294
Chapter 7 Sequences, Minor Cycles, and Event StratigraphY
- Individual sedimentary cyc1es typically show a
coarsening (shallowing) or fining (deepening)-u~ward trend both in siliciclastic and carbonate sedIments. They mayaiso be defined as packages of sediment which are composed of at least three different,
but genetically related types of beds or interbeds
which form a repeated succession or cyc1ic sequence
(e.g., ABC, ABC, etc.; Fig. 7.le and f).
The tenn cyclothem (e.g., Weller 1964) is widely used in
North America and some other countries in a purely descriptive way, especially for Pennsylvanian (~arboniferous) coal-bearing sequences (cf. Sect. 7.6~. Thls.tenn describes a basic cycle, i.e., a package of htholoßles representing the smallest cyclic unit of a sequence.
In seismic stratigraphy the tenns sequence cycle,
depositional sequence, parasequence and parasequence set
are common (e.g. van Wagoner et al. 1990, cf. Sects. 7.2.2
and 7.8.5).
7.1.2 Autogenetic and Allogenetic Processes
GeneticaIly, one can distinguish between two groups
of mechanisms leading to rhythmic and cyc1ic sequences.
- A utocyclic sequences (or autogenetic sequences,
Dott 1988) are controlled primarily by processes taking place in the sedimentary basin itself, such as migration and superposition of channel systems, delta
lobe switching, storms, mass flows. Individual beds
and bedsets of these sequences usually show only
limited stratigraphic continuity.
- Allocyclic sequences (or allogenetic sequences) are
caused mainly by variations external to the considered sedimentary basin, such as c1imatic changes,
tectonic movements in the source area, and global sea
level variations. These processes tend to generate
cyc1ic phenomena of a greater lateral continuity and
more precise time period than autocyc1ic processes.
Most characteristicaIly, aIlocyc1ic processes may
operate simultaneously in different basins and thus
allow long-distance correlations.
However, it is often not possible to distinguish
sharply between autocyc1ic and aIlocyc1ic processes.
For example, regional tectonics may affect both the
drainage area outside the depositional basin as weIl
as tectonic structures within the basin and thus initiate the influx or redeposition of coarse-grained sediments. The occurrence and frequency of essentially
autocyc1ic rnass flows and turbidites is commonly
also controlled by aIlocyc1ic eustatic sea-level
changes. In fact, there are many depositional sequences displaying the results of both allocyclic and
autocyc1ic phenomena. Nevertheless, the terms
autogenetic and allogenetic still appear useful when
applied in the sense that one of these two processes
is dominating.
7.1.3 Scales of Rhythmic and Cyclic Phenomena
Rhythmic and cyc1ic sequences can be c1assified in
different ways, e.g. according to the time period necessary to form one cyc1e (cf. Sect. 7.8), or by distinguishing different lithologies and environments (e.g.
peritidal carbonate cyc1es), etc.
As a first approach, a simple, primarily descriptive
c1assification of rhythmic and cyc1ic phenomena is
shown in Fig. 7.2. This scheme is based mainly on
the thicknesses of beds and larger sedimentary cyc1es. Here, rhythmic and cyc1ic sediments are subdivided into four groups:
- Varve-scale laminations.
- Bed-scale rhythms and cyc1es:
- Field-scale sedimentary cyc1es (inc1uding the third
and fourth order cyc1es (parasequences) of sequence stratigraphy.
- Macro-scale cyc1ic sequences (supercyc1es and
megacycles).
This c1assification can, except for the macro-scale
sequences, be easily applied in the field and in weIl
logs without any information on genesis, sedimentation rates, and associated time spans.
Varve-scale laminations or bed-scale alternations
are weIl-known features, but none are the product of
only one specific process in a certain environment,
and each rnay represent quite different time periods.
This is also true of field-scale sequences which may
be regarded as the typical outcrop cyc1e, several meters to tens of meters thick. If these cyc1es are fully
or partially marine, most of them are today inte:-preted as representing relative sea level changes In
the order of 100 ka to several Ma. In settings of
higher overall sedimentation rates, e.g. in lacustrine
environments cyc1es of the same thickness are controlled by pr~cesses of shorter periods, e.g. c1irnati.c
fluctuations on the order of 20 to 100 ka. These penods, on the other hand, are believed to be the cause
of rnany rhythmic, marine marl-limestone sequences.
Macro-scale cyc1es (1st and 2nd order, Sect. 7.8)
normally cannot be seen in single field exposures.
They comprise successions of considerable thickness
(100 m up to several kilometers) and represent long
time periods (usually between 10 Ma and more than
100 Ma). The different orders, hierarchy, and origins
of cyc1ic sequences are further discussed in Section
7.8.
