366
Chapter 7 Sequences, Minor Cycles, and Event Stratigraphy
One can distinguish several end member types:
(1) Regressive, shallowing-upward cycles (Fig. 7.40c;
e.g. Strasser 1991). These common cyc1es begin with
subtidal carbonate muds and bioclastic sands, possibly
some reefs and stromatolites, followed by intertidal
and supratidal deposits (algal mats, relics of
hypersaline or freshwater ponds. These are truncated
by an erosional unconformity, accompanied in places
by paleosols (e.g. calcrete), beach ridges, and relics of
eolian deposits.
(2) Transgressive Lofer cycles or Loferites (Fig.
7 .40b) mainly representing deepening-upward sequences. Here, sediment accumulation on top of a surface of emergence (± karstified, paleosols) starts with
supratidal and intertidal algal mats (± dolornitic) followed by subtidal bioc1astic carbonates (calcarenites).
These are overlain by a thin tidal section indicating
again upward shallowing or truncation by the next unconforrnity. Loferites appear to be less cornmon than
the dorninantly shallowing-upward type. Types 1 and
2 are sometimes mixed and form symmetrical
transgressive-regressive cyc1es (Balog et al. 1997).
(3) Subtidal carbonate cycles associated with or without emergence (Os leger and Read 1991; Elrick 1995).
The tops of these cycles exhibit incomplete
shallowing-upward and no surface of emergence. Exposed subtidal cyc1es show a distinct surface of erosion followed by a transgressive trend. On bioc1astic
open-shelf (rarnp) margins, the nature of the cyc1es
tends to change in a systematic way from the shoreface
to deeper water (Fig. 7.41; Boreen and James 1995).
This is demonstrated for Tertiary cool-water carbonates of
southeastern Australia. Close to the shoreline, minor sealevel oscillations cause subaerial exposure surfaces and
hardgrounds on top of cross-bedded grainstones. Farther
basinward, bioturbated grainstones and packstones are
capped by hardgrounds cemented below the seafloor during
lowstand. In the zone of storm action, shallowing-upward
cyc1es ofbioc1astic tempestites follow (also see Jennette and
Prior 1993). Finally, in deeper water the sediments respond
to sea-Ievel changes by rhythmically bedded marls and calcareous c1ays, both ofthem containing cool-water organisms
such as bryozoans or bryozoan bioherms.
The different end member types and the fact that rnany
cyc1es observed in the field show transitions between
the idealized models are not surprising in view of the
depositional environment in which the cyc1ic sediments accurnulated. These inc1ude rnigrating channels,
skeletal shoals and barrier systems, strong wave and
current action of changing directions, affecting the
subtidal, intertidal and supratidal zones. The cycle
thicknesses as well as the thicknesses of individual
layers within a cyc1e rnay change from locality to locality. Unconforrnities representing eyc1e boundaries are
present only in part of the cyc1es.
Both fie1d observations and modelling results have shown
that the nature and thicknesses of minor cyc1es are also affected by their position within 3rd order sequences. In a
greenhouse world with low-amplitude sea-1eve1 oscillations,
the generation of minor cyc1es on a subsiding carbonate platform may more or 1ess persist throughout a complete 3rd order sequence. However, the cyc1es thin during lowering sea
level and thicken and display higher proportions of subtidal
sediments during rising sea level (Koerschner and Read
1989; Pittet 1994). If the rate oflong-term sea-Ievel fall is
equal to subsidence, only intertidal and supratidal sediments
can accumulate, interrupted by unconformities or paleosols.
In an icehouse state of the globe, drastic sea-Ievel falls
during the glacials always cause emergence of shallow platforms, and fluctuating highstands often lead to "missed
beats" of the platform carbonates if the platform remains
emerged or is not fully flooded. Complete sequences for
these periods can be expected only at the middle and lower
slopes of carbonate platforms ifthese are not affected by current action. This is one of the reasons why the carbonate
slopes of the Bahama Platform were investigated by the
Ocean Drilling Project (e.g. Eberli et al. 1997; Betzler et al.
