Magnetic Logs from the Lopra-1/1A and Vestmanna-1 wells (Faroe Islands)

• Main Authors: Waagstein, R., Abrahamsen, N.
• Format: Article in Journal/Newspaper
• Language: English
• Published: 2006
• Subjects: Magnetic logging; rock magnetism; susceptibility; NRM; magnetic reversals; palaeomagnetism; Faroe Islands; Lopra; Vestmanna; North Atlantic; Geological Survey of Denmark and Greenland Bulletin
• Online Access: https://pure.au.dk/portal/en/publications/a8371b20-b1f1-11db-bee9-02004c4f4f50

Susceptibility results from cores (representing basalt, lapilli-tuffs and tuffs) and magnetic logs from the Lopra-1/1A well are presented. The basalts fall into a high- and a low-susceptibility group with no overlap. The high-susceptibility basalts (7 cores) have susceptibilities between 4 and 85 x 10 -3 SI and consist of basalt with <1% vesicles from thick massive units. The low-susceptibility basalts vary from 0.6 to 1.5 x 10 -3 SI (7 cores). They are intergranular, intersertal or hypocrystalline and contain no or very little (<1%) visible magnetite, and are generally more altered than the high-susceptibility basalts. The susceptibility of 9 volcaniclastites of lapilli-tuff or tuff varies from 0.4 to 4 x 10 -3 SI. The cores reveal a bimodal distribution of magnetic susceptibily in the Lopra well. Low susceptibilities ranging from 0.4 to 4 in the well are characteristic to both altered basalts poor in magnetite, lapilli-tuffs and tuffs. Thus single measurements of susceptibility are of little use in discriminating between these three types of rock. Susceptibility logs from the Lopra-1/1A well show that the variation below 3315 m clearly distinguishes between volcaniclastics (hyaloclastites) with low and fairly constant susceptibility and basalt beds of between 5 and 10 m thickness (with high susceptibility). The volcaniclastics comprise some 60-70 % of the sequence between 3315 m and 3515 m with the maximum continuous sediment layer being 80 m thick. A 1½ m core of solid basalt at 2381 m and sidewall cores of basalt from the Lopra-1/1A well have a mean susceptibility of 22.1 ± 3.5 ´ 10 -3 SI (σ = 23.6, N = 46), while samples of hyaloclastite (lapilli-tuff and tuff) have a mean susceptibility of 0.85 ´ 10 -3 SI (σ = 0.39, N = 17). The mean values of the rock magnetic parameters for 303 basalt plugs from the Vestmanna-1 well are: Q ave = 13.3 ± 0.6 (σ = 11), S ave = 11.8 ± 0.6 ´ 10 -3 SI (σ = 11) and J ave = 4.64 ± 0.25 A/m (σ = 4.4). The polarity of the lowermost (hidden) part of the c. 4½ km thick lower ...