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C. Lo Iacono, J. Guille´n / Continental Shelf Research 28 (2008) 245–256
favorable conditions for the preservation of these bedforms bathymetric depressions of the light gravelly bands and
would be the presence of a thin layer of fine sediment that asymmetry in grain size between the up-flow and the down-
buried the paleo-shelf sedimentary structures and protected flow edges previously described in other areas were not
them from erosive and destructive actions (Brew, 1996) or observed on the Marettimo shelf. Finally, the always
the occurrence of a very low-energy environment during abrupt boundary between sandy and coarse areas and the
the sea level rise (Wattrus and Rausch, 2001). Neither of absence of ripples are differences with respect to other
these conditions occurred on the Marettimo shelf, and sorted bedforms areas (Cacchione et al., 1984; Cirac et al.,
consequently the third hypothesis is not supported by 2000; Murray and Thieler, 2004; Goff et al., 2005; Ferrini
observations. and Flood, 2005, Fig. 1). These differences are probably
related to the reduced availability of sandy sediments in the
Dune wavelength (1–3 m) of the Marettimo coarse area because of the absence of riverine sedimentary inputs.
bedforms displays a range similar to ones observed on In contrast, many other areas with sorted bedforms are
different shelves around the world, in which wave action located in depositional environments with large amounts of
has been described as the main forcing factor (Table 1). sandy sediments (Schwab et al., 2000; Goff et al., 2005;
As on the Marettimo shelf, the wavelength of coarse Diesing et al., 2006. For a complete review see Table 1 in
bedforms on other shelves seems to be unrelated to Coco et al., 2007b).
sediment grain size or to water depth (Figs. 2 and 6). On
the Scotian shelf, for instance, wave-generated gravel In spite of these particularities, it is assumed that the
bedforms (D50=4–45 mm) with a wavelength of mechanisms of formation of Marettimo sorted bedforms
1.3–3.0 m and a height of about 0.2–0.3 m were observed, are equivalent to those described in other areas (Murray
distributed in a water depth of 15–65 m (Forbes and Boyd, and Thieler, 2004; Coco et al., 2007b). Therefore, they are
1987). On this shelf, wave conditions suitable for sediment likely to be transverse bedforms generated by alongshelf
transport of a 17 mm particle diameter at 30 m water depth currents. The change from a linear (shallow) to a patchy
(Hs ¼ 5.3 m and Tp ¼ 8 s) occurred for several days every and bifurcated (deeper) shape is probably related to local
year. Differences between the high (Scotian) and moderate differences in hydrodynamic conditions, with stronger
(Mediterranean) energy shelves are related to the number currents developing in the deeper sector of the shelf (Coco
of storm events during which the dunes are active. et al., 2007b). Field observations on the Marettimo shelf
However, the morphological characteristics of the dune support the long-term stability of coarse bedforms also
fields and the bedform generation mechanism are the same reported in previous works (Murray and Thieler, 2004;
for both environments. Goff et al., 2005; Diesing et al., 2006). The comparison
between the two SSS mosaics taken in the area in 1989 and
4.4. Forcing mechanisms for sorted bedforms 2002 shows a morphological stability of sorted bedforms
(Fig. 11). In addition, the very high degree of bioturbation
Marettimo sorted bedforms display some differential and the total absence of ripples and dunes above the
characteristics with respect to previously described sorted gravelly bands also suggest long-term inactivity of
bedforms along the inner shelves of the North Sea and the sorted bedforms. Both observations indicate that the
Atlantic Ocean (Murray and Thieler, 2004; Goff et al., mobility of sorted bedforms is probably related to events
2005; Diesing et al., 2006). First, on the Marettimo shelf with a long-term (413 years) recurrence period. However,
sinuous sandy tongues lie on a gravel and sandy gravel area the low precision of the positioning system of the 1989
(Fig. 4), whereas coarse gravelly bands lying over finer survey does not allow an accurate assessment of the
sandy bottom have been observed in other areas. Second, mobility of sorted bedforms, which in some cases can move
Table 1
Morphological and sedimentological characteristics of gravelly dunes on different continental shelves
Reference Shelf area Dune Depth Textural character Forcing mechanism
wavelength range (m)
(m) Storm waves
Storm waves (Hs:5 m, Tp: 6–11 s)
Lewis (1979) New Zealand 1–3 20–50 Coarse grained Storm waves (Hs:5 m, Tp: 11 s)
Hunter et al. (1982) Bering Sea 2 12 Gravels Storm waves (Hs:9 m)
Forbes and Boyd (1987) Nova Scotia 1.3–3 15–65 Gravels and pebbles
Langhorne et al. (1986) Southern 5 10 Pebbles Storm waves (Hs:5 m, Tp: 12 s)
Enlgand
Cirac et al. (2000) North 2 50 Coarse sands and Storm waves (Hs:5 m); currents (70–100 cm/s)
Aquitanian gravels Episodic storm waves (Hs4 5 m, Tp4 11 s)
Anthony and Leth (2002) North Sea 2–3 20–30 Sandy gravels
This paper Mediterranean 1–2.5 10–50 Gravels and pebbles
Sea
