Greinar (Vísindafélag Íslendinga) - 01.01.1977, Blaðsíða 132

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tenth of that normal for the earth’s crust, so that to establish
equilibrium of heat flow, the temperature below such layers
and correspondingly farther down, has to rise. The Moho sinks
and the sea floor rises gradually as the deposited layer gets
thicker. The Oligocene transgression is considered to be an
example.
If before sedimentation, the temperature gradient in the
oceanic crust was 5°C/100 m, then the deposition of a 300 m
thick (35) organic layer of 1/10 the conductivity of the previous
crust would, by heat flow equilibrium, raise the temperature
in the crust and the upper parts of the mantle by 150°C. In
general, shallowing of the sea by 3 km would result from a
sinking of the Moho by about 8—10 km.
This would have been quite similar in the Upper Cretaceous,
the thick organic deposition leading to an extreme trans-
gressipn.
Counteracting these effects of raised temperature are just
such global stresses as pointed out in Chapter 3. This should
not need any further explanation now. Only a relatively strong
global stress field of climatic origin can convert a trans-
gression into a regression. A global stress field manifests it-
self in various ways. The weak filling of a geosyncline must
be sensitive to such stresses. And there we have significant
evidence. The great Upper Cretaceous transgression was re-
verted to a regression with the very beginning of Alpine
phases of uplift, and the Oligocene transgression ended with
the Miocene phase of Alpine uplift. There is no point in enu-
merating here such cases throughout the geological history,
the theoretical treatment and basis ensures the indicated logi-
cal connection. Critics may unearth some apparent exceptions,
not realizing then the lack of mathematical exactness in the
form and sediment filling of the various parts of a geo-
syncline, or other deviations from a mathematical model, which
are the very essence of geological bodies, and which can hide
a simple but exact principle.
We mean, then, to see a genetic connection between trans-
gressions and organic sedimentation, on one hand, and re-