304
Chapter 7 Sequences, Minor Cycles, and Event Stratigraphy
a
1-0 SEDIMENT ACCOMMODATION MODELS
(FOR TIME INTERVAL M WITHIN ONE HALF-CYCLEl
b
SL-RISE AND FALL (llSLF< llSUB)
--7I\-;2~",,-A\--if'-1- -:,\---T- J
- I.J.'
I
I
WD2 = 0 (EMERGENCE)
I />SLR;
I
WD, "SLF "l. ~;LUS SED.
SEA LEVEL 1 Lt.ACC, 1
2~ t- J _ _ _ 1_
. ~ BYPASSING
-l~--lri-r ~D2 (RISE)
- I
~
.'
I t.SU
1
8 IAr LF i i
0' k' luM1?,0 t.ISED
wb,
t ~.3-1--l'ivVD3
JLt.SLF
I
~ ~ ... FALL)
t.ACC~"-l----I
... f /
-~-"SED. SURFACE
i'::>LF
t.SED
BASIC EQUA TIONS
t.ACC
t.SUB 1 I
LlACC = llSLR + llSUB (1)
.-.lt-t -. . -
or llACC = llSUB - llSLR (2)
2_71-- - - - - - --llWD=WD 2 -WD 1 =llACC-llSED (3)
Fig. 7.4. Sediment accommodation model demonstrating the interplay between relative sea-level
(base-level) rise, ÄSLR, and fall, ÄSLF, subsidence
of basin floor (pre-existing sediment surface),
ÄSUB, and sediment accumulation (vertical
Sediment Buildup vs. Time: Three Principal
Variables
The following sediment buildup-time models also consider only one location within a basin, but the time span
comprises at least one full base-level cycle. Taking into
account the three parameters controlling accommodation space and water depth, it is obvious that there are
three principal modes of gaining or losing accommodation space to store water and sediment. This is shown
in Fig. 7.5 where only one parameter (sea level, subsidence, or sediment supply) is varied while the other two
parameters are kept constant. Modes a and b (constant
subsidence) are those which are usually referred to in
sequence stratigraphy, particularly so for medium and
high-frequency base-level cycles. Mode c (varying regional and global subsidence) is the mechanism advocated to interpret long-term base-level changes (cf. Fig.
7.35). The situation becomes more complex iftwo or
even three of the controlling factors vary with time.
Sediment Buildup vs. Time: Variation in Sea Level
and Sediment Supply
These models can be applied to time periods of constant
subsidence. We consider three types of sea- or baselevel change:
aggradation), ÄSED, in time interval, Ät. ÄACC,
Gain or loss of accommodation (vertical space) to
store sediment; ÄWD = WD 1 -WD 2 = increase or decrease in water depth in Ät. a SLR and SLF,
SUB>SLF. b SLF, SUB
case oflow-frequency sea-level changes (Fig. 7.6a), the
rate of subsidence, SUB, is commonly greater than the
maximum rate of relative sea-level fall, SLF max' Therefore, older sea floor (present prior to the beginning of
the cycle) cannot be eroded during the base-level cycle.
If sediment supply and potential sediment buildup,
SED p , are always greater than SUB + SLR, the basin
will be filled up with sediment all the time, but the rates
of sediment buildup decrease during the phase offalling
base level (indicated by isochrones). Surplus sediment
is exported to other areas. With SED ::::; SUB, the location in question will display relatively long phases of
deepening and shallowing, but erosion of sediment deposited within the cycle does not take place. These variants of the model more or less characterize the longterm evolution of intracratonic basins where all three
parameters, i.e. subsidence, sea- or base-level change,
and sediment supply, operated at relatively low rates.
