450
12.12.3 Conclusions
The question remains whether high-frequency climatic forcing, on the 105-year time scale, will be
detectable in the pre-Pleistocene stratigraphic re�
cord. To detect change on such a frequency requires
stratigraphic mapping on at least as refined a time
scale. This scale of periodicity corresponds to the
cyclicity of many major geomorphic processes, such
as the return period of major channels in large-scale
fluvial systems (sediment groups 8 and 9 of Table
9.1}, and is at or below the resolving power of most
chronostratigraphic correlation methods, including
magnetostratigraphy.
12.13 Discussion
This review has demonstrated that in very few fluvial
settings are lithofacies criteria sufficiently distinctive that they can be relied upon, in the absence of
supplementary evidence, to indicate climatiC setting.
Probably the most climatically distinctive lithofacies
assemblage is that of the sandy terminal fan ( ephemeral braid delta of Blair and McPherson 1992), which
is characteristic of arid basin settings. The downstream change fr om channelized to sheet flow and
the decrease in sand body thickness and the scale of
hydrodynamic sedimentary structures are particularly characteristic. The architecture of the distal
deposits, consisting of sandstone sheets composed
of flood cycles and interbedded with fine-grained
deposits showing evidence of evaporation, is very
distinctive (Fig. 12.2B). Most other lithofacies and
architectural features of fluvial deposits have been
shown to be nondiagnostic of climate. Mass-flow
deposits and other upper fl ow-regime fluvial sediments, while very characteristic of alluvial fans in
hot-arid climates (Fig. 12.1A), are not confined to
this climatic setting (Table 12.1), although it is possible that their abundance, and therefore their preservation potential, may be higher in this type of
setting. A quantitative testing of this possibility has
yet to be attempted.
However, a consideration of every aspect of a
fluvial assemblage, including features of the overbank environment such as chemical and biochemical sediments, soils, minor sedimentary structures,
and bed color, may yield useful clues as to climate.
Conflicting criteria may indicate that the climate of
the basin center is different from that of its margin,
or that at least the water table is at a different level.
What Does Fluvial Lithofacies Reveal About Climate?
Climates may also be different on either side of a
major mountain range. The major criteria are summarized in Tables 12.2 and 12.3, and Figs. 12.1-12.3
are schematic diagrams oflithofacies assemblages in
which climatically sensitive criteria are identified.
As noted earlier in this chapter, the evolution of
land vegetation has probably had a profound effect
on fluvial styles. The lack of roots to bind bank
sediments in pre-Devonian time would have led to a
tendency for rapid bank erosion and channel widening, with the supply of abundant bedload to the
system. This would have favored braided over other
fluvial styles, regardless of the discharge regime.
Long (1978) considered Schumm's (1968a) speculations on this matter, in his review of Proterozoic
fluvial sedimentation. He confirmed that rivers with
an interpreted meandering style are indeed rare in
the Precambrian sedimentary record, but much
work has been carried out on Precambrian sedimentation since the late 1970s, and this subject should be
reviewed again. In the meantime, the climatic ambiguity of fluvial lithofucies, and the absence of coal
and of much other relevant fossil material means
pre-Devonian climates are very difficult to interpret
from the fluvial record.
It may be possible to make quite subtle climatic
interpretations based on a summation of all the
available evidence. For example, Hallam (1984) defined an ('intermediate" type of climate, which shows
evidence of both aridity and humidity. The sediments contain ((thin seams of lignite or coal, modest
in thickness and localised in distribution, and minor
evaporites, such as scattered thin gypsum lenses and
seams, [these] could both form in intermediate,
savannah-type environments where wet and dry sea�
sons are of roughly equal importance." The coexist�
ence of trees and evaporites indicates a savannah or
Mediterranean-type climate with alternating wet
and dry seasons.
In a general discussion of the distribution of
various climatic indicators, especially coal, Ziegler et
al. (1987) provided this interpretation of the Nubian
Sandstone of North Africa, based on many sources:
Cross-bedding type and paleocurrent variance are
interpreted in terms of braided or low-sinuosity
streams. Flood activity is indicated by repetition of
channel-based scour surfaces and their lag deposits.
