Greinar (Vísindafélag Íslendinga) - 01.01.1977, Blaðsíða 105

Chapter 7
REMARKS CONCERMNG THE TERTIARY HISTORY
OF THE NORTH ATLANTIC AREA.
CONTENTS
The North Atlantic was a relatively shallow ocean during Eocene and
Lower Oligocene time, as i.a. shown by shallow-water invertebrates. Red
clay is indieative of a deep ocean, only by the present and Pleistocene
low temperature of the bottom water. For earlier periods with warm
bottom water a different C02 — equilibrium has to be taken into account,
and the red clay is no safe indication of abyssal depth.
In the Oligocene a 150—200 km broad zig-zag zone, a prototype of the
present mid-ocean ridge, emerged and was much denuded. This zone was
submerged in the great regression at the end of the Oligocene.
For a long time there has been great reluctance among most
geoscientists to take great vertical movements of the ocean
floor into consideration. The main scientific reason for this is
the belief that isostasv would be violated. Isostasy actually
made an end to the old practise of geologists, not least pale-
ontologists, to postulate landbridges wherever they seemed
to simplify a problem. The claim of isostasy was naturally
a useful restriction on wild speculations. But, as so often,
there came an oversimplification in the other direction. Even
if modern evidence for great vertical movement of large sea
floor areas seemed to be clear, most geoscientists tried some-
how to explain the evidence away, instead of realizing the
obvious conclusion, i.e. if true, these vertical movements simply
could not violate isostasy. Instead, they posed a problem
which had to be solved. We have now pointed out in Chapter 6,
how the dense nanocrystals solve the problem in principle.
But it is desirable to consider the vertical movements of
the ocean floor also from other points of view. Here we shall
especially be concerned with the North Atlantic, first use a parti-