Jökull - 01.12.1991, Blaðsíða 49

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Paleomagnetic stratigraphy of the Mosfellssveit area,
SW-Iceland : a pilot study
LEÓ KRISTJÁNSSON
Science Institute, University of Iceland, Dunhagi 3, 107 Reykjavík
HAUKUR JÓHANNESSON
Icelandic Museum of Natural History, Hlemmur 3-5 , 105 Reykjavrk
INGVAR BIRGIR FRIÐLEIFSSON
Orkustofnun, National Energy Authority, Grensásvegur 9, 108 Reykjavík
ABSTRACT
A pilot stratigraphical study has been carried out of
the basalt lava and hyaloclastite pile between
Gufunes and Hafrahlíð in the Mosfellssveit area,
Southwest Iceland, using paleomagnetic remanence
polarities for correlation. About 200 units, mostly
lavas, were sampled and measured for this purpose.
Due to inadequate outcrops and to tectonic distur-
bances, the results are not entirely satisfactory, espe-
cially as regards the usefulness of the remanence
data for geomagnetic secular variation studies. We
provide descriptions and maps of the sampling pro-
files used in constructing a tentative polarity column
for the area, which is predominantly of reverse
polarity corresponding to the Matuyama geomagnet-
ic chron. Two normal-polarity intervals in the lava
pile are correlated with similar zones in the nearby
Mt. Esja. There is evidence of at least eiglit glacia-
tions in the area.
INTRODUCTION
A large number of paleomagnetic projects have been
undertaken in Iceland in the last thirty years, mostly
in conjunction with stratigraphic mapping efforts.
Among these is the study of Kristjánsson et al. (1980)
in the Esja-Akrafjall area of SW-Iceland, where K-Ar
dating substantiated earlier suggestions that the Esja
volcanism is of Lower to Middle Matuyama age.
Further confirmation of the age estimates of
Kristjánsson et al. (1980) has recently been provided
by K-Ar dates quoted by Geirsdóttir (1991). The
presence of two normal geomagnetic polarity zones
in the Esja succession was also confirmed, the older
one of which is the “N3” of Einarsson (1957).
Kristjánsson et al. (1980) were, however, unable to
determine whether it should be correlated with the
Olduvai or the Reunion subchron but they favoured
the latter.
Knowledge of the Quatemary geology of the
region from Reykjavík to Esja is potentially of some
economic importance, since aquifers at 1-2 km depth
in the region provide geothermal heating for a popu-
lation of approx. 130,000. A map of its surface geol-
ogy has been available (Tryggvason and Jónsson
1958) but it is indeed remarkable that no stratigraph-
ic work has been published up to now. Paleomagnetic
polarity determinations on lava samples in outcrops
and geothermal drill cores were made by the late Th.
Sigurgeirsson around 1954 according to his note-
books in our possession, but the results were not writ-
ten up in detail. Some parts of the region were
mapped by geology students (e.g. theses by Torfason
1974, Theodórsdóttir 1972, Thors 1969,1974).
The present paleomagnetic project was initiated
in 1972 by sampling in profile UL and was gradually
i'
JÖKULL,No. 41, 1991 47