Greinar (Vísindafélag Íslendinga) - 01.01.1977, Page 33

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Tjörnes, and we do not know sofar the time interval between
the youngest tilted pre-zonal basalts and the first zonal
ones. Whether the sea invaded the zones during this interval
is also an open question, but one of great importance, as we
have indicated earlier.
We have actually one indication that the sea did invade the
zone during this tilting which probably meant subsidence in
the zones at the same time. I refer to the Breiðavík marine depo-
sits which we will consider in greater detail below.
In the forming of the zones we have the vestiges of a major
tectonic 'phase, leading up to zonal volcanism which means a
radical change from all the much earlier, very extensive volcanic
activity, characterized in several regions by numerous dykes
and dyke swarms, and in general for instance by the fact that
central volcanoes were rather evenly distributed (21), their
traces showing no indication of zonal character of volcanism
within the boundaries of present Iceland. We thus find ample
evidence for dividing the rock suites into those of zonal and
pre-zonal activity and time.
Our oldest zonal dolerites disappear in the area of the high
and deeply dissected Tertiary Plateau Basalts of the middle
Northern Iceland, to reappear in the Eastern zonal position
east of Bárðardalur. They form the normally polarized surface
layers of Fljótsheiði, overlain by a small patch of reverse rocks
in the north (22), and the normal group extends far to the
south, where it is overlain by a reverse volcanic ruin Kálf-
borg (22), the lavas from which extend for a considerable dis-
tance towards the north.
The normal lava group we have been discussing, and denot-
ing it (NSn), forms also Hvammsheiði and occurs in Tjörnes,
where we shall consider its position later. This normal group
extends in fact all the way south to Vatnajökull, forming wide
parts of the base of the glacier, to reappear south and south-
west of the glacier in Fljótshverfi and Síða, ending here in
high escarpments (17) which may be due to both faulting and
coastal erosion.
The normal group also extends in under the voluminous tuff-