Page 218 - Physical Principles of Sedimentary Basin Analysis

200 Subsidence

v
w w
s

l
l

s
Figure 7.5. An increase in the sea level creates isostatic subsidence.

7.3 Subsidence from eustatic sea level changes
Eustacy is world-wide fluctuations in the sea level due to a changing capacity of the world
basins or a changing volume of ocean water. Global sea level changes have consequences
for basin subsidence when isostasy is assumed. Figure 7.5 shows a situation where the sea
level has increased by an amount v. The basin is simply filled with more water until the sea
level has risen with a distance v relative to the initial sea level. The assumption of isostasy
says that pressures at the same depths in the asthenosphere are the same. We therefore
have
w w + l l + a s = w (v + w + s) + l l (7.16)
where s is the subsidence caused by the sea level rise, and l is the thickness of the litho-
sphere (see Figure 7.5). The densities of water, lithosphere and asthenosphere are w , l
and a , respectively. Equation (7.16) for isostatic equilibrium gives a basin subsidence
w
s = v (7.17)
a − w
from a sea level rise v, and the total increase in water depth after the sea level rise becomes

a
v + s = v. (7.18)
a − w
If the asthenosphere and the water have the densities m = 3300 kg m −3 and w =
1000 kg m −3 , respectively, the isostatic subsidence s is 0.43 times the sea level change
and the change in total water depth v + s is 1.43 times the sea level change. Attempts to
map eustatic sea level fluctuations suggest that they are a maximum of 200 m (Haq et al.,
1987), which means that they are not very important for the subsidence of a basin.

7.4 Basin subsidence by crustal thinning
Many sedimentary basins, for instance those in the North Sea and the Mid-Norwegian
Margin, were formed by stretching and thinning of the continental crust. These basins have
gone through several periods of extension, and the most recent one was at the end of the
Cretaceous period. The crust is less dense than the mantle and crustal thinning therefore