Lowstand terraces on Tyrrhenian Sea steep continental slopes

Abstract

Wave-built terraces formed during sea-level Stillstands at depths lower than present are found along margins with fairly steep morphology and no- to ill-developed continental shelf, for instance the margin of volcanic islands. Terraces deposits are up to 20–30 m thick and are flat-topped with well-defined edges. They extend up to 1 km in dip section. The deposits are composed of intrabasinal sediments, mainly bioclastic sand and silt and are probably due to deposition below the storm wave-base during large-scale energy episodes. Two areas of the Tyrrhenian Sea have been analysed, one in a volcanic context (the Pontine archipelago) and the other in a highly active structural context (the Calabrian Arch). Similar wave-built terraces were found in both areas, with a main terrace placed at a depth of 100–150 m, which is thought to be Würmian. Older terraces are also present, emplaced during the preceding lowstands in the Late Pleistocene. A possible present-day terrace is found at about 20 m depth in the Pontine archipelago. A mechanism of subsequent formation below wave base-level and removal because of sea level lowering has been hypothesised during long-lasting periods of Late Pleistocene sea level falls. Terraces are thus thought to preserve only at the minimum lowstand position and to remain relict on the slope during sea level rise and highstand. The analysis of the distribution and depths of these terraces allows some neotectonic interpretation. For example, in the volcanic archipelago, whose western sector has probably risen at an average rate of 2.5 mm/yr over the last 20,000 years.