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254 C. Lo Iacono, J. Guille´n / Continental Shelf Research 28 (2008) 245–256
Fig. 11. Side scan sonar image of the sorted bedforms acquired in 1989. a dune wavelength ranging between 1 and 2.5 m, a
Dotted lines correspond to a drawing coming from the SSS image
obtained in the same area in 2002 (Fig. 8).
tens of meters for decades (Goff et al., 2005; Diesing et al.,
2006).
The hydrodynamic forcing of the Marettimo sorted
bedforms remains unclear. The experience of local fisher-
men indicates the absence of strong and persistent currents
in this area. As regards wave action, stronger storms
affecting the eastern portion of the Marettimo shelf come
from the southeast and develop wave crests almost
perpendicular to the coast and transverse to the sorted
bedforms. The major storms that occurred during the
1989–2002 period have a maximum wave height of 6 m and
a maximum period of 10 s. However, field observations
suggest that these hydrodynamic conditions were not
strong enough to significantly alter the sorted bedform
shape, and that higher-energy conditions would be
required to modify these sedimentary structures.
5. Concluding remarks
It has been shown that gravel bedforms can develop in a
tideless and moderate wave energy environment like that of
Mediterranean shelves. Gravelly and pebbly dunes
(D50 ¼ 2–16 mm) and sorted bedforms were identified
and measured on the Marettimo Island shelf (NW Sicily,
Italy). These first observations of coarse bedforms on a
Mediterranean inner shelf indicate that such features are
unusual in this environment and that their occurrence on
the Marettimo shelf was favored by specific conditions: a
narrow and relatively steep shelf, almost nil continental
delivery of sediment, and a very long fetch for western and
southeastern wave directions.
Gravelly and pebbly dune fields are distributed in
patches around the island in shallow water (10–50 m), with
symmetrical dune shape and a dune crest lineation almost
parallel to the coastline. As observed on other continental
shelves around the world, wave action during storms is the
only plausible hydrodynamic mechanism for the formation
and maintenance of most gravel and pebble dunes observed
on the Marettimo shelf. Dunefields located on the
shallower part of the shelf can potentially be active for
some storm events every year. However, waves able to
generate and maintain dunefields located in deeper areas
and with coarser sediments are associated with extra-
ordinary storm events with a long recurrence period
(several decades).
Sorted bedforms are developed only in the eastern sector
of the Marettimo shelf. Whereas the occurrence of intense
alongshelf currents has never been reported in this sector of
the shelf, lineations of the sorted bedforms are parallel to
the wave-crest direction of the strongest storms affecting
the area (coming from SE). The presumed long-term
stability of part of the Marettimo coarse bedforms has
also been proved by the comparison of two SSS images,
13 years apart, depicting the same morphological config-
uration of sorted bedforms.
Therefore, although potential mechanisms for the
generation and maintenance of Mediterranean coarse
bedforms are equivalent to those defined in other areas,
they display lower mobility than on higher-wave-energy
shelves and behave as relict morphological features for
long periods of time.
Acknowledgments
This work was carried out in the framework of the
GebecSud project, supported by the MIUR (Ministero
Istruzione Universita` e Ricerca, legge 488/92, Cluster 10—
Ambiente Marino) and directed by the Geology and
Geodesy Department of Palermo University. We are
grateful to D. Bowman (Ben-Gurion University of
the Negev, Israel), P. Puig (ICM-CSIC, Barcelona),
D. Huntley (Editor of the manuscript), and two anon-
ymous referees for their constructive and helpful sugges-
tions that enabled us to improve this manuscript.
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