Jökull - 01.12.1994, Page 8

are only 4% of the section and most of the beds belong
to a sedimentary horizon in the uppermost profiles, es-
pecially in section DL. If the thick sedimentary hori-
zon is excluded the sedimentary beds make up only 1-
2% of the lava succession. The average thickness of
lava flows varies by type. The compound flows are on
the average 18 m thick, olivine basalt flows 12 m, por-
phyritic flows 10 m and tholeiite flows 8 m thick, re-
spectively. The sedimentary beds are on the average
0.4 m thick, ranging from 5 cm up to 30 m.
Close to the base of section DO (Figs. 2,3), be-
tween flows 18 and 19, is a minor unconformity (see
Fig. 1) marking the upper limit of the lava flows
which belong to the Alftafjörður central volcano.
Some of the lava flows in sections DO, DA and DB
Fig. 3-7. Detailed stratigraphy of the sampled ísafjarðardjúp
profiles of Fig. 2. The paleomagnetic polarity (both from
field and laboratory measurements) is shown. Explanations
are in Fig. 3.
are very conspicuous due to unusual thickness and
structure and can be traced visually for tens of kilo-
meters. In the upper part of section DF and in the
lower part of DG is a pronounced group of compound
lava flows (Fig. 1) which is an ideal marker horizon.
Close to the top of our section is a 30 m thick sedi-
mentary horizon (between DL 2 and DL 3) which also
may be a good marker horizon. The present authors
consider it to be contemporaneous with the Brjáns-
lækur sediments which occur between flows JF 46
and JF 48 of McDougall et al. (1984).
Tectonics and alteration
The general strike of the lava flows is NE-SW in
the area south of Isafjarðardjúp (Fig. 1) but changes to
3.-7. mynd. Gerð jarðlaga í sniðum á 2. mynd. Sýnt erfyrir
hvert hraunlag hvort segulstefna hefur mœlst "rétt" eða
"öfug".
6
JOKULL, No. 44