320
Chapter 7 Sequences, Minor Cycles, and Event Stratigraphy
Denudation of fonner coastal-marine sediments will become more pronounced on coastlines with uplift. In contrast,
flexural subsidence landward ofthe hinge line can lead to the
deposition and preservation of coastal plain sediments separating the two lag horizons (Fig. 7.15b, uppennost eycle) ..
Eroded sand-sized particles are redeposited as stonn layers
(tempestites), or as sandy-shelly bars. Calcareous or iron ooids
fonned in shallow water during high sea-Ievel stand may be
transported seaward and come to rest on submarine erosional
surfaces (Fig. 7 .16c). Later, pre-existing channels are filled
with transgressive sediments, and soi! horizons andkarstified
surfaces are covered by younger deposits (Fig. 7.16d and e).
Coastal onlap during transgression (cf. Fig. 7.3f) is indistinet
because ofthe gentle gradient ofthe sea floor.
The depth range of erosion and the intensity ofreworking generally decrease basinward until the erosional
surfaces (Type 1 sequence boundaries) merge into sediments which are no longer affected by storm-wave erosion. The same (diachronous) erosional surface can sit
landward on a transgressive unit and basinward on the
subsequent regressive unit. The position ofthe Type 2
boundaries at the transition from HST to LST is not as
clear, due to the diachronous nature of the erosional
unconformities (Fig. 7 .15b). Vertical sections from this
cross-section may show sharp-based shoreface sequences with erosional gaps (Fig. 7 . 13c).
Beyond the depth range of the storm-wave base, the
chronostratigraphic sequence should be complete apart
from locations where erosive bottom currents operate.
Between the inflection point and peak of sea-Ievel rise,
the sequence may contain condensed horizons due to
reduced terrigenous sediment input. As mentioned
above, this horizon would be an appropriate alternative
as sequence boundary in central parts of the basin.
Figure 7.15c shows the same cross section as in (b)
but in the way of a sediment thickness/space diagram.
Here, the erosional unconformities ofthe successive TR cycles merge landward and form composite beds as
discussed above. It is important to note that the number
of cycles which can be identified in vertical sections
differs along the cross section.
In the central part ofthe basin (right-hand side ofFig. 7.15c)
it may be difficult to recognize cyclicity when sandy regressive deposits or distinct condensed seetions are missing. Similarly, the true number of cycles cannot be found at the very
edge ofthe basin. Even somewhat farther seaward, the number
of readi!y visible cycles varies due to the differing basinward
extension of the unconfonnities. Such local differences are
one of the major reasons why the number and correlation of
T-R cycles for a certain region and time period often remain
controversial.
Some authors have pointed out that not all reworked horizons found in shallow-sea sediments are associated with relative sea-Ievel changes. Part ofthese horizons mayresult from
episodic changes in the current system of the basin leading to
long-Iasting intervals of non-deposition and erosion (e.g.
Bloos 1990). Manyregional studies in various shallow-marine
environments have contributed to our knowledge in this field
(see, e.g., Aigner and Dott 1990; Steel et al. 1995; Emery and
Myers 1996; Miall 1997).
The duration and extent of stratigraphic gaps as weIl as
regional differences in sediment aggradation are better
demonstrated by isochrones marking time lines in the
basin fill (Fig. 7.15d). This model shows a situation
similar to that ofFigure 15a-c. It is based on two sinusoidal sea-Ievel cycles and limited sand supply, especially during relative sea-Ievel rise. The effects of bypassing, erosion and redistribution of sediment lead to
asymmetric "field water-depth" curves (cf. Fig. 7.7c,
Sect. 7.2.4). At locations land 11 on the inner shelfthe
field water-depth curves show an apparently long deepening and rapidly shallowing-upward phase, while sediment bypassed to location 111 generates thick
shallowing-upward units. Irregular changes in sea-Ievel,
sediment supply, and local subsidence may modify the
results of these idealized models.
7.3.3 Shelf-Break-Slope Margins,
Moderate Sediment Supply
Shelves with a distinct sheltbreak and continental-slope
to deep-sea transition usually develop along passive
continental margins (Sect. 12.2). They result not only
from the transition from continental to oceanic crost,
but to some extent also from up- and outbuilding of
sediment towards deeper water. The second process
predominates in the formation of carbonate platforms
(Sect. 7.5). Modem seismic stratigraphy and sequence
stratigraphy was developed mainly in cross sections of
the continental margins of the Atlantic ocean and the
Gulf ofMexico. Many ofthe published "classic" models more or less reflect the conditions ofthis basin type:
- Differential subsidence across a passive margin of
considerable age;
- Relative sea-Ievel fall exceeds subsidence on wide
shelf areas and often leads to emergence ofthe shelfbreak and uppermost slope;
- Moderate to relatively high terrigenous sediment supply:
Surplus of sediment on the inner shelf in relation to
long-term subsidence;
Slope failure during the lowstand systems tract.
The resulting third order sequences and their boundaries
are defined in Section 7.2.2.
Idealized examples have been frequently described (e.g. Wagoner et al. 1990; Vail et al. 1991; Emery and Myers 1996;
Miall 1997). It should be noted, however, that the models
developed for passive margin basins cannot be applied to
deviating basin settings without significant modifications.
This has to be conddered, for example, when shallow basins
such as epieontinental seas, fore land basins, or basins subjeeted to rapid subsidenee and loeal uplift are investigated. In
the following, abrief overview of the eoneeptual framework
for passive eontinental margin settings is given.
