Classification of Bounding Surfaces
define paleocurrent north as the top of the diagram,
so that, for example, arrows pointing horizontally to
the left indicate flow due west (azimuth 270°). In
some cases, it renders the profile more easy to read if
the paleocurrent data are oriented with respect to the
outcrop face. Readings parallel to the face then appear as horizontal arrows pointing left or right, flow
directly into the face is shown as a vertical arrow
pointing upward, and so on. The second method has
the advantage that the reader can more easily integrate the architecture of the face and the corresponding paleocurrent readings when assessing the
profile visually. The disadvantage comes when it is
necessary to compare profiles taken from outcrops
that have different orientations. Comparisons between paleocurrent directions in different outcrops
are then not as obvious visually.
Summary paleocurrent rose diagrams may usefully be added to the profile, if space permits. A rose
for each element is a particularly useful addition.
Summary statistics can be prepared as a table for
inclusion in the text.
The use of paleocurrent analysis as a mapping
tool is discussed in Sect. 9.5.7.
4.5 Classification of Bounding Surfaces
The principles of the hierarchical classification of
depositional units are detailed in Chap. 3. In this
section, it is shown how the ideas can be applied
more specifically to fluvial deposits. A version of
Table 3.2 that fo cuses on fluvial deposits is included
here as Table 4.2, and incorporates a column summarizing the nine major types of bedding and
bounding surface that characterize fluvial deposits.
Most of these types are illustrated in Figs. 4.2 and 4.3.
The method was developed initially fo:r sandstone
deposits, but further work by S.A. Smith (1990) and
Soegaard (1990, 1992) has demonstrated the applicability of the classification to conglomerates as well.
DeCelles et al. (1991) used the system, with some
modifications, to study alluvial-fan deposits. As S.A.
Smith (1990) pointed out, the textural and structural
monotony of conglomerates may make recognition
and correlation of bounding surfaces more difficult
in these types of deposit.
An .l lternative classification of depositional units
and bounding surfaces was erected by Bridge
(1993a), but based on very similar principles to those
discussed here. A comparison between the two nomenclature schemes is provided below, and a discus81
sion of the differences between the schemes was
given by Miall (1995a).
Bridge
Jvliall
microscale set (e.g., ripple)
mesoscale set (e.g., dune)
micro/mesoscale coset
macroscale inclined
stratum
macroscale inclined
strata set
group of macroscale sets
group of macroscale sets
1st-order unit
1st-order unit
2nd-order unit
3rd-order unit (macroform increment)
4th-order unit
(macro fo rm)
5th-order unit
(channel)
6th-order unit (e.g.,
channel-belt)
A question of scale arises in the use of such classifications, because rivers and their dep osits vary
enormously in their size. However, Bristow and Best
(1993) have pointed out that rivers show a considerable degree of self-similarity over the complete range
of scales (see also Sect. 8.3), and the classification
described here can be applied to rivers of all scales
providing due attention is paid to river size. For
example, Fig. 4.4 illustrates the range of depositional
scales in the Mississippi, one of the world's largest
rivers. It is instructive to compare this with the subdivisions of alluvial fans shown in Fig. 4.3. One of the
challenges in the study of ancient rocks is to determine the scale of the rivers responsible for forming
the deposits1 based on architectural studies of the
preserved record.
First- and second-order surfaces record boundaries within microform and meso form deposits. The
definition of first-order surfaces is unchanged from
Allen (1983a). They represent cross-bed set bounding surfaces (Fig. 4.5). Little or no internal erosion is
apparent at these boundaries, and they represent the
virtually continuous sedimentation of trains of
bedforms of similar type. Subtle modifications in
attitude, with minor erosion, may be caused by reactivation following stage changes (Collinson 1970), or
may be the result of changes in bedform orientation
(Haszeldine 1983b). In core, these surfaces may not
be very prominent, but the presence of a bounding
surface can be recognized by truncation or wedgeout of cross-bed fo resets.
Second-order surfaces are simple coset bounding
surfaces, in the sense of McKee 8.nd Weir (1953).
