370
Chapter 7 Sequences, Minor CycIes, and Event StratigraphY
1995). More articles dealing with this topic are found in
Einseie et al. (1991, including Cotillon), House and Gale
(1995).
Shoreface-To-Basin Correlation
in Epicontinental Sea
In some regions where good outcrops and precise stratigraphy allow bed and sequence correlation over long
distances, it has been tried to correlate proximal
shoreface deposits with distal basinal limestone-marl
couplets. One of these examples is the Upper Cretaceous ofthe Western Interior seaway ofNorth America, representing an epicontinental sea transformed into
a foreland basin. Here, a refined biostratigraphyand
radiometric dated bentonite layers have enabled such a
correlation over > 1000 km (Fig. 7.42).
The shoreface deposits consist of progradational
parasequences, 10 to 20 m in thickness, capped by
transgressive lags. The limestone-marl couplets are 0.5 to 1
m in thickness and contain some bentonites. In the transitional zone between these two end members, mudstones,
marls, and layers of calcareous concretions were deposited.
The periodicites ofthese cycles are within the Milankovitch
frequency band. The transgressive lags appear to be timeequivalents ofthe basinallimestones (Eider et al. 1994).
Cycle Correlation on Carbonate Platforms
The sediments of shallow carbonate platforms, attached to land or isolated, often vary considerably
from the outer rim to the inner, mostly lagoonal part
(Sect. 3.3.2). Benthic carbonate production strongly
depends on the water depth, and sediment distribution
controlled by wave action and tidal currents and is
therefore irregular. Considering these complications
and the discussion in Section 7.9.4, one can expect
widely traceable cyclic sequences on carbonate platforms only if these result from high-frequency relative
sea-level changes.
One of several examples where such an attempt has
been made is the well-exposed Middle Devoniancarbonate platform of the Rocky Mountains (Fig. 7.43).
The platform evolved from a ramp setting to a rimmed
platform with a relatively deep lagoon and back to a
flat-topped platform (Fig. 7.43d). The relationship between subsidence, 2nd and 3rd-order sea-Ievel
changes, creating accommodation space for sediment
buildup, is shown in Fig. 7.43c. Sediment accumulaFig. 7.43. High-resolution sequence stratigraphy of
Middle Devonian platform carbonates of eastern
Great Basin, USA. a Location map. b Platform
cross-section and overview of four 3rd order sequences (1-4). c Development of accommodation
space in relation to subsidence and sea-level changes.
tion more or less kept pace with subsidence (keep-up
platform with limited prograding).
The 3rd-order sequences (in total 270-400 m in
thickness) and also part of the minor peritidal and
subtidal cycles (a total number of around 80-100,
mostly about 2.5 m thick) can be correlated over a distance of 400-500 km. Transgressive systems tracts
showa slight deepening-upward trend with thickening
peritidal cycles overlain by subtidal cycles. Highstand
systems tracts exhibit the opposite trend.
Instead of sequence boundaries (unconformities)
and other distinctive surfaces of 3rd order sequences,
here specific sequence boundary zones, SBZ, and maximum flooding zones, MFZ, are present (cf. Fig. 7.44b
and c). SB-zones commonly consist of several relatively thin cycles with distinct surfaces of emergence;
MF -zones are characterized by thicker cycles including
subtidal cycles and non-cyclic sections. Lowstand systems tracts were not identified. The slope and deeper
basin in front of the platform are characterized by
well-bedded limestones (calciturbidites and pelagic
sediments).
The carbonates were deposited during a greenhouse state of
the Earth in a low-energy environment during a time span of
1.5-2.5 Ma which can be subdivided into four third-order
sequences (Elrick 1995 and 1996). The peritidal cycles make
up about 90% of all cycles measured and are capped by tidal
laminites. Eighty percent of the peritidal cycles show evidence of subaerial exposure and subsequent upward deepening. The specific sections in Fig. 7.43e demonstrate how the
high-frequency cycles vary laterally and within 3rd order
cycles (Fig. 7.43b). The amplitudes of the high-frequency
sea-Ievel changes may have been up to 10m. The real nurnber of minor cycles is uncertain, because during emergence
as well as during relatively deep submergence (deep
subtidal), some ofthe cyclic "beats" may have been missed.
Furthermore, partial dolomitization ofthe platform carbonates has obscured the presence of minor cycles. Because of
these difficulties and uncertainties in the biostratigraphic
dating, the periods of the minor cycles are poorly known (50130 ka).
Other studies on ancient peritidal carbonate cycles gave
results similar to those presented above (e.g. Holland' and
Patzkowsky 1998). Minor cycles of partially mixed
carbonate-siliciclastic nature were studied on the outer platform ofthe Permian reef complex ofTexas (Osleger 1998).
Recently drilled cores from the Little Bahama Bank (Pliocene to Pleistocene) confirmed and refined the results of previous studies on ancient carbonate rocks. The aggradation of
modem (icehouse state) platform carbonates is interrupted
frequently and for relatively long time periods by the superposition ofhigh-amplitude, high-frequency sea-Ievel oscillations onto those oflower frequency (McN eill et al. 1998).
d Some stages of platform evolution. f Selected sections of peritidal and subtidal minor cycles
(parasequences) of sequences 2 and 3. (After Elrick
1996, simplified and modified, local names and other
details omitted)
Chapter 7 Sequences, Minor CycIes, and Event StratigraphY
1995). More articles dealing with this topic are found in
Einseie et al. (1991, including Cotillon), House and Gale
(1995).
