Age of submarine debris avalanches and tephrostratigraphy offshore Ischia Island, Tyrrhenian Sea, Italy

We present an "event stratigraphy" framework built for the last 23 cal ka marine record in the southern offshore of Ischia Island based on AMS C-14 dating and tephrostratigraphic analysis of 11 gravity cores. Two collapse events have been recovered in the record: a) the Ischia submarine debris avalanche/debris flow (DA/DF), dated between similar to 3 ka B.P. and 2.4 ka B.P. and possibly between 2.7 ka B.P. and 2.4 ka B.P. (event DF1); b) a former, pre-Holocene, DA/DF older than 23 cal ka B.P. (event DF2). The Ischia DA, with an estimated volume of 1.5 km(3), incorporates thousands of blocks that are still detectable on the sea-floor until 45-50 km far from the island. Our results indicate an age of emplacement younger than previously thought and support the hypothesis that a major catastrophic event occurred when the island was already inhabited by Greek settlers (i.e. after the 7th century BC). Three ash layers have been recognised in the post-DF1 avalanche sequence and correlated with Ischian eruptions occurred between Middle Ages and Roman times. Two tephras recovered in the pre-DF1 succession have been correlated with explosive activity occurred on Ischia and Procida islands from similar to 23 ka to similar to 17.5 ka B.P. The results presented here improve the chronostratigraphic reconstruction of the main eruptive and collapse events that affected Ischia volcano during Late Pleistocene-Holocene and their dispersal at sea The occurrence of at least two major collapsing events in the past 23 kyr confirms the close genetic relationship between gravity failures and Mt. