Chapter 5
Lithofacies
5.1 !ntroductio11
Most fluvial deposits are clastic. A simple tripartite
subdivision into gravel, sand, and fine-grained
(mud, silt, and very fine-grained sand) lithofacies is
a useful first step 'in description and classification.
Most beds may be classified readily into one or the
other of these groups, reflecting a natural separation
of processes and sorting of the sediment load. Some
mixed lithofacies, such as pebbly sandstones, do oc�
cur but can normally be classified according to their
dominant grain-size class.
This chapter is intended priffiarily as descriptive.
Other texts, such as Middleton and Southard (1977),
Collinson and Thompson (1982), and Allen (1984),
deal with the mechanics of sediment transport and
the theory of bedform generation in some depth, and
it is not the purpose of the present book to duplicate
this material. Some of the major recent advances in
this area are touched on in Chap. 2. Recent work on
bedform genesis and classification (especially
Ashley 1990) is discussed in this chapter only where
it provides a useful genetic underpinning for the
lithofacies classification. The major lithofacies types
are listed in Table 4.1. Most were initially proposed
by Miall (1977, 1978c); references to other definitions are given where appropriate.
5.2 Gravel Facies
5.2.1 Depositional Processes in Gravel-Bed Rivers
5.2.1.1 Introduction
Gravel transport takes place under a wide spectrum
of physical conditions and leads to a range of textural and structural variations in the ensuing deposits. The major controlling factors are the condition
of the flow, either laminar or turbulent, and the
sediment concentration. Specific depositional re·
sponses (lithofacies) are associated with particular
processes, but individual flows may evolve from one
set of conditions to another, so that the final deposit
may reveal a range of textures and structures 'Within
one lithesome. Figure 5.1 illustrates a model that
attempts to summarize the relationships between
the various processes and their results.
The main differentiation is between two types of
process: (1) flows that are turbulent and have low
sediment concentrations, in which transport and
deposition take place by traction and very limited
suspension, and (2) flows in which the higher vis·
cosities associated with high sediment concentra·
tions dampen turbulence, leading to laminar flow
and grain support by buoyancy. This second class of
event is given the general term sediment-gravity
flow.
Useful work on gravel sedimentation by traction
currents and the resulting lithofacies has been car·
ried out in modern rivers by Williams and Rust
(1969), Rust (1972), N.D. Smith (1974), Boothroyd
and Ashley (1975), Lewin (1976), Hein and Walker
(1977), Allen (1983c), Southard et al. (1984), and
Carling (1990). Ashmore (1991) carried out a very
useful series of model experiments. Sediment-gravity flows are locally important, particularly in alluvial fan settings, and have been studied by
Blissenbach (1954), Bluck (1967), johnson (1970),
Larsen and Steel (1978), Schultz (1984), Blair ( 1987),
Maizels (1989, 1993), Hubert and Filipov (1989), and
Blair and McPherson (1992, 1994).
5.2.7.2 Traction Currents, Fluid Flows
The processes of gravel transport and gravel bedform, channel and bar formation are less well under·
stood than those of sand-bed rivers. The reasons for
this are partly observational. It is much more difficult to observe gravel-bed rivers during active bedload transport because the high transport energies
make in-place measurement and sampling hazardous, because direct observations may be prevented
by the opaqueness of the water, which commonly
Lithofacies
5.1 !ntroductio11
Most fluvial deposits are clastic. A simple tripartite
subdivision into gravel, sand, and fine-grained
(mud, silt, and very fine-grained sand) lithofacies is
a useful first step 'in description and classification.
Most beds may be classified readily into one or the
other of these groups, reflecting a natural separation
of processes and sorting of the sediment load. Some
mixed lithofacies, such as pebbly sandstones, do oc�
cur but can normally be classified according to their
dominant grain-size class.
This chapter is intended priffiarily as descriptive.
Other texts, such as Middleton and Southard (1977),
Collinson and Thompson (1982), and Allen (1984),
deal with the mechanics of sediment transport and
the theory of bedform generation in some depth, and
it is not the purpose of the present book to duplicate
this material. Some of the major recent advances in
this area are touched on in Chap. 2. Recent work on
bedform genesis and classification (especially
Ashley 1990) is discussed in this chapter only where
it provides a useful genetic underpinning for the
lithofacies classification. The major lithofacies types
are listed in Table 4.1. Most were initially proposed
by Miall (1977, 1978c); references to other definitions are given where appropriate.
5.2 Gravel Facies
5.2.1 Depositional Processes in Gravel-Bed Rivers
5.2.1.1 Introduction
Gravel transport takes place under a wide spectrum
of physical conditions and leads to a range of textural and structural variations in the ensuing deposits. The major controlling factors are the condition
of the flow, either laminar or turbulent, and the
sediment concentration. Specific depositional re·
sponses (lithofacies) are associated with particular
processes, but individual flows may evolve from one
set of conditions to another, so that the final deposit
may reveal a range of textures and structures 'Within
one lithesome. Figure 5.1 illustrates a model that
attempts to summarize the relationships between
the various processes and their results.
The main differentiation is between two types of
process: (1) flows that are turbulent and have low
sediment concentrations, in which transport and
deposition take place by traction and very limited
suspension, and (2) flows in which the higher vis·
cosities associated with high sediment concentra·
tions dampen turbulence, leading to laminar flow
and grain support by buoyancy. This second class of
event is given the general term sediment-gravity
flow.
Useful work on gravel sedimentation by traction
currents and the resulting lithofacies has been car·
ried out in modern rivers by Williams and Rust
(1969), Rust (1972), N.D. Smith (1974), Boothroyd
and Ashley (1975), Lewin (1976), Hein and Walker
(1977), Allen (1983c), Southard et al. (1984), and
Carling (1990). Ashmore (1991) carried out a very
useful series of model experiments. Sediment-gravity flows are locally important, particularly in alluvial fan settings, and have been studied by
Blissenbach (1954), Bluck (1967), johnson (1970),
Larsen and Steel (1978), Schultz (1984), Blair ( 1987),
Maizels (1989, 1993), Hubert and Filipov (1989), and
Blair and McPherson (1992, 1994).
5.2.7.2 Traction Currents, Fluid Flows
The processes of gravel transport and gravel bedform, channel and bar formation are less well under·
stood than those of sand-bed rivers. The reasons for
this are partly observational. It is much more difficult to observe gravel-bed rivers during active bedload transport because the high transport energies
make in-place measurement and sampling hazardous, because direct observations may be prevented
by the opaqueness of the water, which commonly