Page 175 - Carbonate Platforms Facies, Sequences, and Evolution

ent
ing
and
may
tops.
until
lishes
above
Monte
at
between
Rotondo
drowning
carbonate
the
drowning,
have
Regardless
are
the
Pliensbachian
of
and
drowning
Monte
least
discussion
CAUSES
many
upper
to
the
Sassotetto,
the
sediment
then
of
combined
OF
time
seamount
the
and
to
the
discounts
early
Rotondo.
of
Tithonian
The very fine-grained
Pliensbachian
THE
complete
Timing of complete drowning
the
they
as
cause
thinly-bedded micrites like
production
The
Subsidence and sea-level changes
and
complete
the
texture
first
and
seamounts
Tithonian.
the
complete,
drowning
top sediments
Monte
and
the
they were created by the normal faulting.
possibility
or
latest
commonly
at
The
DROWNING
of
drowning.
remained
time
those just west
complete, irreversible drowning of the seamounts.
rise can conceivably cause drowning, but the preced­
made the transition from incipient drowning to com­
pointed out, the nearby Abruzzi platform also ex­
Tithonian through lower Cretaceous Maiolica lime­
seamount seems to have gone from a state of incipi­
Un­
et at., 1970). The evidence from the Maiolica estab­
at substantial water depths in an open-ocean setting,
Oxfordian.
sometime
a paradox. Figure 10 shows how a variety of factors
crustal subsidence. As D'Argenio & Alvarez (1980)
are so high that the drowning of a reef or platform is
Oxfordian to Kimmeridgian by Cecca et al. (1981).
accumulation
Monte Sassotetto that have been assigned an age of
perienced crustal thinning and subsidence, but the
terminal
As Schlager (1981) has pointed out, normal rates of
contain
stones filled the basins to the level of the seamount
succeeded by 50 m of Diaspri cherts
more condensed on top of other seamounts, such as
plete drowning at different times, but a number of
As mentioned above, the seamounts seem to have
rapid
Monte
These deeper-water sediments are either absent or
shallow depths for a prolonged period of time after
upper
incipient
seamounts remained in a state of incipient drowning
usually rapid crustal subsidence or eustatic sea-level
of
with no evidence of shallow-water fauna (Colacicchi
planktonic fossils of the Maiolica suggest deposition
at
abundant
shallow-water fossils that are absent from the Diaspri
Rotondo
of
the
any
Fig.
tops
zone
small,
because
Calcare
amount
network
of
reducing
10).
Isolated carbonate seamounts,
cause
seamounts
Steinmetz,
of
of
(Monte
the
of
the
The
subsidence
Italy
to have played a
1984).
the
appear
Massiccio
maximum
Bove)
rapid
on the seamount top.
to
sediment
distance
extensional
that
convex-up
have
that
incipient
whatsoever,
platform
Sediment
eustatic
carbonate
from
may
faults
Configurations of the seamounts
had
shape
any
that
have
of
flat
created
reasonable to suppose (but difficult to
drowning.
drowning
a
production
accumulates
oscillations
point
the incipient drowning of these seamounts.
tops
broke
accumulation
some
resulted
of
prove)
on
could
on
up
of
on
series
(Hine
incipient drowning (see Fig. 3) in the Pliensbachian,
However,
platform there was not drowned, so subsidence alone
cannot be the cause of the drowning in the northern
sea-level changes (Haq et at., 1987) shows no rapid,
large-amplitude sea-level changes at the onset of the
occur. The most recent estimate of Jurassic eustatic
Apennines. However, it is equally clear that without
not
so it appears that eustatic sea-level changes were not
seamounts with relatively small tops. Some of these
(Monte
major role in the drowning. The
the
Cucco, for instance) whereas others had irregular
One of the important factors in determining the
the
secondary normal faults dropping down the edges of
the seamounts, creating a roughly convex-up surface
that
these sea-level changes played a role in causing the
from incipient drowning to complete drowning, it is
the
Tithonian (see Fig. 3) correspond to the transition
consider what factors may have suppressed sediment
production and accumulation rates in order to initiate
complete, terminal drowning. It is thus necessary to
The dimensions and shapes of the seamounts appear
sediment accumulation rates on Cay Sal Bank, which
Steinmetz (1984) as an important factor in the low
size in sediment removal has been cited by Hine &
is a possible modern analogue to these seamounts in
161
the
the northern Apennines. Cay Sal Bank is relatively
the bank is below 10 m of water, putting it below the
seamount top to the edge. The role of platform-top
of
seamount tops is the rate of sediment removal (see
from
the
of sediment removal. The small size of the seamount
tops also favours the removal of sediment simply by
the
seamount tops would clearly have enhanced the rate
bank top is so low that the bank is not able to build
has no effective fringing reefs, and much of
& ·