Page 194 - Carbonate Platforms Facies, Sequences, and Evolution

180
Age
lime
with
base
in the
of the
which
clastic
facies.
sands,
region.
drapes
cements
the
(Fig. 9b).
channels.
carbonate
the
(Ramalho,
Ota
tidal
Leeward,
of the
grade
development
top
Tidal
discontinuous
and
mudstones
of
unit
flat
Oxfordian in age.
part
buildup
a
buildup
tidal
grainstones,
fault-bounded block
is
of
back-reef
of
limestones
(Fig.
flat
the
reefal
Ota limestones,
partly
and
8)
clearly
poorly-sorted
limestones
Description and interpretation
sands
(Manuppella
zone
and
Cabac;:os
are
was
&
Abadia
and wackestones.
also
such
lagoonal
lagoonal,
exhibiting
shown in
crossed
Balac6
an
(association
spur-and-groove
Fig.
formation
by
formation.
7)
to
occurs as
characterized
10.
P.M. Ellis, R. C. L.
and Campbellina striata, together with the foramin­
differentiated into bimodally-sorted sand flat intra­
The association of the dasyclads Clypeina jurassica,
Oxfordian. A 250m deep uncored water well drilled
buildup may be Lower Kimmeridgian or even Upper
marine sedimentation did not occur prior to the late
1984), so it is possible that the unexposed base of the
Ota limestone apparently did not reach the
Moreira,
A
narrow high-energy reefal barrier zone occurs on its
Facies zonation indicative of an aggradational shelf
northeast of Monte Redondo (Fig. 8) indicates that
the
internal fill to palaeokarst within it, it is suggested
an
The
that the buildup was deposited contemporaneously
ifera Labyrinth,ina mirabalis and Alveosepta jaccardi
uplifted
1971). As the Amaral formation partly
is typical of the Upper Kimmeridgian of Portugal
interfingering of these sediment types suggests the
and intraclastic-bioclastic grainstones (Fig. 9a). The
grapestone
shallowing-upwards sequences, mostly composed of
an upward increasing number of irregular birdseyes,
The reefal belt
algal-stabilized debris, together with algal bindstones
exhibits a high proportion of coral framestones and
system
low-energy
are
western margin, behind which are situated back-reef
beachrock
The lagoonal sediments are chiefly composed of
limestones (association 9). The presence of oncolitic
channel lag deposits indicates the development of a
into thin sheets of laminoid fenestral
tidal
bioturbated lime mudstones and wackestones with
by
8)
to
day
narrow
despite
with
particles
presents
have
fault
Wilson and R. R.
the
on
local
local
due
been
Leinfelder (1987).
the
to
restricted lime mud
Leinfelder
(Leinfelder & Wilson,
active
Enos & Perkins, 1979).
major part
enrichment
western
of
of
1989).
a
during
intraformational
winnowing.
limit
I
despite its high productivity (Fig.
of
.
--
�
�
�
the
skeletal
late Kimmeridgian subaerial exposure
Local
the
Not
Lime
Fault
subaerial
or
reefal
• Amaral
L.:.:.J Tertiary
� Cabacos
I Lourinha
� Fenestral
limestones
� Coral -rich
� limestones
PM lntraclastic
r7l Cretaceous
Ota Limestone
LiJ grainstones
Fm.
Fig. 8. Geological map of the area around Ota. From
differentiated
c:
�
and
wackestones � .
�·
�
X
o"'
3
::>
� �-
;:J"'
-co
mudstones& 9:""0
3
A prominent black pebble horizon related to intra­
0
0 0:
:""
uplifted basement high and apparently re­
�
of this palaeoescarpment coincided with the present­
may be explained by the existence of a bypass escarp­
linear mud banks as in the modern Florida Bay ( cf.
(Leinfelder,
1987; see Fig. 9) allows accurate facies correlation.
horizons and early diagenetic freshwater influence
-wackestone facies (association
inorganic
mud-crack
(at least throughout the exposed part of the unit),
ment margin, preventing progradational reef growth
may indicate the development of shallow, stabilized
10). The position
reefal zone
(Fig. 8). On seismic sections, this fault can be seen
Kimmeridgian
exposure
It is clear that the facies pattern did not shift laterally
The Ota Limestone is probably located on a very
and possible eustatic and tectonic oscillations. This
fairly small, isolated