Jökull - 01.01.2005, Page 17

Reviewed research article
Stratigraphy and paleomagnetism of lava sequences in the
Suðurdalur area, Fljótsdalur, Eastern Iceland
Leó Kristjánsson
Institute of Earth Sciences, University of Iceland, Askja, Sturlugata 7, 101 Reykjavík; leo@raunvis.hi.is
Ágúst Guðmundsson
JFS Geological Services, Rauðagerði 31, 108 Reykjavík; jardis@geoice.net
Abstract— The basal stratigraphy of the south Fljótsdalur valley region, Eastern Iceland, was formed during
the past 7 Ma. The 1.6 km thick stratigraphic pile (from the southwestern end of the Lögurinn lake up to the
base of mt. Snæfell) is chiefly composed of basalt lava flows, but some differentiated volcanics as well as thick
layers of clastic sediments are also seen. Here we review previous geological and geophysical work in the area,
and briefly describe eleven of the main rock suites which make up the local succession. Laboratory paleomag-
netic studies on 76 lava sites in the lower half of the stratigraphic sequence in the Suðurdalur tributary valley
of Fljótsdalur are described; the resulting polarity column was extended upwards using field measurements.
Distinct lava groups and polarity reversals are employed in correlating the Suðurdalur sequences with those in
the Norðurdalur valley and the Bessastaðaá river where previous geological, paleomagnetic and radiometric
dating results are available.
INTRODUCTION
Geological setting of Eastern Iceland
The axial region of the Mid-Atlantic Ridge, which
marks the rifting plate boundary between the Eurasian
and North American plates, passes through central
Iceland. Within the island and the surrounding shelf
the boundary is somewhat diffuse, being chiefly man-
ifested as two zones of active volcanism trending
southwest-northeast in the southern part of the coun-
try and one north-south zone in the north. The overall
spreading rate of the plates is about 1 cm in each di-
rection. The thick sequences of subaerial lava flows
and minor sediments which are exposed above sea
level in Iceland, have been generated at the axial vol-
canic zones in the past 15 Ma (see review by Sæ-
mundsson, 1986). The development of these zones
during that period has not yet been satisfactorily de-
lineated, and many aspects of the overall influence
of a supposed “plume” in the Earth’s mantle under-
neath Iceland on its geological history are also un-
certain. Stratigraphic research and radiometric dating
indicates that relative to the crust, the axial volcanic
zones have undertaken one or more displacements to-
wards the east in the 15Ma time interval. These zones
which are of the order of 50 km in width, may be en-
visaged as being composed of several active volcanic
systems (Figure 1). Each system commonly includes
a fissure/dike swarm and a central-volcano complex
(Walker, 1974) whose lifetime may be 0.5–1 Ma. At
the central-volcano complexes, the geology is char-
acterized by intense extrusive and intrusive volcanism
(including differentiated rock types), hydrothermal al-
teration, and local tectonics. The axial zones are con-
stantly subsiding as a consequence of extrusive vol-
canism and rifting, so that the exposed lava pile tilts
towards them.
In this paper, “Eastern Iceland” will be used for
the region east of the volcanic zones, northward and
eastward of the major ice cap Vatnajökull. “Fljóts-
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