Jökull - 01.12.2006, Síða 41

Reviewed research article
A paleomagnetic study of stratigraphic relations in the lava pile
of Norðurárdalur and Austurdalur, Skagafjörður, North Iceland
Leó Kristjánsson1, Ágúst Guðmundsson2, Árni Hjartarson3, and Haraldur Hallsteinsson2
1Institute of Earth Sciences, University of Iceland, Askja, Sturlugata 7, 101 Reykjavík
2JFS Geological Services, Rauðagerði 31, 108 Reykjavík
3ÍSOR-Iceland Geosurvey, Grensásvegur 9, 108 Reykjavík
email: leo@raunvis.hi.is
Abstract — We present results of stratigraphic mapping in volcanics of Late Miocene age in valleys south of
the Skagafjörður fjord, North Iceland. The strata of six mountainside profiles in the area are shown in detail
in a composite diagram. In these profiles, 250 lava flows have been sampled for laboratory paleomagnetic
measurements of remanence directions and intensities. The directions were employed as an aid in stratigraphic
correlations between the profiles, along with significant sedimentary horizons and other geological evidence.
It appears that the build-up of the lava pile in the area was rather episodic, often with 2-6 lavas having been
emplaced in rapid succession. A composite section consists of a lava pile of over 1.7 km thickness, recording
9 geomagnetic polarity reversals as compared to 17 in a similar pile of the same age range in the valleys of
Eyjafjörður. A single radiometric age determination in the uppermost part of our composite section yields a
date of 5.2 million years, a considerably younger age than expected from previous studies to the north and
east of the Skagafjörður valleys. We discuss various implications of these results, including in particular the
possible presence of unconformities in the area.
INTRODUCTION
The geological structure of the older parts of the
Tertiary lava pile of Iceland is generally well-exposed
due to uplift and Pleistocene erosion. Systematic
stratigraphic mapping of these areas was initiated in
the late 1950’s by G. P. L. Walker, working in the
fjords of East Iceland. For correlations he relied on
features such as extensive horizons of clastic sedimen-
tary rocks including acid tuffs from explosive erup-
tions. Additionally, he introduced the use of groups
of lava flows recognizable in the field by their distinct
petrography. These groups of plagioclase-porphyritic
lavas and olivine tholeiites can in some cases be traced
for tens of kilometers.
In subsequent stratigraphic studies in Iceland, by
Walker and his associates (e.g., Watkins and Walker
1977) as well as by others, Walker’s geological meth-
ods were augmented by two geophysical techniques,
one being radiometric dating by the K-Ar (and later
40Ar-39Ar) method. Dating results which have be-
come available from several parts of Iceland since
1966, have greatly aided in the construction of a gen-
eral picture of the country’s geological history since
15 million years (Ma) ago. However, the accuracy
of the dating has been variable due to geological and
technical reasons, often connected with the effects of
secondary hydrothermal alteration of the lava pile. In-
dividual age determinations may be uncertain by up to
JÖKULL No. 56 39