Page 21 - Carbonate Platforms Facies, Sequences, and Evolution

(2)
the
flat
the
ments
The
The
lack
of
maintains
structures
Limestone
Limestone
sea waters
system
Throughout
of
western and
provided by:
a
the platform.
slump
are
also
(Fig.
with
its
development
Incipient platform
with later tectonics.
12
depositional
of
platform
eastern
occur
1).
now-vanished
structures
terrigenous
(1) lack of
non-cyclic
Laminated Dolomite
growth
indicative
Ceroide platform evolution
Ceroide Limestone platform
asymmetric
of
margins
along
the
characters
(Cocozza,
calcareous
of
an
material
of
occurring
evaporites
the
As a result
deposited
the
in
margins
1969)
land-derived
facies
in
the
in
and
isolated
a
platform
the
the
(Fig.
and
western
12
towards
and/or slow sea-level fall under an arid climate.
platforms
reported by them from the western stromatolitic -
sabkha system flanked to the w'est and east by open­
are here interpreted as the result of plastic move­
and algal (Epiphyton) buildups. Moreover, the oc­
and sediment thickness in the platform interior indi­
consisting of a wide, internal shallow-lagoon/tidal­
edge
Laminated Dolomite, actually occur 20-50 m below
region and referred by Bechstadt et al. (1985) to the
(Balassone et al., 1985). Slump structures have also
siliciclastics;
been reported from the Laminated Dolomite in the
Bechstadt et al. (1985). The deformation structures
Fig. 6a and similar to that of Fig. 6c and, therefore,
the Ceroide Limestone in the ramp-like margin of
connected
suggestive of a steeper margin there than in the east.
eastern
either the laminated or massive mudstone facies of
not exposed. However, evidence of their existence is
Ceroide
The vertical and lateral distribution of the facies and
of the present
similar
internal-lagoon/tidal-flat
The first isolated platform stage is represented by
cate regular, moderate subsidence of the platform
tidal-flat
structural setting, the supposed basin sediments are
currence of normal marine organisms to the west is
Laminated
interior
western and eastern edges of the platform; and (3)
the presence on the western edge of archaeocyathids
evaporitic facies are associated with that shown in
by
the contact with the Cabitza �imestone and affect
II).
the
Dolomite and the rather uniform cyclic deposition
platform
Its
the
time
forms
1987).
The
viously
period,
climate.
Sardinia
exposed
thickness
Within
(Rozanov
was 5 Ma.
(Fig.
deposition.
growth
and
for
bedding
(D'Argenio,
&
the
12
western
of
(Nappe
the
included
Platform exposure
1970;
appears
III).
the
gradually
in
Carbonate deposition during early rifting, Italy
15 Ma (Harland et al.,
to
Sokolov,
margin
platform
structural
balanced by carbonate
Zone)
deposits
Owing
the
was
have
Ceroide
Bernoulli,
1984),
decreases
to
of
the Lenian
reasons,
and prevented lateral outgrowth.
built
been
the
production
This
siliciclastic
amount of fine-grained siliciClastics.
the
by
complex
toward
whose
Limestone
modern
stage
under
the
basin.
bedded marine lime mudstones containing
1972; Wilson,
bedded, open-lagoon facies (Buggerru atea).
organic
controlled
production kept up with irregular subsidence.
internal
was
differential
In
thickness
duration
by
indicate
the great thickness variability imply differential sub­
re­
stra­
pre­
the values reported from the Tethyan Triassic plat­
sub­
This behaviour suggests that carbonate
19
time interval is assumed here to have lasted 5 Ma).
The eastern margin, wider than the western one,
The limestone
(algae and minor archaeocyathids) and oolitic shoals.
clear trend towards a deeper basin (Gandin et al.,
rapid relative sea-level rise that favoured up-building
reefs
a
from Upper Botomian to Lower-Middle Toyonian
sidence of the different areas. Rate of sedimentation
cart be estimated to be in the order of 30-160 mm
eastern
this
area too, although the transition to the basin is not
Cambrian in the Russian subdivision of Cambrian
tigraphy, the non-cyclic recurrence of the facies and
a
(Toyonian is the upper stage of Lower
duction, the occurrence of siliciclastic material and
is
shoal was flanked by a deeper basin, whose existence
can be inferred by the occurrence of a narrow belt of
The high rates of sedimentation compare well with
platforms
To the west the
1975).
had a ramp-like profile. It was mainly constructed by
a small
10- 3 yr assuming that the time interval bf deposition
according to Rozanov & Sokolov (1984) is a short
1982). The Toyonian, which
The deposition of Ceroide Limestone ended when
The depositional behavior of the Ceroide platform
not all the platform sections were exposed
sidence occurring during the previous depositional
most of the platform emerged in late early Cambrian
a humid
deposits reflects an irregular and rapid subsidence