Jökull - 01.12.1988, Page 41

chemical contents are usually low and the water is
suitable for most common use.
THE TERTIARY - EARLY
QUATERNARY BEDROCK
The bedrock of the Tertiary - Early Quatemary
formations is predominantly built up of stratified
basaltic lavaflows, with a number of central vol-
canoes (silicic centres) dispersed in the strata. The
strata are usually slightly tilted in accordance with
the tectonic history of Iceland, but in some areas the
tilting even exceeds ten degrees (Sœmundsson,
1980). The past volcanic activity occurred in dis-
tinct, elongated volcanic systems, accompanied by
swarms of open fissures, as is the case in the
presently active zones. The fissures are now in most
cases tightly closed, but in some regions they have
been rejuvenated during later tectonic events. In
some regions new fissure zones have been formed in
the course of the later tectonic development,
although not of the same intensity as those con-
nected with the volcanism. These "young" fissure
swarms represent zones of strongly increased per-
meability as well as anisotropic elements in the
structure of the bedrock.
The thickness of the lavaflows in the basalt
sequences is variable. Intercalations are usually
much thinner than the lavaflows, consisting of ash
layers, windblown sand and some times of remnants
of soils. The columnar parts of the lavaflows have an
effective porosity only in the narrow fissures
between the joints, and they may be nearly closed
through alteration, tightening and deformation due
to the overburden. The scoriaceous parts, especially
at the contact of lavaflows, have a higher eífective
porosity and permeability, but they are usually much
thinner than the columnar part. The glassy and
vesicular scoriae are more prone to alteration than
the massive columns. The originally higher permea-
bility in the scoriaceous parts can thus be more
strongly reduced than the permeability in the mas-
sive parts.
The intercalative layers have usually had a rather
high effective porosity, but their lithological nature
makes them highly susceptible to geothermal
alteration, which can reduce their permeability, until
it eventually becomes negligible. In principle the
same applies to sedimentary layers more abundant
in the stratigraphically higher parts of the
sequences, especially in the Early Quatemary. In the
younger formations the rocks have not been buried
as deeply as in the older ones, so the degree of
alteration is much less, and the permeabilities
correspondingly less reduced. Parts of the older
(Tertiary) formations have never been buried deep
enough to be subject to any noteworthy alteration
and tightening. This is the case in some regions in
the westem and southwestem parts of the
Northwestem Peninsula (Vestfirðir), as well as in
some more or less isolated spots at the top of the
higher mountainmasses elsewhere. Here the permea-
bility of the bedrock is still remarkably high and
effluent springs issue far down on the mountain
slopes.
The stratigraphical, lithological and tectonic
stmcture of the central volcanoes is very diversified,
showing inhomogeneities and anisotropies. In these
complex systems some small-scale aquifers with
higher permeability may occur, but they are seldom
of great extension and often very irregular. Accord-
ingly, only small springs, if any, are to be found
under these hydrogeological conditions. Reju-
venated or secondarily created fissure swarms are
regionally of importance, although the part they
play in the geothermal hydrology of the country is
much greater. Most springs connected with the
fissure zones in the older formations are indeed
small, but they can be very valuable for local uses.
SEDIMENTARY AQUIFERS
During the last glaciation Iceland was to a large
extent covered by glaciers, which carried most of
the loose sediments on the surface out to the sea.
The present sedimentary cover is therefore mostly
postglacial in age. There is a difference in the most
common sedimentary aquifers between the older
geological formations and the younger ones. In the
Tertiary - Early Quatemary regions the most com-
mon sediments are deposits left by the retreating
glaciers, river gravels, rockslides and a thin
JÖKULL, No. 38, 1988 39