Page 273 - Physical Principles of Sedimentary Basin Analysis

7.16 Stretching and thinning of the sediments 255
Table 7.3. An example of computation of β-factors
where thickness of the sediments are taken into
account. This is an alternative version of the case in
Table 7.2.

i s i i w i f i c i β max,i β i
1 1 2200 0 2.2 35030 1.00 1.56
2 1800 2200 300 2.2 22515 1.56 1.11
3 2300 2250 550 2.1 20271 1.73 1.90
4 10000 2550 950 1.5 10667 3.28 1.33
5 15000 2560 1050 1.5 8002 4.38 −

became stretched and thinned. A larger factor β 1 is therefore needed to make the extra
accommodation space for these early sediments, see note 7.16.The β-factor for the last
rift phase has become less when accounting for the sediment thickness, but the late Juras-
sic rifting has a larger β-factor. It is also reasonable that the accommodation spaces for
the thick Cretaceous formations are made by the Jurassic rifting. If the thick Cretaceous
formations were to have experienced the late Cretaceous and Paleocene rift phase with a
β-factor nearly as large as 2, then they would have been unrealistically thick prior to rifting.
It is possible to calculate how the lithosphere develops through the geohistory once the
β-factors have been estimated. Figure 7.36 shows the lithosphere at the beginning and
end of the three last rift phases and at the present time. The β-factors are computed by
equations (7.203)to(7.206) for a series of columns along the transect. We can now see
how the crust and the basin are stretched and thinned, and how extension creates a thermal
transient in the lithosphere. Especially, the Late Jurassic rifting creates a noticeable thermal
transient. Figure 7.36 also indicates that the thermal transients for rifting are considerably
reduced in the time interval between the rift phases. The sedimentary basin though the
geohistory is shown in Figure 7.37. It shows the deposition, stretching and thinning of
the sediments. This modeling is a mass-conservative handling of the basin through the
rift phases (see Wangen and Faleide, 2008). The state of the basin is also shown at the
beginning of each rift phase and at the present time.

Note 7.16 Example: The use of equations (7.203)to(7.206) compared to the example
in Note 7.15 demonstrates what is different with this algorithm. The two first steps are
the same – the computation of the factors f i and the estimate (7.192) for the initial crustal
thickness. The equations (7.203)to(7.206) are then used to make the β-factors for i = 1to
4. These β-factors are then multiplied together to make the products β max,i , which finally
gives the crustal thickness c i = c 0 /β max,i . There are differences in the results for these two
versions of computing the β-factors. Table 7.3 shows that the inclusion of the sediments
makes the first rift phase more important, and the last rift phase less important. The early
sediments must have been thicker because they have gone through several rift phases. The
first rift phase must therefore have been stronger to make the extra accommodation space.