Discussion
ity of forest vegetation and especially the lowland
rain fo rest. "We propose a savannah environment
with its typical seasonal rainfall pattern for the
Nubian Sandstone and its equivalents in NE Africa
and Arabia."
Dubiel et al. (1991) were able to trace a lateral
gradation from seasonally dry fl oodplains to per�
manently waterlogged swamps and ponds. In the
Monitor Butte Member of the Chinle Formation
purple-mottled sandstones contain large, irregular
mottles of dark purple, lavender, yellow, and white
that refl ect varying concentrations of iron-bearing
minerals. These mottles reflect variations in the water table, giving rise to alternating oxidizing and
reducing conditions, with redistribution of iron and
the fo rmation of gleyed paleosols. The purplemottled units can be traced laterally into gray and
purple siltstones with plant debris, and then into
gray to black, thinly laminated organic-rich shales
with fish debris deposited in permanent ponds,
marshes, and lakes. A seasonal climate is suggested,
with the ponds occupying permanently saturated
areas in topographically low parts of the basin.
Jerzykiewicz and Sweet (1988) distinguished
three climatic regimes in the Upper CretaceousPaleocene sediments of the Foothills of Alberta. A
humid fl uvial facies association consists of organicrich to coal-bearing floodplain deposits associated
with meandering channels of high sinuosity. A semiarid fl uvial · facies association includes varicolored
floodplain mudstones with mature caliche paleosol
horizons. The alluvial plain was drained by broad
mobile channels. Periodic heavy rainfall modified
the floodplain into a braidplain with numerous
ephemeral streams eroding the caliche soil. An intermediate fl uvial facies association can also be distinguished, consisting of gray or greenish mudstone,
barren of coal and devoid of caliche, associated with
a low"sinuosity meander-belt facies. They demonstrated the stratigraphic and geographic shifting of
these climatic belts across Alberta through the Late
Cretaceous and early Tertiary.
451
An example of a climatic interpretation of a preDevonian fluvial system is that by Trewin (1993b).
He described sand sheets of mixed eolian-fluvial
origin. Discharge is interpreted as fl ashy on the basis
of a dominance of trough cross-bedded sandstone,
the great lateral extent of thin units, the extreme
rarity of channel features, strongly unimodal current directions, and planar upper bounding surfa ces. These deposits contain abundant trace fo ssils.
Red grain coatings indicate oxidizing conditions.
Evidence of desiccation is very rare, but in the absence of significant land vegetation an arid climate is
not necessarily implied by this evidence. A high
water table is suggested by the trace fo ssils, which
indicate invertebrate activity. Trewin (1993b) stated:
"Before the advent of land plants sandy alluvial
outwash areas were associated with a wider range of
climate than is seen at the present day. Flashy discharge, and the lack of sediment binding resulted in
lateral expansion of streams in times of flood to
produce laterally extensive thin sandstone dominated by trough cross-bedding in medium- to
coarse-grained sands.n Eolian reworking took place
between floods or during periods of low discharge.
In conclusion, it is clear that climatic interpretations can be made from clastic fluvial lithofacies, but
this is best carried out in a regional context, with due
regard for the tectonic setting and incorporating
data from all interbedded nonmarine facies, especially chemical and biochemical sediments. Many
fe atures of clastic fluvial lithofacies, and the architectural elements and successions of which they are a
part, are indicative of a discharge regime, the relationship of which to climate may be indirect or complex. In many cases climatic and tectonic signatures
may be ambiguously similar. Nonetheless, a careful
attention to the question of local climate may yield
valuable information and insights to assist in regional basin studies and paleogeographic reconstruction.
ity of forest vegetation and especially the lowland
rain fo rest. "We propose a savannah environment
with its typical seasonal rainfall pattern for the
Nubian Sandstone and its equivalents in NE Africa
and Arabia."
Dubiel et al. (1991) were able to trace a lateral
gradation from seasonally dry fl oodplains to per�
manently waterlogged swamps and ponds. In the
Monitor Butte Member of the Chinle Formation
purple-mottled sandstones contain large, irregular
mottles of dark purple, lavender, yellow, and white
that refl ect varying concentrations of iron-bearing
minerals. These mottles reflect variations in the water table, giving rise to alternating oxidizing and
reducing conditions, with redistribution of iron and
the fo rmation of gleyed paleosols. The purplemottled units can be traced laterally into gray and
purple siltstones with plant debris, and then into
gray to black, thinly laminated organic-rich shales
with fish debris deposited in permanent ponds,
marshes, and lakes. A seasonal climate is suggested,
with the ponds occupying permanently saturated
areas in topographically low parts of the basin.
Jerzykiewicz and Sweet (1988) distinguished
three climatic regimes in the Upper CretaceousPaleocene sediments of the Foothills of Alberta. A
humid fl uvial facies association consists of organicrich to coal-bearing floodplain deposits associated
with meandering channels of high sinuosity. A semiarid fl uvial · facies association includes varicolored
floodplain mudstones with mature caliche paleosol
horizons. The alluvial plain was drained by broad
mobile channels. Periodic heavy rainfall modified
the floodplain into a braidplain with numerous
ephemeral streams eroding the caliche soil. An intermediate fl uvial facies association can also be distinguished, consisting of gray or greenish mudstone,
barren of coal and devoid of caliche, associated with
a low"sinuosity meander-belt facies. They demonstrated the stratigraphic and geographic shifting of
these climatic belts across Alberta through the Late
Cretaceous and early Tertiary.
451
An example of a climatic interpretation of a preDevonian fluvial system is that by Trewin (1993b).
He described sand sheets of mixed eolian-fluvial
origin. Discharge is interpreted as fl ashy on the basis
of a dominance of trough cross-bedded sandstone,
the great lateral extent of thin units, the extreme
rarity of channel features, strongly unimodal current directions, and planar upper bounding surfa ces. These deposits contain abundant trace fo ssils.
Red grain coatings indicate oxidizing conditions.
Evidence of desiccation is very rare, but in the absence of significant land vegetation an arid climate is
not necessarily implied by this evidence. A high
water table is suggested by the trace fo ssils, which
indicate invertebrate activity. Trewin (1993b) stated:
"Before the advent of land plants sandy alluvial
outwash areas were associated with a wider range of
climate than is seen at the present day. Flashy discharge, and the lack of sediment binding resulted in
lateral expansion of streams in times of flood to
produce laterally extensive thin sandstone dominated by trough cross-bedding in medium- to
coarse-grained sands.n Eolian reworking took place
between floods or during periods of low discharge.
In conclusion, it is clear that climatic interpretations can be made from clastic fluvial lithofacies, but
this is best carried out in a regional context, with due
regard for the tectonic setting and incorporating
data from all interbedded nonmarine facies, especially chemical and biochemical sediments. Many
fe atures of clastic fluvial lithofacies, and the architectural elements and successions of which they are a
part, are indicative of a discharge regime, the relationship of which to climate may be indirect or complex. In many cases climatic and tectonic signatures
may be ambiguously similar. Nonetheless, a careful
attention to the question of local climate may yield
valuable information and insights to assist in regional basin studies and paleogeographic reconstruction.