Page 244 - Carbonate Platforms Facies, Sequences, and Evolution

the
irregular
(Fig. 25).
cemented
Orbitolina
surrounded
Chartreuse)
transgression
by
Transgressive
by
beds
in
transgressive
a
shallowing-upward
in
together
echinoids,
condensed sections).
sequences composed
one
the
in
facies
facies
or
a
coating
are
sequence
and
minifera and dasydad algae.
of
more
southern
the
iron
Palorbitolina,
of argillaceous
ferruginous
observed
the northern Vercors
characterized
oxides,
Vercors,
maximum
benthic
by
either
limestone. Thickness of the transgressive sequence: =
and
facies rich
The maximum trans­
trans­
di­
fora­
rectly on an emergence surface or at the top of a
gradual vertical deepening of the environment from
locally
gression stage generally corresponds to marls which,
in
especially on the outer-shelf, may include glauconitic
gression level is usually taken up (late Barremian
the
the
lower
beds that are locally rich in ammonites (likelihood of
shallowing-upward
hardground
one parasequence to the next. The interval between
this,
Two
whereas
pretation.
established
comparison
major
in
Barremian-early
The Jura and northern Subal pine chains
On the
the
EUSTATIC
COMPARISONS
other
observations
Aptian.
studied may be of eustatic
the variations in depth
WITH
sorted wackestone with large foraminifera ( Orbitolina, centre), algae and gastropods. (Scale bar :
hand,
The
between the sea-level
SEA-LEVEL
1 mm)
HAQ
region studied and
slight
since an analysis of subsidence curves,
very different for the early Barremian.
may be drawn
ET
the
variation
CURVE
AL.
from
sequence Ail (lower Orbitolina beds) interbedded between the lower and the upper member of the massive Urgonian
'S
top D of the sequence; (B) bimodal, medium-very coarse fossil fragments, rounded intraclasts and limonitized oncoids
with fine peloidal-foraminifera wackestone; (C) well-sorted limonitized peloidal grainstone from hardground; (D) poorly­
25 m. (B to D) Selected microfacies in thin section from base B to
timescale rather than any true differences in inter­
which still exist at these levels are due to the difficult­
Fig. 25. Transgressive sequence. (A) Outcrop photograph showing argillaceous limestones and marls of the transgressive
curve
eustatic
229
origin in the first case,
they would appear to arise from tectonic
recorded in the region
Because of
the region studied (Arnaud, 1988) but also farther
not only in
causes in second case. This last hypothesis is probable
(Fig. 26) are similar for the Hauterivian and the late
sea-level curve of Haq et al. (1987). The two curves
the
the two curves are
ies encountered in accurately locating events on a
differences