296
Chapter 7 Sequences, Minor Cycles, and Event StratigraphY
It is still very difficult to prove whether or not a
given cyclic sequence is really caused by a mechanism with a constant time period. In general, the average cycle duration can only be detennined by dating the beginning and end of a cyclic sequence (cf.
Sect. 7.9). Even in the case of one dominating constant period as in the astronomical time sc ale
(Milankovitch cycles; "periodites"), the thicknesses
and nature of individual beds, A and B, or cycles
may vary due to changing sediment composition or
fluctuating sedimentation rates (Fig. 7.1 c and d). In
addition, the superposition of cyclic phenomena of
different time periods (cf. Sects. 7.2.3, 7.8 and 7.9)
may result in complicated combined effects on climate change and sedimentary processes. The last effect may, for example, lag behind an orbital signal.
For these reasons it remains difficult to discover
whether a specific depositional system responded to
a true periodic process and actually formed
"periodites". Thus, many allocyclic phenomena in
sediments appear to be quasi-periodic rather than
strictly periodic.
7.1.5 Symmetry and Asymmetry of Sedimentary
Cycles
Sedimentary cycles, including cyclothems, may be
either symmetric or asymmetric (e.g., coarseningupward or fining-upward sequences), as weIl as complete or incomplete (Fig. 7.1 e and f). Symmetric sequences seem to be relatively rare in the ancient record. One example are the "bundles" in some
limestone-marl successions which originate from the
superposition of two or several periodically recurring
processes (Fig. 7.lg; Schwarzacher 1993). Asymmetric sequences occur everywhere including the deep
sea. Truncated sequences due to sub aerial and submarine erosion are an important topic in sequence
stratigraphy (Sects. 7.2 through 7.9). The so-called
punctuated aggradational cycles (Goodwin and Anderson 1985) represent truncated, shallowing-upward
sequences deposited in shallow water environments
under a fluctuating sea level.
7.1.6 Cycle Hierarchy
When cyclic phenomena of different frequencies and
natures are superposed, the resulting sedimentary
record may show a complex multi-cyclic pattern.
This reflects not only the added effects of two or
more cyclic processes, but the different processes
also exert some influence on each other. Usually, it is
the cycle of the shortest period which has the most
conspicuous effect. The term cycle hierarchy includes such an interplay between various factors controlling the actual stratigraphic record. (For more
details and examples see Sect. 7.8).
Sedimentary cycles are caused not only by various
physical mechanisms but are also influenced by biological processes, such as the appearance and mass
production of new organisms in specific environments, or the mass extinction of pre-existing groups.
Such repeated biological events may leave behind a
signature in the sedimentary record.
7.1.7 Biological Response to Sedimentological
Events and Cycles
One of the best means for discriminating between
subaquatic cyclic beds and event beds is the study of
benthic organisms and their burrows preserved in
beds and interbeds. Gradual environmental changes,
e.g. within the Milankovitch cycles, are accommodated by shifts in the faunal and floral spectrum and
species dominance. Short physical events, in contrast, are experienced by organisms as catastrophes,
wiping out the existing bottom life. Re-establishment
of the original bottom community can begin only
after the depositional event. In the case of tempestites
and turbidites, the post-event community may differ
from the background fauna, if the new substrate on
the sea floor provides different conditions for
epifauna and infauna.
Such taphonomic feedback systems also operate in
regressive and transgressive situations in shallow
seas. During regressions, the muddy fraction is winnowed and transported into deeper water, leaving
behind a shell layer which can be settled by a new
epifauna. Similarly, transgressive surfaces are often
marked by skeletal concentrations. In this situation,
the coarse biogenic particles are produced in place
and their preservation is enhanced by diagenesis.
Reduced siliciclastic influx during transgressions may allow the establishment of sessile epifaunal organisms (such
as some species of corals and oysters), who live in quiet
waters near or below the storm wave base and achieve stability through their large, massive skeletons. Rare storm
events may rework such outsized bioclasts and mix them
with diagenetically formed concretions (see below). Thus,
over significant periods of time, the production of outsized
bioclasts and concretional diaclasts may eventually generate thick shell beds or mounds. These may reflect repeated
episodes ofburial and winnowing and even form in regions
where overall fine-grained substrate conditions persist.
Epibenthic shell beds occur throughout the Phanerozoic
record. They have been frequently used as marker beds,
because they occur over considerable distances and commonly coincide with biozonal boundaries. For more details
see, e.g., Brett and Seilacher (1991), Seilacher (1991),
Kidwell (1991), Sepkoski et al. (1991), Brett and Baird
(1997).
