Page 272 - Physical Principles of Sedimentary Basin Analysis

254 Subsidence

The last β-factor depends on the difference f 5 s 5 − f 4 s 4 , which is close to being proportional
to the difference s 5 −s 4 . This difference is the accumulation space needed for sediments that
are filled in after the last rift phase. The preceding β-factors are similarly dependent on the
accumulation space needed for the sediments that follow the rift phase, and they are inflated
by β-factors of the rift phases that follow. The β-factor for the first rift phase is therefore
dependent on the β-factors of the second, third and fourth rift phase. How sensitive these
β-factors are for changes in the paleo-water depth is studied in Exercise 7.33.
Figure 7.34a shows the total cumulative β-factors (7.183) across the profile 7.32, where
stretching and thinning of the sediments are not accounted for. Figure 7.34bshowsthe
β-factors (7.203)to(7.206), which includes the thinning of the sedimentary load during
each rifting interval. We notice that the final stretching is the same for both β-factors,
but the amount of stretching during each rift phase is different. The first rift phase now
becomes much more important, which is reasonable because the early (Paleozoic) sedi-
ments have gone through three rift phases and therefore were much thicker before they

6
rift phase 1: 400 Ma to 360 Ma
rift phase 2: 310 Ma to 260 Ma
rift phase 3: 160 Ma to 140 Ma
5
rift phase 4: 80 Ma to 56 Ma
(total) beta−factor [−] 4 4

3

2
3
2
1
1
0 100 200 300
distance [km]
(a)
6
rift phase 1: 400 Ma to 360 Ma
rift phase 2: 310 Ma to 260 Ma
rift phase 3: 160 Ma to 140 Ma
rift phase 4: 80 Ma to 56 Ma
5
(total) beta−factor [−] 4 4

3
2
1 2 3
1
0 100 200 300
distance [km]
(b)
Figure 7.34. (a) β-factors when the sediments are not stretched. (b) β-factors that account for
stretching.