Prevalence of soil horizons and common preservation of flora suggest well-vegetated alluvial plains,
but the common tree Dadoxylon was probably
restricted to channel margins and swamps. Floral
diversity is not high, and the ubiquitous braided
stream deposits would seem to rule out the possibil�
12.12.3 Conclusions
The question remains whether high-frequency climatic forcing, on the 105-year time scale, will be
detectable in the pre-Pleistocene stratigraphic re�
cord. To detect change on such a frequency requires
stratigraphic mapping on at least as refined a time
scale. This scale of periodicity corresponds to the
cyclicity of many major geomorphic processes, such
as the return period of major channels in large-scale
fluvial systems (sediment groups 8 and 9 of Table
9.1}, and is at or below the resolving power of most
chronostratigraphic correlation methods, including
magnetostratigraphy.
12.13 Discussion
This review has demonstrated that in very few fluvial
settings are lithofacies criteria sufficiently distinctive that they can be relied upon, in the absence of
supplementary evidence, to indicate climatiC setting.
Probably the most climatically distinctive lithofacies
assemblage is that of the sandy terminal fan ( ephemeral braid delta of Blair and McPherson 1992), which
is characteristic of arid basin settings. The downstream change fr om channelized to sheet flow and
the decrease in sand body thickness and the scale of
hydrodynamic sedimentary structures are particularly characteristic. The architecture of the distal
deposits, consisting of sandstone sheets composed
of flood cycles and interbedded with fine-grained
deposits showing evidence of evaporation, is very
distinctive (Fig. 12.2B). Most other lithofacies and
architectural features of fluvial deposits have been
shown to be nondiagnostic of climate. Mass-flow
deposits and other upper fl ow-regime fluvial sediments, while very characteristic of alluvial fans in
hot-arid climates (Fig. 12.1A), are not confined to
this climatic setting (Table 12.1), although it is possible that their abundance, and therefore their preservation potential, may be higher in this type of
setting. A quantitative testing of this possibility has
yet to be attempted.
However, a consideration of every aspect of a
fluvial assemblage, including features of the overbank environment such as chemical and biochemical sediments, soils, minor sedimentary structures,
and bed color, may yield useful clues as to climate.
Conflicting criteria may indicate that the climate of
the basin center is different from that of its margin,
or that at least the water table is at a different level.
What Does Fluvial Lithofacies Reveal About Climate?
Climates may also be different on either side of a
major mountain range. The major criteria are summarized in Tables 12.2 and 12.3, and Figs. 12.1-12.3
are schematic diagrams oflithofacies assemblages in
which climatically sensitive criteria are identified.
As noted earlier in this chapter, the evolution of
land vegetation has probably had a profound effect
on fluvial styles. The lack of roots to bind bank
sediments in pre-Devonian time would have led to a
tendency for rapid bank erosion and channel widening, with the supply of abundant bedload to the
system. This would have favored braided over other
fluvial styles, regardless of the discharge regime.
Long (1978) considered Schumm's (1968a) speculations on this matter, in his review of Proterozoic
fluvial sedimentation. He confirmed that rivers with
an interpreted meandering style are indeed rare in
the Precambrian sedimentary record, but much
work has been carried out on Precambrian sedimentation since the late 1970s, and this subject should be
reviewed again. In the meantime, the climatic ambiguity of fluvial lithofucies, and the absence of coal
and of much other relevant fossil material means
pre-Devonian climates are very difficult to interpret
from the fluvial record.
It may be possible to make quite subtle climatic
interpretations based on a summation of all the
available evidence. For example, Hallam (1984) defined an ('intermediate" type of climate, which shows
evidence of both aridity and humidity. The sediments contain ((thin seams of lignite or coal, modest
in thickness and localised in distribution, and minor
evaporites, such as scattered thin gypsum lenses and
seams, [these] could both form in intermediate,
savannah-type environments where wet and dry sea�
sons are of roughly equal importance." The coexist�
ence of trees and evaporites indicates a savannah or
Mediterranean-type climate with alternating wet
and dry seasons.
In a general discussion of the distribution of
various climatic indicators, especially coal, Ziegler et
al. (1987) provided this interpretation of the Nubian
Sandstone of North Africa, based on many sources:
Cross-bedding type and paleocurrent variance are
interpreted in terms of braided or low-sinuosity
streams. Flood activity is indicated by repetition of
channel-based scour surfaces and their lag deposits.
Prevalence of soil horizons and common preservation of flora suggest well-vegetated alluvial plains,
but the common tree Dadoxylon was probably
restricted to channel margins and swamps. Floral
diversity is not high, and the ubiquitous braided
stream deposits would seem to rule out the possibil