Page 2 - Colantoni_alii_1993

Bathymetry and general morphology Marsala, where often they are cut by erosional
Figure 1 was originally drawn at a scale of surfaces. Here a single, sharp shelf break is
1:100,000 utilizing a grid of profiles integrated replaced by a succession of smaller steps.
by data from the Italian Hydrographic Institute Around Adventure Bank the shelf break
(Bortoluzzi and Ligi, 1986, 1987, 1988; Ligi and ranges between 83 m and 170 m. The deepest
Bortoluzzi, 1989a, 1989b). Contour interval is 10 sectors are in the east and in the southeast where
m between 0 and 100 m, while in the deeper more sediments have accumulated. The depth-
water 50 m is used. Good detail is therefore sedimentation relationship is probably due to
ensured. The 100 m contour line, which is close subsidence and tectonic tilting of the entire bank.
to the shelf edge, describes the outline of the Good examples of prograding sediment wedges
three geographic units. are seen on the seismics , while in other parts of
1. The inner Sicilian continental shelf that the Bank, in particular along the western side, the
joins the islands of Favignana and Levanzo with shelf break is shallow, sharp and due to faulting.
the mainland, shows a generally gentle slope with In many places the shelf edge is incised by
numerous shallow banks and minor islets that rise the heads of canyons and valleys that cut the
from the seafloor as erosional remnants. slopes and duct sediments into the basins. Local
Minimum depths are recorded on shoals (Secca fan systems, such as those recognized south and
del Toro, -6 m; Secca dei Pesci, -21 m; and NW west of Marettimo and west of the Adventure
of Levanzo, -29 m). The shelf is more regular off Bank, are also form coincident with topographic
Mazzara del Vallo where it has more depostional gradient decreases.
character. Extended sliding and slumping of soft
2. The shelf around Marettimo Island is sediments occurs on the bottom and flanks of the
separated from the inner Sicilian shelf by a main valley north of the Islands.
NNW-SSE trending channel. A sill at about -250 Sea floor types
m divides the channel in two valleys facing
3.5 kHz profiles show the
opposite directions.
micromorphology and lithology of the seafloor.
The Marettimo shelf is fairly flat, without Three main types of acoustic response have been
important shoals and extends more south-
recognized (Fig. 2).
eastward than to the north and to the north-east,
where it is narrow and steep (Colantoni et al., 1) Rough seafloors not penetrated by
acoustic energy are found in the inner shelf and
1993). An isolated flat topped relief with a
where rocks outcrop. A special seabed is
minimu depth of 107 m is also present to the
north of Marettimo. represented by the wide meadows of Posidonia
Oceanica that flourish between Favignana and the
The same direction of the Marettimo
mainland. These are also present locally on the
channel is shown by the Mazzara del Vallo Adventure Bank.
channel the divides the SW Sicilian shelf from
2) Smooth seafloors scarcely penetrated by
the Adventure Bank. The channel is very narrow acoustic energy characterize the outer shelf and
and along its axis has three close depressions that
reach the depth of 148 m, 167 m and 152 m. the largest portion of the Adventure Bank, where
a thin sedimentary cover overlies erosional
3. The Adventure Bank is about 90 km surfaces.
wide and has an average depth of 80-90 meters.
Its surface is quite flat but there are some small 3) Smooth seafloors well penetrated by
acoustic energy are characterized by numerous,
shallow shoals in which sedimentary and
volcanic rocks of different age outcrop. Their continuous and parallel reflections in the sub-
depth ranges between 18 m and 74 m (Colantoni bottom. They are found in the deep areas beyond
the edge of the shelf. Major sedimentation of
et al., 1985).
fine-grained sediments occurs in these areas. A
The depth of the shelf edge in the northern special facies of this seafloor type, shown
part of the area (Egadi Islands) varies from the 80
separately in Figure 2, is represented by
m, recorded SW of Marettimo, to 167 m NW of acoustically transparent sea beds. This bottom
Levanzo. Average depth is about 120 m. Seismic
type occurs in small, closed basins where
profiles show that the shelf break as a generally
sediments ponding has taken place and in areas
straight, sharp break with a tectonic character. where mass gravity deposits have come to rest.
Progradational features are very scarce and only
located in the area south of Favignana and off

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