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Variability of depositional setting along the north-western Sicily continental shelf (Italy) ... 151
acoustically transparent lenticular bodies, with an erosive
base. These morphologies lie at 2-3 km seaward the
mouths of the Freddo and the Jato Rivers, within a band
trending W-E in subsurface depths from 75 to 25 ms
(t.w.t.), where the thickness of the HST exceeds 25 m
close to the river mouths. Pockmarks in the subsurface
occur up-dip the most significant wipe-out zones and they
point out the existence of gas-charged fluid expulsion.
The TST has four depocenters, aligned parallel to
the shoreline off major river mouths, as in the central
gulf, or at the base of the steep paleocliff in the eastern
gulf off Capo Rama. Close to the western edge of the
Castellammare Gulf the continental shelf is narrower
and the seabed is a marine transgressive surface cut
above the pre-Quaternary rocky substrate, at places
floored by a very thin, shallow sedimentary cover.
5.4. Structural features
During the Pleistocene, faults, as well as folded
structures, exerted a control on the morphology of the
present day coastal areas and shelf-slope system, as
for the submarine canyons and the mass failure pro-
cesses.
In the Termini Gulf E-W to NE-SW-trending normal
faults are prevailing. Very recent NW-SE vertical faults
displace the buried Quaternary succession in the west-
ern sector, being also seismically active.
In the Palermo Gulf NNW-SSE fault systems, with
a downthrow ranging from some tens to 150 m, are
prevalent off Monte Pellegrino and in the southeastern
side (off the head of the Eleuterio canyon), whilst to the
north, between Capo Gallo and La Barra, faults appear
to have a dominant NNE-SSW trend. Pockmarks and
mounds are often aligned with the fault systems trends.
In the Monti di Palermo offshore main fault sys-
tems are and near vertical, oriented ENE-WSW (see La
Barra structural high) or NNW-SSE, dipping NNEward.
WNW-ESE trending fault systems were recognized par-
allel to the coastline. In the Carini Bay, inner continental
shelf is characterized by N-S and NNE-SSW fault sys-
tems with variable downthrow.
In the Gulf of Castellammare the continental shelf
develops in the southern part of a tectonic depression that
formed during the Quaternary (Fig. 11). It is bounded to
the East by N-S to NNW-SSE, with some hundreds of
meters of downthrow, along which Meso-Cenozoic car-
bonate succession is juxtaposed to Plio-Quatermary de-
posits; to the South pre-Pliocene deposits are intensely
folded and dissected by both reverse and extensional N-S
to NNW-SSE faults, even very recent; to the West by
NNE-SSW, NS and NNW-SSE faults.
6. DISCUSSION
Several evidences highlight that the North Sicily
continental margin is tectonically immature. The Plio-
Quaternary tectonic evolution of the NW Sicily margin
produced the development of a morphostructural setting
characterized by a series of depressions (intraslope ba-
sins) separated by structural highs along the margin
(Agate et al., 1993), as result of differential uplift and
subsidence during Neogene to Recent times.
Recent tectonic activity is testified by neotectonic el- erwise, beneath the LQDS, oblique seawards dipping
ements and seismic activity (Fig. 1), calculated uplift rates,
depositional alternating with structural edge, present lack
of highstand-transgressive systems tracts in the oldest
depositional sequences recognized along the margin.
Furthermore, the occurrence of pockmarks and
mounds probably consisting of authigenic carbonates
above faulted and folded strata suggests a local rela-
tionship between structural control and fluid escape.
Some of the tectonic features could have represented a
preferential escape route for fluids as evidenced by the
pockmarks. Pockmarks may in turn trigger mass failures
as evidenced by the headscarps observed below them
(Lo Iacono et al., 2011). The structural features are pos-
sibly associated with the recent tectonics mapped on
land (Catalano et al., 2011 a, b) as well as the wide-
spread seismicity of the margin.
As a matter of fact tectonic activity persists today
with the occurrence of shallow (<25 km) seismic events
of low to moderate magnitude along an E-W trending
belt located along the continental slope. The focal
mechanisms related to the main seismic shocks are in
agreement with a dominant NE-SW fault trend coupled
with a NW-SE compressive offset direction. A minor
cluster in the Monti di Palermo offshore well match with
the extensional ENE-WSW trending extensional faults.
Uplift of the northern Sicily shelf and surrounding
areas during the Pleistocene is testified also by the
Pleistocene regressive coastal deposits in the western
sector of the margin (Mauz et al., 1997; Arces et al.,
2000), and the Pleistocene deposits, outcropping close
to the study area, which are uplifted tens of metres (Hu-
gonie, 1982). Moreover, uplift of the continental margin,
as a consequence of thermal perturbation due to the
thinning of the lithosphere which is not compensated by
local isostasy, is predicted by forward modeling (Pepe et
al., 2003), thus constraining the late Neogene to Recent
tectonic and stratigraphic evolution of the margin.
Therefore, it is evident that along the North Sicily
continental margin eustatic and tectonic processes
combine to cause both relatively short- and long-term
sea-level changes which control the space available for
sediments, forming a number of depositional sequences
from the early (?) Pleistocene to the Holocene.
6.1. Effects of eustatic sea-level changes
Subsequent, rapid sea level changes impacted the
continental margins as consequence of astronomically
forced, worldwide climatic changes occurred during
Quaternary age. As documented by several studies on
Oxigen isotopes (Chappel & Schackleton, 1986; Imbrie
et al., 1984; Williams et al., 1988), middle-late Quater-
nary sea-level changes have been asymmetrical fluctua-
tions characterized by slow falling stages (1 mm/y) fol-
lowed by fast rising stage (1 cm/y); average amplitude of
cycles was 100-150 m and the period was about 100-
200 ky. As we have illustrated, the internal geometry of
the LQDS well documents the sedimentary response to
specific segments of the last glacio-eustatic fluctuation.
The effects of eustatic changes appear less evi-
dent in the more ancient deposits where complete dep-
ositional sequences have not been detected. According
to us, the interpreted seismic profiles does not image
past shelfal TST and/or HST deposits and subsequent
erosional surfaces are not clearly distinguishable. Oth-
layers only are preserved with small, interbedded, pro-