The model can also be applied to sea-level cycles of
higher frequency as long as SLF max
variation and four different rates of sediment buildup,
SED, which are assumed to remain constant for each
model variant. With SED 1 being slow, the water depth
at the location considered will become deeper with time
in a stepwise manner. When SED2, SED3, and SED4,
Chapter 7 Sequences, Minor Cycles, and Event Stratigraphy
a
1-0 SEDIMENT ACCOMMODATION MODELS
(FOR TIME INTERVAL M WITHIN ONE HALF-CYCLEl
b
SL-RISE AND FALL (llSLF< llSUB)
--7I\-;2~",,-A\--if'-1- -:,\---T- J
- I.J.'
I
I
WD2 = 0 (EMERGENCE)
I />SLR;
I
WD, "SLF "l. ~;LUS SED.
SEA LEVEL 1 Lt.ACC, 1
2~ t- J _ _ _ 1_
. ~ BYPASSING
-l~--lri-r ~D2 (RISE)
- I
~
.'
I t.SU
1
8 IAr LF i i
0' k' luM1?,0 t.ISED
wb,
t ~.3-1--l'ivVD3
JLt.SLF
I
~ ~ ... FALL)
t.ACC~"-l----I
... f /
-~-"SED. SURFACE
i'::>LF
t.SED
BASIC EQUA TIONS
t.ACC
t.SUB 1 I
LlACC = llSLR + llSUB (1)
.-.lt-t -. . -
or llACC = llSUB - llSLR (2)
2_71-- - - - - - --llWD=WD 2 -WD 1 =llACC-llSED (3)
Fig. 7.4. Sediment accommodation model demonstrating the interplay between relative sea-level
(base-level) rise, ÄSLR, and fall, ÄSLF, subsidence
of basin floor (pre-existing sediment surface),
ÄSUB, and sediment accumulation (vertical
Sediment Buildup vs. Time: Three Principal
Variables
The following sediment buildup-time models also consider only one location within a basin, but the time span
comprises at least one full base-level cycle. Taking into
account the three parameters controlling accommodation space and water depth, it is obvious that there are
three principal modes of gaining or losing accommodation space to store water and sediment. This is shown
in Fig. 7.5 where only one parameter (sea level, subsidence, or sediment supply) is varied while the other two
parameters are kept constant. Modes a and b (constant
subsidence) are those which are usually referred to in
sequence stratigraphy, particularly so for medium and
high-frequency base-level cycles. Mode c (varying regional and global subsidence) is the mechanism advocated to interpret long-term base-level changes (cf. Fig.
7.35). The situation becomes more complex iftwo or
even three of the controlling factors vary with time.
Sediment Buildup vs. Time: Variation in Sea Level
and Sediment Supply
These models can be applied to time periods of constant
subsidence. We consider three types of sea- or baselevel change:
aggradation), ÄSED, in time interval, Ät. ÄACC,
Gain or loss of accommodation (vertical space) to
store sediment; ÄWD = WD 1 -WD 2 = increase or decrease in water depth in Ät. a SLR and SLF,
SUB>SLF. b SLF, SUB
case oflow-frequency sea-level changes (Fig. 7.6a), the
rate of subsidence, SUB, is commonly greater than the
maximum rate of relative sea-level fall, SLF max' Therefore, older sea floor (present prior to the beginning of
the cycle) cannot be eroded during the base-level cycle.
If sediment supply and potential sediment buildup,
SED p , are always greater than SUB + SLR, the basin
will be filled up with sediment all the time, but the rates
of sediment buildup decrease during the phase offalling
base level (indicated by isochrones). Surplus sediment
is exported to other areas. With SED ::::; SUB, the location in question will display relatively long phases of
deepening and shallowing, but erosion of sediment deposited within the cycle does not take place. These variants of the model more or less characterize the longterm evolution of intracratonic basins where all three
parameters, i.e. subsidence, sea- or base-level change,
and sediment supply, operated at relatively low rates.
The model can also be applied to sea-level cycles of
higher frequency as long as SLF max
variation and four different rates of sediment buildup,
SED, which are assumed to remain constant for each
model variant. With SED 1 being slow, the water depth
at the location considered will become deeper with time
in a stepwise manner. When SED2, SED3, and SED4,