L
Associated Clastic, Chemical, and Biochemical Sediments
sand bed. Ancient examples of pedogenic mud aggregates have now been described by Rust and
Nanson (1989), Ekes (1993), and Talbot et a!. (1994).
12.8 The Significance of Color
The significance of color in nonmarine sediments
has been the subject of much debate (Van Houten
1973; Wallcer 1967; Turner 1980) that will not be
repeated here. Drab colors (gray, green, brown) are
generated largely by ferrous oxides or carbonates
(e.g., siderite) in reducing environments, whereas
red, yellow and orange are created by ferric oxides,
notably hematite, in oxidizing environments.
Studies in hot arid environments and in humid
tropical regions show that red beds can be generated
by diagenetic processes following deposition
(Walker 1967, 1974). Yellow and brown iron oxides,
in initial stages of hematite production, are present
above the water table in the modern alluvium of the
Colorado River on the arid shores of the Gulf of
California, and well below the water table in alluvium in Puerto Rico, under hot, humid conditions.
Therefore, red beds are not necessarily an indication
of arid desert climates, as has long been thought.
Turner (1980) summarized mineralogical, diagenetic, and grain-size data relating to post-depositional generation of red-bed coloration, drawing on
the work of Walker. He distinguished between (a) a
desert-evaporite red-bed association in which red
beds are associated with eolian sands, desert fluvial
sediments, and evaporites formed in playas and inland sabkhas, and (b) a moist-climate red-bed association in which red beds are interbedded and
interfinger with coal-bearing strata.
The oxidation state is commonly set during early
diagenesis rather than at the time of deposition, and
so caution must be used in making environmental
interpretations from color. However, many red units
contain evidence of an arid or semiarid climate, such
as pedogenic carbonates and facies associations indicating ephemeral deposition, whereas drab units
are typically associated with coals or carbonaceous
shales, indicating saturated environments of deposition and a humid climate. The preservation of organic matter is a function of organic productivity,
burial rate, the depth of the water table, and the rate
of groundwater movement.
The main attribute controlling color differences
is the level of the water table. An oxidizing environment is favored by a low water table, such as occurs
in arid climates and those with seasonal dry periods.
439
Such a setting favors pedogenic carbonate development (Retallack 1981, 1986). A reducing environment occurs in saturated environments and is
necessary for the preservation of plant matter. These
differences are commonly preserved following
deposition. In fact, different facies associations with
the expected contrasting colors may be interbedded
within the same basin fill, indicating variations in
water table across the basin during or shortly after
deposition (e.g., Besly 1988). The existence of these
complex color patterns also indicates that, in such
cases at least, colors are set during early diagenesis
and not subsequently modified.
Although colors reflect levels of water saturation,
the climatic implications of this may not be easily
determined, especially where saturation levels vary
across a Pasin. This is discussed further in a later
section.
12.9 Associated Cl astic, Chemical,
and Biochemical Sediments
Many other nonmarine deposits constitute much
clearer climatic indicators than do.fluviallithofacies.
Lakes are sensitive indicators of climate because of
the delicate balance that commonly exists between
climate and the water level and chemistry of lakes
(Katz 1990). Large-scale eolian depositional systems
may indicate aridity, and large ancient ergs preserve
evidence of persistent wind patterns that may be
related to the patterns of global atmospheric circulation (Parrish and Peterson 1988). Chemical and
biochemical sediments that occur in interfluve envi�
ronments, particularly paleosols, coal, and evapor�
ites, also contain useful signatures of the climate at
the time of deposition. It is not the purpose of this
chapter to review these climatic indicators at length,
most of which have been extensively reviewed elsewhere. However, some brief comments are included
here in order to provide the basis for an integrated
interpretation of climate from an ancient alluvial
suite.
12.9.1 Coal
McCabe (1984) stated:
"The necessary Conditions for peat accumulation can ... be
regarded as a balance between plant production and organic decay. Both are a function of climate. Plant production is fastest in hot humid areas and is slowest in dry
or cold areas. Organic decay is principally a fu nction of
Associated Clastic, Chemical, and Biochemical Sediments
sand bed. Ancient examples of pedogenic mud aggregates have now been described by Rust and
Nanson (1989), Ekes (1993), and Talbot et a!. (1994).
