Rit (Vísindafélag Íslendinga) - 01.06.1967, Blaðsíða 79

75
Interpretation of seismic layers
The characteristic layering which has been found in the
upper part of the crust in Iceland, can be interpreted in more
than one way. The seismic boundaries could correspond to
stratigraphic horizons, hut they could also, especially the
deeper ones, be due to alteration of the basalts. Some argu-
ments in favour of a stratigraphic interpretation will he given
here.
In Iceland the P-velocities which are measured on surface
formations show an increase with increasing age. Quaternary
volcanic rocks (layer 0) in the neovolcanic zone have low
velocities, in the range 2.2 to 3.4 km/sec. Tertiary surface
basalts usually fall in the range 3.7 to about 4.5 km/sec
(layer 1). In southeast Iceland three profiles give surface
velocities which are higher, ahout 5 km/sec (layer 2). In
this part of the country the lavas are exposed at a deeper ero-
sion level than is known for any other part of the country
(Bödvarsson and Walker, 1964).
Additional support for a stratigraphic interpretation of the
seismic boundaries comes from measurements in the Faeroe
Islands (Pálmason, 1965). They are built up of Tertiary
flood hasalts which belong to the same geological province as
Iceland. The visible pile of basalts in the islands has been
divided into three formations, each approximately 1000 me-
ters thick. Direct measurements of the P-wave velocities in
the three formations, where they are exposed at the surface,
gives the correlation between velocities and stratigraphic
formations, which is shown in Fig 7. The seismic layers with
P-velocities 3.9 and 4.9 km/sec correspond to layers 1 and 2
in Iceland. If this stratigraphic correlation were valid both
in Iceland and in the Faeroes, it would mean that much of
the basalts of the Faeroes is older than the Tertiary surface
basalts of Iceland. This is indeed indicated by recent K/Ar
dating of Tertiary hasalts from the Faeroes (Rasmussen and
Noe-Nygaard, 1966) and from Iceland (Dagley et al., 1967).