Jökull - 01.12.1974, Blaðsíða 42

K/Ar Ages of Rocks from HúsafelL Western Iceland,
and the Development of the Húsafell Central Volcano
KRISTJÁN SÆMUNDSSON,
NATIONAL ENERGY AUTHORITY, REYKJAVIK, ICELAND
HORST NOLL,
GEOLOGICAL INSTITUTE, UNIVERSITY OF COLOGNE, W-GERMANY
ABSTRACT
The paper describes the uppermost 1000 m
of a south-easterly tilted lava pile wliich borders
the Reykjanes—Langjökull volcanic tone on the
west in the area of Húsafell. Paleomagnetic
stratigraphy and KfAr age determinations allow
a correlation with tlie geopolarity time scale.
The section described begins with the Mammoth
event of the Gauss epoch and is continuous up
to the earliest part of the Matuyana epoch. It
includes a central volcano which began its activ-
ity during the Kaena event 2.9 m. years ago and
ended early in the Matuyama epoch. Acid vol-
canism occurred in three distinct phases, which
are separated by intervals of basaltic volcan-
ism. The first acid phase is represented by
dacite flows ancl an ignimbrite which are ex-
posed in a section running marginally to the
wsst of the volcanic centre. Tlie seconcl acid
phase, which was active during the last stage
of the Gauss epoch, includes besides a large
number of rhyolite floivs also an ignimbrite
sheet which may have measured more than 20
kms by volume. A large number of minor
basaltic intrusions accompanies this phase,
among them southeastwardly dipping sheets. A
small segment of a possible caldera fracture
indicates that a caldera may have collapsed
consequently upon the ignimbrite eruption of
this pliase. The third acid phase developed
after lava shields had almost completely screen-
ed the slopes of the volcano. This phase pro-
duced both acid and intermediate lavas, and
a minor ignimbrite. It is also characterized by
numerous acid and composite dykes and plugs.
The core area of this phase overlaps with that
40 JÖKULL 24. ÁR
of the second acid phase but may have been
shifted well to the west of it.
The first recorcl of a glacial horizon is found
near the base of the Mammoth event of the
Gauss epoch 3.1 m. years ago. From tliere up-
wards altogether 8 glacial horizons occur un-
til the end of the third acid phase. This gives
an average of one glacial event per 100,000
years. Only two glacial horizons are associated
with extensive hyaloclastites which is taken to
indicate that most of the glacial events were of
short duration. The rate of growth of the lava
pile during the Gauss epoch based on mapping
just to the SW of the volcano was found to be
close to 100 m per 100,000 years. Groivth rates
in the core area of the volcano may have been
as much as twice this value. A stratigraphic
break occurs at the top of the third acid phase.
It is overlain unconformably by a group of
hyaloclastites and lavas of more than 200 m
thickness ivhich spread into the Húsafell area
during the Gilsá event 1.8 m. to 1.6 m. years ago.
Before that time several 100 m had been stripp-
ed off by erosion and the Húsafell area had be-
come part of an extensive peneplain which is
still recognizable to the west and north. The
valleys which dissect the area are younger than
ihe Gilsá event.
1. INTRODUCTION
The subject of this study is the uppermost
part of a lava pile which borders the Reykjanes—
Langjökull volcanic zone on the west. This lava
pile shows a regional tilt of 5—8° towards the
volcanic zone so that increasingly older rocks