laminae. Sand dykes (or clastic dykes) are intrusions of
sand upwards into cracks in a finer-grained sediment,
due to porewater overpressure. The overpressure
reduces the friction between the grains and injects
water with sand into vertical fractures produced by
high overpressures or follows bedding planes as sills.
Overpressurised porewater with sand may rise right to
the surface and form sand volcanoes.
Well-sorted sand has at the time of deposition an
initial porosity of 40–45%, whereas recently deposited
mud contains 50–80% water. When sedimentation is
rapid, the mud has little time to lose its excess water,
and a sand bed deposited on top may then sink down
into the underlying silt and clay and form load
structures. On the lower surface of a sand layer we
often see pillow-shaped depressions surrounded by
clay which has oozed up around them. If this process
continues, it will form isolated sand pockets in the
underlying clay: ball-and-pillow structures. Primary
structures such as flute casts often sink in the underlying mud and are deformed by loading (Fig. 2.20).
This mechanism also operates on a larger scale, for
example channel sand will sink down into surrounding
clay (see “deltas”). Poorly compacted clay and silt will
be lighter than surrounding sediments, and flow
upwards to form clay diapirs.
Gravitational deformation occurs in sediments
which are deposited on slopes. These forces can be
resolved into a vector normal to the bedding, and a
shear stress parallel to it. The vector which acts along
the bedding is proportional to the sine of the angle of
dip, and acts as a compaction force which can slide
and fold the beds. In the upper part of the slide,
tensional deformation is prominent, producing
faulting, while compression occurs near the base of
the slide and produces folding. The result is called
slumping (Fig. 2.21). Slumping occurs most readily
where we have rapid sedimentation and therefore relatively thick beds with a high water content and low
shear strength. Deformation takes place when the
shear stress exceeds the shear strength. The shear
stress increases with the thickness of the unconsolidated sediments, but there is not usually a
corresponding increase in shear strength with increasing thickness. Slumping may resemble convolute lamination, but normally affects more than one bed.
Gravitational deformation also leads to faults on a
greater or lesser scale. Sliding of large volumes of
sediment down slopes produces slope scars. Growth
faults and other types of “listric” faults are a result of
large-scale gravitational deformation in the upper part
of an area which is under tension.
Dessication cracks or mud cracks are examples of
contraction or shrinkage of sedimentary beds due to
dehydration. Cracks frequently form regular polygons,
often hexagons or orthogonal sets. Dessication cracks
form only in clay and silt, and the cracks often become
filled with sand, resulting in good contrast. The formation of dessication cracks requires that the beds be
exposed to the air so that the sediments can dry out.
Fig. 2.20 Load cast structures at the base of a sandstone bed in
the Late Precambrian Ring Formation at Rena, Southern
Norway. The picture covers an area of 3 Â 4 m. (Bjørlykke
et al. 1976)
Fig. 2.21 Sediment beds which have been folded immediately
after deposition (through slumping) due to sliding on submarine
slopes. Note that the overlying beds are undeformed showing
that this is not tectonic folding. From the Ridge Basin (MiocenePliocene), California. Scale, John Crowell
2 Introduction to Sedimentology
55
sand upwards into cracks in a finer-grained sediment,
due to porewater overpressure. The overpressure
reduces the friction between the grains and injects
water with sand into vertical fractures produced by
high overpressures or follows bedding planes as sills.
Overpressurised porewater with sand may rise right to
the surface and form sand volcanoes.
Well-sorted sand has at the time of deposition an
initial porosity of 40–45%, whereas recently deposited
mud contains 50–80% water. When sedimentation is
rapid, the mud has little time to lose its excess water,
and a sand bed deposited on top may then sink down
into the underlying silt and clay and form load
structures. On the lower surface of a sand layer we
often see pillow-shaped depressions surrounded by
clay which has oozed up around them. If this process
continues, it will form isolated sand pockets in the
underlying clay: ball-and-pillow structures. Primary
structures such as flute casts often sink in the underlying mud and are deformed by loading (Fig. 2.20).
This mechanism also operates on a larger scale, for
example channel sand will sink down into surrounding
clay (see “deltas”). Poorly compacted clay and silt will
be lighter than surrounding sediments, and flow
upwards to form clay diapirs.
Gravitational deformation occurs in sediments
which are deposited on slopes. These forces can be
resolved into a vector normal to the bedding, and a
shear stress parallel to it. The vector which acts along
the bedding is proportional to the sine of the angle of
dip, and acts as a compaction force which can slide
and fold the beds. In the upper part of the slide,
tensional deformation is prominent, producing
faulting, while compression occurs near the base of
the slide and produces folding. The result is called
slumping (Fig. 2.21). Slumping occurs most readily
where we have rapid sedimentation and therefore relatively thick beds with a high water content and low
shear strength. Deformation takes place when the
shear stress exceeds the shear strength. The shear
stress increases with the thickness of the unconsolidated sediments, but there is not usually a
corresponding increase in shear strength with increasing thickness. Slumping may resemble convolute lamination, but normally affects more than one bed.
Gravitational deformation also leads to faults on a
greater or lesser scale. Sliding of large volumes of
sediment down slopes produces slope scars. Growth
faults and other types of “listric” faults are a result of
large-scale gravitational deformation in the upper part
of an area which is under tension.
Dessication cracks or mud cracks are examples of
contraction or shrinkage of sedimentary beds due to
dehydration. Cracks frequently form regular polygons,
often hexagons or orthogonal sets. Dessication cracks
form only in clay and silt, and the cracks often become
filled with sand, resulting in good contrast. The formation of dessication cracks requires that the beds be
exposed to the air so that the sediments can dry out.
Fig. 2.20 Load cast structures at the base of a sandstone bed in
the Late Precambrian Ring Formation at Rena, Southern
Norway. The picture covers an area of 3 Â 4 m. (Bjørlykke
et al. 1976)
Fig. 2.21 Sediment beds which have been folded immediately
after deposition (through slumping) due to sliding on submarine
slopes. Note that the overlying beds are undeformed showing
that this is not tectonic folding. From the Ridge Basin (MiocenePliocene), California. Scale, John Crowell
2 Introduction to Sedimentology
55