Jökull - 01.12.1980, Blaðsíða 14

ation of the latter, fault breccias and intensive
secondary mineralization at the faults, and a
juxtaposition at the faults of rock suites of
varying age and dips. An age gap of 5 m. y
was inferred across the Húsavík faults.
Sœmundsson suggested that the Tjörnes beds
were probably formed in a subsiding trough
initiated approximately north of Eyjafjöröur
as the Eastern zone became established. A new
spreading axis across the Tjörnes Fracture
Zone, the Grímsey shoal, then poured lavas
over the sediment filled trough during the
upper Gauss epoch. Finally, opening up of the
Axarfjörður spreading zone produced lava
flows of late Matuyama epoch age, covering
the Breiðavík beds, which had been deposited
in a minor tectonic depression, a forerunner of
the Axarfjörður trough.
An analysis of earthquakes and general
features of the Tjörnes Fracture Zone was
published by Ward (1970, 1971). Scháfer (1972)
inferred a right lateral transform fault in the
area. Active volcanism north of Tjörnes has
been reported by Thoroddsen (1902, 1925),
Wolff (1931), and Thorarinsson (1965). P. Ein-
arsson (1976) analyzed the seismicity of the
Tjörnes Fracture Zone and demonstrated that
the transform motion between the Kolbeinsey
Ridge and the Eastern zone occurs along three
or more parallel strike slip faults. Walker
(1975) did not accept the view that sectors of
the rift zone had been offset by northwesterly
trending faults and postulated an en echelon
arrangement of spreading sectors from the
Eastern zone to Tjörnes. McMaster et al. (1977)
studied the insular shelf off North Iceland.
They interpreted the arrangement of
volcano-tectonic features within the Tjörnes
Fracture Zone as a transient or leaky type of
transform fault and stated that Walker’s en
echelon rift zone configuration was confirmed
by their studies. Recent contributions to the
geological history of the Eastern zone (Aðal-
steinsson 1974, Watkins et al. 1975, Alberlsson
1976, Watkins and Walker 1977) indicate that
volcanism in the Eastern zone has been virtu-
ally continuous since about 13.5 Ma.
Radiometric dating and lithological sludies
Aronson and Sœmúndsson (1975) studied the
plateau basalts beneath the Tjörnes beds at
Kaldakvísl and the sections at Héðinshöfði,
and reported radiometric ages from this part
of the Tjörnes sequence. The Kaldakvísl ages
confirmed a Tertiary age for the rocks in
question, but the low ages obtained from the
Héðinshöfði locality indicated a possible cor-
relation with the Furuvík beds.
The first radiometric ages within the Tjör-
nes sequence above the basal basalts were
presented by Albertsson (1976, 1977, 1978), who
initiated an exhaustive sampling programme
to this effect in 1972. Albertsson correlated the
sequence with the geomagnetic polarity time
scale and concluded that the alternative con-
sidered less likely by Th. Einarsson et al. (1967)
was partly confirmed. According to Albertsson
(1976, 1978) the Mactra/Serripes Zone boun-
dary dates from ca. 3 Ma., and the oldest tillite
bed of the sequence (in Furuvík) is ca. 2 Ma.
old. The Tjörnes sequence was reviewed by
Th. Einarsson (1977, 1978) in the light of the
new radiometric ages, and palynological
results from the Tjörnes sequence were also
presented.
Eiríksson (1979) examined the stratigraphy
and origin of the sedimentary rocks of the
upper part of the Tjörnes sequence. Lithos-
tratigraphical holostratotypes were defined
for the Breiðavík Group, which includes the
Furuvik beds and all higher strata of the
Tjörnes sequence. Informal lithozones were
defined for the lower part of the Tjörnes
sequence. According to Eiríksson the Breiðavík
Group is characterized by at least eleven or
twelve lithological cycles deposited during as
many climatic glacial — interglacial cycles.
DISCUSSION
Past geological research on Tjörnes has
answered many questions about the origin,
age, and tectonic evolution of the area. How-
ever, the answers are probably outnumbered
12 JÖKULL 30. ÁR