94
cross-bedded sandstone or conglomerate lithofacies as element SB or GB, respectively, whereas
better exposure of the internal geometry of the
beds may lead to the recognition that the unit is
actually part of a macroform, such as DA or LA.
2. Interbedding: Elements may be interbedded at
the scale of the individual lithofacies set or coset.
For example, alluvial fanglomerates, comprising
stacked GB elements, commonly contain thin
stringers of cross-bedded sandstone (element
SB). Many sandstone macroforms are draped by
thin mudstone lenses, representing abandonment or preservation of FF deposits. At what
point is element subdivision cut off and generalized? This is a common problem in facies definition, and the solution depends only on practical
criteria of data availability and descriptive convenience.
3. Intergradation: Elements tend to grade laterally
into each other. For example, a mid-channel sand
flat in a braided system may accrete laterally
along its flanks and in a downstream direction at
its .downstream end. Therefore, a DA unit may
pass laterally along strike into an LA unit, or the
same transition may occur down depositional
dip, indicating an evolution of macroform geometry with time (Fig. 6.24c).
It is not suggested that the classifications offered
in Tables 4.3 and 4.4 should be rigidly followed in all
future fluvial research. Some other classifications
that have been published are noted above. However,
it is claimed that the method of systematic observation and classification required to construct such
descriptive frameworks should be an integral part of
future research endeavours. Other researchers may
wish to construct their own classifications and
codes. However, it is incumbent on all researchers,
where they modify an existing nomenclature for
their own purposes_ , to explain and justify such revisions. Use of the same lithofacies and element codes
and bounding-surface rank numbers in ways different from those proposed by earlier workers can lead
to unnecessary confusion.
4.7 Classification of Channels
and Larger Bodies
Lithosomes bounded by fifth-order surfaces, corresponding to individual channel fills and channel-fill
complexes, may be classified using the proposals by
Methods of Architectural-Element Analysis
Friend et al. (1979) and Friend (1983). According to
these authors, there are essentially three types of
fifth-order lithosomes: sheet sandstones, formed by
non channelized flow, for example during ephemeral
flash floods; fixed channels, where the channel is
laterally stable; and mobile-channel belts, where a
broad channel or channel complex occupies a fixed
position for a lengthy period of time, and develops
minor channels and bars within a broadly defined
basal scour surface. Mobile channels may evolve by
lateral migration or in-channel avulsion, or a combination of the two processes. Fixed channels give rise
to ribbon sandstones, and Friend et al. (1979) proposed that these be defmed as those linear sandstone
bodies having a width/depth ratio of less than 15.
Sheet sandstones and mobile-channel-belt sandstones have ratios greater than 15) and are corn�
monly in excess of 100.
Sheet sandstones formed by sheet floods commonly have flat bounding surfaces showing little
evidence of erosion. This type of sheet sandstone,
and ribbon sandstones formed bY aggradation of
flXed channels) commonly have simple fills,_in which
lithosomes and bounding surfaces of third- and
fourth-order are absent. By contrast, sheet sandstones formed as the fill of mobile channels typically
have complex fills composed of the deposits of bar
complexes and minor channels.
Channel stacking patterns depend on such factors
as total sediment load, channel migration, and
switching rates, and basin subsidence rate (Chap.
10). Friend ( 1983) suggested that where the sediment
load is dominated by coarse, bed�load materials,
overbank areas would be poorly developed and
channel fills would be superimposed directly, with
floodplain deposits thin or absent. By contrast, a
significant suspension load may lead to significant
deposition of floodplain successions, and the
isolation of channel fills from each other. These
variations led Friend (1983) to propose a sixfold
classification of fifth-order lithosomes (Fig. 2.35),
although no examples of stacked fixed channels
had been identified at the time he proposed his
classification (nor has such a depositional pattern
been identified since, according to the writer's
knowledge),
The description and classification of the channels
and their component elements are part of the basis
for the erection of local ''summaries of the environment", or overall fades models for particular fluvial
styles. This process, and the results, are the subject of
Chap. 8. However, it is worth pointing out, at this
stage, that such interpretations are by no means
cross-bedded sandstone or conglomerate lithofacies as element SB or GB, respectively, whereas
better exposure of the internal geometry of the
beds may lead to the recognition that the unit is
actually part of a macroform, such as DA or LA.
