438
information regarding climate. Lenses of gravel may
be deposited in areas close to the channel by rare
flood events (Costa 1974) or by ice push (Martini et
al. 1993).
The most common detrital sediments on floodplains in all climatic settings are fine silts and muds.
These are normally finely laminated, but commonly
the lamination is disturbed by pedogenesis, by roots,
or by invertebrate bioturbation. The style oflamination in these fine-grained clastics normally reveals
nothing about climate, although Broadhurst (1988)
developed an argument suggesting that laminated
siltstone-claystone couplets in a Carboniferous succession in Britain are seasonal in origin. He recorded
some 45 couplets in 9 m of section (average couplet
thickness 20 em). The siltstone-claystone units are
interpreted as lacustrine· or overbank deposits, and
interbedded saridstones as distal ·crevasse-splays. A
bivalve colony shows evidence ofbeing buried by the
deposits and not replenished. Given the 20-year average life span of modern bivalves, the persistence of
the colony through eight or nine of the overlying
couplets suggests that the couplets may be seasonal.
The sedimentation rate calculated from this evidence is 30 em/year, which Broadhurst (1988) regarded as high, but in fact it is in agreement with the
average rate for seasonal deposits (group 5 of the
depositional hierarchy, Chap. 3). These deposits correlate with and are interbedded with coals, which
indicates permanent saturation, and which appears
to conflict with the seasonality suggested by the couplets. Broadhurst (1988) suggested that the coals
were probably fo rmed in permanent swamps, while
the rivers were affected by seasonal discharge fluctuations.
In many arid systems fi ne-grained sediments
deposited on distal interfluve mudflats are interbedded with lacustrine or playa margin sediments
(Figs. 7.27, 7.28). Layers of very fine grained sandstone and siltstone may display a millimeter-scale
lamination and commonly show graded bedding,
indicating rapid deceleration of flow into very shallow standing bodies of water (Smoot 1983; Flint
1985; Mertz and Hubert 1990; Hartley 1993). Evaporite crystallization shortly after deposition may
disrupt bedding (Mertz and Hubert 1990). Couplets
of interbedded clastic and chemical sediments
may reflect seasonal variations in precipitation,
evaporation, and teinperature, as in the siltstonecarbonate cycles in parts of the Chinle Formation
(Triassic) of the Colorado Plateau (Dubiel et a!.
1991). Clastic-chemical cycles also develop in lakemargin settings under the influence of longer-term
climatic rhythms, as discussed in a later section.
What Does Fluvial Lithofacies Reveal About Climate?
Several types of minor sedimentary structure
carry useful climatic signatures (Fig. 12.3). Desiccation features are common in hot, arid fluvial systems, including large mudcracks and mud curls on
bedding planes (Miall et al. 1978; Tunbridge 1981,
1984; Rust and Legun 1983; Smoot 1983; Flint 1985;
Stear 1985; Glennie 1987; Mertz and Hubert 1990;
Hartley 1993). Cracks may be up to at least a meter
deep and a decimeter or more in width. Even larger
cracks, up to 1.5 m deep, have been recorded in
evaporitic eolian sandstones (Kocurek and Hunter
1986). They are similar to ice-wedge casts formed in
frigid climates (Bryant 1983), and both processes
may form polygonal crack networks on beddingplane surfaces. Supplementary evidence might be
needed to differentiate such features. Rip-up clasts
and intraclast breccias are very common features of
hot, arid environments (even in high intertidal settings) because of. the rapid lithification of freshly
deposited muds caused by desiccation (Smoot 1983;
Stear 1985).
Parts of the arid Lake Eyre drainage basin of
Australia are characterized by an unusual combina�
tion of fluvial styles. A series of seasonally active
anastomosed channels containing permanent pools
of water cross a broad system of shallow braided
channels which appeared to Rust (1981) to be relict
channels dating from an earlier, wetter, climatic
phase. This change in fluvial style has, in fact, long
been regarded as a classic example of "river meta�
morphosis" in response to climate change (Schumm
1969). However, detailed studies by Nanson et al.
(1986) and Rust and Nanson (1989) showed that the
braided channels and intervening bars are composed of mud aggregates forming sand-sized particles that retain their coherence during fluvial
transport and fu nction as sand grains. The high
"sand" load causes the rivers to develop the braided
patterll during flOods. The day aggregates are interpreted as peds fo rmed by volume changes of the
floodplain mud in response to wetting and drying
cycles in the arid to semiarid climate. Earlier work
had suggested that the presence of salts is required
for aggregate fo rmation, but Nanson et al. (1986)
disputed this, pointing out that the evaporite concentration is low. Flume experiments indicated
that mud aggregates can form flow-regime type
bedforms, and it was suggested that mud beds in the
ancient record should be reexamined, as some may
be mud braids, not floodplain deposits (Rust and
Nanson 1989). In the modern Lake Eyre basin the
anastomosed channels are deeper channels which
have cut through lower levels in the muds, where the
mud has greater cohesion, and into an underlying
information regarding climate. Lenses of gravel may
be deposited in areas close to the channel by rare
flood events (Costa 1974) or by ice push (Martini et
al. 1993).
