Page 59 - Carbonate Platforms Facies, Sequences, and Evolution

with
Dale
of the
Figure
suggest
worked
bioclast
bioclasts
majority
1) which
9
that these
of bioclasts.
components;
Interpretation
a�d
'
bedform.
shows
differs
Bioclasts show a
and micritization
the environfuent.
brachiopods
sediment
deposition
Bedform dynamics
wackestone
This
that
was
in
and
from
buted to bioturbation.
EVOLUTION
a
from
the
fragmentary
that
OF
crinoids
intraclasts
COMPLEX
alternation
arcuate preferential alignment of
bedform surface to occur.
generally
THE
into the sediment of the bedform.
is
are
bioclasts
clinoforms
surrounding
low
retarded allowing
also
During
with
Dale compr�se alternations of microfacies
are disarticulated and fragmented
as relatively
within
SHOAL
areas.
energy
of other microfacies
interpreted
with
1 and
en­
bioclasts is attri­
the depo­
occurrence of intraclasts also indicates input of re­
interpreted as the establishment of low energy con­
vironment. However, the presence of highly disar­
in
suggest unusual salinity or a low oxygen content of
The wackestone texture and the
common occurrence of sediment mottling and the
with the local occurrence of microfacies 3 in the Pin
Sorted bioclast peloid grainstone and
present.
occurrence of whole bioclasts with minimal abrasion
transported into the lower energy environment. The
in bedform migration, rates of sediment burial were
rria'y have been reworked elsewhere and
in
ticulated, fra,gmented and rounded bioclasts suggests
low allowing micritization of grains and cementation
a
The
range of preservation states ; the
fracture surfaces are also present. This microfacies
commonly displays a mottled texture which is picked
the developm�nt of an arcuate preferential alignment
out by a pat�hy distribution of carbonate mud and
shoal
sequence 3 at Hope Cement Works Quarry and Pin
high degree of rounding. However, whole unabraded
that it is dominated by molluscs and calcified spicules
angular
minor
The dominance of molluscs and calcitized spicules
Microfacies 4 is
bedform
of microfaci�s 3, particularly in bottomset beds, is
ditions allowing deposition and infiltration of micrite
migration' rates of sediment burial were much higher
sition of 'fresh' bioclastic sediment. The occurrence
terms of episodic bedform migration. During pauses
2
up
by
2 in
At
been
shoal
along
partly
dation
during
with
section.
generally
sediment
upbuilding
Platform-margin bioclastic shoals,
strike
of
which
sedimentation
500-700 m
eroded.
resulted
to
is
important
Bradwell
UK
Quarry, Fig. 9b).
development ' the
in
the shoal complex.
rate
Hope Cement
restricted
transport.
the
storm -tide influence.
along
to
Dale,
spill
was
bedform prograded basin
inferred
west
This,
the
Works
bedform
basinwards.
to
over
development of the bedforms.
At
shelf
the
Re-establishment
shelfward
suggests
during
Quarry.
together
Pin
of
the topography of the
areas adjacent
a
shelf
represent
and
that
margin.
with
the
complex as a whole which is discussed in
relative sea-level rise. Following
'catching-up' with
Dale
the
phase
to the
the
stratification indicates both shelfward and basin
bedform
break,
Thus
progra­
This is shown sche­
dipping clinoforms and bimodal smaller-scale cross­
or downlap of clinoforms onto the reactivation sur­
dominant ebb tidal current. The predominant basin­
shoal sand bodies, and to the sequences overlying
matically in Fig. 12, which is based on shoal sequence
main
At all localities there is evidence of a tWo-stage
This two-stage
bedform
after the development of these truncation surfaces is
basinward
which truncate underlying clinoforms are interpreted
as reactivation surfaces formed by high energy events
was
marked asymmetry of the shoal complex suggests a
ward transport of sediment across the shelf margin
prevailing
ward
large
scale of the bedform may suggest a tidal influence on
Within the clinoforms, low angle erosion surfaces
reflects a,limitation of vertical growth, possibly by
sea-level stillstand, or that sedimentation had caught
this the
sea-level rise. The onset of basinward progradation
initial
when
relative
bedform represented by shoal-sequence 2 prograded
and
supply of sediment to the basinal area to the north.
accretion. At the Hope Cement Works Quarry the
wards with minimal vertical
and abandonment of these bedforms, as illustrated
change. The mechanism of nucleation, development
as emergence or flooding due to relative sea-level
by the shoal sequences, has important consequences
for the development and architecture of the shoal
51
the next
by sediment production or by extrinsic factors such
the deposition of the shoal complex. However, the
face (e.g. shoal sequence 3 at Hope Cement Works
either by the conformable deposition of clinoforms,
offshelf wind or storm driven currents may also have
The extent of progradation may have been limited
complex probably developed under a mixed
the