376
1. Gustavs
• Theoretically calculating the effects of compaction on sediment thickness
and fluid motion within an idealized mud sequence.
This chapter is a partial account of such study, concentrating on the successive development of a complex porosity model for the uppermost half meter of
Holocene muds in the Arkona Basin (Fig. 1), and an application of this model
and a scenario of compaction effects to an idealized mud sequence.
2
Factors that Influence Porosity
One can divide factors controlling porosity into two main classes. The first class
groups all facies components, retaining those properties of the solid compounds
that influence the water-binding capacity of the sediment. For instance, porosity
is directly proportional to grain-size sorting and content of organic matter, biogenic silica and clay minerals, while it shows an indirect proportionality to mean
grain diameter.
The second group consists of diagenetic factors. Ignoring chemical effects
playing a minor part in the stage of early diagenesis, this class consists of physical
factors that are acting in connection with increasing burial and through time.
3
Compaction and its Classical Modeling
Physical diagenesis or compaction results in a continuous porosity decrease
with increasing burial. This process, being mainly a function of effective pressure and time, causes pore water movements and a reduction of sediment thickness. The most simple and most frequently used mathematical description of
compaction is an empirical, in most cases exponential function in a porosity-depth diagram, called compaction curve (Athy 1930; Weller 1959; Roll 1974;
Sclater and Christie 1980; Baldwin and Butler 1985; Huang and Gradstein 1990).
This method is still the most common way of mathematically considering compaction effects for mass balances (e.g. Wold 1992).
Two parameters characterize the compaction curve - the initial porosity at
the sediment surface, and the rate of porosity reduction with depth (compaction
constant). Usually, these two parameters are constant and specific for each sediment type. This assumption implies a homogeneous composition of each sediment type that is not fulfilled in the case of Baltic muds.
A second fact to be considered is the interrelation between depth interval and
compaction curve parameters. The so-called compaction constant is not strongly constant, but depends on the depth interval of measured porosities (near-surface, some meters, or deep burial). The wide disparity between the compaction
constants for kilometer intervals estimated by Sclater and Christie (1980) of
about 0.005 km- I , and values of about 0.05 m- I (=50 km- I ) for the near-surface
meter interval estimated from our own measurements for Baltic muds illustrates
this behaviour.
1. Gustavs
• Theoretically calculating the effects of compaction on sediment thickness
and fluid motion within an idealized mud sequence.
This chapter is a partial account of such study, concentrating on the successive development of a complex porosity model for the uppermost half meter of
Holocene muds in the Arkona Basin (Fig. 1), and an application of this model
and a scenario of compaction effects to an idealized mud sequence.
2
Factors that Influence Porosity
One can divide factors controlling porosity into two main classes. The first class
groups all facies components, retaining those properties of the solid compounds
that influence the water-binding capacity of the sediment. For instance, porosity
is directly proportional to grain-size sorting and content of organic matter, biogenic silica and clay minerals, while it shows an indirect proportionality to mean
grain diameter.
The second group consists of diagenetic factors. Ignoring chemical effects
playing a minor part in the stage of early diagenesis, this class consists of physical
factors that are acting in connection with increasing burial and through time.
3
Compaction and its Classical Modeling
Physical diagenesis or compaction results in a continuous porosity decrease
with increasing burial. This process, being mainly a function of effective pressure and time, causes pore water movements and a reduction of sediment thickness. The most simple and most frequently used mathematical description of
compaction is an empirical, in most cases exponential function in a porosity-depth diagram, called compaction curve (Athy 1930; Weller 1959; Roll 1974;
Sclater and Christie 1980; Baldwin and Butler 1985; Huang and Gradstein 1990).
This method is still the most common way of mathematically considering compaction effects for mass balances (e.g. Wold 1992).
Two parameters characterize the compaction curve - the initial porosity at
the sediment surface, and the rate of porosity reduction with depth (compaction
constant). Usually, these two parameters are constant and specific for each sediment type. This assumption implies a homogeneous composition of each sediment type that is not fulfilled in the case of Baltic muds.
A second fact to be considered is the interrelation between depth interval and
compaction curve parameters. The so-called compaction constant is not strongly constant, but depends on the depth interval of measured porosities (near-surface, some meters, or deep burial). The wide disparity between the compaction
constants for kilometer intervals estimated by Sclater and Christie (1980) of
about 0.005 km- I , and values of about 0.05 m- I (=50 km- I ) for the near-surface
meter interval estimated from our own measurements for Baltic muds illustrates
this behaviour.