Palaeomagnetic results from the Lopra-1/1A re-entry well, Faroe Islands 51© GEUS, 2006. Geological Survey of Denmark and Greenland Bulletin 9, 51–65. Available at: www.geus.dk/publications/bull Palaeomagnetic results from the Lopra-1/1A re-entry well, Faroe Islands Niels Abrahamsen The palaeomagnetic dating and evolution of the Faroe Islands are discussed in the context of new density and rock magnetic results from the deepened Lopra-1/1A well. The reversal chronology of the c. 6½ km thick basalt succession is also described. The polarity record of the Faroe Islands may now be correlated in detail with the Geomagnetic Polarity Time Scale. The lowermost (hidden) part of the lower basalt formation correlates with Chron C26r (Selandian age), the top (exposed) part of the lower basalt formation correlates with Chrons C26n, C25r and C25n (Selandian and Thanetian age) and the middle and upper basalt formations correlate with Chron C24n.3r (Ypresian). Inclinations indicate a far-sided position of the palaeomagnetic poles, which is characteristic of results from most Palaeogene volcanics from the northern North Atlantic region. The density, magnetic susceptibility and magnetic remanence of 20 specimens from one solid core (1½ m in length) and 26 sidewall cores from the well between –2219 and –3531 m below sea level (b.s.l.) suggest that the volcanic materials can be divided into two characteristic groups: solid unal- tered basalts and altered basalts and tuffs. The magnetic properties are typically log-normally distribu- ted and the carriers of remanence are Ti-poor Ti-magnetites with Curie temperatures close to 580°C. The inclination of the 1½ m core at 2380 m b.s.l. is dominantly negative (two plugs at the very top of the core do show normal polarity, but they are likely to be misoriented as all specimens appear to be from one flow). Magnetic logging (magnetic susceptibility and field intensity) down to 3515 m b.s.l. was made in Lopra-1/1A together with other geophysical logs but did not yield conclusive inclination data. Keywords: Palaeomagnetism, rock magnetism, magnetic reversals, plate tectonics, Faroe Islands, Lopra-1/1A well, North Atlantic, large igneous province _________________________________________________________________________________________________ Department of Earth Sciences, University of Aarhus, Finlandsgade 8, DK-8200 Aarhus N, Denmark. E-mail: Abraham@geo.au.dk Review of the geology The Faroe Islands are situated on the eastern side of the northern North Atlantic between the Shetland Islands and Iceland on the northern part of the NE–SW-trending elon- gated Faroe Rise (Fig. 1). The volcanic islands are a result of the hotspot-related plume activity recorded by the Brito- Arctic Large Igneous Province (LIP) (Lawver & Muller 1994; Larsen & Saunders 1998; T.B. Larsen et al. 1999; Burke & Torsvik 2004) that stretches from present-day cen- tral West Greenland to the north-western parts of the UK. Seismic (e.g. Richardson et al. 1998) and gravity inves- tigations (e.g. Saxov & Abrahamsen 1964) suggest that the invisible basement of the islands is composed of con- tinental lithospheric crust, somewhat thinned by lithos- pheric stretching processes during the continental break- up that formed the North Atlantic. The exposed part of the Faroe Islands is composed of a c. 3 km thick pile of Palaeogene flood basalts (Rasmussen & Noe-Nygaard 1969, 1970; Noe-Nygaard & Rasmus- sen 1984) situated above a c. 3½ km unexposed volcanic sequence below sea level (Fig. 2). Only minor sedimenta- GEUS Bulletin no 9 - 7 juli.pmd 07-07-2006, 14:1951 52 ry layers are intercalated in the whole volcanic sequence. The volcanic sequence, more than 6½ km in total thick- ness (Waagstein 1988; L.M. Larsen et al. 1999), is divid- ed into three parts, the lower (> 4½ km thick), the mid- dle (1.4 km thick) and the upper basalt (> 0.9 km thick) formations. The basalts are cut by numerous dykes and a few sills. An up to 10 m thick coal-bearing formation of lacustrine claystones and shales was deposited on top of the slightly eroded surface of the lower basalt formation (the A-level). Two other stratigraphical levels, B (in the middle formation) and C (separating the middle and up- per formations), are also useful for stratigraphical pur- poses (Fig. 2). The purpose of the present contribution is to present 60° Norway 50° 50° 30° 30° 15° 15° 0° 0° 15° 15° 30° 30° 30°W 30°W 15°W 15°W 0°W 0°W Icelandd Icelandd Rockall Rockall Plateau Plateau 70° 70° 50° 60° 70 50° 70° 30° 15° 0° 15° 30° C al ed on ia n fr on t 30°W Greenland Norway Faroe Islands 15°W C al ed on ia n fr on t Iceland UK Ridg e DK 0°W Onshore basalt and sills Offshore basalt flows and sills Seaward-Dipping Reflector sequences Rockall Plateau Spr ea din g r idg e Fig. 1. Index map of the eastern North Atlantic showing the Faroe Islands (modified from Larsen et al. 1995). N2.2 C25n C24n.3n C24n.1n C23n.2n C28n C29n Magnetic polarity chrons (1) 52 - 54 - 56 - 58 - 50 - 48 - Ma 60 - 62 - 64 - 66 - 68 - 70 - C26n C25n C22n C21n C24n.3n C24n.1n C23n.2n C27n C28n C29n C30n C31n C32n.1n FAROE ISLANDS Stratigraphy (4) –4.5 km Lower fm Middle fm Upper fm - 0 Sea level 1 2 3 4 5 6 2.9 km D an ia n Yp re sia n S el an di an T ha ne tia n R1 R1 R2 R2 R3 N3 N3.2 N2.1 N3.1 N3r N2 N2.2 N2r C24r C25r C26r (3) Reversals (2) Fig. 2. Compilation of magnetic reversals within the c. 6½ km thick basalt pile of the Faroe Islands, showing stratigraphy and the correlation with the Geomagnetic Polarity Time Scale. The four columns are based upon information compiled from: (1) Ogg (1995); (2) Abrahamsen (1965, 1967), Abrahamsen et al. (1984), Waagstein (1988) and Riisager et al. 2002a; (3) Tarling & Gale (1968); (4) Rasmussen & Noe-Nygaard (1970), Waagstein (1988) and L.M. Larsen et al. (1999). GEUS Bulletin no 9 - 7 juli.pmd 07-07-2006, 14:1952 53 the magnetic results from a core of basaltic rock obtained from the Lopra-1/1A reentry well and to discuss these data in relation to other palaeomagnetic results from the Faroe Islands. Previous work Magnetic investigations in relation to the Faroe Islands have been made since the early 1960s (Abrahamsen 1965, 1967; Saxov & Abrahamsen 1966; Tarling & Gale 1968; Tarling 1970; Schrøder 1971; Løvlie 1975; Løvlie & Kvin- gedal 1975; Abrahamsen et al.1984; Schönharting & Abrahamsen 1984; Tarling et al. 1988). Density determi- nations (Saxov & Abrahamsen 1964) as well as gravity measurements (Saxov 1969) and seismic investigations (Pálmason 1965; Bott et al. 1974, 1976; Casten 1974; Nielsen et al. 1981; Richardson et al. 1998) have been made on and around the islands. Geophysical logs from the Lopra-1/1A and Vestmanna-1 boreholes have been published by Nielsen et al. (1984), Boldreel (2006, this volume) and Abrahamsen & Waagstein (2006, this vol- ume). Geothermal measurements were described by Bal- ling et al. (1984) and Balling et al. (2006, this volume). Palaeomagnetic results from the Faroe Islands have been published by Abrahamsen (1965, 1967), Tarling & Gale (1968), Tarling (1970), Løvlie (1975), Løvlie & Kvinge- dal (1975), Abrahamsen et al. (1984), Schönharting & Abrahamsen (1984) and Riisager et al. (2002a, b). A com- parison of palaeomagnetic results from East Greenland and other results from the Palaeogene of the North Atlan- tic Igneous Province (NAIP) was published by Tarling et al. (1988) and a critical review of palaeomagnetic poles from the Eurasian part of the NAIP together with a new pole for the Faroe Islands was presented by Riisager et al. (2002a). A summary of all published palaeomagnetic di- rectional data from the Faroe Islands is shown in Table 1. The magnetic results for the exposed part of the basalt succession were extended by the wells at Vestmanna-1 and Lopra-1 in 1980–1981 (Abrahamsen et al. 1984; Schön- harting & Abrahamsen 1984) and by the re-entry of the Lopra-1/1A hole in 1996, the results of which are pre- sented in this paper. Despite intentions, the re-entry hole at Lopra-1/1A reached a depth of 3565 m without pene- trating to the base of the lower basalt formation volcanics. The polarity sequence and the compiled total strati- graphic column of the Faroe Islands as now known are shown in Fig. 2, together with the Geomagnetic Polarity Time Scale (GPTS). Essentially we find three intervals of reverse magnetic polarity (R1, R2, R3) with two normal Table 1. Palaeomagnetic results from the Faroe Islands All formations All formations ubf, Torshavn ubf, Argir mbf, Argisfossar mbf, Vestmanna core lbf, Vestmanna core mbf + lbf, Vestm. core mbf + lbf lbf, Lopra-1; 862 mbf lbf, Lopra-1; 1219 mbf lbf, Lopra-1; 1923 mbf lbf, Lopra-1; 2178 mbf lbf, Lopra-1A; 2380 mbf Average, Nos 1–10, except * Average, Nos 1, 2, 3 & 6 N(Dg) 33 1809 34 8 18 275 28 303 548(43) 6 8 5 7 20 (10) (4) 176.0 185.0 171.9 175.1 156.0 7.7 181.2 ± 69.0 ± 66.4 –72.2 –53.9* –36.0* –61.8 ± 63.4 ± 61.9* ± 60.9 –75.0 –62.0 –73.0 –55.0* –71.7 ± 67.2 ± 67.3 6 1.9 3.5 2.2 1.2 6.3 1.2 4.5 1 3 2.0 1.4 6.3 53 258 53.4 19.4 46.1 24.5 709 213 80.0 76.7 84.0 62.3 48 70.8 72.8 70.9 71.4 78.7 Plat °N 159.0 161.0 218.0 182.4 154.7 164.4 Plon °E 10.3 3.1 6.3 3.1 1.9 10.0 1.9 6.0 8.0 A 95 ° 52.5 48.9 57.3 34.4* 20.0* 43.0 45.0 43.1* 41.9 61.8 43.2 58.6 35.5* 56.5 50.0 50.1 Palaeo- lat (°N) 1 2 3 * * 4 5 * 6 7 8 9 * 10 11 12 No. R&N R&N R R R R R&N R&N R&N R ?R R ?R R R&N R&N (1) (2) (3) (3) (4) (5) (5) (5) (6) (7) (7) (7) (7) (8) (8) (8) ReferenceFormation/site/core depth Decl ° Incl ° k 14 δ95 ° α 95 ° Polarity lbf, mbf, ubf: lower, middle and upper basalt formations; No.: number in palaeolatitude figure; * Not used in average; N: number of samples; Dg: directional groups; Decl: mean of cleaned declination; Incl: mean of cleaned inclination; α95 : cone of 95% confidence. For core data the inclination statistics of Kono (1980) were used for k and α 95 (Tarling 1983); k: Fisher precision parameter; Plat: latitude of apparent palaeomagnetic pole; Plon: longitude of apparent palaeomagnetic pole; δ95 and A 95 : error angles of app. latitude and app. palaeopole at 95% confidence level; (1) Abrahamsen 1967; (2) Tarling 