Jökull - 01.12.1992, Blaðsíða 34

Fig. 1. Map of the main geolog-
ical features of the area east and
south of Flúnaflói bay. Central vol-
canoes shown are Pr: Prestbakki,
Vi: Víðidalur, Va: Vatnsdalur, St:
Stakkfell, Sk: Skagi. This map is
largely based on unpublished ob-
servations by one of the authors
(H.J.) —Kort af helstu atriðum í
jarðfrœði Húnaflóasvœðisins.
6-7 million years ago (Sæmundsson 1974, Jóhann-
esson 1980). The dips within the flexure zone are
8-28°compared with a regional dip of 2-6°. Thick
sequences of silty sediments and hyaloclastites are ex-
posed in the lowest parts of the flexure on the westem
side of the Skagi peninsula.
LOCAL GEOLOGY
Field methods. The mapping involved definition
of lava types, measurements of the thickness of indi-
vidual lava flows and sedimentary beds, identification
of secondary mineralization, measurements of mag-
netic polarity by portable fluxgate magnetometer, and
determination of strike and dip.
The lava flows were classified according to Walk-
ers (1959,1971) field classification scheme. The rock
types of that scheme are as follows: tholeiite, olivine
basalt (single flows), compound flows (lava shields),
basaltic andesite (i.e. basaltic icelandite), andesite (i.e.
icelandite), dacite and rhyolite. Ignimbrites are classi-
fied separately. The sediments are commonly thin red
beds believed to be paleosoil, lake sediments (mostly
siltstone) and occasional conglomerates. The sedi-
mentary beds are usually a few tens of cm thick but
may be up to a few tens of meters.
Stratigraphy. Eight profiles (TL, KL, KM, KN,
TM, TN, TO and TP) comprising 303 lava flows of
total thickness 2125 m were measured. The location
of the profiles is shown in Fig. 2 apart from TO and
TP which are on the eastem slopes of mt. Svínadals-
fjall close to the Grund farm (about 12 km southwest
from TM). Individual sections are shown in Figs. 3-5.
Correlation between profiles was relatively straightfor-
ward and is based both on petrographic characteristics
of the lava flows and on their paleomagnetic polarity.
The section can be divided into distinct formations
(Fig. 6). The lowest formation (in profiles TL and KL)
consists of relatively thin tholeiite flows which belong
32 JÖKULL, No. 42, 1992