Jökull - 01.12.1994, Síða 7

(Hald et al. 1971). Irregularities in dip and strike
occur around the central volcanoes; they usually dis-
play a great compositional variety in rock types and
the thickness of individual rock series increases to-
wards them.
km upper section is at Reykjarfjörður on the east side
of the Vestfirðir peninsula, with profiles stretching
south along the coast to Hrútafjörður. Included in the
work of McDougall et al. (1984) were a number of
K/Ar-age determinations of basaltic lava flows.
Previous work on stratigraphic sections
Kristjánsson et al. (1975) mapped a 2-km section
in the Amarfjörður area. Jóhannesson (1980) mapped
a section from the Króksfjörður central volcano to the
southeast into the Dalir district.
Extensive stratigraphic mapping was carried out
by McDougall et al. (1984) on a composite section
through the Vestfirðir in two parts, lower and upper.
The base of the 4-km lower section is at Súganda-
fjörður and Skálavík, with the section running south
from there to Arnarfjörður and ending in the eastern
part of Barðaströnd. An isolated segment of two pro-
files was also mapped farther east. The base of the 3-
Fig. 2. Location of profiles sampled for paleomagnetic mea-
surements. See Table 1. NW-SE line: cross-section of Fig. 9.
2. mynd. Staðsetning sniða til sýnatöku vegna bergsegul-
mœlinga, sjá 1. töflu Þverskurður eftir NV-SA línu er
sýndur á 9. mynd.
GEOLOGICAL METHODS AND
RESULTS
Field methods
The mapping involved definition of lava type,
measurements of the thickness of individual lava
flows and sedimentary beds, identification of sec-
ondary mineralization, measurements of magnetic po-
larity by portable fluxgate magnetometer and deter-
mination of strike and dip.
The lava flows were classified according to a mod-
ified Walker's (1959) classification scheme. The rock
types are as follows: tholeiite, olivine basalt (single
flows), compound flows (lava shields), feldspar-por-
phyritic lavas, basaltic andesite (i.e. basaltic ice-
landites), andesites (i.e. icelandite), dacite and rhyolite.
Ignimbrites are classified separately. The sedimentary
beds in ísafjarðardjúp are commonly thin red lateritic
beds believed to be paleosoil, with lake sediments
(mostly siltstone), acid tuff and conglomerates also oc-
curring. The sedimentary beds are usually few tens of
cm thick but they can reach 10 m thickness or more.
Stratigraphic profiles and correlations between them
Twelve profile sections (DO, DA,.... DM) com-
prising 320 lava flows of 3900 m total thickness were
sampled. The location of the profile sections is shown
in Fig. 2. Individual sections are shown in Figs. 3-7
and a 2600 m thick composite section in Fig. 8. Cor-
relations between neighboring sections were relative-
ly easy to trace in the field, even up to distances of the
order of 10 km. The correlations are based both on
petrographic characteristics of the lava flows and on
their paleomagnetic polarity.
The most common rock types are tholeiites and
olivine basalt amounting to 30% each of the total
thickness. Compound lava flows are 22% and por-
phyritic flows 11%, the latter group including the two
ankaramitic flows DL9 and DL9A. Sedimentary beds
JÖKULL, No. 44
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