Page 60 - Carbonate Platforms Facies, Sequences, and Evolution

52
low
Dale
Where
energy
energy
processes:
emergence
(Figs
the
in sea-level.
energy,
4
such
(storm?)
Hope Cement
&
a
b
conditions
as
sw
sw
9a)
20-25m ?1
a
(deeper
over
Works
event;
lower sequence
the
where
(2)
water?)
Quarry
the
Controls on shoal complex evolution
paleokarst or
bedform
20m
main
R. L.
setting;
20m
episodic relative sea-level rises of at least
development
shelf
calcrete.
of
and
followed
-
bedform
20-25
initially rapid relative transgression followed by still­
faces of the shoal sequences suggest that abandon­
(1) erosional truncation, where the top­
took place during deposition of the shoal complex.
Features associated with the upper bounding sur­
ography of the bedform was planed-off by a high
low
This
implies that the bedforms developed during a relative
boundary of the shoal
sequences is exposed, it is marked by evidence of
sea-level rise which flooded an emergent platform.
m
ment of these bedforms occurred as a result of three
The bedform dynamics discussed above suggest an
stand. The dimensions of the bedforms suggest that
by
(3)
emergence, due to shoal up-building or relative fall
is
onlapped by microfacies 4 which was deposited in a
onlap and burial of the bedform. This is seen at Pin
provides evidence
interior
is
of
the
Gawthorpe and P.
shoal
50m.
offset
which
The
the
shelf
margin
unclear
varying
in
wedge
in
-
a
immediately
relationships
shoal
and bedform
from
margin
owing
the
Gutteridge
Younger
Derbyshire
sequences
to
could have
and leeward types
shaped,
basinward-stacking
predominant
form
parallel
shoal
several
of
complex
currents. Clinoforms have an oblique, concave-up geometry with asymptotic bottomsets.
basinward
poor
Stage
of
underlying
along depositional strike.
is
(Hine
nucleate
to
a
Stage .2:
1:
carbonate
shoal
thinning
thus
basinward
hundred
'reef'
sequences
direction
exposure.
Sea
away
progradation is seen
one
or
sequence.
platform
are
Accretion
in
close to the
et al., 1981).
of
depositional
metres
level
Sea level
Progradation
sequences.
from
These
sediment
The
NE
NE
relative
to
would
the
afforded protection of the
emergent
to
vertical accretion of the bedform with the result that it progrades basinward, under the influence of storm and tidal
strike
modern
shoal
be
shoal sequences (Fig. 6) form a total thickness of
se­
car­
=
Stage 1 predominantly vertical accretion of bedform associated with relative rise of sea-level. Clinoforms are sigmoidal and
Fig. 12. Shoal sequence evolution based on examples from Pin Dale, Hope Cement Works Quarry and Bradwell Dale. (a)
convex-up, with downlap onto lower sequence boundary which is often a paleokarst. (b) Stage 2 stillstand of sea-level limits
younger
margin towards the shelf interior. They form a belt
the
are
progressive
bedform in the underlying sequence. The architecture
progressively
of the architecture of the shoal complex. The three
transport
margin. A leeward setting of the northern margin of
quences coalesce to produce a shoal complex which
There is no
The facies
evidence of a persistent barrier parallel to the shelf
kilometres in width and several kilometres in length
bonate platform margins of the windward-protected
shelf
several
crest of the
in
area
shelf