7.2 Basic Concepts
7.1.8 Diagenetic Overprint
Primary bedding features, inc1uding sedimentary
structures, bedding planes, bedding rhythms and
larger sedimentary cyc1es, can be significantly modified by diagenetic overprints, particularly in carbonates and siliceous sediments. Examples of these effects are described in Sects. 5.3.5 and 13.3. Even
minor variations in primary composition and pore
space are sufficient to cause significant diagenetic
modifications. As a result, primary structures and
bedding phenomena may be enhanced, modified, or
in some cases obliterated. Some important processes
are:
- Selective dissolution of unstable minerals and their
reprecipitation as stable cement minerals in
carbonate-bearing and siliceous strata. With increasing burial depths (often at several hundred meters)
dissolved carbonate or opaline silica
is exported from the potential interbed and
reprecipitated as pore cement in either the overlying
or underlying bed (cf. Sect. 13.4.2 and Fig. 13.24).
As a result, the beds become lithified and resist further mechanical compaction, while the interbeds continue to compact and lose dissolved matter by molecular diffusion. Solution seams and stylolites are sites
where intensive dissolution has taken place. In skeletal carbonates, early diagenetic differential dissolution and cementation may lead to the selective
lithification of specific layers. Emergence of marine
carbonates accelerates both dissolution and cementation (cf. Sect. 13.3.7).
- Hardgrounds form at the sediment surface in various calcareous environments. They are often associated with phases of non-deposition, winnowing and
moderate erosion. They preferentially occur below
warm bottom waters of enhanced salinity. Cemented
hardgrounds with erosional surfaces may occur repeatedly in a sequence and thus indicate a kind of
rhythmicity or cyc1icity.
- Concretions of carbonate, silica, and phosphate
generally form early in diagenesis near the sea floor,
but they also grow at greater depth below the
sediment-water interface (cf. Sect. 13.4.1). Concretions are frequently concentrated in layers which deviated in their original composition and/or texture
from the underlying and overlying sediments. They
may have had higher contents in carbonate, biogenie
silica, or organic carbon, or relatively high porosities
and somewhat differing grain size distributions. For
this reason, most concretion horizons reflect subtle
primary depositional variations and are found parallel
to the primary bedding.
- Nodular limes tones are generated by early
concretionary cementation of bioturbated carbonate
muds and by pressure dissolution during the later
stages of diagenesis. Flint layers in chalk may de297
velop independently from the primary bedding
rhythm (Ricken and Eder 1991). Cementation of
graded beds can also affect a thin layer below the
event bed, thus forming an underbed, which apparently enhances the thickness of the event bed.
- Weathering. Further alterations of rhythmic and
cyclic sequences are caused when seetions are exposed to weathering. In the case of limestone-marl
altemations, strata with carbonate contents 65 to
85% rapidly disintegrate as a result of physical
weathering, while layers with higher carbonate contents commonly resist. Hence, rhythmic bedding is
often more conspicuous in field exposures than in
drill cores.
7.1.9 Summary (General Characteristics of
Cyclic Sediments)
- Cyc1ic sediments comprise a wide range from
thin bedding couplets via meter-scale phenomena to very thick (macro-scale) sedimentary
sequences.
- Some of the smaller-scale, discyc1ic or quasiperiodic features are caused by autogenetic
processes operating in the basin itself, whereas
most of the medium- to large-scale sequences
result from regional (e.g. tectonism, c1imate) or
global processes (e.g. eustasy).
- Sedimentary cyc1es are often asymmetrie (e.g.
coarsening-up), sometimes ± symmetrie.
- The thicknesses of various cyc1es depend not
only on their time period (conventionaUy decreasing from the first to the sixth order), but
also on the rates of their sediment buildup.
Thin cyc1es may represent the same period as
thick cyc1es.
- Epibenthos and infauna distinctly respond to
gradual or episodic rapid change in sediment
deposition (event deposits).
- Diagenesis often modifies and enhances the
primary cyciic signal.
7.2 Sequence Stratigraphy:
Basic Concepts
7.2.1 Introduction
During the last two decades sequence stratigraphy
has become a very popular field in sedimentary geology, both in academia and hydrocarbon exploration.
