Jökull - 01.01.2001, Blaðsíða 35

Paleomagnetic studies in Skarðsheiði, SW-Iceland
Previous paleomagnetic research
In his pioneering work in South–Western Iceland,
Einarsson (1957, 1962) used magnetic remanence–
polarity measurements on hand samples in the field
to set up a scheme of polarity zones. They were num-
bered back in time from the present, as N1, R1, N2,
R2 and so on. His mapping, however, did not in-
clude the delineation of petrographically distinct lava
groups as used very successfully by G.P.L. Walker
(1959) and later investigators. Unfortunately, Einars-
son’s regional mapping has not been followed up to
sufficient extent: some small areas in South–Western
and Western Iceland have been mapped stratigraph-
ically, but the descriptions of many such areas are
mostly to be found in student theses and in internal
institute reports.
Sigurgeirsson (1957) studied paleomagnetic pole
positions in lava flows in South–Western Iceland
by laboratory measurements on oriented samples,
concentrating on lavas at Einarsson’s polarity zone
boundaries. His results constituted the first–ever ev-
idence of “intermediate” pole positions in middle and
low latitudes. The largest numbers of such intermedi-
ate poles were found at the R3–N3 boundary at sev-
eral sites in the Esja and Kjós areas (Figure 1), which
have been subsequently studied by other investiga-
tors (Kristjánsson and Sigurgeirsson, 1993; Gogui-
tchaichvili et al., 1999). Another site of several inter-
mediate poles was investigated in the Skorradalur val-
ley east of Borgarfjörður (Figure 1) which Einarsson
(1957, Figure 3) and Sigurgeirsson assumed to repre-
sent the same polarity boundary as the N4–R3 in Esja
and Kjós, see Kristjánsson (1995).
Wilson et al. (1972) published polarity results and
some diagrams of pole positions from a paleomag-
netic study of several profiles through the lava pile
of South–Western Iceland, based on Einarsson’s map-
ping. They essentially confirmed his results as regards
the polarity of lavas in the profiles. However, no revi-
sion of Einarsson’s (1957, 1962) polarity zone num-
bering scheme was attempted, nor a correlation with
the time scale of geomagnetic polarity reversals then
available in the literature.
Extensive K–Ar dating by McDougall et al.
(1977) of a composite section in valleys northeast of
Borgarfjörður supported correlations previously sug-
gested (e.g., Sæmundsson and Noll, 1974) between
polarity zones in the lava pile and the Geomagnetic
Polarity Time Scale in the time interval 1.5 to 3.5 M.y.
ago, i.e. to below the R5–N5 which was correlated
with the Gilbert–Gauss boundary. These correlations
also agree fairly well with the polarity mapping by
Einarsson (1962) in the Borgarfjörður valleys, where
Einarsson’s locally designated r5–n5 boundary in his
map V may be the upper Mammoth transition rather
than the Gilbert–Gauss transition. It corresponds to
the zone boundary at NT 10/11 in McDougall et al.
(1977).
The main comprehensive project of stratigraphic
mapping in South–Western Iceland published to date
is described in the paper of Kristjánsson et al. (1980)
in the Esja mountain south of Hvalfjörður and in
Akrafjall to the north (Figure 1). This work was based
on previous geological studies by Friðleifsson (1973)
and Franzson (1978) respectively. Figures 3 and 4 of
Kristjánsson et al. (1980) indicate that their compos-
ite section reaches from the upper part of Einarsson’s
(1957) R5 zone up to the the lower part of his N2.
The paper by Kristjánsson et al. (1980) included a
K–Ar date of 1.85

0.18 M.y. obtained at the base of
N2. It was therefore tentatively concluded that N2 in
Esja represents the Olduvai subchron, that the rela-
tively thin N3 zone represents the Reunion, the N4–
R3 is the Gauss–Matuyama boundary, and R4 is the
Mammoth subchron. The Kaena subchron appears to
have been missed by Einarsson but Kristjánsson et al.
(1980) assigned a thin group of reversely magnetized
lavas (EY 7–8) close to sea level south of Hvalfjörður,
to this subchron.
The only K–Ar dates published since 1980 on lava
flows in SW–Iceland are those quoted by Geirsdóttir
(1991) on flows FA 51 and FB 01 in Akrafjall, not far
above the base of the R4 zone. Flow FA 51 which
is just underneath a conglomerate horizon yielded an
age of 3.12

0.23 M.y. whereas FB 01 (in a profile
about 4 km north–west of FA, not shown in Figure
1) above that horizon, yielded 2.87

0.23 M.y. These
ages do not conflict with the view of Kristjánsson et
al. (1980) that R4 is the Mammoth subchron but addi-
tional, and more accurate, dates in these sections are
JÖKULL No. 50 35