Jökull - 01.12.1988, Page 57

K/Ar Ages of Rocks from the Flatey Borehole
in the Offshore Skjálfandi Basin, North Iceland
KRISTINN J. ALBERTSSON
and
JÓN EIRÍKSSON
Department ofGeosciences, Science Institute,
University oflceland, 101 Reykjavík, Iceland
ABSTRACT
K/Ar age determinations of lava flows penetrated
in the FL-1 borehole section on Flatey confirm the
conclusions drawn by previous workers. The whole
sequence was accumulated within the Matuyama
geomagnetic chron. All the lava flows are reversely
magnetized. The uppermost lava flow was extruded
at 0.81 ± 0.08 Ma ago, i.e. some time after the
Jaramillo geomagnetic subchron. The middle flow,
ofwhich two samples yielded ages of 1.39 ± 0.10 Ma
and 1.67 ±0.18 Ma, is significantly older and was
erupted some time between the Jaramillo and Oldu-
vai geomagnetic subchrons. The lowermost flow
predates the Olduvai geomagnetic subchron and
shows an age of 1.96 + 0.33 Ma. Comparison
between the Flatey section and the Tjörnes sequence
indicates that the glacial horizons beneath the old-
est Flatey lava flow may correspond to the oldest
glacial horizons on Tjörnes.
INTRODU CTION
The island Flatey lies off the north coast of Ice-
land in the Skjálfandi Bay (Fig. 1), and is tectoni-
cally separated from the mainland by faults associ-
ated with the southem margin of the Tjömes Frac-
ture Zone (Ward, 1971; Sœmundsson, 1974). A very
distinct WNW trending fault south of Flatey was
revealed by shallow seismic work by Thors (1982).
Although the island is made up of two lava flows
visible above sea level, separated by a conglomer-
ate, its location within a negative gravity anomaly
(.Pálmason, 1974) indicated the existence of low
density rocks beneath.
A 554 m deep borehole was drilléd in Flatey in
1982, and a nearly continuous 47.6 mm diameter
core was recovered (Gunnarsson et al., 1984). Only
three lava flows of total thickness of about 40 m
were penetrated, including the two flows already
seen in outcrops on the island. A third flow was
encountered at a depth of 382 m in the core. The
remainder of the core, over 500 m, is made up of
sedimentary rocks. It has been estimated that the
total thickness of sediments beneath Flatey amounts
to 2 km, increasing to the west (Gunnarsson et ai,
1984). This is supported by the existence of over
1000 m thick sediments with intercalated lava units
of Tertiary and Quatemary age which are exposed
on the Tjömes horst to the east of Skjálfandi
0Bárðarson, 1925; T. Einarsson, 1958; Strauch,
1963; Th. Einarsson et al., 1967; Albertsson, 1978;
Eiríksson, 1985).
The Flatey core was analyzed and interpreted by
Eiríksson et al. (1987), who found evidence of 5 gla-
ciations within the sequence, which is terminated by
an erosional surface with glacial striations on the
topmost lava flow. A cyclical accumulation was
demonstrated with diamictites, deformation stmc-
tures, shear planes and associated erosional uncon-
formities in the core being typically followed by
poorly sorted conglomerates, sandstones and then by
JÖKULL, No. 38, 1988 55