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

DEFINITION

The definition of "Submerged Depositional Terrace", for the deposit object of this atlas, has been
proposed since the beginning of the initiative and it is confirmed at the end of the research, even if
the word "terrace" is usually associated to fluvial coastal features and involves a strong erosive com-
ponent. The definition of SDT has, for us, a descriptive and not a genetic meaning, and it indicates a
deposit made significant by the steadiness of the morphologic-stratigraphic features, even in very dif-
ferent geologic-physiographic contests.

The features that define a SDT are the following:
- It is not a purely morphologic unity, but a submerged sedimentar y body, with a clinostrati-
fied internal structure having a prograding growth towards the basin.
- The external morphology is characterized by a quite flat roof, a well-defined edge, a fron-
tal slope with a gradient similar to the angle of rest of the sediments, that in the lower part gra-
dually connects with the substratum (generally the upper continental slope)
- The thickness distribution is always parallel to the coast, while the slope extension is far infe-
rior to the longitudinal one.
- The external shape is not modified by the postdepositional erosion, with a frontal slope
having the same gradient and the same shape of the internal stratification.

Despite the word terrace is used in geomorphology to indicate features produced by marine or flu-
vial erosion, this definition does not have got genetic implications. As an example, the word terrace is
used for submarine morphologies of complex origin and great dimension like, for example, the dic-
tion of continental platform/terrace.

The depositional nature of the morphologies we have studied is underlined by the adjective "depo-
sitional" that, in addition to differentiate the SDT from the fluvial and marine-coastal terraces, shows
the primary character of these bodies, originated by lateral accretion and unmodified afterwards.

The census and the atlas have taken into account only the features at present submerged, leaving
out both the buried features (present whitin the continental margin) outcropping features of which we
have mentioned only one example for comparison. This choice has been made considering our need to
limit the compilation to deposits with similar stratigraphic situation; it is however obvious that also
more ancient or differently located features can be defined as submerged depositional terraces, if they
are found similar in shape, dimension and features respect to the SDT present on the sea floor.

In order to point out the similarities, we have made a summarizing table that synthesizes the main
features of the SDT described in this atlas (TAB.1). The comparison among the different cases allo-
wed a semi-quantitative definition of the morphologic and depositional charactery of the SDT (depth,
internal and external geometry, thickness, acoustic facies) that are here after described.

LOCATION AND DEPTH OF THE SDT

The SDTs are found on tracts of continental margin characterized by reduced or absent shelf
and/or by steep seafloor; such conditions are common on margins controlled by tectonic lineaments
or great erosive structures. Volcanic coasts and/or islands are also characterized by very steep sea
floor (ex. Pontine Archipelago, Aeolian Isles, Linosa, Egadi). The development of the SDT is clearly
conditioned by the gradient of the basal surface on which it rests (between 0.5° and 2°, rarely up some
grades) as well as by the extension of the shelf (less than 10 km); the depositional edge of the SDT
often coincides with the platform edge.

It has often been noted the association of the SDT with erosive features lying behind (paleocliffs)
or below (abrasion platforms); this fact indicates the importance of the morphology for the formation
and the preservation of these bodies. Especially as for insular volcanic areas, the presence of abrasion
platforms interrupt the steep morphology of the sea floor and shows a preferential place for the deve-
lopment and the preservation of the SDT that would otherwise be remobilized by gravitational phe-
nomena.

The depths of the SDT are mostly between -100 and -200 metres, with a depositional edge between
-120/-150; these depths are consistent with the ones reached by the sea levels during the last glaciou-