Jökull - 01.12.1963, Page 48

vclocity as low as 2.4 km/sec, whereas older Qua-
ternary rocks have velocities up to 3.3 km/sec.
Layer 1 is generally founcl at the surface in
the Tertiary basalt districts in northern Iceland
ancl in much of eastern Iceland. According to
the surface geology of these areas it consists of
flood basalts. These are rnore or less horizontal
lava flows with thin intercalations of sediments.
This layer is also generally found under the
Quaternary surface layer in the Neovolcanic
zone. The height, referred to sea level, of the
interface between layers 0 and 1 in the Neo-
volcanic zone has been plotted in Fig. 2. This
interface is found to be below the present sea
level in the northern part of the area studied.
Layer 2 is in general founcl below layer 1
both in the Tertiary areas and in the Neo-
volcanic zone. In southeastern Icelancl where
profiles 28—30 are located it is found at the
surface. The surface rocks are here large basic
and acid intrusions as well as flood basalts.
In Breiddalur, where profile 27 is located, layer
2 is also founcl close to the surface (possibly 100—
200 meters below sea level).
Layer 3 is found on every profile of sufficient
length at a depth from 1.2 km to about 4.5 krn.
It has not been found anywhere at the surface.
In Fig. 3 the depths below sea level to this layer
have been plottecl. Data from five profiles of
Tryggvason and Báth (1961) have also been in-
cluded. In southeastern Iceland it is founcl at
a depth of 1.25 to 1.82 km, but in northern
Icelancl it is usually founcl at 3.0—4.5 km depth.
In southwestern Iceland three profiles of
Tryggvason and Báth (1961) give a depth of 1.6
to 1.9 km below sea level to layer 3. Preliminary
measurements by the present author indicate a
depth of over 4 km to layer 3 in the central
part of southern Iceland.
In Fig. 4 a schematic structure section A—B
through the basalt lava in northern Iceland is
presented. It is based on profiles on or near
to line A—B shown in Fig. 1.
On profile 2 in Vatnsdalur it was observed
that the Pg waves at 18 to 22 km clistance ar-
rived too early compared with neighbouring
stations. This was found to be due to an ano-
malous structure under these stations. A 0.6 km
thick layer with P velocity about 3.7 km/sec
was found overlying a substratum with P velo-
city about 6.2 km/sec. If this substratum is layer
3, this is the shallowest depth at which it has
been found in Iceland so far.
Interpretation of layers.
The nature of layers 0 and 1 niust be
considered fairly well known as these cor-
respond to surface formations in the Neovol-
canic zone and the Tertiary basalt districts
44
JOKULL 1963