422
dence may be complex, however, because sedimentation at any one location in a basin may be affected
by both local and regional factors. For example, the
channel and floodplain processes of tributary rivers
may reflect local microclimates, which may vary
across a basin (e.g., rain-shadow effects), whereas a
trunk river may bear the discharge and sedimentload signatures of a distant source area that may be
characterized by a quite different climate (see also
Sect. I 1.3.1). These and other complexities are introduced below and are discussed in Sect. 12.10.
12.2 Climatic Variables
What is dimate, a:hd how might it be-reco. rded in
terrigenous detrital sediments? The two most important attributes of climate are rainfall and temperature. Wind, its strength, direction, and consistency,
are important climatic attributes in the study of
eolian deposits, but not of other nonmarine sediments. The production, dispersal, and deposition of
terrigenous detrital sediment depends on water, and
therefore on rainfall, its surface runoff in fluvial
channels, and in overbank flow. Both the amount of
rainfall and its distribution with time (e.g., seasonal,
monsoonal, fl ashy) are climatic factors that have an
important influence on sedimentation. Temperature is important as a control on the physical behavior of water freezing, thawing, and evaporation.
Temperature also controls the rate of various chemical reactions that affect syndepositional diagenesis.
Rainfall and temperature together control the
amount and type of vegetation, which influences
depositional styles in channels and overbank areas
and, through biochemical processes, the type of
weathering and erosion processes in fluvial source
areas, thereby affecting sediment yield.
Climatologists have classified global climates in
many different ways, recognizing up to as many as 14
different climate types that reflect the ranges of variability and seasonality of temperature and rainfall
on the present-day earth (Huggett 1991; Barty and
Chorley 1992), but most of these subtleties are only
crudely represented in the lithofacies record.
Perlmutter and Matthews (1989) used the following
temperature categories in their discussion of
Milankovitch forcing: tropical, temperate, and polar, and the following rainfall categories: very humid,
humid, sub humid, dry, arid. Mack and )ames (1994)
proposed a simple climatic differentiation of the
earth related tq latitudinal position and discussed
What Does Fluvial Lithofacies Reveal About Climate?
how paleosol distribution is related to this classification. Their four broad climatic zones are wet equatorial, dry subtropical, moist midlatitude, and polar.
Complications in this zonation are caused by the
regional distribution of land and water masses, and
by topographic effects.
From the point of view of the preservability of
different bedform styles, jones (1977) recognized
five basic types of fluvial discharge pattern:
1. Perennial rivers with relatively steady discharge
and a single maximum peak annual flood; common in temperate climates, e.g., Rhine
2. Perennial rivers with double or many maximum
discharge events; common in tropical and temperate regions with seasonal variations in rainfall,
e.g., Platte
3. Perennial rivers with a pronounced low-discharge period and a long fl ood period; common
in tropical areas with monsoons, e.g., Brahmaputra
4. Perennial rivers with pronounced low-discharge
period and short flood period; common in highlatitude rivers and those draining alpine mountains, where there is a short spring flood period
and a long period of slowly tapering summer
discharge, e.g. Tana, Norway; sandur rivers in
Iceland and Alaska
5. Ephemeral rivers with flashy discharge; characteristic of arid regions
These categories cannot yet be resolved in terms of
the facies styles described in Chap. 8. Given the obvious relationships between discharge regime, fluvial
style, and climate discussed in these paragraphs, it
is perhaps surprising that climate is not singled
out as a significant controlling parameter in the
major geomorphic classification of fluvial floodplains proposed by Nanson and Croke (1992), although several variables affected by climate are
discussed, such as the style of flooding and the influence of vegetation.