1999).
Principally, peritidal carbonate cyc1es can result either
from autocyclic (intra-basinal) or allocyc1ic processes
(sea-Ievel changes). Shallowing-upward sequences displaying little submarine erosion and no emergence can
form in slowly subsiding areas without sea-Ieve1
changes (Fig. 7.40d). Carbonate sediment produced in
shallow-marine environments is transported toward the
tidal flats and builds sediment up to sea level (Phase
1). With decreasing sediment supply, the sediment surface maintains its position near sea level for some time
(Phase 2), but then drops due to continued subsidence
(Phase 3). After a certain lag time, sediment buildup
starts again and forms the base of the next cyc1e.
In contrast, the combination of emergence, karstification, or pedugenesis with a following deepeningupward trend, as observed in many cases, is good evidence for relative sea-Ieve1 change (Fig. 7 .40e).
An example of the autogenie type of meter-scale carbonate
cyc1es has been described from the Upper Cambrian ofNewfoundland (Cowan and James 1996). The muddy to oolitic
shallowing-upward cycles are capped by hardgrounds and
small microbial bioherms which grew more or less simultaneously on a large platform and protected the underlying
ooids and muds from erosion.
The Mesozoic allogenetic examples of carbonate cycles
were generated by low-amplitude sea-Ievel oscillations (a
few up to 10 m) compared with high-amplitude Pleistocene
sea-Ievel changes. The transgressive, deepening-upward
Lofer cycles require relatively deep-reaching erosion to remove the regressive part ofthe cycles. A simple facies model
for symmetrie and asymmetrie peritidal-Iagoonal cycles is
shown in Fig. 7.40f. On the landward side ofthe lagoon, cyc1ic sediments are truncated by erosion and tend to be intensively dolomitized as a result of evaporative pumping (Sect.
6.4.2).
Many shallow-water carbonate cycles seem to be delicately
balanced between high-frequency eustatic variations and
autocyclic processes related to laterally migrating tidal flats.
Chapter 7 Sequences, Minor Cycles, and Event Stratigraphy
One can distinguish several end member types:
(1) Regressive, shallowing-upward cycles (Fig. 7.40c;
e.g. Strasser 1991). These common cyc1es begin with
subtidal carbonate muds and bioclastic sands, possibly
some reefs and stromatolites, followed by intertidal
and supratidal deposits (algal mats, relics of
hypersaline or freshwater ponds. These are truncated
by an erosional unconformity, accompanied in places
by paleosols (e.g. calcrete), beach ridges, and relics of
eolian deposits.
(2) Transgressive Lofer cycles or Loferites (Fig.
7 .40b) mainly representing deepening-upward sequences. Here, sediment accumulation on top of a surface of emergence (± karstified, paleosols) starts with
supratidal and intertidal algal mats (± dolornitic) followed by subtidal bioc1astic carbonates (calcarenites).
These are overlain by a thin tidal section indicating
again upward shallowing or truncation by the next unconforrnity. Loferites appear to be less cornmon than
the dorninantly shallowing-upward type. Types 1 and
2 are sometimes mixed and form symmetrical
transgressive-regressive cyc1es (Balog et al. 1997).
(3) Subtidal carbonate cycles associated with or without emergence (Os leger and Read 1991; Elrick 1995).
The tops of these cycles exhibit incomplete
shallowing-upward and no surface of emergence. Exposed subtidal cyc1es show a distinct surface of erosion followed by a transgressive trend. On bioc1astic
open-shelf (rarnp) margins, the nature of the cyc1es
tends to change in a systematic way from the shoreface
to deeper water (Fig. 7.41; Boreen and James 1995).
