662
same porosity-depth curve as in Fig. 13.9. As a result of
compaction caused by the new sediment increment (Layer
1, hl), the original pore water in the middle layer (Layer 2)
moves upward beyond its base and top and penetrates far
into the uppermost layer (Layer 1). This process can have
some bearing on chemical diagenesis when the ascending
pore water differs in chemistry from that of the other layers. Even more important may be the case in which the
lowermost layer (Layer 3) deviates in its pore water chemistry (e.g. non-saline or highly saline) from the following
layers with normal marine pore water (Fig. 13.lOc).
Compaction Flow as Function of the
Sedimentation Rate
During deposition, the range and velocity of ascending pore water flow and its potential for solute transport are mainly a function of the sedimentation rate.
This relationship is exemplified in Fig. 13.1 Oa (based
on the same porosity-depth curve as Fig. 13.9).
Here, a two-layer sequence with the layers hl and h2 is
overlain, in the course oftime, bya new increment of sediment with the thickness h l. Under conditions of normal
consolidation, the former sediment layers, h land h2, are
compacted to h2 and h3, respectively, and their former porosities, nl and n2, are reduced to n2 and n 3 , until a new
state of equilibrium is established. In contrast to the model
in Fig. 13.9, the thickness, h" of compact solid matter is
kept constant in all three succeeding sections, i.e., they become thinner with increasing burial depth. The loss of pore
water in terms ofthe height, hp, from the freshly compacted
sections h2 and h3 can easily be found graphically. Because
of the preservation of mass, the sum of h 3 and hp2, for example, is equal to the thickness, h2, ofthe former, less compacted section.
The height, ~, of expelled pore water can also be calculated with the aid ofthe compaction ratio, CR (Fig.
13.6a, Eq. 1). Ifh/h2 = CR1,2 and h/h3 = CR2,3 etc., we
find
h l
h 2
h =h -h = - - - - -
p2
2
3
CR
CR
1,2
2,3
(13.5)
and
(13.6)
Sequences subdivided into more sections can be
treated in the same way.
The ~ values of expelled pore water signify the
heights of colurnns or the thicknesses of layers of pure
water. In reality, the ascending pore water passes
through pores which occupy only a fraction ofthe sediment volume. Hence, the range of ascending pore waChapter 13 Diagenesis
ter flow, h f , crossing the boundaries of the sediment
sections in question is greater than ~. It is generally
h p
h=- f n
For our example we find
h = ~ and h = h p2
fl
n 1
f2
n 2
Velocity of Ascending Pore Water
(13.7)
The velocity, v, ofthe pore water ascending in the porous sediment is then approximately (t = time interval)
(13.8)
because the sedimentation rate is SR = h/t.
Equation (13.8) shows that the velocity of the
upstreaming pore water is controlled by the sedimentation rate and is inversely related to the thickness, h, of
the uppermost sediment layer considered. For the example in Fig. 13.10 we find (using n 1 = 0.3)
0.84SR w ,
where S~ = sedimentation rate ofwet sediment in the
uppermost section with h l = 1000 m.
For S~ = 10, 100, 1000 mlMa, the velocity ofascending pore water in our example is VI = 8.4,84, and
840 mlMa, or 2.7x10' 13 , 2.7x10· 12 , and 2.7xl0· 11 m/s,
respectively. In the underlying sediment intervals, the
velocity of compaction flow becomes successively less
with depth.
It should be mentioned here that one can distinguish between
different sedimentation rates (measured, for example, in
rnIMa; cf. Sect. 10.1): the sedimentation rate ofwet sediment
calculated from hl, the sedimentation rate for dry, compact
sediment derived from h" and the elevation, dh, of the
sediment-water interface above the basin floor (Fig. 13.10).
