Main Components of the Fluvial Sequence Model
times of low sediment load, which are nOt Synchronous with changes in base level. This is particularly
evident landward of the limit of base-level influence.
Postglacial terraces within inland river valleys reveal
a history of alternating aggradation and channel
incision, reflec.:: ting climate changes, all of which occurred during the last postglacial rise in sea level
(Blum 1994). The implications of this have yet to be
resolved for inland basins where aggradation occurs
(because of tectonic subsidence), rather than incisiOn and terrace formation. However, it would seem
to suggest that no simple relationship between major
bounding surfaces and base-level change should be
expeCted.
Patterns of terrace fragments and incised valleys
are probably associated with many nonfuarine sequence boundaries in the geological record, where
they may be related to tectonic, eustatic, or climatic
processes. For example, the Mannville Group (Lower
Cretaceous) of Alberta is characterized by several
regional unconformity surfaces with incised valleys,
many of them containing produCing oil fields
(Hopkins 1981, 1985; Chap. 15). Stratigraphic separation of the various channel and terrace fragments,
which also include Jurassic remnants, is a difficult
task in the subsurface.
In another part of the Alberta Basin careful
observation and interpretation made it possible
to demonstrate the importance of fluvial incision
and sediment transport during falling base level,
even in the absence of any recognizable preserved fluvial deposits. Conglomerates of the Upper
Cretaceous Cardium Formation are interpreted
as shoreface deposits (Flint et al. 1986), reworked
from original fluvial channel deposits by Wave pro�
cesses during transgression (Arnott 1992). Depresw
siOns in the transgressive surface, which here
corresponds to the sequence boundary, are inter�
preted as incised fluvial valleys widened by marine
processes, which also reworked all remaining fluvial
deposits.
Good outcrops in Petrified Forest National Park
of Arizona allowed Kraus artd Middleton (1987) to
map a major intraformational disconformity surface
within the Upper Triassic Chinle Formation. At least
two .periods of incision and aggradation ate present,
comparable in geometry to those shown in Fig.
13.11 but probably of different origin. Kraus and
Middleton (1987) attributed the erosion to lowering
of base level, which led to gullying of a mature
alluvial landscape characterized by well-developed
paleosols. The fill of the incised valley is mud dominated. It cdntairts several generations ofless mature
465
paleosols, suggesting more rapid alluviati6h during
episOdes of raised base level.
Fluvial deposits are, by their very nature, charac�
terized by multiple channel scours. One of the diffi�
culties that must be tackled in a sequence analysis of
entirely nonmarine deposits is the distinction of sequence boundary erosion surfaces from autogenic
channel scours. Major changes in fluvial style or
grain size acrosS the boundary may be indicative, as
in the sequences described by Steel and Ryseth
(1990; see Figs. 9.21-9.23) and Olsen et al. (1995).
Rogers (1994) recognized two major erosion surfaces in a nonmarine succession in Montana based
on erosional relief, changes in grain size, and the
presence of coarse channel-lag deposits with evidence of pervasive oxidation. However, such sedimentological cha:nges may not occur. A much more
subtle sequence boundary was recognized by
Bromley (199lb) within a monotonous succession
of brald-bar Sandstones in the Kayenta Formation
(Jurassic) of western Colorado. Cut-and-fill erosional fe atures along this surface are unusually steep
sided, suggesting that the sand was at least partially
consolidated at the time of incision. Petrographic
examinatiOn of the sandstone immediately below
this surface revealed limited grain-to-grain contacts
and abundant euhedral, syntaxial quartz overgrowths, suggesting cementation prior to compaction, in contrast to other sandstones within this deposit. Cementation can probably be attribUted to
silcrete development on an exposed desert surface,
Paleocurrent directions are markedly different
above and below this surface. Bromley (1991b) classified the surface as sixth-order in rank (in the classification system used in this book) and attributed it to
tectonic adjustments of the alluvial plain. In this
particular case salt tectonics, originating in the Paleozoic evaporites of the underlying Paradox Basin,
was thought to have caused shifts in the paleoslope
and changes in fluvial dispersal, resulting in lengthy
exposure of the alluvial plain ih the area under study.
In another study of a monotonous braided sheet
sandstone unit L6pez-G6mez and Arche (1993) were
able to distinguish sixth-order surfaces on the basis
of detailed mapping oflarge cliff sections. they subdivided their Triassic unit in Spain into six sequences bounded by these surfaces and related the
development of the sequences to periodic basinmargin fault movement.
These studies by Kraus and Middleton (1987),
Bromley (1991b), L6pez-G6mez and Arche (1993),
and Rogers (1994) identified sequence bouhdaries
located entirely within nonmarine deposits and
times of low sediment load, which are nOt Synchronous with changes in base level. This is particularly
evident landward of the limit of base-level influence.
