Jökull - 01.12.1992, Síða 33

STRATIGRAPHY, AGE AND PALEOMAGNETISM OF
LANGIDALUR, NORTHERNICELAND
Leó Kristjánsson,
Science Institute, University oflceland, Reykjavik
Haukur Jóhannesson,
Icelandic Museum ofNatural Histoiy, Reykjavik
Ian McDougall,
Research School ofEarth Sciences, Australian National University, Canberra, Australia
abstract
We present the results of stratigraphic mapping,
Paleomagnetic measurements and radiometric dat-
"l8 of lava profiles of 2.1 km total thickness in the
hangidalur valley, Húnavatnssýsla, Northern Iceland.
Seven reversals of paleomagnetic polarity occur in a
coniposite lava section of 1.15 km thickness. Most
°f the lavas were emplaced hetween 8.2 and and 7.3
17l,Hion years ago. The area is fairly typical for the
con\position of the lava pile in Iceland, hut instances
°í large tectonic tilts are encountered due to a NNW
s"'iking flexure. Unusual lava types, prohably origi-
nating in nearby volcanic centers, were used as marker
layers during the mapping.
INTRODUCTION
History of the project. The study area is lo-
cated in Northem Iceland (Fig. 1). We are not aware
°f previous geological mapping in this area, apart
from descriptive work on particular features by Th.
Thoroddsen and other pioneers. Some distance to
the west, students of the University of Iceland have
carried out mapping in the Vatnsnes peninsula (Jón
Eiríksson, pers. comm. 1980), and to the south of
the area the bedrock geology and tectonics has been
Inapped in connection with the Blanda hydroelectric
Plant (Guðmundsson et al., 1982). The first stage of
the present mapping effort was carried out in 1976-78
by Á. Guðmundsson and J. Helgason as an extension
of the stratigraphic study of central Northern Iceland
by Sæmundsson et al. (1980), but reliable correlations
to that region were not established. This work was
then discontinued and not resumed until 1984-86.
Geological setting. Iceland is situated on the rift-
ing plate boundary in the Northeastem Atlantic Ocean.
The boundary is marked by a wide rift zone which
stretches SW-NE across the island. The bedrock strata
in general increase in age with increasing distance
from the rift zone, along with progressively greater
tectonic tilts. However, major lateral shifts of the rift
axis, and the likely presence of more than one simulta-
neously active volcanic zones in the past, complicate
this pattern in many regions.
The region around Húnaflói Bay in westem N-
Iceland is characterized by an undulating basalt lava
pile with some contemporaneous central volcanoes.
The lava succession consists mainly of tholeiitic lava
flows intercalated with sedimentary beds, usually thin
red partings but occasional thicker beds of siltstones
and acidic tuffs or ignimbrites occur. A few interme-
diate to acidic lava flows are found.
A flexure zone borders the eastem side of Húnaflói
Bay (Fig. 1) and stretches inland towards southeast,
fading out gradually. The flexure zone dips towards
a syncline axis which trends N-S between Vatnsnes
and Skagi. The syncline is believed to be the rem-
nants of a former rift zone which became extinct about
JÖKULL, No. 42, 1992 31