Main Components of the Fluvial Sequence Model
1994). Such deposits are better classified with the
transgressive systems tract and are discussed in the
next section.
13.3.3 Transgressive Systems Tract
The transgressive systems tract is defined as the beds
between the first widespread transgressive surface
below and the maximum flooding surface above
(Posamentier and Vail 1988; Zaitlin et a!. 1994). In
practice, there is considerable variation in the composition of the lower part of stratigraphic sequences.
As rioted above, in some cases little or no fluvial
deposit may be preserved at the base of the sequence,
although there may be evidence for their fo rmer
presence, as in the -example discussed by Arnott
(1992). In this case a marine transgressive systems
tract rests directly on the sequence boundary. Incised valleys may be filled with fluvial or estuarine
strata, depending on the balance between the rate
of generation of accommodation space and the rate
of sediment supply (Zaitlin et a!. 1994). There may
be an upward transition from fluvial to estuarine
, deposits corresponding to the boundary between the
LST and the TST, although this is likely to be
diachronous, as the rate of generation of accommodation space increases, and the coastline undergoes
transgression and retrogression (Fig. 13.17). Alternatively, there may be simply a change in fluvial
style, as suggested in some hypothetical fl uvial sequence models (Fig. 13.4). In the Holocene deposits
of the Rhine-Meuse delta Tornqvist (1993, 1994) and
Tornqvist et al. (1993) demonstrated that a rapid rise
in base level led to the development of an anastomosed fluvial style with a high avulsion frequency,
whereas a lower rate resulted in the fo rmation of a
meandering channel system (Fig. 13.16).
The thickness of the transgressive systems tract
depends on the balance between sediment supply
and the rate of generation of accommodation space
(Fig. 13.17). A rapid rise in base level may result in a
very thin TST. In the case of the Castlegate Sandstone
of Utah, there is virtually no recognizable TST in
distal regions, east of Green River, where the marine
shales of the Buck Tongue overlie the LST sandstones with a sharp contact (Fig. 9.18), whereas in
updip areas, such as near Sunnyside, much of the
Upper Castlegate Sandstone is assigned to the transgres sive systems tract (Fig. 9.18). In the modern
Canturbury Plains of New Zealand, transgression,
resulting from tectonic subsidence and eustatic sealevel rise, is leading to shoreface erosion and the
471
incision of river valleys that drain into the retreating
coastline. This is occurring because of high coastal
wave energies, and is resulting in an extension of the
marine ravinement surface inland up the major river
valleys (Leckie 1994).
The key to the recognition of the transgressive
systems tract and the maximum flooding surface
may be the recognition of marine infl uence in otherwise nonmarine successions (Shanley and McCabe
1994). The evidence may consist of tidal or wavefo rmed sedimentary structures or marine faunas
(Figs. 13.5, 13.18). The landward extent of this marine influence depends on the balance of such factors
as wave power, tide range, sediment supply, and
subsidence rate. A detailed description of tidal features in transgressive deposits of Upper Cretaceous
age in southern Utah is given by Shanley et a!. (1992),
who mapped tidal fades some 65 km inland of coeval
shoreline deposits. Similar fe atures are present in
the Castlegate Sandstone of central Utah and are
illustrated here. The most prominent sedimentary
indicators of tidal influence are sand waves (Allen
1980), tidal bedding (Reineck and Wunderlich
1968), and inclined heterolithic strata (Thomas et a!.
1987), examples of which are illustrated in Fig. 13.18.
Tidal sand waves are commonly characterized by
sigmoidal bedding and rhythmically spaced mud
drapes, which are distinctive structures that may be
recognizable even in small outcrops or core.
Marine influence may extend inland fo r tens of
kilometers, but a limit is reached at the time of
highest base level) when the maximum flooding surface is fo rmed. This marine limit may be well within
the depositional basin) especially in foreland basins,
where such proximal deposits have a high preservation potential. Updip from the marine limit, the
transgressive systems tract may be recognizable only
by a change in fluvial style. The study by Shanley et
al. (1992) in southern Utah (Fig. 13.17) and that of
the Castlegate Sandstone (Olsen et al. 1995; and as
reported here: Fig. 9.18) both show upward changes
in fl uvial style. In Price Canyon the Castlegate Sandstone can be subdivided into two members (Olsen
et al. 1995). There is a lower sandstone member,
which consists of a succession of superimposed
braided sandstone sheets with very little in the way
of preserved overbank deposits [e.g., Castlegate-A
location of Miall (1993)] and an upper member
containing thicker and more extensive mudstone
deposits (Fig. 13.14). There is little evidence of a
change in channel style [contrary to the interpretations of Olsen et al. (1995)], and the change may
simply reflect an inct:ease in the rate of generation of
1994). Such deposits are better classified with the
transgressive systems tract and are discussed in the
next section.
