306
Chapter 7 Sequences, Minor Cycles, and Event Stratigraphy
In summary, the models with long-term SED>SUB or
SED=SUB (and SLFmax>SUB) generally show time
intervals (during sea-level rise) of a deepening basin
and deposition of all the incoming sediment alternating
with intervals of excess sediment supply and erosion
during sea-level fall. In settings of differential subsidence, a combination ofthe models a and bin Fig. 7.6
has to be applied to interpret the phenomena of 3rd order sea-level changes (Sect. 7.2.4).
- High-frequency sea-level change. Figure 7.6c deals
with high-frequency sea-level oscillations ofrelatively
high amplitude as known from the Quaternary. The rate
of (constant) potential sediment buildup, SEDp, is
higher than that of subsidence, SUB, assumed to be
relatively low. Rapid sea-level fall allows substantial
erosion of previously deposited material and creates
long stratigraphic gaps (cf. model b). Erosion may
lower the total sedimentary surface being exposed or
affected by wave action, or incise valleys. As in model
b (SED2-4), the thickness ofthe resulting sedimentary
section is solely controlled by subsidence (once the sea
floor has reached the elevation of the sea-level
lowstand). The sediments preserved at this location only
represent the lowermost part of the sea-level rise; the
eroded sediments may leave behind some lags. A situation similar to this model has been observed in late
Quaternary sediments ofthe Rhone delta (Tesson et al.
1990).
- Varying sediment supply. In the models discussed so
far, the rate of sediment influx into the basin was assumed to be constant through time. Figure 7.7 demonstrates a case in which terrigenous sediment supply varies. Sediment influx is high during falling sea level, but
low during the transgressive phase. Such an assumption
agrees with many observations in nature. In the long
term, subsidence is compensated for by sediment accumulation.
The model ofFig. 7.7 represents a ramp setting along the margin of an epicontinental sea. The amplitude of sea level
change is 50 m and the duration of one transgression-regression cyele is 4 Ma. These assumptions imply SLF max>SUB
(SUB=1O m/Ma). The storm-wave base in the basin is asFig. 7.6. One-dimensional models demonstrating the
evolution of accommodation space/water depth at
one location within basin vs. time. The eustahc sealevel (base-level) variation is assumed to be sinusoidal, while the rates of subsidence, SUB, and sediment supply, SS, or potential sediment buildup,
SEDp, are kept constant. a Low-frequency (1st and
2nd order) sea-level change: SUB and mostly also
SEDp>SLFmax (= maximum rate of sea-level fall at
inflection point). Accommodation space, ACC, always increases but at changing rate. b Medium-frequency (3rd order) sea-level variation: SLFmax>SUB
sumed to be relatively shallow (20 m) because the basin area
and thus the fetch for the buildup of waves is limited. Location P is situated slightly above the storm-wave base oflow
sea level, point Q is in deeper water (Fig. 7.7a). The sediment
thickness-time diagram (Fig. 7.7b) shows the evolution at
point P. The sections ofP and Q (Fig. 7.7c) show sediment
thicknesses (linear scale) and isochrones (non-linear).
In contrast to the "constant supply models", the "varying supply" scenario leads to a prolonged phase of
deepening and slow sediment buildup during rising and
high sea level. The sedimentation rate tends to be lowest between the inflection point and the peak of
highstand and may therefore generate condensed sections, especially at location Q, consisting largely of
hemipelagic sediment. Depending on the depositional
environment, marls, limestones, black shales, skeletal
concentrations, or thin beds rich in authigenic minerals,
such as phosphorite or glauconite may form (Loutit et
al. 1988).
During falling sea level, sediment buildup at points
P and Q proceeds in markedly different ways (Fig.
7.7 c). At P the phase of rapid sediment buildup is truncated by a long interval of erosion (SB 1).
Ifthe previously deposited sediments are cohesive (elays, silty
elays, marls) and contain layers which are already
diageneticallyindurated (e.g. hardgrounds, limestone or siderite concretions, limestone layers), submarine erosion often
ends at such horizons (cf. Fig. 7.16a, band c). As a result,
lags ofthe eroded material may be left behind (e.g., reworked
fossils and concretions, iron ooids, elasts of semi-solid
mudstone ). Sessile fauna requiring hard substrate may establish new communities, ineluding rock-boring species. Storm
wave erosion may proceed stepwise from shallower to deeper
indurated layers and locally cut channels.
The sequence cycle on top ofthe unconforrnity exhibits
a thin deepening-upward unit, a more or less condensed
section, and a thicker shallowing-upward unit which is
truncated by the next unconforrnity. The "field-water
depth" curve, directly derived from the thicknesses and
paleo-water depths ofthe individual units, has an asymmetric shape (in contrast to the symmetric model curve).
At location Q in deeper water, transgressive and
highstand sediments tend to be thinner than at P, but
sediment bypassed and eroded at P during SLF can in
and SEDp>SUB with four scenarios for different (but
constant with time) SS and SED . c High-frequency
(4th and 5th order) and high-ampfitude sea-level vanation: SLF max»SUB and SEDp>SUB. Possibly, a
small portion only of the sediment buildup during
SLR is preserved while most of the previously deposited and newly incoming material is eroded and bypassed into deeper water (e.g. known from
prograding Quaternary deltas). ER, sediment section
which may be eroded by the lowering sea level, either locally (incised valleys) or in wider areas. ONSET ER, onset of erosion; BYP, sediment bypassing.
