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Chapter 7 Sequences, Minor Cycles, and Event Stratigraphy
7.9.6 Discyclic Bedding and
Other Depositional Events
Examples of Discyclic Bedding
The most prominent examples of discyclic, event-related rhythmic bedding are:
- Flood event deposits on alluvial plains (inundites;
Sects. 2.2.1 and 2.2.3).
- Siliciclastic and calcareous tempestites and
tsunamiites (Sect. 3.1.2).
- Gravity rnass flow deposits on land and below the
sea, including exceptionally large debris and mud
flow deposits such as olistostromes (Sect. 5.4.2).
- All types of turbidites, sandy and muddy
(siliciclastic, calcareous, and siliceous; Sect. 5.4.2).
- Tephra layers, particularly widely dispersed ashfalls
(Sect. 2.4).
These types of event beds do not always form conspicuous rhythmic bedding. Sometimes they only form rare
intercalations within the "normal host sediments". Frequently, one event bed rnay follow an earlier one without an intercalated interbed, representing the more or
less autochthonous background sedimentation. Flood
deposits are often not separated by a distinct hiatus
(e.g. soil horizon), unless theyare rare (see below). A
sequence of fine-grained, thin mud turbidites, for example, is commonly not referred to as rhythmic bedding, but rather as massive siltstone or claystone (Kelts
and Arthur 1981). Similarly, lithographie limestones
consisting of very fine-grained calcite particles
(micrite) do not show distinct rhythmic bedding although their slate-like appearance results from gravity
mass flows and turbidites, including slope breccias.
Lithographie limestones usually formed in specific (semi-)
enclosed andJor silled strike-slip related basins in arid regions where terrigenous input was limited or absent (e.g.
Oost and de Boer 1994). Such limestones are known from
paleo-Iatitudes between 15° and 40° in both hemispheres.
Micrite produced by algae and bacteria in shallow water and
on neighboring platforms was episodically transported into
more or less stagnant, hypersaline deeper water. In these basins, the event beds including displaced biota could be well
preserved. More clayey interbeds and bioturbation are commonly absent.
Other Depositional Events
Some of the above mentioned event beds result from
rare or exceptional processes, such as tsunami- and
bolide-related deposits or the break of natural darns
causing catastrophic floods. Individual thin silt layers
in continental deposits may originate from extreme
dust storrns. Sandy and silty sediments on land and
below the sea can record earthquake shocks by "soft
sediment deformation" (referred to as seismites) without being substantially displaced. Such event beds
sometimes provide excellent marker beds for stratigraphie correlation in limited areas. However, most of
these rare event beds are not sufficiently extensive and
frequent to generate rhythrnic bedding. (F or more details see, e.g. Clifton 1988; Einsele 1996; Poag 1997).
Aseries of catastrophic floods, generated by repeated outbursts of glacier-dammed lakes in the late Wisconsin (15-12
ka B.P.), has been reported from the Columbia Plateau,
Washington, USA (Smith 1993). The floods commonly left
behind one distinct graded bed (some decimeters up to 1 m
in thickness) with a bioturbated top or soil; the flood events
are separated bynormal fluvial deposits. In certain areas one
flood event is represented by a number of thinner beds. According to dated ash layers from Mt. Helens, about 40 major
flood events occurred within a time span of3 to 4 ka. A catastrophic flood affecting the valley ofthe Snake River, Idaho,
occurred after the outflow of the Pleistocene Lake
Bonneville, Utah, about 15 ka ago (Jarrett and Malde 1987;
cf. Fig. 11.6).
Recurrence Intervals of Event Beds
Depositional events generally occur in irregular, stochastic intervals, but these are controlled to some extent by processes which take a certain period of time,
such as the accumulation of sediment on a slope before
failure can occur. Similarly, depositional events in a
fluvial or shallow-marine basin cannot bring about
sediment aggradation faster than allowed by accommodation space controlled by subsidence and base- or
sea-level change.
Although the recurrence times of cyc1ic and episodic phenomena in sedimentary sequences span a very wide scale,
many common features in marine environments such as
tempestites, turbidites, limestone-marl rhythms and banded
chert layers occur in about the same, relatively narrow range
of frequencies, i.e., between 1000 and several 100000 years.
In fact, the thicknesses of individual bedding couplets are
often in the same range, regardless ofwhether they are generated by depositional events or gradual cyc1ic changes. When
both types of bedding rhythms occur in the same sequence,
the discrimination of cyc1ic and discyclic bedding and the
determination ofthe cyc1e number may become problematic.
However, this is only true for environments where the sedimentation rates are of the same order of magnitude, as for
example in wide areas of the oceans. Most lake sediments
accumulate much faster, with the result that cyc1e periods
similar to those in the sea generate thick packages of sediment, but event deposits maintain their dimensions or even
tend to decrease in size.
The following considerations on the recurrence intervals of discyclic event beds (Fig. 7.45) take into account only the common types of macro-scale rhythmic
bedding.
