6.4 SEDIMENTARY MODELS, INCREMENTS, AND CYCLES
283
The sedimentary facies of a depositional sequence are grouped into three systems
tracts (a translation of Walther's faciesbezirk). These are defined according to relative
sea level at the time of deposition, namely, low stand, transgressive, and high stand
tracts, corresponding to low, rising, and highest sea level (Fig. 6.81). The highest sea
level is termed a maximum flooding surface. This normally results in the deposition of
a laterally extensive condensed sequence. High stands correspond to what used to be
termed transgressions, and low stands, to regressions. Similarly Busch's terms "genetic
increment" and "genetic sequence," defined earlier, are renamed parasequence and
parasequenee set, respectively (Van Wagoner et aL, 1988, 1990). One depositional sequence may be composed of the deposits of high and low stand systems tracts (transgressions and regressions) arranged in many parasequences (increments). Deposition
will only take place where there is accommodation space, the vertical interval between
sea level and the sea floor.
Depositional sequences are associated with two types of chronostratigraphic surface.
Bedding, or stratal, surfaces occur within a sequence. Unconformities, and their relative correlative conformities, occur at, and define, sequence boundaries. Unconformities may be recognized on seismic sections in their own right. Their presence may also
be inferred where stratal surfaces onlap, toplap, offlap, downlap, or are truncated by an
unconformity (these terms are illustrated in Fig. 6.81).
Unconformities are of three types, illustrated in Fig. 6.82, and are related to high
stands and low stands of sea level:
9 Type I unconformities occur when sea level is below the shelf edge, leading to the extensive erosion of the shelf, with incision by fluvial channels.
9 Type 2 unconformities occur when sea level is at or landward of the shelf edge, with
overlying transgressive marine sediments or deeper water deposits and no evidence
of a low stand systems tract.
9 Type 3 unconformities occur when sea level is at or landward of the shelf edge, with
coastal deposits above and below the unconformity, either with coastal deposits above
and below the unconformity, or with deeper water deposits overlain by shallower.
It is argued that these three types of unconformity can be recognized and correlated in
sedimentary basins around the world. Vail et al. (1977) and Haq et al. (1988) believe that
these unconformities correspond to global changes of sea level, and that it is possible to
construct a curve of global changing sea level (Fig. 6.83). Type 1, 2, and 3 unconformities
are used to define a hierarchy of cycles with periodicities of 15, 11, and 4 my, according
to (Vail and Todd, 1981), or 50, 3-50, and 0.5-3 my, according to Neal et al. (1993). The
first-order cycle is attributed to continental breakup, the second to changes in the rate of
subsidence and uplift due to plate movements, the third is the sequence cycle, described
more fully later. Fourth and higher order cycles are attributed to climatic changes.
The third-order sequence cycle is illustrated in Plate 2. Conventionally the cycle starts
with the lowest sea level at which time a low stand systems tract overlies a type 1 unconformity (Plate 2A). Sandy lobate submarine fans are deposited at the foot of the continental slope. As sea level begins to rise turbidite sands and shales are deposited on
the continental slope (Plate 2B). Continuing submergence increases the accommodation space between basin floor and sea level, allowing upward-coarsening deltas to prograde out across the basin (Plate 2C). Once sea level has risen over the shelf edge it will
migrate rapidly across the shelf, cutting off sediment supply to the basin, and deposit a
283
The sedimentary facies of a depositional sequence are grouped into three systems
tracts (a translation of Walther's faciesbezirk). These are defined according to relative
sea level at the time of deposition, namely, low stand, transgressive, and high stand
tracts, corresponding to low, rising, and highest sea level (Fig. 6.81). The highest sea
level is termed a maximum flooding surface. This normally results in the deposition of
a laterally extensive condensed sequence. High stands correspond to what used to be
termed transgressions, and low stands, to regressions. Similarly Busch's terms "genetic
increment" and "genetic sequence," defined earlier, are renamed parasequence and
parasequenee set, respectively (Van Wagoner et aL, 1988, 1990). One depositional sequence may be composed of the deposits of high and low stand systems tracts (transgressions and regressions) arranged in many parasequences (increments). Deposition
will only take place where there is accommodation space, the vertical interval between
sea level and the sea floor.
Depositional sequences are associated with two types of chronostratigraphic surface.
Bedding, or stratal, surfaces occur within a sequence. Unconformities, and their relative correlative conformities, occur at, and define, sequence boundaries. Unconformities may be recognized on seismic sections in their own right. Their presence may also
be inferred where stratal surfaces onlap, toplap, offlap, downlap, or are truncated by an
unconformity (these terms are illustrated in Fig. 6.81).
Unconformities are of three types, illustrated in Fig. 6.82, and are related to high
stands and low stands of sea level:
9 Type I unconformities occur when sea level is below the shelf edge, leading to the extensive erosion of the shelf, with incision by fluvial channels.
9 Type 2 unconformities occur when sea level is at or landward of the shelf edge, with
overlying transgressive marine sediments or deeper water deposits and no evidence
of a low stand systems tract.
9 Type 3 unconformities occur when sea level is at or landward of the shelf edge, with
coastal deposits above and below the unconformity, either with coastal deposits above
and below the unconformity, or with deeper water deposits overlain by shallower.
It is argued that these three types of unconformity can be recognized and correlated in
sedimentary basins around the world. Vail et al. (1977) and Haq et al. (1988) believe that
these unconformities correspond to global changes of sea level, and that it is possible to
construct a curve of global changing sea level (Fig. 6.83). Type 1, 2, and 3 unconformities
are used to define a hierarchy of cycles with periodicities of 15, 11, and 4 my, according
to (Vail and Todd, 1981), or 50, 3-50, and 0.5-3 my, according to Neal et al. (1993). The
first-order cycle is attributed to continental breakup, the second to changes in the rate of
subsidence and uplift due to plate movements, the third is the sequence cycle, described
more fully later. Fourth and higher order cycles are attributed to climatic changes.
The third-order sequence cycle is illustrated in Plate 2. Conventionally the cycle starts
with the lowest sea level at which time a low stand systems tract overlies a type 1 unconformity (Plate 2A). Sandy lobate submarine fans are deposited at the foot of the continental slope. As sea level begins to rise turbidite sands and shales are deposited on
the continental slope (Plate 2B). Continuing submergence increases the accommodation space between basin floor and sea level, allowing upward-coarsening deltas to prograde out across the basin (Plate 2C). Once sea level has risen over the shelf edge it will
migrate rapidly across the shelf, cutting off sediment supply to the basin, and deposit a