13.3.3 Transgressive Systems Tract
The transgressive systems tract is defined as the beds
between the first widespread transgressive surface
below and the maximum flooding surface above
(Posamentier and Vail 1988; Zaitlin et a!. 1994). In
practice, there is considerable variation in the composition of the lower part of stratigraphic sequences.
As rioted above, in some cases little or no fluvial
deposit may be preserved at the base of the sequence,
although there may be evidence for their fo rmer
presence, as in the -example discussed by Arnott
(1992). In this case a marine transgressive systems
tract rests directly on the sequence boundary. Incised valleys may be filled with fluvial or estuarine
strata, depending on the balance between the rate
of generation of accommodation space and the rate
of sediment supply (Zaitlin et a!. 1994). There may
be an upward transition from fluvial to estuarine
, deposits corresponding to the boundary between the
LST and the TST, although this is likely to be
diachronous, as the rate of generation of accommodation space increases, and the coastline undergoes
transgression and retrogression (Fig. 13.17). Alternatively, there may be simply a change in fluvial
style, as suggested in some hypothetical fl uvial sequence models (Fig. 13.4). In the Holocene deposits
of the Rhine-Meuse delta Tornqvist (1993, 1994) and
Tornqvist et al. (1993) demonstrated that a rapid rise
in base level led to the development of an anastomosed fluvial style with a high avulsion frequency,
whereas a lower rate resulted in the fo rmation of a
meandering channel system (Fig. 13.16).
The thickness of the transgressive systems tract
depends on the balance between sediment supply
and the rate of generation of accommodation space
(Fig. 13.17). A rapid rise in base level may result in a
very thin TST. In the case of the Castlegate Sandstone
of Utah, there is virtually no recognizable TST in
distal regions, east of Green River, where the marine
shales of the Buck Tongue overlie the LST sandstones with a sharp contact (Fig. 9.18), whereas in
updip areas, such as near Sunnyside, much of the
Upper Castlegate Sandstone is assigned to the transgres sive systems tract (Fig. 9.18). In the modern
Canturbury Plains of New Zealand, transgression,
resulting from tectonic subsidence and eustatic sealevel rise, is leading to shoreface erosion and the
471
incision of river valleys that drain into the retreating
coastline. This is occurring because of high coastal
wave energies, and is resulting in an extension of the
marine ravinement surface inland up the major river
valleys (Leckie 1994).
The key to the recognition of the transgressive
systems tract and the maximum flooding surface
may be the recognition of marine infl uence in otherwise nonmarine successions (Shanley and McCabe
1994). The evidence may consist of tidal or wavefo rmed sedimentary structures or marine faunas
(Figs. 13.5, 13.18). The landward extent of this marine influence depends on the balance of such factors
as wave power, tide range, sediment supply, and
subsidence rate. A detailed description of tidal features in transgressive deposits of Upper Cretaceous
age in southern Utah is given by Shanley et a!. (1992),
who mapped tidal fades some 65 km inland of coeval
shoreline deposits. Similar fe atures are present in
the Castlegate Sandstone of central Utah and are
illustrated here. The most prominent sedimentary
indicators of tidal influence are sand waves (Allen
1980), tidal bedding (Reineck and Wunderlich
1968), and inclined heterolithic strata (Thomas et a!.
1987), examples of which are illustrated in Fig. 13.18.
Tidal sand waves are commonly characterized by
sigmoidal bedding and rhythmically spaced mud
drapes, which are distinctive structures that may be
recognizable even in small outcrops or core.
Marine influence may extend inland fo r tens of
kilometers, but a limit is reached at the time of
highest base level) when the maximum flooding surface is fo rmed. This marine limit may be well within
the depositional basin) especially in foreland basins,
where such proximal deposits have a high preservation potential. Updip from the marine limit, the
transgressive systems tract may be recognizable only
by a change in fluvial style. The study by Shanley et
al. (1992) in southern Utah (Fig. 13.17) and that of
the Castlegate Sandstone (Olsen et al. 1995; and as
reported here: Fig. 9.18) both show upward changes
in fl uvial style. In Price Canyon the Castlegate Sandstone can be subdivided into two members (Olsen
et al. 1995). There is a lower sandstone member,
which consists of a succession of superimposed
braided sandstone sheets with very little in the way
of preserved overbank deposits [e.g., Castlegate-A
location of Miall (1993)] and an upper member
containing thicker and more extensive mudstone
deposits (Fig. 13.14). There is little evidence of a
change in channel style [contrary to the interpretations of Olsen et al. (1995)], and the change may
simply reflect an inct:ease in the rate of generation of