Jökull - 01.01.2004, Blaðsíða 57

Reviewed research article
A reconnaissance study of paleomagnetic directions in the
Tjörnes beds, Northern Iceland
Leó Kristjánsson
Institute of Earth Sciences, University of Iceland, Askja, Sturlugötu 7, 107 Reykjavík, email: leo@raunvis.hi.is
Abstract — Paleomagnetic direction measurements were carried out on core samples collected at 30 sites in
sediments and one lava site, on the west coast of the Tjörnes peninsula, Northern Iceland. The sediments are
mostly sandstones, more coarse-grained than younger sediments in the peninsula sampled by Eiríksson et al.
(1990). The remanence in the present collection also has a lower intensity and stability during alternating
field demagnetization. Several sites yield reasonably consistent directions, and both polarities are found. More
extensive collections and improved experimental techniques are needed before the reversals can be used in local
stratigraphic work.
HISTORICAL INTRODUCTION
General geology
The fossil-bearing sediments in the Tjörnes peninsula
of Northern Iceland are the country’s most extensive
and best known sediment sequence. These sediments
which are largely of shallow marine origin, have been
the focus of research by many geologists, paleontolo-
gists and other scientists since the mid-eighteenth cen-
tury. A bibliographic review of this research was pub-
lished by Eiríksson (1981a), and more recent develop-
ments have been summarized e.g. by Eiríksson et al.
(1990) and Buchardt and Símonarson (2003).
The pioneer geologists Þorvaldur Thoroddsen and
Helgi Pjeturss around 1900, Guðmundur G. Bárðar-
son (1925) and subsequent authors have considered
the age of the entire Tjörnes sequence to be Pliocene
to Pleistocene. Eiríksson (1981b) divides the sedi-
ment sequence into two main parts: the younger one is
the Breiðavík Group which contains a number of lava
flows and is underlain by a series of lavas named after
the small Höskuldsvík inlet. The lower part of the se-
quence is the Tjörnes beds, which only contains one
occurrence of lava flows. It is generally divided into
three zones: from top to bottom the Serripes, Mactra
and Tapes zones according to characteristic mollusc
fossils.
Various methods have been employed in attempts
to obtain definite ages, either absolute or relative,
of parts of the sequence. These methods have in-
volved e.g. climate variations, K-Ar radiometric dat-
ing of lavas, and faunal changes. An approach used at
Tjörnes by Th. Einarsson et al. (1967) as well as by
several later authors (see Eiríksson, 1981, Buchardt
and Símonarson, 2003) was based on correlation of
paleomagnetic polarities with a global time scale of
geomagnetic reversals. Unfortunately, the polarity
time scale of the 1960’s and its later versions (de-
rived partially from dated volcanics worldwide and
partially from ocean-floor anomaly lineations) have
turned out to underestimate the actual number of re-
versals. Therefore it is becoming increasingly clear
that paleomagnetic polarities in sections where only a
few reversals occur, are not to be relied on by them-
selves for long-distance (> 20 km) correlation or age
estimates. However, extended polarity patterns and
particular smaller-timescale features such as rever-
sal paths can be used for short-distance correlations.
Here, new paleomagnetic direction results from sites
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