Jökull - 01.01.2005, Page 24

Leó Kristjánsson and Ágúst Guðmundsson
to a highly eroded central volcano near the east margin
of the Vatnajökull glacier. This suite is stratigraph-
ically more or less parallel to the LA-suite, formed
during the same period.
HD-suite (Hafursárandesít) is a widespread lava
flow of intermediate composition (andesite or dacite)
forming the base of mt. Snæfell. It is probably less
than 0.8 Ma old and covers the LA-suite uncon-
formably. A conglomerate layer, variable in thickness
and consolidation, separates the HD-andesite from the
LA-suite.
Hydrothermal alteration
The basalts of Iceland contain abundant secondary ze-
olites and other amygdale and joint-filling minerals.
Walker (1960) found that these minerals have a dis-
tinct subhorizontal zonal distribution in Eastern Ice-
land, see Table 1. The zeolitization is thought to have
begun during the buildup of the lava pile and to have
lasted until glaciers cut down the valleys. The alter-
ation zones are named after prevalent amygdale min-
erals, which are best developed in olivine-rich basalts.
Boundaries of these zones in Fljótsdalur are shown in
Figure 2.
Of the profiles sampled by us in Suðurdalur (see
below), MK and ST begin in the analcime zone of al-
teration, and VV begins not far below the top of the
scolecite zone (Guðmundsson, 1978). Both VV and
MK reach to the uppermost part of the chabazite zone.
Table 1. Zeolite zonation in the basalt lava pile of
Iceland. – Lagskipting ummyndunarsteinda.
Depth below original Alteration zone
surface of lava pile m
0–200 Minor alteration, empty vesicles
200–800 Chabazite-thomsonite zone
800–1000 Analcime zone
1000–1600 Mesolite-scolecite zone
1600–2000 Laumontite zone
Note: The depths are only approximate, and Walker (1983)
uses 600 m instead of 800 m for the position of the top of
the analcime zone.
Tectonics
The lava pile of the Fljótsdalur area may be consid-
ered as an isoclinal body dipping gently westwards.
In its lowest part exposed at Hengifossá and Bessa-
staðaá and in the mountains Múli and Víðivallaháls,
dips 8-9◦ towards 280◦E. Variations in the downdip
direction are typically ±10◦. The dip angle gradually
decreases with increasing elevation, from 4–5◦ at 400
m a.s.l. to less than 3◦ at 600 m.
The lava pile is affected by two relatively dense
sub-vertical fault and dyke systems, both striking in
direction 0◦ to 15◦E. See maps in Guðmundsson
(1978) for details. Nearly 100 faults have been ob-
served and mapped in the Fljótsdalur area, see Fig-
ure 2 for examples. Faulted zones are often 1 to 5 m
wide and contain fault breccia. The observed down-
throws range between 1 and 40 m. At Bessastaðaá,
85 m movement was observed on one fault and down-
throws up to 30–50 m are found in Víðivallaháls. The
typical spacing of the faults is 200–250 m.
Basaltic dykes are common in the Fljótsdalur lava
pile; about 60 of these have been located. The typi-
cal thickness of the dykes is 3–5 m, but thinner and
thicker (up to 10 m) dykes occur. The dykes are
usually continuous and densely jointed (horizontal
columns). Thin fault breccia is found at places along
the dykes. The longest two dykes can be traced over
15 and 30 km respectively.
PALEOMAGNETIC STUDY
Description of profiles
The work reported here represents a reconnaissance
paleomagnetic survey which only covers a small part
of the area mapped by Guðmundsson (1978). Sam-
ples have been collected from the following four pro-
files (Figures 4 and 5) which are thought to overlap in
time. In order of decreasing age:
VV – Exposures created by a 1979 debris flow a
few hundred m north of the farm Víðivellir ytri I. 20
lavas plus one flow unit VV 3B were sampled; these
are overlain by poorly exposed tholeiite flows.
VS – Small stream at a cottage 2 km north of the
Sturluá river, 6 sites were sampled. The profile spans
24 JÖKULL No. 55