Jökull - 01.12.1982, Page 114

ance in seismic profiles. The bottom unit (1.) is
represented in profiles A and B by landward-dipp-
ing reflectors at 300-400 ms. This is overlain by a
unit (2.), approximately 60 ms thick, characterised
by jumbled reflections and faulting, especially in
the southern (landward) part of the profiles. A third
unit is a transparent layer which reaches a thickness
of about 80 ms in profiles C and D. Finally the
topography is smoothed by a thin blanket of sedi-
ment (4.) which thickens to form a terrace-like fea-
ture (submerged beach?) seen near the middle of
profiles C and D and the southern end of profiles E
and F.
No direct information is available on the age of
the four seismic units observed. The uppermost unit
is likely to be of Holocene age and the underlying
units Pleistocene. The faultingobserved in the sedi-
ments is probably due mainly to tectonic move-
ments on the Húsavík fault system. The island of
Flatey is made up of SW-dipping, mid-Pleistocene
lava fiows and is thus younger than the Tertiary
rocks of the adjacent land (Birgisdóttir 1982). Profile
G shows faulted sediments lapping against the
deeper part of this formation.
Basement rocks in Skjálfandaflói
In the western part of the bay of Skjálfandaflói
basaltic basement is seen to form an escarpment
below which it eventually disappears under sedi-
ments. The escarpment swings from the ESE-dir-
ection of the Flatey fault to a more southerly orient-
ation, following the coastline. A north-trending
ridge of crystalline rock crosses the middle of the
bay. This feature is reflected in the topography only
in profile K (Fig. 3) but is seen buried in sediments
farther south. To the north of profile K the ridge
form is traceable in sediments. The sediment-filled
troughs to the east and west of the ridge are pre-
sumably glacially carved valleys and the ridge itself
an erosional feature. The apparent transition in the
the composition of the ridge, from crystalline to
sedimentary rock occurs near a line joining the
Húsavík faults and the Flatey fault and probably
represents a structural boundary.
Igneous rocks are indicated as an offshore cont-
inuation of the headlands to the north of Húsavík
faults. The headlands are made up ofTertiary lavas
overlain by the Pliocene Tjörnes sediments (Einars-
son 1968, Aronson and Sœmundsson 1975, Eiríksson
1980). Reflectors in the northern part of profiles K,
L, N are thought to correspond to these sediments.
Skjálfandaflói sediments
Skjálfandaflói may be described as a double vall-
ey with the two troughs separated by a narrow ridge
which is now almost completely buried in sedi-
ments. A similar morphology is found in the glaci-
ally carved valley and bay of Skagafjördur, some
100 km to the west, but here the central ridge is still
emergent. The irregular, jumbled attitude of re-
flectors in the lower parts of profiles H through O in
Skjálfandaflói suggests that these represent glacial
sediments. Profiles H and O show these sediments
to continue up the flank of Grímseyjargrunn and
correlate with sediments to the west of Flatey.
The youngest sediments in Skjálfandaflói are re-
presented by a transparent sequence reaching a
thickness of over 100 ms in the inner reaches of the
bay. The sequence is probably made up ofoutwash
Fig. 4. Profile O, showing reflectors in Skjálfandaflói sediments continuing up th flank of Grímseyjargrunn.
Section of median ridge shaded.
Mynd 4. Snið 0,frá Flateyjarsundi að ósum Laxár. Þykk setlög í Jlóanum hylja hrygg úr storkubergi (R).
110 JÖKULL 32. ÁR