12.8 The Significance of Color
The significance of color in nonmarine sediments
has been the subject of much debate (Van Houten
1973; Wallcer 1967; Turner 1980) that will not be
repeated here. Drab colors (gray, green, brown) are
generated largely by ferrous oxides or carbonates
(e.g., siderite) in reducing environments, whereas
red, yellow and orange are created by ferric oxides,
notably hematite, in oxidizing environments.
Studies in hot arid environments and in humid
tropical regions show that red beds can be generated
by diagenetic processes following deposition
(Walker 1967, 1974). Yellow and brown iron oxides,
in initial stages of hematite production, are present
above the water table in the modern alluvium of the
Colorado River on the arid shores of the Gulf of
California, and well below the water table in alluvium in Puerto Rico, under hot, humid conditions.
Therefore, red beds are not necessarily an indication
of arid desert climates, as has long been thought.
Turner (1980) summarized mineralogical, diagenetic, and grain-size data relating to post-depositional generation of red-bed coloration, drawing on
the work of Walker. He distinguished between (a) a
desert-evaporite red-bed association in which red
beds are associated with eolian sands, desert fluvial
sediments, and evaporites formed in playas and inland sabkhas, and (b) a moist-climate red-bed association in which red beds are interbedded and
interfinger with coal-bearing strata.
The oxidation state is commonly set during early
diagenesis rather than at the time of deposition, and
so caution must be used in making environmental
interpretations from color. However, many red units
contain evidence of an arid or semiarid climate, such
as pedogenic carbonates and facies associations indicating ephemeral deposition, whereas drab units
are typically associated with coals or carbonaceous
shales, indicating saturated environments of deposition and a humid climate. The preservation of organic matter is a function of organic productivity,
burial rate, the depth of the water table, and the rate
of groundwater movement.
The main attribute controlling color differences
is the level of the water table. An oxidizing environment is favored by a low water table, such as occurs
in arid climates and those with seasonal dry periods.
439
Such a setting favors pedogenic carbonate development (Retallack 1981, 1986). A reducing environment occurs in saturated environments and is
necessary for the preservation of plant matter. These
differences are commonly preserved following
deposition. In fact, different facies associations with
the expected contrasting colors may be interbedded
within the same basin fill, indicating variations in
water table across the basin during or shortly after
deposition (e.g., Besly 1988). The existence of these
complex color patterns also indicates that, in such
cases at least, colors are set during early diagenesis
and not subsequently modified.
Although colors reflect levels of water saturation,
the climatic implications of this may not be easily
determined, especially where saturation levels vary
across a Pasin. This is discussed further in a later
section.
12.9 Associated Cl astic, Chemical,
and Biochemical Sediments
Many other nonmarine deposits constitute much
clearer climatic indicators than do.fluviallithofacies.
Lakes are sensitive indicators of climate because of
the delicate balance that commonly exists between
climate and the water level and chemistry of lakes
(Katz 1990). Large-scale eolian depositional systems
may indicate aridity, and large ancient ergs preserve
evidence of persistent wind patterns that may be
related to the patterns of global atmospheric circulation (Parrish and Peterson 1988). Chemical and
biochemical sediments that occur in interfluve envi�
ronments, particularly paleosols, coal, and evapor�
ites, also contain useful signatures of the climate at
the time of deposition. It is not the purpose of this
chapter to review these climatic indicators at length,
most of which have been extensively reviewed elsewhere. However, some brief comments are included
here in order to provide the basis for an integrated
interpretation of climate from an ancient alluvial
suite.
12.9.1 Coal
McCabe (1984) stated:
"The necessary Conditions for peat accumulation can ... be
regarded as a balance between plant production and organic decay. Both are a function of climate. Plant production is fastest in hot humid areas and is slowest in dry
or cold areas. Organic decay is principally a fu nction of