Page 68 - Carbonate Platforms Facies, Sequences, and Evolution

in
up
60
that
In
water
to
major
during
300 kyr
stepwise
patterns
patterns,
many
Schlager,
Goodwin
(Walk den
diagenetic
commonly
cyclothems
penetrative
Asymmetry
fluctuations
&
conformable
et
Furthermore,
Goldhammer
is
of
removed
eustatic
1981)
et
(e.g.
al. ,
found
glaciations
lOmkyr- 1
modelling
upward-shallowing
the
observed
C-A
'Cyclothem 2'
vadose
stepwise subsidence.
subsidence
al.,
and
from
typical
but Turcotte
that
cycles
in
(e.g.
seem
past
1983;
Anderson,
modification
of
&
Pennysylvanian
directly
the
to
period asymmetrical pulse.
Wilson,
alone
the
palaeokarsts
1987).
Modelling using 'Cyclothem'
A-B-C
boundary.
1985)
(e.g.
should
diagenetic
will
have
a sine wave
Pleistocene
and
cycles,
field
In
Donovan
oceans
1974;
Using
capping
Walkden,
symmetrical
and
The problem of cyclothem asymmetry
&
Willemann
superimposed
match the geological record well,
match
and
(e.g,
other
some
shallow-water
G.M.
produce
1987,
if the Quaternary is to be used as a guide.
simulates
deep-sea
many
through
Walkden,
upon
James,
subtidal
(1983)
terminated
Jones,
p.
modification
cases
'Cyclothem
forms
Imbrie
oscillations
30m
sequences with a dis­
x
super­
sudden falls in base-level that precluded the develop­
1979;
carbonate
they lack significant emergence phenomena (Fig. 4).
oxygen
&
1979;
display
1987;
terms of its geometry, therefore, a simple sine wave
may not be a satisfactory analogue of glacioeustacy
faster
of
Grotzinger
asymmetrical regressive or 'shallowing-up' patterns
In particular, the symmetry of a sine wave is not
representative of the Quaternary situation in which
strongly
al. (1987) managed to reproduce complex pentacyclic
1',
except that
Imbrie, 1980; Imbrie, 1985), suggest similar rates. In
facies
143;
than they began (Imbrie & Imbrie, 1980). There is
Quaternary
Goldhammer et al., 1987, p. 868) indicating relatively
noted
no reason to suppose that smaller scale glacioeustatic
isotope curves, which mainly reflect the volume of
rocks. Holocene records infer glacioeustatic rises of
ment of intertidal facies. This cannot be modelled by
sea-level did not produce the strong asymmetry seen
steady
in
this
boundary is an emergent surface showing subaerial
are
(1986) and Read et al. (1986) opted for asymmetrical
cycles using fast rises and slow falls. Goldhammer et
imposing a sinusoidal oscillation on top of a longer
the
Walkden and G.D.
alone
rise rat�
restored.
cycles of
permitting
inundation.
record (Fig.
stratigraphic
could
sedimentation
30m
Walkden
Once
5).
symmetrical
The
result
rates
again
subsidence
in
time-depth
the
and
magnitude,
and
deepening
moderate
during the upwave
of
and
the shape of the cyclic wave itself.
c.
achieve
presence
marine
small-scale
analogues
30
in
magnitude
reducing to c.
is
the
which
shallowing
produced regardless of the sedimentation rate used.
deepening
of
base facies for a short period (dark grey) followed by
em kyr- 1
inundation
used
field
10cm
subsidence floods the platform and establishes sub wave­
Fig. 4. 'Cyclothem 1' run using rapid subsidence of 10m
subtidal facies, indicative of sharp downward move­
the Ginsburg model, but his principle of a sedimen­
is marked, but the cyclothems produced are chrono­
tation lag remains. Subsequent modelling has incor­
subtidal facies then peritidal (lightest grey). This pattern is
every 400 kyr. At the facies settings used here this sudden
supratidal fiats before normal production rates were
facies. This would produce a 'lag' in sedimentation,
asymmetrical
Ginsburg (1971) suggested that steady subsidence
of
porated either a lag time of a few thousand years
(Read et al., 1986; Grotzinger, 1986) or a lag depth
of 1m or so (Goldhammer et al., 1987) as a means of
of
penetrative meteoric vadose effects directly capping
ment of sea-level, rules out universal application of
create patterns with high depth diversity. Emergence
reflect
the
at
on
10m magnitude. These rises outpace low to average
to
either more complex facies dynamics or a change in
cyclothems if sediment production was much slower
following progradation of intertidal and supratidal
To reproduce asymmetry requires
for
background subsidence. This produces a maximum
input cycles and lack the asymmetry of the geological