Page 69 - Carbonate Platforms Facies, Sequences, and Evolution

is
of
as
the
lated
by
nential
if
reduced
upwave.
small
scale
produces
thousands
shown
asymmetry
using
cyclothems.
However,
the
deepest part
of
by
a
when
in
Even
of
function
lag
higher magnitude
in
is
being used (Fig. 6).
magnitude
years,
rapid
cycles
cyclothems
that
which
the
used in these
of
the
cycles,
given
Pleistocene
that
the
a
upwave
decays
models
and
extreme
have
and
long
over
moderate
symmetry
upwave
case
a
the cyclothem more
is
slow
term
or
cyclothem boundaries. Scale in tens of metres.
Holocene
few
not
Fig. 5. 'Cyclothem 2' run with symmetrical wave­
darker the tone the deeper the facies. Cyclothem
markedly
sediment column and by slight subaerial pitting at
less
boundaries are marked by the tick to the left of the
occupies
magnitude
and consequent low depth
difficult to distinguish in monochrome. In general the
tens
modelled
producing symmetrical cyclothems. Settings are as Fig.
asym-
sym-
di­
asymmetry is only simu­
ap­
'Cyclothem 3', where the lag is an adjustable expo­
remainder. This pattern could preserve the chrono­
1.
Peritidal facies (white) and subtidal facies (light blue) are
of
downwave
excluding intertidal deposits at the base of synthetic
versity that obvious asymmetry can be achieved. For
only eliminate the
time-defined or depth-defined lags of
A more effective means of modelling asymmetry
proximately 25% of the cycle and the downwave the
(Imbrie & Imbrie, 1980). 'Cyclothem 4' has a preset
eustacy
are
stratigraphical property of the simulation but it has
metrically positioned. It is therefore only with cycles
been combined with the sediment lag. The version
effective over a significant timespan of the sea-level
greatly
intertidal window during transgression, leaving the
in
influence,
.
Cyclic sedimentation: simulation programs
between 70 and 80 m.
large cycle magnitudes.
failing to balance subsidence.
Fig. 6. 'Cyclothem 3' showing the effect of an initial
such high facies diversity, and cyclothem boundaries
cycle creating well-developed asymmetric cyclothems.
cyclothems and subtidal facies are also much reduced.
sedimentation lag. Variables are otherwise identical to
Fig. 5. Peritidal facies are always absent at the base of
Peritidal facies are present at the tops of cyclothems as
become generally more subdued towards the top of the
Settings as in Fig. 5. Peritidal facies and sometimes also
metrical A-B-C, A-B-C patterns with interven­
Fig. 7. 'Cyclothem 4' using an asymmetric 30m x 300kyr
to a fast transgression and a combined sedimentation lag.
61
subtidal facies are missing at the base of cyclothems owing
ing emergent surfaces (Fig. 7), even with relatively
column (numbered) owing to sedimentation + emergence
Cyclothems are otherwise not markedly asymmetrical with
regressive intertidal deposits would form at the top
Asymmetrical cycles increase the probability that