Page 72 - Carbonate Platforms Facies, Sequences, and Evolution

a
a
of
of
al.
64
and
time
plain
these
an
depth
points
in
adjacent
broad
(Fig.
(1987)
mud
simulation
is random,
and
'castellated'
only
asymmetric.
actual
generalized
11).
sequences (e.g.
and
square
'Walther
Program features
sediment column,
irregular
is
along
1'
stochastically
sediment
peat.
attributed
growing
so
necessitates
expression
the
sand
this
to
already
is
that
delta
Facies
and
Elliott, 1975,
some
shallow-water
nominally representing an 8 X
sheet
periods in the range 235-880
the
in
lobe
eustacy.
given
northern
multi-distributary fluvial-dominated
a
1986).
accumulation.
means
by
distal or proximal,
occupied
Full
transition
of
advances
patterns
G.M.
by
delta
The
'Walther
margin
linear,
8 km or 80 x
in
also calls for feedback between facies type,
kyr (Driese &
2'
the
cycles
of
within
wave-influenced
80
these to take place in the sediment column.
it
generating
window, a reasonably convincing pattern of south­
choice, controlled by the need for clarity and sim­
carbonate, prodelta mud, delta front sand-silt, delta­
1974; Heckel 1986; Walkden, 1987) and their prob­
any
able glacioeustatic origin has already been discussed.
seawards.
individual square is mostly uni-directional but timing
and transition can only occur when an
new
The routine uses a 64 square grid,
Dott,
runs produce quite different distributions and arrival
The introduction of specific clastic facies into the
are not merely an animation of the real situation. It
water
times of facies. Sand advance commonly breaks into
separate lobes but in general the patterns approach
or
eventual
a
generating
plicity, was to emulate some of the essential features
produced
which
rudimentary facies map of a delta that develops with
'Cyclothem'. The main differences lie in the use of a
The style and format of Walther is close to that of
screen window in which 'clastic' facies are generated
A
and an additional routine that enables 'compaction'
By minor program alteration any one square in
facies. By seeding new facies at randomly determined
the
facies appearing in this reference square that finds
had
noted in carbonate successions by Goldhammer et
sector of shelf, and five colour-coded facies, namely
This pattern is appropriate for modelling Yoredale
the window can be set as the reference point for the
km
is
and it is the
precise timing and rate of accumulation of specific
ward prograding facies belts is created and separate
almost
sine wave is used in the program to control eustacy,
Walkden and G.D.
Whilst
mensional
succession
expression
in
accumulation
the
Walkden
facies
noted
the
rates
delta
by
(e.g.
mosaic,
point for the sediment column.
window
fast
Walther
in
1973) gives the program its name.
sand
changing
sediment column.
1884
provides
and
with
a
This
delta in 'Walther'. Facies are colour coded in the
slow
time,
two
link
front sand-silt; (4) delta-plain mud; (5), delta-top peat.
simulations but are numbered here to aid recognition in
be­
Fig. 11. Two successive stages in the development of the
and shallow-water facies are only permitted to pro­
di­
grade across the reference square when an appro­
(Middleton,
water depth at the column and delta development in
using the rising and falling sea-level bar adopted in
the
depth dimension is simulated at the sediment column
tween lateral facies distribution and vertical facies
'Cyclothem'. There are feedback routines that link
priate water depth is developed above the sediment
monochrome. (1) Carbonate; (2) pro-delta mud; (3) delta
peat)
The black bar to the left of the sediment column represents
the window. Facies types can be allocated specific
column. Emergence resulting from a fall of sea-level
sea-level and the oblong on the delta map is the reference
at the sediment column strands the delta locally, and