Page 79 - Carbonate Platforms Facies, Sequences, and Evolution

a
to
of
are
the
ting
This
fully
a
period
created
brief
mentation
by
short-period
possible
reasonably
permits
cycles
than cycles in
The results of
the
demonstrate
the Pleistocene.
and
insufficient
high-magnitude
A key to the
cycle magnitude
throughout
& Goldhammer,
period
the
wave
fifth-order
may
wide
megacyclic packet,
to
two
of
and
the long-period wave,
their
20
are
by Goldhammer et al.
not
magnitude
000
flood
range
successive
that if the
fall
Examples are shown in Fig.
subsidence
of
series of
preservation
the
recognition
plotted
to reproduce their asymmetric
year
19.
emergence
of
highs
of
(1987). In
rates
experiments
a
of
that
possible
magnitude
against
diminishing
period
fewer
platform
the
and
less than half the total number of cycles.
sedimentation rate
of
sufficiently to
pentacyclic
each
to
sine
occurs
the
(Fig.
It is also important to note from modelling
employed
the stratigraphical record lies in
variations
comprising a rapid rise and slow fall. This relation­
'zone of pentacycles' that extends from high magni­
ship, with only a small departure was used success­
Using 'Milankovich' and the same orders of sedi­
the platform is flooded, such low-magnitude short­
emergence of the platform surface so that the num­
fifth­
other.
zero.
these simulations are
presented in Fig. 18 where, for a given magnitude of
using fifth-order waves right down to 10 000 years.
Goldhammer et al. , together with their asymmetric
fourth-order cycle, we have found that it is possible
clusters
magnitude down to 10 000 years or less with a range
during the fourth-order (100 000 year) sea-level low
tude 20 000 year cycles with a wide range in possible
that
18),
period and magnitude of the
wedge-shaped
effectively
cyclothems
the
produce
the conclusion that the Middle Triassic was dominated
supported
wave
1988). 'Milankovich' can be used
against a fourth-order 100 000 year asymmetric wave
loss of cycles below the platform surface (e.g. Read
by Milankovich periodicities closely comparable with
by
in
deposition must be confined to
fifth-order
addition to this, during the fourth-order high when
ber of evident cyclothems can be further reduced to
short-period cycles fail to develop
sedimentation has nearly filled the accommodation
and as many as one third of the cycles go unrecorded
because they are lost beyond the platform edge. In
marked thinning of cyclothems towards the top of a
the strongly asymmetric pattern recorded in the field
order to achieve
when
order (20 000 year) cycle is small enough its upwaves
although a true cycle ratio of around 5: 1 lies within
Fig.
( a )
( b )
18.
�
�
c
E
"
0
�
,
�
,
"'
"'
E
�
"'
"
,.,
c
�
0
�
�
�
,
"
"'
"'
�
"
E
e
,
,.,
"'
3
4
2
6
7
5
9
8
6
10
7
5
8
Cyclic sedimentation: simulation programs
8
12.5
qo
5th
o<�-
p o o � � s � ll-
._.<o
10
10
...
10
�
�
«-
N ON
._<I-
12
of runs are seen in Figs 17 & 19.
8.3
.,_o
12
as shown on 'x' and 'y' coordinates.
�q
as shown on 'x' and 'y' coordinates.
�>"
14
7.1
*"""
14
"'"'
C Ycl Es
v.E ' � � of p p.c�<.Ei S
16
16
6.2
4th : 5th order cycle ratio
5th ord er cycle period (kyr)
E AI E R G E N r Ft R s r
order cycle poriod {kyr)
18
5.5
(b) Sedimentation lag depth of 1m; fourth-order
Graphs constructed using empirical data from
'Milankovich' runs to show the range of settings under
20
(a) No sedimentation lag on inundation, fourth-order
5
asymmetric cycle set at 4 m x 100 kyr, fifth-order settings
asymmetric cycle set at 4 m x 100 kyr, fifth-order settings
71
which a series of pentacyclic packets comprising five or so
thinning-up minor cyclothems can be produced. Examples