442
Bromley ( 199la,b) mapped an eolian rippled-siltstone unit interbedded with braided sandstones in
the Kayenta Formation (Jurassic) of western Colorado. Similar mixed facies were described by Luttrell
(1993), based on her regional study of this unit.
Minor eolian lenses interbedded with fluvial conglomerates or sandstones have been recorded in several studies. For example, in his study of the Triassic
Budleigh Salterton Pebble Beds of SW England,
Smith (1990) noted sand lenses with distinct texture
and petrography. Unlike the fl uvial sands in the
system, they contain no mica and are well sorted and
free of pebbles. Shepherd (1987) docnmented eolian
lenses within the modern lateral-accretion deposits
of Rio Puerco, New Mexico, distinguishing them
from fluvial units on the basis of textural analyses.
Clemmensen and Tirsgaard (1990) recognized a distinctive type of bounding surface, which they termed
«sand-drift surfaces", separating eolian from fl uvial
deposits.
Flint (1985) noted an absence of eolian sands in
his study of the arid mid-Tertiary Pacencia Group of
Chile. He attributed this to armoring of the sediment
surface by evaporite crystallization, which prevented
eolian defl ation.
12.9.5 Palustrine Limestones
Freshwater limestones containing a pedogenic overprint are present in some alluvial successions, and
indicate the presence of temporary ponds in the
overbank environment (Platt and Wright 1992).
They occur in a variety of climatic setttings, the
nature of which may be indicated by associated features, such as evaporite nodules, desiccation features, karst solution, or thin coals.
12.10 Contrasting Climatic lndicators
In many recent regional studies sedimentological
analysis has suggested that different parts of a sedimentary basin were characterized by different saturation levels, suggesting variations in the balance
between rainfall, runoff, infiltration, and evaporation. In other cases it can be argued that the basin
and its fluvial source area were characterized by
different climates. Such differences are commonly
observed in present-day sedimentary basins, but
their presence requires subtlety and care in the interpretation of climatic indicators.
What Does Fluvial Lithofacies Reveal About Climate?
A good example of apparently conflicting climatic indicators occurs in the Late Carboniferous to
Early Permian coal-bearing succession of central
England and was highlighted by Besly (1988). He
documented a diachronous interbedding between
an alluvial coal-bearing succession, an alluvial-plain
red-bed association, and an alluvial-fan red-bed
association. The transition from gray to mottled to
red between these assocations indicates improved
drainage of the floodplain. Additional details on the
paleosol catena that indicates a lateral transition
from dry (red) to wet (gray) beds, corresponding to
fan progradation, was provided by Besly and Fielding (1989). According to Besly (1988), "the transition
from the deposition of coal-bearing sediments to
that of red beds resulted from an improvement in
drainage conditions at the site of deposition. This
was initially achieved by the progradation of a topographically elevated, well-drained floodplain area
over poorly drained alluvial backswamps. Repeated
progradation led to a progressive drop in groundwater table levels, possibly reinforced by seasonal fl uctuations ... which eventually led to the accumulation
of a continuous red-bed sequence." Besly (1988)
commented that <{while the Late Carboniferous redbed sequences do allow climatic reconstruction, they
do not in all cases indicate aridity." He noted that the
fairly rapid onset of more arid conditions during the
Westphalian D and Stephanian corresponds to the
acme of coal formation in basins within the mountain belt to the southeast. This contrast suggests the
growth of a rain shadow in the lee of the Variscan
mountains.
Blakey and Gubitosa (1984) found what appeared
to be conflicting evidence of climate in the Chinle
Formation (Triassic) of the C olorado P. lateau. An
abundant and diverse fo ssil fauna and flora suggests
a .humid climate, whereas the presence of caliches
and other signs of overbank aridity suggest a dry
climate. Their suggested explanation is that the rivers were perennial, fed by a humid source area, but
that the depositional basin itself was more arid) with
at least a seasonal dry period dominating conditions
on the floodplains. These suggestions have recently
been confirmed and amplified by Dubiel et a!.
(1991). The juxtaposition of humid upland source
areas and arid basins is a common one, especially
where the upland region generates orographic rain�
fall, leading to a permanent rain shadow downwind.
The San joaquin valley in California and the San Luis
valley of Colorado are good modern examples of
such contrasting cl.imatic regimes.
