Page 58 - Carbonate Platforms Facies, Sequences, and Evolution

50
and
cant
clasts
blage
stems
facies
calcite
energy
absent.
change
coating
grading
together
generally
are
carbonate
is
algae.
interpreted
wackestone
have
Description
pause
Interpretation
with
of the
cement
high
in
in
locally present.
mud
not
quency of rolling.
sub-parallel
the
representing
Peloids
pr�sent.
show some evidence
terms
been
similar
to
intraclasts
energy
the sand body complex
disarticulated
and
through
Ari
fully
to
of
presence
and
the
that
sedimentation.
of
�edding
grainstone-packstone (Fig. llc)
indicates
sediment from sheltered
marine
in
micritized.
conditions.
micritized
differing
to lower energy conditions.
isopachous
This
and
grain-supporting texture, high degree
sediment
areas
The
Microfacies 2: Bioclast intraclast oncolite
of sponge borings.
interpreted as a marine phreatic cement.
Some,
fragmented
oncolites
reworking
conditions.
normal
Allocherri§ are poorly to well-sorted.
microfacies
of
allochems
adjacent
difference
1
following
Bio­
oncolite nuclei suggests a high fre­
However, the asymmetric thick­
This microfacies contains -a diverse allochem assem­
of disarticu­
local packstone texture. The allochem assemblage
Minor constituents include bryozoans, foraminifera
(Table 1) is dominated by brachiopods and crinoids.
are
and bioclast wackestone intraclasts are also present.
Oncolites have a bioclast nw;leus with a complete
coating which shows asymmetric thickening. There
some whole brachiopod valves are present. Bioclasts
Oncolites
although
microfacies 1 is that the allochems within this micro­
radial-fibrous
is
is local preferential alignment of shells and crinoid
size
have been bored by sponges which suggests a signifi­
indicate
complete
This microfacies is a grainstone with development of
The bioclastic
a
occurrence of packstone is attributed to infiltration
low
conditions when the oncolites were stationary. The
to
The
ening of the coating indicates alternating low energy
sedimentation
lation, fragmentation and rounding of the allochems,
rates. In contrast to microfacies 1, microfacies 2 was
. . A significant contrast with
is
in microfacies 1 the radial-fibrous calcite cement is
however,
Bedload transport and winnowing are suggested by
allow sufficient time for micritization to take place.
the orientation of bioclasts and stacking of shells. As
subject to a higher sedimentation rate which did not
by
less
R. L. Gawthorpe and P.
local
stage
blage
blage
ment.
tation
implies
nucleus
of
micritized
packstone
sponge
mentation
micritized,
Description
Description
and
that
(Table
peloids.
Interpretation
abundant
and
and
Gittteridge
the
environment, or
rates
However,
borings.
to
bioclasts, is present.
incomplete
some
to microfacies
although
wackestone (Fig. lla)
the
In
1
were
allochems
crinoids,
Allochems
coatings
and
partial
some
wackestone.
display
with
some
variable
increasing
have
reworking are present and the
show
The
This microfacies is transitional in
generally
have
2 and
size
an
bryozoans,
sponge
making it more difficult to be moved.
accretion
degree
differing
specimens
is
been
high,
of
early
Microfacies 3: Bioclast oncolite packstone­
a
allochem
Microfacies 4: Bioclast wackestone (Fig. lld)
borings.
represents
stage
shown
histories
some oncolites. The coating of some oncolites con­
clasts are disarticulated and fragmented with a vari­
assem­
texture, picked out by a patchy distribution of car­
Bio­
foraminifera,
1) is dominated by brachiopods with
This microfacies contains a similar allochem assem­
texture between
of bioclasts suggests a high-energy environ­
marine conditions. The disarticulation and fragmen­
stone texture suggests final deposition in a moderate­
packstone-wacke­
algae, bioclast wackestone intraclasts, oncolites and
generally not been
able degree of rounding. Oncolites have a bioclast
and micritized and bm .'!d grains indicates that sedi­
complete accretion of uniform thickness followed by
tains several horizons which have been penetrated
in
an asymmetric thickening of the coating. A further
partially
of
bonate mud and a preferential arcuate alignment of
mottled
of the oncolite
rounding
of
This wackestone has an allochem assemblage (Table
to low-energy environment. Bioturbation is indicated
although
arcuate alignment of bioclasts. The paucity of peloids
by mottling of the sediment and the occurrence of an
fully
coatings from symmetrical to asymmetrical can be
explained either as a decrease in the energy of the
episodic, as indicated by bored layers within oncolitic
coatings. The change in the nature of the oncolitic