The most common detrital sediments on floodplains in all climatic settings are fine silts and muds.
These are normally finely laminated, but commonly
the lamination is disturbed by pedogenesis, by roots,
or by invertebrate bioturbation. The style oflamination in these fine-grained clastics normally reveals
nothing about climate, although Broadhurst (1988)
developed an argument suggesting that laminated
siltstone-claystone couplets in a Carboniferous succession in Britain are seasonal in origin. He recorded
some 45 couplets in 9 m of section (average couplet
thickness 20 em). The siltstone-claystone units are
interpreted as lacustrine· or overbank deposits, and
interbedded saridstones as distal ·crevasse-splays. A
bivalve colony shows evidence ofbeing buried by the
deposits and not replenished. Given the 20-year average life span of modern bivalves, the persistence of
the colony through eight or nine of the overlying
couplets suggests that the couplets may be seasonal.
The sedimentation rate calculated from this evidence is 30 em/year, which Broadhurst (1988) regarded as high, but in fact it is in agreement with the
average rate for seasonal deposits (group 5 of the
depositional hierarchy, Chap. 3). These deposits correlate with and are interbedded with coals, which
indicates permanent saturation, and which appears
to conflict with the seasonality suggested by the couplets. Broadhurst (1988) suggested that the coals
were probably fo rmed in permanent swamps, while
the rivers were affected by seasonal discharge fluctuations.
In many arid systems fi ne-grained sediments
deposited on distal interfluve mudflats are interbedded with lacustrine or playa margin sediments
(Figs. 7.27, 7.28). Layers of very fine grained sandstone and siltstone may display a millimeter-scale
lamination and commonly show graded bedding,
indicating rapid deceleration of flow into very shallow standing bodies of water (Smoot 1983; Flint
1985; Mertz and Hubert 1990; Hartley 1993). Evaporite crystallization shortly after deposition may
disrupt bedding (Mertz and Hubert 1990). Couplets
of interbedded clastic and chemical sediments
may reflect seasonal variations in precipitation,
evaporation, and teinperature, as in the siltstonecarbonate cycles in parts of the Chinle Formation
(Triassic) of the Colorado Plateau (Dubiel et a!.
1991). Clastic-chemical cycles also develop in lakemargin settings under the influence of longer-term
climatic rhythms, as discussed in a later section.
What Does Fluvial Lithofacies Reveal About Climate?
Several types of minor sedimentary structure
carry useful climatic signatures (Fig. 12.3). Desiccation features are common in hot, arid fluvial systems, including large mudcracks and mud curls on
bedding planes (Miall et al. 1978; Tunbridge 1981,
1984; Rust and Legun 1983; Smoot 1983; Flint 1985;
Stear 1985; Glennie 1987; Mertz and Hubert 1990;
Hartley 1993). Cracks may be up to at least a meter
deep and a decimeter or more in width. Even larger
cracks, up to 1.5 m deep, have been recorded in
evaporitic eolian sandstones (Kocurek and Hunter
1986). They are similar to ice-wedge casts formed in
frigid climates (Bryant 1983), and both processes
may form polygonal crack networks on beddingplane surfaces. Supplementary evidence might be
needed to differentiate such features. Rip-up clasts
and intraclast breccias are very common features of
hot, arid environments (even in high intertidal settings) because of. the rapid lithification of freshly
deposited muds caused by desiccation (Smoot 1983;
Stear 1985).
Parts of the arid Lake Eyre drainage basin of
Australia are characterized by an unusual combina�
tion of fluvial styles. A series of seasonally active
anastomosed channels containing permanent pools
of water cross a broad system of shallow braided
channels which appeared to Rust (1981) to be relict
channels dating from an earlier, wetter, climatic
phase. This change in fluvial style has, in fact, long
been regarded as a classic example of "river meta�
morphosis" in response to climate change (Schumm
1969). However, detailed studies by Nanson et al.
(1986) and Rust and Nanson (1989) showed that the
braided channels and intervening bars are composed of mud aggregates forming sand-sized particles that retain their coherence during fluvial
transport and fu nction as sand grains. The high
"sand" load causes the rivers to develop the braided
patterll during flOods. The day aggregates are interpreted as peds fo rmed by volume changes of the
floodplain mud in response to wetting and drying
cycles in the arid to semiarid climate. Earlier work
had suggested that the presence of salts is required
for aggregate fo rmation, but Nanson et al. (1986)
disputed this, pointing out that the evaporite concentration is low. Flume experiments indicated
that mud aggregates can form flow-regime type
bedforms, and it was suggested that mud beds in the
ancient record should be reexamined, as some may
be mud braids, not floodplain deposits (Rust and
Nanson 1989). In the modern Lake Eyre basin the
anastomosed channels are deeper channels which
have cut through lower levels in the muds, where the
mud has greater cohesion, and into an underlying