Time Scales of Nonmarine Sequences and Their Causes
energy landscape undergoing slow accumulation of
floodplain deposits, limited channel aggradation,
and closely-spaced, well-developed soil profiles
(Shanley and McCabe 1994). Given no change in
source-area conditions, however, the sediment supply into the basin will continue, and vigorous channel systems will eventually be re-established. Under
these conditions, channel bodies will form that show
reduced vertical separation relative to the TST, leading to lateral amalgamation of sandstone units and
high net-to-gross sandstone ratios (Wright and
Marriott 1993; Olsen et al. 1995; Fig. 13.4). Basin ward
progradation of coastal depositional systems leads
to downlap of deltaic and barrier-strandplain deposits onto the maximum flooding surface (Fig. 13.17).
A good nonmarine example of - this was described by
Ray (1982), who mapped the progradation of an
alluvial plain and deltaic system into lake deposits
(Fig. 9.44).
lt seems likely that the HST will be poorly represented in most nonmarine basins, because the
highstand is immediately followed by the next cycle
of falling base level, which may result in the removal
of much or all of the just-formed HST deposits by
subaerial erosion. A minor increase in the sandshale ratio immediately below the sequence boundary may be the only indication of the highstand
phase, as in the Castlegate Sandstone of Utah (Olsen
et a!. 1995; Fig. 9.18).
Care must be taken to evaluate all the evidence in
interpreting such data as net-to-gross sandstone ratios. Changes in this parameter may not always be
attributable to changes in the rate of generation of
accommodation space. G.A. Smith (1994) described
a case where an increase in the proportion of channel
sandstones in a section seems to be related not to
changes in the rate of generation of accommodation
space, but to increased sediment runoff resulting
from increased rainfalL In the case of sequences
driven by orbital forcing mechanisms, where both
base-level change and climate change may be involved, unraveling the complexity of causes and effects is likely to be a continuing challenge. Some
aspects of this problem are discussed in Sect. 12.12.2.
In the model of Shanley and McCabe (1994) a greater
degree of channel amalgamation is shown in the TST
than in the HST (Fig. 13.5), the opposite of that
shown in the model of Wright and Maq:iott (Fig.
13.4). Shanley and McCabe (1994) suggested that
where rising base level is the main control on the rate
and style of channel stacking, the rate of generation
of accommodation will be small during transgression in inland areas while the coastline is still distant,
475
and will increase only when transgression has
brought the coastline farther inland, where the baselevel rise on the lower reaches of the river produces a
more rapid increase in accommodation. In this
model the rate of generation of accommodation is
greater during the highstand than during transgression and results in low net-to-gross sandstone ratios.
However, this line of reasoning omits the influence
of upstream factors, and must therefore not be followed dogmatically. One must also be cautious in
using systems-tract terminology derived from marine processes for the labeling of nonmarine events.
There may be a considerable lag in the transmission
of a transgression upstream to inland positions (Fig.
13.17). The inland reaches of the river will not
"know" that a transgression is occurring, and it is
questionable, therefore, whether the deposits
formed during the initial stages of the marine transgression should be included with the TST.
13.3.6 Falling-stage Systems Tract
This systems tract has not yet been identified in any
ancient fluvial deposits. In nonmarine settings a fall
in base level is a time when the generation of accommodation space is negative, and erosion occurs. This
constrasts with shoreline environments, in which
"sharp-based shoreface bodies" form ('stranded
parasequences" that have a high preservation potential (Plint 1988; Hunt and Tucker 1992; Posamentier
et a!. 1992).
Terraces flanking incised valleys may correspond
to the FSST. The origin of fluvial terraces is complex,
however. Blum (1992; Fig. 13.11) argned that climate
changes may have been more important than baselevel changes in the development of postglacial terraces of the Colorado River, Texas (this topic is
discussed at greater length in Chap. 12). As argued
by Arnott (1992), detritus generated by fluvial erosion during the falling stage may provide an important sediment supply for shoreline sedimentation,
even when all traces of the river system are removed
by the falling-stage erosion itself.
