5.1 INTRODUCTION
This chapter describes the internal megascopic features of a sediment. These are termed
sedimentary structures, and are distinguished from the microscopic structural features
of a sediment, termed the fabric, described in Chapter 3. Sedimentary structures are
arbitrarily divided into primary and secondary classes. Primary structures are those
generated in a sediment during or shortly after deposition. They result mainly from
the physical processes described in Chapter 4. Examples of primary structures include
ripples, cross-bedding, and slumps. Secondary sedimentary structures are those that
formed sometime after sedimentation. They result from essentially chemical processes,
such as those which lead to the diagenetic formation of concretions. Primary sedimentary structures are divisible into inorganic structures, including those already mentioned, and organic structures, such as burrows, trails, and borings. Table 5.1 shows the
relationships between the various structures just defined. In common with most classifications of geological data, this one will not stand careful scrutiny. The divisions between the various groups are ill defined and debatable. Nevertheless, it provides a useful framework on which to build the analysis of sedimentary structures contained in this
chapter.
First, consider the definition of a sedimentary structure more carefully. Colloquially,
a sedimentary structure is deemed to be a primary depositional feature of a sediment
that is large enough to be seen by the naked eye, yet small enough to be carried by a
group of healthy students, or at least to be contained in one quarry. A channel is thus
generally considered to be a sedimentary structure -- just. A sand dune is generally considered a sedimentary structure; but what about an offshore bar or a barrier island that
is just a large and very complex dune? Surely a coral reef is a sedimentary structure?
But is it organic or inorganic? These questions highlight the problem of defining exactly
what is a sedimentary structure.
Conventionally a sedimentary structure is considered as a smaller scale feature that is
best illustrated by the examples of ripples, cross-beds, and slumps already mentioned.
This chapter is concerned with such phenomena, both inorganic sedimentary structures
and biogenic ones.
The observation, interpretation, and classification of inorganic sedimentary structures are considered first. Sedimentary structures can be studied at outcrop and in cliffs,
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This chapter describes the internal megascopic features of a sediment. These are termed
sedimentary structures, and are distinguished from the microscopic structural features
of a sediment, termed the fabric, described in Chapter 3. Sedimentary structures are
arbitrarily divided into primary and secondary classes. Primary structures are those
generated in a sediment during or shortly after deposition. They result mainly from
the physical processes described in Chapter 4. Examples of primary structures include
ripples, cross-bedding, and slumps. Secondary sedimentary structures are those that
formed sometime after sedimentation. They result from essentially chemical processes,
such as those which lead to the diagenetic formation of concretions. Primary sedimentary structures are divisible into inorganic structures, including those already mentioned, and organic structures, such as burrows, trails, and borings. Table 5.1 shows the
relationships between the various structures just defined. In common with most classifications of geological data, this one will not stand careful scrutiny. The divisions between the various groups are ill defined and debatable. Nevertheless, it provides a useful framework on which to build the analysis of sedimentary structures contained in this
chapter.
First, consider the definition of a sedimentary structure more carefully. Colloquially,
a sedimentary structure is deemed to be a primary depositional feature of a sediment
that is large enough to be seen by the naked eye, yet small enough to be carried by a
group of healthy students, or at least to be contained in one quarry. A channel is thus
generally considered to be a sedimentary structure -- just. A sand dune is generally considered a sedimentary structure; but what about an offshore bar or a barrier island that
is just a large and very complex dune? Surely a coral reef is a sedimentary structure?
But is it organic or inorganic? These questions highlight the problem of defining exactly
what is a sedimentary structure.
Conventionally a sedimentary structure is considered as a smaller scale feature that is
best illustrated by the examples of ripples, cross-beds, and slumps already mentioned.
This chapter is concerned with such phenomena, both inorganic sedimentary structures
and biogenic ones.
The observation, interpretation, and classification of inorganic sedimentary structures are considered first. Sedimentary structures can be studied at outcrop and in cliffs,
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