1970; (3) Løvlie & Kvingedahl 1975; (4) Løvlie 1975; (5) Abrahamsen et al. 1984; (6) Riisager et al. 2002a; (7) Schönharting & Abrahamsen 1984; (8) This work. GEUS Bulletin no 9 - 7 juli.pmd 07-07-2006, 14:1953 54 polarity intervals in between (N2 and N3). Minor differ- ences between columns (2) and (3) in Fig. 2 are likely to be due to somewhat different positions of the profiles in- vestigated on Suðuroy, the southernmost of the Faroe Is- lands. According to recent high-precision 40Ar-39Ar datings (Storey et al. 1996; L.M. Larsen et al. 1999), the basalt formations in the Faroe Islands as well as the contempo- raneous East Greenland basalts can be divided into an older part with ages of about 59–56 Ma, followed (after a pause or a period with much reduced volcanic activity) by a young- er part, with ages of 56–55.5 Ma for the Faroes and 56– 54.5 for East Greenland. Based upon these radiometric datings, the polarity record of the Faroe Islands may now be correlated to the GPTS as shown in Fig. 2. The lowermost (hidden) part of the lower basalt formation correlates with Chron C26r (Selandian age), the upper (exposed) part of the lower basalt formation correlates with Chrons C26n, C25r and C25n (Selandian and Thanetian) and the middle and up- per basalt formations correlate with Chron C24r (Ypre- sian). This correlation follows the suggestion by Waag- stein (1988), who revised the original interpretation of Abrahamsen et al. (1984) by suggesting that R3 belongs to Chron C26r rather than to C24r. More details in rela- tion to magnetic inclinations from the Lopra-1/1A data are discussed below. Assuming the geomagnetic field to have been a central, axial dipole field, the palaeolatitude may be determined from the characteristic (primary) inclination of the vol- canics, combining both polarities. A compilation of all inclination values obtained from the Faroe Islands is list- ed in Table 1. Using inclination statistics (Kono 1980; Tarling 1983) the fisherian mean of published inclina- tions (group numbers 1–10, Table 1) is 67.2° ± 1.4° (equi- valent to a palaeolatitude of 50.0° ± 2.1°), whereas the average of the palaeolatitudes listed is 50.9° ± 2.3° (± 1 sigma). Further discussion of the shallow inclinations and the palaeolatitude question will be given below. Palaeogeography Many palaeogeographic reconstructions of the North At- lantic have been published since the early work of Bullard et al. (1965) (e.g. Ziegler 1990; Knott et al. 1993; L.M. Larsen et al. 1999; Torsvik et al. 2001; Mosar et al. 2002). Before about 60 Ma, the supposed mantle hotspot (just south-east of Iceland at the present day) lay under the volcanic areas of Disko and Nuussuaq in West Greenland (O’Connor et al. 2000; Nielsen et al. 2002; Chambers et al. 2005), far from the Faroe Islands that are situated just north-west of the continental margin of Europe. The whole volcanic pile of the Faroe Islands, more than 6½ km thick, was formed in the time interval between Chron 26 (61.65 Ma) and Chron 24n.3n (c. 53.286 Ma) (Chron ages are the orbitally tuned age calibration of Gradstein et al. 2004, table 5.2). During this time interval, the hotspot moved eastwards under Greenland as Greenland moved west- north-west relative to Europe and the North Atlantic grad- ually opened between the Faroe Islands and Greenland. Absolute declinations are known from only four of the palaeomagnetic investigations from the Faroe Islands (Ta- 1 3 6 21 3 6 N 21 3 6 Fig. 3. Palaeomagnetic directions from the Faroe Islands (from Table 1, results Nos 1, 2, 3 and 6) with α95 circle. The axial dipole field direction is indicated by a cross. Fig. 4. Apparent palaeomagnetic pole positions (solid circles) with 95% significance circle (Table 1, poles Nos 1, 2, 3 and 6). All poles appear ‘farsided’ as seen from the Faroe Islands (diamond). Further discussion in the text. 6 2 1 3 6 6 1 1 3 3 6 2 1 3 GEUS Bulletin no 9 - 7 juli.pmd 07-07-2006, 14:1954 55 ble 1, Nos 1, 2, 3 and 6). If both normal and reverse po- larities are combined and assumed to be normal direc- tions towards the north with steep down-dip (positive) inclinations, the four directions appear as shown in Fig. 3. The equivalent apparent palaeomagnetic pole positions are shown in Fig. 4. All four poles are seen to be ‘farsided’ (Wilson 1971; Merrill et al. 1998), the apparent palaeo- magnetic poles falling beyond the geographic pole as seen from the Faroe Islands. An equivalent histogram of all published apparent palaeolatitudes (Table 1) is also illus- trated in Fig. 5, most of which show low values as com- pared to the present-day latitude. Lopra-1/1A investigations Sampling, instruments and techniques used The material investigated from the extended Lopra-1/1A well consists of two types: core plugs from the solid core (2380.0 to 2381.3 m) and sidewall cores (between 2219 and 3531 m). The solid core is 1.4 m long and in several pieces, but some fit together, as shown in Fig. 6. After marking the core with an upward directed arrow in the core lab at GEUS, 20 plugs with a diameter