Page 171 - Carbonate Platforms Facies, Sequences, and Evolution

for
dent
time
least
Two
typical
million
Myr- 1 .
in
on
onlapped
drowning
(Schlager,
prolonged
of
ment rates
the
the
of
The
years
possible
by
subaerial
have inferred
another
period
time
DROWNING
these
the basins to form.
using
lOOm
above rate estimates:
of
resulting
of complete drowning.
ing the basins to form.
OF
exposure
subaerial
the
sediments.
(1)
shallow-water
right is about 10-12 m wide.
conclusions
Pliensbachian
of
spanned
fault
time
THE
seamounts
by
incipient
This
(no
the
downward.
exposure
can be
bilities are not mutually exclusive.
the
was
scale
that was produced on the seamount top.
carbonate
gives
more
of
This
a
1981). (It should be stressed
Calcare
drowning
displacement
than
Haq
preceded
4
means
that
SEAMOUNTS
rate
or
by
beneath the basinal areas, which suggests that en­
between the very end of the Sinemurian and some­
the actual fault displace­
halted the production of carbonate sediment, allow­
Of course, these two possi­
rates are only estimates, and they are strongly depen­
ex­
tended through most of the Jurassic for many of the
were much greater than the estimated
1987), a 600 m escarpment was formed, eroded and
5
tops in the middle Jurassic, but there is no evidence
minimum; or (2) environmental factors or subaerial
Colacicchi et al.
that these
Pliensbachian.)
environments
top and basinal sediments indicates that the complete
The sedimentary record contained in the seamount
escarpment growth rate of between 120 and 150 m
a
is
signs of diminishing, so it is estimated they extend at
of the seamount
minimum
drawn from the
vironmental factors or rapid displacements enabled
evidence for the stage of incipient drowning is first
exposure from a sea-level drop greatly reduced or
et al.,
seamounts. The sedimentological and stratigraphical
that
Massiccio
the 1000 m Myr- 1 sediment production rate that is
between 178 and 208 m Myr- 1 , significantly less than
discussed and then the evidence regarding the timing
down from the former seamount top, and show no
(1970), Cecca et al. (1981) and Farinacci et al. (1981)
the
One
state
rapid
1978)
piece
where
water,
posited
detrital
bearing
Instead,
of
of
drowning
deposited
the
framework
Formation,
Formation.
might
on
the
which
on
and
seamount tops.
seamount tops,
Isolated carbonate seamounts, Italy
the
for
The
bases of the
Sinemurian-early
often
incipient
sediments
the
Incipient drowning
immediately
evidence
indicate that
Calcare Massiccio. The Calcare Massiccio is approximately horizontally bedded here.
the
sediments
therefore
subsidence immediately
that
second
found
relatively
post-faulting
crustal
drowning
marginal
referred
after the
of the Bugarone Formation.
will
have
drowning
in
seamount
of
expect
on the seamount tops
to
Jurassic
source
in
be
the
as
Pliensbachian,
the
been
shallow
of
tops,
Jurassic
that
Appennines would have experienced
record
subsidence
the
the
a
the
escarpments.
the
the
is
basinal
after the faulting.
water
Lower: Closer view of the preserved margin on Monte Bove showing
swept
relatively
considered
of evidence
on
seamounts
sediments
extension begins,
off
is
is
marginal escarpment. Dashed line outlines Pliensbachian sediments that rest in the base of a groove eroded into the
Rotondo escarpment; the grooved surface shows original erosional features of the escarpment. Young normal fault is
These sediments contain detritus
approximately perpendicular to marginal escarpment. Upper right: View looking up one of the erosional grooves of the
ence immediately after faulting and that this subsid­
rapid subsid­
Pliensbachian and younger sediments onlapping the grooved marginal escarpment. The widest part of the groove on the
de­
ence would be the primary cause of the drowning.
From the above discussion of the stratigraphical and
isostatically adjusted to the reduction in thickness.
and the basins, it seems quite clear that in the late
Fig. 7. [Opposite] Details of grooved marginal escarpments. Upper left: Closer view of the western portion of the Monte
the detrital sediments found in the
can
The sedimentological and stratigraphical evidence
sedimentological differences between the seamounts
the
adjacent basins contain a similar record of incipient
Apennines, shown in Fig. 3. The first evidence to be
the
continental margins (Steckler & Watts, 1978; Bond
Apennines basin was subjected to extension along a
network of normal faults. Observations from other
& Kominz, 1984) and theoretical studies (McKenzie,
generally
but
northern
then tapers off as the crust becomes thermally and
prolonged
northern
the
In cases
shallow
means that there was not significant
best be understood with reference to a stratigraphic
Marmarone
157
considered comes from the post-faulting sediments
northern
beds, and equivalent aged sediments that occur at
The third
the
Bugarone
Corniola
presence of hermatypic corals from the upper portion