Page 70 - Carbonate Platforms Facies, Sequences, and Evolution

x
to
be
by
62
the
The
(Fig.
from
upon
To
cases
faster
More
overall
250
well
steps. of
Read
the
persistent
a
eustacy
rate
kyr)
randomized
these
et
response
reduce
of
rate
increased
complex
few
at.
are
Holocene
of
the
subsidence
shoreline
shown
ment column (Fig. 8).
facies
(Cyclothem
is
(1986)
metres,
diversity
rate
by
lacking,
background
subsidence
5).
carbonate
who
horizontal
facies
by prograding intertidal facies.
opportunity
further emphasizes the pattern.
Effects of changes in subsidence
patterns
appears
enables
With
relative
therefore
for
indicating
are
subsidence
pointed
of
'stretches'
developed during regression,
small
deeper
or
should be used, the upwave
out
because
the
boundaries
sharp
progradation
sea-level
sedimentation
a
of
there
of
intertidal
that
can
is
may
water
introduced
lOb) and marginally increases their facies di­
importance. Since 'Cyclothem' is principally a time­
matched directly to the facies responses in the sedi­
a 'square' wave form in which the fall is also rela­
pro­
crossed and recrossed, especially during regression.
of subsidence increases the thickness of cyclothems
eustatic
of an eustatic rise and more in depositing sediments.
Pleistocene studies (e.g. Donovan & Jones, 1979).
Using steady subsidence, any increase in the rate
of accumulation of peritidal facies to less than that
to
Alternatively, cycles of moderate magnitude (30m
tidal
approaches the sediment surface. Reducing the rate
flats across the platform. The presumed location of
of subtidal facies, for which there is some support
downwave the fall rate is at its fastest as sea-level
rates,
considerable
development of intertidal facies may greatly depend
the modelled site (proximal or distal) in relation to a
be
which
faster
significantly outpaces sedimentation so that during
background subsidence are reintroduced in addition
subsidence
movements of sea-level. This problem was addressed
no
relative
guarantee that in the real situation it would be filled
be
versity. The increased thickness occurs because the
the
is clear that although the intertidal 'window' might
gradation, the size of this window must be reduced
wave with less time taken up by exposure at the end
in the simulatipns. This could be done by moving to
The graph of background subsidence together ·with
if
depth simulation and not a real facies simulation, it
stepwise
of cyclothems and yet it has been noted that in many
tively abrupt, for which there is some support from
the
This
cycle
G.M. Walkden and G.D.
Using
become
reducing
surface),
resultant
calculated
subsidence
magnitude
eustatic
steady
the
(cf. Schlager,
is
of
eustatic cycle.
does
Walkden
magnitudes
cannot keep pace
observation
the
otherwise as in Fig. 5.
established
upwave
cyclothems.
subsidence
allowed,
the average magnitude
subsidence
of
produce
during
eustatic
give
Only
sedimentation
contradicts
the
was filled
the
when
wave.
the
rate,
cyclothem thickness
Effects of changes in cycle magnitude
it
alternatively
cyclothems
straight
upwave.
can
of
magnitude
be
away
Thereafter,
1981; Kendall & Schlager, 1981).
of
with stepped subsidence of 10m in 400kyr as used in
suggestion
modelled
less known
Note that exposure periods are also affected. Variables
frequently briefly reversed owing to sudden subsidence.
as
Fig. 4. Eustatic cycles remain clearly identifiable through
the subsidence .effect but the shallowing-up sequences are
facies diversity and regular thickness, whilst larger
small cycle magnitudes produce cyclothems with low
and sedimentation rates,
greater
With fast sedimentation rates it is only the first
cyclothem in any run that reflects to any degree the
facies diversity but identical thickness (Figs 9a, b).
Exposure
the
Fig. 8. 'Cyclothem 5' combining the a symmetric 30m wav e
surfaces are thus poorly developed and if too much
and the platform finally drowns
sedimentation
by
there was no drop of sea-level below the platform
no significant exposure of the sediment surface (i.e.
the
or
result of fast sedimentation rates; and (2) there was
eustatic cycles from the
Leeder (1988) that it should be possible to calculate
of
a
assumed that; (1) all accommodation created during
safely