Ustica Island (Tyrrhenian Sea, Italy): from shoaling to emergent stage

Abstract

Ustica is a volcanic island located in the southern Tyrrhenian sea, ~60 km NW of Sicily. As usual for volcanic ocean islands, its exposed part (8.6 km2, 248 m max elevation, mostly of Pleistocene age), is a small fraction of the whole edifice which rises from ~2000 m depth. Its 5-pointed-star shape is slightly elongated in a NE direction. A new geological field survey was carried out at scale 1:10000 and locally at 1:2000, establishing informal stratigraphic units that on the whole fit a common scheme of evolution for volcanic ocean islands. In this framework, the whole pre-existing stratigraphy has been revised. Ustica has a variety of volcanic deposits from submarine (basaltic effusive to explosive) to subaereal (effusive, explosive and highly explosive -Plinian?). Moreover, Ustica is one of the few places in the world where a transition of deposits from shoaling to emergent stage crop out. In fact, its oldest deposits consist of: (a) a flank-facies association of submarine lavas (variably-shaped pillows, pillow breccias and hyaloclastites) with biocalcarenite-biocalcirudite lenses, dipping coastward in the E, S and W outer parts of the island; this association is arranged in steep foreset beds (lava deltas) and is capped by flat-lying transitional to subaereal massive lava flows and surf-shaped boulder conglomerates; the geometry of this association may suggest a progressive island uplift or sea lowering during this period; (b) shallow-water to emergent tuff cone deposits in the NW part of the island. In the centre of the island, subsequent activity built a pile, now deeply eroded, of subaereal basaltic lava flows capped by a scoria cone. A previously unknown outcrop where a pumice fall layer is exposed, allows a distinction into two members of a unit that was known as formed by pyroclastic surges only. Higher in the succession, the Ustica Pumice formation (for which 4 members are defined) is underlain by a palaeosoil, and is likely the remnant of a caldera-forming eruption, which possibly dismantled the northern part of the island. It may be suggested that a later flank collapse affected the W side of the island, exposing seamount-stage deposits and allowing small fissural eruptions. The northern part of the island was then rebuilt by hydro-magmatic littoral deposits, subaereal aa flows, and thick columnar lava flows. At the NE corner of the island, a littoral tuff cone (the only cone of the island which is roughly preserved in its original shape) gave the last volcanic activity in Ustica. Acknowledgement: LBM benefited of a grant from Milan University under the supervision of Prof. G.Pasquaré in the framework of a joint venture with SGN.

Full text

Geophysical Research Abstracts, Vol. 5, 07094, 2003
c
European Geophysical Society 2003
USTICA ISLAND (TYRRHENIAN SEA, ITALY):
FROM SHOALING TO EMERGENT STAGE
L.B. Marinoni (1), G. Pasquaré (2), L. Vezzoli (3)
(1) Milan, Italy, (2) University of Milan-1, (3) University of Insubria; e-mail:
LBM2@interfree.it
Ustica is a volcanic island located in the southern Tyrrhenian sea,˜60 km NW of Sicily.
As usual for volcanic ocean islands, its exposed part (8.6 km2, 248 m max elevation,
mostly of Pleistocene age), is a small fraction of the whole edifice which rises from
˜2000 m depth. Its 5-pointed-star shape is slightly elongated in a NE direction. A new
geological field survey was carried out at scale 1:10000 and locally at 1:2000, estab-
lishing informal stratigraphic units that on the whole fit a common scheme of evolution
for volcanic ocean islands. In this framework, the whole pre-existing stratigraphy has
been revised. Ustica has a variety of volcanic deposits from submarine (basaltic ef-
fusive to explosive) to subaereal (effusive, explosive and highly explosive -Plinian?).
Moreover, Ustica is one of the few places in the world where a transition of deposits
from shoaling to emergent stage crop out. In fact, its oldest deposits consist of: (a) a
flank-facies association of submarine lavas (variably-shaped pillows, pillow breccias
and hyaloclastites) with biocalcarenite-biocalcirudite lenses, dipping coastward in the
E, S and W outer parts of the island; this association is arranged in steep foreset beds
(lava deltas) and is capped by flat-lying transitional to subaereal massive lava flows
and surf-shaped boulder conglomerates; the geometry of this association may suggest
a progressive island uplift or sea lowering during this period; (b) shallow-water to
emergent tuff cone deposits in the NW part of the island. In the centre of the island,
subsequent activity built a pile, now deeply eroded, of subaereal basaltic lava flows
capped by a scoria cone. A previously unknown outcrop where a pumice fall layer is
exposed, allows a distinction into two members of a unit that was known as formed
by pyroclastic surges only. Higher in the succession, the Ustica Pumice formation (for
which 4 members are defined) is underlain by a palaeosoil, and is likely the remnant of
a caldera-forming eruption, which possibly dismantled the northern part of the island.

It may be suggested that a later flank collapse affected the W side of the island, expos-
ing seamount-stage deposits and allowing small fissural eruptions. The northern part
of the island was then rebuilt by hydro-magmatic littoral deposits, subaereal aa flows,
and thick columnar lava flows. At the NE corner of the island, a littoral tuff cone (the
only cone of the island which is roughly preserved in its original shape) gave the last
volcanic activity in Ustica.
Acknowledgement: LBM benefited of a grant from Milan University under the super-
vision of Prof. G.Pasquaré in the framework of a joint venture with SGN.