Asso ciated Clastic, Chemical, and Biochemical Sediments
with kaolinite indicating humid climates and illite
m ore common in more arid climates. This reflects
the leaching of soluble cations, which decreases with
aridity. Quartz/feldspar ratios in soils also indicate
humidity, with feldspar being destroyed by acidic
weathering in humid climates. Indicators of temperature are harder to find. In general, soil becomes
red der in warmer climates, but the amount of rainfall and the age of the soil complicate this relationship.
However, the climatic signature may be very difficult to interpret. The difficulties discussed above
with regard to the interpretation of color apply
equally to paleosols. Wright (1990) pointed out, with
regard to the calcretes that develop in semiarid
areas, that variations in exposure time (determined
by subsidence.and channel return time) and local
variations in rainfall and calciwn content all lead to
markedly different rates of soil development.
Given these difficulties, some generalizations
may be made, using the soil-type terms and descriptions adapted by Retallack (1988) from the U.S.
Department of Agriculture classification. Aridisols
develop in arid climates, and are characterized by
light coloration and a thin calcareous layer close to
the surface of the profile, and may include pedogenic
gypsum or other evaporite minerals. Vertisols contain abundant swelling clays (smectite) to depths of a
meter or more and may display slickensides or clastic dykes. This type of soil develops in climatic settings wjth seasonal variations from saturated to dry.
Oxisols are a type of soil developed in humid tropical
settings and consist of a thick, well-differentiated to
uniform profile with a clayey texture, red, highly
oxidized subsurface horizons, and an absence of
easily weathered detrital minerals. Many other soil
types are listed by Retallack (1988).
Mack et al. (1993) developed a paleosol classification better adapted to the study of ancient rocks than
those developed for agricultural purposes, and Mack
and )ames (1994) discussed the paleoclimatic significance of this classification.
12.9.3 Evaporites
Evaporite minerals are deposited within fluvial substrates by the evaporation of groundwaters and on
the surface of playa mudflats during the evaporation
of sheet floods. Thin beds, laminae, nodules, and
individual crystals (or crystal casts) of evaporite,
particularly gypsum and halite, are very common in
the deposits of arid fluvial systems, especially in the
distal regions where the braidplain or terminal fan
441
merges imperceptibly into a playa lake or arid tidal
flat (Miall et al. 1978; Smoot 1983; Glennie 1987;
Mertz and Hubert 1990; Dreyer 1993b). Mertz and
Hubert (1990) reported a gradual increase in humidity during deposition of the Triassic-Jurassic
Blomidon Formation of Nova Scotia, based in part
on the upward change in intergranular cements
fr om halite and gypsum to calcite.
12.9.4 Eolian Interbeds
Many fluvial deposits formed in arid settings contain
interbedded layers or lenses of eolian sand. Largescale interbedding may occur at the margins of ergs,
or where these are crossed by ephemeral river systems, but minor eolian units can occur in any fluvial
system where aridity and wind strength are sufficient for the eolian reworking of fluvially transported detritus. Glennie (1970) published a classic
study of desert sedimentary environments that contains much descriptive information on fluvial-eolian
interactions, and a detailed local study was presented by Langford (1989).
Langford and Chan (1989) recognized four major
facies in mixed eolian-fluvial systems, based on their
study of the Cutler-Ceda Mesa Sandstones (Permian) of Utah. 1. Large-scale cross-stratified eolian
sandstone; 2. the deposits of wet interdune areas,
consisting of interbedded lenses of sandstone and
limestones. The sandstones are structureless or reworked into wave ripples. Gypsum crystal casts and
bioturbation are common; 3. Fluvial channel sandstones. Common lithofacies are St and Sh1 which
may be organized into accretionary macroforms
(elements DA and LA), consisting of bundles of
trough cross-bedded sandstone separated by thirdorder surfaces; 4. overbank interdune sandstones
and shales in thin sheets and lenses up to 2 m thick.
Mudcracks and bioturbation are common. Sandstones consist of lithofacies Sr, Sh, and Sm. Small
channel forms up to 2 m thick may be present,
formed from minor overbank flows. Isolated eolian
dune lenses up to 1.5 m thick may also be present,
formed by dune migration over desiccated interfluve
surfaces between floods. Langford and Chan (1989)
traced some fluvial flooding surfaces over areas of>
400 km2, indicating extensive flooding episodes.
