Chapter 6
Archi tectllral Elements Formed Within Chamtels
6.1 Introduction
A ba�ic set of eight elements was recognized by Miall
(1985), and has been updated, as noted below. The
purpose of this chapter is to provide a brief summary
description and key illustrations of the main element
types. Further descriptions are provided in the casestudies that illustrate the discussion of fluvial style in
Chap. 8.
Most of these elements are macroforms which, in
the definition of jackson (1975), are the product of
the cumulative effect of sedimentation over periods
of tens to thousands of years. They include major
channels and bars (such as point bars, side bars, sand
flats, and islands), flood sheets, and sedimentgravity-flow lobes. It is the plan view of these macroform elements that generates the familiar channel
styles, so commonly illustrated by low-level aerial
photographs of modern rivers.
Macro forms are "group 6n deposits, in the classification of Table 3.2. In principle, they are bounded
by fo urth-order surfaces (Table 4.2). However,
where an element rests on the floor of a channel its
base is defined by the base of the channel, which is a
surface of fifth-order rank or higher (e.g., sequence
boundary of seventh order). In addition, elements
are commonly truncated by overlying channels, in
which case their upper bounding surface is the fifthorder surface at the base of that channel. Fourthorder surfaces are, for this reason, commonly not
preserved. Where they are present, they may commonly be recognized by their convex-up shape
which, in part, parallels stratification in the underlying beds, and indicates that they are surfaces of
accretion. Large channel-fill sheets may consist of
stacked macroforms separated by fourth-order surfaces. All the elements may contain internal growth
increments, defined by third-order surfaces. Typically, these show evidence of cut-and-fill erosional
relief. They may rest on mud drapes capping the
underlying element, and they may be followed by
poorly sorted beds forming the base of the next
element, such as gravel lags.
Bristow and Best (1993, p. 2) summarized the
complexities in channel-fill sedimentation that may
occur during changing stage:
"Where bars exist for periods of time in excess of a single
flood event they will experience a complex history of erosional and depositional modifications related to changes
in stage. At higher flow stages when the largest volumes of
sediment are tranported, the channels are often scoured,
bars may be reduced in height or in some cases completely
eroded. However, during fal ling stage maximum deposition occurs as discharge and flow competence are reduced.
Channel beds aggrade, the high stage bedforms may be
modified and new bars may be formed or enlarged as
sediment is deposited. As discharge continues to fall, bars
may become emergent and dissected by low stage channels. Additionally, the nature of the falling limb recession
(rate and length of recession) will be important not only in
the reworking of higher stage sediments, but also in the
deposition and spatial distribution of the finer grained
sediments (silts and clays) ... "
Modifications in the original element scheme of
Miall (1985) are as follows: The proposed code for
midchannel macroforms that accrete downstream
was changed from FM (foreset macroform) to DA
(downstream accretion) by Miall (1988a) because
the old code seemed to lead to confusion, and because DA is comparable to LA, in terms of the ele�
ment code and the element it defmes. A new
in-channel element, HO, is introduced here, bringing the total to nine. These are listed in Chap. 4
(Table 4.3). Figure 4.13 is a summary illustration of
the eight in-channel gravel and sand elements that
are the focus of this chapter. Examples of a typical
range of macroforms and other architectural elements in a sheet-braided fluvial deposit are illustrated in Fig. 6.1.
·
6.2 Channels (Element CH)
Most coarse (gravel, sand) deposits in fluvial systems
are deposited in channels, therefore designation of a
channel element in an ancient unit is not a particularly significant step forward. On outcrop profiles,
use of the CH coding is useful only where the chan-
Archi tectllral Elements Formed Within Chamtels
6.1 Introduction
A ba�ic set of eight elements was recognized by Miall
(1985), and has been updated, as noted below. The
purpose of this chapter is to provide a brief summary
description and key illustrations of the main element
types. Further descriptions are provided in the casestudies that illustrate the discussion of fluvial style in
Chap. 8.
Most of these elements are macroforms which, in
the definition of jackson (1975), are the product of
the cumulative effect of sedimentation over periods
of tens to thousands of years. They include major
channels and bars (such as point bars, side bars, sand
flats, and islands), flood sheets, and sedimentgravity-flow lobes. It is the plan view of these macroform elements that generates the familiar channel
styles, so commonly illustrated by low-level aerial
photographs of modern rivers.
Macro forms are "group 6n deposits, in the classification of Table 3.2. In principle, they are bounded
by fo urth-order surfaces (Table 4.2). However,
where an element rests on the floor of a channel its
base is defined by the base of the channel, which is a
surface of fifth-order rank or higher (e.g., sequence
boundary of seventh order). In addition, elements
are commonly truncated by overlying channels, in
which case their upper bounding surface is the fifthorder surface at the base of that channel. Fourthorder surfaces are, for this reason, commonly not
preserved. Where they are present, they may commonly be recognized by their convex-up shape
which, in part, parallels stratification in the underlying beds, and indicates that they are surfaces of
accretion. Large channel-fill sheets may consist of
stacked macroforms separated by fourth-order surfaces. All the elements may contain internal growth
increments, defined by third-order surfaces. Typically, these show evidence of cut-and-fill erosional
relief. They may rest on mud drapes capping the
underlying element, and they may be followed by
poorly sorted beds forming the base of the next
element, such as gravel lags.
Bristow and Best (1993, p. 2) summarized the
complexities in channel-fill sedimentation that may
occur during changing stage:
"Where bars exist for periods of time in excess of a single
flood event they will experience a complex history of erosional and depositional modifications related to changes
in stage. At higher flow stages when the largest volumes of
sediment are tranported, the channels are often scoured,
bars may be reduced in height or in some cases completely
eroded. However, during fal ling stage maximum deposition occurs as discharge and flow competence are reduced.
Channel beds aggrade, the high stage bedforms may be
modified and new bars may be formed or enlarged as
sediment is deposited. As discharge continues to fall, bars
may become emergent and dissected by low stage channels. Additionally, the nature of the falling limb recession
(rate and length of recession) will be important not only in
the reworking of higher stage sediments, but also in the
deposition and spatial distribution of the finer grained
sediments (silts and clays) ... "
Modifications in the original element scheme of
Miall (1985) are as follows: The proposed code for
midchannel macroforms that accrete downstream
was changed from FM (foreset macroform) to DA
(downstream accretion) by Miall (1988a) because
the old code seemed to lead to confusion, and because DA is comparable to LA, in terms of the ele�
ment code and the element it defmes. A new
in-channel element, HO, is introduced here, bringing the total to nine. These are listed in Chap. 4
(Table 4.3). Figure 4.13 is a summary illustration of
the eight in-channel gravel and sand elements that
are the focus of this chapter. Examples of a typical
range of macroforms and other architectural elements in a sheet-braided fluvial deposit are illustrated in Fig. 6.1.
·
6.2 Channels (Element CH)
Most coarse (gravel, sand) deposits in fluvial systems
are deposited in channels, therefore designation of a
channel element in an ancient unit is not a particularly significant step forward. On outcrop profiles,
use of the CH coding is useful only where the chan-