Late ralHAccretion Deposits (Element LA)
Fig. 6.38. View across a modern point-bar surface, looking upstream. A field of 2-D dunes that was migrating
toward the viewer has been abandoned by falling water.
The point bar is accreting toward the left Note that the
ration eddies at sharp bends (Figs. 2.37, 6.40). For
modern examples1 see: Nanson (1980), Jackson
(1981); for ancient examples: Miall (!979a), Stewart
(1983), D.G. Smith (1987), Wood (1989).
LA deposits do not retain a constant geometry or
composition around any given meander bend. As a
result, the classic fi ning-upward profile (Allen 1964,
1965a,b) may not be present. In gravelly rivers, Bluck
(1971), Lewin (1976), and Bridge and Jarvis (1976)
showed that the coarsest part of the point bar is
located at the upstream end of the bar (bar head)
and may migrate downstream over sandy bar tail
deposits. Wood (1989) interpreted facies variations
in some ancient point-bar deposits on this basis.
jackson (1976b) found that in the Wabash River
(sand and pebbly sand) the helical flow patterns
responsible for the fining-upward point bar profile
tend to develop only in the downstream part of a
meander bend (Fig. 6.4!). Carson (1986) made a
similar observation on gravel-bed rivers in New
Zealand. Nanson and Page (1983) showed that
within tight meanders flow separation may occur at
the downstream end of a point bar. Eddy currents
161
crest lines of these dunes are oriented down the dip of the
point-bar accretion surface. (Photo courtesy of M.O.
Hayes and R. Levey)
there form significant deposits of fine sand, silt, and
mud in "concave bench complexes" (Figs. 2.37, 6.40).
In some muddy rivers, it has been found that accretionary development of mud, silt, and fine sand may
take place on both sides of the channel. The deposits
on the inside of the meander bend are referred to as
an "inner accretionary bank." This element type is
common in some of the muddy, ephemeral rivers of
interior Australia (Taylor and Woodyer 1978; M.R.
Gibling, pers. comm. 1995). A detailed study of the
ridge-and-swale topography that develops in the uppermost levels of point bars was provided by Gibling
and Rust (1993). They described variations in some
exhumed Carboniferous examples, and related the
variations to slight differences in the style of scrollbar accretion.
Variations in the style of meander·evolution (Fig.
6.34) lead to subtle but important differences in the
internal architecture of the resulting LA deposit.
These variations were described by Willis (1989),
who used a computer model to simulate various
meander configurations. Figure 6.42 illustrates his
cross sections through a meander that evolves by
Fig. 6.38. View across a modern point-bar surface, looking upstream. A field of 2-D dunes that was migrating
toward the viewer has been abandoned by falling water.
The point bar is accreting toward the left Note that the
ration eddies at sharp bends (Figs. 2.37, 6.40). For
modern examples1 see: Nanson (1980), Jackson
(1981); for ancient examples: Miall (!979a), Stewart
(1983), D.G. Smith (1987), Wood (1989).
LA deposits do not retain a constant geometry or
composition around any given meander bend. As a
result, the classic fi ning-upward profile (Allen 1964,
1965a,b) may not be present. In gravelly rivers, Bluck
(1971), Lewin (1976), and Bridge and Jarvis (1976)
showed that the coarsest part of the point bar is
located at the upstream end of the bar (bar head)
and may migrate downstream over sandy bar tail
deposits. Wood (1989) interpreted facies variations
in some ancient point-bar deposits on this basis.
jackson (1976b) found that in the Wabash River
(sand and pebbly sand) the helical flow patterns
responsible for the fining-upward point bar profile
tend to develop only in the downstream part of a
meander bend (Fig. 6.4!). Carson (1986) made a
similar observation on gravel-bed rivers in New
Zealand. Nanson and Page (1983) showed that
within tight meanders flow separation may occur at
the downstream end of a point bar. Eddy currents
161
crest lines of these dunes are oriented down the dip of the
point-bar accretion surface. (Photo courtesy of M.O.
Hayes and R. Levey)
there form significant deposits of fine sand, silt, and
mud in "concave bench complexes" (Figs. 2.37, 6.40).
In some muddy rivers, it has been found that accretionary development of mud, silt, and fine sand may
take place on both sides of the channel. The deposits
on the inside of the meander bend are referred to as
an "inner accretionary bank." This element type is
common in some of the muddy, ephemeral rivers of
interior Australia (Taylor and Woodyer 1978; M.R.
Gibling, pers. comm. 1995). A detailed study of the
ridge-and-swale topography that develops in the uppermost levels of point bars was provided by Gibling
and Rust (1993). They described variations in some
exhumed Carboniferous examples, and related the
variations to slight differences in the style of scrollbar accretion.
Variations in the style of meander·evolution (Fig.
6.34) lead to subtle but important differences in the
internal architecture of the resulting LA deposit.
These variations were described by Willis (1989),
who used a computer model to simulate various
meander configurations. Figure 6.42 illustrates his
cross sections through a meander that evolves by