Page 300 - Carbonate Platforms Facies, Sequences, and Evolution

is
in
the
port
(Fig.
small
(Ul,
gently
Vitoria
1985b).
(Bilbao
shallow
sidence
Hornijo
situation
Guernica
to
reef
the region
provenance
Carbonate
thickness,
considered
sedimentation
of the region.
sequence
zones
deposition.
to
uppermost
formation of the
(Rat,
was
steeply
U3
(Ramales
mounds
sedimentary
the low-lying, fluvial
intra-platform
sedimentary
platforms
1959)
occurred
belong
dominated
(Fig.
Incipient
(Ramales,
shallow trough was
inclined
trapped
to
typified
in
the Sili6-Soba, Vitoria and
grew
sector).
basins
33)
(e.g. facies number 9 in Fig. 6).
sectors,
sedimentary
on
open
that
between
those
The
coastal areas.
(Santander
platform
so that another low subsiding zone,
another
Aptian-lowermost
Gorbea,
Fig.
area.
clinoforms
of little thickness and rudistid
sector,
relative
area,
Synclinorium
A
terials as the shoreline fluctuated with time.
32),
platform
dominated by
transition
transition
Aralar
margins
low
Fig.
(Garda-Rodrigo
gentle slopes, with few, if any, signs of mass-trans­
marine realms (Arija-Nograro sector) was charac­
sub­
High-and low­
energy tidal deposits alternated with the fluvial ma­
probably cor-
have been the main connection through to the open­
sub-sequence
characterized the platform margin, just prior to the
while
(10°-40°)
and NW-SE orientations. The Sili6-Soba trough
areas
between platforms and basins were characterized by
areas
Bilbao was probably determined by the intersection
of several deep-seated, normal faults, with NE-SW
(Fig. 32). The confluence of the three in the area of
In some places like
marly
32)
(Garcia-Monctejar,
thick mud mounds which typified
with
troughs
during the sedimentation of the U2 sequence were
The main geomorphologic features of the seafloor
areas
limestone
at the beginning of the U2 time. Nevertheless, soon
Fernandez-Alvarez, 1973). The Synclinorium trough
of deltaic sandstones of western provenance, which
suggest the presence of a shallow intra-platform basin
afterwards shallow-water limestones, very reduced
where much of the siliciclastic material of southern
terized by the important development of marshes in
time (U/ time); later on it received a thick succession
&
ing Bay of Biscay trough, situated to the northwest
and
was filled with marls during the early Upper Aptian
32); it represented a back-reef area (lagoon)
predominance is found in the northeastern outcrops,
Fig. 32)
was predominantly filled with marls. It is thought to
Albian). The
to the northwest
similar
Marls with ammonites in the northwestern part of
of
to
land
(Figs
the
for the
general
occurred
Sebastian
22,
mudstones
correspond
erosion,
basin
26
in
of Voort
(Gu
the late Aptian.
&
Aptian-Albian carbonate episode, Spain
the
and
terrigenous
continental-marine
afterwards
27,
(1964)
transgression,
SS,
(probably
to
and
areas
karstification
La
cliff in the middle of the picture is
Fig.
the
of
was
materials,
measured in the Rio Miera area.
thickness of the u2 sequence, =
32),
environment.
and
of the sequence U3 (Lower-early
punctuated
respectively).
north
are
by
Demanda
The
Guernica
unconformity. From Garcfa-Mondejar (1985).
a
Palaeozoic
of the
and
Fig. 33. Platform-basin transition in the Ramales
Coastal
characterized
and
inferred
phase
by
lower unit of prograding limestones (U2) separated from
basinal sediments of the Soba trough. The section of the
enance. Close to San Sebastian they contain carbon­
to dispersal systems of northern prov­
the Hornijo area (Rat, 1959) face to the right (south) the
present
emergent
aceous deposits and can be considered of transitional
Ebro,
uplifted
outcrops, might thus have been present as early as
to
an upper tabular unit with mud mounds (U3) by an angular
= 250 m thick. It shows a
sedimentary sector. Limestones of the 'Ramales barrier' in
deduced for that area. Fine-grained sandstones and
San
responding to an individualized basement block, is
sedimentation and a relatively wide surface subjected
Middle Albian)
was characterized by a rather reduced area of marine
dolomitization
279
a
onlap
The palaeogeography of the basin at the beginning
Fig. 32) to give an apron of fluvial sediments. In the
blocks must have existed to the south and southeast
Apart from this hypothetical northern source area
west the Asturian Palaeozoic Massif continued to be
1500 m, has been
Miera (Fig. 6), derived from the west. The maximum
emergent, as testified by the deltaic deposits of Rio
of