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ARTICLE IN PRESS
252 C. Lo Iacono, J. Guille´n / Continental Shelf Research 28 (2008) 245–256
Fig. 8. SSS image of Marettimo sorted bedforms acquired in 2002. Gray
backscatter tones correspond to gravelly sediments, while dark backscatter
tones correspond to medium and coarse sands. Approximate depth: 45 m.
For bedforms location see Fig. 4.
and pebbly sediments for the first 5–10 cm of subseafloor,
overlying sandy sediments (Fig. 8).
Potential mechanisms for coarse sediment motion were
evaluated. A rough estimate of the critical bed shear
stresses needed for the entrainment of sediment with grain
sizes of 2–16 mm (Eq. (2)) ranges from 1.1 to 14.1 N mÀ2.
According to the estimated shear stress, the current
velocity 1 m above the bottom (Eq. (4)) necessary to move
sediment ranging in size from 2 to 16 mm varies from 0.8 to
2.2 m sÀ1 when a flat bottom is considered and from 0.3 to
1.2 m sÀ1 when the bedform roughness is included in the
equation (Fig. 9). These estimations of shear stress and
current intensity are in agreement with laboratory experi-
ments (Neill, 1967; Langhorne et al., 1986) as well as field
observations (Carling et al., 2006) and they fall within the
dunes field of the bedform existence diagram (D50 vs mean
current velocity) defined by Southard and Boguchwal
(1990). However, current velocities required for the
entrainment of the Marettimo coarse sediments have rarely
been reported on NW Mediterranean shelves. For instance,
Palanques et al. (2002) and Guille´ n et al. (2006) reported
maximum near-bottom mean currents of about 0.8 m sÀ1
on the NW Mediterranean continental shelf during strong
storms.
4.3. Forcing mechanisms for gravelly dunes
The symmetric morphology and the coast-parallel crest
lines of the observed gravelly dunes suggest that the wave-
generated forcing mechanism is the most likely. Further-
more, the largest and coarsest dunes, observed along the
northwestern portion of the shelf (Figs. 4 and 6), are
oriented toward the west and northwest, corresponding to
the directions of the strongest storm events and facing the
long fetch (Figs. 2 and 3). Waves thus appear to be the
Fig. 9. Critical current velocities (Eq. (4)) for the grain size range sampled
on the Marettimo shelf assuming a flat bottom (solid line) and a roughness
value proportional to a bedform height of 0.15 m (dotted line).
Fig. 10. Thresholds for sediment entrainment (Eq. (1)) across the shelf for
different grain sizes and wave conditions.
main forcing mechanism of dune generation in the study
area, although currents could play a complementary role.
Threshold critical values for initiation of sediment motion
would require wave conditions with Hs43 m and T47 s
(Fig. 10). Potential wave conditions (Hs43 m) for sedi-
ment entrainment and dune activity on the Marettimo shelf
occur during 2.5% of the time and the shallower
Marettimo dunes could be active during these events.
However, the extreme wave conditions observed during the
last two decades (20/01/1998; Hs ¼ 6.6 m; Tp ¼ 11.1 s and
24/11/1991; Hs ¼ 6.2 m; Tp ¼ 9.1 s) could not explain the
formation of the coarsest dunes at water depths greater
than 25 m (Fig. 10).
Several possibilities can be put forward to explain the
origin of pebble dunes on the Marettimo shelf: (a) there are
episodic, high-energy storm events that have not yet been
recorded in the study area during the 17 years of wave buoy
records; (b) the threshold hydrodynamic conditions for
dune formation are lower than estimated because of the
increased turbulence due to the high roughness associated
with the coarse sediment or because grain entrainment only
affects the finer fractions (Forbes and Boyd, 1987; Carling
et al., 2006; Kuhnle et al., 2006); and (c) deeper dunes are
relict and were generated at shallow depths during the
Holocene post-glacial marine transgression. However,