Page 77 - Carbonate Platforms Facies, Sequences, and Evolution

be
the
(e.g.
pally
1987;
cyclic
Read
as
ments
emerge
House,
&
simulates
for
Objectives
(e.g.
units
Imbrie,
evidence
from
1985).
predictable
PROGRAMS
simulating
as
hammer et al., 1987).
grouped
studies
Goldhammer,
of
Although
Schwarzacher
facies
into
1985; Chappell
1988;
hitherto
Schwarzacher & Fischer,
'CROLL'
there
sedimentary sequence forms an
&
astronomically-forced
shallow-
thems, the programs 'Milankovich'
coals and limestones are commonly split.
clusters
diversity
1982;
have
that Milankovich-band
& Fischer,
to
assumed,
'MILANKOVICH '
Fig. 14. 'Walther 2' run with stepped subsidence
numbers and preset variables are as in Figs 11 &
important
been
within
1982;
Shackleton,
Schwarzacher,
through the geological past (Winograd et al. ,
Olsen,
comprising 4m every 14kyr. The graphic log, facies
AND
12. The
sediments from a wide range of ages and environ­
functions. These are modelled as waves, and adopt-
cyclo­
sudden subsidence causes a reversed facies transition that
Gold­
can affect any facies with the exception of that at the base
Unlike 'Cyclothem' and 'Walther' which are princi­
of the cyclothem. This means that all lithologies including
deep-water
1986;
especially
Milankovich-type
1988;
1984;
record (Grotzinger, 1986; Goldhammer et al. , 1987;
As its name suggests, the program 'Milankovich'
Laskar, 1989), some convincing patterns comprising
1975,
recent
recognition of repetitive patterns in certain types of
Weedon, 1989) may be recognizable in the geological
part of
periodicities
warnings that the Milankovich periodicities may not
(megacycles)
and 'Croll' are
intended for modelling cyclothem distributions. The
et
as
up
the
first
from
Croll
to
The
1975),
forced
al. ,
to
periods
controls
the
Deeside
a
Weedon,
was
'Vail'-type
acceptable
developed.
cyclothems
which
the
three
well
1984;
key
shallow-water
accommodate
factors
it
programs
program
village
born
cycles
is
recognize
Middle
this
Cyclic sedimentation: simulation programs
of
a
facies,
(Goldhammer et al. ,
grouped
might
and
beyond
feature
House,
so that the
(Vail
is
carbonate
simulation.
and
range,
'Croll' is
the
et
into
directly
described
deeper-water environment
Triassic
1985;
the
different sedimentation rate.
By
the
in
Modelling using 'Milankovich'
brought
al. ,
Banchory,
in
each
connection
1986; Raiswell, 1988).
setting.
of
nevertheless
up
can
scaled as required at 390 780 or 3900
derived,
designed
which
this
program can
screen
Arthur
in
1977).
presence
where
defining
megacyclic
be
to
et
range and deposition rate of each facies.
Three
Milankovich
between
affect
In
of
and
the
the
paper
al. ,
northern
all
including
band
waves
order
packets
that is asymmetrical, with an upwave that is 25%
emergence
assigned
simulate
the total period. Each can be assigned an indepen­
user­
re­
The name of the program acknowledges the self­
Up to three inde­
pendent waves of user-definable period and magni­
orbitally­
the sediment pile is not possible, but where orbitally­
of
place the single wave in 'Cyclothem' including one
ing the basic assumption that it is these signals that
dent period and magnitude through the menu, and
are
kyr. Subsidence
also simulate
direct control on sea-level. The program shares most
control glacioeustacy the sum of these is used as a
there is no longer independent control on the depth
the
features of style and presentation with 'Cyclothem'
defined variables based upon absolute values can be
to
driven variations and climatic change (Croll, 1864).
1986;
of
sediment
upper and lower thresholds the peaks and troughs of
69
In certain shallow-water carbonate sequences such
made scientist James Croll LLD FRS, who was the
type. This may be more directly through temperature
a
tude are simultaneously drawn and the sum of these
can again be set in a steady or stepwise mode but
present
Scottish
appropriate
the sum wave can be used to trigger changes between
were
Italy
a
or climate rather than through eustacy (e.g. Arthur
Lofer sequence of the northern Alps (Schwarzacher,
authors have lived for the last 20 years and where
1m-scale
1987) and the Upper Triassic
of