Postglacial terraces within inland river valleys reveal
a history of alternating aggradation and channel
incision, reflec.:: ting climate changes, all of which occurred during the last postglacial rise in sea level
(Blum 1994). The implications of this have yet to be
resolved for inland basins where aggradation occurs
(because of tectonic subsidence), rather than incisiOn and terrace formation. However, it would seem
to suggest that no simple relationship between major
bounding surfaces and base-level change should be
expeCted.
Patterns of terrace fragments and incised valleys
are probably associated with many nonfuarine sequence boundaries in the geological record, where
they may be related to tectonic, eustatic, or climatic
processes. For example, the Mannville Group (Lower
Cretaceous) of Alberta is characterized by several
regional unconformity surfaces with incised valleys,
many of them containing produCing oil fields
(Hopkins 1981, 1985; Chap. 15). Stratigraphic separation of the various channel and terrace fragments,
which also include Jurassic remnants, is a difficult
task in the subsurface.
In another part of the Alberta Basin careful
observation and interpretation made it possible
to demonstrate the importance of fluvial incision
and sediment transport during falling base level,
even in the absence of any recognizable preserved fluvial deposits. Conglomerates of the Upper
Cretaceous Cardium Formation are interpreted
as shoreface deposits (Flint et al. 1986), reworked
from original fluvial channel deposits by Wave pro�
cesses during transgression (Arnott 1992). Depresw
siOns in the transgressive surface, which here
corresponds to the sequence boundary, are inter�
preted as incised fluvial valleys widened by marine
processes, which also reworked all remaining fluvial
deposits.
Good outcrops in Petrified Forest National Park
of Arizona allowed Kraus artd Middleton (1987) to
map a major intraformational disconformity surface
within the Upper Triassic Chinle Formation. At least
two .periods of incision and aggradation ate present,
comparable in geometry to those shown in Fig.
13.11 but probably of different origin. Kraus and
Middleton (1987) attributed the erosion to lowering
of base level, which led to gullying of a mature
alluvial landscape characterized by well-developed
paleosols. The fill of the incised valley is mud dominated. It cdntairts several generations ofless mature
465
paleosols, suggesting more rapid alluviati6h during
episOdes of raised base level.
Fluvial deposits are, by their very nature, charac�
terized by multiple channel scours. One of the diffi�
culties that must be tackled in a sequence analysis of
entirely nonmarine deposits is the distinction of sequence boundary erosion surfaces from autogenic
channel scours. Major changes in fluvial style or
grain size acrosS the boundary may be indicative, as
in the sequences described by Steel and Ryseth
(1990; see Figs. 9.21-9.23) and Olsen et al. (1995).
Rogers (1994) recognized two major erosion surfaces in a nonmarine succession in Montana based
on erosional relief, changes in grain size, and the
presence of coarse channel-lag deposits with evidence of pervasive oxidation. However, such sedimentological cha:nges may not occur. A much more
subtle sequence boundary was recognized by
Bromley (199lb) within a monotonous succession
of brald-bar Sandstones in the Kayenta Formation
(Jurassic) of western Colorado. Cut-and-fill erosional fe atures along this surface are unusually steep
sided, suggesting that the sand was at least partially
consolidated at the time of incision. Petrographic
examinatiOn of the sandstone immediately below
this surface revealed limited grain-to-grain contacts
and abundant euhedral, syntaxial quartz overgrowths, suggesting cementation prior to compaction, in contrast to other sandstones within this deposit. Cementation can probably be attribUted to
silcrete development on an exposed desert surface,
Paleocurrent directions are markedly different
above and below this surface. Bromley (1991b) classified the surface as sixth-order in rank (in the classification system used in this book) and attributed it to
tectonic adjustments of the alluvial plain. In this
particular case salt tectonics, originating in the Paleozoic evaporites of the underlying Paradox Basin,
was thought to have caused shifts in the paleoslope
and changes in fluvial dispersal, resulting in lengthy
exposure of the alluvial plain ih the area under study.
In another study of a monotonous braided sheet
sandstone unit L6pez-G6mez and Arche (1993) were
able to distinguish sixth-order surfaces on the basis
of detailed mapping oflarge cliff sections. they subdivided their Triassic unit in Spain into six sequences bounded by these surfaces and related the
development of the sequences to periodic basinmargin fault movement.
These studies by Kraus and Middleton (1987),
Bromley (1991b), L6pez-G6mez and Arche (1993),
and Rogers (1994) identified sequence bouhdaries
located entirely within nonmarine deposits and