Page 80 - Carbonate Platforms Facies, Sequences, and Evolution

72
no
ling
fore
10: 1
cycles
water
quence
periods
is
simulate
Wagoner
cyclothem
and
appropriate
reproduced,
et
exclusive
rate
rcttio
towards
al. ,
and
possible,
considerably
is the most likely.
successions,
of
a
stratigraphy
reason
seen
by
parasequences
are less easily modelled.
1988),
type
fourth-order
for
in
reduces
2
although
and
depending
sequence
using
the
(e.g.
from
an
sedimentation
together
upon
platform
assuming
area
sequence
in
300kyr)
sequences
with
boundary
G.M.
that
a
is
chosen,
interplay
Fig.
and
modelling
(e.g.
the
of
which good asymmetric 'pentacycles' can form.
sediments
magnitude
15
type
Since the program is designed for modelling shallow­
before the fourth-order rise takes place. This there­
A further application of 'Milankovich' is in model­
third­
some of the general concepts surrounding se­
boundaries,
Goldhammer et al. (1987) was in fact the effect of
The implication of this modelling is that there is
two
known Milankovich periods. A cycle ratio of up to
in
(erosional) sequence boundary. The typical thinning
cycles, as in Fig. 15. This situation cannot be achieved
cycles may be lost at the sequence boundary itself.
several
readily
of parasequences accompanying increasing exposure
of
with large magnitude fifth-order cycles because the
order (e.g. 3 Myr) cycles of moderate magnitude will
symmetrical cycle periods. For example, combining
where emergence and erosion are not characteristic,
maximum accommodation they create is never filled
1
Van
'Milankovich' it can be seen that a ratio of =5·5-6·5
by
5:1
using
is
the
but
agic
Walkden and G.D.
strate
where
an
number
resenting
the
facies
of
independent
continuous.
Walkden
standard
such
100000
effects
sedimentation
as
different
of
and is influenced by
orbital-forcing
Modelling using 'Croll'
100
a
rates
accumulation
case lm.
of
scales.
compared.
basinal
Milankovich
(eccentricity),
dilution cycles of Einsele, 1982).
the
combining
simulation
rates,
This
number of
it
is
we
is
three
41 000
progressively increases.
factors
have
frequencies
(a) Fifth-order cycle 7 m x
(b) Fifth-order cycle 5 m x
facies,
limestone-shale
not
13 kyr; fourth-order cycle 4 m x
possible
16 kyr; fourth-order cycle 4 m X
such
100 kyr. Two to three cycles are
Fig. 19. Two 'Milankovich' runs
is regularly lost because it fails to
Sedimentation lag depth in each
cycle in each packet is only briefly
emergent. The degree of exposure
in­
se­
The program 'Croll' is primarily intended to demon­
forcing functions of different periods to model pel­
rep­
The effect of summing symmetrical waves of dif­
ferent periods is to produce 'beats' at specific mul­
Milankovich-type
simulations assume radically different appearances
(Fig. 20b-d). These transformations could be used
the
produce emergence of the platform.
sedimentation rate (Fig. 20a). However, by adjusting
cluded the asymmetric wave facility. Emergence of
et al. , 1987). Since 'Croll' is not a sea-level simulation
the sediment pile cannot occur so that sedimentation
to
Since the three facies can be given
climatic change and sea-level is extremely complex
as
quences. The relationship between orbitally-forced
well as total insolation (e.g. Imbrie, 1985; Crowley
oceanic conditions and volume and location of ice as
(obliquity)
to trigger facies changes ensures that the sediment
demonstrated
response also has a broadly symmetrical pattern on a
changes in sediment influx (productivity cycles and
simulate both changes in carbonate productivity and
tiples of the input frequencies. Using the sum wave
and 23 000 (precession) years are used with a uniform
at each subsequent minor boundary
kyr. Note that at least one cycle
regularly lost and the first or second