Jökull - 01.12.1974, Síða 59

a well established observation tliat zeolitization
begins below an overburden of at least 200 m
within the lava succession (Walker, 1960; Ade
Hall et al., 1971) gradually increasing in inten-
sity and diversity of mineral phases downwards.
The central volcano poses somewhat of an ano-
maly because of the elevated temperatures af-
fecting its core and surroundings. West of tlie
central volcano in the slopes south of Reykja-
dalsá and north of Hvítá, zeolitization is mod-
erate and includes minerals characteristic of
the chabazite-thomsonite zone. Analcite and
mesolite are conspicuously lacking. The lava
pile from the valley bottoms up to about 400 m
altitude respresents most of the cliabazite-thom-
sonite zone. Above this an additional thickness
of some 200 m is required before reaching the
original top of the lava pile at about 600 m
altitude at the time when the isotherms rose
highest. The Húsafell central volcano thus al-
most certainly became buried by flood basalts
up to about 600 m altitude after its activity
ceased. We envisage this to have happened
during tlie early Matuyama epoch as a result
of volcanism continuing further east and piling
lava flows up against the slopes of the volcano
and finally burying it altogether. Most of tliis
lava cover was eroded again during a long
period of denudation prior to the Gilsá event.
The reason that tlie stacking of lava flows came
to an end is logically to be sought in the
moving of the Húsafell area towards the west
away from a stationary rift axis located to the
east. As a result of the long continued denuda-
tion the Húsafell area became part of an ex-
tensive peneplain which is still recognizable in
the level surfaces of the hills and highlands to
the west and north.
The Bæjarfell—Kaldidalur group marked a
new sequence of events: The Húsafell area
hitherto suffering denudation became again in-
undated by lava flows. This volcanic activity
led during the Gilsá event to a voluminous
group of lavas which spread unusually far west.
The valleys which dissect the peneplain have
extended into the Húsafell area later than
Gilsá because rocks of that age are dissected.
Tlie oldest valley filling preserved in the area
dates from the Jaramillo-Brunhes interval. This
is the lava cap on top of Búrfell on the western
fringe of tlie mapped area. After headward
erosion of the valleys reached tlie border of the
active volcanic zone a number of lava flows
were channelled into tliem during interglacials
and Postglacial time.
ACKNOWLEDGEMENTS
The field work was sponsored by the National
Energy Authority, the community of Mýra- and
Borgarfjarðarsýsla, the German Science Founda-
tion and a special fund of the University of
Cologne. The seven KfAr datings from the
Teledyne Isotopes, Westwood, N. ]., were fund-
ed by a grant from the German Science Founda-
tion.
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