Page 3 - Colantoni_alii_1993

Sea floor lithology skeletal material. Another source of biogenic
The many samples collected by grabs and,
whenever possible by subaqua diving, have
shown that the coastal wedge is composed of
course terrigenous sediments, while deposits on
the shelf are mainly composed of biogenic
debris (algal remains, Molluscs, Bryozoan and
Serpulids debris, Foraminifera). Bio-calcarenites
and bio-calcirudites, the cover large parts of the
Adventure Bank and Sicilian shelf derived from
the same lithified materials. They appear to have
been eroded and locally reddened in a subaerial
environment and therefore must have been
formed at least before the last glacial phase
(Upper Pleistocene). Tentatively, these rocks can
also be related to the “Calcareniti di Marsala”
(Lower Pleistocene), which outcrops along on
the Sicilian coast and the islands.
Many older lithological types crop out
through the cover of the loose or lithified
biogenic deposits and form shallow banks and
shoals that have been reached and sampled by
diving.
In the Egadi area, rocks other than
calcarenites have been sampled:
- Triassic-Jurassic dolomites: NW of Favignana.
- Cretaceous calcilutites (Scaglia): scoglio
Porcelli.
On the Adventure Bank (Colantoni et al.,
1985) older rocks are:
- Triassic-Jurassic dolomites: Talbot Bank.
- Oligocene-Miocene quartz-arenites: Ante
Talbot Bank.
- Upper Miocene calcarenites: Pantelleria
Vecchia Bank, Nereo Bank.
- Alkaline basalts: Anfitrite Bank.
- Tholeitic basalts: Tetide Bank.
The sediments
Forty-three grab samples has been
analyzed fro grain size. Nine textural types have
been recognized (Tab. 1 and Fig. 3).
Sediments covering the shelf are mainly
poorly sorted coarse and medium sands (0-2
phi), while bi-modal sandy and silty clays
dominate in deeper water. This is consistent with
their biogenic origin. Production of biogenic
sediment is centered in the extensive meadows
of Posidonia oceanica which is inhabited by
many organisms with calcareous skeletons or
shells. Dominant epibionts are Bryozoan,
encrusting algae, Serpulides, Foraminifera,
Gastropods and corals, which produce abundant
95
debris are the “Coralligenous” formations which
are widely encountered in the circa-littoral stage
(below 30-40 m). These beds are dominated by
Algae and Corals.
In our samples, the biogenic content of
sands is always more than 90%. Fragments of
Molluscs and Bryozoan are more frequently
found in the coarser fractions, while the finer
fractions are rather composed of Foraminifera.
Strong currents rework and move this bio-
clastic material and much of it is transported off
the shelf into deeper water. Only a thin carpet of
sediments normally rests on the shelf. On the
Adventure Bank, which is swept by steady east-
flowing Atlantic water and is subject to W-NW
storms, some sandy ribbons, dune-type
formations and deposits, accumulated against
the obstacles of rocky banks, occur (Fig. 2).
Concluding remarks
The general setting and shelf margins of
west and southwest Sicily are defined by
tectonic elements within the substratum. The
main NNW-SSE and E-W Plio-Quaternary
tectonic (faults) lineations are clearly shown by
the bathymetry and morphology.
The tectonized substratum is deeply
eroded. Rocky outcrops in several places form
islands, shallow banks and shoals where rocks
from Triassic to Pleistocene have been sampled.
The sedimentary cover is extremely thin.
Progradational deposits form only in very
limited sectors, while sediment carried out away
from the shelf feeds numerous submarine valley
and canyon distribution systems.
On the Adventure Bank, localized
sedimentary structures indicate the presence of
strong current activity.
Sediments are largely of biogenic origin.
Outcropping older bio-calcarenites show that the
same type of sediment generation extends back
to early Pleistocene times without significant
variation.
In the Plio-Quaternary subsiding basins,
depositional sequences are preserved, while on
the stable or uplifted shelf, erosional phases
prevail. In absence of significant subsidence,
eustatic changes have not allowed much
sediment accommodation. High-stand deposits
have been erode during low-stand phases,
leaving the shelf almost bare of sediments.