Bioturbation and bleaching indicate incipient pedogenesis, but true calcretes are rare. Dreikanters
(ventifacts) may be present on eolian deflation surfaces, but they require lengthy exposure to the wind
to develop and therefore may not be common in
areas dominated by fluvial sedimentation.
with kaolinite indicating humid climates and illite
m ore common in more arid climates. This reflects
the leaching of soluble cations, which decreases with
aridity. Quartz/feldspar ratios in soils also indicate
humidity, with feldspar being destroyed by acidic
weathering in humid climates. Indicators of temperature are harder to find. In general, soil becomes
red der in warmer climates, but the amount of rainfall and the age of the soil complicate this relationship.
However, the climatic signature may be very difficult to interpret. The difficulties discussed above
with regard to the interpretation of color apply
equally to paleosols. Wright (1990) pointed out, with
regard to the calcretes that develop in semiarid
areas, that variations in exposure time (determined
by subsidence.and channel return time) and local
variations in rainfall and calciwn content all lead to
markedly different rates of soil development.
Given these difficulties, some generalizations
may be made, using the soil-type terms and descriptions adapted by Retallack (1988) from the U.S.
Department of Agriculture classification. Aridisols
develop in arid climates, and are characterized by
light coloration and a thin calcareous layer close to
the surface of the profile, and may include pedogenic
gypsum or other evaporite minerals. Vertisols contain abundant swelling clays (smectite) to depths of a
meter or more and may display slickensides or clastic dykes. This type of soil develops in climatic settings wjth seasonal variations from saturated to dry.
Oxisols are a type of soil developed in humid tropical
settings and consist of a thick, well-differentiated to
uniform profile with a clayey texture, red, highly
oxidized subsurface horizons, and an absence of
easily weathered detrital minerals. Many other soil
types are listed by Retallack (1988).
Mack et al. (1993) developed a paleosol classification better adapted to the study of ancient rocks than
those developed for agricultural purposes, and Mack
and )ames (1994) discussed the paleoclimatic significance of this classification.
12.9.3 Evaporites
Evaporite minerals are deposited within fluvial substrates by the evaporation of groundwaters and on
the surface of playa mudflats during the evaporation
of sheet floods. Thin beds, laminae, nodules, and
individual crystals (or crystal casts) of evaporite,
particularly gypsum and halite, are very common in
the deposits of arid fluvial systems, especially in the
distal regions where the braidplain or terminal fan
441
merges imperceptibly into a playa lake or arid tidal
flat (Miall et al. 1978; Smoot 1983; Glennie 1987;
Mertz and Hubert 1990; Dreyer 1993b). Mertz and
Hubert (1990) reported a gradual increase in humidity during deposition of the Triassic-Jurassic
Blomidon Formation of Nova Scotia, based in part
on the upward change in intergranular cements
fr om halite and gypsum to calcite.
12.9.4 Eolian Interbeds
Many fluvial deposits formed in arid settings contain
interbedded layers or lenses of eolian sand. Largescale interbedding may occur at the margins of ergs,
or where these are crossed by ephemeral river systems, but minor eolian units can occur in any fluvial
system where aridity and wind strength are sufficient for the eolian reworking of fluvially transported detritus. Glennie (1970) published a classic
study of desert sedimentary environments that contains much descriptive information on fluvial-eolian
interactions, and a detailed local study was presented by Langford (1989).
Langford and Chan (1989) recognized four major
facies in mixed eolian-fluvial systems, based on their
study of the Cutler-Ceda Mesa Sandstones (Permian) of Utah. 1. Large-scale cross-stratified eolian
sandstone; 2. the deposits of wet interdune areas,
consisting of interbedded lenses of sandstone and
limestones. The sandstones are structureless or reworked into wave ripples. Gypsum crystal casts and
bioturbation are common; 3. Fluvial channel sandstones. Common lithofacies are St and Sh1 which
may be organized into accretionary macroforms
(elements DA and LA), consisting of bundles of
trough cross-bedded sandstone separated by thirdorder surfaces; 4. overbank interdune sandstones
and shales in thin sheets and lenses up to 2 m thick.
Mudcracks and bioturbation are common. Sandstones consist of lithofacies Sr, Sh, and Sm. Small
channel forms up to 2 m thick may be present,
formed from minor overbank flows. Isolated eolian
dune lenses up to 1.5 m thick may also be present,
formed by dune migration over desiccated interfluve
surfaces between floods. Langford and Chan (1989)
traced some fluvial flooding surfaces over areas of>
400 km2, indicating extensive flooding episodes.
Bioturbation and bleaching indicate incipient pedogenesis, but true calcretes are rare. Dreikanters
(ventifacts) may be present on eolian deflation surfaces, but they require lengthy exposure to the wind
to develop and therefore may not be common in
areas dominated by fluvial sedimentation.