7.9 Cyclo- and Event Stratigraphy
greatly. Large debris flows on alluvial fans commonly
occur at intervals of tens to hundreds ofyears. On the
slopes of fan deltas, avalanching and gravity mass
flows of some size may take place with about the same
frequency, apart from over-supplied proglacial settings. Overbank fines on flood plains may form either
seasonally or at very irregular intervals oftens to hundreds ofyears.
In marine settings, as in continental ones, some preevent sediment storage is commonly necessary to produce event beds. The recurrence intervals of both
sandy and bioclastic tempestites as well as mediumsize gravity mass flows, turbidite sands and muds commonly range from several hundreds to several thousands of years.
In specific cases, very short interva1s of 10 to 100 years have
been found. An examp1e are typhoon-triggered turbidites in
the Su1u Sea, Philippines, mostly 10-30 mm in thickness and
deposited during the past 20 ka (Schönfe1d and Kudrass
1991). Further data are summarized and discussed by Einse1e
(1993 and 1996).
The channel fills of deep-sea fans partly record more
depositional events than the sediments offan lobes. On
the other hand, reworking and amalgamation ofpreviously deposited beds may reduce the initial number of
events (e.g. that oftempestites).
The frequencies of shallow-water tempestites and
deep-sea turbidites are commonly much higher than
the Milankovitch periodicities (Fig. 7.45). Only relatively rare events, for example large mass flow deposits and thick turbidites on distal fan lobes and basin
plains, range within the Milankovitch frequency band.
In front ofthe Niger delta, for example, episodic sediment transport beyond the shelf edge has been controlled in the Neogene by 4th and 5th-order glacioeustatic sea-level fluctuations (Damuth 1997). Very
large mass flows, as known from the modem oceans or
ancient megaturbidites, represent sediment masses
which have required millions ofyears to accumulate in
the area where they were remobilized.
When relative sea-level is the dominant faCtor controlling the depositional system, then the early
lowstand represents the time at which event deposits
are most abundant (e.g. Posamentier and Allen 1993;
Einseie 1996). In particular, the effects of high-frequency, high-amplitude sea-level oscillations cannot be
ruled out by any other process. They also control to
some extent the recurrence interval of certain "bundIes" of event deposits, such as the Pliocene turbidite
sequences of 20 ka precession cycles in Greece
(Welt je and de Boer 1993). Especially large mass
flows, such as "megaturbidites" and calcareous
megabreccias, seem to be largely associated with sealevel fall. Sea level-controlled valley incision mayaiso
provide large amounts of sediment during the late
lowstand phase and thus enable sediment gravity
flows.
7.9.7 Non-Depositional and Erosional Events
(Skeletal Horizons, Lags etc.)
General Aspects
375
Decreasing sedimentation rates cause condensation,
sediment-starvation, omission or non-deposition, and
may be finally replaced by erosion. Most of these
terms are commonly used for marine sediments, but
they can also be applied to continental deposits. Nondepositional events are often not strictly isochronous
and should therefore be used with caution in stratigraphie correlation.
Non-deposition and erosion do not create distinct
beds, but ratherproduce stratal discontinuities, specific
surfaces or event horizons. Such horizons can occur on
land and below the sea under a considerable number of
different circumstances. Here, we only consider surfaces which affect large areas within a sedimentary
basin or even several basins simultaneously. In contrast to depositional events, event horizons often span
a considerable period of time. F or this reason, various
organisms may be actively involved in characterizing
these horizons, apart from physical processes. ExampIes of this interrelationship are soil horizons
(paleosols) on land or recolonized shell beds in the
sea. The most important representatives of nondepositional and erosional event horizons are
- Clastic lag sediments,
- Biogenie skeletal concentrations,
- Buried soils (paleosols, cf. Sect. 9.1.2).
The first two groups consist of lithoclasts, i.e. relics of
reworked older sediment, commonly coarse sand to
gravel-size particles, andJor reworked skeletal particles, partially abraded or broken. Both lithoclasts and
skeletal particles originate either from local sources
(e.g. intrabasinallithoclasts) or more distant sourees.
They have resisted further physical, chemical and
biologically-mediated breakdown and transport proces ses and therefore have been left behind as lag sediments. Some of these lag sediments are comprised
solely of lithoclasts or skeletal particles (skeletal concentrations ); many are mixed.
Continental Non-Depositional Events
In the continental realm, widespread soil horizons
(paleosols) present on surfaces of sediment starvation
or non-deposition are most important. Other event horizons, such as erosionallags in fluvial channels, winnowed desert pavements, or truncation surfaces in
eolian sands are mostly oflimited lateral extent and are
not considered further here.
