Greinar (Vísindafélag Íslendinga) - 01.01.1977, Síða 131

Chapter 9
TRANSGRESSIONS AND REGRESSIONS.
CAUSES OF THE VERTICAL MOVEMENTS OF THE OCEAN
FLOOR AND PARTS OF THE CONTINENTS.
CONTENTS
Great transgressions, such as the Upper Cretaceous and the Oligocene
ones, are considered to be due to thick organic sediments in the shallow
oceans. The sediments caused general heating in the uppermost mantle,
a consequential sinking of the Moho and an uplift of the ocean floor.
The following regressions are taken to be due to global stresses which
caused the Moho to rise again, and the sea floor to sink. Evidences of
such stresses are pointed out.
We pointed out in Chapter 6 that the Moho would move up
and down, —which corresponded to sinking or rising of the
ocean floor — depending on the condition: temperature versus
stress in the uppermost mantle. As to the temperature, that of
the deep sea water is here only a minor factor. Rise of the
bottom temperature by 10°C, decreases in the beginning the
geothermal gradient at a shallow crustal depth. That means
accumulation of the conducted heat from below, until equili-
brium of heat flow is reestablished by rise of temperature
farther down. This causes the Moho to sink a little and the
sea floor to rise.
But quantitatively, this is a far too small effect to explain
transgressions. But a significant factor immediately presents
itself. A thick layer of organic deposition in a shallow ocean
is clearly such a factor.
We know the relatively thick layer of organic sediments of
Eocene and Oligocene time, the great Oligocene transgression
and the regression which followed. The organic sediments
have no doubt a very low thermal conductivity1), about one
1) G. W. C. Kaye and T. H. Laby, 9th ed. 1941 give the following values
in the units 10 3cal/cm sec degree: Earth’s crust 4; porcelain 2.5; diatoma-
ceous brick 0.3.
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