Page 147 - Carbonate Platforms Facies, Sequences, and Evolution

and
the
rare
The
beds
1980;
1911;
An
rapid
event
again;
where
on the
general
Ardeche.
literature
are
carbonate
associated
winnowed
shorelines.
the
and
deposition
have
pelmicrites
during
depocentres
Upper
gap
change
marls
Purser,
Contini,
on
than
remaining
the
margin
to
with
the
are
with
important
away.
been
slopes
and
the
the
1975;
platforms
Shallower conditions
other
highs
1987).
major
following
coeval
were
in
Some
in agreement
marly
may
the
transition
part
ammonite speciation which
regression
widespread
ammonites
platform
Contini
western
with
preserved.
events,
with
be
Aalenian-Lower
of
washed
was
1987; see also Haq et al., 1987).
from
sedimentation in the depocentre.
and
and
the
lenticular
northwest
the
In Ardeche,
gaps,
the
border
by
limestones
in Enay
occurred
although the eastward
early
strong
beds
regression
Emersion
&
Bajocian
progradation
was
sponge
general
responsible
to
Europe
is
of
by currents
active
Variabilis
basin
prevailed in the more
during
sediments
along
during
second half of the Toarcian (Elmi et al., 1986).
for
particularly
Europe and on both rims of West Tethys: disconti­
depo­
model as the fall of sea-level was followed by non­
Aubenas Sub-basins the sediments are made of bio­
with shallowing of the basins while deepening of the
the
coarse
(lower part of the Upper Calcareous Series). Only
it resulted in a
sandy, bioclastic limestones ending the S5 sequence
Mangold,
documented and erosion was active along the ridges.
shallowing seems to have been regional (in northwest
centre was filled with marly hemipelagic limestones.
nuity no. 4 in Gabilly et al., 1985; Elmi & Benshili,
in
The regression is
Zone
second part of the middle Toarcian (Variabilis Zone)
the
middle
Aalenian. Although less frequently reported in the
spicules.
It led to a noteworthy differentiation in
the
The middle Aalenian eustatic regressive event is
well documented and it led to the growth of broad
(Haug,
stable
The uplift was short lived and the margin subsided
lenticular
which
supply of a certain amount of iron which precipitated
along the shore. All these features can be correlated
carried the fine sediments to the eastern depocentre
zones (Les Vans area), leading to accumulation and
the
preserved in small protected areas. In the Privas and
eustatic
(Elmi, 1967). Data concerning the middle Aalenian
marly
The Bajocian deepening (during the S6 sequence)
did not result in a resumption of the sedimentation
locally
was
have been
accumulated
probably
progradation of crinoidal sands. The winnowing of
&
on
on
a
north
shelves
sinking
lOf).
the
occurred
remained
bordering
and
steep
Mountains,
Bathonian).
Late Triassic and Jurassic platf orms,
hemipelagic
its accretion.
the
along
During
steep,
platform
building
marls
France
the
a
margin.
ated with ammonites.
northeast
such
Burgundy).
and
crossed
The
down-dropped
as
middle
In
by
Late movements of tilt-blocks
gentle slopes
complex
France
Saint-Ambroix
the
a
and
the
blocks;
limestones
carbonate
Basin
late
(Paris
more stable continental crust, general
bottom
Courry
basins
are
network
they
(Fig.
Basin,
narrow strips along the shores and the ridges.
platform
Bathonian,
tectonic sub­
variations and by resedimentation processes (gravity­
(Rosieres, Escrinet­
Formation, Fig. 5) or by alternating marls and lime­
centimetre-sized quartz pebbles and feldspars associ­
135
From the middle Bathonian to the beginning of the
slides and slumps which are, however, better known
The slopes are also characterized by
was
sidence (Brunet & Le Pichon, 1980) was overcome
to a transgression and to a deepening which occurred
by the major sea-level changes which controlled the
relatively high-energy environment which prevented
reduced to a narrow strip with steep slopes and a
bordering
Jura
spread over large regions instead of being limited to
Tethys, tectonics and eustacy operated together. On
in slightly younger deposits (late Bajocian and early
palaeogeographical evolution, and shelf conditions
In contrast, wide carbonate platforms
faults. This documents some late tilting movements
Privas) covered by marly limestones (Argevillieres
a
reworked quartz pebbles, slides and slumps; Figs 6
developed at this time in the less mobile platform of
(Elmi, 1983b, 1984). Transition to the basin normally
10f and
along the eastern down-dropped segments of some
thickness occurred. They were complicated by facies
Callovian (Macrocephalus Zone), rapid changes in
large shelf developed in the Les Vans area as a result
stones (Saint-Priest Formation, Fig. 5). Some slopes
preserved
was missing across large areas of the ridge but some
of the widening of the Pa·iolive ridge. Sedimentation
Escarpment
Redon Formation of the north Aubenas Sub-basin
(Ehni, 1985a). The environment may have been a
did not keep pace with the subsidence. Small gulleys
or umbilics appeared where a nektonic fauna seems
to have been trapped, as suggested by the marly Plat
contain
Fig. 21, IV). Thin lenticular micrites were deposited
morphology
of narrow
In the more subsiding areas marly sedimentation