!52
large Sp, St sets.
c.
5
D.
Fig. 6;25. Ancient examples of DA elements showing the
distinctive, internal, downstream-dipping bounding surfaces, down which trains of medium-scale bedforms migrated. Letters are lithofacies codes (Chap. 5}, numerals
are rank of bounding surfaces (Chap. 4). A Carboniferous
sandstones (e.g., Figs. 4.8, 4.15), a fact that appeared
to be missed by those focusing on vertical-proflle
studies.
The essential characteristic of a downstreamaccretion element are that it consists of several
(possibly many) cosets of downstream-oriented
flow-regime bedforms dynamically related to each
other by a hierarchy of internal downstream-dipping bounding surfaces (Figs. 6.24, 6.25). These reveal the former existence of an active, nonperiodic,
possibly irregularly shaped bar form comparable in
height and width to the channel in which it formed.
The bars contain second- and third-order surfaces
that generally dip gently ( < 10") downstream (Banks
1973; Allen 1983a; Haszeldine 1983a,b; Kirk 1983;
.
Miall 1988a,c, 1992a, 1994), although oblique or
gentle upstream dips, around and over low-relief bar
cores (sand shoal of Allen 1983a) may also be present. Between these surfaces are sets or cosets of Sp,
St, Sh, Sl, or Sr. The Sh and Sl laminae are organized
parallel or subparallel to the internal bounding surfaces. Detailed paleocurrent studies show that the
flow-regime bedforms advance generally down the
slopes defined by the second- and third-order surfaces (Haszeldine 1983a,b) or oblique to the surfaces
draping the bar cores (Allen 1983a; Mia!! 1994).
These data reveal a picture of fields of bedforms
driving across, around and down the bar forms (Fig.
6.24c).
Most sandstone elements are accumulated by
both vertical aggradation and lateral accretion. In
DA and LA elements, the evidence of accretion is
Architectural Elements Formed Within Channels
0
10m
�
example, England (Haszeldine 1983a,b); B Poison Canyon
Sandstone, Morrison Formation, New Mexico (Miall
1988a); C Example from Kayenta Formation, Colorado
(Miall 1988c); D part of a "sand wave" in the modern
Brahmaputra River (Coleman 1969). (Miall 1988a)
obvious, and forms the main basis for the definition
of the element, but in some cases the accretionary
geometry may be very subtle and difficult to define
in small outcrops. Figure 6.21 illustrates a crevasse�
splay sandstone sheet that contains some evi9.ence of
lateral progradation, yet is classified here in the SB
element because of the dominance of vertical aggradation. For more precise work, use of Gibling and
Rust's (1990) "aggradation index" may be useful to
assist in discriminating betwen the two styles of
accumulation.
It has been shown that midchannel macroforms
may accrete downstream at their downstream ends
and laterally along their flanks (Bristow 1987, 1993).
Upstream accretion on the upstream flank has also
been recorded (Bristow 1987, 1993). Preserved
macroforms may therefore grade laterally from DA
to LA architecture (Allen 1983a; Miall 1993, 1994), as
suggested in the accompanying model diagram (Fig.
6.24c). Care must be taken in the interpretation of
two-dimensional outcrops to distinguish these geometries, using all available paleocurrent informa �
tion. It is suggested that where a gradation between
the LA and DA end members can be documented a
cutoff be employed as follows (Fig. 6.24c). Where the
orientation of the accretion surface and that of the
cross�bedding within the same element are within
about 60° of each other, it indicates that the element
grew by accretion in a direction parallel or oblique to
local flow, and the element is designated a DA unit,
even if local flow is oriented at a high angle to the
regional trend. Where the orientations of the accre-
large Sp, St sets.
c.
5
D.
Fig. 6;25. Ancient examples of DA elements showing the
distinctive, internal, downstream-dipping bounding surfaces, down which trains of medium-scale bedforms migrated. Letters are lithofacies codes (Chap. 5}, numerals
are rank of bounding surfaces (Chap. 4). A Carboniferous
sandstones (e.g., Figs. 4.8, 4.15), a fact that appeared
to be missed by those focusing on vertical-proflle
studies.
The essential characteristic of a downstreamaccretion element are that it consists of several
(possibly many) cosets of downstream-oriented
flow-regime bedforms dynamically related to each
other by a hierarchy of internal downstream-dipping bounding surfaces (Figs. 6.24, 6.25). These reveal the former existence of an active, nonperiodic,
possibly irregularly shaped bar form comparable in
height and width to the channel in which it formed.
The bars contain second- and third-order surfaces
that generally dip gently ( < 10") downstream (Banks
1973; Allen 1983a; Haszeldine 1983a,b; Kirk 1983;
.
Miall 1988a,c, 1992a, 1994), although oblique or
gentle upstream dips, around and over low-relief bar
cores (sand shoal of Allen 1983a) may also be present. Between these surfaces are sets or cosets of Sp,
St, Sh, Sl, or Sr. The Sh and Sl laminae are organized
parallel or subparallel to the internal bounding surfaces. Detailed paleocurrent studies show that the
flow-regime bedforms advance generally down the
slopes defined by the second- and third-order surfaces (Haszeldine 1983a,b) or oblique to the surfaces
draping the bar cores (Allen 1983a; Mia!! 1994).
These data reveal a picture of fields of bedforms
driving across, around and down the bar forms (Fig.
6.24c).
Most sandstone elements are accumulated by
both vertical aggradation and lateral accretion. In
DA and LA elements, the evidence of accretion is
Architectural Elements Formed Within Channels
0
10m
�
example, England (Haszeldine 1983a,b); B Poison Canyon
Sandstone, Morrison Formation, New Mexico (Miall
1988a); C Example from Kayenta Formation, Colorado
(Miall 1988c); D part of a "sand wave" in the modern
Brahmaputra River (Coleman 1969). (Miall 1988a)
obvious, and forms the main basis for the definition
of the element, but in some cases the accretionary
geometry may be very subtle and difficult to define
in small outcrops. Figure 6.21 illustrates a crevasse�
splay sandstone sheet that contains some evi9.ence of
lateral progradation, yet is classified here in the SB
element because of the dominance of vertical aggradation. For more precise work, use of Gibling and
Rust's (1990) "aggradation index" may be useful to
assist in discriminating betwen the two styles of
accumulation.
It has been shown that midchannel macroforms
may accrete downstream at their downstream ends
and laterally along their flanks (Bristow 1987, 1993).
Upstream accretion on the upstream flank has also
been recorded (Bristow 1987, 1993). Preserved
macroforms may therefore grade laterally from DA
to LA architecture (Allen 1983a; Miall 1993, 1994), as
suggested in the accompanying model diagram (Fig.
6.24c). Care must be taken in the interpretation of
two-dimensional outcrops to distinguish these geometries, using all available paleocurrent informa �
tion. It is suggested that where a gradation between
the LA and DA end members can be documented a
cutoff be employed as follows (Fig. 6.24c). Where the
orientation of the accretion surface and that of the
cross�bedding within the same element are within
about 60° of each other, it indicates that the element
grew by accretion in a direction parallel or oblique to
local flow, and the element is designated a DA unit,
even if local flow is oriented at a high angle to the
regional trend. Where the orientations of the accre-