These indicate changes in flow conditions, or a
change in flow direction1 but no significant time
break (Fig. 4.6). Lithofacies above and below the
define paleocurrent north as the top of the diagram,
so that, for example, arrows pointing horizontally to
the left indicate flow due west (azimuth 270°). In
some cases, it renders the profile more easy to read if
the paleocurrent data are oriented with respect to the
outcrop face. Readings parallel to the face then appear as horizontal arrows pointing left or right, flow
directly into the face is shown as a vertical arrow
pointing upward, and so on. The second method has
the advantage that the reader can more easily integrate the architecture of the face and the corresponding paleocurrent readings when assessing the
profile visually. The disadvantage comes when it is
necessary to compare profiles taken from outcrops
that have different orientations. Comparisons between paleocurrent directions in different outcrops
are then not as obvious visually.
Summary paleocurrent rose diagrams may usefully be added to the profile, if space permits. A rose
for each element is a particularly useful addition.
Summary statistics can be prepared as a table for
inclusion in the text.
The use of paleocurrent analysis as a mapping
tool is discussed in Sect. 9.5.7.
4.5 Classification of Bounding Surfaces
The principles of the hierarchical classification of
depositional units are detailed in Chap. 3. In this
section, it is shown how the ideas can be applied
more specifically to fluvial deposits. A version of
Table 3.2 that fo cuses on fluvial deposits is included
here as Table 4.2, and incorporates a column summarizing the nine major types of bedding and
bounding surface that characterize fluvial deposits.
Most of these types are illustrated in Figs. 4.2 and 4.3.
The method was developed initially fo:r sandstone
deposits, but further work by S.A. Smith (1990) and
Soegaard (1990, 1992) has demonstrated the applicability of the classification to conglomerates as well.
DeCelles et al. (1991) used the system, with some
modifications, to study alluvial-fan deposits. As S.A.
Smith (1990) pointed out, the textural and structural
monotony of conglomerates may make recognition
and correlation of bounding surfaces more difficult
in these types of deposit.
An .l lternative classification of depositional units
and bounding surfaces was erected by Bridge
(1993a), but based on very similar principles to those
discussed here. A comparison between the two nomenclature schemes is provided below, and a discus81
sion of the differences between the schemes was
given by Miall (1995a).
Bridge
Jvliall
microscale set (e.g., ripple)
mesoscale set (e.g., dune)
micro/mesoscale coset
macroscale inclined
stratum
macroscale inclined
strata set
group of macroscale sets
group of macroscale sets
1st-order unit
1st-order unit
2nd-order unit
3rd-order unit (macroform increment)
4th-order unit
(macro fo rm)
5th-order unit
(channel)
6th-order unit (e.g.,
channel-belt)
A question of scale arises in the use of such classifications, because rivers and their dep osits vary
enormously in their size. However, Bristow and Best
(1993) have pointed out that rivers show a considerable degree of self-similarity over the complete range
of scales (see also Sect. 8.3), and the classification
described here can be applied to rivers of all scales
providing due attention is paid to river size. For
example, Fig. 4.4 illustrates the range of depositional
scales in the Mississippi, one of the world's largest
rivers. It is instructive to compare this with the subdivisions of alluvial fans shown in Fig. 4.3. One of the
challenges in the study of ancient rocks is to determine the scale of the rivers responsible for forming
the deposits1 based on architectural studies of the
preserved record.
First- and second-order surfaces record boundaries within microform and meso form deposits. The
definition of first-order surfaces is unchanged from
Allen (1983a). They represent cross-bed set bounding surfaces (Fig. 4.5). Little or no internal erosion is
apparent at these boundaries, and they represent the
virtually continuous sedimentation of trains of
bedforms of similar type. Subtle modifications in
attitude, with minor erosion, may be caused by reactivation following stage changes (Collinson 1970), or
may be the result of changes in bedform orientation
(Haszeldine 1983b). In core, these surfaces may not
be very prominent, but the presence of a bounding
surface can be recognized by truncation or wedgeout of cross-bed fo resets.
Second-order surfaces are simple coset bounding
surfaces, in the sense of McKee 8.nd Weir (1953).
These indicate changes in flow conditions, or a
change in flow direction1 but no significant time
break (Fig. 4.6). Lithofacies above and below the