Page 3 - terraces-chap10

DEPOSITIONAL SUBMERGED TERRACES OFFSHORE OF FAVIGNANA 71

of the shelf edge: from the bottom currents, from balancing, from different subsidence rate and from
the morphology of the shelf lying behind.

During the period the morphology of the area depends from the formation of the isthmus between
Levanzo and Favignana and between Favignana and the Sicily, from the long littoral bars and from the
incision of the morphological height of the "Secca del Toro" to the South of Favignana (Fig. 3)
(AGNESI et alii, 1993).

This coastal environment so changeable and subjected to strong bottom currents (testified by seve-
ral sedimentary structures identified with Side Scan Sonar records nearer to the coast) supplied the
sediments setting up the terrace.

Furthermore, the trend of general uplifting of the area somewhere has produced conditions of
strong erosion that make the keeping hard for these depositional structures and which have reshaped
the depositional terrace with erosion at the top, piles at the base dislocation of the blocks.

Fig. 5 - Uniboom profile crossing the N-S canyon westward of Favignana. It's the northern examined profile
(Fig. 4). The opening of the depositional terrace is at a depth of 80 m, in correspondence of the shelf-edge.

The steepnees of the slope (amplified by the the vertical exaggeration of the seismic profile, 17x) produces a small
development of the sediment thicknesses and of the extension perpendicularly to the wedge.

The end seems to be identified at - 160 m, while the edge, just signed for the terrace surface, is located at about -
94 m.

The coarse grain size of the sediments and the high gradient of the reflectors, prevent the penetration of the sei-
smic signal and the identification of the internal structures of the depositional terrace.

The withdrawing of the head of the canyon causes a marked lateral instability of the sediments that collect at the
base of the terrace, forming a convex wedge which partly obstructs the buttom of the canyon. On the opposite side of
the canyon, placed against the Marettimo shelf break, it's visible another submerged depositional terrace, even less
developed, and placed at different depths.

Fig. 6 - On the eastern side of the canyon on the Favignana shelf (Fig. 2), three phases of growth of the margin are recognized,
identified by breaks of the slope of the sea-bottom..

The internal structure is scarcely recognizable for the low penetration of the acoustic signaland the strong gradient of the reflectors.
At the base of the frontal scarp of the terrace, a morphological height of sedimentary origin is observed, probably eroded by the cur-
rents that occur at the bottom of the canyon. The layers of the weak internal reflectors does'nt seem to be related to the deposits of the
eastern slopes. At the opening (at -94 m) a depositional oblique-tangent structure eroded at the top is visible; offshore the deposition conti-
nues with a slight angular discordance and with prograding oblique-sigmoidal reflectors, in which it has been recognizable a standstill and
a following restarting of the progradational event.
The edge is placed at -106 m and the end, not very clear, seems to be at -178 m.

Fig. 7 - The shelf is composed by a substrate cut by a rough erosional surface on which prograding deposits lay with a clear morpho-
logical break.

Also in this profile, like in the previous one, three depositional phase can be recognized, marked by light flexures on the sea-floor. The
terrace is characterized by transparent acoustic units. It's evident the erosive action of the currents at the base of the slope which were acti-
ve also during the first prograding phase and have created a low morphology (transfered westward during the time). These currents probably
interested the frontal scarp of the depositional terrace, which (unlike the cases shown in the former figures) seem to have greater gradients
than the inner reflectors.

The opening is at -106 m, the edge is at -108 m and the close at -144 m.

Fig. 8 - The prograding structure lays laterally on a terraced slope. The acoustic units are very transparent and it's difficult to reco-
gnize the internal structure. Like in 5 b) at the opening (-104 m) the surface is eroded while towards the edge the progradation is a little
more clear. Unfortunately a temporary stopping of the acoustic signal acquisition has prevented to point out the base of the depositional
terrace; in this record the edge is the only recognizable element, placed at -114 m, while the end of the depositional body (at about -160
m) is partially buried by a light morphological height similar to that noticed in Fig. 5 b).

In the bottom of the channel the same low erosive morphology is observed.

Fig. 9 - The shelf break is marked and very steep; a relict depositional structure lays at the base of the scarp. In this case the ero-
sion seems to have interested deeply the body of the depositional terrace till to leve only the basal parts. Only a thin portion (50 m of thick-