444
regional uplift of such areas as western North
America and Tibet has also been documented. This
has led to a lengthy and complex debate regarding
the role of uplift in generating the cooling trend. This
debate is largely beyond the scope of this chapter,
but because discussions of fluvial lithofacies assem�
blages and petrology have formed part of the evidence, some relevant parts of the controversy are
briefly summarized here.
Ruddiman and Kutzbach (1990) summarized extensive paleobotanical evidence for widespread glo�
bal cooling during the Cenozoic and developed
numerical climate models to analyze global climates
with and without the effects of large-scale plateau
uplift. They pointed out that areas such as southern
China, Burma, and Thailand, which did not undergo
significant uplift during the Cenozoic, show no
botanical evidence of codling during the Cenozoic,
which would seem to confirm the tectonic causes of
cooling. However, they did not point .out that these
areas are located in equatorial regions, which are
not necessarily strongly affected by global climate
change. Their field evidence and modeling data support the concept of plateau uplift as a cause for global
cooling, but the data are inadequate to verify this as
a cause (or at least a trigger) of the Late Cenozoic
glaciation. Regional uplift is also· cited as a cause of
increased aridification because of rai�-shadow effects and because of changes to global circulation
patterns.
Molnar and England ( 1990) stated: "Late Cenozoic uplift has been inferred for mountain ranges
throughout the world, yet globally synchronous
changes in plate motions, if they have occurred, have
been smalL Climate change, on the other hand, has
been a global phenomenon." Indicators of cooling
climate in sediments include increased grain size,
increased detrital volume and sedimentation rates
reflecting increased erosion rates (Stanley and
Wayne 1972; Stanley 1976), changes in fauna and
flora, and changes in oxygen and carbon isotope
ratios. These changes could mean regional or global
climate change, or uplift without climate change.
Rapid uplift in the past 2 million years has been
interpreted for the Himalayas and elsewhere, but
Molnar and England (1990) suggested that this may
be incorrect. Plate tectonic reconstructions of IndiaAsia convergence suggest that the Tibetan Plateau
would have risen gradually during the past 50 million years. This would have set off a long-term slow
cooling, amplified by fe edback effects, that was then
enhanced in the Late Cenozoic by Milankovitch processes, leading to glaciation.
What Does Fluvial Lithofacies Reveal About Climate?
Hoffman and Grotzinger (1993) speculated that
the climatic belt in which a rising orogen develops
actually influences the tectonic style of the orogen
and the architecture of the adjacent foreland basin.
Areas of high precipitation, as in monsoonal belts,
are characterized by rapid erosional unroofing, leading to rapid uplift, deep erosion, and the development of a foreland basin overfilled with nonmarine
sediments (but of low preservation potential). The
Grenville orogen (1.1 Ga) of eastern North America
may be cited as a possible example. This contrasts
with orogens such as the Alleghenian, which may
have developed in arid climates. Erosional unroofing would not compensate uplift, leading to preservation of the fold-thrust belt and an underfilled
foreland basin containing deep-water sediments.
Sinclair and Allen (1992) pointed out the fe edback
effects of elevating a fo ld-thrust belt, resulting in
greater orographic rainfall, increased rates of erosional unroofing, and consequently enhanced uplift
rates.
In a recent lengthy review, Eyles (1993) demonstrated convincingly the importance of tectonic uplift in generating the climatic cooling necessary for
the triggering of widespread continental glaciation
throughout geologic time. Many of the major Phanerozoic and Proterozoic glacial episodes are correlated with times of regional continental extension
and rifting. However, glaciofluvial deposits represent a very small fr action of the glacially generated
clastic successions in the ancient record.
12.12 Orbital Forcing
12.12.1 Sedimentary Evidence of Orbital Forcing
Rhythmicity in glaciation is now attributed to orbital
forcing, the so-called Milankovitch effects, named
after the Serbian mathematician who first developed
the quantitative relationships between the orbital
behavior of the earth and its effects on incoming
solar radiation. There is now a very considerable
literature on the theory and sedimentological implications of orbital forcing, most of which is beyond
the scope of this book (e.g., Berger eta!. 1984; Fischer
et al. 1990; de Boer and Smith 1994a). This section
makes only very brief reference to the theory and
focuses on the few examples of studies in the nonmarine record where some evidence of the influence
of orbital forcing may be demonstrated, or at least
postulated.
regional uplift of such areas as western North
America and Tibet has also been documented. This
has led to a lengthy and complex debate regarding
the role of uplift in generating the cooling trend. This
debate is largely beyond the scope of this chapter,
but because discussions of fluvial lithofacies assem�
blages and petrology have formed part of the evidence, some relevant parts of the controversy are
briefly summarized here.
