Page 74 - Carbonate Platforms Facies, Sequences, and Evolution

66
the
delta
there
initially
window
More
through
is
gradation
interfacing
a
generalized
for
of
rapid
increased
rarely
with
outpaces
slower
facies,
the
synthesis.
delta-top
sufficient
and
subsidence
subsidence,
overall
basic
facies
coals
rate
sedimentation
compaction on cyclothem thickness (b
or the
extends
and
longer runs accurately in real time.
of
are
program
but
opportunity
the
so
as
thick
G.M.
for
initially
Effects of tectonic and compactive subsidence
the
that
equivalent
levels out sedimentation begins to catch up.
full
peats
it
casionally appear during downwave or lowstand.
developed. However, if these early cyclothems con­
'Walther 1' is preset with a symmetrical 20m magni­
Shal­
pro­
oc­
no
wave
sedimentation rates for all facies (Fig. 12). Upwave
tude flat-topped eustatic wave and is given uniform
thin or possibly missing. During the first few cycles
falling sea-level and delta-top facies are consequently
lowing is rapid as the sediment surface rises to meet
are at the very least thought provoking. The program
tain appreciable quantities of compactible prodelta
runs slowly when the machine code delta routine is
sedimentation,
effect
depositional
programming and 'Walther' can provide only a very
added compactive subsidence (Fig. 12). In particular
Nevertheless its simulations
mud, later cyclothems become thicker as a result of
rarely
development makes great demands on basic
of
for
The
that
Walkden and G.D.
duced.
to prograde across the reference square if the water depth is 2m or less.
coal
cyclothems
is
'Cyclothem'.
inclusion
a
Walkden
and
of
karstification
This
Emergence
the transgressive
Development of coals
of
peat
and
portions of cyclothems.
to
would
the
component
early
be
in the
of
Proximal and distal cyclothems
distal
subaerial
the
delta-dominated
unusual
regressive
in
fully developed Yoredale-type sequences.
proximal
diagenesis
thicker in upper cyclothems despite the fact that they do not compact. In the case of the limestone this is because
and c). Variables as follows: Subsidence rate 3cm kyr- 1 , sedimentation rate
facies succession
as
Fig. 12. A completed run of 'Walther 1' (a) together with enlargements of the sediment column showing the effect of
10cm kyr- 1 , cycles 20 m x 400 kyr. Facies are numbered as in Fig. 11, and a graphic log has been superimposed on the
where the muds (2, 4) and coal (5) have been compacted. This lost thickness has been transferred to the upper part of the
situations and demonstrates the reason for the nor­
compactional subsidence reduces the degree of emergence at the cycle boundary and flooding occurs earlier. In the case of
sediment column to aid facies recognition. Note how sand (3) and limestone (1) dominate in the lower part of the column
column in the form of increased accommodation creating cyclothems that are 50 % thicker. Sand and limestones are also
the sand it is because of a feedback routine between delta development and water depth which will only permit the delta top
cyclo­
(1972) and Ramsbottom (1973), are readily repro­
strates. Conversely, overall faster rates of sedimen­
the limestones are relatively thick would bring car­
of delta-top facies, as resetting these values demon­
therefore has the effect of increasing the importance
tation or slower subsidence increases the importance
thems, as noted in Yoredale sequences by Johnson
cyclothems where
The above effects are similar to those achieved by
situations respectively. Carbonate-dominated distal
setting the reference square to proximal and distal
proximal
delta-front
in
bonates into the meteoric vadose zone with potential
'ideal' Yoredale
only limited significance, the main justification being
has
mal Jack of a diagenetic cap over limestones within