Page 67 - Carbonate Platforms Facies, Sequences, and Evolution

of,
the
has
must
rate.
steps
water
screen
scaling
caused
record.
effects.
say,
or
be
changing
period is
variables:
Any
of
by
45 em kyr- 1 .
5:5
depth
accepted
width
Once
read
a
the
cycle
of
can
Magnitude and
100 kyr,
Program variables
would
During
progressive,
sediment column
carbonate cyclothems.
subsidence,
eustatic
individual
environments.
can
3·9 Myr
ranges, cycles down to 10
throughout
achieve
controlling factor on facies
therefore
magnitude
separate accumulation rates,
be
simultaneous
supratidal facies is deposited,
as
the
eustacy
model
be
Time
the
user-defined limits for tide base,
emergence
a
and
determines
and
for
a
and
the
same
late
operation
minimum
depth facies are
randomized
sediment sequence in which a
directly in metres per thousands of years.
single
rate
the sediment
occupied
simulated,
sediment
of
but by scaling the screen
comes penetrated by stylized dissolution pits.
by a line directly tied to the sea-level curve.
'facies'
of
or
period of eustatic cycles are
line
'Milankovich' such scaling is done automatically.
within
the surface be­
sub­
type
rises and falls as the curve is drawn and the depth at
and storm wave base. The rate of accumulation for
selected. Facies are colour coded with reference to
long as sedimentation rate is kept uniform during a
during
normal wave base
The depth of water at the sediment column is shown
This
each facies can be independently set between 1 and
order of magnitude, which also involves appropriate
lithostratigraphic
Using a
but is shown proportionally by penetrative erosive
by an
run, each completed cyclothem can be regarded as a
emergence is not expressed chronostratigraphically
column
given a vertical 'riser' component and an horizontal
as one controlling factor upon facies succession. As
Subsidence is not defined in absolute values but is
the
practical
involved and the facies patterns created are a record
development has been
three
near but not perfect chrono-stratigraphic record of
sidence but in larger steps. Subsidence, whether by
kyr may be examined. In
assigned
program may simply be used to demonstrate depth
of
depth
Dinantian
In 'Cyclothem' no clastic sediments are necessarily
set
sedimentation
principal
to a subsidence rate of 2.5 em kyr- 1 , whilst a ratio
'tread' in screen pixels. A ratio of 1: 1 corresponds
of changing depth with time at the sediment surface
a
and
time
(e.g.
from
1981;
alone
Cisne
cussed
overall
average
margins
invoked
on
assigned
platform
stepwise,
sediment
graphical
the
produced
Namurian
certainties
Dinantian
controlled
subsidence
took
(1986)
Sadler,
in
In order to
Schlager,
edge.
rates
over
record,
over.
accretion
realistic
episodic
sediment
Cyclic sedimentation: simulation programs
regimes
in
simulations all
Holocene
sediment
controlled
1981;
the
Britain
This
thicknesses,
showed
is
the program is running.
the
some
rates
by
cyclothem development.
and
These
thrust
(e.g.
these
values.
Brigantian stages (Leeder,
of
how
by
late
difficult to model accurately,
thermally-subsiding
tectonically-produced
rates
Mitchell
pattern
Klein
which extend between
normal range for subsidence in
duration
accumulation
faulting,
&
1
which
loading
(Walkden,
lie
Wilson,
subsidence
&
eventually
Bearing
was
of
in the choice of sedimentation rates.
making
as
and
passive
and
in
a
rates.
and Westphalian as thermal
well
Carboniferous basins. Blair & Bilodeau
Dinantian,
subsidence
a
lost
1974;
rates
cyclothems
1987),
a
It
Willard
Reading,
Bott
extensional
in
Asbian
is
are
mind
obtain meaningful results from
probably
over
'Cyclothem'
within
and
might
factor
time
(1988) dis­
un­
erosion and non-depositional hiatuses in the strati­
accumulation rates, pointing out that much internally­
ex­
1986).
be
suggested that steps of up to 8 m might have affected
fault
10 em kyr- 1
and
input variables must be
showed that basinal rates were a little faster, up to
of
averages vary according to facies type and location.
given range to explore the effects of change whilst
the
subsidence
Schlager,
1988) average platform
continental
kyr- 1 . Leeder (1988) and Leeder & McMahon (1988)
also
8 em kyr- 1 , but in general all rates slowed into the
basins
real
especially for ancient
perienced on all platforms at some time in the late
Rates and patterns of eustatic change are far more
59
the
known that such figures can be more than an order
active basinal extension, were in the range 3-6cm
1981). Recent accumulation rates for
of magnitude slower than rates measured or deduced
well
(1989)
(1987)
Since ancient subsidence rates are measured from
in
model. Goldhammer et al. (1987) argued for lower
can
distinction
the
between sediment production rates and net platform
Whilst it is possible that Holocene rates are atypical
carbonates range between 0·5 and >1m kyr- 1 , and
because of very rapid sea-level rises, the difference
is more likely to reflect the importance of reworking,
importance of the distinction should be recognized