11. Association or any tendency to statistical periodicity in the features of the strata in a profile – bed
types, structures.
These observations form the basis for defining
facies, which are a synthesis of all the data listed
above which can be used to group certain types of
rocks. They may be genetic facies, i.e. strata which
one assumes have formed in the same manner. All
strata which contain criteria which indicate that they
were deposited in shallow water can be described (in
reality interpreted) as shallow water facies. In the
same way we have fluvial facies, deep water facies,
evaporite facies, and so on. The facies concept can
also be used to distinguish between different rock
compositions (lithologies), e.g. carbonate facies, sandstone facies.
2.16 Sedimentary Structures
By sedimentary structures we mean structures in sedimentary rocks which have formed during or just after
deposition. We distinguish between primary structures
which are formed at the time of deposition of the
sediments, and secondary structures which are formed
after deposition.
2.17 Layering and Lamination
Most sedimentary rocks exhibit some lamination or
bedding, but we also have massive (unlaminated)
rocks. Lamination records variations in the sediment
composition as successive depositional layers are
draped over the contours of the sedimentary surface.
The variations may reflect different grain sizes,
sorting, mineral composition or organic matter.
Laminae are less than 1 cm thick. Units greater than
1 cm are called beds. A bed will contain sediments
which have been deposited by the same sedimentary
processes. Some sedimentary processes, for example
deposition of a turbidite bed, may be fairly rapid.
Migration of a sand dune to give cross-bedding
takes somewhat longer, while deposition of a clay
bed from suspended material may take a very long
time.
Graded beds have a grain size which tends to
decrease upwards within the bed. The opposite is
called inverse grading. Normal grading may be due
to deposition from suspension, when the largest
particles tend to fall to the bottom first, as with
turbidites, or to flow velocities dropping off during
deposition in a river.
Inverse grading may be due to increasing flow
velocity but if the increase in velocity is too high, the
result will be erosion. The supply of coarse material
during transport and deposition may also produce
inverse grading. In high density sediment currents
(debris flows) we may get inverse grading; smaller
particles sink more easily to the bottom between the
large particles. Massive beds, or at any rate reasonably
massive beds without visible lamination or bedding,
may be formed during very rapid deposition of
sediments from suspension. X-ray photography of
apparently massive sediments nevertheless usually
reveals the presence of weak lamination even in
sand. Massive sand beds occur due to rapid fall out
from suspension. In mudstones the primary lamination
may be destroyed by intense bioturbation.
On the surface of laminated sediment we may find
erosion structures.
Water running over a plane surface, e.g. beach
sand, will produce small-scale, branched (dendritic)
erosion marks called rill marks. The flow of water
may be due to runoff from big waves or from groundwater seepage at low tide. These structures are good
indicators of inter- or supratidal environments, but
they are seldom preserved because they are usually
destroyed when the water level rises again. Raindrop
imprints are often preserved on bedding surfaces, and
are good indicators of subaerial exposure.
2.18 Bedforms
Bedforms are morphological features resulting from
the interaction between particular types of flow and the
sediment grains on the bottom. A specific type of
bedform will only form within a limited flow velocity
range and is also dependent on the availability of grain
sizes which can be moved by that flow.
Current ripples form in fine-grained sand when the
velocity exceeds the lower limit for sediment
movement.
Ripples and dunes have a stoss side upstream where
erosion takes place and a lee side on which deposition
50
K. Bjørlykke
types, structures.
These observations form the basis for defining
facies, which are a synthesis of all the data listed
above which can be used to group certain types of
rocks. They may be genetic facies, i.e. strata which
one assumes have formed in the same manner. All
strata which contain criteria which indicate that they
were deposited in shallow water can be described (in
reality interpreted) as shallow water facies. In the
same way we have fluvial facies, deep water facies,
evaporite facies, and so on. The facies concept can
also be used to distinguish between different rock
compositions (lithologies), e.g. carbonate facies, sandstone facies.
2.16 Sedimentary Structures
By sedimentary structures we mean structures in sedimentary rocks which have formed during or just after
deposition. We distinguish between primary structures
which are formed at the time of deposition of the
sediments, and secondary structures which are formed
after deposition.
2.17 Layering and Lamination
Most sedimentary rocks exhibit some lamination or
bedding, but we also have massive (unlaminated)
rocks. Lamination records variations in the sediment
composition as successive depositional layers are
draped over the contours of the sedimentary surface.
The variations may reflect different grain sizes,
sorting, mineral composition or organic matter.
Laminae are less than 1 cm thick. Units greater than
1 cm are called beds. A bed will contain sediments
which have been deposited by the same sedimentary
processes. Some sedimentary processes, for example
deposition of a turbidite bed, may be fairly rapid.
Migration of a sand dune to give cross-bedding
takes somewhat longer, while deposition of a clay
bed from suspended material may take a very long
time.
Graded beds have a grain size which tends to
decrease upwards within the bed. The opposite is
called inverse grading. Normal grading may be due
to deposition from suspension, when the largest
particles tend to fall to the bottom first, as with
turbidites, or to flow velocities dropping off during
deposition in a river.
Inverse grading may be due to increasing flow
velocity but if the increase in velocity is too high, the
result will be erosion. The supply of coarse material
during transport and deposition may also produce
inverse grading. In high density sediment currents
(debris flows) we may get inverse grading; smaller
particles sink more easily to the bottom between the
large particles. Massive beds, or at any rate reasonably
massive beds without visible lamination or bedding,
may be formed during very rapid deposition of
sediments from suspension. X-ray photography of
apparently massive sediments nevertheless usually
reveals the presence of weak lamination even in
sand. Massive sand beds occur due to rapid fall out
from suspension. In mudstones the primary lamination
may be destroyed by intense bioturbation.
On the surface of laminated sediment we may find
erosion structures.
Water running over a plane surface, e.g. beach
sand, will produce small-scale, branched (dendritic)
erosion marks called rill marks. The flow of water
may be due to runoff from big waves or from groundwater seepage at low tide. These structures are good
indicators of inter- or supratidal environments, but
they are seldom preserved because they are usually
destroyed when the water level rises again. Raindrop
imprints are often preserved on bedding surfaces, and
are good indicators of subaerial exposure.
2.18 Bedforms
Bedforms are morphological features resulting from
the interaction between particular types of flow and the
sediment grains on the bottom. A specific type of
bedform will only form within a limited flow velocity
range and is also dependent on the availability of grain
sizes which can be moved by that flow.
Current ripples form in fine-grained sand when the
velocity exceeds the lower limit for sediment
movement.
Ripples and dunes have a stoss side upstream where
erosion takes place and a lee side on which deposition
50
K. Bjørlykke