of 2.5 cm were drilled orthogonal to the main core and cut to a stand- ard length of 2.2 cm. The major part of the present mag- netic investigation is concentrated upon these 20 plugs. In addition some rotary sidewall cores were investigated. The pieces from the sidewall cores had a diameter of 2.33 cm and varied in length, which limited the possibility of fitting these samples into the magnetic instrument holders. Bulk density To avoid problems with air bubbles adhering to the rela- tively small specimens if they were weighted in water, the bulk density was determined by weighing in air only (to an accuracy of ± 0.001 g), then determining the volume by measuring the shape of the specimens (to an accuracy of ± 0.02 cm). The likely accuracy in the finally deter- mined density is about ± 2–3%, depending on the rough- 0 20 40 60 80 Result No. (Table 1) Pa la eo la tit ud e 1 2 3 4 5 6 7 8 9 10 Present latitude Fig. 5. Histogram of apparent palaeolatitudes according to Table 1 (poles Nos 1–10). Most results appear systematically low com- pared to the present-day latitude. For further discussion see the text. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110 115 120 125 Fragments 130 cm Fragments Fragments Fragments 0 Fig. 6. Sketch from a photograph of the Lopra-1/1A core between 2380 and 2381.4 m. The fragments containing the numbered 1- inch core plugs are indicated. The absolute azimuths of the indi- vidual segments and fragments are not known. The top segment containing plugs Nos 1 and 2 appears to have been turned upside- down before the core was archived. GEUS Bulletin no 9 - 7 juli.pmd 07-07-2006, 14:1955 56 ness of the shape. A total of 34 specimens were determined (Table 2). Susceptibility Two types of susceptibility instruments were used. Initial whole core measurements were made by a handheld Czech kappameter KT5 (sensitivity ± 0.00001 SI) before drill- ing plugs from the core. A Molspin bulk susceptibility bridge (sensitivity ± 0.000001 SI) was then used to meas- ure the susceptibility of the core plugs and to monitor possible chemical changes during thermal demagnetisa- tion experiments. A total of 46 specimens were measured (Table 2). Remanence The direction (declination and inclination) and intensity of the natural remanent magnetisation (NRM) was deter- mined using a Molspin spinner magnetometer. The plugs with preferred dimensions of 2.2 cm in length and a di- ameter of 2.5 cm (plugs from the solid core) were all meas- ured and demagnetised in detail, see below. The NRM of the sidewall cores was also measured but, due to the vari- able length of the core pieces, only one (SWC57) was in- vestigated in detail (Table 2). The sensitivity of the Mol- spin spinner is ± 0.02 mA/m and the direction of the re- manence within the plug was determined to within ± 1°. The declination and inclination is given with respect to the local specimen coordinates, assuming the axis of the plug (= specimen) to be approximately horizontal (i.e. orthogonal to the Lopra-1/1A drill hole). As the azimuth of the vertical core is not known, the true magnetic decli- 1V 1H 1 2 3 4 5 6 6V 6H 7 8 9 10 11 12 13 14 15 16 17 18 19 20 20V 20H 2380.08 2380.08 2380.08 2380.12 2380.17 2380.21 2380.28 2380.58 2380.58 2380.58 2380.62 2380.66 2380.69 2380.75 2380.79 2380.91 2380.94 2380.97 2381.00 2381.05 2381.09 2381.12 2381.22 2381.25 2381.25 2381.25 basalt basalt basalt basalt basalt basalt basalt basalt basalt basalt basalt basalt basalt basalt basalt basalt basalt basalt basalt basalt basalt basalt basalt basalt basalt basalt 69.040 36.000 31.79 28.66 30.38 28.78 29.76 28.22 38.240 44.907 31.37 26.24 28.84 29.92 27.31 29.37 30.07 27.08 26.96 30.99 29.11 30.24 29.87 25.87 71.848 27.740 2.992 2.981 2.885 2.868 2.922 2.869 2.92 0.05 0.02 6 17997 3258 327 1247 673 2319 2553 1227 1391 971 1539 333 1782 1628 984 359 887 322 2616 2705 2256 3808 851 20 Table 2a. Lopra-1/1A: magnetic susceptibility, NRM, Q-ratio, density Solid core (d = 25 mm) Sample No. Depth m Rock type Weight g NRM corr mA/m Density g/cm3 46.26 41.62 17.32 3.903 6.945 4.340 11.773 18.850 11.459 16.865 33.756 34.636 32.987 37.825 36.450 38.704 43.323 31.722 35.241 41.288 36.890 43.15 28.40 14.00 2.98 22 Suscept- bridge 10–3 SI 10.868 4.728 2.105 4.513 3.897 4.951 3.404 2.691 2.073 0.723 1.117 0.254 1.184 1.123 0.639 0.208 0.703 0.230 1.592 1.843 2.44 2.51 0.56 20 Q-ratio NRM/(F × Sus) a: Mean Standard deviation Mean error N GEUS Bulletin no 9 - 7 juli.pmd 07-07-2006, 14:1956 57 nation is not known. A further description of the palaeo- magnetic experimental standard laboratory procedures may be found in e.g. Butler (1992). AF-demagnetisation After measuring the initial NRM intensity, 16 of the 20 plugs were AF-demagnetised in stepwise increasing alter- nating magnetic fields (Table 3) using a Molspin AF-de- magnetiser. Minimum and maximum AF-fields were 2.5 mT (25 Oe) and 100 mT (1000 Oe), respectively. Thermal demagnetisation Stepwise thermal demagnetisation was made in a Schon- stedt furnace on four plugs from the solid core. An