extent in freshwater sediments. In poorly oxygenated
environments there is, however, little bioturbation.
In high energy environments, e.g. above the fairweather wave base, we only have vertical trace fossils,
e.g. Skolithos or Diplocraterion. The high current
velocity prevents these organisms from crawling
around on the bottom; they have to burrow down into
the sediment and live by filtering nutrients out of the
water. To remain at the same level beneath the sediment surface they must move upwards or downwards
in their holes, depending on whether erosion or sedimentation is proceeding in the area. Structures or
fillings which reflect such adjustments are called
“spreiten”.
In modern marine environments we can study a
number of organisms which create bioturbation
structures. The most common are worms like
Arenicola which make U-shaped traces in fine-grained
sand and silt. Such traces (arenicolites) are also to be
found in older rocks. Burrowing bivalves create various types of trace as well. Arthropods like crabs and
prawns make burrowing structures in beach sand
(ophiomorpha). These vertical trace fossils are
grouped together in an ichnofacies called the Skolithos
facies. Thallassinoids are more horizontally aligned
networks of arthropod burrows.
Below the wave base and in other protected
environments, for example the intertidal zone, we
find traces in the horizontal plane from organisms
which live off blue-green algae and other organic
material on the surface of the sediment. In this neritic
zone (Fig. 2.22a,b) we find a number of different types
of traces from organisms which eat their way through
sediments, and which form different patterns. This is
called the Cruziana facies. Rusophycus and Cruziana
are typical of the neritic zone and represent horizontal
traces of arthropods which feed on the sediment surface. Rhizocorallium and Teichichnus are other trace
fossils that occur below the Cruziana facies. In deeper
water where wave and current energy is even lower,
we find Zoophycus and Nereites facies. It is important
to remember that these environments are primarily a
function of wave and current energy, and cannot simply be correlated with absolute depth. In shallow
enclosed seas with a shallow wave base, e.g. the modern Baltic, we find an effective wave base of only 5–10
m in many areas, while elsewhere we may have stronger currents along parts of the deeper trenches. In the
epicontinental Cambro-Silurian marine sedimentary
sequence of the Oslo area we find Nereites facies in
the shales, but the water depth was probably not more
than 100–200 m, perhaps even less. In the deep oceans
the Nereites facies may correspond to a depth of several thousand metres.
Trace fossils are very useful facies indicators and
should be noted whenever sedimentary sections are
examined for facies interpretations. Certain trace
fossils can be linked with animals that have fairly
specific environmental requirements.
2.23 Facies and Sedimentary
Environments
The word “facies” is used in a number of geological
disciplines. A term such as “metamorphic facies” is
thoroughly entrenched. Sedimentary facies have also
long been identified in sedimentology to distinguish
between sedimentary rocks which differ in appearance
and have formed in different ways. The term facies can
be used both descriptively and genetically. We use
terms like “sandy facies”, “shaly facies”, “carbonate
facies” when we are describing properties of the rock
that can be observed or analysed objectively. We use
terms such as “shallow water facies”, “deep water
facies”, “turbidite facies”, “deltaic facies”, “intertidal
facies”, “aeolian facies”, “reef facies” etc., depending
on which environment we believe the rocks represent.
In these examples, the word “facies” represents an
interpretation, and is therefore not very suitable for
describing sedimentary rocks objectively. For this reason it is important that we define the objective criteria
(observations) on which we are basing our
interpretations.
What we can do, then, is first describe and take
measurements on a series of beds in the field, and on
the basis of certain criteria divide the series into facies.
The criteria will generally be texture, sedimentary
structures, mineral composition, and, if present, also
fossils. Interpreting a facies in terms of depositional
environment is often very difficult, especially since
few criteria are unambiguously diagnostic of one particular environment. In some cases it may be useful to
use statistical methods for distinguishing between different facies and for describing facies sequences.
What we observe and measure is a selection of the
properties of the rock. When we describe a sedimentary rock, we observe the results of processes which
58
K. Bjørlykke
environments there is, however, little bioturbation.
In high energy environments, e.g. above the fairweather wave base, we only have vertical trace fossils,
e.g. Skolithos or Diplocraterion. The high current
velocity prevents these organisms from crawling
around on the bottom; they have to burrow down into
the sediment and live by filtering nutrients out of the
water. To remain at the same level beneath the sediment surface they must move upwards or downwards
in their holes, depending on whether erosion or sedimentation is proceeding in the area. Structures or
fillings which reflect such adjustments are called
“spreiten”.
In modern marine environments we can study a
number of organisms which create bioturbation
structures. The most common are worms like
Arenicola which make U-shaped traces in fine-grained
sand and silt. Such traces (arenicolites) are also to be
found in older rocks. Burrowing bivalves create various types of trace as well. Arthropods like crabs and
prawns make burrowing structures in beach sand
(ophiomorpha). These vertical trace fossils are
grouped together in an ichnofacies called the Skolithos
facies. Thallassinoids are more horizontally aligned
networks of arthropod burrows.
Below the wave base and in other protected
environments, for example the intertidal zone, we
find traces in the horizontal plane from organisms
which live off blue-green algae and other organic
material on the surface of the sediment. In this neritic
zone (Fig. 2.22a,b) we find a number of different types
of traces from organisms which eat their way through
sediments, and which form different patterns. This is
called the Cruziana facies. Rusophycus and Cruziana
are typical of the neritic zone and represent horizontal
traces of arthropods which feed on the sediment surface. Rhizocorallium and Teichichnus are other trace
fossils that occur below the Cruziana facies. In deeper
water where wave and current energy is even lower,
we find Zoophycus and Nereites facies. It is important
to remember that these environments are primarily a
function of wave and current energy, and cannot simply be correlated with absolute depth. In shallow
enclosed seas with a shallow wave base, e.g. the modern Baltic, we find an effective wave base of only 5–10
m in many areas, while elsewhere we may have stronger currents along parts of the deeper trenches. In the
epicontinental Cambro-Silurian marine sedimentary
sequence of the Oslo area we find Nereites facies in
the shales, but the water depth was probably not more
than 100–200 m, perhaps even less. In the deep oceans
the Nereites facies may correspond to a depth of several thousand metres.
Trace fossils are very useful facies indicators and
should be noted whenever sedimentary sections are
examined for facies interpretations. Certain trace
fossils can be linked with animals that have fairly
specific environmental requirements.
2.23 Facies and Sedimentary
Environments
The word “facies” is used in a number of geological
disciplines. A term such as “metamorphic facies” is
thoroughly entrenched. Sedimentary facies have also
long been identified in sedimentology to distinguish
between sedimentary rocks which differ in appearance
and have formed in different ways. The term facies can
be used both descriptively and genetically. We use
terms like “sandy facies”, “shaly facies”, “carbonate
facies” when we are describing properties of the rock
that can be observed or analysed objectively. We use
terms such as “shallow water facies”, “deep water
facies”, “turbidite facies”, “deltaic facies”, “intertidal
facies”, “aeolian facies”, “reef facies” etc., depending
on which environment we believe the rocks represent.
In these examples, the word “facies” represents an
interpretation, and is therefore not very suitable for
describing sedimentary rocks objectively. For this reason it is important that we define the objective criteria
(observations) on which we are basing our
interpretations.
What we can do, then, is first describe and take
measurements on a series of beds in the field, and on
the basis of certain criteria divide the series into facies.
The criteria will generally be texture, sedimentary
structures, mineral composition, and, if present, also
fossils. Interpreting a facies in terms of depositional
environment is often very difficult, especially since
few criteria are unambiguously diagnostic of one particular environment. In some cases it may be useful to
use statistical methods for distinguishing between different facies and for describing facies sequences.
What we observe and measure is a selection of the
properties of the rock. When we describe a sedimentary rock, we observe the results of processes which
58
K. Bjørlykke