8.2 Methods for Determining Subsidence
c1uding erosion, and eustatie sea-level ehanges from
these deviations, if the "normal" subsidenee curve
can be inferred from the uncorrected curve (Fig.
8.6h).
This method can be used when the subsidence
history comprises intervals for which the sea level
can be assumed to have been at average level or constantly high or low. In addition, the basin investigated must have been fi11ed up to sea level with sediment during these intervals. Then the curve for a permanently sediment~filled basin is found by interpolation, and the amount of deviation for a particular time
span can be quantified. With the aid of sedimentation
rates derived from stratigraphie dating and
sedimentologieal-paleontological criteria for determining paleo-water depth, an attempt can be made to
discriminate between the different factors causing
such deviations. Equations (8.15), (8.18), and (8.19)
through (8.21) may enable a rough quantitative evaluation of these factors.
The purely thermo-tectonic part, T, of total subsidence can be calculated for a certain increment of
sediment and time from the uncorrected total subsidence with the equation (Steckler et al. 1988):
T = h s1 Pm - Ps + WD + Pm EU
(8.22)
Pm - Pw
Pm -Pw
where h s1 is the decompacted sediment thickness;
other symbols as above.
A kind of summary of a11 these methods and results is presented in Fig. 8.7 showing the subsidence
his tory at a location where a drill hole has provided
the stratigraphy, age deterrninations, and paleo-water
depth interpretation of the section. Here, the fo11owing points are of interest:
- Curve A is the exponential thermo-tectonic subsidence of an air-filled basin. It indicates the minimum
amount of subsidence which is not affected by any
other mechanism.
- Curve B signifies the subsidence of a basin fi11ed
with water up to a fixed sea level, without sediment
accumulation. The water load magnifies the subsidence of an air-filled basin by a factor of about 1.4.
- Curve C represents total subsidence of a tectonica11y subsiding basin (Curve A) which is permanently filled with sediments up to a fixed sea level.
The subsidence in relation to a water-fi11ed basin is
magnified by a factor about 2.5 (for a relatively high
sediment density of Ps = 2.4 g/cm 3 , cf. Sect. 8.1.1).
- The regular subsidence curve (C) is modified during certain times by the processes discussed above:
(1) Incomplete sediment fi11ing (Curve D): subsidence decreases.
(2) Erosion of emerged sediment (Curve E): subsidence decreases.
399
(3) Long-term relative sea-level changes (Curve
F): during rise subsidence inereases, during fall it
decreases. (High-frequency variations are not considered here.)
These modifying factors can partly counterbalance
each other, for example, when increasing paleo-water
depth coineides with a high sea-level stand.
Curve G derived from the present thicknesses of
the stratigraphie units does not show the true subsidence history. Neglecting sediment compaction, the
older units yield subsidence rates too low and the
younger units rates too high in comparison with trne
subsidence (cf. Fig. 8.4). In addition, this irregular
curve includes the cumulative effects of the other
modifying factors.
An evaluation of the role of a11 factors controlling
subsidence is possible when the sedimentary fi11 of a
basin is largely preserved and we11 investigated. In
addition, the nature and thickness of the underlying
ernst should be known.
8.2.5 Summary (Subsidence History
Analysis)
To determine the subsidence history of sedimentary basins (at a certain location), the sedimentary
section and the underlying basement (ernst)
should be we11 known (bio- or chronostratigraphy, paleoenvironrnent and paleo-water depth,
relative sea-level changes). The backstripping
technique inc1udes
- Decompaction of sediment units, using appropriate porosity-depth curves.
- Isostatic adjustment (if tectonic subsidence is
the objective).
- Corrections for the effects of changing water
depth (or elevation above sea level), varying
sedimentation rates, relative sea-Ievel changes,
or intermittent sediment erosion.
For a given thermo-tectonic subsidence curve,
purely water-fi11ed basins subside about 1.45
times and sediment-filled basins 2 to 2.5 times
faster than air-filled basins (cf. Fig. 11.10).
c1uding erosion, and eustatie sea-level ehanges from
these deviations, if the "normal" subsidenee curve
can be inferred from the uncorrected curve (Fig.
