Page 65 - Carbonate Platforms Facies, Sequences, and Evolution

or
also
The
up to
The
be
relating
veloped
longer
more
to
determined
three
four
independent
stratigraphy.
used
'Milankovich'
by
using
to
a
sine
period
and
complex
troughs of the sum wave.
similar
wave
programs,
stratigraphic
'Croll'
Vail-type
demonstrate
style
combination
'Walther'
facility
were
of
astronomical forcing
analysis
some
and
eustatic
into
shallow-water carbonate environment.
'Cyclothem',
and
a
depth
simulates
basic
functions
a
rate as the sum wave builds across the screen. The
cycles
and
successively
of
in
Fig. 2. Main screen mode of 'Croll'. Three Milankovich
de­
'Cyclothem' simulates cyclicity in a marine environ­
the sedimentation rate in each field can be separately set.
(top of screen) and are summed in the centre of the screen.
darker). Field thresholds are adjustable up and down and
The upper field of the screen is red (dark grey), the middle
'Walther',
sediment column is therefore an analogue of the peaks and
The sediment column picks up the appropriate colour and
variety of sedimentary settings. The programs may
periods at 23, 41 and 100 kyr have been run simultaneously
concepts
is for autochthonous sediments such as carbonates.
of depth and time. Its most appropriate application
ment in which facies type is expressed as a function
ness or by notionally reassigning the colour-coded
field is white (light grey) and the lower field is blue (slightly
presentation.
sequence
and stratal complexity on different scales and in a
facies, the programs can be used to model cyclicity
simplified
delta
mixed
carbonate-deltaic environment where facies type is
any
version of 'Cyclothem' to simulate orbitally forced
period, and sums these to control a simulated pelagic
progradation. 'Croll' generates sine waves simulating
succession. 'Milankovich' incorporates this multiple
a
is
in
of
the
The
level
these.
1987).
drawn
the
eustatic
through
dynamic
made
use
which
analysing
nominally
modulated
compnsmg
an
of
background
Such
Their
change
programs are
by
or sedimentary
cycles,
the
programs
are
between
an
Cyclic sedimentation: simulation programs
horizontally
3·9Myr,
angle
cyclicity,
based
successive
a
simulations
PROGRAM
but
is
depth
diagrams
term
interactive
subsidence,
but
from
successively
is
drawn.
upon
are
eustatic
a
where
range
left
are
eustatic
includes
an
sea-level/sediment-surface
The
of
routine.
cyclothem
less
veloped sediment column is 120m.
wave.
proportional
rate
rocks created by a
format
accreted,
The
and the height of
and
expressed in analogue form as a wave.
facies
to
attention
to right,
sea-level
and
excellent
a
that
Our
magnitude ·and
sediment
cyclic
the
or eustatic rise are creating accomodation,
and
lithostratigraphic
PRESENTATION
main
cycle is defined as the process or sequence
relationship.
waves
appropriate
surface becomes exposed when sea-level drops.
cycle.
focuses
and the
means
rate
rate
period
pattern
and
as­
of en­
process.
the fully de­
subsidence and the sum of tectonic and eustatic sea­
57
use of this term is chronostratigraphic but it can be
cyclothem is here defined as a sequence of sediments
the discussion of modelling and results, a distinction
vironmental changes that creates a cyclothem. The
cyclothem may be symmetrical or asymmetrical. A
a
A
Throughout the description of the programs and
expresses
subsidence and eustacy in a less conceptual manner
on
Our
by graphing background subsidence as a rising curve
menu (Fig. 3) screens. The menu shows the current
user-defined parameters and enables their resetting
sediment
column builds alongside this curve when subsidence
sociated sediment packets are drawn obliquely below
Much computer-simulated cyclicity has been based
are
of
for
until now upon the format of the 'Fischer diagram'
subsidence (e.g. Fischer, 1964; Goldhammer et at.,
screen
facies selected from a pre-determined range, appears
colour-coded
sea-level window in which separate graphs of tectonic
column,
displays the real time simulation and is divided into
two sectors. Most of the screen is occupied by the
of
of
and its
of
of
on the right. The width of the sea-level window is
All programs use full colour main (Figs 1 & 2) and
The range of user-defined variables differs in each
accumulation of each. In most cases variables can be