Page 210 - Carbonate Platforms Facies, Sequences, and Evolution

4
the
the
196
like
along
which
during
related
cap
Jurassic,
changes.
Late
the
to
the
comparable
to
experienced
but
during
Tectonic setting
A thin ( =60
southwestwards
the
their
in
the
Palaeogeography
whole
development
the
Jurassic
the
tectonic
of
the
highest
North Atlantic (see Fig.
the
tectonic
Upper
m) Upper
on to the
mented by salt withdrawal.
sequence in the Sintra area.
1).
4-5000 km
Ramalhal
wave
curring during the late Jurassic.
subsidence
lowest
This
newly
settings.
Jurassic
and silts of the Abadia Formation.
Lusitanian
area
length
of palaeogeography, tectonic setting
southward prograding
Oxfordian-Kimmeridgian
The
during
of
energy
faced
this
energies,
opened
Basin
are
the
as
the
was
P.M. Ellis, R. C. L.
in a siliciclastic starved part of the basin (Fig. 20).
strome and ooid grainstone developed as a sheet­
Kimmeridgian coral bio­
aug­
Triassic-Hettangian tectonic subsidence to that oc­
grainstone­
flexures or faults. For this reason, the eastern build­
shelf break prevented basinward progradation. The
southeast
that the southwest margin of the present-day basin,
sedimentation at Ramalhal was able to keep up with
(see Fig. 20) was the most sheltered, thus explaining
the difference in thickness of the two buildup types
subsidence histories of the two settings account for
Clearly, carbonate
dominated buildups to the northwest developed on
carbonate
Atlantic as it then existed. Thus it seems probable
ups, such as Montejunto and Ota, developed good
lateral facies zonation patterns, which were unable
effectively added
waves
Therefore the eastern
southern
and northern parts of the Basin were likely to have
the east were bounded on their basinward sides by
the flanks of a rising salt structure, whereas those on
opened
North
to migrate laterally because tectonically-controlled
approaching from the southwest would have had the
opportunity to develop over the longest fetch, indeed
clearly
the eastern and northwestern margins of the basin
As indicated at the beginning of the paper, during
Upper
(Fig. 22). It is probable that both regions experienced
The differences between the nature of buildups on
a subsidence rate of some 75 mm yr- 1 (at least 1500 m
of these buildups is now reviewed under the headings
The possible controls of the formation and demise
and sea-level
slope marls
is
a
in
the
ate
this
which
the
that
cerning
sections
mented
Wilson and R.R.
AN
in
throughout
Abadia
the
could
problem
Kimmeridgian
combination
during
controlled highs.
Sea-level changes
most
4.5 of Hag et al.
ages
is situated in the
slope
Lusitanian
sedimentation
of
have
must
carbonate
of
the
Portugal
of
Leinfelder
(Fig.
the
eustatic
Upper
ATLANTIC
latest
Basin.
system
await
parts
latest
means
to
obscured
continued
of
An
breccias
Oxfordian timespan of 2Myr).
basin
and
more
that
on
the
and
Oxfordian,
was
effects
Oxfordian
prograde
salt-
Oxfordian
was
The eustatic sea-level curve of Hag et al.
caution
tectonic
additional
Upper
detailed
and
and
and
PERSPECTIVE
eustatic sea-level changes (Wilson et at. , 1989).
must
produced
caused
plotted on the summary diagram of buildup devel­
Kimmeridgian, parts of the Lusitanian Basin experi­
opment presented in Fig. 4. Lack of precision con­
fill. This highstand may correlate with cycles LZA-
of the Abadia formation, but shallow-water carbon­
biostra­
drowning of buildups and the appearance of resedi­
Throughout much of the Basin, this event was fol­
conclusions made in Portugal. Despite the very dif­
examine the nature of the carbonate buildups com­
fault­
exercised in correlating the Hag curve and its associ­
(1987) is
problem
effects.
early
of compacted carbonate accumulated during the late
Jurassic
by
and so would
at the base of the Tojeira member, the base of which
earliest
correlate better with cycle LZA-4.4. Resolution of
tigraphical studies. It is tempting to suggest that the
ferent sizes of the two regions, far more is known
synchronous
ated sequence boundaries with depositional events
boundary on seismic sections (Fig. 14) appears to be
during the late Jurassic in Portugal was the 700 m
enced extremely rapid rates of tectonic subsidence
rise followed by a relative stillstand that permitted
by
lowed by the southwards progradation of siliciclastics
The most significant change of relative sea-level
prising the 'Mesozoic reef trend' beneath the Atlantic
The purpose of the final section of the paper is to
continental shelf of North America in the light of
4), yet the lower sequence
be
southwards
and the Mem Martins formation to form as a passive