ini- tially moderate AF-demagnetisation of up to between 7.5 and 15 mT removed recently induced viscous magnetisa- tion components, most likely acquired during the drilling operations (details in Table 2), after which the thermal demagnetisation was applied. b: a & b: 1.43 0.55 0.71 0.64 0.61 0.69 1.60 1.83 0.72 1.13 0.62 0.64 0.83 0.39 0.58 0.58 0.88 77.4 0.623 4.864 0.032 0.046 0.060 0.015 1.406 1.240 9.224 5.022 2.344 2.030 1.080 2.346 0.074 0.470 0.024 2.433 *) Excluding sample No. 59 59 57 46 44 43 40 39 38 37 36 34 33 31 30 26 25 19 16 15 13 12 9 6 5 5 4 2219.00 2275.00 2441.00 2456.00 2475.00 2558.00 2559.80 2560.20 2562.00 2570.00 2610.00 2630.00 2690.00 2780.00 2970.00 3030.00 3233.50 3328.00 3382.00 3438.00 3464.50 3500.50 3512.50 3514.50 3514.50 3531.00 Sample No. Depth m basalt basalt basalt basalt, ves. basalt, ves. basalt, ves. basalt, ves. tuff, lapilli tuff basalt basalt, alt. tuff, lapilli tuf, lapilli basalt tuf, lapilli basalt tuff basalt basalt tuf, lapilli tuf, lapilli basalt tuff tuf, lapilli tuf, lapilli basalt Rock type 18.966 22.222 23.857 33.653 18.421 17.973 22.920 21.428 29.838 9.353 18.741 13.720 8.082 21.170 18.744 18.515 9.915 18.471 14.066 19.505 17.287 16.494 23.953 13.372 15.648 27.341 Weight g 2.361 2.857 2.865 2.488 1.981 2.708 2.711 2.336 2.345 2.379 3.167 2.404 2.276 2.929 2.553 2.808 2.572 2.875 2.919 2.549 2.605 3.001 2.648 2.511 2.607 2.940 Density g/cm3 4406 2180 9027 0.7 1.3 1.5 0.4 38.5 3412 587 365 7171 304 1146 106 1.8 12.0 0.8 38.2 NRM mA/m 1.43 87.96 46.65 0.55 0.71 0.64 0.61 0.69 69.17 1.60 1.83 76.89 3.77 26.67 0.72 1.13 63.36 0.62 0.64 0.83 0.39 0.58 0.58 0.88 Suscept- bridge 10–3 SI 87.96 46.65 69.17 76.89 3.77 26.67 63.36 High > 2 × 10–3 10–3 SI Low < 2 × 10–3 10–3 SI Q-ratio NRM/(F × Sus) Mean Standard deviation Mean error N Mean Standard deviation Mean error N 2.63 0.27 0.05 26 2.68 0.27 0.05 32 1516 2585 593 19 1895 3276 525 39 16.20 28.80 5.88 24 34.47 21.00 3.90 29 34.5 21.4 4.0 29 0.85 0.39 0.10 17 1.85 2.34 0.55 18*) 2.16 2.45 0.40 38*) Table 2b. Lopra-1/1A: magnetic susceptibility, NRM, Q-ratio, density Sidewall cores (d = 23.3 mm) 22.1 23.6 3.5 46 GEUS Bulletin no 9 - 7 juli.pmd 07-07-2006, 14:1957 58 the limit between the two groups being about 2 × 10–3 SI. The higher group yields an average susceptibility of 34 ± 4 (σ = 21) × 10–3 SI and the lower group an average suscep- tibility of 0.85 ± 0.1 (σ = 0.39) × 10–3 SI. The more strongly magnetised group is represented mostly by unaltered basalts, whereas the less strongly mag- netised group is more typical of most sediments including tuffaceous sediments, as well as vitrinites and deuterically altered or weathered basalts. In the present case the differ- ence between high and low values in the basalts is likely to be caused by alterations of the primary Ti-magnetites, since Ti-magnetite is the main carrier of the remanence (see below). NRM intensity The intensity of the NRM (natural remanent magnetisa- tion) is listed in Table 2 and shown in Fig. 7. Values of the Rev.: Norm: All Rev: Mean inclination Standard deviation N Mean inclination Standard deviation N Mean inclination Standard deviation N –71.22 1.99 18 76.00 5 2 –71.70 2.85 20 α 95 = 1.95 k = 709 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 2380.08 2380.12 2380.17 2380.21 2380.28 2380.58 2380.62 2380.66 2380.69 2380.75 2380.79 2380.91 2380.94 2380.97 2381.00 2381.05 2381.09 2381.12 2381.22 2381.25 (?N) (?N) R R R R R R R R R R R R R R R R R R Polarity 31.79 28.66 30.38 28.78 29.76 28.22 31.37 26.24 28.84 29.92 27.21 29.37 30.07 27.08 26.96 30.99 29.11 30.24 29.87 25.87 Weight g 0–40 0–40 0–70 0–7.5 0–95 0–40 0–70 0–40 0–50 0–630 0–50 0–600 0–50 0–40 0–50 0–630 0–70 0–630 0–50 0–50 Treatment AF mT/°C 18.00 3.26 0.33 1.25 0.67 2.32 2.55 1.23 1.39 0.97 1.54 0.33 1.78 1.63 0.98 0.36 0.89 0.32 2.62 2.71 335 350 358 175 345 51 62 75 80 289 106 7 169 204 195 334 163 334 32 24 81 71 –70 –70 –69 –67 –71 –69 –71 –75 –72 –72 –70 –71 –70 –74 –73 –74 –73 –71 6 2 2 4 1 1 1 1 1 3 2 2 2 1 2 2 3 2 3 4 NRM intensity A/m Decl (rel.) Characteristic direction MAD Degrees Demag interval AF thermal mT Tmax °C 15–50 5–40 15–70 10–95 5–40 5–70 5–40 5–50 5–50 5–50 5–40 5–50 10–70 5–50 0–50 T630 T600 T630 T630 Sample No. Depth m Incl Table 3. Lopra-1/1A: palaeomagnetic results Results Bulk density The bulk density of sidewall cores and plugs are listed in Table 2. The mean bulk density of the solid basaltic core was well-determined as 2.92 ± 0.02 (standard deviation (σ) = 0.05) g/cm3, although the determination was based on only six specimens. The bulk density of the SWC-cores was lower and much more scattered, 2.63 ± 0.05 (σ = 0.27) g/cm3. The low bulk density and high scatter is likely to be due to differences in porosity and the abundance of secondary minerals. Susceptibility The magnetic susceptibility is also listed in Table 2 and shown in Fig. 7. In contrast to for instance the bulk den- sity, the magnetic susceptibility and remanence intensity may vary considerably, and they are known typically to be logarithmically