7.2 Basic Concepts
Instead or in addition to the Type 1 and 2 sequence boundaries defined by the EXXON Group, other surfaces have been
proposed as sequence boundaries forthe delineation of cycles.
Several authors use the maximum tlooding surface, MFS, as
sequence boundary (Galloway 1989; HelIand-Hansen 1996),
because this surface is isochronous and may represent both an
easily recognizable lithology and fossil-rich horizon. It therefore often depends on the depositional environment which sort
of sequence boundary is best applicable. The rules for the
generation of sequence boundaries cannot be applied to deep,
slowly subsiding ocean basins where bottom currents largely
control sediment distribution (cf. Sect. 5.5). Some authors
have pointed out that sequence boundaries only develop over
limited areas within an individual basin and do not form extensive, basin-wide surfaces (e.g. Cartwright et al. 1993).
Parasequences (PS) represent smaller units than 3rdorder sequenee eycles. Their time periods are in the
order ofthe Milankoviteh frequeney band (Seet. 7.8.5
and 7.9.2). Parasequenee sets are sueeessions of genetieally related parasequenees whieh form distinetive
staeking patterns whieh either pro grade or step back.
Chronostratigraphic sections or charts (Wheeler
diagrams, Fig. 7.3h). The units of a stratigraphie cross
seetion are subdivided by isochrones and plotted with
a linear time seale. This ehart displays areas of deposition and non-deposition for individual time units and
thus also the duration of stratigraphie gaps and eondensed seetions. It does not show the sediment thieknesses.
Thethicknesses ofsequences (eycles). Inareasoflimited aeeommodation spaee (e.g. in shallow water), the
thiekness of a stratigraphie sequenee (eycle) is eommonly eontrolled by subsidence takingplace during one
cycle period. Ifthe aceommodation space for sediment
is always large (e.g. in deep water), the cycle thickness
depends on both eycle period and sedimentation rate.
7.2.3 One-Dimensional Models (l-D)
One-dimensional (1-D) models describe processes taking place at one location within a basin or affeeting
larger areas of a uniformly subsiding and sediment
Fig. 7.3. Definition of various terms in sequence
stratigraphy. a The coneept of base-level change for
fluvial systems (inclined surface) as compared with
lake- and sea-level change (horizontal surface).
b Terms used to describe sea-Ievel ehanges (here a
sinusoidal eurve) through time. c The equilibrium
point charaeterizes a location (and time) where the
rate of eustatic sea-level fall, SLF, is equal to the rate
of subsidence, SUB. Relative sea-level change is
RSL = eust. SLF-SUB, which may be either positive
or negative. d Allomember represented by a facies
traet, eonsisting of facies types 1, 2, 3, bounded by
diachrones. e Principal stratal patterns relevant in
303
aggrading basin. With respect to sediment aeeumulation, two processes are of eminent importance: (1) gain
or loss of accommodation space to store sediment and
(2) erosion of older sediment and bypassing of new
incorning or in plaee produeed sediment from one location to another. The second process will be further diseussed in Section 7.2.4. In all models of water-filled
basins the sea-level only is shown. In reality the sealevel should be replaced by the wave base moving up
and down approximately in the same way as the water
surfaee (cf. Fig. 7.7). It is commonly the (storm) wave
base, including coastal currents, which controls the water depth where an equilibrium between deposition and
erosion is established. For reasons of simplicity this fact
is ornitted here.
The Accommodation Space Concept
The I-D model ofa water-filled basin (Fig. 7.4) eonsiders a short increment oftime, ~t, within a half-cycle of
base- or sea-level change. The model demonstrates the
fundamental relationship between sea-level change,
subsidence, vertical sediment accumulation
(aggradation) and the resulting changes in accommodation space and water depth. Sea-level rise within a certain time interval, ~t, commonly leads to a gain in accommodation space (Fig. 7.4a, equation 1), but not necessarily to an increase in water depth. This is also controlled by the actual sediment accumulation during ~t
on top ofthe previous basin floor (equation 3). Falling
sea level reduces the accommodation space only when
the rate of sea-level fall is greater than that of subsidenee (Fig. 7.4b, equation 2). If sediment supply or
potential sediment buildup is greater than the available
accommo:iation spaee, surplus sediment is bypassed to
locations of deeper water.
The relationship between eustatic sea-Ievel variation
and subsidence and thus the potential for sediment accumulation (or erosion) changes with time, i.e. during
sea-level or base-level cycles. This behavior is discussed in the following models in which time is one of
the variables controlling the depositional system.
sequence stratigraphy. f Some stratal termination
patterns used in seismic stratigraphy. g Systems
tracts and stacking of parasequences of a complete
sequence in a shelf-slope setting (cross section, after
Steekler et al. 1993, modified). The sequenee begins
with a type I sequence boundary, SB1, and overlies
older highstand deposits. It terminates with a type 2
sequence boundary, SB2, and is overlain by a shelf
margin wedge, SMW. The horizontal bars in the
Wheeler plot h indicate zones of deposition during
successive time steps (parasequences); dots show the
positions of the depositional shelf breaks. F or further
explanation see text
Instead or in addition to the Type 1 and 2 sequence boundaries defined by the EXXON Group, other surfaces have been
proposed as sequence boundaries forthe delineation of cycles.
