Main Components of the Fluvial Sequence Model
1988; Fortuin and de Smet 1991), possibly even exceeding the rates of sea�level change induced by
glacioeustasy. Long-term rates of vertical tectonic
movement reflecting regional plate-tectonic processes may be punctuated by short-term rates of very
rapid subsidence, reflecting movements on individual thrust sheets or nappes. Therefore, there may
not be a zone A in some fo reland basins. In fact,
erosional sequence boundaries are present in all the
studies of nonmarine sequences in foreland basins
published to date (e.g., Fig. 13.9), suggesting an absence of zone A. It is not yet known whether the
channel clustering effect suggested in Fig. 13.8 actually occurs.
Figure 13.10 illustrates an interpretation of the
sequence architecture of the Castlegate Sandstone
and associated units in the Book Cliffs, based on the
work of Yoshida et al. (in press; Fig. 9.18). It is
suggested that the basin was affected by a pattern of
long-term fl exural subsidence which was greatest in
proximal regions (sections toward the left side of the
diagram) and decreased distally, and which fl uctuated in rate over a time scale of a few millions of
years. Superimposed on this was a pattern of higherfre quency base-level fl uctuation which may have
been tectonic or eustatic in origin. Erosional (type-1)
sequence boundaries in distal areas help to define
the position of zone B of the foreland basin, but, as
shown in Fig. 13.10, the lateral extent of these erosion surfaces varies considerably. The boundary
between zones A and B appears to have moved proximally and distally. During deposition of the Lower
Castlegate Sandstone and the Bluecastle Sandstone
the long-term rate of subsidence was slow and zone
A was absent, or was restricted to the most proximal
areas west of the project area.
Away fr om the main meander belt, in interfluve
areas, type 2 unconformities may be characterized
by well-developed paleosols, which require stability
of the land surface and low sediment influx for their
development (Sect. 7.4.2). Posamentier and Allen
(1993) suggested the Boulder Creek Formation of
British Columbia as a possible example. In this unit
15 paleosols are associated with a major regional
unconformity (Leckie et al. 1989). Superimposed
paleosols also mark unconformity surfaces in
Eocene-Oligocene deposits of South Dakota and
have been interpreted as indicating two episodes of
either base-level fall and rise, or changes in sediment
supply relating to volcanism or climate change
(Evans and Terry 1994). Other examples of paleosols
marking sequence-boundary disconformities have
been described by Hanneman and Wideman (1991)
463
and Gibling and Bird (1994). Hanneman et al. (1994)
discussed the identification of sequence-bounding
paleosols in the subsurface using seismic and welllog data (see Sects. 9.5.2, 9.5.4). In the Breathitt
Group of eastern Kentucky interfluves are underlain
by carbonaceous siltstones, underclays, and rooted
surfaces (Aitken and Flint 1995). There is little to
distinguish such deposits from floodplain deposits
of the transgressive and highstand systems tracts,
and Aitken and Flint (1995) were able to designate
sequence boundaries in the interfluve areas only by
physically tracing them from the base of incised
valleys.
Tandon and Gibling (1994) argued for a climatic
control on paleosols at sequence boundaries in the
late Paleozoic cyclothems they studied in - Nova
Scotia. They interpreted calcareous paleosols as indicating seasonally dry climates, contrasting with
the humid climates indicated by coal-bearing deposits of the highstand systems tracts. These cyclothems
are, of course, ascribed to glacioeustatic controls
(Crowell 1978). During major episodes of continental glaciation large-scale shifts in the earth's climatic
belts occur, and it is to be expected that the effects of
eustasy will be overlayered with the sedimentary
effects of climate change. In this case, a northern
hemisphere seasonal dryness is correlated with
southern hemisphere glacial coldness (sea-level
lowstand), and a northern hemisphere warm, humid
climate is correlated with a southern hemisphere
warm interglacial climate (sea-level highstand). As
discussed in Sect. 12.12.2, with reference to the late
Cenozoic record, the complexities of these linked
changes in the controlling mechanisms are only just
beginning to be properly understood.
Throughout the earlier studies of sequence
stratigraphy the assumption has been made that the
regional unconformities that constitute sequence
boundaries have chronostratigraphic significance
and can be used for regional, even global, correlation. A discussion of this premise is beyond the scope
of this book. It should be noted, however, that it has
long been known that the processes that generate
nonmarine erosion surfaces are not instantaneous
or regionally synchronous in their effect. For example, an erosion surface generated by a fall in base
level develops by headward erosion as the longitudinal river profiles adjust to the new lower leveL A
change in slope from the old to the new, lower slope
may generate a "nickpoint", which migrates landward with time (Butcher 1990; see Sect. !1.2.2). The
time required for this migration is on the order of at
least a few thousand years, which is a significant
1988; Fortuin and de Smet 1991), possibly even exceeding the rates of sea�level change induced by
glacioeustasy. Long-term rates of vertical tectonic
movement reflecting regional plate-tectonic processes may be punctuated by short-term rates of very
rapid subsidence, reflecting movements on individual thrust sheets or nappes. Therefore, there may
not be a zone A in some fo reland basins. In fact,
erosional sequence boundaries are present in all the
studies of nonmarine sequences in foreland basins
published to date (e.g., Fig. 13.9), suggesting an absence of zone A. It is not yet known whether the
channel clustering effect suggested in Fig. 13.8 actually occurs.
Figure 13.10 illustrates an interpretation of the
sequence architecture of the Castlegate Sandstone
and associated units in the Book Cliffs, based on the
work of Yoshida et al. (in press; Fig. 9.18). It is
suggested that the basin was affected by a pattern of
long-term fl exural subsidence which was greatest in
proximal regions (sections toward the left side of the
diagram) and decreased distally, and which fl uctuated in rate over a time scale of a few millions of
years. Superimposed on this was a pattern of higherfre quency base-level fl uctuation which may have
been tectonic or eustatic in origin. Erosional (type-1)
sequence boundaries in distal areas help to define
the position of zone B of the foreland basin, but, as
shown in Fig. 13.10, the lateral extent of these erosion surfaces varies considerably. The boundary
between zones A and B appears to have moved proximally and distally. During deposition of the Lower
Castlegate Sandstone and the Bluecastle Sandstone
the long-term rate of subsidence was slow and zone
A was absent, or was restricted to the most proximal
areas west of the project area.
Away fr om the main meander belt, in interfluve
areas, type 2 unconformities may be characterized
by well-developed paleosols, which require stability
of the land surface and low sediment influx for their
development (Sect. 7.4.2). Posamentier and Allen
(1993) suggested the Boulder Creek Formation of
British Columbia as a possible example. In this unit
15 paleosols are associated with a major regional
unconformity (Leckie et al. 1989). Superimposed
paleosols also mark unconformity surfaces in
Eocene-Oligocene deposits of South Dakota and
have been interpreted as indicating two episodes of
either base-level fall and rise, or changes in sediment
supply relating to volcanism or climate change
(Evans and Terry 1994). Other examples of paleosols
marking sequence-boundary disconformities have
been described by Hanneman and Wideman (1991)
463
and Gibling and Bird (1994). Hanneman et al. (1994)
discussed the identification of sequence-bounding
paleosols in the subsurface using seismic and welllog data (see Sects. 9.5.2, 9.5.4). In the Breathitt
Group of eastern Kentucky interfluves are underlain
by carbonaceous siltstones, underclays, and rooted
surfaces (Aitken and Flint 1995). There is little to
distinguish such deposits from floodplain deposits
of the transgressive and highstand systems tracts,
and Aitken and Flint (1995) were able to designate
sequence boundaries in the interfluve areas only by
physically tracing them from the base of incised
valleys.
Tandon and Gibling (1994) argued for a climatic
control on paleosols at sequence boundaries in the
late Paleozoic cyclothems they studied in - Nova
Scotia. They interpreted calcareous paleosols as indicating seasonally dry climates, contrasting with
the humid climates indicated by coal-bearing deposits of the highstand systems tracts. These cyclothems
are, of course, ascribed to glacioeustatic controls
(Crowell 1978). During major episodes of continental glaciation large-scale shifts in the earth's climatic
belts occur, and it is to be expected that the effects of
eustasy will be overlayered with the sedimentary
effects of climate change. In this case, a northern
hemisphere seasonal dryness is correlated with
southern hemisphere glacial coldness (sea-level
lowstand), and a northern hemisphere warm, humid
climate is correlated with a southern hemisphere
warm interglacial climate (sea-level highstand). As
discussed in Sect. 12.12.2, with reference to the late
Cenozoic record, the complexities of these linked
changes in the controlling mechanisms are only just
beginning to be properly understood.
Throughout the earlier studies of sequence
stratigraphy the assumption has been made that the
regional unconformities that constitute sequence
boundaries have chronostratigraphic significance
and can be used for regional, even global, correlation. A discussion of this premise is beyond the scope
of this book. It should be noted, however, that it has
long been known that the processes that generate
nonmarine erosion surfaces are not instantaneous
or regionally synchronous in their effect. For example, an erosion surface generated by a fall in base
level develops by headward erosion as the longitudinal river profiles adjust to the new lower leveL A
change in slope from the old to the new, lower slope
may generate a "nickpoint", which migrates landward with time (Butcher 1990; see Sect. !1.2.2). The
time required for this migration is on the order of at
least a few thousand years, which is a significant