432
Conglomerates dominated by mass� flow deposits
(Fig. 12.1A) may be somewhat more likely to have
been deposited under arid climatic conditions, the
Trollheim fan being the extreme example (Blair and
McPherson 1992), but other criteria should be
sought to test a climatic interpretation. Paraglacial
alluvial fans may also contain signif'icant debris-flow
deposits. In most other humid environments lowerflow-regime stream-flow conglomerates with interbedded cross-bedded sandstones are dominant (Fig.
12.1B). As discussed in succeeding sections, the
sandstones, fi ne-grained sediments, and biochemical sediments that commonly are interbedded with
fans and braid deltas contain many other criteria,
none of which are definitive, but which, in combination, should provide reasonably certain indications
of broad climatic characteristics (Fig. 12.3, Tables
12.2, 12.3).
12.5.2 The Influence of Climate
on Texture and Composition of Gravels
Climatic variables affect not only the production and
transportation of clastic debris, but also the nature
of the debris itself (Basu 1985; Johnsson and Basu
1993). Styles of weathering and erosion are governed
largely by levels of water saturation, by temperature,
and by biochemical processes. Mechanical weathering is favored in cold and arid climates; chemical
weathering is more important in humid environments, especially in tropical settings, where organic
activity is at its most intense. These variations have
measureable effects on the petrology and texture of
gravel and sand in fluvial systems. Detrital sandstone composition (including the detrital matrix of
conglomerates) may Contain much climatic information. This is a specialized area of study that is
beyond the scope of this book, but some key points
are noted in this section.
Several workers have commented on the abundance of carbonate sedimentary clasts and sandsized detritus in deposits interpreted to have been
deposited in cold or arid climates, the inference
being that in humid climates chemical weathering
would have dissolved such material before significant transportation and accumulation could occur
(Miall 1970a; Besly 1988). Some arid braidplain and
playa-margin deposits consist largely of detrital
mud and sand fragments composed almost entirely
of carbonate, including hematite-stained calcareous
granules derived from reworking of soil nodules
(Wells 1983), or detrital limestone and dolomite
What Does Fluvial Lithofacies Reveal About Climate?
grains eroded from older sedimentary sources (Miall
et al. 1978). Stanley and Wayne (1972) found that in
the Pleistocene deposits of Nebraska, transport of
mechanically weak clast types (limestone, anorthosite cobbles, sandstone, hornblende schist, and mica
schist) was favored during glacial periods, when
chemical weathering was at a minimum and transport rates were rapid.
Darby et al. (1990) set out to determine the ways
in which gravels deposited in humid tropical environments might differ from those deposited in other
climatic settings, but they were unable to identify
any definitive facies criteria. However, they found
that deep tropical weathering of clasts produced
much in-place matrix, and clasts tended to be
reduced to ghosts. Evans (1991) attributed gravel
roundness to spheroidal weathering prior to erosion
and transportation. Jwaniw (1984) interpreted wellrounded clasts in his deposits as the product of
extreme weathering in a humid tropical climate.
Kochel and Johnson (1984) found that during the
long periods between depositional events on modern fans in Virginia unstable minerals tend to
weather, producing clay ftlms around the weathered
clasts and stable minerals. By contrast, in another
study of tropical alluvial fans, Brierley et al. (1993)
found no evidence of deep weathering of clasts and
no thick weathered zones, and suggested that this
reflects the reduced importance of deep chemical
weathering in the seasonally dry, savannah-type climate of Papua-New Guinea.
Periglacial and boreal environments are characterized by several distinctive petrological and textural fe atures. As .noted above, the chemically more
easily weathered clast ty-pes survive transportation
in greater abundance in dry or cold climates, where
rates of chemical weathering are lower (Stanley and
Wayne 1972). A composition-shape inversion may
develop in channel deposits, whereby resistant clasts
of plutonic and igneous rock retain their rounded
shapes, whereas softer sedimentary clasts are subject
to frost breakage and therefore tend to be angular
(Martini et al. 1993). Clay balls, including till balls,
are common in channel-lag deposits and in the
lenses of gravel that are commonly pushed by seasonal ice onto floodplains. Such heterolithic clasts
are commonly generated by the severe erosion that
occurs during jokulhlaups (Ashley et al. 1985; Martini et al. 1993). Casshyap and Tewari (1982) described large disrupted beds of diamictite in Late
Paleozoic fluvioglacial gravels in India, which they
interpreted as ice-rafted blocks of frozen morainic
material. Collinson (1971b) described distinctive
Conglomerates dominated by mass� flow deposits
(Fig. 12.1A) may be somewhat more likely to have
been deposited under arid climatic conditions, the
Trollheim fan being the extreme example (Blair and
McPherson 1992), but other criteria should be
sought to test a climatic interpretation. Paraglacial
alluvial fans may also contain signif'icant debris-flow
deposits. In most other humid environments lowerflow-regime stream-flow conglomerates with interbedded cross-bedded sandstones are dominant (Fig.