Soil horizons are common on wide fluvial plains and
their older terraces, dried lake basins, and isolated pla-
greatly. Large debris flows on alluvial fans commonly
occur at intervals of tens to hundreds ofyears. On the
slopes of fan deltas, avalanching and gravity mass
flows of some size may take place with about the same
frequency, apart from over-supplied proglacial settings. Overbank fines on flood plains may form either
seasonally or at very irregular intervals oftens to hundreds ofyears.
In marine settings, as in continental ones, some preevent sediment storage is commonly necessary to produce event beds. The recurrence intervals of both
sandy and bioclastic tempestites as well as mediumsize gravity mass flows, turbidite sands and muds commonly range from several hundreds to several thousands of years.
In specific cases, very short interva1s of 10 to 100 years have
been found. An examp1e are typhoon-triggered turbidites in
the Su1u Sea, Philippines, mostly 10-30 mm in thickness and
deposited during the past 20 ka (Schönfe1d and Kudrass
1991). Further data are summarized and discussed by Einse1e
(1993 and 1996).
The channel fills of deep-sea fans partly record more
depositional events than the sediments offan lobes. On
the other hand, reworking and amalgamation ofpreviously deposited beds may reduce the initial number of
events (e.g. that oftempestites).
The frequencies of shallow-water tempestites and
deep-sea turbidites are commonly much higher than
the Milankovitch periodicities (Fig. 7.45). Only relatively rare events, for example large mass flow deposits and thick turbidites on distal fan lobes and basin
plains, range within the Milankovitch frequency band.
In front ofthe Niger delta, for example, episodic sediment transport beyond the shelf edge has been controlled in the Neogene by 4th and 5th-order glacioeustatic sea-level fluctuations (Damuth 1997). Very
large mass flows, as known from the modem oceans or
ancient megaturbidites, represent sediment masses
which have required millions ofyears to accumulate in
the area where they were remobilized.
When relative sea-level is the dominant faCtor controlling the depositional system, then the early
lowstand represents the time at which event deposits
are most abundant (e.g. Posamentier and Allen 1993;
Einseie 1996). In particular, the effects of high-frequency, high-amplitude sea-level oscillations cannot be
ruled out by any other process. They also control to
some extent the recurrence interval of certain "bundIes" of event deposits, such as the Pliocene turbidite
sequences of 20 ka precession cycles in Greece
(Welt je and de Boer 1993). Especially large mass
flows, such as "megaturbidites" and calcareous
megabreccias, seem to be largely associated with sealevel fall. Sea level-controlled valley incision mayaiso
provide large amounts of sediment during the late
lowstand phase and thus enable sediment gravity
flows.
7.9.7 Non-Depositional and Erosional Events
(Skeletal Horizons, Lags etc.)
General Aspects
375
Decreasing sedimentation rates cause condensation,
sediment-starvation, omission or non-deposition, and
may be finally replaced by erosion. Most of these
terms are commonly used for marine sediments, but
they can also be applied to continental deposits. Nondepositional events are often not strictly isochronous
and should therefore be used with caution in stratigraphie correlation.
Non-deposition and erosion do not create distinct
beds, but ratherproduce stratal discontinuities, specific
surfaces or event horizons. Such horizons can occur on
land and below the sea under a considerable number of
different circumstances. Here, we only consider surfaces which affect large areas within a sedimentary
basin or even several basins simultaneously. In contrast to depositional events, event horizons often span
a considerable period of time. F or this reason, various
organisms may be actively involved in characterizing
these horizons, apart from physical processes. ExampIes of this interrelationship are soil horizons
(paleosols) on land or recolonized shell beds in the
sea. The most important representatives of nondepositional and erosional event horizons are
- Clastic lag sediments,
- Biogenie skeletal concentrations,
- Buried soils (paleosols, cf. Sect. 9.1.2).
The first two groups consist of lithoclasts, i.e. relics of
reworked older sediment, commonly coarse sand to
gravel-size particles, andJor reworked skeletal particles, partially abraded or broken. Both lithoclasts and
skeletal particles originate either from local sources
(e.g. intrabasinallithoclasts) or more distant sourees.
They have resisted further physical, chemical and
biologically-mediated breakdown and transport proces ses and therefore have been left behind as lag sediments. Some of these lag sediments are comprised
solely of lithoclasts or skeletal particles (skeletal concentrations ); many are mixed.
Continental Non-Depositional Events
In the continental realm, widespread soil horizons
(paleosols) present on surfaces of sediment starvation
or non-deposition are most important. Other event horizons, such as erosionallags in fluvial channels, winnowed desert pavements, or truncation surfaces in
eolian sands are mostly oflimited lateral extent and are
not considered further here.
Soil horizons are common on wide fluvial plains and
their older terraces, dried lake basins, and isolated pla-