668
equivalent to approximately 3000 m ofwater column. Today,
about 70 to 80% of the overburden load is carried by pore
Chapter 13 Diagenesis
water and only 20 to 30% by the sandstone framework
(BjerIykke 1983).
13.2.9 Summary (Compaction and Subsurface Flow Regimes)
- We distinguish between normally consolidated
sediments with pore water pressure equal to the
hydrostatic pressure, underconsolidated sediments (overpressured pore water), and
overconsolidated sediments (e.g. after erosion).
Sediment compaction is generated by both loading and chemical-mineralogical processes. It is
accompanied by increasing bulk densitities and
decreasing porosities and permeabilities. With
the aid of analytical equations or the compaction
ratio, these parameters can be approximated.
Differential compaction can explain a number of
small- and large-scale sedimentary structures.
- Compaction flow results from expelled pore water and occurs in overpressured systems (mainly
caused by ongoing sedimentation or tectonic
stress). The velocities of compaction flow are
very slow and normally related to the sedimenta13.3 General Processes in
Chemical Diagenesis
13.3.1 Introduction
Chemical diagenesis is an extremely complex, but very
interesting and important topic in sedimentology and
economic geology. There is an immense number of
publications on the different aspects of chemical
diagenesis in various rock types from many regions all
over the world. Chemical diagenesis affects silicic1astic and biogenic materials (skeletal remains and
organic matter) of greatly varying solubility and thermodynamic stability, and it inc1udes mineral reactions
with pore fluids ranging from meteoric waters to
highly concentrated brines. Some aspects ofthe chemical diagenesis of carbonates and evaporites are briefly
described in Sects. 3.4.7 and 6.4.6.
Pore fluids are the main mediator of all reactions in
chemical diagenesis. Solid-solid reactions are negligible under the low temperature conditions characterizing diagenetic processes. The flow and chemistry of
pore fluids are therefore the most important criteria for
distinguishing between various diagenetic realms (cf.
Fig. 13.16) at both shallow and deep burial depths.
Primary minerals may be depleted in trace elements,
which are enriched in secondary mineral phases or
have migrated in solution to other sites. Stable isotopes
and fluid inclusions present in secondary minerals may
reveal their origin (cf. Sect. 13.8.1). Some inorganic
tion rate. Deeply buried, fine-grained sedimentary bodies have lost more water than equivalent to their present-day volume.
Other subsurface flow regimes include temperature/pressure driven thermobaric flow and
density-driven convective flow. These flow systems often exert a great influence on deepburial chemical diagenesis.
- Meteoric flow, however, operating in continental depositional systems or in at least partially
emerged marine sediments, is mostly more effective in dissolving and reprecipitating mineral
phases than the other flow regimes.
The directions of all kinds of subsurface flow
are strongly influenced by refraction which normally leads to lateral flow toward the basin
margin and to the development of separated
flow systems within an individual basin.
processes are closely related to the decomposition and
diagenesis of organic matter.
Generally, one can distinguish between two end
members of diagenetic scenarios which can be realized
in early and late diagenesis (cf. Fig. l3.16b,c):
- Open diagenetic systems, which exchange dissolved
matter with sea water or other parts ofthe sedimentary
basin (allochemica/ diagenesis).
- Closed systems, which preserve their bulk chemical
composition (isochemica/ diagenesis), although they
exhibit internal solution and recrystallization processes.
Specific processes in diagenesis include:
- Dissolution of pre-existing minerals, including pressure solution and earlier cement minerals.
- Leaching of primary minerals and their partial replacement by secondary minerals (e.g. the transformation of feldspar into expanded hydrous minerals, such
as kaolinite and other clay minerals).
- Full replacement of primary minerals by secondary
minerals (e.g. calcite by dolomite, matrix dolomite).
- Overgrowth of detrital grains (e.g. new quartz on
primary quartz).
- Pore-filling cements.
