al., 2007). While weathering may have continued until the mid Creataceous in southern and central Scandinavia and on structural highs offshore, the coastal areas of Norway were buried by sediments during the remaining part of the Jurassic and the Early Creatac- eous. Maturity and diagenetic minerals in samples from the Stabbfjorden Basin indicate less than 2 km burial. A sediment thickness of 2 km in the Stabbfjorden area might indicate that the strata continued far eastwards into Sweden as suggested above. Following burial, the sedimentary succession was uplifted and eroded. From the maps presented by Riis (1996) it can be estimated that the Stabbfjorden area was uplifted ca. 500 m in the Late Cretaceous and Paleogene and ca. 800 m in the Neogene (mainly Pliocene and Pleistocene). Riis (1996) proposed that maximum burial of the basins in Beitstadfjorden, Frohavet and Andøya occurred in the Eocene, and that most of the erosion took place during the Pliocene–Pleistocene glaciations. This model may also apply for the Stabbfjorden Basin. Shallow gas occurs in Quaternary mud along the margins of Vestfjorden, northwest of Ternholman. Common features are blanking of the boomer data due to a high content of free gas in the sediments, gas chimneys, and enhanced/bright reflectors caused by trapping of gas between sand, silt and clay layers (Fig. 15). Also pockmarks occur in this area. Shallow gas is most abundant in the lowermost part of the Quaternary succession, immediately above the bedrock surface, and in most cases the gas occurs above sedimentary bedrock. These observations suggest that the gas is of thermogenic origin, and that it originates from mature sedimentary strata in Vestfjorden. Thermogenic gas may indicate leaking hydrocarbon accumulations in the subsurface. Where gas occurs in Quaternary sediments above basement rocks, it might have migrated laterally along the bedrock–sediment interface before rising into the Quaternary succession. 8. Conclusions Seismic data acquisition and erratic block sampling along the Nordland coast has resulted in the discovery of two sedimentary basins of Mesozoic age:    The NE-trending Stabbfjorden Basin west of Meløya is a 28 km long and 3–10 km-wide half graben with an up to 800 m thick, dipping sedimentary succession. Two faults, 8 km and 420 km long, occur along the southeastern boundary of the half graben. The northwestern boundary is defined by a 9 km-long fault, while further east, the northwestern boundary is a primary unconformity. The sedimentary rocks of the half graben locally occur only 100–200 m from the nearest skerries. The Lyngværfjorden Basin northeast of Træna is elongated in NNE direction. Its length is about 12 km, whereas its width is up to 3 km and depth possibly more than 350 m. The structural pattern of the basin is complex, with rapid lateral changes of strike and dip and many small faults displacing the layering. Ice-transported erratic blocks found on nearby islands and skerries, along the transport direction of the lateglacial ice-streams, comprise conglomerates, sandstones, ARTICLE IN PRESS R. Bøe et al. / Marine and Petroleum Geology 25 (2008) 235–253 251 Fig. 15. Seismic profile NGU0304020 exhibiting shallow gas in stratified Quaternary sediments. Upper panel: boomer record, filtered at 600–3000 Hz, with clear indications of shallow gas in the Quaternary succession. Lower panel: sleevegun record, filtered at 100–600 Hz, showing the presence of layered, sedimentary bedrock below shallow gas occurrences. See Fig. 3 for location of the seismic line.   siltstones, and mudstones with numerous shells and coal fragments. Microfossil analyses show that they range in age from Barremian to Triassic, although the majority is of Middle-Late Jurassic age. The majority of the samples reflect shallow-marine conditions and the lithofacies can be correlated with the well-known tectonic phases and sea-level changes that have been reported for the Middle Jurassic–Early Cretaceous offshore Nordland. The strata show now signs of syndepositional faulting, and the successions are thus interpreted as mainly pre-rift. Thin-section analysis shows that the samples of Middle Jurassic sandstone have a composition quite similar to sandstones of the same age in the Halten-Dønna area, although the erratic blocks often have higher feldspar contents. Barremian sandstone from the Stabbfjorden Basin has a composition and texture similar to the Cretaceous Lange Formation in well 6610/3-1R 100 km west of the Stabbfjorden Basin. The maturity of organic material determined from vitrinite reflectance shows that it can be classified as immature. The diagenetic minerals in the sandstone samples together with the lack of quartz-cementation is also consistent with shallow burial (probably o2 km), and it can be concluded that the Stabbfjorden and Lyngværfjorden Basins have no exploration potential. However, shallow gas in Quaternary mud along the margin of the nearby Vestfjorden Basin is probably of thermogenic origin. Acknowledgments We would like to thank BP, Shell, Statoil and NGU for financial support to the project ‘‘The recycling of an orogen: provenance and routing of detritus from Norway to the Mid-Norwegian margin’’ and for permission to publish these results. Erik Lundin and Håkon Fossen are thanked for constructive comments to the manuscript. References Birkelund, T., Thusu, B., Vigran, J., 1978. Jurassic-Cretaceous biostratigraphy of Norway, with comments on the British Rasenia cymadoce Zone. Palaeontology 21, 31–63. Blystad, P., Brekke, H., Færseth, R.H., Larsen, B.T., Skogseid, J., Tørudbakken, B., 1995. 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