In this chapter the following broad climatic zonations are used, based on features observable from the
lithofacies record:
-H ot-arid
-H ot-humid
- Tropical to semitropical with wet-dry seasonality
- Temperate-humid
- Boreal-paraglacial
There are several complications involved in determining climate from the fluvial sedimentary
record, as follows:
dence may be complex, however, because sedimentation at any one location in a basin may be affected
by both local and regional factors. For example, the
channel and floodplain processes of tributary rivers
may reflect local microclimates, which may vary
across a basin (e.g., rain-shadow effects), whereas a
trunk river may bear the discharge and sedimentload signatures of a distant source area that may be
characterized by a quite different climate (see also
Sect. I 1.3.1). These and other complexities are introduced below and are discussed in Sect. 12.10.
12.2 Climatic Variables
What is dimate, a:hd how might it be-reco. rded in
terrigenous detrital sediments? The two most important attributes of climate are rainfall and temperature. Wind, its strength, direction, and consistency,
are important climatic attributes in the study of
eolian deposits, but not of other nonmarine sediments. The production, dispersal, and deposition of
terrigenous detrital sediment depends on water, and
therefore on rainfall, its surface runoff in fluvial
channels, and in overbank flow. Both the amount of
rainfall and its distribution with time (e.g., seasonal,
monsoonal, fl ashy) are climatic factors that have an
important influence on sedimentation. Temperature is important as a control on the physical behavior of water freezing, thawing, and evaporation.
Temperature also controls the rate of various chemical reactions that affect syndepositional diagenesis.
Rainfall and temperature together control the
amount and type of vegetation, which influences
depositional styles in channels and overbank areas
and, through biochemical processes, the type of
weathering and erosion processes in fluvial source
areas, thereby affecting sediment yield.
Climatologists have classified global climates in
many different ways, recognizing up to as many as 14
different climate types that reflect the ranges of variability and seasonality of temperature and rainfall
on the present-day earth (Huggett 1991; Barty and
Chorley 1992), but most of these subtleties are only
crudely represented in the lithofacies record.
Perlmutter and Matthews (1989) used the following
temperature categories in their discussion of
Milankovitch forcing: tropical, temperate, and polar, and the following rainfall categories: very humid,
humid, sub humid, dry, arid. Mack and )ames (1994)
proposed a simple climatic differentiation of the
earth related tq latitudinal position and discussed
What Does Fluvial Lithofacies Reveal About Climate?
how paleosol distribution is related to this classification. Their four broad climatic zones are wet equatorial, dry subtropical, moist midlatitude, and polar.
Complications in this zonation are caused by the
regional distribution of land and water masses, and
by topographic effects.
From the point of view of the preservability of
different bedform styles, jones (1977) recognized
five basic types of fluvial discharge pattern:
1. Perennial rivers with relatively steady discharge
and a single maximum peak annual flood; common in temperate climates, e.g., Rhine
2. Perennial rivers with double or many maximum
discharge events; common in tropical and temperate regions with seasonal variations in rainfall,
e.g., Platte
3. Perennial rivers with a pronounced low-discharge period and a long fl ood period; common
in tropical areas with monsoons, e.g., Brahmaputra
4. Perennial rivers with pronounced low-discharge
period and short flood period; common in highlatitude rivers and those draining alpine mountains, where there is a short spring flood period
and a long period of slowly tapering summer
discharge, e.g. Tana, Norway; sandur rivers in
Iceland and Alaska
5. Ephemeral rivers with flashy discharge; characteristic of arid regions
These categories cannot yet be resolved in terms of
the facies styles described in Chap. 8. Given the obvious relationships between discharge regime, fluvial
style, and climate discussed in these paragraphs, it
is perhaps surprising that climate is not singled
out as a significant controlling parameter in the
major geomorphic classification of fluvial floodplains proposed by Nanson and Croke (1992), although several variables affected by climate are
discussed, such as the style of flooding and the influence of vegetation.
In this chapter the following broad climatic zonations are used, based on features observable from the
lithofacies record:
-H ot-arid
-H ot-humid
- Tropical to semitropical with wet-dry seasonality
- Temperate-humid
- Boreal-paraglacial
There are several complications involved in determining climate from the fluvial sedimentary
record, as follows: