fossils are consequently a great help in reconstructing
the environment in which the sediments were deposited. Palaeoecology is the study of ecological
conditions as we are able to reconstruct them on the
basis of remains or traces of plants and animals in
rocks. Traces of animals in sediments have proved to
be very useful environmental indicators.
Studies of recent and older sedimentary rocks provide a fruitful two-way exchange of information in
sedimentology. From studies of recent environments
we can learn about the conditions that particular processes require. In older rocks, however, we can study
sedimentary sections which encompass many millions
of years of sedimentation, offering us a completely
different record of the way sedimentological processes
can vary as a function of geologic time. As a result,
studies of older rocks also contribute to our understanding of the recent environment and offer nonuniformitarian explanations.
When we study rocks, we should attempt to give
objective descriptions of the composition, structure
etc. of the rocks, and on the basis of these try to
interpret how they were formed. However, it is impossible to give a completely exhaustive, objective
description of a rock. Nevertheless it is often most
fruitful to have a theory or hypothesis against which
to test our observations. Data collection can then be
focused on observations and measurements which can
support or disprove the hypothesis.
We know from experience that we have a tendency
to observe what we are looking for, or what we anticipate finding.
Early descriptions of sedimentary sequences contain few observations about sedimentary structures
which we would consider fairly conspicuous and
important today. We observe sedimentary structures
because we have learned to recognise them and understand their genetic significance.
Many sedimentologists use a standard checklist for
what they should observe in the field, so that their
descriptions are as comparable as possible. Nevertheless, it is important that field observations do not
become too much of a routine. Facies analysis should
be a creative process and the various depositional
models should be kept in mind when making the
observations. It is also desirable to quantify field
observations as far as possible, for example by surveys
in the field and a range of laboratory analyses. These
might be texture analyses (e.g. grain-size distribution),
microscope analyses (perhaps using a scanning
electron microscope) or chemical analyses. Pure
descriptions of sedimentary rocks are useful because
they increase the data base on which we can build our
interpretations.
Systematic studies of recent environments of sedimentation are important to find connections between
the environment and the sediments which accumulate.
The environment governs the sedimentological processes which determine what sort of sediments are
formed and deposited. The connection we are trying
to understand in modern environments is thus: environment ! process ! sediment.
Today a large number of modern environments of
sedimentation have been studied in great detail. These
include aeolian and fluvial environments, deltas, beach
zones, tidal flats and carbonate banks. Deep sea
environments have naturally not been so easy to
study, but modern sampling and remote sensing equipment and underwater TV cameras have made it easier
to gather observations from this environment, too. In
recent years systematic drilling through sedimentary
layers on the ocean floor (Deep Sea Drilling Program –
DSDP, and Ocean Drilling Program – ODP) has
provided an entirely new picture of the geology of
the ocean depths. Specially constructed diving ships
(e.g. ALVIN) make it possible for geologists to
observe the ocean floor directly at depths of up to
about 3,500 m and take samples of surface sediments.
In addition geophysical, particularly seismic, surveys
provide one of the most important bases for understanding the stratigraphy and geometry of sedimentary
basins.
In studying older rocks we base our approach on
certain features which we can observe or measure, and
attempt to interpret the processes that produced them.
Particular variations in grain size and sedimentary
structures in profiles can be interpreted as having
been formed through particular processes, e.g. aeolian,
tidal or deltaic processes. The recognition of such
sedimentary processes helps us to reconstruct the
environments. The sequence of interpretation in studies of older sedimentary rocks is thus: description of
sedimentary rock ! processes ! environment.
Applied geology has always been important, even
for purely scientific research. The interests of economic recovery of raw materials from sedimentary
rocks create a demand for sedimentologists and sedimentological research. Exploration for and recovery
of raw materials also provide important scientific
information. Sedimentary rocks contain raw materials
32
K. Bjørlykke
the environment in which the sediments were deposited. Palaeoecology is the study of ecological
conditions as we are able to reconstruct them on the
basis of remains or traces of plants and animals in
rocks. Traces of animals in sediments have proved to
be very useful environmental indicators.
Studies of recent and older sedimentary rocks provide a fruitful two-way exchange of information in
sedimentology. From studies of recent environments
we can learn about the conditions that particular processes require. In older rocks, however, we can study
sedimentary sections which encompass many millions
of years of sedimentation, offering us a completely
different record of the way sedimentological processes
can vary as a function of geologic time. As a result,
studies of older rocks also contribute to our understanding of the recent environment and offer nonuniformitarian explanations.
When we study rocks, we should attempt to give
objective descriptions of the composition, structure
etc. of the rocks, and on the basis of these try to
interpret how they were formed. However, it is impossible to give a completely exhaustive, objective
description of a rock. Nevertheless it is often most
fruitful to have a theory or hypothesis against which
to test our observations. Data collection can then be
focused on observations and measurements which can
support or disprove the hypothesis.
We know from experience that we have a tendency
to observe what we are looking for, or what we anticipate finding.
Early descriptions of sedimentary sequences contain few observations about sedimentary structures
which we would consider fairly conspicuous and
important today. We observe sedimentary structures
because we have learned to recognise them and understand their genetic significance.
Many sedimentologists use a standard checklist for
what they should observe in the field, so that their
descriptions are as comparable as possible. Nevertheless, it is important that field observations do not
become too much of a routine. Facies analysis should
be a creative process and the various depositional
models should be kept in mind when making the
observations. It is also desirable to quantify field
observations as far as possible, for example by surveys
in the field and a range of laboratory analyses. These
might be texture analyses (e.g. grain-size distribution),
microscope analyses (perhaps using a scanning
electron microscope) or chemical analyses. Pure
descriptions of sedimentary rocks are useful because
they increase the data base on which we can build our
interpretations.
Systematic studies of recent environments of sedimentation are important to find connections between
the environment and the sediments which accumulate.
The environment governs the sedimentological processes which determine what sort of sediments are
formed and deposited. The connection we are trying
to understand in modern environments is thus: environment ! process ! sediment.
Today a large number of modern environments of
sedimentation have been studied in great detail. These
include aeolian and fluvial environments, deltas, beach
zones, tidal flats and carbonate banks. Deep sea
environments have naturally not been so easy to
study, but modern sampling and remote sensing equipment and underwater TV cameras have made it easier
to gather observations from this environment, too. In
recent years systematic drilling through sedimentary
layers on the ocean floor (Deep Sea Drilling Program –
DSDP, and Ocean Drilling Program – ODP) has
provided an entirely new picture of the geology of
the ocean depths. Specially constructed diving ships
(e.g. ALVIN) make it possible for geologists to
observe the ocean floor directly at depths of up to
about 3,500 m and take samples of surface sediments.
In addition geophysical, particularly seismic, surveys
provide one of the most important bases for understanding the stratigraphy and geometry of sedimentary
basins.
In studying older rocks we base our approach on
certain features which we can observe or measure, and
attempt to interpret the processes that produced them.
Particular variations in grain size and sedimentary
structures in profiles can be interpreted as having
been formed through particular processes, e.g. aeolian,
tidal or deltaic processes. The recognition of such
sedimentary processes helps us to reconstruct the
environments. The sequence of interpretation in studies of older sedimentary rocks is thus: description of
sedimentary rock ! processes ! environment.
Applied geology has always been important, even
for purely scientific research. The interests of economic recovery of raw materials from sedimentary
rocks create a demand for sedimentologists and sedimentological research. Exploration for and recovery
of raw materials also provide important scientific
information. Sedimentary rocks contain raw materials
32
K. Bjørlykke