Chapter 7 Sequences, Minor Cycles, and Event StratigraphY
- Individual sedimentary cyc1es typically show a
coarsening (shallowing) or fining (deepening)-u~ward trend both in siliciclastic and carbonate sedIments. They mayaiso be defined as packages of sediment which are composed of at least three different,
but genetically related types of beds or interbeds
which form a repeated succession or cyc1ic sequence
(e.g., ABC, ABC, etc.; Fig. 7.le and f).
The tenn cyclothem (e.g., Weller 1964) is widely used in
North America and some other countries in a purely descriptive way, especially for Pennsylvanian (~arboniferous) coal-bearing sequences (cf. Sect. 7.6~. Thls.tenn describes a basic cycle, i.e., a package of htholoßles representing the smallest cyclic unit of a sequence.
In seismic stratigraphy the tenns sequence cycle,
depositional sequence, parasequence and parasequence set
are common (e.g. van Wagoner et al. 1990, cf. Sects. 7.2.2
and 7.8.5).
7.1.2 Autogenetic and Allogenetic Processes
GeneticaIly, one can distinguish between two groups
of mechanisms leading to rhythmic and cyc1ic sequences.
- A utocyclic sequences (or autogenetic sequences,
Dott 1988) are controlled primarily by processes taking place in the sedimentary basin itself, such as migration and superposition of channel systems, delta
lobe switching, storms, mass flows. Individual beds
and bedsets of these sequences usually show only
limited stratigraphic continuity.
- Allocyclic sequences (or allogenetic sequences) are
caused mainly by variations external to the considered sedimentary basin, such as c1imatic changes,
tectonic movements in the source area, and global sea
level variations. These processes tend to generate
cyc1ic phenomena of a greater lateral continuity and
more precise time period than autocyc1ic processes.
Most characteristicaIly, aIlocyc1ic processes may
operate simultaneously in different basins and thus
allow long-distance correlations.
However, it is often not possible to distinguish
sharply between autocyc1ic and aIlocyc1ic processes.
For example, regional tectonics may affect both the
drainage area outside the depositional basin as weIl
as tectonic structures within the basin and thus initiate the influx or redeposition of coarse-grained sediments. The occurrence and frequency of essentially
autocyc1ic rnass flows and turbidites is commonly
also controlled by aIlocyc1ic eustatic sea-level
changes. In fact, there are many depositional sequences displaying the results of both allocyclic and
autocyc1ic phenomena. Nevertheless, the terms
autogenetic and allogenetic still appear useful when
applied in the sense that one of these two processes
is dominating.
7.1.3 Scales of Rhythmic and Cyclic Phenomena
Rhythmic and cyc1ic sequences can be c1assified in
different ways, e.g. according to the time period necessary to form one cyc1e (cf. Sect. 7.8), or by distinguishing different lithologies and environments (e.g.
peritidal carbonate cyc1es), etc.
As a first approach, a simple, primarily descriptive
c1assification of rhythmic and cyc1ic phenomena is
shown in Fig. 7.2. This scheme is based mainly on
the thicknesses of beds and larger sedimentary cyc1es. Here, rhythmic and cyc1ic sediments are subdivided into four groups:
- Varve-scale laminations.
- Bed-scale rhythms and cyc1es:
- Field-scale sedimentary cyc1es (inc1uding the third
and fourth order cyc1es (parasequences) of sequence stratigraphy.
- Macro-scale cyc1ic sequences (supercyc1es and
megacycles).
This c1assification can, except for the macro-scale
sequences, be easily applied in the field and in weIl
logs without any information on genesis, sedimentation rates, and associated time spans.
Varve-scale laminations or bed-scale alternations
are weIl-known features, but none are the product of
only one specific process in a certain environment,
and each rnay represent quite different time periods.
This is also true of field-scale sequences which may
be regarded as the typical outcrop cyc1e, several meters to tens of meters thick. If these cyc1es are fully
or partially marine, most of them are today inte:-preted as representing relative sea level changes In
the order of 100 ka to several Ma. In settings of
higher overall sedimentation rates, e.g. in lacustrine
environments cyc1es of the same thickness are controlled by pr~cesses of shorter periods, e.g. c1irnati.c
fluctuations on the order of 20 to 100 ka. These penods, on the other hand, are believed to be the cause
of rnany rhythmic, marine marl-limestone sequences.
Macro-scale cyc1es (1st and 2nd order, Sect. 7.8)
normally cannot be seen in single field exposures.
They comprise successions of considerable thickness
(100 m up to several kilometers) and represent long
time periods (usually between 10 Ma and more than
100 Ma). The different orders, hierarchy, and origins
of cyc1ic sequences are further discussed in Section
7.8.