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

seaward shift of depositional environments, emersion and subaerial exposure/erosion of the conti-
nental shelf.

The identification of specific features that can be directly tied to eustatic minimum (or to stillstand
during sea-level rise) is crucial for seismostratigraphic interpretation of continental margins, as this is
chiefly based on the reconstruction of depositional geometries.

If one moves from steep and underfeed coast, where TDSs are often found, to well-feed continen-
tal margins, the situation is far more interesting. Wedge-shaped deposits very similar in dimension and
form to TDS outcropping on the present-day seafloor are found in continental margins, usually made
up of monotonous parallel sedimentary units as thick as hundreds of meters. In the most favourable
situations, their formation at the eustatic minimum is testified by a position at the paleo-shelf break,
where the erosional unconformity looses its erosional characters becoming a correlative conformity.
Where the seismostratigraphy is ill-defined, TDS may represent the only feature pointing out the posi-
tion reached by sea level at eustatic minimum, as in figure 2, taken from the Latium margin, a little
nord of Civitavecchia (Capolinaro).

APPLICATIONS 3: COMPARISON WITH OUTCROP

One of the common characters among the studied areas where TDSs have been found is the steep-
ness of the basal surface on which they rest. Because of that TDSs are mainly present in continental
margins affected by volcanism and/or tectonics; It is likely that their common presence along the
Italian coast is probably due to the active geodynamic setting of the Italian seas.

If active geodynamic setting is likely to produce TDSs, an involvement of the latter on regional
uplift has to be expected so that they can be found on outcrop.

Especially Quaternary deposits are likely to host such depositional bodies, as the strong eustatic
fluctuations may enhance their formation.

On the article of MASSARI (this volume) a clinostratified deposit outcropping in Sicily is described.
From a photograph in a figure of this article, a line-drawing has been realised, that has been deformed
adding the same amount of vertical exaggeration that affects high-resolution seismic profiles. In other
words, the lower part of the figure would be the seismic images of the deposit described by MASSARI,
if depicted by marine seismic prospection as those presented in this volume.

As it can be seen by a comparison Fig. 3, the overall appearance of the deposits is similar to the
TDS reported in this atlas for the sigmoidal geometry of the stratification, the steepness of the fore-
sets up to 16°, the presence of erosion/reactivation surfaces within the deposit and for the dimension
of the depositional body (30-40 m thick, some hundred m long normal to progradation, some km
parallel to progradation).

FINAL REMARKS

The submerged depositional terraces, as we choose to define these forms, are a morpho-depositio-
nal feature rather common on the Italian seas, and in fact all the Italian research groups working in
marine geology have somehow surveyed them.

These forms are of high scientific interest because of their "singularity and significance", as they
are easy to recognise, quite small in dimension, constant in depositional character and have a very pre-
cise stratigraphic position (at eustatic minimum).

Despite this fact, such features are barely described in the scientific literature, often only casually
depicted. It is possible that the fact that they are more likely to form (or to preserve) on continental
margin affected by volcanism or active tectonics, let these forms to be rare if not absent on oceanic
coasts or in coast with meso- macro-tidal regime. On this hypothesis TDS may only be frequent on
"Mediterranean" latu sensu geological setting, i.e. active margin, marginal basins, volcanic islands with
microtidal siliciclastic sedimentation. The co-operation among the main research groups of Italian
marine geologists to define and characterise these depositional forms, as summarised in this volume,
may therefore represent an original contribution of the Italian research to highlight a depositional fea-
ture having a possible great general geological significance.