Jökull - 01.12.1974, Page 43

are explosed towards the west. During the build-
ing up of the uppermost ~ 1000 m of the lava
pile glaciations occurred repeatedly and a cen-
tral volcano was active.
The mapping was done in tlie summers 1966
and 1969. Since 1970 this mapping has been
extended areally by the first author with the
help of students to cover the greater part the
flood basalts occupying the area between Snæ-
fellsnes and the Reykjanes—Langjökull volcanic
zone. The Húsafell section was first described
by Tr. Einarsson (1962) and the magnetic po-
larity zoning for the most part correctly estab-
lished. A correlation to the geopolarity time
scale was suggested by Noll and Sæinundsson
(in Schwarzbach ancl Noll, 1971) based on a
revised stratigraphy and magnetic polarity deter-
minations. Piper (1971) included Húsafell in his
interpretation of the polarity zones of SW-Ice-
land but unfortunately his correlation of the
Húsafell section is untenable. A later revision
(.Piper, 1973) is still markedly different from
our results.
The purpose of this paper is 1) to outline
the stratigraphy and structure of the Húsafell
area, 2) to report K/Ar age determinations
which enable a correlation to be made of the
Húsafell section with the geopolarity time scale,
3) to estimate, once tlie absolute ages are known,
the rate of lava production, the onset and fre-
quency of glaciations and the life span of the
central volcano, 4) to outline tlie nature of, and
the time span represented by unconformities in
the upper part of the section.
The commercially obtained K/Ar ages report-
ed here are the first from a continuous well
mapped succession in western Iceland. The fix-
ed age of this succession provides means of
time correlations in a broad zone of flood
basalts adjacent to the Reykjanes—Langjökull
volcanic zone, which is broadly consistent with
Piper’s (1971) time correlation of Tr. Einars-
son’s (1957 and 1962) paleomagnetic stratigrapliy
of this area.
2. LITHOLOGY AND
GEOLOGICAL STRUCTURE
The following is a description of the lava pile,
which is divided into three parts: i.e. the central
volcano and the successions below and above it.
The lowest tillite bed found in the lava pile
of this area so far was chosen as the base of
the section.
Petrology is not considered in this paper but
some general remarks are relevant. Major ele-
ment analysis lias been carried out on 22 sam-
ples from the Húsafell section. The analyses
were made by Karl Grönvold and 13 of them
are listed in Grönvold (1972). The samples were
so chosen as to cover the entire compositional
ranges represented by each of the three phases
of acid volcanism. The rocks grade in composi-
tion from olivine tholeiite through tholeiite,
andesite and dacite to rhyolite. Variation dia-
grams sliow that the chemical trend is indis-
tinguishable from that of Thingmúli (Char-
michael, 1964) and several other central vol-
canoes of the tholeiitic suite (Grönvold, 1972).
2.1 THE ROCKS BELOW THE CENTRAL
VOLCANO
The lowest tillite bed of the section (number
1 on the geological map, Plate 1) is found near
Giljafoss, 10 km west of Húsafell. At this lo-
cality the tillite is 10 m thick and overlies a
well polished basement which is striated NW—
SE. Striated boulders occur in the tillite but
they are rare as is the case also for other tillites
higher in the succession. Above the unsorted
tillite is a 2 m thick well sorted and stratified
conglomerate evidently of fluvial origin. The
fine matrix of the tillite is composed mainly
of comminuted basaltic glass wliich is altered
to palagonite and zeolites. The high glass con-
tent is equally characteristic of subsequent til-
lites and detrital beds and the alteration of the
glass is responsible for their strong induration.
We interpret the glass content as being largely
derived from contemporaneous subglacial vol-
canism. The Giljafoss tillite does not inter-
finger with truly volcanic hyaloclastites but
many later tillite beds show this relationship.
When followed along the strike, thickness varia-
tions are observed and locally the conglomerate
on top of it is present alone. The association
of tillites and fluvial conglomerates is the rule
also among glacial horizons higher in the suc-
cession.
Between the Giljafoss tillite and the first acid
rocks indicating the birth of the central volcano
a sequence of some 160 m of basaltic lava flows
occurs. The lowermost 40 m are of a plagio-
JÖKULL 24. ÁR 41