Page 78 - Carbonate Platforms Facies, Sequences, and Evolution

70
20
the
Fig.
The
tively
minor
additional
cyclothems
tectonically
short
surface
around five.
result is a
(e.g.
(Fig.
cyclothems,
period
longer
showed
(Goldhammer et a/.,
a
of
17).
the
Cisne,
Goldhammer
total
period
These
marked
eustatic
et a/.
rise
number
G.M.
and
of
simulations
build as far as the top of the upwave (Fig.
asymmetric
rate of the cycle always significantly outpaces the
noted that
degree of exposure at minor cyclothem boundaries
fall
upward-thinning
perfect upward-thinning packet
which
increases as the time spent during deposition decreases.
as a result of the successive up-building of the sedi­
kyr) which has a rise rate that significantly out­
succession of minor cyclothems will result. Note how the
pattern using a computer simulation. A single mega­
cycle of this type is easily synthesized using a rela­
mega­
1986) by introducing step­
and a stationary 7 m x 20 kyr wave. Provided that the rise
of
the component minor cyclothems and modelled the
to an absolute datum line so that sediments can only
15). As
surface, separate increments of sediment are added,
(e.g.
of
ment column which extends the period of exposure.
but the amount deposited with each cycle decreases
the wave repeatedly floods then exposes the platform
wise subsidence (Fig. 16), or eustatically through an
wave
paces sedimentation and a static position in relation
sedimentation rate (here set at 7 em kyr- 1 ) then a thinning
1987) that will rapidly drown
provide
dependent upon the balance between sedimentation
this pattern repeat itself some means of achieving
rate, cycle period and cycle magnitude, and will vary
with a change in any one of these. In order to make
sudden subsidence is necessary. This can be achieved
15. A 'Milankovich' simulation using a 390 kyr screen
is
p
lm.
Walkden and G.D.
useful
Fig. 17.
diversity
Walkden
predictive
within minor
the tops of the packets.
indistinguishable from Fig.
16.
observations
:�I
arranged in 2· 5 m packets or clusters.
for
cyclothems
�
at
,,
use
1
,,
,,
in
the
_:_::
fold thinning-upward clusters. The run is almost
'"
·�
I· · �� ·
"
��
the
� ·
bases
Sedimentation lag depth
n
�
�
Cl:
liol
"';" �
�
cycle and 2·5 m steps at 100 kyr intervals. The result is a
megacyclic packets and the greatest degree of sub­
reproduce their five-fold thinning-up clusters by set-
where one should expect to see the greatest depth
field
of
wave and a 4 m x 100 kyr asymmetric wave to produce five­
Fig. 16. 'Milankovich' run using a 5 m x 20 kyr symmetrical
repeated pattern of five thinning-upward minor cyclothems
Goldhammer et a/. (1987) found that they could
aerial alteration above the minor cyclothems towards
'Milankovich' run using an 8 m x 20 kyr symmetric