THE SEDIMENTATION ALONG THE ROMAN COAST BETWEEN MIDDLE AND UPPER PLEISTOCENE: THE INTERPLAY OF EUSTATISM, TECTONICS AND VOLCANISM – NEW DATA AND REVIEW

Abstract

The Quaternary stratigraphy of the Roman coastal area is one of the most studied for the excellent exposures of the sedimentary succession, which offer the opportunity to analyse the interplay between eustatism, tectonics and volcanism. We present an updated interpretation based on new stratigraphic and geomorphologic data. Sedimentary successions have been organised as Unconformity-bounded Stratigraphic Units, based on the hierarchy of the bounding unconformities. Major regional unconformities are related to eustatic sea level changes and regional uplift, whereas tectonic uplift and subsidence produced high relief but localised unconformities. Major regional unconformities are related to eustatic sea level changes and regional uplift, whereas tectonic uplift and subsidence produced high relief but localised unconformities. The sedimentary record is best preserved during high stands of the sea level, as constrained by the several available age determinations of volcanic deposits. The transition from marine to continental environments occurred between approximately 850 and 700 ka, when the Roman area hosted the deltaic sedimentation from a paleo-Tiber river (Ponte Galeria Synthem). The uplift of the NW-trending Mt. Mario rise, isolated the deltaic sedimentary wedge and forced the paleo-Tiber toward the SE, inside a subsiding valley wherein a thick succession of fluvial conglomerates was deposited. As a consequence of the river diversion, a large lake or swamp probably developed in the area, bearing an influence upon the early phreatoplinian activity of the Colli
Albani volcano which started at about 600 ka (Santa Cecilia Synthem; ca. 700-550 ka). The growth of the volcano progressively shifted
the river back northward, across the Monte Mario rise (after ca. 550 ka) approximately where the present day river has its course. The
Valle Giulia Synthem (ca. 550-450 ka) is made up of clastic and volcaniclastic fluvial deposits, as well as travertine related to both active tectonics (up to 20 m of local uplift) and hydrothermal activity. The subsequent Torrino Synthem (ca. 450-350 ka) is mostly made up
of large volume ignimbrites and lavas erupted both from the Colli Albani volcano to the S and the Sabatini volcanoes to the N. Regional
and local tectonics was relatively quiescent. This synthem is cut by the erosional unconformity formed during the low stand of the sea
level relative to the oxygen isotopic stage 10. The overlying succession has been named Quartaccio Synthem, characterised at the
base by the Villa Senni eruption unit (ca. 350 ka), a complex ignimbrite succession erupted from the Colli Albani volcano, which caused
the last and largest collapse of its caldera. The following rise of the sea level produced an abrasion surface at the top of the ignimbrite
overlain by dune sand (Nuova California subsynthem, i.e. Duna Rossa antica auct.). These deposits are cut by NE-trending faults
between Pomezia and Castelporziano and uplifted by 140+/-5 m during ca. 70 ka, at the rate of 2 mm/y. The uplift produced a local
high relief unconformity wherein the Aurelia and the Vitinia subsynthems were deposited until ca. 280 ka. The top of the Quartaccio
Synthem is presently at elevations comprised between 40 m and 60 m a.s.l. Successively, along with the waning of the volcanic activity,
regional uplift resumed at a rate of approximately 0.2 mm/y. This allowed the preservation of terraced deposits, along the coastal belt
both to the N and S of the Tiber delta and along the Tiber Valley, referred to two different isotopic stages. Those related to the oxygen
isotopic stage 7 (Campo Selva Synthem), are today preserved between 30 and 40 m a.s.l. whereas those related to the oxygen isotopic
stage 5 (Saccopastore Synthem), occur between 15 and 30 m a.s.l. The computed regional uplift rate is well in agreement with known
data for the Tyrrhenian terrace along the Italian coast.