normal distributed (e.g. Tarling 1983; Abrahamsen & Nordgerd 1994). This is also the case here, as two log-normal distributions are found. The core sus- ceptibility varies between 4 and 46 × 10–3 SI, and the sus- ceptibility of the SWC-cores varies even more, between 0.4 and 88 × 10–3 SI. The data thus fall into two populations, GEUS Bulletin no 9 - 7 juli.pmd 07-07-2006, 14:1958 59 order of 1 A/m are typical for the unaltered basalts, whereas tuffs and altered basalts may have lower values. The mean NRM intensity value of the core plugs is 2.26 ± 0.85 (σ = 3.8) A/m, and the mean NRM intensity value of the SWC- cores is 1.52 ± 0.59 (σ = 2.6) A/m. Due to the high scatter, the mean NRM intensities of the two groups are not sig- nificantly different. The combined populations plotted logarithmically (Fig. 9) again show two overlapping log- normal distributions, as do the susceptibilities (Fig. 8). Q-ratio The Q-ratio (Koenigsberger ratio) illustrated in Fig. 10 is the ratio between the remanent (JNRM) and the induced (Ji = k·F) magnetisation, Q = JNRM/Ji = JNRM/(k·F), F being the intensity of the local geomagnetic field, F ≈ 0.05 mT. For basaltic rocks, values between 0.2 and 10 are characteris- tic. Generally, the higher value the more fresh and unal- tered the samples are. Mean values for Q is found to be 2.4 ± 0.6 (σ = 2.5) for the core plugs, and 1.8 ± 0.6 (σ = 2.3) for the SWC-cores, respectively (omitting a single extraordinary high value of Q = 77 for SWC59). The Q-ratios of the two groups are not significantly different, but again the combined population has a tendency to two log-normal distributions. Magnetic carriers Two examples of isothermal remanent magnetisation (IRM) acquisition of plugs Nos 3 and 7 are shown in Fig. 11. Both specimens show magnetic saturation around 0.1 T, which indicate that the dominant carrier of the rema- nence is magnetite or Ti-magnetite, although maghemite may also be present. The thermal demagnetisations (see below) show blocking temperatures between 560 and Lopra-1: Susceptibility frequency (SI) 0 2 4 6 8 10 Log (susceptibility × 10–3 SI) N = 46 Lopra-1: NRM frequency 0 2 4 6 8 10 Log (NRM, mA/m) N = 39 Lopra-1: Q-ratio frequency 0 2 4 6 8 10 Log (Q-ratio) N = 38 –0.5 0.0 0.5 1.0 1.5 2.0 –1 0 1 2 3 4 5 –2 –1 0 1 2 N um be r N um be r N um be r Fig. 7. Histograms of susceptibility, NRM intensity and Q-ratio. All appear bimodal on a logarithmic scale. 2200 2400 2600 2800 3000 3200 3400 3600 Lopra-1: SWC and core 0 1 10 100 Depth (m) core Su sc ep tib ili ty ( 10 –3 S l) Fig. 8. Magnetic susceptibility of side- wall cores (diamonds) and core plugs (dotted line shows extent), logarithmic scale. GEUS Bulletin no 9 - 7 juli.pmd 07-07-2006, 14:1959 60 580°C, indicating that the Ti-content is low, pure mag- netite having a Curie temperature of 580°C (e.g. Dunlop & Özdemir 1997). AF and thermal demagnetisations The NRM values of the 20 cores and the SWC-cores are listed in Tables 2a & b, and examples of characteristic results of the AF and thermal demagnetisation experiments performed are illustrated in Fig. 12. Thermal demagneti- sations were made on cores Nos 10, 12, 16 and 18 and AF-demagnetisations were made on the remaining 16 cores. Values chosen for the AF-field were in most cases 0, 5, 7.5, 10, 15, 20, 25, 30, 40, 50 and 60 mT. Some plugs were further demagnetised to 70, 80, 90 and 100 mT. Four thermally demagnetised plugs were first AF-demag- netised in 2.5, 5, 7.5, and 10 mT fields, to remove the recent drillstem-induced viscous remanence (see below), and then stepwise demagnetised at temperatures of 150, 250, 350, 450, 550, 570, 600 and 630°C. The examples in Fig. 12 show stereographic plots (left) of the direction of the unit vector of the remanent mag- netisation (solid signature: positive inclination, open sig- nature: negative inclination). All except the first example show characteristic stable negative inclinations. To the right, the corresponding intensity decay of the sample is shown (normalised to the initial value J0 = JNRM), the hor- izontal scale indicating the peak value of the applied al- ternating field in Oe (× 0.1 mT), or the temperature in C. Inclination Prior to the demagnetisation experiments, about half of the plugs showed a low coercivity NRM with positive in- clination (down-dip), which is most likely due to a drill- stem induced viscous remanent magnetisation (VRM). The VRM was easily removed by AF-demagnetisation in low fields, typically between 2.5 and 5 mT. Based upon the AF- and Thermal demagnetisation da- ta, the characteristic (stable) remanent magnetisations for each plug were determined by the principal component analysis (PCA) method of Kirschvink (1980), as imple- mented in the IAPD-programme by Torsvik (1986). In all cases a stable characteristic, supposed primary, mag- 0 1 10 100 1000 100 000 10 000 core N R M ( m A /m ) Depth (m) 2200 2400 2600 2800 3000 3200 3400 3600 Lopra-1: SWC and core Fig. 9. NRM intensity of side-wall cores (diamonds) and core plugs (dotted line shows extent), logarithmic