In certain cases shrinkage structures may also form
underwater through dehydration of clay minerals
(smectite) as a result of variation in the salinity of
the porewater (syneresis). Shrinkage structures are
less regular than desiccation cracks and are not usually
interconnected. In the smectite-rich Eocene sediments
from the North Sea basin and the Norwegian Sea,
seismic data show large (several hundred metres
wide) polygons which have been interpreted as shrinkage cracks.
When the porewater in sediments freezes to ice and
remelts, we also find expansion and contraction which
results in polygonal surface marks and associated vertical ice wedges.
2.21 Concretions
Concretions are round, flat or elongated structures
which consist of cement which has been chemically
precipitated in the pores of the sediment. The most
common types of concretion are carbonate (calcite and
siderite) and silica (chert). Sulphides, particularly
pyrite, also form concretions.
A characteristic feature of concretions is that any
laminations in the sediment pass through the
concretions. This shows that the concretion has been
formed through passive filling of its pores. As the
overburden increases, the sediments around the concretion will be subject to compaction, while the concretion cannot be compressed because the pores are
full of cement. A concretion therefore has a cement
content which corresponds to its porosity at the time of
formation. Carbonate concretions in clay may have a
carbonate content reflecting 50–70% porosity if the
matrix does not contain carbonate. Concretions in
calcareous rocks, i.e. marls, contain clastic or biogenic
carbonate in addition to carbonate cement, and therefore have more carbonate than the matrix. In these
cases the carbonate content cannot be taken as an
indication of the porosity at the time of formation.
Concretions often contain fossils, showing no sign of
compaction while the same fossils are severely
deformed and sometimes also dissolved outside the
concretion. In carbonate sediments, particularly
chalk, there are silica concretions (chert). These are
formed through precipitation of finely divided amorphous silica to form a type of chert called flint.
The source of the silica is usually amorphous biogenic
silica, frequently sponge spicules.
2.22 Trace Fossils
Trace fossils are structures in sedimentary rocks which
have been left by organisms that lived on and/or
burrowed in the sediment. Such organisms are
extremely sensitive to changes in the composition of
the nutrient content, the sedimentation rate and bottom
currents, and are therefore useful indications of the
environment (Fig. 2.22a,b). Trace fossils are therefore
good indicators of the depositional environments.
They can be classified taxonomically, i.e. according
to the animal which left the traces, but it may not be
possible to determine which animal was responsible.
The same species may form several different types of
trace depending on the sediment composition and on
its mode of life. Furthermore, different animals may
leave traces which are so similar that they are classified as one trace fossil. For these reasons a descriptive,
morphological classification of trace fossils is used.
Trace fossils can also be classified according to
where they occur in relation to the bed:
1. On top of beds (e.g. a thin sand or carbonate layer
Epichnia).
2. Within the bed (Endichnia).
3. On the lower surface of the bed (Hypichnia).
4. Outside the bed (Exichnia).
The impression made by an animal may create a
mould or a cast in the overlying bed. Organisms which
burrow into sediments often secrete a cement which
ensures that the walls of their burrows do not collapse.
These secretions also contribute to preservation.
When worms eat sediment, for example, it passes
through their digestive organs and their burrow refills
with a sediment which has a somewhat different composition from the surrounding sediments. This is particularly noticeable in burrows at the interface
between two strata with different compositions. The
amount of bioturbation reflects nutritive conditions
and the sedimentation rate. With very rapid sedimentation there will be less time for organisms to burrow
through the sediments. Where we have very slow
sedimentation or an hiatus, the sediments will often
be thoroughly churned up by bioturbation, and thereby
homogenised. Bioturbation is most widespread in
marine environments, but can also be found to a lesser
56
K. Bjørlykke
underwater through dehydration of clay minerals
(smectite) as a result of variation in the salinity of
the porewater (syneresis). Shrinkage structures are
less regular than desiccation cracks and are not usually
interconnected. In the smectite-rich Eocene sediments
from the North Sea basin and the Norwegian Sea,
seismic data show large (several hundred metres
wide) polygons which have been interpreted as shrinkage cracks.
When the porewater in sediments freezes to ice and
remelts, we also find expansion and contraction which
results in polygonal surface marks and associated vertical ice wedges.
2.21 Concretions
Concretions are round, flat or elongated structures
which consist of cement which has been chemically
precipitated in the pores of the sediment. The most
common types of concretion are carbonate (calcite and
siderite) and silica (chert). Sulphides, particularly
pyrite, also form concretions.
A characteristic feature of concretions is that any
laminations in the sediment pass through the
concretions. This shows that the concretion has been
formed through passive filling of its pores. As the
overburden increases, the sediments around the concretion will be subject to compaction, while the concretion cannot be compressed because the pores are
full of cement. A concretion therefore has a cement
content which corresponds to its porosity at the time of
formation. Carbonate concretions in clay may have a
carbonate content reflecting 50–70% porosity if the
matrix does not contain carbonate. Concretions in
calcareous rocks, i.e. marls, contain clastic or biogenic
carbonate in addition to carbonate cement, and therefore have more carbonate than the matrix. In these
cases the carbonate content cannot be taken as an
indication of the porosity at the time of formation.
Concretions often contain fossils, showing no sign of
compaction while the same fossils are severely
deformed and sometimes also dissolved outside the
concretion. In carbonate sediments, particularly
chalk, there are silica concretions (chert). These are
formed through precipitation of finely divided amorphous silica to form a type of chert called flint.
The source of the silica is usually amorphous biogenic
silica, frequently sponge spicules.
2.22 Trace Fossils
Trace fossils are structures in sedimentary rocks which
have been left by organisms that lived on and/or
burrowed in the sediment. Such organisms are
extremely sensitive to changes in the composition of
the nutrient content, the sedimentation rate and bottom
currents, and are therefore useful indications of the
environment (Fig. 2.22a,b). Trace fossils are therefore
good indicators of the depositional environments.
They can be classified taxonomically, i.e. according
to the animal which left the traces, but it may not be
possible to determine which animal was responsible.
The same species may form several different types of
trace depending on the sediment composition and on
its mode of life. Furthermore, different animals may
leave traces which are so similar that they are classified as one trace fossil. For these reasons a descriptive,
morphological classification of trace fossils is used.
Trace fossils can also be classified according to
where they occur in relation to the bed:
1. On top of beds (e.g. a thin sand or carbonate layer
Epichnia).
2. Within the bed (Endichnia).
3. On the lower surface of the bed (Hypichnia).
4. Outside the bed (Exichnia).
The impression made by an animal may create a
mould or a cast in the overlying bed. Organisms which
burrow into sediments often secrete a cement which
ensures that the walls of their burrows do not collapse.
These secretions also contribute to preservation.
When worms eat sediment, for example, it passes
through their digestive organs and their burrow refills
with a sediment which has a somewhat different composition from the surrounding sediments. This is particularly noticeable in burrows at the interface
between two strata with different compositions. The
amount of bioturbation reflects nutritive conditions
and the sedimentation rate. With very rapid sedimentation there will be less time for organisms to burrow
through the sediments. Where we have very slow
sedimentation or an hiatus, the sediments will often
be thoroughly churned up by bioturbation, and thereby
homogenised. Bioturbation is most widespread in
marine environments, but can also be found to a lesser
56
K. Bjørlykke