This is demonstrated for Tertiary cool-water carbonates of
southeastern Australia. Close to the shoreline, minor sealevel oscillations cause subaerial exposure surfaces and
hardgrounds on top of cross-bedded grainstones. Farther
basinward, bioturbated grainstones and packstones are
capped by hardgrounds cemented below the seafloor during
lowstand. In the zone of storm action, shallowing-upward
cyc1es ofbioc1astic tempestites follow (also see Jennette and
Prior 1993). Finally, in deeper water the sediments respond
to sea-Ievel changes by rhythmically bedded marls and calcareous c1ays, both ofthem containing cool-water organisms
such as bryozoans or bryozoan bioherms.
The different end member types and the fact that rnany
cyc1es observed in the field show transitions between
the idealized models are not surprising in view of the
depositional environment in which the cyc1ic sediments accurnulated. These inc1ude rnigrating channels,
skeletal shoals and barrier systems, strong wave and
current action of changing directions, affecting the
subtidal, intertidal and supratidal zones. The cycle
thicknesses as well as the thicknesses of individual
layers within a cyc1e rnay change from locality to locality. Unconforrnities representing eyc1e boundaries are
present only in part of the cyc1es.
Both fie1d observations and modelling results have shown
that the nature and thicknesses of minor cyc1es are also affected by their position within 3rd order sequences. In a
greenhouse world with low-amplitude sea-1eve1 oscillations,
the generation of minor cyc1es on a subsiding carbonate platform may more or 1ess persist throughout a complete 3rd order sequence. However, the cyc1es thin during lowering sea
level and thicken and display higher proportions of subtidal
sediments during rising sea level (Koerschner and Read
1989; Pittet 1994). If the rate oflong-term sea-Ievel fall is
equal to subsidence, only intertidal and supratidal sediments
can accumulate, interrupted by unconformities or paleosols.
In an icehouse state of the globe, drastic sea-Ievel falls
during the glacials always cause emergence of shallow platforms, and fluctuating highstands often lead to "missed
beats" of the platform carbonates if the platform remains
emerged or is not fully flooded. Complete sequences for
these periods can be expected only at the middle and lower
slopes of carbonate platforms ifthese are not affected by current action. This is one of the reasons why the carbonate
slopes of the Bahama Platform were investigated by the
Ocean Drilling Project (e.g. Eberli et al. 1997; Betzler et al.
1999).
Principally, peritidal carbonate cyc1es can result either
from autocyclic (intra-basinal) or allocyc1ic processes
(sea-Ievel changes). Shallowing-upward sequences displaying little submarine erosion and no emergence can
form in slowly subsiding areas without sea-Ieve1
changes (Fig. 7.40d). Carbonate sediment produced in
shallow-marine environments is transported toward the
tidal flats and builds sediment up to sea level (Phase
1). With decreasing sediment supply, the sediment surface maintains its position near sea level for some time
(Phase 2), but then drops due to continued subsidence
(Phase 3). After a certain lag time, sediment buildup
starts again and forms the base of the next cyc1e.
In contrast, the combination of emergence, karstification, or pedugenesis with a following deepeningupward trend, as observed in many cases, is good evidence for relative sea-Ieve1 change (Fig. 7 .40e).
An example of the autogenie type of meter-scale carbonate
cyc1es has been described from the Upper Cambrian ofNewfoundland (Cowan and James 1996). The muddy to oolitic
shallowing-upward cycles are capped by hardgrounds and
small microbial bioherms which grew more or less simultaneously on a large platform and protected the underlying
ooids and muds from erosion.
The Mesozoic allogenetic examples of carbonate cycles
were generated by low-amplitude sea-Ievel oscillations (a
few up to 10 m) compared with high-amplitude Pleistocene
sea-Ievel changes. The transgressive, deepening-upward
Lofer cycles require relatively deep-reaching erosion to remove the regressive part ofthe cycles. A simple facies model
for symmetrie and asymmetrie peritidal-Iagoonal cycles is
shown in Fig. 7.40f. On the landward side ofthe lagoon, cyc1ic sediments are truncated by erosion and tend to be intensively dolomitized as a result of evaporative pumping (Sect.
6.4.2).
Many shallow-water carbonate cycles seem to be delicately
balanced between high-frequency eustatic variations and
autocyclic processes related to laterally migrating tidal flats.