650
strength is less than that which would result from the
total overburden pressure. Excess pore water pressure builds up, which may approach and counteract
the effective vertical pressure induced by the
(buoyancy-reduced) weight of solid matter in the sediment. As a result, the weight of the overlying sedimentary column is no longer carried alone by the
solid grain structure (grain-to-grain stress), but to a
great part or nearly entirely by the practically incompressible pore water. This condition causes a drastic
reduction in shear strength (friction) and is therefore
one of the most important factors generating gravity
mass movements (cf. Sect. 5.4.l). Underconsolidation and excess pore water pressures in the
subsurface are weIl known phenomena in the exploration of hydrocarbons and deep groundwater (see
below).
(3) Overconsolidation. Cohesive, commonly claybearing sediments, may exhibit astate of mechanical
overconsolidation, i.e., astate in which their fabric
strength is higher than under equilibrium with overburden pressure. This frequently occurs when the
upper part of a sedimentary column is removed by
erosion (Figs. 13.lc and 13.3d). Due to a kind of
"memory effect", the underlying sediment maintains
its compaction state achieved under the former
higher overburden pressure. This signifies that its
bulk density is too high and its porosity too low with
respect to the lowered sediment-water interface. The
erosion-induced reduction in vertical stress has practically no influence on the compaction state. Furthermore, renewed deposition on top of the erosion surface (Stage 3 in Fig. l3.lc) does not affect the compaction state of the underlying material as long as the
newly added sediment does not surpass the former
top of the sequence.
Reeent fine-grained marine sediments frequently show
apparent overeonsolidation at or slightly below the sea
floor. They have, for example, shear strength values higher
than normal for this depth. This phenomenon is not related
to submarine erosion, but to the onset of ehemieal
diagenesis (the beginning of eementation) .
As sediment builds up over time, a fine-grained sequence may undergo changes in its consolidation
state, as demonstrated in the model in Fig. 13.2.
Emergence of sediments above sea level and/or the
lowering of the groundwater table lead to an increase
in effective overburden pressure and therefore initiates renewed compaction or underconsolidation. Water loss by evaporation near the land surface, associated with high capillary pressures, can produce
"evaporative hardening" of fine-grained materials.
Present-day erosion on land generally leads to
overconsolidated shales and mudrocks, while earlier
erosional events followed by subsequent deposition
mostly become masked.
Chapter 13 Diagenesis
13.1.3 Mechanical and Chemical Compaction
(Overview)
The porosity-depth relationship of usually nonuniform sedimentary columns is generally controlled not
only by gravitational, mechanical compaction, but
also by chemical-mineralogical processes. This applies in particular to coarse-grained materials (sand
and gravel) which are but little affected by mechanical diagenesis. The models of Fig. 13.3 demonstrate
the principal behavior of some major sediment types.
(1) Compaction 0/ clays and sands. Purely mechanical (or gravitational) compaction of a fine-grained,
mainly siliciclastic sediment leads to a porosity-depth
relations hip as indicated in Fig. l3.3a. The regularly
decreasing porosity versus depth is interrupted by
intercalated sands (thick beds or relatively thin sandy
turbidites) which cause a drastic drop in the porosity
curve in the upper part of the sequence. However, at
greater burial depth, as long as the sands are not affected by pressure solution and cementation (see below), they may maintain a higher porosity than their
neighboring shales and mudrocks.
If the same sequence is also affected by chemical
diagenesis, the porosity-depth relationship may be
modified in such a way as that shown in Fig. 13.3b.
At aburial depth of several hundreds of meters, the
less stable portions of the fine-grained material, for
example various bioclasts, start to dissolve and
reprecipitate as pore cement. At greater depths, unstable clay minerals "mature" into stable ones and
lose their crystal water (dehydration). As a result, the
porosity curve shifts to lower values. Sandy interbeds
in particular undergo shaIlow-burial cementation and,
in addition, deep-burial pressure solution at the contacts of individual grains. Both processes strongly
reduce the pore space and permeability of such beds.
As stated by many authors (e.g. Chilingarian 1983,
BfÜekmann 1989), the absolute values of porosity at eertain burial depths are reversely related to grain size and, in
addition, depend on the type of sediment particles (cf. Fig.
13.4). Clays rieh i!J. smeetite, montmorillonite, and mixedlayer clay minerals generally exhibit higher porosities than
illitie or kaolinitie clays. Normally eonsolidated clays rieh
in exehangeab1e Na+, for exarnp1e montmorillonitie material, usually bind more pore water and henee have higher
porosities than Ca++-rieh clays.