scale. Lopra-1: SWC and core 0 2 4 6 8 10 12 14 16 18 20 Depth (m) core 77.4 Q -r at io * 2200 2400 2600 2800 3000 3200 3400 3600 * Q-ratio = (NRM/F × susceptibility) Fig. 10. Q-ratio of side-wall cores (diamonds) and core plugs (dotted line shows extent). GEUS Bulletin no 9 - 7 juli.pmd 07-07-2006, 14:1960 61 netisation was isolated, as listed in Table 3, and illustrated in the stereogram of Fig. 13. (Bearing in mind that the azimuth of the core is not known, only the inclinations are diagnostic, declinations being relative.) After cleaning, plugs Nos 3 to 20 show typical steep negative inclinations. Only plugs 1 and 2 show normal inclinations and they are both from the topmost 10 cm long core-piece. The broken core is from the massive cen- tre of a very thick flow and it is most unlikely that the inclination should shift the sign within the core. It is there- fore suggested that the top part of the core has been turned upside-down, most likely during the initial handling at the core site. As the azimuth of the core is not known, ordinary Fisher statistics are not applicable, but the modified inclination statistics of Kono (1980) may be used. Supposing all 20 plugs to have negative inclinations, the mean value is found to be: Im = –71.7°, with α95 = 1.95° (k = 709, N = 20) provided that the drilling was truly vertical. This would give an unusually accurate determination of the palaeofield inclination. However, as the geomagnetic secular varia- tion cannot be recorded from one flow only, this low value IRM Lopra-1 0 100 200 300 400 500 600 700 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Field (T) Lopra-1 plug 7 Lopra-1 plug 3 M ag ne tic in te ns ity ( A /m ) 1 0 0 100 mt J/JO J/JO 1 0 0 100 mt 1 0 0 100 mt 1 0 0 100 mt 1 0 0 100 mt 1 0 0 100 mt AF AF AF + Th AF + Th AF AF J/JO J/JO J/JO J/JO Sample: L1-2 Sample: L1-14 Sample: L1-3 Sample: L1-18 Sample: L1-10 Sample: L1-20 Fig. 11. Isothermal remanent magnetisation (IRM) of plugs Nos 3 and 7. Both specimens show magnetic saturation around 0.1 T, indicating that the dominating carrier of the remanent magneti- sation is magnetite. Fig. 12. Examples of typical behaviour of samples during AF and thermal demagnetisa- tion. Plugs Nos 2 (AF 0–40 mT), 3 (AF 0–70 mT) and 10 (AF 0–10 mT, combined with heating up to 630°C). Plugs Nos 14 (AF 0– 40 mT), 18 (AF 0–10 mT, combined with heating up to 630°C) and 20 (AF 0–50 mT). GEUS Bulletin no 9 - 7 juli.pmd 07-07-2006, 14:1961 62 of α95 does not give a realistic estimate for the accuracy of the average palaeomagnetic field inclination. Furthermore an angle of c. 2.5° from the vertical towards the south- east has been obtained from the GHMT-log of the hole at 2380 m (R. Waagstein, personal communication 2005). The inclination I0 of the geocentral axial dipole (GAD) field at the site of Lopra/Suðuroy with a latitude of 61.4°N is equivalent to a value of I0 = 74.8°, which is about 3° steeper than that found for the core. Based on McElhinny & McFadden’s (1997) analysis of a large number of vol- canic data from the last 5 Ma in the global palaeomagntic database, the expected geomagnetic dispersion of a VGP (virtual geomagnetic pole) at the latitude of the Faroe Is- lands may further be estimated to be c. 20°. Earlier palaeo- magnetic investigations have typically given systematical- ly lower mean values (see Figs 3, 5) for the palaeomagne- tic inclination (Table 1) except the one of –72° for a site near Torshavn (Løvlie & Kvingedal 1975). If we suppose the value from the Lopra-1/1A core of Im = –71.7° to be the optimum one, this would correspond to an axial di- pole palaeolatitude for the Lopra-1/1A site of 56.2°N at the time of extrusion. Most Cenozoic palaeopoles tend to be ‘farsided’ (Wilson 1971; Merrill et al. 1998), i.e. the palaeofield recorded in the rocks shows a more shallow inclination than does the present-day geomagnetic field at the site and biased shallow inclinations are also the case for most of the Palaeogene volcanic palaeomagnetic data from the North Atlantic region. This phenomenon may be due either to northward plate tectonic movements af- ter the formation of the sample, non-symmetric behav- iour of the geomagnetic field at the time of formation or unusual magnetic properties of the rocks investigated – or a combination of all three effects. A systematic error due to the latter cause (magnetic refraction) is not likely, as this requires rather strong values of the magnetic prop- erties of the lavas (e.g. Knudsen et al. 2003). If the palaeo- geomagnetic field was exactly a geocentral axial dipole field (the GAD-hypothesis), this would imply that the lithos- pheric plate carrying the Faroe Islands had moved about 5.2° northward during the last c. 60 Ma with an average northward component of velocity of c. 1 cm/year. An oc- topole contribution of the order of 10% (i.e. g3°/g1° = 0.1) to the central axial dipole field would alone suffice to explain the observed farsidedness of the Faroe Islands. An octopole contribution of this order of magnitude has been considered