7.10 General Discussion
7.10 General Discussion (Sequence and
Event Stratigraphy)
7.10.1 Principal Achievements in Sequence,
Cyclo- and Event-Stratigraphy
(1) Modem sequence stratigraphy, cyclo- and eventstratigraphy have provided concepts which have revolutionized classical stratigraphy. These concepts can be
applied to deposits of all ages, from the Precambrian to
the Quatemary, and to almost all depositional environments: marine and continental, inc1uding silicic1astic,
carbonate, siliceous, and mixed sediments. The identification, detailed mapping, and correlation of sequence
boundaries and other characteristic surfaces (formerly
largely neglected and overlooked) enables a better understanding and interpretation of large-scale vertical
and lateral facies relationships in a depositional system. In particular, coastal and shallow-marine environments respond sensitively to relative sea-level changes
by shifts in their depocenters of sediment accumulation. Basinward and landward migrating facies tracts,
inc1uding coastal sands and carbonate buildups, create
both reservoirs and seal rocks for hydrocarbons and
groundwater. Many prominent coal deposits of the
world are associated with repeated flooding events
affecting marine deltas, coastal plains, and incised valleys.
(2) Sequence stratigraphy is a complicate topic, however, although the basic concept (the sediment accommodation model) is straightforward and easy to take
up. To understand sea level-induced facies architecture, the various factors controlling a depositional system (tectonic setting, differential subsidence,
depositional environment, allochthonous and
autochthonous sediment supply, frequencyand amplitude of sea-level changes, etc.) have to be evaluated.
For this reason, a considerable number of different
"dynamic" facies models has been proposed.
(3) In order to deduce sea-level changes from the resulting depositional sequences, transgressive and regressive sections as weIl as sequence boundaries must
be identified and dated. A combination of data from
field observations, drilling cores, welllogging and reflection seismic profiles, revealing, e.g., large-scale
features of low-angle coastal onlap and offlap, is the
best way to study the architecture and genesis of sequence cyc1es (e.g., Baum and Vail1987; Weimer and
Posamentier 1993).
(4) Carbonate systems fed by autochthonous sediment,
especially rimmed carbonate platforms following rising and falling sea levels, respond to sea-level changes
in a different way as silicic1astic systems fed by
terrigenous sediment supply. Mixed carbonate381
silicic1astic systems are often particularly sensitive to
sea-level changes.
(5) Subaerial and submarine erosional unconforrnities
in shallow~marine sediments (Type 1 sequence boundaries, SB 1) only occur when the rate of sea-level fall is
greater than the rate of sea-floor subsidence. High-frequency sea-level changes therefore have a much
greater potential of generating SB I' s than low-frequency oscillations (e.g. ofthe 3rd and 2nd order). The
basinward extent of SB I' s in basin cross sections depends on the amplitude of sea-level changes and the
slope of the sediment surface. Flat carbonate platforms, for example, document low-amplitude sea-level
oscillations over wide areas. In regions of slow subsidence, global sea-level fall is normally recorded as
relative fall, but in areas of faster subsidence, it often
appears as a relative rise. Similarly, over-supplied basins are more sensitive to sea- or base-level changes
than under-supplied ones. Local subsidence or uplift
creates relative sea-level changes even during times of
a globally ± stable sea level.
(6) In somewhat deeperwater, sea-level highstands are
frequently recorded by condensed sections (e.g.
hemipelagic to pelagic sediments, black shales,
authigenic minerals, andJor composite skeletal horizons). Some ofthe bio-events (incursion epiboles) and
eco-events (e.g. redox cyc1es) may occur at the same
time.
(7) Deep-sea environments often reflect sea-level
changes indirectly. They may record an increased number of gravity mass movements and turbidity currents,
or a greater activity of bottom currents triggered by
relative sea-level fall. Sea-level rise frequently favored
anoxia in deep waters and intermediate waters.
(8) When sea-level changes of different orders are superimposed, it is usually the high-amplitude oscillation
(4th to 5th order) which is recorded best in field exposures and drill holes of shallow-water environments.
The signature of these oscillations appears to be best
preserved in low-energy, slowly subsiding basins.
Then, weIl supplied silicic1astic systems develop seaward prograding facies tracts, or. carbonate buildups
show peritidal cyc1es at their tops and characteristic
facies changes on their slopes. However, the nature of
these minor cycles (or parasequences) changes within
the systems tracts of lower order sequences. These in
turn can be weIl identified in seismic lines across basin
margins.
