7.9 Cyclo- and Event Stratigraphy
(1) Autogenie mode. The shell (or elast) eoneentrations are produeed by reworking of in situ skeletal remains and winnowing ofthe fine-grained matrix (Fig.
7.46b). The hard substrate ofthe shell bedallows eolonization by new speeies (gravel dwellers, "live-dead
interaetion"), until a soft sea bed is re-established
through eontinued normal sediment aggradation.
(2) Allogenie mode. The event eoneentrations result
from a single brief hydraulie episode, sueh as a storm
event. The storm provides "exotie" shells and/or
lithoelasts eroded in other (mostly shallower) parts of
the basin, but mayaiso exhume loeal shells of soft-bottom dwellers and mix them with the alloehthonous
shells (Fig. 7.46e). As in (1), the hard substrate ofthe
shell bed, often lithified to hardground, may subsequently beeome eolonized by epifauna.
One of the end members of a great variety of event
eoneentrations are shell beds, either of the auto genie or
allogenie mode, whieh have been formed by one single
storm or eurrent event. An example of this type are the
basallayers of ealcareous tempestites alternating with
"norrnally" aeeumulated host sediments. However,
many skeletal eoneentrations are modified by repeated
reworking (multi-event eoneentrations) during longer
times of limited or interrupted sediment buildup (Fig.
7.46d). The resulting eomposite, hiatal, or lag eoneentrations mayaiso eontain autoehthonous minerals sueh
as glaueonite and phosphorite nodules, indieating alternating periods of deposition and reworking.
Hiatal and lag concentrations show indistinct discontinuities
as weil as strong bio-erosion, abrasion, and fragmentation of
bioclasts. The shell orientation (biofabric) caused by storms
and unidirectional currents is predominantly convex-up (Fig.
47a); wave action sometimes leads to edge-wise, fan-shape,
or chaotic fabrics. The faunal associations involved in the
various types of skeletal event beds vary significantly
through Phanerozoic time due to the evolution of organisrns
producing bioclasts.
More details about the various types of skeletal concentrations are described in special articles (e.g. KidweIl 1991 and
1993; Fürsich and Oschmann 1993; Abbott 1997).
Biological and Ecological Events
Stratigraphers and paleontologists have frequently deseribed biologieal and eeologieal events (bio-events,
eeo-events) whieh may or may not be assoeiated with
physieally or ehemieally eontrolled stratigraphie
events. A typieal example is the sudden appearanee of
new taxa or faunal elements whieh were not present
earlier in the area studied. These phenomena usually
refleet drastie ehanges in the environmental eonditions,
sueh as the opening of oeeanie gateways, modified
eurrent systems, variations in nutrient supply and redox eonditions (eeo-events, anoxie events, ete.). Ifthe
resulting event beds are eontroiled by both physieal
377
and biologieal proeesses they may be referred to as
"eomposite" events. Bio- and eeo-events in the sense
diseussed here are usually not global phenomena, but
they often provide good marker horizons traeeable
over hundreds to thousands of square kilometers
within a basin, or even from one basin to another.
More details and examples have been described, e.g., by
Ernst et al. (1983), Kauffinan (1988), Kauffinan etal. (1991),
Paul et al. (1994), Wilrnsen et al. 1996), Sageman et al.
(1997).
The term "epibole" describes the unusual abundance of a
single taxon or several taxa that are normally rare or absent
(Brett and Baird 1997). In the context of this chapter, two
categories of epiboles are of partieular interest: (I) ecological
epiboles reflect brief episodes of expansion in particular speeies ("population bursts") resulting from short-period favorable environmental conditions (e.g. changing redox conditions) which mayeven cross facies boundaries; (2) incursion
epiboles are characterized by the sudden, brief, and widespread appearance of certain species or groups of species.
They may represent episodes in which formerly separated
biogeographie provinces or basins are connected, e.g.
through oceanic gateways. Incursion epiboles are therefore
often associated with a relatively high sea level. In general,
the improvement or deterioration of environmental conditions for certain organisrns is often associated with changes
in relative sea level and/or terrestrial sediment influx.
Long persisting ecozones, such as those controlled by the
substrate and oxygen content ofbottom water, may be overprinted by short-term bio-events. For example in the
CenomanianiTuronian ofthe Western Interior Basin, North
America, the bio-event horizons consist of either thin or
thick (meters-scale) intervals ofbenthic communities which
exhibit changes in diversity and abundance. These intervals
can be widely used as marker beds (Sageman et al. 1997).
Global biological events in the evolution oflife, including
evolutionary radiations and mass extinctions, usually span
longer time periods than the stratigraphie events discussed
here (e.g. Walliser 1995).
7.9.8 Minor Cycles, Event Beds and Lags
in 3rd Order Sequence Cycles
In this seetion we ask the question whether or not the
various types of minor eyeles or parasequenees, rhythmie bedding, depositional and non-depositional event
horizons, diseussed above, take a eertain position
within the systems traets of third order sequenees. If
this question ean be answered positively, eyelo- and
event-stratigraphy ean be used for a better identifieation, sub division, and understanding of the longer-period sequenee eyeles. Figure 7.47 presents an overview
of the preferential positions of various stratigraphie
events, minor eyeles and eyelie bedding within a third
order sequenee. The model seetions of this figure for
event stratigraphy show the most important phenomena
to be observed in shallow- and deep-marine environments as weIl as some trends in eontinental basins. F or
marine environments the following "rules" ean be applied:
(1) Autogenie mode. The shell (or elast) eoneentrations are produeed by reworking of in situ skeletal remains and winnowing ofthe fine-grained matrix (Fig.
