Stratigraphy and paleomagnetism of a 2.9-km composite lava section in Eyjafj�rdur, Northern Iceland: a reconnaissance study

Abstract

Eight mountainside profiles in lava flows south of the fjord Eyjafjördur, Northern Iceland, were sampled for paleomagnetic studies. The sampling was concurrent with pilot stratigraphic mapping of the lava sequences in these and several supplementary profiles. The eight profiles are correlated with minor overlaps so that they form a composite section of 2.9 km thickness, estimated to cover the age range between about 9 and 5 million years ago (Hardarson et al. 1999). Paleomagnetic measurements made on 319 lavas generally yield primary remanence directions of high stability and within-unit consistency. Evidence for at least 17 reversals of the geomagnetic field is seen, as well as numerous field excursions. Frequent clustering of directions in successive lavas indicates that the volcanism was episodic.

Paleomagnetism and magnetic anomalies in Iceland

“…However, later research has indicated that the volcanic productivity is quite episodic, and dating of a thick normal-polarity zone in Northwestern Iceland (McDougall et al, 1984) did not yield the expected agreement with the two thick zones dated previously. In some cases the paleomagnetic directions in successive lavas are serially correlated, indicating that the time interval between these lavas is less than 1 kyear (Kristjansson et al, , 2004. In other cases there may be hiatuses of 100 kyear and more between lava flows, not detectable from geologic observations or from radiometric dating at the present level of resolution.…”

“…Other examples are found in papers by Sigurgeirsson (1957), Kristjansson (1995) and Kristjansson et al (2004). Field mapping of remanence directions in late-glacial lavas of the Reykjanes peninsula, initiated by A. Gudmundsson in the mid-1970s, revealed that in several hills all exposed lava flows were erupted during a geomagnetic transition or major excursion of the field (Levi et al, 1990).…”

“…Hence, there is no worthwhile gain in quality by applying further demagnetization (see e.g. Watkins and Walker, 1977;Kristjansson et al, 2003Kristjansson et al, , 2004 or sophisticated methods of statistical processing. As an example of the excellent reproducibility of directional results, it may be mentioned that a series of 22 basalt lavas in Southern Iceland was sampled and measured both by Kristjansson et al (1988) and Tanaka et al (1995).…”

“…However, variations by up to 10 • in mean pole positions between parts of the large 1964-65 Eastern Iceland paleomagnetic survey were noted by Wilson and McElhinny (1974); these were unlikely to be due to chance in a collection of independent data. Differences of this size have also been noted subsequently, even between collections of up to 300 lava flows spanning several reversals (see Kristjansson et al, 2003Kristjansson et al, , 2004 but the reasons for these variations remain obscure. It is possible that they are partly caused by regional magnetic anomalies of crustal origin, presence of clustered atypical directions due to episodicity of the volcanism, inaccurate tectonic corrections, or some small directional bias in the orientation of samples during collection and measurement.…”

“…However, no In the stippled area, basement rocks are less than 0.8 Ma old, and we also indicate the position of the 10 Ma isochron (Saemundsson, 1986). The numbers refer to publications where the composite surveys are reported, as follows: (1) Watkins and Walker (1977); (2) Helgason (1982), Bleil et al (1982); (3) Kristjansson et al (1995); (4) McDougall et al (1976b) and Kristjansson and Gudmundsson (2005); (5) Helgason and Duncan (2001); (6) Saemundsson et al (1980); (7) Kristjansson et al (1992); (8) Kristjansson et al (2004); (9) Doell (1972), Kristjansson et al (1988), Eiriksson et al (1990); (10) McDougall et al (1984); (11) Kristjansson and Johannesson (1996); (12) Kristjansson et al (2003); (13) ; (14) Kristjansson and Johannesson (1999); (15) Kristjansson et al (1980); (16) Kristjansson et al (1991); (17) Kristjansson and Gudmundsson (2001). certain correlations between distant parts of the country, using e.g. particular geomagnetic epochs or extensive clastic horizons, have been found so far.…”