Page 83 - Carbonate Platforms Facies, Sequences, and Evolution

Y
of
of
are
and
This
a
them
stage
some
stand
imply
areas.
the
system
flooding
Mixed
of
diversity.
an
Walkden,
as
and
highstand,
of
a
If
the
little relative
naturally
growing
smaller
situation
thickness
subsidence
thick
ice
is
Brigantian,
whole
faster
but
cycle.
cycles
rate,
asymmetric
coals
are
but
body.
sourcelands,
asymmetric
may
would
sea-level fall
These
carbonate-clastic
miss
enhanced
cycle.
could
and
accord with field observations.
on
encouraged anywhere between
and
not
subsidence
including
have
unstable
the
Veevers
and the sedimentation
the
the
do
if
form.
&
where
it
were
It
failed
Yoredale
not
increase
However,
to
evidently
generalizations
Powell
confirms that the basal limestones seen in Y
Thick coals are not a normal feature of Y
the
faster this
compactional
downwave
formed
follows
successions mainly represent the upwave phase
are
as
1987) would lead to an increase in cyclo­
feature of distal Yoredale cyclothems where lime­
cyclothems. Using a symmetrical wave, peat preser­
clastics. In proximal situations the tops of the lime­
emergence, decreasing the number of recorded cyclo­
during lowstand. Peat accumulation is best encour­
highstand and low­
sub­
result of a slow downwave being balanced by sub­
thems over a given period. Changes in cycle magni­
cycles of the Asbian that start about 5-9 kyr earlier.
smaller
cyclothems
facies
sediments were once again released from the source
acknowledged Yoredale cyclicity from the beginning
place the more effective would be the inhibition of
oredale
of
stones should be marked by a diachronous transition
necessarily
modelling
to shale created after the cycle peaked and clastic
tude and asymmetry prelude the onset of the main
Gondwanan glaciation timed at the beginning of the
dynamics of the
and slow downwave stretches the delta-top window.
sidence. In these circumstances delta-top facies are
produce
responsible,
be responses to changes in distribution and massing
oredale
(1987)
vation seems less likely if it accumulated only during
sidence, created the balance necessary to stretch the
in
subsidence is taking place and when cycle magnitude
aged where an asymmetric cycle with a fast upwave
is relatively small. The ideal pattern is where there is
Emergence features should only be a typical
a
compactional
than late Upper Carboniferous cycles and that coals
stone deposition continued well into the downwave
took
only
that
Namurian by Powell & Veevers (1987), and would
formed where the combination of cycle magnitude
oredale cycles could have had larger magnitudes
the cycle when clastic influx was reduced owing to
slow
effect
ments,
secular
platform
on
including
may
the
wave frequencies.
control
be
limestone-shale
Cyclic sedimentation: simulation programs
an
sedimentation
stepwise
has
relative
environments,
leads
couplets
been
in
to
tectonic
or
indication
in
gressive-dominated cyclothems.
of
importance
that
of
and regressive portions of cyclothems.
facies
more
subsidence.
operation,
off-platform
than
such
produced using periods other than 20 and 100 kyr. (a)
thresholds or the sum wave magnitude, bunches of any
The two periods are 19 and 23 kyr. (b) The two periods
number of beds up to seven can be produced from these
are 23 and 41 kyr. Screen width 390 kyr. By adjusting the
environ­
cyclothems whilst fast sedimentation leads to trans­
Fig. 22. Two runs of 'Croll' to show that bunching can be
transgressive
75
delta-top window. Sedimentation rates also have an
Grouping and bunching of emergent surfaces in
In general,
regressive-dominated
perhaps
Correct
one
as