Jökull - 01.07.2003, Síða 25

Paleomagnetic observations on Late Quaternary basalts, SW-Iceland
THE REYKJAVÍK GRAY LAVAS
Introduction
The “Reykjavík gray basalt lavas” were a subject of
considerable discussion among geologists in the first
half of the 20th century. They are a sequence of
olivine tholeiites (sometimes containing feldspar phe-
nocrysts), exposed at many locations in Reykjavík and
nearby coastal areas as well as on higher ground to
the east in the general region of Figure 1; see the map
of Jóhannesson and Sæmundsson (1988) for details.
They are usually fresh-looking, light gray and finely
porous. The individual units are thin (typically a few
m) and separated by scoria; only at a few locations
are any sediments seen between them. Outcrops are
discontinuous because of later erosion and sediment
cover. One reason for interest in the gray lavas by
geologists was clear evidence for glacial action at the
surface of some of the lavas (Pjeturss, 1909); this gave
rise to the designation “(inter)glacial dolerites” used
by several authors during the next 40 years or so. An-
other reason was the occurrence of fossiliferous sedi-
ments below and above the lava sequence; these were
named the Elliðavogur and Fossvogur sediments re-
spectively. The former sediments are mostly of ma-
rine origin but contain river deposits and lignite at the
top. The latter sediments, described in detail by Geirs-
dóttir and Eiríksson (1994) have been dated by
C at
about 11 kyr (see Andersen et al., 1989; Hjartarson,
1989; Sveinbjörnsdóttir et al., 1993).
After 1950 new geological research (largely un-
published) has been carried out on the Quaternary ge-
ology of the Reykjavík area. This research was mo-
tivated at least in part by a need for knowledge about
local aquifers. Early efforts to separate the Reykja-
vík lavas into distinct groups for stratigraphic pur-
poses, on the basis of their chemical composition,
were abandoned due to the observed heterogeneity
of units of similar age (K. Grönvold, pers. comm.,
1973). The lavas have been supposed to originate
in shield volcanoes of low relief, and some locations
of these volcanoes have been suggested. Hjartarson
(1980) published a diagram where various lava units
of the gray-basalt sequence are grouped into a tenta-
tive stratigraphic scheme according to e.g. their litho-
logical aspects (phenocryst content, etc.), geographic
position and altitude, and relative ages where known.
Available information on many of the present sam-
pling sites is insufficient for relating them definitely to
Hjartarson’s (1980) scheme, and more recent studies
(Hjartarson, 1993; Sigfússon, 2002) indicate alterna-
tive possibilities of separating the gray lavas into age
groups.
Sampling and measurements, 1964–1984
The first paleomagnetic measurements on the Reykja-
vík gray lavas were obtained by a 1964 LiverpoolUni-
versity/University of Iceland expedition. Two sam-
ples were collected from each of 8 successive lava
units in a quarry at the Öskjuhlíð hill (site 8 in Fig-
ure 1) and they all give similar directions. The results
have not been published in detail. An average direc-
tion is given at the top of Table 1a.
Around 1972 the question was raised whether pa-
leomagnetic measurements might be of help in unrav-
eling the stratigraphy of the gray lavas. At the request
of K. Sæmundsson of the National Energy Authority,
ten sites were sampled in the Reykjavík area for pa-
leomagnetic measurements in 1973–1974. Site loca-
tions are indicated in Table 1 and Figure 1. At each
site, four oriented 25 mm core samples were collected
by the present author from each of 2–4 lava units. Re-
manence measurements on the cores were made by
technicians at the University of Rhode Island, U.S.A.,
after alternating field (AF) demagnetization at 10 and
20 mT. Results on the direction and intensity of these
lavas are shown in Table 1a and in Figure 2a. In the
Table, the units from each site have been combined
wherever their remanence directions agreed within a
few degrees.
Kristjánsson et al. (1980) published overall statis-
tical data on the above sites. Further collections were
made by the author in 1983–1984, in collaboration
with Á. Hjartarson of the National Energy Authority.
Usually only one unit was sampled at each locality.
The localities are shown in Figure 1 and listed in Ta-
ble 1b. Remanence measurements were made at the
University of Iceland, generally using 10, 15 and 20
mT AF treatment. Mean remanence directions and in-
tensities in these lavas are given in Table 1b.
JÖKULL No. 52, 2003 23