Chapter 7 Sequences, Minor Cycles, and Event Stratigraphy
In summary, the models with long-term SED>SUB or
SED=SUB (and SLFmax>SUB) generally show time
intervals (during sea-level rise) of a deepening basin
and deposition of all the incoming sediment alternating
with intervals of excess sediment supply and erosion
during sea-level fall. In settings of differential subsidence, a combination ofthe models a and bin Fig. 7.6
has to be applied to interpret the phenomena of 3rd order sea-level changes (Sect. 7.2.4).
- High-frequency sea-level change. Figure 7.6c deals
with high-frequency sea-level oscillations ofrelatively
high amplitude as known from the Quaternary. The rate
of (constant) potential sediment buildup, SEDp, is
higher than that of subsidence, SUB, assumed to be
relatively low. Rapid sea-level fall allows substantial
erosion of previously deposited material and creates
long stratigraphic gaps (cf. model b). Erosion may
lower the total sedimentary surface being exposed or
affected by wave action, or incise valleys. As in model
b (SED2-4), the thickness ofthe resulting sedimentary
section is solely controlled by subsidence (once the sea
floor has reached the elevation of the sea-level
lowstand). The sediments preserved at this location only
represent the lowermost part of the sea-level rise; the
eroded sediments may leave behind some lags. A situation similar to this model has been observed in late
Quaternary sediments ofthe Rhone delta (Tesson et al.
1990).
- Varying sediment supply. In the models discussed so
far, the rate of sediment influx into the basin was assumed to be constant through time. Figure 7.7 demonstrates a case in which terrigenous sediment supply varies. Sediment influx is high during falling sea level, but
low during the transgressive phase. Such an assumption
agrees with many observations in nature. In the long
term, subsidence is compensated for by sediment accumulation.
The model ofFig. 7.7 represents a ramp setting along the margin of an epicontinental sea. The amplitude of sea level
change is 50 m and the duration of one transgression-regression cyele is 4 Ma. These assumptions imply SLF max>SUB
(SUB=1O m/Ma). The storm-wave base in the basin is asFig. 7.6. One-dimensional models demonstrating the
evolution of accommodation space/water depth at
one location within basin vs. time. The eustahc sealevel (base-level) variation is assumed to be sinusoidal, while the rates of subsidence, SUB, and sediment supply, SS, or potential sediment buildup,
SEDp, are kept constant. a Low-frequency (1st and
2nd order) sea-level change: SUB and mostly also
SEDp>SLFmax (= maximum rate of sea-level fall at
inflection point). Accommodation space, ACC, always increases but at changing rate. b Medium-frequency (3rd order) sea-level variation: SLFmax>SUB
sumed to be relatively shallow (20 m) because the basin area
and thus the fetch for the buildup of waves is limited. Location P is situated slightly above the storm-wave base oflow
sea level, point Q is in deeper water (Fig. 7.7a). The sediment
thickness-time diagram (Fig. 7.7b) shows the evolution at
point P. The sections ofP and Q (Fig. 7.7c) show sediment
thicknesses (linear scale) and isochrones (non-linear).
In contrast to the "constant supply models", the "varying supply" scenario leads to a prolonged phase of
deepening and slow sediment buildup during rising and
high sea level. The sedimentation rate tends to be lowest between the inflection point and the peak of
highstand and may therefore generate condensed sections, especially at location Q, consisting largely of
hemipelagic sediment. Depending on the depositional
environment, marls, limestones, black shales, skeletal
concentrations, or thin beds rich in authigenic minerals,
such as phosphorite or glauconite may form (Loutit et
al. 1988).
During falling sea level, sediment buildup at points
P and Q proceeds in markedly different ways (Fig.
7.7 c). At P the phase of rapid sediment buildup is truncated by a long interval of erosion (SB 1).
Ifthe previously deposited sediments are cohesive (elays, silty
elays, marls) and contain layers which are already
diageneticallyindurated (e.g. hardgrounds, limestone or siderite concretions, limestone layers), submarine erosion often
ends at such horizons (cf. Fig. 7.16a, band c). As a result,
lags ofthe eroded material may be left behind (e.g., reworked
fossils and concretions, iron ooids, elasts of semi-solid
mudstone ). Sessile fauna requiring hard substrate may establish new communities, ineluding rock-boring species. Storm
wave erosion may proceed stepwise from shallower to deeper
indurated layers and locally cut channels.
The sequence cycle on top ofthe unconforrnity exhibits
a thin deepening-upward unit, a more or less condensed
section, and a thicker shallowing-upward unit which is
truncated by the next unconforrnity. The "field-water
depth" curve, directly derived from the thicknesses and
paleo-water depths ofthe individual units, has an asymmetric shape (in contrast to the symmetric model curve).
At location Q in deeper water, transgressive and
highstand sediments tend to be thinner than at P, but
sediment bypassed and eroded at P during SLF can in
and SEDp>SUB with four scenarios for different (but
constant with time) SS and SED . c High-frequency
(4th and 5th order) and high-ampfitude sea-level vanation: SLF max»SUB and SEDp>SUB. Possibly, a
small portion only of the sediment buildup during
SLR is preserved while most of the previously deposited and newly incoming material is eroded and bypassed into deeper water (e.g. known from
prograding Quaternary deltas). ER, sediment section
which may be eroded by the lowering sea level, either locally (incised valleys) or in wider areas. ONSET ER, onset of erosion; BYP, sediment bypassing.