Greinar (Vísindafélag Íslendinga) - 01.01.1977, Síða 30

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ween that subsidence and the eruptions may have been short,
in which case the corresponding marine sediments might be
thin, and economically unimportant. And the outcrop in Bjarn-
arfell rather supports that alternative. But that single outcrop
does not tell everything, and drilling close to the uplifted
plateau i.e. within the uplifted masses to test the occurrence
of the inferred marine sediments would at any rate be of geo-
logical interest.
In 1965 the present author made gravity measurements in
a wide flat area south of the uplifted mountains. He construct-
ed the hitherto unpublished, isostatically corrected map, given
in Fig. 6. The reference level 50 m above sea-level was chosen,
as in that way uncertainties of the density in the Bouguer cor-
rection are practically ruled out. We see that gravity falls
rather rapidly from the western outskirts of the Hreppar ba-
salts, i.e. here the magnetic group (N1n), see definition below,
towards the uplifted plateau. This may mean a depression filled
with sediments, thickening towards the plateau and towards
the plateau and towards the west, where we have now the re-
cent volcanic zone. A drill hole at the foot of, but within the
primary breccia mass of Laugarvatnsf jall, might be of special
interest. A sediment thickness of 100-140 m seems not excluded,
a part of which might be tuff.
The light-grey fresh basaltic lavas on top of the plateaus
are sometimes relatively coarse and of an open crystal texture,
and have traditionally been called dolerites in the geological
literature on Iceland, although they are often of fine grain.
But also in that case they are easy to recognize.
In Ármannsfell and adjoining blocks, we have another up-
lifted part of the dolerite plateau of just the same character-
istics as in the parts we have described. On the surface of Ár-
mannsfell one finds sediments which could only have been
formed on a lower general land surface (17), i.e. before the
uplift of that mountain block in the group, the clear contact
of the Mosfellsheiði dolerite at the fully developed southern
slope of Gagnheiði, which is a continuation of the Ármanns-
fell block, shows that the plateau as such was formed a long