Chapter 7 Sequences, Minor Cycles, and Event StratigraphY
It is still very difficult to prove whether or not a
given cyclic sequence is really caused by a mechanism with a constant time period. In general, the average cycle duration can only be detennined by dating the beginning and end of a cyclic sequence (cf.
Sect. 7.9). Even in the case of one dominating constant period as in the astronomical time sc ale
(Milankovitch cycles; "periodites"), the thicknesses
and nature of individual beds, A and B, or cycles
may vary due to changing sediment composition or
fluctuating sedimentation rates (Fig. 7.1 c and d). In
addition, the superposition of cyclic phenomena of
different time periods (cf. Sects. 7.2.3, 7.8 and 7.9)
may result in complicated combined effects on climate change and sedimentary processes. The last effect may, for example, lag behind an orbital signal.
For these reasons it remains difficult to discover
whether a specific depositional system responded to
a true periodic process and actually formed
"periodites". Thus, many allocyclic phenomena in
sediments appear to be quasi-periodic rather than
strictly periodic.
7.1.5 Symmetry and Asymmetry of Sedimentary
Cycles
Sedimentary cycles, including cyclothems, may be
either symmetric or asymmetric (e.g., coarseningupward or fining-upward sequences), as weIl as complete or incomplete (Fig. 7.1 e and f). Symmetric sequences seem to be relatively rare in the ancient record. One example are the "bundles" in some
limestone-marl successions which originate from the
superposition of two or several periodically recurring
processes (Fig. 7.lg; Schwarzacher 1993). Asymmetric sequences occur everywhere including the deep
sea. Truncated sequences due to sub aerial and submarine erosion are an important topic in sequence
stratigraphy (Sects. 7.2 through 7.9). The so-called
punctuated aggradational cycles (Goodwin and Anderson 1985) represent truncated, shallowing-upward
sequences deposited in shallow water environments
under a fluctuating sea level.
7.1.6 Cycle Hierarchy
When cyclic phenomena of different frequencies and
natures are superposed, the resulting sedimentary
record may show a complex multi-cyclic pattern.
This reflects not only the added effects of two or
more cyclic processes, but the different processes
also exert some influence on each other. Usually, it is
the cycle of the shortest period which has the most
conspicuous effect. The term cycle hierarchy includes such an interplay between various factors controlling the actual stratigraphic record. (For more
details and examples see Sect. 7.8).
Sedimentary cycles are caused not only by various
physical mechanisms but are also influenced by biological processes, such as the appearance and mass
production of new organisms in specific environments, or the mass extinction of pre-existing groups.
Such repeated biological events may leave behind a
signature in the sedimentary record.
7.1.7 Biological Response to Sedimentological
Events and Cycles
One of the best means for discriminating between
subaquatic cyclic beds and event beds is the study of
benthic organisms and their burrows preserved in
beds and interbeds. Gradual environmental changes,
e.g. within the Milankovitch cycles, are accommodated by shifts in the faunal and floral spectrum and
species dominance. Short physical events, in contrast, are experienced by organisms as catastrophes,
wiping out the existing bottom life. Re-establishment
of the original bottom community can begin only
after the depositional event. In the case of tempestites
and turbidites, the post-event community may differ
from the background fauna, if the new substrate on
the sea floor provides different conditions for
epifauna and infauna.
Such taphonomic feedback systems also operate in
regressive and transgressive situations in shallow
seas. During regressions, the muddy fraction is winnowed and transported into deeper water, leaving
behind a shell layer which can be settled by a new
epifauna. Similarly, transgressive surfaces are often
marked by skeletal concentrations. In this situation,
the coarse biogenic particles are produced in place
and their preservation is enhanced by diagenesis.
Reduced siliciclastic influx during transgressions may allow the establishment of sessile epifaunal organisms (such
as some species of corals and oysters), who live in quiet
waters near or below the storm wave base and achieve stability through their large, massive skeletons. Rare storm
events may rework such outsized bioclasts and mix them
with diagenetically formed concretions (see below). Thus,
over significant periods of time, the production of outsized
bioclasts and concretional diaclasts may eventually generate thick shell beds or mounds. These may reflect repeated
episodes ofburial and winnowing and even form in regions
where overall fine-grained substrate conditions persist.
Epibenthic shell beds occur throughout the Phanerozoic
record. They have been frequently used as marker beds,
because they occur over considerable distances and commonly coincide with biozonal boundaries. For more details
see, e.g., Brett and Seilacher (1991), Seilacher (1991),
Kidwell (1991), Sepkoski et al. (1991), Brett and Baird
(1997).