Jökull - 01.01.2005, Page 19

Stratigraphy and paleomagnetism of Suðurdalur, Eastern Iceland
dalur” here refers to an area surrounding the south-
western part of the Lögurinn lake (Figure 1), includ-
ing the main tributary valley Norðurdalur (with the
Jökulsá glacial river), the shorter Suðurdalur valley
and the ridges Múli and Víðivallaháls.
Detailed mapping of the geology in parts of East-
ern and South-Eastern Iceland was carried out by
G.P.L. Walker and his students between 1955 and
the early 1970’s (e.g., Walker, 1959, 1960, 1974;
Carmichael, 1964). Subsequently, geological studies
have been published from other limited areas in the
region. As several areas in between these remain to
be mapped or dated radiometrically, a comprehensive
picture of the geological structure of Eastern Iceland
is not yet available. The overall composition of the
lava pile is approximately 80–85%basaltic lava flows,
10% acidic and intermediate rocks, and 5–10% sedi-
ment interbeds. In the Fljótsdalur area, the sediment
percentage is higher than this value (cf. Geirsdóttir
and Eiríksson, 1994).
Previous geological work in the Fljótsdalur area
The detailed mapping by Walker and his students
which mostly took place in fjords and coastal valleys
of central Eastern Iceland, did not include paleomag-
netic measurements. In connection with a large pale-
omagnetic collection effort in 1964–1965 (Dagley et
al., 1967; Watkins andWalker, 1977), Walker mapped
six profiles for sampling on the western side of the
Norðurdalur valley of Fljótsdalur. The positions of the
base of four of these profiles are indicated in Figure
1, i.e. Q (at Bessastaðaá river), R, S (Grundarlækur
brook), and T (Kleifará stream). The remaining two
profiles U and V lie 5–6 km and 10 km southwest of
T, respectively. The six Norðurdalur profiles make up
a composite section of about 110 lava flows of 1.4 km
thickness (including sediments). The entire exposed
basaltic lava pile in Eastern Iceland, generated be-
tween 13 and 2 Ma ago, is about 9 km thick. The
average accumulation rate of the lava pile in Eastern
Iceland is therefore of the order of 0.8 km of thickness
per Ma (Watkins and Walker, 1977). The lava pile has
a westerly dip which is generally less than 10◦.
Vilmundardóttir (1972) made geological observa-
tions and collected samples for geochemical analyses
from seven profiles at the west side of Norðurdalur.
These profiles coincided more or less with those of
Walker. She also sampled along Jökulsá and in some
hills in the area. These analyses are unpublished.
Einarsson (1972) attempted to correlate magnetic
polarity zones between Fljótsdalur and Jökuldalur.
However, these turned out to have widely different
ages according to Watkins et al. (1975) and Mc-
Dougall et al. (1976). The latter authors extended the
previously mentioned 1964–1965 mapping and pale-
omagnetic sampling considerably upstream in Bessa-
staðaá (profile Q), and they also sampled a new short
profile W in the Hengifossá river. Mussett et al.
(1980) published Ar-Ar dates on five samples from
the Norðurdalur profiles Q, R, T and V.
Inspection of G.P.L. Walker’s profile correlations
in the paper by Watkins and Walker (1977) reveals
that they do not always agree with the magnetic po-
larities, e.g. between R and S. Watkins and Walker
suggested correlations between several polarity zones
in profiles Q throughV in Norðurdalur and contempo-
rary versions of the geomagnetic polarity time scale,
but it should be noted that some of these zones were
only excursions or otherwise doubtful (Kristjánsson,
1983, p. 99).
Figure 1. Map showing the location of the Suðurdalur profiles mapped by us, sections A-B etc., see Figure
2. Profiles mapped by Guðmundsson (1978) in Suðurdalur and Norðurdalur are numbered. Profiles sampled
by Dagley et al. (1967), Watkins and Walker (1977) and other authors in Norðurdalur are shown with capital
letters in squares. Geomagnetic polarity zone boundaries inferred from these studies are indicated by brown
contours. In the inset map of Iceland the main volcanic systems of the axial volcanic zones have been drawn,
after Sæmundsson (1986). Black box denotes the study area. – Kort af Suðurdal og nágrenni. Sýnasöfnunar- og
þverskurðarsnið eru merkt með upphafsstöfum. Númer eiga við snið í kortlagningu Ágústar Guðmundssonar
(1978).
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