Greinar (Vísindafélag Íslendinga) - 01.01.1977, Side 111

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from the characteristic shallow sea sediments of epicontinen-
tal seas (coarseness, cross-bedding etc.). The great scarcity
of red clay layers in epicontinental sediments is also highly
suggestive of shallowness. But such an empirical rule must
not be used as a physical law the other way round.
We are now free to accept a shallow sea in the Northern
Atlantic, and then the near-general, equal thickness of sedi-
ments found by John and Maurice Ewing (35) becomes im-
mediately clear as an organic deposition in a relatively shallow
ocean, during Eocene and Lower Oligocene time. The thinning
out of these sediments towards the mid-ocean ridges, is then
also obvious, as a result of emergence of an 150—200 km broad
worldwide zigzag zone, coaxial with the present ridges. This
can only be an uplift of tectonic nature, and as we have found
that in Iceland the uplift took place in about Middle Oligocene
time, we seem justified to assume the same time for the uplift
in the North Atlantic. The data of (35), moreover, suggest the
same time for all mid-ocean ridges, and thus, a global event.
This emergence would essentially explain the subaerial de-
nudation, which we inferred from (36), if the emergence took
place before the Upper Oligocene. Thus, both in this way, and
from the morphologic history of the Plateau Basalts in Iceland,
we find Middle Oligocene uplift to be the most harmonious
conclusion.
We have at the same time got an answer to the question, at
which time marine invertebrates could perhaps not have mig-
rated from western North America to Europe. Such might have
been the case during the existence of the emerged median
zone, if there were no northerly gaps in that land zone. Migra-
tion could have taken place in the Eocene and the Lower Oli-
gocene to start again in the Miocene, if conditions were other-
wise such that larvae could survive the transport to about
Britain, either along a chain of islands or right across the
ocean with a northerly warm current. In the Pliocene the con-
ditions for migration were at any rate favourable, as pointed
out in (29).
Dating of the thick general sea-floor sediments registered