In continental settings the frequency of extreme
rainstorrns and floods causing event deposits varies
Chapter 7 Sequences, Minor Cycles, and Event Stratigraphy
7.9.6 Discyclic Bedding and
Other Depositional Events
Examples of Discyclic Bedding
The most prominent examples of discyclic, event-related rhythmic bedding are:
- Flood event deposits on alluvial plains (inundites;
Sects. 2.2.1 and 2.2.3).
- Siliciclastic and calcareous tempestites and
tsunamiites (Sect. 3.1.2).
- Gravity rnass flow deposits on land and below the
sea, including exceptionally large debris and mud
flow deposits such as olistostromes (Sect. 5.4.2).
- All types of turbidites, sandy and muddy
(siliciclastic, calcareous, and siliceous; Sect. 5.4.2).
- Tephra layers, particularly widely dispersed ashfalls
(Sect. 2.4).
These types of event beds do not always form conspicuous rhythmic bedding. Sometimes they only form rare
intercalations within the "normal host sediments". Frequently, one event bed rnay follow an earlier one without an intercalated interbed, representing the more or
less autochthonous background sedimentation. Flood
deposits are often not separated by a distinct hiatus
(e.g. soil horizon), unless theyare rare (see below). A
sequence of fine-grained, thin mud turbidites, for example, is commonly not referred to as rhythmic bedding, but rather as massive siltstone or claystone (Kelts
and Arthur 1981). Similarly, lithographie limestones
consisting of very fine-grained calcite particles
(micrite) do not show distinct rhythmic bedding although their slate-like appearance results from gravity
mass flows and turbidites, including slope breccias.
Lithographie limestones usually formed in specific (semi-)
enclosed andJor silled strike-slip related basins in arid regions where terrigenous input was limited or absent (e.g.
Oost and de Boer 1994). Such limestones are known from
paleo-Iatitudes between 15° and 40° in both hemispheres.
Micrite produced by algae and bacteria in shallow water and
on neighboring platforms was episodically transported into
more or less stagnant, hypersaline deeper water. In these basins, the event beds including displaced biota could be well
preserved. More clayey interbeds and bioturbation are commonly absent.
Other Depositional Events
Some of the above mentioned event beds result from
rare or exceptional processes, such as tsunami- and
bolide-related deposits or the break of natural darns
causing catastrophic floods. Individual thin silt layers
in continental deposits may originate from extreme
dust storrns. Sandy and silty sediments on land and
below the sea can record earthquake shocks by "soft
sediment deformation" (referred to as seismites) without being substantially displaced. Such event beds
sometimes provide excellent marker beds for stratigraphie correlation in limited areas. However, most of
these rare event beds are not sufficiently extensive and
frequent to generate rhythrnic bedding. (F or more details see, e.g. Clifton 1988; Einsele 1996; Poag 1997).
Aseries of catastrophic floods, generated by repeated outbursts of glacier-dammed lakes in the late Wisconsin (15-12
ka B.P.), has been reported from the Columbia Plateau,
Washington, USA (Smith 1993). The floods commonly left
behind one distinct graded bed (some decimeters up to 1 m
in thickness) with a bioturbated top or soil; the flood events
are separated bynormal fluvial deposits. In certain areas one
flood event is represented by a number of thinner beds. According to dated ash layers from Mt. Helens, about 40 major
flood events occurred within a time span of3 to 4 ka. A catastrophic flood affecting the valley ofthe Snake River, Idaho,
occurred after the outflow of the Pleistocene Lake
Bonneville, Utah, about 15 ka ago (Jarrett and Malde 1987;
cf. Fig. 11.6).
Recurrence Intervals of Event Beds
Depositional events generally occur in irregular, stochastic intervals, but these are controlled to some extent by processes which take a certain period of time,
such as the accumulation of sediment on a slope before
failure can occur. Similarly, depositional events in a
fluvial or shallow-marine basin cannot bring about
sediment aggradation faster than allowed by accommodation space controlled by subsidence and base- or
sea-level change.
Although the recurrence times of cyc1ic and episodic phenomena in sedimentary sequences span a very wide scale,
many common features in marine environments such as
tempestites, turbidites, limestone-marl rhythms and banded
chert layers occur in about the same, relatively narrow range
of frequencies, i.e., between 1000 and several 100000 years.
In fact, the thicknesses of individual bedding couplets are
often in the same range, regardless ofwhether they are generated by depositional events or gradual cyc1ic changes. When
both types of bedding rhythms occur in the same sequence,
the discrimination of cyc1ic and discyclic bedding and the
determination ofthe cyc1e number may become problematic.
However, this is only true for environments where the sedimentation rates are of the same order of magnitude, as for
example in wide areas of the oceans. Most lake sediments
accumulate much faster, with the result that cyc1e periods
similar to those in the sea generate thick packages of sediment, but event deposits maintain their dimensions or even
tend to decrease in size.
The following considerations on the recurrence intervals of discyclic event beds (Fig. 7.45) take into account only the common types of macro-scale rhythmic
bedding.
In continental settings the frequency of extreme
rainstorrns and floods causing event deposits varies