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114 CHIOCCI F.L. - D’ANGELO S. - ROMAGNOLI C. - RICCI LUCCHI F. - GRUPPO DI DISCUSSIONE TDS

assumed according to the acoustic facies in S.B.P 3.5 kHz profiles and the inclination of the foreset
of the depositional bodies.

The most common texture is the medium-coarse sand, as suggested by the lack of high-frequency
seismic wave penetration and by the acoustic facies with low-continuity, low-amplitude reflections; dif-
ferent sediments can be found in the SDT: rudites as volcanic scoriae at Aeolian Isles to pelites as
sandy silts of the west Sardinia edge.

The seafloor cores stratigraphy shows a coarsening up trend; also the bioclastic fraction (often with
glacial affinity fauna) increases towards the top and towards the basin, sometimes in form of biorudi-
tic floor or of organogenic crusts.

It is important to observe how paralic or terrestrial facies have never been found; towards the top
the sandy deposits sharply pass to thinner, hemipelagic mud, ascribed to the sedimentation during the
phases of sea level rise and highstand.

The SDT lithologies are variable, as they reflect the different geology of the coast; the bioclastic
component is always well represented.

The location of the alimentation sources strictly controls development of the SDT. As for the SDT
developed at the shelf-edge, the alimentation can be assumed to be linear. The feeding from subaerial
basin seems to be very rare, as SDT are not usually associated to paleodrainage able to carry fluvial or
littoral sediments down to the platform edges (north-west Sicily, Egadi).

As for volcanic areas, the development of thick SDT is often connected to a good availability of
volcanogenic sediment deriving from the erosion of eruptive centers and of pyroclastic deposits
oucropping on the facing cliffs. An example of extremely localized alimentation, tied to the dismant-
ling of a little exogenous dome, has been observed for the Basiluzzo Isle (Panarea, Aeolian Isles), at
the foot of which there is a SDT with a limited extension but with geometric and morphologic featu-
res similar to other observed cases.

GENESIS AND AGE

The origin of the SDT is referable to sea level stillstands significantly lower than present. The
depositional bodies with a depth of -100/150 m and located at the edge of platforms represent relict
structures formed, probably, during the last glacio-eustatic lowstand (about 18 ka ago); this interpre-
tation comes from their bathymetric position that is consistent with the depth reached by the sea level
during the last glacial acme, and it is also sustained by radiometric dating and biostratigraphic data
(ex.: SDT of Tuscany and Pontine Archipelago and in Calabria).

The chronological attribution of those SDTs found at a depth less than 100m it is les clear: apart
from vertical dislocations due to tectonic uplift, their present day depth is tought to reflect stillstands
of the relative sea level during the last post-glacial transgression. The retrogradational setting obser-
ved in some SDT might suggest a deposition happened during a discontinuous sea level rise.

The observation of the depositional characters of the SDT suggests, for their formation, a high
energy environment with limited clastic contribution from the inland basin (that could become locally
significant); deposition occurred on steep sea floor affected by littoral drift, able to re-distribute the
sediment. The SDT form along coastlines dominated by the wave motion and subject, because of the
insularity or the lack of continental shelf, to a great amount energy during the storms; in particular,
wave energy concentration and fetch seem to play an important role in controlling the distribution of
SDT and the depth of its depositional parameters (as in the case of the SDTs present on the Aeolian
Isles).

In most cases, unfortunately, it is not possible to have direct data on the depositional facies and
fauna of the SDT; this makes their attribution to a specific depositional environment uncertain.

The most common interpretation given by the authors for the SDTs observed at different depths
along the lines of the Italian coasts is that they are littoral or deltaic wedges (e. g. SDT on Tuscany
Archipelago). This hypothesis is based on the followings: (1) the excellent correlability with paleo sea-
level (especially those at -120m, the lowermost level reached by the sea during the last glacial acme);
(2) the sedimentary facies observed in the corings, that suggest depositional processes highly selecti-
ve typical of the beach environment; (3) the strong analogies of the external and internal geometries