(9) For continental environments, the basic accommodation model is also valid, but sea-level change has to
be replaced by the base-level concept. This explains,
for example, the prograding or backstepping of fluvial
facies belts and the architecture of either vertically or
7.10 General Discussion (Sequence and
Event Stratigraphy)
7.10.1 Principal Achievements in Sequence,
Cyclo- and Event-Stratigraphy
(1) Modem sequence stratigraphy, cyclo- and eventstratigraphy have provided concepts which have revolutionized classical stratigraphy. These concepts can be
applied to deposits of all ages, from the Precambrian to
the Quatemary, and to almost all depositional environments: marine and continental, inc1uding silicic1astic,
carbonate, siliceous, and mixed sediments. The identification, detailed mapping, and correlation of sequence
boundaries and other characteristic surfaces (formerly
largely neglected and overlooked) enables a better understanding and interpretation of large-scale vertical
and lateral facies relationships in a depositional system. In particular, coastal and shallow-marine environments respond sensitively to relative sea-level changes
by shifts in their depocenters of sediment accumulation. Basinward and landward migrating facies tracts,
inc1uding coastal sands and carbonate buildups, create
both reservoirs and seal rocks for hydrocarbons and
groundwater. Many prominent coal deposits of the
world are associated with repeated flooding events
affecting marine deltas, coastal plains, and incised valleys.
(2) Sequence stratigraphy is a complicate topic, however, although the basic concept (the sediment accommodation model) is straightforward and easy to take
up. To understand sea level-induced facies architecture, the various factors controlling a depositional system (tectonic setting, differential subsidence,
depositional environment, allochthonous and
autochthonous sediment supply, frequencyand amplitude of sea-level changes, etc.) have to be evaluated.
For this reason, a considerable number of different
"dynamic" facies models has been proposed.
(3) In order to deduce sea-level changes from the resulting depositional sequences, transgressive and regressive sections as weIl as sequence boundaries must
be identified and dated. A combination of data from
field observations, drilling cores, welllogging and reflection seismic profiles, revealing, e.g., large-scale
features of low-angle coastal onlap and offlap, is the
best way to study the architecture and genesis of sequence cyc1es (e.g., Baum and Vail1987; Weimer and
Posamentier 1993).
(4) Carbonate systems fed by autochthonous sediment,
especially rimmed carbonate platforms following rising and falling sea levels, respond to sea-level changes
in a different way as silicic1astic systems fed by
terrigenous sediment supply. Mixed carbonate381
silicic1astic systems are often particularly sensitive to
sea-level changes.
(5) Subaerial and submarine erosional unconforrnities
in shallow~marine sediments (Type 1 sequence boundaries, SB 1) only occur when the rate of sea-level fall is
greater than the rate of sea-floor subsidence. High-frequency sea-level changes therefore have a much
greater potential of generating SB I' s than low-frequency oscillations (e.g. ofthe 3rd and 2nd order). The
basinward extent of SB I' s in basin cross sections depends on the amplitude of sea-level changes and the
slope of the sediment surface. Flat carbonate platforms, for example, document low-amplitude sea-level
oscillations over wide areas. In regions of slow subsidence, global sea-level fall is normally recorded as
relative fall, but in areas of faster subsidence, it often
appears as a relative rise. Similarly, over-supplied basins are more sensitive to sea- or base-level changes
than under-supplied ones. Local subsidence or uplift
creates relative sea-level changes even during times of
a globally ± stable sea level.
(6) In somewhat deeperwater, sea-level highstands are
frequently recorded by condensed sections (e.g.
hemipelagic to pelagic sediments, black shales,
authigenic minerals, andJor composite skeletal horizons). Some ofthe bio-events (incursion epiboles) and
eco-events (e.g. redox cyc1es) may occur at the same
time.
(7) Deep-sea environments often reflect sea-level
changes indirectly. They may record an increased number of gravity mass movements and turbidity currents,
or a greater activity of bottom currents triggered by
relative sea-level fall. Sea-level rise frequently favored
anoxia in deep waters and intermediate waters.
(8) When sea-level changes of different orders are superimposed, it is usually the high-amplitude oscillation
(4th to 5th order) which is recorded best in field exposures and drill holes of shallow-water environments.
The signature of these oscillations appears to be best
preserved in low-energy, slowly subsiding basins.
Then, weIl supplied silicic1astic systems develop seaward prograding facies tracts, or. carbonate buildups
show peritidal cyc1es at their tops and characteristic
facies changes on their slopes. However, the nature of
these minor cycles (or parasequences) changes within
the systems tracts of lower order sequences. These in
turn can be weIl identified in seismic lines across basin
margins.
(9) For continental environments, the basic accommodation model is also valid, but sea-level change has to
be replaced by the base-level concept. This explains,
for example, the prograding or backstepping of fluvial
facies belts and the architecture of either vertically or