2. Interbedding: Elements may be interbedded at
the scale of the individual lithofacies set or coset.
For example, alluvial fanglomerates, comprising
stacked GB elements, commonly contain thin
stringers of cross-bedded sandstone (element
SB). Many sandstone macroforms are draped by
thin mudstone lenses, representing abandonment or preservation of FF deposits. At what
point is element subdivision cut off and generalized? This is a common problem in facies definition, and the solution depends only on practical
criteria of data availability and descriptive convenience.
3. Intergradation: Elements tend to grade laterally
into each other. For example, a mid-channel sand
flat in a braided system may accrete laterally
along its flanks and in a downstream direction at
its .downstream end. Therefore, a DA unit may
pass laterally along strike into an LA unit, or the
same transition may occur down depositional
dip, indicating an evolution of macroform geometry with time (Fig. 6.24c).
It is not suggested that the classifications offered
in Tables 4.3 and 4.4 should be rigidly followed in all
future fluvial research. Some other classifications
that have been published are noted above. However,
it is claimed that the method of systematic observation and classification required to construct such
descriptive frameworks should be an integral part of
future research endeavours. Other researchers may
wish to construct their own classifications and
codes. However, it is incumbent on all researchers,
where they modify an existing nomenclature for
their own purposes_ , to explain and justify such revisions. Use of the same lithofacies and element codes
and bounding-surface rank numbers in ways different from those proposed by earlier workers can lead
to unnecessary confusion.
4.7 Classification of Channels
and Larger Bodies
Lithosomes bounded by fifth-order surfaces, corresponding to individual channel fills and channel-fill
complexes, may be classified using the proposals by
Methods of Architectural-Element Analysis
Friend et al. (1979) and Friend (1983). According to
these authors, there are essentially three types of
fifth-order lithosomes: sheet sandstones, formed by
non channelized flow, for example during ephemeral
flash floods; fixed channels, where the channel is
laterally stable; and mobile-channel belts, where a
broad channel or channel complex occupies a fixed
position for a lengthy period of time, and develops
minor channels and bars within a broadly defined
basal scour surface. Mobile channels may evolve by
lateral migration or in-channel avulsion, or a combination of the two processes. Fixed channels give rise
to ribbon sandstones, and Friend et al. (1979) proposed that these be defmed as those linear sandstone
bodies having a width/depth ratio of less than 15.
Sheet sandstones and mobile-channel-belt sandstones have ratios greater than 15) and are corn�
monly in excess of 100.
Sheet sandstones formed by sheet floods commonly have flat bounding surfaces showing little
evidence of erosion. This type of sheet sandstone,
and ribbon sandstones formed bY aggradation of
flXed channels) commonly have simple fills,_in which
lithosomes and bounding surfaces of third- and
fourth-order are absent. By contrast, sheet sandstones formed as the fill of mobile channels typically
have complex fills composed of the deposits of bar
complexes and minor channels.
Channel stacking patterns depend on such factors
as total sediment load, channel migration, and
switching rates, and basin subsidence rate (Chap.
10). Friend ( 1983) suggested that where the sediment
load is dominated by coarse, bed�load materials,
overbank areas would be poorly developed and
channel fills would be superimposed directly, with
floodplain deposits thin or absent. By contrast, a
significant suspension load may lead to significant
deposition of floodplain successions, and the
isolation of channel fills from each other. These
variations led Friend (1983) to propose a sixfold
classification of fifth-order lithosomes (Fig. 2.35),
although no examples of stacked fixed channels
had been identified at the time he proposed his
classification (nor has such a depositional pattern
been identified since, according to the writer's
knowledge),
The description and classification of the channels
and their component elements are part of the basis
for the erection of local ''summaries of the environment", or overall fades models for particular fluvial
styles. This process, and the results, are the subject of
Chap. 8. However, it is worth pointing out, at this
stage, that such interpretations are by no means