Geology Reference
In-Depth Information
Fig. 11.2
Method of calculating
the total vertical movement of a
terrestrial basin (from Linol
2013
). Subsidence or uplift
(Y
total
) equals the variation of
altitude of the basin
Continent
Bathy (t
2
)
s surface
(Alti), sediment thickness (Thick)
and bathymetry (Bathy),
corrected to known eustatic sea
level fluctuations (
'
Thick (t
2
)
ʔ
sl)
Alti (t
2
)
Total Vertical
Movement (+Y
total
)
Bathy (t
1
)
Thick (t
1
)
Alti (t
1
)
Sea level rise (
Δ
sl)
Y
total
= Alti t
2
- (Bathy t
2
+ Thick t
2
) - Alti t
1
- (Bathy t
1
+ Thick t
1
) +
Δ
sl
11.2.1 Method of Backstripping
11.2.1.1 Decompaction
The amount of decompaction is calculated using empirical
porosity-depth relationships, linked to the lithological
properties of each stratigraphic unit (c.f. Allen and Allen
2005
). In this case, we use field estimation of the proportion
of clay (and silt) compared to sand (Fig.
11.3
) to determine
the value of these physical parameters: porosity-depth
coefficient, surface porosity and dry sediment density (see
Tables
11.1
-
11.8
). The decompacted sedimentary thick-
nesses are then calculated from the general decompaction
equation:
Backstripping corrects for sediment compaction at the
respective depths and times of deposition of each strati-
graphic unit, and is then adjusted to sea level fluctuations
and paleo-bathymetry to quantify a total subsidence of the
basin basement. By removing the sediment and water
loadings, backstripping also calculates the tectonic subsi-
dence that corresponds to the vertical displacement of the
basin induced by tectonic and thermal processes only (e.g.
Vail et al.
1991
; Allen and Allen
2005
). Although this
method is well-known for marine settings, no such proce-
dure exists in the literature for terrestrial sequences where
overburden removal or erosion occurred. Here, we first
describe a new method of backstripping especially adapted
for the continental CB (Linol
2013
).
In continental domain, the total subsidence of a basin
integrates variations of accommodation (i.e. the space avail-
able for sediments accumulation) and the paleo-altitudes of
the basin surface, i.e. base level above sea level (Fig.
11.2
).
The accommodation simply corresponds to the thickness
of the deposited (decompacted) sediments plus the paleo-
bathymetry. The latter is evaluated from the sedimento-
logical data. For example, tidal bundles form at shallow
depth, less than 10-20 m (e.g. Dalrymple
1992
). At greater
depths, such as for turbidites, the paleo-bathymetry is more
difficult to assess, but in a lacustrine context it normally
ranges from 0 m to about 2,000 m (e.g. the Black Sea) and,
in general, anoxic layers start to form around 100-200 m
depth (e.g. Murray et al.
2007
). By contrast, paleo-bathymery
is nil if the basin emerges and records terrestrial sedimen-
tation (e.g. aeolian dunes), fluvial sequences and paleosols.
EpS
¼
z2
z1
‐
Φ
0
=
ð
c
Þ
ð
exp
ð
c
:
z
1
Þ
exp
ð
c
:
z
2
Þ
Þ
‐
‐
‐
z
0
1
Þ
z
0
2
ÞÞ
þ Φ
0
=
ð
c
Þ ð
exp
ð
‐
c
:
exp
ð
‐
c
:
ð
11
:
1
Þ
where EpS is the thickness of the stratigraphic unit before
compaction; z
1
and z
2
the present base and top of the unit
(after compaction), and z
0
1
and z
0
2
before compaction, respec-
tively; c is the porosity-depth coefficient;
Φ
0
is the surface
porosity, and
is the porosity at depth z. The porosity
exponentially decreases with depth as:
Φ
Φ ¼ Φ
0
exp
ðÞ
c
:
z
ð
11
:
2
Þ
‐
11.2.1.2 Water and Sediment Loadings
The water and sediment loadings are calculated in using
a standard 1D Airy-compensated model. The density
contrast of water and sediment reflects the density of the
mantle:
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