Discussion
1990, Chap, 8). The primary mechanism is probably
variations in global sea-floor spreading rates superw
imposed on the first-order cycle discussed above.
Such variations reflect continental-scale adjustments to spreading patterns in response to plate
rifting and collision events. The mechanism was first
suggested by Hallam (1963), and Pitman (1978) is
credited with the theoretical research that lead to
general acceptance of the idea.
Cycles of base-level change can also be generated
by crustal movements at continental margins. On
extensional margins crustal thinning and post-rift
cooling lead to subsidence on a time scale of tens of
millions of years (McKenzie 1978; Watts 1981), and
riftA111 art d extensional-margins deposits may include significant thicknesses of fluvial deposits
(Sect. 1 !.4.2). Clastic wedges are also generated by
convergent and strikewslip tectonics, especially in
foreland basins (Sect. 1 !.4.6), where wedges of nonmarine ((molasse'' spanning several millions of years
to more than 10 million years are common.
13.4.3 Third-order Cycles
The nonmarine sequences and sequence models dew
scribed earlier in this chapter are based mainly on
examples of third- and fo urthworder cycles that have
been described in the outcrop or subsur f ace.
It is this writer's view that most, perhaps even all
thirdworder cycles are tectonic in origirt (Miall 1991c;
YOshida et al., in press). Regional tectonic episodes
localized along major faults generate basin-margin
uplifts and cycles of basin subsidence on time scales
of several millions of years. Tectonism is triggered by
the regional response to plate�tectonic movements
and may include minor changes in sea�floor spreading vectors and the adjustment of major fault blocks
and terrane fragments on the continents. Stresses
are transmitted laterally by intraplate stress (Sect.
1!.3.5), and in this way may cause the development
of synchronous cycles of uplift and subsidence over
areas as large as entire continental plates. Several
examples are given in Sect. 9.4 (group 10 deposits of
Table 9.1), and the plate-tectonic settings of these
various types of sequence are discussed in Sect. 11.4.
13.4.4 Fourth- and Fifth-order Cycles
Fourth- and fifth-order cycles correspond to groups
9 and 8, respectively, in Table 9.1, where several
nonmarine examples are listed. It seems increasingly
likely that many fourth- and fifth-order cycles in
477
tectonically active basins, such as fo reland basins,
are caused by short�term episodes of ctustal loading
(Peper et a!. 1992; Plint et a!. 1993; Yoshida et a!., in
press). If this is the case, these cycles are likely to be
localized in extent and confined to individual basins.
The mechanics of basin movement and consequent
changes in sediment delivery, depositional slope,
and accumulation rates are the subject of much theoretical study at the present time (e.g., Jordan and
Flemings 1991).
Other cycles are driven by orbital fo rcing of
climate. Minor climatic cycles may simply lead to
variations in discharge within a river system,
whereas major cycles may cause regional or hemi�
spheric Continental glaciation and consequent
changes in base level. This has long been thought to
have been the cause of the classic cy-clothems of
Europe and North America. Minor cycles may have
occurred at any time, whereas those involving
glacioeustatic base-level change probably are confined to the Neogene, the Late Paleozoic (the time of
the Gondwana glaciation), and various times in
the Precambrian. An additional discussion of orbital
forcing is given in Sect. 12.12, where examples of
minor and major cycles are described.
13.5 Discussion
Sequence stratigraphy is a subject undergoing rapid
evolution at this time; and there is a danger that this
chapter in the book, in particular, may be out of date
before it is published. Although a consensus seems
to be emerging regarding the application of sequence concepts to the nonmarine record, very few
well-documented examples of nonmarine sequences
have yet been described. There is room for a considerable amount of additional research in this area.
Quantitative modeling is also required, of the type
discussed in Sect. 11.3.6, to more fully explore
the relationship between tectonic processes, basin
movements, sediment supply, sediment delivery,
and facies changes within sedimentary basins.
The example of the Castlegate Sandstone (Fig.
9.18) illustrates some of the difficulties in applying
sequence concepts to fluvial deposits (Yoshida et
al., in press). Where marine deposits interfinger
with the fl uvial succession, ClS in the area east of
Trail Canyon, recognition of sequence boundaries,
fl ooding surfaces, and systems tracts is relatively
straightforward (Van Wagoner et a!. 1990, 1991).