Bromley ( 199la,b) mapped an eolian rippled-siltstone unit interbedded with braided sandstones in
the Kayenta Formation (Jurassic) of western Colorado. Similar mixed facies were described by Luttrell
(1993), based on her regional study of this unit.
Minor eolian lenses interbedded with fluvial conglomerates or sandstones have been recorded in several studies. For example, in his study of the Triassic
Budleigh Salterton Pebble Beds of SW England,
Smith (1990) noted sand lenses with distinct texture
and petrography. Unlike the fl uvial sands in the
system, they contain no mica and are well sorted and
free of pebbles. Shepherd (1987) docnmented eolian
lenses within the modern lateral-accretion deposits
of Rio Puerco, New Mexico, distinguishing them
from fluvial units on the basis of textural analyses.
Clemmensen and Tirsgaard (1990) recognized a distinctive type of bounding surface, which they termed
«sand-drift surfaces", separating eolian from fl uvial
deposits.
Flint (1985) noted an absence of eolian sands in
his study of the arid mid-Tertiary Pacencia Group of
Chile. He attributed this to armoring of the sediment
surface by evaporite crystallization, which prevented
eolian defl ation.
12.9.5 Palustrine Limestones
Freshwater limestones containing a pedogenic overprint are present in some alluvial successions, and
indicate the presence of temporary ponds in the
overbank environment (Platt and Wright 1992).
They occur in a variety of climatic setttings, the
nature of which may be indicated by associated features, such as evaporite nodules, desiccation features, karst solution, or thin coals.
12.10 Contrasting Climatic lndicators
In many recent regional studies sedimentological
analysis has suggested that different parts of a sedimentary basin were characterized by different saturation levels, suggesting variations in the balance
between rainfall, runoff, infiltration, and evaporation. In other cases it can be argued that the basin
and its fluvial source area were characterized by
different climates. Such differences are commonly
observed in present-day sedimentary basins, but
their presence requires subtlety and care in the interpretation of climatic indicators.
What Does Fluvial Lithofacies Reveal About Climate?
A good example of apparently conflicting climatic indicators occurs in the Late Carboniferous to
Early Permian coal-bearing succession of central
England and was highlighted by Besly (1988). He
documented a diachronous interbedding between
an alluvial coal-bearing succession, an alluvial-plain
red-bed association, and an alluvial-fan red-bed
association. The transition from gray to mottled to
red between these assocations indicates improved
drainage of the floodplain. Additional details on the
paleosol catena that indicates a lateral transition
from dry (red) to wet (gray) beds, corresponding to
fan progradation, was provided by Besly and Fielding (1989). According to Besly (1988), "the transition
from the deposition of coal-bearing sediments to
that of red beds resulted from an improvement in
drainage conditions at the site of deposition. This
was initially achieved by the progradation of a topographically elevated, well-drained floodplain area
over poorly drained alluvial backswamps. Repeated
progradation led to a progressive drop in groundwater table levels, possibly reinforced by seasonal fl uctuations ... which eventually led to the accumulation
of a continuous red-bed sequence." Besly (1988)
commented that <{while the Late Carboniferous redbed sequences do allow climatic reconstruction, they
do not in all cases indicate aridity." He noted that the
fairly rapid onset of more arid conditions during the
Westphalian D and Stephanian corresponds to the
acme of coal formation in basins within the mountain belt to the southeast. This contrast suggests the
growth of a rain shadow in the lee of the Variscan
mountains.
Blakey and Gubitosa (1984) found what appeared
to be conflicting evidence of climate in the Chinle
Formation (Triassic) of the C olorado P. lateau. An
abundant and diverse fo ssil fauna and flora suggests
a .humid climate, whereas the presence of caliches
and other signs of overbank aridity suggest a dry
climate. Their suggested explanation is that the rivers were perennial, fed by a humid source area, but
that the depositional basin itself was more arid) with
at least a seasonal dry period dominating conditions
on the floodplains. These suggestions have recently
been confirmed and amplified by Dubiel et a!.
(1991). The juxtaposition of humid upland source
areas and arid basins is a common one, especially
where the upland region generates orographic rain�
fall, leading to a permanent rain shadow downwind.
The San joaquin valley in California and the San Luis
valley of Colorado are good modern examples of
such contrasting cl.imatic regimes.