]54
Architectural Elements Formed Within Channels
Pla.t te-type macroform
A
B
South Platte-type macroform
slipface
v
channal
channt�l
Upper South Pia tte-type macroform
�L
Major slipface
Fig. 6.27. The macroform models of Crowley (1983). The
Platte-type macroform is comparable to the downstreamaccretion element described here. The South Platte-type
(notably their low depositional dip) are not emphasized, and they could be mistaken for bedfon;n
reactivation surfaces. Bristow { 1993) provided a
three-dimensional facies model for the bars of the
Brahmaputra River, based on natural cutbank outcrops along the modern channel, but the model only
relates to the upper few meters of these very large bar
forms (Fig. 6.31).
In the ancient record, detailed architectural studies rarely reveal simple ''textbook )) examples of
macroforms, such as those shown in the model diagrams in this section. Bar accretion and channel
o
25
so
m
SCALE
plan views not to scalu
macroform is comparable to the alternate bars described
in Sect. 6.7
scour can lead to changes in channel orientation,
resulting in complex accretionary geometries, commonly with superimposed minor channels scoured
out during stage changes. Miall ( 1994) documented a
suite of LA and DA units in the Upper Cretaceous
Castlegate Formation of Utah, which illustrate some
of the resulting complexities. An example is illustrated in Fig. 6.32.
In some cross sectional configurations certain
rypes of lateral-acretion deposit could be confused
with downstream-accretion elements, as noted in
the next section.