8.6h).
This method can be used when the subsidence
history comprises intervals for which the sea level
can be assumed to have been at average level or constantly high or low. In addition, the basin investigated must have been fi11ed up to sea level with sediment during these intervals. Then the curve for a permanently sediment~filled basin is found by interpolation, and the amount of deviation for a particular time
span can be quantified. With the aid of sedimentation
rates derived from stratigraphie dating and
sedimentologieal-paleontological criteria for determining paleo-water depth, an attempt can be made to
discriminate between the different factors causing
such deviations. Equations (8.15), (8.18), and (8.19)
through (8.21) may enable a rough quantitative evaluation of these factors.
The purely thermo-tectonic part, T, of total subsidence can be calculated for a certain increment of
sediment and time from the uncorrected total subsidence with the equation (Steckler et al. 1988):
T = h s1 Pm - Ps + WD + Pm EU
(8.22)
Pm - Pw
Pm -Pw
where h s1 is the decompacted sediment thickness;
other symbols as above.
A kind of summary of a11 these methods and results is presented in Fig. 8.7 showing the subsidence
his tory at a location where a drill hole has provided
the stratigraphy, age deterrninations, and paleo-water
depth interpretation of the section. Here, the fo11owing points are of interest:
- Curve A is the exponential thermo-tectonic subsidence of an air-filled basin. It indicates the minimum
amount of subsidence which is not affected by any
other mechanism.
- Curve B signifies the subsidence of a basin fi11ed
with water up to a fixed sea level, without sediment
accumulation. The water load magnifies the subsidence of an air-filled basin by a factor of about 1.4.
- Curve C represents total subsidence of a tectonica11y subsiding basin (Curve A) which is permanently filled with sediments up to a fixed sea level.
The subsidence in relation to a water-fi11ed basin is
magnified by a factor about 2.5 (for a relatively high
sediment density of Ps = 2.4 g/cm 3 , cf. Sect. 8.1.1).
- The regular subsidence curve (C) is modified during certain times by the processes discussed above:
(1) Incomplete sediment fi11ing (Curve D): subsidence decreases.
(2) Erosion of emerged sediment (Curve E): subsidence decreases.
399
(3) Long-term relative sea-level changes (Curve
F): during rise subsidence inereases, during fall it
decreases. (High-frequency variations are not considered here.)
These modifying factors can partly counterbalance
each other, for example, when increasing paleo-water
depth coineides with a high sea-level stand.
Curve G derived from the present thicknesses of
the stratigraphie units does not show the true subsidence history. Neglecting sediment compaction, the
older units yield subsidence rates too low and the
younger units rates too high in comparison with trne
subsidence (cf. Fig. 8.4). In addition, this irregular
curve includes the cumulative effects of the other
modifying factors.
An evaluation of the role of a11 factors controlling
subsidence is possible when the sedimentary fi11 of a
basin is largely preserved and we11 investigated. In
addition, the nature and thickness of the underlying
ernst should be known.
8.2.5 Summary (Subsidence History
Analysis)
To determine the subsidence history of sedimentary basins (at a certain location), the sedimentary
section and the underlying basement (ernst)
should be we11 known (bio- or chronostratigraphy, paleoenvironrnent and paleo-water depth,
relative sea-level changes). The backstripping
technique inc1udes
- Decompaction of sediment units, using appropriate porosity-depth curves.
- Isostatic adjustment (if tectonic subsidence is
the objective).
- Corrections for the effects of changing water
depth (or elevation above sea level), varying
sedimentation rates, relative sea-Ievel changes,
or intermittent sediment erosion.
For a given thermo-tectonic subsidence curve,
purely water-fi11ed basins subside about 1.45
times and sediment-filled basins 2 to 2.5 times
faster than air-filled basins (cf. Fig. 11.10).