12.1B). As discussed in succeeding sections, the
sandstones, fi ne-grained sediments, and biochemical sediments that commonly are interbedded with
fans and braid deltas contain many other criteria,
none of which are definitive, but which, in combination, should provide reasonably certain indications
of broad climatic characteristics (Fig. 12.3, Tables
12.2, 12.3).
12.5.2 The Influence of Climate
on Texture and Composition of Gravels
Climatic variables affect not only the production and
transportation of clastic debris, but also the nature
of the debris itself (Basu 1985; Johnsson and Basu
1993). Styles of weathering and erosion are governed
largely by levels of water saturation, by temperature,
and by biochemical processes. Mechanical weathering is favored in cold and arid climates; chemical
weathering is more important in humid environments, especially in tropical settings, where organic
activity is at its most intense. These variations have
measureable effects on the petrology and texture of
gravel and sand in fluvial systems. Detrital sandstone composition (including the detrital matrix of
conglomerates) may Contain much climatic information. This is a specialized area of study that is
beyond the scope of this book, but some key points
are noted in this section.
Several workers have commented on the abundance of carbonate sedimentary clasts and sandsized detritus in deposits interpreted to have been
deposited in cold or arid climates, the inference
being that in humid climates chemical weathering
would have dissolved such material before significant transportation and accumulation could occur
(Miall 1970a; Besly 1988). Some arid braidplain and
playa-margin deposits consist largely of detrital
mud and sand fragments composed almost entirely
of carbonate, including hematite-stained calcareous
granules derived from reworking of soil nodules
(Wells 1983), or detrital limestone and dolomite
What Does Fluvial Lithofacies Reveal About Climate?
grains eroded from older sedimentary sources (Miall
et al. 1978). Stanley and Wayne (1972) found that in
the Pleistocene deposits of Nebraska, transport of
mechanically weak clast types (limestone, anorthosite cobbles, sandstone, hornblende schist, and mica
schist) was favored during glacial periods, when
chemical weathering was at a minimum and transport rates were rapid.
Darby et al. (1990) set out to determine the ways
in which gravels deposited in humid tropical environments might differ from those deposited in other
climatic settings, but they were unable to identify
any definitive facies criteria. However, they found
that deep tropical weathering of clasts produced
much in-place matrix, and clasts tended to be
reduced to ghosts. Evans (1991) attributed gravel
roundness to spheroidal weathering prior to erosion
and transportation. Jwaniw (1984) interpreted wellrounded clasts in his deposits as the product of
extreme weathering in a humid tropical climate.
Kochel and Johnson (1984) found that during the
long periods between depositional events on modern fans in Virginia unstable minerals tend to
weather, producing clay ftlms around the weathered
clasts and stable minerals. By contrast, in another
study of tropical alluvial fans, Brierley et al. (1993)
found no evidence of deep weathering of clasts and
no thick weathered zones, and suggested that this
reflects the reduced importance of deep chemical
weathering in the seasonally dry, savannah-type climate of Papua-New Guinea.
Periglacial and boreal environments are characterized by several distinctive petrological and textural fe atures. As .noted above, the chemically more
easily weathered clast ty-pes survive transportation
in greater abundance in dry or cold climates, where
rates of chemical weathering are lower (Stanley and
Wayne 1972). A composition-shape inversion may
develop in channel deposits, whereby resistant clasts
of plutonic and igneous rock retain their rounded
shapes, whereas softer sedimentary clasts are subject
to frost breakage and therefore tend to be angular
(Martini et al. 1993). Clay balls, including till balls,
are common in channel-lag deposits and in the
lenses of gravel that are commonly pushed by seasonal ice onto floodplains. Such heterolithic clasts
are commonly generated by the severe erosion that
occurs during jokulhlaups (Ashley et al. 1985; Martini et al. 1993). Casshyap and Tewari (1982) described large disrupted beds of diamictite in Late
Paleozoic fluvioglacial gravels in India, which they
interpreted as ice-rafted blocks of frozen morainic
material. Collinson (1971b) described distinctive