Jökull - 01.01.2014, Page 3

Paleomagnetic studies, Northwest Iceland
new vision: continuous volcano-tectonic processes in-
cluding rifting and subsidence at the active zones
passing through Iceland from southwest to northeast.
A further important change of views was brought
about by K-Ar age determinations on some of the old-
est lava flows in Northwest- and East Iceland (Moor-
bath et al., 1968) which indicated ages of only 14–
16 and 12–13 Ma respectively. It was also suggested
tentatively in the late 1960s (Sæmundsson, 1967, p.
158) that a volcanic zone had in the past been active
west of the present zone in West Iceland. This is now
generally agreed to have been the case, with the activ-
ity moving to its current location at around 7 Ma. A
similar eastwards jump has been suggested to have oc-
curred at about 15 Ma (Hardarson et al., 1997; Pringle
et al., 1999).
The dating by Moorbath et al. (1968) in Northwest
Iceland was done on material from a single locality at
a stratigraphic height of some 500 m above the oldest
lignite beds, so that neither the age of the lignites nor
of the 300 m thick underlying exposed lava pile was
determined. The rate of buildup of the pile was also
unknown. In the large 1964–1965 field study in East
Iceland summarized by Watkins and Walker (1977),
a pile of some 900 lavas (including up to 200 flow
units), 8.8 km in total thickness, was inferred to have
been built up in a time interval estimated to reach from
about 13.5–2 Ma. This translates into an average rate
of 0.7–0.8 km per million years or 10 m per 12–14000
years.
In a pilot paleomagnetic project on lava flows
around six outcrops of the oldest lignite-bearing sed-
iments south of the long fjord Ísafjarðardjúp (Figure
1), Kristjánsson (1967, 1968) found that the sediments
tended to be associated with a zone of a few normal-
polarity lavas within a thicker reverse zone. This as
well as the fact that the lignite sediments are often
absent in exposed locations where they could be ex-
pected, lent some support to the possibility that the
interval represented by the lignite-bearing sediments
was relatively short, say of the order of 0.2 M.y. rather
than say >2 M.y.
Research in 1970–1985
Few radiometric age studies have been published on
Icelandic lavas after 1968, the major contribution be-
ing due to I. McDougall of the Australian National
University in connection with comprehensive strati-
graphic and paleomagnetic mapping efforts in several
areas in 1972–1978. In particular, his total of some
180 dates included more than 70 lavas in two com-
posite sections through the west and east coastal re-
gions of the Northwest peninsula (McDougall et al.,
1984). The western composite section which ran from
Skálavík south to Breiðafjörður (along the blue line of
Figure 1), was made up from non-overlapping parts
of over 15 hillside profiles, about 450 sampled lava
flows. The profiles were correlated by stratigraphic
mapping based on tectonic dip, unusually thick sedi-
ments, petrographically distinct lava groups, and geo-
magnetic polarity zones mapped in the field.
The lignite-bearing sediment beds in Northwest-
Iceland include laterite, clay, tephra, and sandstone,
but the order and proportions of these types vary
greatly between outcrops (Sigurðsson and Sæmunds-
son, 1984). It has not been certain to what extent
the lignite beds may be considered as distinct hori-
zons. With the work of McDougall et al. (1984) it
was established that in the western part of the penin-
sula there were at least three different sets of relatively
thick sediment outcrops. The oldest and best known
of these includes occurrences south of Ísafjarðardjúp:
in the Skálavík and Bolungarvík inlets, in Súganda-
fjörður, and in Önundarfjörður. They consist of vari-
ous altered volcanoclastics, rarely exceeding 15 m in
thickness where exposed. It has also always been as-
sumed (Sigurðsson and Sæmundsson, 1984) that out-
crops in Aðalvík and Fljótavík north of Ísafjarðar-
djúp and on the promontories south of Dýrafjörður
and Arnarfjörður (Hardarson et al., 1997) belong to
the same episode. A second set of sediment outcrops
runs from southwest to northeast just east of Patreks-
fjörður, Tálknafjörður, Arnarfjörður and Dýrafjörður,
and a third one with a more east-westerly trend fol-
lows the coast of Breiðafjörður (Jóhannesson and Sæ-
mundsson, 2009). Some fossiliferous sediment occur-
rences may not belong to any of the above three sets.
Thus, a well-known plant-fossil locality within profile
TE south of Arnarfjörður (Figure 1) lies at least 200 m
above a lignite outcrop at the end of that promontory
(Figure 3 of Kristjánsson, 2009).
JÖKULL No. 64, 2014 3