- Recrystallization of pre-existing, normally finer
grained particles.
equivalent to approximately 3000 m ofwater column. Today,
about 70 to 80% of the overburden load is carried by pore
Chapter 13 Diagenesis
water and only 20 to 30% by the sandstone framework
(BjerIykke 1983).
13.2.9 Summary (Compaction and Subsurface Flow Regimes)
- We distinguish between normally consolidated
sediments with pore water pressure equal to the
hydrostatic pressure, underconsolidated sediments (overpressured pore water), and
overconsolidated sediments (e.g. after erosion).
Sediment compaction is generated by both loading and chemical-mineralogical processes. It is
accompanied by increasing bulk densitities and
decreasing porosities and permeabilities. With
the aid of analytical equations or the compaction
ratio, these parameters can be approximated.
Differential compaction can explain a number of
small- and large-scale sedimentary structures.
- Compaction flow results from expelled pore water and occurs in overpressured systems (mainly
caused by ongoing sedimentation or tectonic
stress). The velocities of compaction flow are
very slow and normally related to the sedimenta13.3 General Processes in
Chemical Diagenesis
13.3.1 Introduction
Chemical diagenesis is an extremely complex, but very
interesting and important topic in sedimentology and
economic geology. There is an immense number of
publications on the different aspects of chemical
diagenesis in various rock types from many regions all
over the world. Chemical diagenesis affects silicic1astic and biogenic materials (skeletal remains and
organic matter) of greatly varying solubility and thermodynamic stability, and it inc1udes mineral reactions
with pore fluids ranging from meteoric waters to
highly concentrated brines. Some aspects ofthe chemical diagenesis of carbonates and evaporites are briefly
described in Sects. 3.4.7 and 6.4.6.
Pore fluids are the main mediator of all reactions in
chemical diagenesis. Solid-solid reactions are negligible under the low temperature conditions characterizing diagenetic processes. The flow and chemistry of
pore fluids are therefore the most important criteria for
distinguishing between various diagenetic realms (cf.
Fig. 13.16) at both shallow and deep burial depths.
Primary minerals may be depleted in trace elements,
which are enriched in secondary mineral phases or
have migrated in solution to other sites. Stable isotopes
and fluid inclusions present in secondary minerals may
reveal their origin (cf. Sect. 13.8.1). Some inorganic
tion rate. Deeply buried, fine-grained sedimentary bodies have lost more water than equivalent to their present-day volume.
Other subsurface flow regimes include temperature/pressure driven thermobaric flow and
density-driven convective flow. These flow systems often exert a great influence on deepburial chemical diagenesis.
- Meteoric flow, however, operating in continental depositional systems or in at least partially
emerged marine sediments, is mostly more effective in dissolving and reprecipitating mineral
phases than the other flow regimes.
The directions of all kinds of subsurface flow
are strongly influenced by refraction which normally leads to lateral flow toward the basin
margin and to the development of separated
flow systems within an individual basin.
processes are closely related to the decomposition and
diagenesis of organic matter.
Generally, one can distinguish between two end
members of diagenetic scenarios which can be realized
in early and late diagenesis (cf. Fig. l3.16b,c):
- Open diagenetic systems, which exchange dissolved
matter with sea water or other parts ofthe sedimentary
basin (allochemica/ diagenesis).
- Closed systems, which preserve their bulk chemical
composition (isochemica/ diagenesis), although they
exhibit internal solution and recrystallization processes.
Specific processes in diagenesis include:
- Dissolution of pre-existing minerals, including pressure solution and earlier cement minerals.
- Leaching of primary minerals and their partial replacement by secondary minerals (e.g. the transformation of feldspar into expanded hydrous minerals, such
as kaolinite and other clay minerals).
- Full replacement of primary minerals by secondary
minerals (e.g. calcite by dolomite, matrix dolomite).
- Overgrowth of detrital grains (e.g. new quartz on
primary quartz).
- Pore-filling cements.
- Recrystallization of pre-existing, normally finer
grained particles.