Rit (Vísindafélag Íslendinga) - 01.06.1967, Page 178
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SSW-NNE through the middle of the area. On the eastern
flank of the anticline a broad valley has heen eroded, the
Thjórsárdalur-valley. On the lower slopes of the valley a
thick sequence of normally magnetized lava flows is ex-
posed, which are discordantly overlain by a thick reversely
magnetized sequence of lava flows with interbedded layers
of fluviatile sediments. This sequence probably helongs to the
Gilbert geomagnetic epoch of uppermost Tertiary age. This
sequence is again overlain, at the head of the valley at Hái-
foss, by a sequence of normally magnetized sequence of lava
flows (Trausti Einarsson, 1962). The age of these lava flows
has heen determined by Grasty (pers. comm. Th. Sigurgeirs-
son) by the K/Ar-method as 2,9 m. y., so it seems that this
lava group belongs to the Gauss normal geomagnetic epoch.
In 1964 a 1565 m deep borehole was drilled on Heimaey,
the main island of the Vestmannaeyjar. The topmost layer
is a volcanic breccia of late Quaternary age down to 180 m
below sea level, succeeded by a 640 m thick sequence of
marine sediments. From 820 m down to the bottom of the
hole there is a sequence of hydrothermally altered basalt lavas
(Pálmason et al., 1965). The lowest part of the sediments is
probably of similar age as shelly boulders found in the tuff
formation in Mýrdalur 50 km east of Vestmannaeyjar (Ás-
kelsson, 1960). The mollusc fauna of the stratigraphically
oldest xenoliths is similar to the fauna of the highest part
of the Tjömes sediments, lowermost Pleistocene. Because of
the very much greater alteration of the basalts, it seems there-
fore likely that there is a hiatus between the basalt lavas and
the sedimentary sequence and the former will therefore be
well down in the Tertiary.
The Tertiary rock sequences in Tjömes, Thjórsárdalur and
Vestmannaeyjar make it very reasonable to assume that
Quaternary rock series in the so-called “central graben” of
Iceland are everywhere underlain by the Tertiary Plateau
Basalt formation. This assumption is very much supported
by seismic refraction studies of Gudmundur Pálmason (1963),