Page 148 - Carbonate Platforms Facies, Sequences, and Evolution

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136
The
that
with
which
molds
crease
end of
ground
tinuous
the
late
the
along
of
During
in
transition
eastward.
conditions
sub-basins,
subsequent
the
the
which
slope.
basin
impeded
Oxfordian event
rifting
from
must
history
has
of
This
the
Post rifting dynamics
carbonate,
second
have
Ardeche
shallowing.
climax
been
the
enlarged
middle
ammonites and
a regional eustatic event.
part
and
The
drop in organic productivity.
was
to
general
of
evolution
North
bioclasts,
persisted
towards
is
late
the
nodules.
Sea
the
into
Cevennes
early
benthic
circulation
followed by
no discontinuity is noticeable.
beds thicken
the
recognized for >
Basin
west
a
Jurassic
Revival of the slope: the late Callovian-early
of
consistent
margin
at
and
from the early rifting was replaced by a more con­
Figs 5 & 6). This facies suite (S7c) ends with a hard­
of fauna may have resulted from limited communi­
a shallowing sequence with an upward in­
It seems
(Gracilis Zone), 'the Callovian Triad' accumulated
200 km
Its base overlies a sharp
commonly encrusted but the origin of the coating is
Such protected
Callovian
(see
disconformity on the ridges but, in the pre-existing
bivalves,
glauconite and iron oxides. The final hardground is
change in the dynamic processes; it can be regarded
Major tectonic and sea-level events took place at the
times
as a consequence of the spreading occurring at this
with
the western margin, the structural mosaic inherited
time in the Piemont Zone (Lemoine, 1985). Along
deep
well as foraminifera became very scarce. The scarcity
Callovian (La Gette Formation), but ammonites as
no upper Callovian beds preserved along the outcrop
Ardeche, because of recent denudation. There are
global
by a narrow western shelf which is not preserved in
cation with the more open sea and then a subsequent
of
McKenzie's model (1978). It can be easily compared
strongly
waters. Bottom waters became restricted and local
the
northwest Germany (Pratsch, 1982; Morton, 1987).
not clear. The S7c sequence appears to have recorded
(sequence S8). The margin was probably bordered
beginning of the sequence and that it underwent a
anoxic conditions are evidenced by pyritic internal
along the western borders of the basin. It reflects the
earliest
S.
Elmi
by
for
and
and
data
A
Italy
main
1928;
along
rifting
during
Voulte
study).
stones,
the
(Fig. 21,
southern
microbial
the
area
bigerinids,
Superiore'
seem
the
transitional
Massari
Elmi,
and
rich
depositional
(Elmi, 1980;
similar
as well
stromatolites
to
of
marly
tectonic
Alps
'Terres Noires'
in
as
Jura Mountains;
early
fabrics
1967;
stromatolites
the
slope.
and
hexactinellids,
benthic
in
Ardeche
(1979,
depths
study
beds,
consistent
Rompon
Basin
and
where
Bourseau
the
structure
sponge
have
activity.
Shelf
to
Trento
was
these
with
of >
1981).
Almeras
In
border.
mud
Oxfordian.
the
oncolites.
been
the
covered by nodular limestones
localities
Sturani
the
& Elmi,
Balanocrinus
foraminifera,
deposits
They
The
prevailed
Plateau
Portuguese
features
probably
Similar
mounds
& Elmi,
1981;
are
(Sayn
in
is
may
200-300 m
spicule-bearing
thick marly sedimentation in the basins.
(1971)
basinal
'Pelagic'
stalked
&
lithistid
extensively
during
'Terres
not
the
'Ammonitico
1984).
have
consequences
horizons
areas
around
(Dromart
of
IV and TV) remained active during post­
On this outer shelf, bedded nodular lime­
rupted by sedimentary gaps, small faults with metre­
de­
top of the Lower or Middle Callovian beds, inter­
(protoglo­
Noires'
(La Voulte, Saint-Bres), where the old escarpments
position of black shales ('Terres Noires') took place
middle
Oxfordian times as sedimentation resumed on the
known
general sea-level fall: sediments reduced or missing
thin-shelled bivalves, radiolarians; Dromart, 1986).
crinoids,
been
('grumeleux' facies
of the lower lumpy series, LLS, Fig. 2) containing
&
see Gaillard, 1983 for a detailed
intraclasts
Birmensdorf'; Enay, 1966). Transitional slopes were
Black
shales with thin evenly-bedded micritic ferruginous
levels ('lithoid' iron ore, Athleta Zone) overlie the
overlie the middle Callovian condensed beds. The
La
Roman,
scale offsets and block-sliding. The transition to the
Dromart &
and the collapse of calcareous beds (La Voulte area)
a
Elmi, 1986; Elmi et al., 1984; Elmi, 1985c). These
controlled by
basement faults; evidence is only given by the sliding
known in late Jurassic beds of southeast France and
are
studied
and Moroccan
strip with the exception of the more external zones
sponges
Pliensbachian. The faunal data are consistent with
similar to those developed on the Jura shelf (French
accumulated around sponge biotherms ('Couches de
on the slope, widespread erosion on the seafloor at
the top of the Middle Callovian reduced beds and
Elmi, 1986). The slope facies are very similar, except
described
oncolites
Rosso
Italian