These calculations show that the velocity of a uniform,
upward directed compaction flow is generally extremely low and less than the sedimentation rate, a fact
which will be discussed further in Section 13.3.
same porosity-depth curve as in Fig. 13.9. As a result of
compaction caused by the new sediment increment (Layer
1, hl), the original pore water in the middle layer (Layer 2)
moves upward beyond its base and top and penetrates far
into the uppermost layer (Layer 1). This process can have
some bearing on chemical diagenesis when the ascending
pore water differs in chemistry from that of the other layers. Even more important may be the case in which the
lowermost layer (Layer 3) deviates in its pore water chemistry (e.g. non-saline or highly saline) from the following
layers with normal marine pore water (Fig. 13.lOc).
Compaction Flow as Function of the
Sedimentation Rate
During deposition, the range and velocity of ascending pore water flow and its potential for solute transport are mainly a function of the sedimentation rate.
This relationship is exemplified in Fig. 13.1 Oa (based
on the same porosity-depth curve as Fig. 13.9).
Here, a two-layer sequence with the layers hl and h2 is
overlain, in the course oftime, bya new increment of sediment with the thickness h l. Under conditions of normal
consolidation, the former sediment layers, h land h2, are
compacted to h2 and h3, respectively, and their former porosities, nl and n2, are reduced to n2 and n 3 , until a new
state of equilibrium is established. In contrast to the model
in Fig. 13.9, the thickness, h" of compact solid matter is
kept constant in all three succeeding sections, i.e., they become thinner with increasing burial depth. The loss of pore
water in terms ofthe height, hp, from the freshly compacted
sections h2 and h3 can easily be found graphically. Because
of the preservation of mass, the sum of h 3 and hp2, for example, is equal to the thickness, h2, ofthe former, less compacted section.
The height, ~, of expelled pore water can also be calculated with the aid ofthe compaction ratio, CR (Fig.
13.6a, Eq. 1). Ifh/h2 = CR1,2 and h/h3 = CR2,3 etc., we
find
h l
h 2
h =h -h = - - - - -
p2
2
3
CR
CR
1,2
2,3
(13.5)
and
(13.6)
Sequences subdivided into more sections can be
treated in the same way.
The ~ values of expelled pore water signify the
heights of colurnns or the thicknesses of layers of pure
water. In reality, the ascending pore water passes
through pores which occupy only a fraction ofthe sediment volume. Hence, the range of ascending pore waChapter 13 Diagenesis
ter flow, h f , crossing the boundaries of the sediment
sections in question is greater than ~. It is generally
h p
h=- f n
For our example we find
h = ~ and h = h p2
fl
n 1
f2
n 2
Velocity of Ascending Pore Water
(13.7)
The velocity, v, ofthe pore water ascending in the porous sediment is then approximately (t = time interval)
(13.8)
because the sedimentation rate is SR = h/t.
Equation (13.8) shows that the velocity of the
upstreaming pore water is controlled by the sedimentation rate and is inversely related to the thickness, h, of
the uppermost sediment layer considered. For the example in Fig. 13.10 we find (using n 1 = 0.3)
0.84SR w ,
where S~ = sedimentation rate ofwet sediment in the
uppermost section with h l = 1000 m.
For S~ = 10, 100, 1000 mlMa, the velocity ofascending pore water in our example is VI = 8.4,84, and
840 mlMa, or 2.7x10' 13 , 2.7x10· 12 , and 2.7xl0· 11 m/s,
respectively. In the underlying sediment intervals, the
velocity of compaction flow becomes successively less
with depth.
It should be mentioned here that one can distinguish between
different sedimentation rates (measured, for example, in
rnIMa; cf. Sect. 10.1): the sedimentation rate ofwet sediment
calculated from hl, the sedimentation rate for dry, compact
sediment derived from h" and the elevation, dh, of the
sediment-water interface above the basin floor (Fig. 13.10).
These calculations show that the velocity of a uniform,
upward directed compaction flow is generally extremely low and less than the sedimentation rate, a fact
which will be discussed further in Section 13.3.