Shoreface-To-Basin Correlation
in Epicontinental Sea
In some regions where good outcrops and precise stratigraphy allow bed and sequence correlation over long
distances, it has been tried to correlate proximal
shoreface deposits with distal basinal limestone-marl
couplets. One of these examples is the Upper Cretaceous ofthe Western Interior seaway ofNorth America, representing an epicontinental sea transformed into
a foreland basin. Here, a refined biostratigraphyand
radiometric dated bentonite layers have enabled such a
correlation over > 1000 km (Fig. 7.42).
The shoreface deposits consist of progradational
parasequences, 10 to 20 m in thickness, capped by
transgressive lags. The limestone-marl couplets are 0.5 to 1
m in thickness and contain some bentonites. In the transitional zone between these two end members, mudstones,
marls, and layers of calcareous concretions were deposited.
The periodicites ofthese cycles are within the Milankovitch
frequency band. The transgressive lags appear to be timeequivalents ofthe basinallimestones (Eider et al. 1994).
Cycle Correlation on Carbonate Platforms
The sediments of shallow carbonate platforms, attached to land or isolated, often vary considerably
from the outer rim to the inner, mostly lagoonal part
(Sect. 3.3.2). Benthic carbonate production strongly
depends on the water depth, and sediment distribution
controlled by wave action and tidal currents and is
therefore irregular. Considering these complications
and the discussion in Section 7.9.4, one can expect
widely traceable cyclic sequences on carbonate platforms only if these result from high-frequency relative
sea-level changes.
One of several examples where such an attempt has
been made is the well-exposed Middle Devoniancarbonate platform of the Rocky Mountains (Fig. 7.43).
The platform evolved from a ramp setting to a rimmed
platform with a relatively deep lagoon and back to a
flat-topped platform (Fig. 7.43d). The relationship between subsidence, 2nd and 3rd-order sea-Ievel
changes, creating accommodation space for sediment
buildup, is shown in Fig. 7.43c. Sediment accumulaFig. 7.43. High-resolution sequence stratigraphy of
Middle Devonian platform carbonates of eastern
Great Basin, USA. a Location map. b Platform
cross-section and overview of four 3rd order sequences (1-4). c Development of accommodation
space in relation to subsidence and sea-level changes.
tion more or less kept pace with subsidence (keep-up
platform with limited prograding).
The 3rd-order sequences (in total 270-400 m in
thickness) and also part of the minor peritidal and
subtidal cycles (a total number of around 80-100,
mostly about 2.5 m thick) can be correlated over a distance of 400-500 km. Transgressive systems tracts
showa slight deepening-upward trend with thickening
peritidal cycles overlain by subtidal cycles. Highstand
systems tracts exhibit the opposite trend.
Instead of sequence boundaries (unconformities)
and other distinctive surfaces of 3rd order sequences,
here specific sequence boundary zones, SBZ, and maximum flooding zones, MFZ, are present (cf. Fig. 7.44b
and c). SB-zones commonly consist of several relatively thin cycles with distinct surfaces of emergence;
MF -zones are characterized by thicker cycles including
subtidal cycles and non-cyclic sections. Lowstand systems tracts were not identified. The slope and deeper
basin in front of the platform are characterized by
well-bedded limestones (calciturbidites and pelagic
sediments).
The carbonates were deposited during a greenhouse state of
the Earth in a low-energy environment during a time span of
1.5-2.5 Ma which can be subdivided into four third-order
sequences (Elrick 1995 and 1996). The peritidal cycles make
up about 90% of all cycles measured and are capped by tidal
laminites. Eighty percent of the peritidal cycles show evidence of subaerial exposure and subsequent upward deepening. The specific sections in Fig. 7.43e demonstrate how the
high-frequency cycles vary laterally and within 3rd order
cycles (Fig. 7.43b). The amplitudes of the high-frequency
sea-Ievel changes may have been up to 10m. The real nurnber of minor cycles is uncertain, because during emergence
as well as during relatively deep submergence (deep
subtidal), some ofthe cyclic "beats" may have been missed.
Furthermore, partial dolomitization ofthe platform carbonates has obscured the presence of minor cycles. Because of
these difficulties and uncertainties in the biostratigraphic
dating, the periods of the minor cycles are poorly known (50130 ka).
Other studies on ancient peritidal carbonate cycles gave
results similar to those presented above (e.g. Holland' and
Patzkowsky 1998). Minor cycles of partially mixed
carbonate-siliciclastic nature were studied on the outer platform ofthe Permian reef complex ofTexas (Osleger 1998).
Recently drilled cores from the Little Bahama Bank (Pliocene to Pleistocene) confirmed and refined the results of previous studies on ancient carbonate rocks. The aggradation of
modem (icehouse state) platform carbonates is interrupted
frequently and for relatively long time periods by the superposition ofhigh-amplitude, high-frequency sea-Ievel oscillations onto those oflower frequency (McN eill et al. 1998).
d Some stages of platform evolution. f Selected sections of peritidal and subtidal minor cycles
(parasequences) of sequences 2 and 3. (After Elrick
1996, simplified and modified, local names and other
details omitted)