Ruddiman and Kutzbach (1990) summarized extensive paleobotanical evidence for widespread glo�
bal cooling during the Cenozoic and developed
numerical climate models to analyze global climates
with and without the effects of large-scale plateau
uplift. They pointed out that areas such as southern
China, Burma, and Thailand, which did not undergo
significant uplift during the Cenozoic, show no
botanical evidence of codling during the Cenozoic,
which would seem to confirm the tectonic causes of
cooling. However, they did not point .out that these
areas are located in equatorial regions, which are
not necessarily strongly affected by global climate
change. Their field evidence and modeling data support the concept of plateau uplift as a cause for global
cooling, but the data are inadequate to verify this as
a cause (or at least a trigger) of the Late Cenozoic
glaciation. Regional uplift is also· cited as a cause of
increased aridification because of rai�-shadow effects and because of changes to global circulation
patterns.
Molnar and England ( 1990) stated: "Late Cenozoic uplift has been inferred for mountain ranges
throughout the world, yet globally synchronous
changes in plate motions, if they have occurred, have
been smalL Climate change, on the other hand, has
been a global phenomenon." Indicators of cooling
climate in sediments include increased grain size,
increased detrital volume and sedimentation rates
reflecting increased erosion rates (Stanley and
Wayne 1972; Stanley 1976), changes in fauna and
flora, and changes in oxygen and carbon isotope
ratios. These changes could mean regional or global
climate change, or uplift without climate change.
Rapid uplift in the past 2 million years has been
interpreted for the Himalayas and elsewhere, but
Molnar and England (1990) suggested that this may
be incorrect. Plate tectonic reconstructions of IndiaAsia convergence suggest that the Tibetan Plateau
would have risen gradually during the past 50 million years. This would have set off a long-term slow
cooling, amplified by fe edback effects, that was then
enhanced in the Late Cenozoic by Milankovitch processes, leading to glaciation.
What Does Fluvial Lithofacies Reveal About Climate?
Hoffman and Grotzinger (1993) speculated that
the climatic belt in which a rising orogen develops
actually influences the tectonic style of the orogen
and the architecture of the adjacent foreland basin.
Areas of high precipitation, as in monsoonal belts,
are characterized by rapid erosional unroofing, leading to rapid uplift, deep erosion, and the development of a foreland basin overfilled with nonmarine
sediments (but of low preservation potential). The
Grenville orogen (1.1 Ga) of eastern North America
may be cited as a possible example. This contrasts
with orogens such as the Alleghenian, which may
have developed in arid climates. Erosional unroofing would not compensate uplift, leading to preservation of the fold-thrust belt and an underfilled
foreland basin containing deep-water sediments.
Sinclair and Allen (1992) pointed out the fe edback
effects of elevating a fo ld-thrust belt, resulting in
greater orographic rainfall, increased rates of erosional unroofing, and consequently enhanced uplift
rates.
In a recent lengthy review, Eyles (1993) demonstrated convincingly the importance of tectonic uplift in generating the climatic cooling necessary for
the triggering of widespread continental glaciation
throughout geologic time. Many of the major Phanerozoic and Proterozoic glacial episodes are correlated with times of regional continental extension
and rifting. However, glaciofluvial deposits represent a very small fr action of the glacially generated
clastic successions in the ancient record.
12.12 Orbital Forcing
12.12.1 Sedimentary Evidence of Orbital Forcing
Rhythmicity in glaciation is now attributed to orbital
forcing, the so-called Milankovitch effects, named
after the Serbian mathematician who first developed
the quantitative relationships between the orbital
behavior of the earth and its effects on incoming
solar radiation. There is now a very considerable
literature on the theory and sedimentological implications of orbital forcing, most of which is beyond
the scope of this book (e.g., Berger eta!. 1984; Fischer
et al. 1990; de Boer and Smith 1994a). This section
makes only very brief reference to the theory and
focuses on the few examples of studies in the nonmarine record where some evidence of the influence
of orbital forcing may be demonstrated, or at least
postulated.