Jökull - 01.12.1967, Page 66

On the Origin of Sedimentary Water beneath Vestmann Islands
JENS TÓMASSON,
NATIONAL ENERGY AUTHORITY, REYKJAVIK, ICELAND
ABSTRACT
The water from the deep toell in Vestmann
Islands has half the salt content of sea-water
but the same hydrogen isotopic composition as
the precipitation on mountain slopes in south-
ern Iceland. This paradox is explained by the
hypothesis that. original sea-water in sediments
has been flushed out ancl replaced by meteoric
water.
INTRODUCTION
The Vestmann Islands are located 10 km
ofí the south coast of Iceland. Only Heimaey,
the main island, is inhabitated with a town of
5000 inhabitants and an important fishing
harbour. Domestic water supply has been a
problem on the island for a long time and
several shallow wells have been drilled in an
attempt to recover fresh ground water but in
all of them there was only a thin layer of fresh
water replaced at depth by sea-water. The fresh
water layer was too thin for economic utiliza-
tion. In 1964 it was, therefore, decided to drill
a deep well in an attempt to recover water
from a possible extension of the basalt forma-
tion on the mainland that woulcl underlie the
Vestmann Islands Formation.
The drilling was stopped in September 1964
when the well was 1565 m deep. (A geological
section of the well is presented in Fig. 1). The
Vestmann Islands Formation reached down to
180 m depth, below which were mainly marine
sediments down to 820 m succeeded by basalt
layers to the bottom of the well. By pumping,
some water was recovered frorn the well, but
it had half the salt concentration of sea-water.
The water had the same hydrogen isotopic
composition, (ð-value) as the precipitation in
the mountains of southern Icelancl (see Arna-
son and Sigurgeirsson 1967). The origin of this
water is discussed in the present paper and
compared with other chloride rich waters in
Iceland.
THE SEDIMENTS
Most of the sediments are tuffaceous sand-
stones and siltstones. The matrix consists of an
opaline mass often with pale green or dark
colour. Its refractive index is between 1.470
and 1.490. In it spherulites of chalcedony are
frequent, and white aggregates of poorly cryst-
allized montmorillonite. The clastic part of the
sediments is composed of volcanic glass, crystals
of plagioclase, pyroxene, olivine and rnagne-
tite, and basaltic fragments. Secondary miner-
als, besides chalcedony and montmorillonite,
include zeolites such as analcime which are
found below 300 m depth. Hematite ancl pvrite
is found sporadically. Calcite is found in all
the sediments of the well but it is in part of
biological origin.
Roughlv 25 per cent. of the sediments appear
to be finely bedded but the rest coarsely bedd-
ed or massive. The iinest sediments consist
mostly of mineral fragments with opaline ma-
trix which can be as much as 80 per cent
by volume. The most coarse grained sandstones
are most closely packed and lack nearly all
matrix. In the coarser sediments the grains are
much more closely packed than in the finer
ones. Again opal is the most common cement-
ing mineral, but calcite occurs sporadically.
The tuffaceous fragments are as a rule much
opalized. Only these sandstones are logged as
“sandstone” in Fig. 1. Tlre conglomerate con-
tains on the other hand opaline matrix.
300 JÖKULL 17. ÁR