Page 82 - Carbonate Platforms Facies, Sequences, and Evolution

74
eral
year
19 kyr
Fisher
within
(1981)
period
period.
against
achieve
a
a
times
simulation
and 2.5 m.
Cretaceous
frequencies.
forcing
using
This
large
model
age
a
is
limestone-shale
41 kyr
faster
bunching
Regular bunching
Such
that
demonstrated
is
containing
function
than
fast. Screen width 390 kyr.
a
period
simple
a
using
also
involving
achieved
sequences
liferating the shale horizons.
period against 23 kyr,
display
facies
within
(Fig.
by
wave
of
a
simulations
five-fold
number of cycles,
stochastic
22).
produced
standard
regular
triggered
longer
adjusting
G.M.
More
confirm
means
grouping
by
frequencies
of the screen is reduced, the upper (dark) field is
the
of
Carboniferous
that
period
'bundling'
Schwarzacher
with
by running
given a slow sedimentation rate whilst the lower field is
couplets could be produced. In this run the central field
Fig. 21. 'Croll' run using 20 and lOO kyr waves and their
pro­
sum to show how 'bunched' limestone-shale (light/dark)
wave peaks (representing muds) are deposited sev­
bunched patterns can be reproduced using a three­
sedi­
of
Using 'Croll' an idealized 5: 1 pattern of 'bunched'
&
ratio of >5: 1 if the mud threshold is high enough to
which they attributed to the operation of a 20 000
a
in
ensure that only the highest peaks trigger the muds.
of
troughs
(Fig. 21). As with 'Milankovich' it is also possible to
cycles is easily reproduced using a 100 kyr and 20 kyr
mentation rate as above so that facies triggered by
100 000 years. This was modelled by Schwarzacher
By analysing bedding plane distribution
a
mathematically-generated
and
and a convincing 5: 1
pattern
Milankovich
complex
bunching pattern is produced running a 23 kyr period
bed thicknesses into cycles on a scale between <1m
of
as
ing
best
that
Walkden and G.D.
Used
or
clearly
fast
greater
scientist.
rise
as
forms
asymmetric
Milankovich
other
it
Brigantian
'Milankovich'
by diagenesis.
The
facies
of
rates
Walkden
subsidence.
properly,
microcomputer.
Asymmetrical
exceeded
asymmetry is not
not asymmetrical.
wave.
A
is indicated which
rose.
and
(late
forms
signals
DISCUSSION
were
the
In
of
diversity
may
Such
programs
'Croll'
statistical methods may be
AND
experiment
slow,
be
such
and
shallowing-up
time between emergent events.
longer cycle
thickness
Dinantian)
are
repetition
cases
always obvious.
of
asymmetry
perhaps
computer modelling
available
are
is
demonstrate
'Cyclothem',
a
cycle
the
to
thicker
by clear emergence features such as palaeo
the
cyclothems
from the sedimentary
is one
that
the
cyclothems
because
period alone
of
CONCLUSIONS
further evidence of
environments that enjoy long term stability, bunch­
results, with applications both in research and teach­
Where there is also a tendency for peritidal de­
character of orbitally-forced sedimentation patterns.
required to extract the
What they also show, however, is that sophisticated
diagnostic
earth
the
ing, are well within the scope of a widely available
'Walther',
especially if the patterns have become masked by
record,
posits to be missing at the tops of cyclothems, again
magnitudes were not great, or that the cycles were
capped
eustatic wave that was not asymmetric and fell about
an
noted in the Asbian, a relatively rapid sea-level fall
cycle
outpaced sedimentation rate, but in some successions
such as the Asbian (Dinatian) of northern England
This may mean
a characteristic product of an eustatic cycle with an
karsts are
rule and reflects a sea-level rise rate that significantly
general
magnitude
independent changes in sedimentation dynamics or
magnitude. Assuming constant sedimentation rates,
useful
because the rate of sea�level fall was already fairly
possible to arrive at a close estimate of actual cycle
cyclothem thickness in these circumstances is entirely
have
determined by the subsidence rate and the period of
obviously asymmetrical than Asbian ones (Walkden,
rapid as it passed the sediment surface, but it is not
increased cycle magnitude and asymmetry leading to
and more
cyclothem
(e.g.
1987). These differences might be accounted for by
the greater facies diversity, or a faster overall rate