In spite of great efforts and accomplishments, there
is still an ongoing debate about some specific problems in sequence stratigraphy (see below). At present
it appears that a generally applicable and fully ac-
7.1.8 Diagenetic Overprint
Primary bedding features, inc1uding sedimentary
structures, bedding planes, bedding rhythms and
larger sedimentary cyc1es, can be significantly modified by diagenetic overprints, particularly in carbonates and siliceous sediments. Examples of these effects are described in Sects. 5.3.5 and 13.3. Even
minor variations in primary composition and pore
space are sufficient to cause significant diagenetic
modifications. As a result, primary structures and
bedding phenomena may be enhanced, modified, or
in some cases obliterated. Some important processes
are:
- Selective dissolution of unstable minerals and their
reprecipitation as stable cement minerals in
carbonate-bearing and siliceous strata. With increasing burial depths (often at several hundred meters)
dissolved carbonate or opaline silica
is exported from the potential interbed and
reprecipitated as pore cement in either the overlying
or underlying bed (cf. Sect. 13.4.2 and Fig. 13.24).
As a result, the beds become lithified and resist further mechanical compaction, while the interbeds continue to compact and lose dissolved matter by molecular diffusion. Solution seams and stylolites are sites
where intensive dissolution has taken place. In skeletal carbonates, early diagenetic differential dissolution and cementation may lead to the selective
lithification of specific layers. Emergence of marine
carbonates accelerates both dissolution and cementation (cf. Sect. 13.3.7).
- Hardgrounds form at the sediment surface in various calcareous environments. They are often associated with phases of non-deposition, winnowing and
moderate erosion. They preferentially occur below
warm bottom waters of enhanced salinity. Cemented
hardgrounds with erosional surfaces may occur repeatedly in a sequence and thus indicate a kind of
rhythmicity or cyc1icity.
- Concretions of carbonate, silica, and phosphate
generally form early in diagenesis near the sea floor,
but they also grow at greater depth below the
sediment-water interface (cf. Sect. 13.4.1). Concretions are frequently concentrated in layers which deviated in their original composition and/or texture
from the underlying and overlying sediments. They
may have had higher contents in carbonate, biogenie
silica, or organic carbon, or relatively high porosities
and somewhat differing grain size distributions. For
this reason, most concretion horizons reflect subtle
primary depositional variations and are found parallel
to the primary bedding.
- Nodular limes tones are generated by early
concretionary cementation of bioturbated carbonate
muds and by pressure dissolution during the later
stages of diagenesis. Flint layers in chalk may de297
velop independently from the primary bedding
rhythm (Ricken and Eder 1991). Cementation of
graded beds can also affect a thin layer below the
event bed, thus forming an underbed, which apparently enhances the thickness of the event bed.
- Weathering. Further alterations of rhythmic and
cyclic sequences are caused when seetions are exposed to weathering. In the case of limestone-marl
altemations, strata with carbonate contents 65 to
85% rapidly disintegrate as a result of physical
weathering, while layers with higher carbonate contents commonly resist. Hence, rhythmic bedding is
often more conspicuous in field exposures than in
drill cores.
7.1.9 Summary (General Characteristics of
Cyclic Sediments)
- Cyc1ic sediments comprise a wide range from
thin bedding couplets via meter-scale phenomena to very thick (macro-scale) sedimentary
sequences.
- Some of the smaller-scale, discyc1ic or quasiperiodic features are caused by autogenetic
processes operating in the basin itself, whereas
most of the medium- to large-scale sequences
result from regional (e.g. tectonism, c1imate) or
global processes (e.g. eustasy).
- Sedimentary cyc1es are often asymmetrie (e.g.
coarsening-up), sometimes ± symmetrie.
- The thicknesses of various cyc1es depend not
only on their time period (conventionaUy decreasing from the first to the sixth order), but
also on the rates of their sediment buildup.
Thin cyc1es may represent the same period as
thick cyc1es.
- Epibenthos and infauna distinctly respond to
gradual or episodic rapid change in sediment
deposition (event deposits).
- Diagenesis often modifies and enhances the
primary cyciic signal.
7.2 Sequence Stratigraphy:
Basic Concepts
7.2.1 Introduction
During the last two decades sequence stratigraphy
has become a very popular field in sedimentary geology, both in academia and hydrocarbon exploration.
In spite of great efforts and accomplishments, there
is still an ongoing debate about some specific problems in sequence stratigraphy (see below). At present
it appears that a generally applicable and fully ac-