13.4 Time Scales of Nonmarine Sequences
and Their Causes
Stratigraphic sequences occur over time scales ranging through five orders of magnitude, from tens of
thousands of years to hundreds of millions of years
energy landscape undergoing slow accumulation of
floodplain deposits, limited channel aggradation,
and closely-spaced, well-developed soil profiles
(Shanley and McCabe 1994). Given no change in
source-area conditions, however, the sediment supply into the basin will continue, and vigorous channel systems will eventually be re-established. Under
these conditions, channel bodies will form that show
reduced vertical separation relative to the TST, leading to lateral amalgamation of sandstone units and
high net-to-gross sandstone ratios (Wright and
Marriott 1993; Olsen et al. 1995; Fig. 13.4). Basin ward
progradation of coastal depositional systems leads
to downlap of deltaic and barrier-strandplain deposits onto the maximum flooding surface (Fig. 13.17).
A good nonmarine example of - this was described by
Ray (1982), who mapped the progradation of an
alluvial plain and deltaic system into lake deposits
(Fig. 9.44).
lt seems likely that the HST will be poorly represented in most nonmarine basins, because the
highstand is immediately followed by the next cycle
of falling base level, which may result in the removal
of much or all of the just-formed HST deposits by
subaerial erosion. A minor increase in the sandshale ratio immediately below the sequence boundary may be the only indication of the highstand
phase, as in the Castlegate Sandstone of Utah (Olsen
et a!. 1995; Fig. 9.18).
Care must be taken to evaluate all the evidence in
interpreting such data as net-to-gross sandstone ratios. Changes in this parameter may not always be
attributable to changes in the rate of generation of
accommodation space. G.A. Smith (1994) described
a case where an increase in the proportion of channel
sandstones in a section seems to be related not to
changes in the rate of generation of accommodation
space, but to increased sediment runoff resulting
from increased rainfalL In the case of sequences
driven by orbital forcing mechanisms, where both
base-level change and climate change may be involved, unraveling the complexity of causes and effects is likely to be a continuing challenge. Some
aspects of this problem are discussed in Sect. 12.12.2.
In the model of Shanley and McCabe (1994) a greater
degree of channel amalgamation is shown in the TST
than in the HST (Fig. 13.5), the opposite of that
shown in the model of Wright and Maq:iott (Fig.
13.4). Shanley and McCabe (1994) suggested that
where rising base level is the main control on the rate
and style of channel stacking, the rate of generation
of accommodation will be small during transgression in inland areas while the coastline is still distant,
475
and will increase only when transgression has
brought the coastline farther inland, where the baselevel rise on the lower reaches of the river produces a
more rapid increase in accommodation. In this
model the rate of generation of accommodation is
greater during the highstand than during transgression and results in low net-to-gross sandstone ratios.
However, this line of reasoning omits the influence
of upstream factors, and must therefore not be followed dogmatically. One must also be cautious in
using systems-tract terminology derived from marine processes for the labeling of nonmarine events.
There may be a considerable lag in the transmission
of a transgression upstream to inland positions (Fig.
13.17). The inland reaches of the river will not
"know" that a transgression is occurring, and it is
questionable, therefore, whether the deposits
formed during the initial stages of the marine transgression should be included with the TST.
13.3.6 Falling-stage Systems Tract
This systems tract has not yet been identified in any
ancient fluvial deposits. In nonmarine settings a fall
in base level is a time when the generation of accommodation space is negative, and erosion occurs. This
constrasts with shoreline environments, in which
"sharp-based shoreface bodies" form ('stranded
parasequences" that have a high preservation potential (Plint 1988; Hunt and Tucker 1992; Posamentier
et a!. 1992).
Terraces flanking incised valleys may correspond
to the FSST. The origin of fluvial terraces is complex,
however. Blum (1992; Fig. 13.11) argned that climate
changes may have been more important than baselevel changes in the development of postglacial terraces of the Colorado River, Texas (this topic is
discussed at greater length in Chap. 12). As argued
by Arnott (1992), detritus generated by fluvial erosion during the falling stage may provide an important sediment supply for shoreline sedimentation,
even when all traces of the river system are removed
by the falling-stage erosion itself.
13.4 Time Scales of Nonmarine Sequences
and Their Causes
Stratigraphic sequences occur over time scales ranging through five orders of magnitude, from tens of
thousands of years to hundreds of millions of years