Jökull - 01.12.1980, Page 13

dary was defined at this level, for which an age
3.0—3.35 Ma was suggested. In a lateredition,
Th. Einarsson (1971) divided the Pleistocene
sequence in Iceland into two units, the Grey
Basalt formation (3.35—0.7 Ma)and the
Palagonite Formation (0.7 Ma — Late
Weichselian, cf. also Th. Einarsson 1973, 1974).
During the late sixties several geothermal
boreholes were sunk in the Húsavík area on
Tjörnes. The borehole sections and strati-
graphical evidence were published in two
reports of the National Energy Authority in
Iceland (Tómasson 1969, Tómasson et. al. 1969).
The inference was that the Tjörnes beds were
penetrated by some of the boreholes, and thus
had a subsurface extension towards south
across the Húsavík fault swarm. There was no
direct evidence of marine sediments, however,
and although the boreholes may have pene-
trated a terrestrial equivalent of the Tjörnes
beds, the relationship is not clear. An altern-
ative interpretation of the Húsavík borehole
sections was presented by Sœmundsson (1974),
who argued against a subsurface extension of
the Tjörnes beds across the faults at Húsavík.
A continued effort has been directed
towards the geology of Tjörnes during the
seventies. Several members of the Soviet
Geodynamic Expedition in Iceland visited
Tjörnes for consecutive field seasons in the
early seventies (Akhmetiev et al. 1978).
Sedimentary and volcanogenic deposits in
Breiðavík were studied by Geptner (1972, 1973,
1976, 1977, Geptner and Lavrushin 1972), who
verified the existence of tillites and volcanic
tuffs in the sequence, and contributed to the
solution of stratigraphical problems in
Breiðavík. Gladenkov 'studied the palaeontology
and stratigraphy of Tjörnes, and presented
correlations of the Tjörnes sequence with
other areas (Gladenkov 1974a, 1974b, 1974c,
1978, Gladenkov et al. 1975, Th. Einarsson and
Gladenkov 1973). The palaeomagnetism of the
Tjörnes sequence was studied by Gladenkov and
Gurari (1976), and the palaeobotany by Akh-
metiev et al. (1975, 1978).
Símonarson (1974) examined fossil material
from the Tjörnes sequence in his study of the
genus Cyrtodaria, and later (1975) presented a
general account of the palaeontology and age
of the Tjörnes and Breiðavík beds.
The palaeoecology of the Tjörnes beds was
studied by Norton (1975), who also discussed
faunal relationships between Tjörnes and the
North Sea basin (1977a, 1977b). Zagwijn
(1974) discussed the Tjörnes sequence and its
faunal, climatic, and age implications in his
paper on the Plio-Pleistocene boundary in the
Netherlands and elsewhere. The problem of
the Neogene/Quaternary boundary in the
Netherlands, England, and Iceland was
recently reviewed by Hey (1977).
The Tjörnes Fracture Zone. Position of Tjörnes with
regard to plate tectonics
The tectonic aspects of Northeast Iceland
were examined by Sœmundsson (1974, 1977,
1978), who interpreted the geological features
of the area in terms of plate tectonics and a
shift of the active rifting zone from Skagi to
the present rifting zone in Northeast Iceland
(Eastern zone), leading to a 75 km wide,
WNW trending transform fault zone (Tjörnes
Fracture Zone). Sæmundsson (1974) described
the Axarfjörður area as a downfaulted trough
that had subsided 1000 m or more relative to
its uplifted western margin (Tjörnes). He
concluded that the 25 km wide Axarfjörður
trough appeared to be undergoing ductile
thinning and tensional faulting with active
volcanism at the margins. This trough is,
according to Sœmundsson’s interpretation,
presently accomplishing most of the spreading
within the Tjörnes Fracture Zone, while the
transcurrent movement is taking place in a
WNW trending zone north of Tjörnes.
Previously, he argued, the strike-slip move-
ment had been taken up by the Húsavík
faults, amounting to some 60 km during the
activity of a former spreading axis in the
Grímsey area during the time from ca. 4—1
Ma. According to Sæmundsson the geological
evidence supporting a right lateral movement
along the Húsavík faults consists of oblique
slickensides on fault planes to the north of the
Húsavík faults and on the eastern continu-
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