Several authors use the maximum tlooding surface, MFS, as
sequence boundary (Galloway 1989; HelIand-Hansen 1996),
because this surface is isochronous and may represent both an
easily recognizable lithology and fossil-rich horizon. It therefore often depends on the depositional environment which sort
of sequence boundary is best applicable. The rules for the
generation of sequence boundaries cannot be applied to deep,
slowly subsiding ocean basins where bottom currents largely
control sediment distribution (cf. Sect. 5.5). Some authors
have pointed out that sequence boundaries only develop over
limited areas within an individual basin and do not form extensive, basin-wide surfaces (e.g. Cartwright et al. 1993).
Parasequences (PS) represent smaller units than 3rdorder sequenee eycles. Their time periods are in the
order ofthe Milankoviteh frequeney band (Seet. 7.8.5
and 7.9.2). Parasequenee sets are sueeessions of genetieally related parasequenees whieh form distinetive
staeking patterns whieh either pro grade or step back.
Chronostratigraphic sections or charts (Wheeler
diagrams, Fig. 7.3h). The units of a stratigraphie cross
seetion are subdivided by isochrones and plotted with
a linear time seale. This ehart displays areas of deposition and non-deposition for individual time units and
thus also the duration of stratigraphie gaps and eondensed seetions. It does not show the sediment thieknesses.
Thethicknesses ofsequences (eycles). Inareasoflimited aeeommodation spaee (e.g. in shallow water), the
thiekness of a stratigraphie sequenee (eycle) is eommonly eontrolled by subsidence takingplace during one
cycle period. Ifthe aceommodation space for sediment
is always large (e.g. in deep water), the cycle thickness
depends on both eycle period and sedimentation rate.
7.2.3 One-Dimensional Models (l-D)
One-dimensional (1-D) models describe processes taking place at one location within a basin or affeeting
larger areas of a uniformly subsiding and sediment
Fig. 7.3. Definition of various terms in sequence
stratigraphy. a The coneept of base-level change for
fluvial systems (inclined surface) as compared with
lake- and sea-level change (horizontal surface).
b Terms used to describe sea-Ievel ehanges (here a
sinusoidal eurve) through time. c The equilibrium
point charaeterizes a location (and time) where the
rate of eustatic sea-level fall, SLF, is equal to the rate
of subsidence, SUB. Relative sea-level change is
RSL = eust. SLF-SUB, which may be either positive
or negative. d Allomember represented by a facies
traet, eonsisting of facies types 1, 2, 3, bounded by
diachrones. e Principal stratal patterns relevant in
303
aggrading basin. With respect to sediment aeeumulation, two processes are of eminent importance: (1) gain
or loss of accommodation space to store sediment and
(2) erosion of older sediment and bypassing of new
incorning or in plaee produeed sediment from one location to another. The second process will be further diseussed in Section 7.2.4. In all models of water-filled
basins the sea-level only is shown. In reality the sealevel should be replaced by the wave base moving up
and down approximately in the same way as the water
surfaee (cf. Fig. 7.7). It is commonly the (storm) wave
base, including coastal currents, which controls the water depth where an equilibrium between deposition and
erosion is established. For reasons of simplicity this fact
is ornitted here.
The Accommodation Space Concept
The I-D model ofa water-filled basin (Fig. 7.4) eonsiders a short increment oftime, ~t, within a half-cycle of
base- or sea-level change. The model demonstrates the
fundamental relationship between sea-level change,
subsidence, vertical sediment accumulation
(aggradation) and the resulting changes in accommodation space and water depth. Sea-level rise within a certain time interval, ~t, commonly leads to a gain in accommodation space (Fig. 7.4a, equation 1), but not necessarily to an increase in water depth. This is also controlled by the actual sediment accumulation during ~t
on top ofthe previous basin floor (equation 3). Falling
sea level reduces the accommodation space only when
the rate of sea-level fall is greater than that of subsidenee (Fig. 7.4b, equation 2). If sediment supply or
potential sediment buildup is greater than the available
accommo:iation spaee, surplus sediment is bypassed to
locations of deeper water.
The relationship between eustatic sea-Ievel variation
and subsidence and thus the potential for sediment accumulation (or erosion) changes with time, i.e. during
sea-level or base-level cycles. This behavior is discussed in the following models in which time is one of
the variables controlling the depositional system.
sequence stratigraphy. f Some stratal termination
patterns used in seismic stratigraphy. g Systems
tracts and stacking of parasequences of a complete
sequence in a shelf-slope setting (cross section, after
Steekler et al. 1993, modified). The sequenee begins
with a type I sequence boundary, SB1, and overlies
older highstand deposits. It terminates with a type 2
sequence boundary, SB2, and is overlain by a shelf
margin wedge, SMW. The horizontal bars in the
Wheeler plot h indicate zones of deposition during
successive time steps (parasequences); dots show the
positions of the depositional shelf breaks. F or further
explanation see text