Page 73 - Carbonate Platforms Facies, Sequences, and Evolution

of
the
the
than
rates.
stitial
The
is
column
paction
modelled
pactional
completion
simulations
upwave
window,
can be rapid
water,
basic
than
likely
is
in
an eustatic
Ramsey
Coal Measures
shallow flooding
in
during
our
but
thickness and 40%
rise,
to
the
subsidence
merely
continues,
be
'Walther'
yr- 1 which considerably
real
& Moslow
promotes
cyclothems
program
that
rates
is
or
of
sedimentological
transgression
simulating flooding
down to 500 m but
a
effective
is
(1987)
the
deposited and goes to completion
above
a reverse transition of facies (knock
situation.
it
representing
simulations
that deltas cannot advance during a
modelling necessarily involves
designed
the
thicker
has
at 20%
to
more
ment by transgressive facies (wipe routine).
of mud at around 100m.
to
major
only
therefore
document
assumptions
According
effect
go
structural and ad­
inter­
difference between depositional thickness and com­
the complete drowning of the delta and its replace­
com­
trans­
em­
the limited thickness of the sediment column, com­
accom­
nom­
Chillingarian (1983), expulsion of interstitial water
modate, and therefore remains reasonably realistic.
rapidly
rate of accumulation is user-defined its only intrinsic
of
well modelled, and large-scale delta abandonment,
Yoredale situations of Ramsbottom (1973) are not
of peat
The routine starts after 20m of sediment has been
Compaction is introduced first to demonstrate the
sediment
up of the clastic supply systems. The sediment that is
rising base levels in the hinterland and the backing
sorbed water in clays continues to be expelled beyond
1 km. The effect of more rapid compaction in our
delta-top mud or peat.
gression and marine conditions prevail at this stage.
programmed characteristic is that it will not compact.
inally a carbonate rather than a mud, but since its
to
Rapid flooding, depending upon scale, causes either
back routine) or
outpaces normal tectonic
as in the shoal water deltas of the Mississippi (e.g.
the effect of active compaction on overall subsidence
pacted thickness in cyclothems, and second to show
Holocene deltaic sediments in Louisiana of > 1 em
unconsolidated
the delta top becomes rapidly
withdrawal of subsurface oil and brines. Because of
ployed in 'Walther' are that during the initial phases
of the abandoned
subsidence, but some of this may be attributable to
negligible. A wipe routine removes the delta from
flooded and clastic sedimentation rates are slowed to
Elliott, 1986), cannot be reproduced within a single
surface, and delta development remains inhibited as
to
A conscious limitation of this program, therefore, is
highly proximal
or
as
As
the
top
wise
lead
gram
in
during
is
volved,
system,
surface.
to
Other
potential.
reworking
inundation,
determined
stepwise,
will
and erosion.
preservation
and is
subsidence,
difference
by
fast falls
and
not
and
subsidence.
permitted
termined by base
Cyclic sedimentation: simulation programs
unrealistic
a
of
'Cyclothem',
the
usually
can
but merely
potential
This
level
steady
opment would continue.
be
between
menu
and preservation
Modelling using 'Walther'
plots
compaction,
and
following
trigger
sea-level
tation rate is in approximate
dynamics
rather
a
of
option.
past
activates
over-emphasis
followed by renewed progradation.
sedimentation
menu options include the
subsidence
potential,
than
inverse
upward
sea-level
of
the
the
randomized.
The
the
renewed
accumulation
conditions will promote delta-top facies.
reversal
subsidence.
rates
under
facies
The
of
these
are
routine can be disabled through the program.
The
change
ultimately
con­
de­
user­
routine and, depending upon the depth change in­
width of facies belts) is adjustable through the pro­
eustatic cycle unless simulated by introducing step­
addressed by Sadler (1981) who noted that sedimen­
that in distal areas beyond the site-modelled devel­
flanks of the hinterland (e.g. Walkden, 1987). Delta­
Once sea-level rise is complete, it is assumed that
knockback
depositional
notoriously difficult to model and steady figures are
Sediment accumulation rates in clastic systems are
compaction
potential,
small-scale
trolling link between the two is sediment reworking
'Walther' does not simulate sediment
quite meaningless for some facies. This problem is
accumulation
65
facies
shallow
magnitude and
period of the cycle. Subsidence can be set as linear
rate of development of the delta (and the relative
facies
conditions in delta-top lagoons and ponds but can
2m above and 1m below sea-level. This simulates
Simulation of a complex interplay of processes such
On emergence of the delta top, the delta
draw routine switches off, although it is recognized
sedimentary
proportion to
the timespan over which it is measured. This reflects
in abundance during fall as the notionally backed up
Local reactivation of the
draw routine switches on. Clastic supply continues
definable, but rates should be chosen with regard to
delta outbuilding would recommence, and the delta
and
top facies are given a depositional window between
overall
sediment is rapidly reworked and cleared from the
trends.