Page 75 - Carbonate Platforms Facies, Sequences, and Evolution

is
the
top
In
rates
delta
these
delta,
period
model,
screen.
sidence
prior to
situation
sediment
outpaced
their own
of
cyclothem.
plain
reality
can
modelled
Lowstand
reduce
This permits
(but
sediments
downwave
as
though,
and
The
which
peat
sea-level
with
a
of
their
sequence
are
who
a
circumstances
quickly
highstand
only
compactive
an
fall.
as
could
because
can
than
faster
for
sediments to accumulate.
lead
results in
because it
different accumulation rate
This
sedimentation
be
to
full
intrinsic
themselves
equally
importance.
process
both the
symmetrical
modelled
sure?),
including peat accumulation,
requires
The
coal-forming environment to develop.
permanent
contribution to
than
features
appear
the
eustatic
in
within
either
sediment
undergoing
development
to continue for
from the muds.
to take
a
sedimentation
a
overall
the
scenario
of
during
of peat
cycle
drowning
extended opportunity for
problem is magnified, but the effect is clearly equiv­
Simu­
window of the delta-top muds, or not at all. Never­
Using 'Walther 1', however, this situation is meta­
before emergence terminates it, providing more op­
theless, it is notable that peat accumulation is nor­
the system vulnerable. Once these additional delta­
sub­
the
'Walther' simulations are that it compacts more than
lated peat is mainly an indication of maturity of the
in
place.
and sea-level are falling during this part of a cycle.
stable and although extensive peats can be produced
a
other delta-top sediments and it can be assigned a
burial,
at lowstand the crucial conditions of either relatively
fast subsidence or relatively slow sedimentation leave
real
longer
the sediment column. Clearly, since our compaction
surface
of
portunity for the delta to mature and for delta-top
mally maximized during the later parts of runs when
compactive subsidence is active. The added effect of
being
compaction reduces the length of emergence during
also
fast overall sedimentation
Thick delta-top sequences are in any case unlikely in
involves dramatic elevation of the delta top during
downwave and lowstand, with (in the real situation)
of
moderate magnitude could therefore be an unlikely
burst of delta development at highstand or a longer
situation is difficult to
rapid
the
scenario for peat formation in sequences that contain
a lot of coal. More stable would be the highstand
coal built on top of rapidly accumulated clastic facies
alent to that produced by an appropriately thicker
of delta-plain facies. For these reasons extensive and
single
consequent dramatic draining, erosion and oxidation
drowning
the
the
the
and
coal
thick
facies
result
routine
reverse.
is
repeated
Stepwise
cycle,
produced
subsidence
Broadhurst
alone
or
the
swamp
by
effects either.
of
and lowstand.
&
delta-top
putting
compactive
are
Cyclic sedimentation: simulation programs
was
such
the slow
Resumed
the
Quantum subsidence
delta-plain
>2 m
facies
quantum
repetition
Simpson,
assigned
will
coals
of
subsidence
normal
interplay
suddenly
downwave
of the delta during emergence.
to
slow
1983)
carbonate mid-continent cycles.
including
may
Depending upon
downwave-lowstand period
upwave-highstand period is 25%
subsidence
progradation
activate
facies
is
prodelta
not
peat.
between
delta and the progradation of the
has
the
or
the
order to fill the accommodation created.
will
be
drowned.
If
occurring
counteract
its
accumulation
at
transition
In 'Walther 2'
where
the stability of the pattern under a range of sedimen­
initiated progradational sequences and tectonically-
eustatically­
wave with a fast rise and slow fall, as developed in
delta plain.
'Cyclothem 4'. This accords with the conclusions of
greatest
Heckel (1986) regarding the origin of mixed clastic
rates
tation rates, and reducing the deleterious elevation
When cycle magnitudes are small these effects are
highstand
67
switching can be simulated as subsidence outpaces
reversed to sand through the knockback routine in
part of the sea-level fall (Fig. 13). This has the effect
A better scenario may be the asymmetrical eustatic
enhanced and sea-level rise effectively proceeds in a
with this type of cycle, a limited degree of delta lobe
of extending the sedimentation window, increasing
enables background
75%. This wave-form produces fast inundation of
sedimentation is possible for most of the time and
delta-top
remaining
the delta regularly matures with the appearance of
Coals may be produced anywhere between highstand
highstand and downwave for the re-establishment of
the magnitude of
the delta top, and there follows much time during
series of steps. Because emergence is never marked,
of the cycle, and
the
least
to low cycle magnitude is employed (Fig. 14). Sudden
knockback
delta-front
eventually reinstate the interrupted facies and the
Columns
effect on 'Walther 2' simulations when a moderate
appeal is the appearance of seam splits in coals (e.g.
facies in the lower or central parts of cyclothems and
of delta-top facies in the upper parts. Of particular
a
delta-top accumulation rates and facies transition is
normally
into