for Precambrian and Palaeozoic as well as Mes- ozoic times (e.g. Kent & Smethurst 1998; Torsvik et al. 2001; Van der Voo & Torsvik 2001). However, Merrill & McFadden’s (2003) analysis of data in the palaeomagnetic global database for the last 5 Ma concluded that a non- dipole bias appears less likely for the younger periods. Therefore, rather than claming that the shallower val- ue of inclination indicates fully either a northward plate movement of 5.2° (the ‘traditional’ palaeomagnetic inter- pretation), or is due entirely to a deficiency in the GAD- hypothesis, a more cautious interpretation may be a com- bination of both, implying that the GAD-hypothesis may not be exactly valid for the early Palaeogene, i.e. that the palaeomagnetic field was not a perfect central and axial dipole field at that time. To solve this palaeomagnetic important question fully, more global data from the period is needed. Reversal stratigraphy and age at Lopra-1/1A As mentioned above, the reversal stratigraphy of the 6½ km thick Faroe basalt formations was re-interpreted by Waagstein (1988), based upon published data then avail- able (Abrahamsen 1967; Tarling & Gale 1968; Schön- harting & Abrahamsen 1984; Abrahamsen et al. 1984), including the former palaeomagnetic results from Lopra- N EE Fig. 13. Characteristic AF-cleaned inclinations of the 20 plugs from the Lopra-1/1A core at depths between 2380 and 2381.4 m. Plugs other than Nos 1 and 2 (with positive inclinations, solid symbols) have negative inclinations (open symbols). The declina- tions are arbitrary since the azimuth of the core is not known. Full circle shows the expected axially centred dipole inclination of 74.7° at the Lopra-1/1A drill site, i.e. 3° steeper than the numer- ical average of –71.7° (dashed circle) of the 20 core plugs (See Table 1). GEUS Bulletin no 9 - 7 juli.pmd 07-07-2006, 14:1962 63 1, the cored information of which at that time reached a depth of 2178 m. All five cores from Lopra-1, at depths of 338, 862, 1219, 1923 and 2178 m, showed negative inclinations, i.e. reversed polarity, although no stable val- ues were obtained after demagnetising the cores from 338 and 1219 m (Schönharting & Abrahamsen 1984). The bottom of Lopra-1 was interpreted by Waagstein (1988) to match marine anomaly 26r, thus superseding two ear- lier alternative correlations discussed by Abrahamsen et al. (1984), in which this level was suggested to match ei- ther marine anomaly 25r or 24r. The present data from Lopra-1/1A, with negative in- clinations in the single core from 2380 m depth, indicates a reversed polarity at this level. Provided that there are no reversals in the unsampled interval above, the present da- ta extend the reversed sequence of the lower basalt forma- tion from the core at TD of the original well (2178 m) to the present level of the solid core at 2380 m. The SWC- cores reach the deeper level of 3531 m. However, as the up–down orientation of the individual SWC-cores is not known, no inclination information has yet been obtained from below 2381 m. Combining all polarity evidence available from the Faroe Islands and comparing with the Paleocene time scale by Berggren et al. (2000), we conclude that the lower part (below sea level) of the lower basalt formation may be correlated with Chron C26r (Selandian age), while the upper (exposed) part of the lower basalt formation corre- lates with Chrons C26n, C25r and C25n (Selandian and Thanetian age). The middle and upper basalt formations correlate with Chron C24r (Ypresian age). Magnetic logging (magnetic susceptibility and field intensity) was also attempted in the Lopra-1/1A well to- gether with other geophysical logs (Boldreel 2006, this volume) but, due to technical problems with the magnet- ic logging tool, no reliable inclination data were obtained (Abrahamsen & Waagstein 2006, this volume). Summary and conclusions A compilation of the palaeomagnetic age, the reversal chro- nology and evolution of the c. 6½ km thick basalt forma- tions of the Faroe Islands is presented, together with new petrophysical results from the Lopra-1/1A well. 1. The polarity record of the Faroe Islands has been cor- related in detail with the Global Polarity Time Scale. The lower part (below sea level) of the lower basalt formation correlates with Chron C26r (Selandian age).The upper (exposed) part of the lower basalt for- mation correlates with Chrons C26n, C25r and C25n (Selandian and Thanetian age). The middle and upper basalt formations correlate with Chron C24r (Ypre- sian age). 2. The inclinations yield farsided positions for the palaeo- magnetic poles, which is characteristic of most Palaeo- gene volcanics and sediments from the North Atlantic region. 3. The density and the rock magnetic properties of a sol- id core (1½ m in length) and 26 sidewall cores from the Lopra-1/1A well between –2219 and –3531 m are bimodal and suggest two characteristic groups of vol- canic materials, solid unaltered basalts and altered ba- salts and tuffs. 4. 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