The fact that some of these features are difficult to
trace updip, away from the coastal plain, raises an
1990, Chap, 8). The primary mechanism is probably
variations in global sea-floor spreading rates superw
imposed on the first-order cycle discussed above.
Such variations reflect continental-scale adjustments to spreading patterns in response to plate
rifting and collision events. The mechanism was first
suggested by Hallam (1963), and Pitman (1978) is
credited with the theoretical research that lead to
general acceptance of the idea.
Cycles of base-level change can also be generated
by crustal movements at continental margins. On
extensional margins crustal thinning and post-rift
cooling lead to subsidence on a time scale of tens of
millions of years (McKenzie 1978; Watts 1981), and
riftA111 art d extensional-margins deposits may include significant thicknesses of fluvial deposits
(Sect. 1 !.4.2). Clastic wedges are also generated by
convergent and strikewslip tectonics, especially in
foreland basins (Sect. 1 !.4.6), where wedges of nonmarine ((molasse'' spanning several millions of years
to more than 10 million years are common.
13.4.3 Third-order Cycles
The nonmarine sequences and sequence models dew
scribed earlier in this chapter are based mainly on
examples of third- and fo urthworder cycles that have
been described in the outcrop or subsur f ace.
It is this writer's view that most, perhaps even all
thirdworder cycles are tectonic in origirt (Miall 1991c;
YOshida et al., in press). Regional tectonic episodes
localized along major faults generate basin-margin
uplifts and cycles of basin subsidence on time scales
of several millions of years. Tectonism is triggered by
the regional response to plate�tectonic movements
and may include minor changes in sea�floor spreading vectors and the adjustment of major fault blocks
and terrane fragments on the continents. Stresses
are transmitted laterally by intraplate stress (Sect.
1!.3.5), and in this way may cause the development
of synchronous cycles of uplift and subsidence over
areas as large as entire continental plates. Several
examples are given in Sect. 9.4 (group 10 deposits of
Table 9.1), and the plate-tectonic settings of these
various types of sequence are discussed in Sect. 11.4.
13.4.4 Fourth- and Fifth-order Cycles
Fourth- and fifth-order cycles correspond to groups
9 and 8, respectively, in Table 9.1, where several
nonmarine examples are listed. It seems increasingly
likely that many fourth- and fifth-order cycles in
477
tectonically active basins, such as fo reland basins,
are caused by short�term episodes of ctustal loading
(Peper et a!. 1992; Plint et a!. 1993; Yoshida et a!., in
press). If this is the case, these cycles are likely to be
localized in extent and confined to individual basins.
The mechanics of basin movement and consequent
changes in sediment delivery, depositional slope,
and accumulation rates are the subject of much theoretical study at the present time (e.g., Jordan and
Flemings 1991).
Other cycles are driven by orbital fo rcing of
climate. Minor climatic cycles may simply lead to
variations in discharge within a river system,
whereas major cycles may cause regional or hemi�
spheric Continental glaciation and consequent
changes in base level. This has long been thought to
have been the cause of the classic cy-clothems of
Europe and North America. Minor cycles may have
occurred at any time, whereas those involving
glacioeustatic base-level change probably are confined to the Neogene, the Late Paleozoic (the time of
the Gondwana glaciation), and various times in
the Precambrian. An additional discussion of orbital
forcing is given in Sect. 12.12, where examples of
minor and major cycles are described.
13.5 Discussion
Sequence stratigraphy is a subject undergoing rapid
evolution at this time; and there is a danger that this
chapter in the book, in particular, may be out of date
before it is published. Although a consensus seems
to be emerging regarding the application of sequence concepts to the nonmarine record, very few
well-documented examples of nonmarine sequences
have yet been described. There is room for a considerable amount of additional research in this area.
Quantitative modeling is also required, of the type
discussed in Sect. 11.3.6, to more fully explore
the relationship between tectonic processes, basin
movements, sediment supply, sediment delivery,
and facies changes within sedimentary basins.
The example of the Castlegate Sandstone (Fig.
9.18) illustrates some of the difficulties in applying
sequence concepts to fluvial deposits (Yoshida et
al., in press). Where marine deposits interfinger
with the fl uvial succession, ClS in the area east of
Trail Canyon, recognition of sequence boundaries,
fl ooding surfaces, and systems tracts is relatively
straightforward (Van Wagoner et a!. 1990, 1991).
The fact that some of these features are difficult to
trace updip, away from the coastal plain, raises an