Some examples of porosity-depth eurves refleeting the
influenee of mierofossi1s and primarily meehanieal eompaetion are shown in Fig. 13.4. The highest initial porosities are in sediments rieh in diatoms and radiolarians, as
long as the delieate opal skeletons res ist meehanieal and
ehemieal destrueüon. They ean maintain porosities higher
than 0.7 (70%) up to several hundreds ofmeters below the
sea floor. In addition, other biogenie oozes may display
abnormally high initial porosities as a result of their
intratest porosity. This may inerease the total sediment porosity by approximately 35 to 45% (Baehman 1984), as
eompared with a sediment eonsisting simp1y of solid grains
strength is less than that which would result from the
total overburden pressure. Excess pore water pressure builds up, which may approach and counteract
the effective vertical pressure induced by the
(buoyancy-reduced) weight of solid matter in the sediment. As a result, the weight of the overlying sedimentary column is no longer carried alone by the
solid grain structure (grain-to-grain stress), but to a
great part or nearly entirely by the practically incompressible pore water. This condition causes a drastic
reduction in shear strength (friction) and is therefore
one of the most important factors generating gravity
mass movements (cf. Sect. 5.4.l). Underconsolidation and excess pore water pressures in the
subsurface are weIl known phenomena in the exploration of hydrocarbons and deep groundwater (see
below).
(3) Overconsolidation. Cohesive, commonly claybearing sediments, may exhibit astate of mechanical
overconsolidation, i.e., astate in which their fabric
strength is higher than under equilibrium with overburden pressure. This frequently occurs when the
upper part of a sedimentary column is removed by
erosion (Figs. 13.lc and 13.3d). Due to a kind of
"memory effect", the underlying sediment maintains
its compaction state achieved under the former
higher overburden pressure. This signifies that its
bulk density is too high and its porosity too low with
respect to the lowered sediment-water interface. The
erosion-induced reduction in vertical stress has practically no influence on the compaction state. Furthermore, renewed deposition on top of the erosion surface (Stage 3 in Fig. l3.lc) does not affect the compaction state of the underlying material as long as the
newly added sediment does not surpass the former
top of the sequence.
Reeent fine-grained marine sediments frequently show
apparent overeonsolidation at or slightly below the sea
floor. They have, for example, shear strength values higher
than normal for this depth. This phenomenon is not related
to submarine erosion, but to the onset of ehemieal
diagenesis (the beginning of eementation) .
As sediment builds up over time, a fine-grained sequence may undergo changes in its consolidation
state, as demonstrated in the model in Fig. 13.2.
Emergence of sediments above sea level and/or the
lowering of the groundwater table lead to an increase
in effective overburden pressure and therefore initiates renewed compaction or underconsolidation. Water loss by evaporation near the land surface, associated with high capillary pressures, can produce
"evaporative hardening" of fine-grained materials.
Present-day erosion on land generally leads to
overconsolidated shales and mudrocks, while earlier
erosional events followed by subsequent deposition
mostly become masked.
Chapter 13 Diagenesis
13.1.3 Mechanical and Chemical Compaction
(Overview)
The porosity-depth relationship of usually nonuniform sedimentary columns is generally controlled not
only by gravitational, mechanical compaction, but
also by chemical-mineralogical processes. This applies in particular to coarse-grained materials (sand
and gravel) which are but little affected by mechanical diagenesis. The models of Fig. 13.3 demonstrate
the principal behavior of some major sediment types.
(1) Compaction 0/ clays and sands. Purely mechanical (or gravitational) compaction of a fine-grained,
mainly siliciclastic sediment leads to a porosity-depth
relations hip as indicated in Fig. l3.3a. The regularly
decreasing porosity versus depth is interrupted by
intercalated sands (thick beds or relatively thin sandy
turbidites) which cause a drastic drop in the porosity
curve in the upper part of the sequence. However, at
greater burial depth, as long as the sands are not affected by pressure solution and cementation (see below), they may maintain a higher porosity than their
neighboring shales and mudrocks.
If the same sequence is also affected by chemical
diagenesis, the porosity-depth relationship may be
modified in such a way as that shown in Fig. 13.3b.
At aburial depth of several hundreds of meters, the
less stable portions of the fine-grained material, for
example various bioclasts, start to dissolve and
reprecipitate as pore cement. At greater depths, unstable clay minerals "mature" into stable ones and
lose their crystal water (dehydration). As a result, the
porosity curve shifts to lower values. Sandy interbeds
in particular undergo shaIlow-burial cementation and,
in addition, deep-burial pressure solution at the contacts of individual grains. Both processes strongly
reduce the pore space and permeability of such beds.
As stated by many authors (e.g. Chilingarian 1983,
BfÜekmann 1989), the absolute values of porosity at eertain burial depths are reversely related to grain size and, in
addition, depend on the type of sediment particles (cf. Fig.
13.4). Clays rieh i!J. smeetite, montmorillonite, and mixedlayer clay minerals generally exhibit higher porosities than
illitie or kaolinitie clays. Normally eonsolidated clays rieh
in exehangeab1e Na+, for exarnp1e montmorillonitie material, usually bind more pore water and henee have higher
porosities than Ca++-rieh clays.
Some examples of porosity-depth eurves refleeting the
influenee of mierofossi1s and primarily meehanieal eompaetion are shown in Fig. 13.4. The highest initial porosities are in sediments rieh in diatoms and radiolarians, as
long as the delieate opal skeletons res ist meehanieal and
ehemieal destrueüon. They ean maintain porosities higher
than 0.7 (70%) up to several hundreds ofmeters below the
sea floor. In addition, other biogenie oozes may display
abnormally high initial porosities as a result of their
intratest porosity. This may inerease the total sediment porosity by approximately 35 to 45% (Baehman 1984), as
eompared with a sediment eonsisting simp1y of solid grains