Chapter 7 Sequences, Minor Cycles, and Event Stratigraphy
Denudation of fonner coastal-marine sediments will become more pronounced on coastlines with uplift. In contrast,
flexural subsidence landward ofthe hinge line can lead to the
deposition and preservation of coastal plain sediments separating the two lag horizons (Fig. 7.15b, uppennost eycle) ..
Eroded sand-sized particles are redeposited as stonn layers
(tempestites), or as sandy-shelly bars. Calcareous or iron ooids
fonned in shallow water during high sea-Ievel stand may be
transported seaward and come to rest on submarine erosional
surfaces (Fig. 7 .16c). Later, pre-existing channels are filled
with transgressive sediments, and soi! horizons andkarstified
surfaces are covered by younger deposits (Fig. 7.16d and e).
Coastal onlap during transgression (cf. Fig. 7.3f) is indistinet
because ofthe gentle gradient ofthe sea floor.
The depth range of erosion and the intensity ofreworking generally decrease basinward until the erosional
surfaces (Type 1 sequence boundaries) merge into sediments which are no longer affected by storm-wave erosion. The same (diachronous) erosional surface can sit
landward on a transgressive unit and basinward on the
subsequent regressive unit. The position ofthe Type 2
boundaries at the transition from HST to LST is not as
clear, due to the diachronous nature of the erosional
unconformities (Fig. 7 .15b). Vertical sections from this
cross-section may show sharp-based shoreface sequences with erosional gaps (Fig. 7 . 13c).
Beyond the depth range of the storm-wave base, the
chronostratigraphic sequence should be complete apart
from locations where erosive bottom currents operate.
Between the inflection point and peak of sea-Ievel rise,
the sequence may contain condensed horizons due to
reduced terrigenous sediment input. As mentioned
above, this horizon would be an appropriate alternative
as sequence boundary in central parts of the basin.
Figure 7.15c shows the same cross section as in (b)
but in the way of a sediment thickness/space diagram.
Here, the erosional unconformities ofthe successive TR cycles merge landward and form composite beds as
discussed above. It is important to note that the number
of cycles which can be identified in vertical sections
differs along the cross section.
In the central part ofthe basin (right-hand side ofFig. 7.15c)
it may be difficult to recognize cyclicity when sandy regressive deposits or distinct condensed seetions are missing. Similarly, the true number of cycles cannot be found at the very
edge ofthe basin. Even somewhat farther seaward, the number
of readi!y visible cycles varies due to the differing basinward
extension of the unconfonnities. Such local differences are
one of the major reasons why the number and correlation of
T-R cycles for a certain region and time period often remain
controversial.
Some authors have pointed out that not all reworked horizons found in shallow-sea sediments are associated with relative sea-Ievel changes. Part ofthese horizons mayresult from
episodic changes in the current system of the basin leading to
long-Iasting intervals of non-deposition and erosion (e.g.
Bloos 1990). Manyregional studies in various shallow-marine
environments have contributed to our knowledge in this field
(see, e.g., Aigner and Dott 1990; Steel et al. 1995; Emery and
Myers 1996; Miall 1997).
The duration and extent of stratigraphic gaps as weIl as
regional differences in sediment aggradation are better
demonstrated by isochrones marking time lines in the
basin fill (Fig. 7.15d). This model shows a situation
similar to that ofFigure 15a-c. It is based on two sinusoidal sea-Ievel cycles and limited sand supply, especially during relative sea-Ievel rise. The effects of bypassing, erosion and redistribution of sediment lead to
asymmetric "field water-depth" curves (cf. Fig. 7.7c,
Sect. 7.2.4). At locations land 11 on the inner shelfthe
field water-depth curves show an apparently long deepening and rapidly shallowing-upward phase, while sediment bypassed to location 111 generates thick
shallowing-upward units. Irregular changes in sea-Ievel,
sediment supply, and local subsidence may modify the
results of these idealized models.
7.3.3 Shelf-Break-Slope Margins,
Moderate Sediment Supply
Shelves with a distinct sheltbreak and continental-slope
to deep-sea transition usually develop along passive
continental margins (Sect. 12.2). They result not only
from the transition from continental to oceanic crost,
but to some extent also from up- and outbuilding of
sediment towards deeper water. The second process
predominates in the formation of carbonate platforms
(Sect. 7.5). Modem seismic stratigraphy and sequence
stratigraphy was developed mainly in cross sections of
the continental margins of the Atlantic ocean and the
Gulf ofMexico. Many ofthe published "classic" models more or less reflect the conditions ofthis basin type:
- Differential subsidence across a passive margin of
considerable age;
- Relative sea-Ievel fall exceeds subsidence on wide
shelf areas and often leads to emergence ofthe shelfbreak and uppermost slope;
- Moderate to relatively high terrigenous sediment supply:
Surplus of sediment on the inner shelf in relation to
long-term subsidence;
Slope failure during the lowstand systems tract.
The resulting third order sequences and their boundaries
are defined in Section 7.2.2.
Idealized examples have been frequently described (e.g. Wagoner et al. 1990; Vail et al. 1991; Emery and Myers 1996;
Miall 1997). It should be noted, however, that the models
developed for passive margin basins cannot be applied to
deviating basin settings without significant modifications.
This has to be conddered, for example, when shallow basins
such as epieontinental seas, fore land basins, or basins subjeeted to rapid subsidenee and loeal uplift are investigated. In
the following, abrief overview of the eoneeptual framework
for passive eontinental margin settings is given.