Jökull - 01.12.1978, Page 105

year as an earthquake takes place greatly ex-
ceeds the expected value for two Poisson
processes, and there is also evidence of a
relationship extending over a longer timespan.
THE GEOLOGY OF THE
GNÚPVERJAHREPPUR AREA,
SOUTHERN ICELAND
Gudmundur Ingi Haraldsson, National Energy
Authority, Björn Jóhann Björnsson, National
Energy Authority
A 1.7 km thick section consisting of basaltic
lava flows and basaltic hyaloclastite units is
described. There are ten major hyaloclastite
units which make up to 50% of the total thic-
kness. Each of these are regarded to represent
a major glaciation. The paleomagnetic
polarity directions of each lava flow and
hyaloclastite unit were measured in the field by
a portable fluxgate magnetometer. A corre-
lation of the stratigraphic column with the
paleomagnetic timescale indicates that the
rocks date from lower Matuyama (2.4 M.y.) to
the Jaramillo event (0.9 M.y.). Two recent
K-Ar age determinations support the corre-
lation. Two dissected central volcanoes are in
the area. The Stóra-Laxá central volcano was
active for 0.5 M.y. in the lower Matuyama.
The Thjórsárdalur central volcanoe was active
from just before the Gilsá event for about 1
M.y. Acid rocks, intensive dyke swarms, high
temperature hydrothermal alteration and
major basaltic intrusions occur in the volca-
noes. The Hreppar anticline stretches from S
to N along the western part of the area. The
strata on the flanks of the anticline dip 3° —
20° towards NW and 3°—9° towards NNE.
Faults have been active up to recent times.
They can be divided into three groups based
on their different trends, N0°—10°E,
N20°—40°E and N60°E. Dykes in the area as
most of the faults have dominantly the
N20°—40°E direction, which is considered to
be the direction of minimum stress during
active volcanism in the area.
A LATE PLEISTOCENE LAVAPILE IN
THE NEIGHBOURHOOD OF
REYKJAVÍK
Árni Hjartarson,
National Energy Authority
In Reykjavík and the surrounding areas a
late Pleistocene lavapile overlies an older vol-
canic basement. This lavapile, which has been
termed the Reykjavík Dolerite Formation,
was earlier believed to have all originated
from the Mosfellsheidi shield volcano. Recent
evidence shows, however, that the lava pile
can be subdivided into a number of lava units
of different ages and origins.
Although the lavas from shield volcanoes
are the most abundant, lavas from fissure
eruptions are also found. At least five separate
lava flows with intercalating and commonly
fossiliferous sediments are found by surface
exploration within Reykjavík and this
number increases when drillhole data and
surface exposures outside Reykjavík are taken
into account. The majority of the lavas be-
longing to the Reykjavík Dolerite Formation
are olivine tholeiites but plagioclase
prophyritic lavas are also found along with
one picritic lava.
These conclusions call for a revision of the
age of the well known fossiliferous sediments
in Ellidavogur and Fossvogur and the estab-
lished ideas of the geological history of the
Reykjavík area.
HYDROGEOLOGICAL MAP OF
ICELAND
Árni Hjartarson,
National Energy Authority
The first hydrogeological map of Iceland is
in preparation. It is based on new inter-
national hydrogeological mapping methods
developed by the Commission for
Hydrogeological Maps of the International
JÖKULL 28. ÁR 103