7.46b). The hard substrate ofthe shell bedallows eolonization by new speeies (gravel dwellers, "live-dead
interaetion"), until a soft sea bed is re-established
through eontinued normal sediment aggradation.
(2) Allogenie mode. The event eoneentrations result
from a single brief hydraulie episode, sueh as a storm
event. The storm provides "exotie" shells and/or
lithoelasts eroded in other (mostly shallower) parts of
the basin, but mayaiso exhume loeal shells of soft-bottom dwellers and mix them with the alloehthonous
shells (Fig. 7.46e). As in (1), the hard substrate ofthe
shell bed, often lithified to hardground, may subsequently beeome eolonized by epifauna.
One of the end members of a great variety of event
eoneentrations are shell beds, either of the auto genie or
allogenie mode, whieh have been formed by one single
storm or eurrent event. An example of this type are the
basallayers of ealcareous tempestites alternating with
"norrnally" aeeumulated host sediments. However,
many skeletal eoneentrations are modified by repeated
reworking (multi-event eoneentrations) during longer
times of limited or interrupted sediment buildup (Fig.
7.46d). The resulting eomposite, hiatal, or lag eoneentrations mayaiso eontain autoehthonous minerals sueh
as glaueonite and phosphorite nodules, indieating alternating periods of deposition and reworking.
Hiatal and lag concentrations show indistinct discontinuities
as weil as strong bio-erosion, abrasion, and fragmentation of
bioclasts. The shell orientation (biofabric) caused by storms
and unidirectional currents is predominantly convex-up (Fig.
47a); wave action sometimes leads to edge-wise, fan-shape,
or chaotic fabrics. The faunal associations involved in the
various types of skeletal event beds vary significantly
through Phanerozoic time due to the evolution of organisrns
producing bioclasts.
More details about the various types of skeletal concentrations are described in special articles (e.g. KidweIl 1991 and
1993; Fürsich and Oschmann 1993; Abbott 1997).
Biological and Ecological Events
Stratigraphers and paleontologists have frequently deseribed biologieal and eeologieal events (bio-events,
eeo-events) whieh may or may not be assoeiated with
physieally or ehemieally eontrolled stratigraphie
events. A typieal example is the sudden appearanee of
new taxa or faunal elements whieh were not present
earlier in the area studied. These phenomena usually
refleet drastie ehanges in the environmental eonditions,
sueh as the opening of oeeanie gateways, modified
eurrent systems, variations in nutrient supply and redox eonditions (eeo-events, anoxie events, ete.). Ifthe
resulting event beds are eontroiled by both physieal
377
and biologieal proeesses they may be referred to as
"eomposite" events. Bio- and eeo-events in the sense
diseussed here are usually not global phenomena, but
they often provide good marker horizons traeeable
over hundreds to thousands of square kilometers
within a basin, or even from one basin to another.
More details and examples have been described, e.g., by
Ernst et al. (1983), Kauffinan (1988), Kauffinan etal. (1991),
Paul et al. (1994), Wilrnsen et al. 1996), Sageman et al.
(1997).
The term "epibole" describes the unusual abundance of a
single taxon or several taxa that are normally rare or absent
(Brett and Baird 1997). In the context of this chapter, two
categories of epiboles are of partieular interest: (I) ecological
epiboles reflect brief episodes of expansion in particular speeies ("population bursts") resulting from short-period favorable environmental conditions (e.g. changing redox conditions) which mayeven cross facies boundaries; (2) incursion
epiboles are characterized by the sudden, brief, and widespread appearance of certain species or groups of species.
They may represent episodes in which formerly separated
biogeographie provinces or basins are connected, e.g.
through oceanic gateways. Incursion epiboles are therefore
often associated with a relatively high sea level. In general,
the improvement or deterioration of environmental conditions for certain organisrns is often associated with changes
in relative sea level and/or terrestrial sediment influx.
Long persisting ecozones, such as those controlled by the
substrate and oxygen content ofbottom water, may be overprinted by short-term bio-events. For example in the
CenomanianiTuronian ofthe Western Interior Basin, North
America, the bio-event horizons consist of either thin or
thick (meters-scale) intervals ofbenthic communities which
exhibit changes in diversity and abundance. These intervals
can be widely used as marker beds (Sageman et al. 1997).
Global biological events in the evolution oflife, including
evolutionary radiations and mass extinctions, usually span
longer time periods than the stratigraphie events discussed
here (e.g. Walliser 1995).
7.9.8 Minor Cycles, Event Beds and Lags
in 3rd Order Sequence Cycles
In this seetion we ask the question whether or not the
various types of minor eyeles or parasequenees, rhythmie bedding, depositional and non-depositional event
horizons, diseussed above, take a eertain position
within the systems traets of third order sequenees. If
this question ean be answered positively, eyelo- and
event-stratigraphy ean be used for a better identifieation, sub division, and understanding of the longer-period sequenee eyeles. Figure 7.47 presents an overview
of the preferential positions of various stratigraphie
events, minor eyeles and eyelie bedding within a third
order sequenee. The model seetions of this figure for
event stratigraphy show the most important phenomena
to be observed in shallow- and deep-marine environments as weIl as some trends in eontinental basins. F or
marine environments the following "rules" ean be applied: