Page 2 - terraces-chap10

70 D'ANGELO S. - LEMBO P. - SACCHI L.

An extensional phase, from Upper Pliocene to early Pleistocene, determined the formation of some
basins and structurals heights with main orientation NNW-SSE and E-W, till now in evolution (AGATE
et alii 1992; ARGNANI 1993; CATALANO & MILIA, 1990); their outlines are pointed out by submarine
m o r p h o l o g y.

Seismostratigraphic analysis clear up a Mesozoic substratum formed by sedimentary deposits evol-
ving from neritic to pelagic facies ( CATALANO, 1988; CATALANO et alii, 1989). This substrate outcrops
underwater along a continous belt around the island and in isolated banks to the South, West and
North of Favignana at depth ranging from -40 to- 70 m. On this substrate often the coralligenous bio-
cenosis sets off.

The recent sedimentary cover is very thin and is made of organogenous medium to coarse sands.
The source of biogenic material are calcareous algae, mollusks, Bryozoa, Serpulides, Foraminifera
etc. (D'ANGELO et alii, 1994) that cover the shelf to depth of -100 m. Beyond this depth the fine por-
tion increases and we find the pelitic sands and the sandy pelites as far as the pelites in the deepest
areas (Fig.2).
All the sediments, also the finest one, have a prevailing biogenic content, while the terrigenous part
is very scarse.
This trend is extended backward in time to early Pleistocene, as it is conformed by the presence of
bio-calcarenites outcropping on the island and on the Sicilian coast (ABATE et alii, 1996).

Fig. 3 - Geomorphological sketch of the area (from ORRÙ & ULZEGA, 1993, modified). Bathymetric lines are provided from
the Hydrographic Institute of Navy (I.I.M.) that, during the last years, has carried out new surveys offshore Western
Sicily. They offer a good detail for the description of the main morphological lineaments.

MORPHOLOGY

On the basis of the data from bathymetric map and from Uniboom and Sub-Bottom records, we
can recognize an abrasion continental shelf that joins with a slight slope the island of Favignana and
Levanzo to the mainland. It extend for about ten Km to depths of -80,-90 m.

Another inner abrasion shelf is shown at depths of -30, -40 m, between Favignana and Levanzo and
southward of Favignana; it joins with the lower one with a regular slope.

Scattered morphological highs are present between the two island and south of Favignana and they
correspond to the outcrops of the Mesozoic carbonatic substrate.

They are shaped at the top by abrasion surfaces and at the base (-25, -30 m) by tidal grooves
(AGNESI et alii 1993) (Fig.3).

The depth of the shelf break is steep and stright, denoting a tectonic origin.
Westward and Northwestward of Favignana it is backward eroded while to the South, Southwest of
the island it is prograding in some places.
The depositional wedge, interpreted such as a submerged depositional terrace, is located in an area
subject to strong erosion and itself is interested to successive erosions; it lays at the edge of the lower
shelf at depths ranging from -80, -90 m. It lenghtens in NNW-SSE direction for about 10 km and it
extends to a maximum of 2.5 km in width, with thicknesses variable up to 40 m depending on the
steepness of the scarp.
It's mainly set up by pelitic sands and sandy pelites (grab sampling).
Northward the terrace thins and ends where the slope is steeper for the backing of the canyon
headscarp which separates the Favignana shelf from that of Marettimo; southward it is cut off by
slumpings.

Fig. 4 - Development of the submerged depositional terrace.

Several depositional phases setting up the terrace can be recognized.
These episodes probably are related to the strong sedimentary stages during the lowstand of the
last glacio-eustatic event characterized by numerous fluctuations.
Their respective geometric relations are different, depending from the different evolution of some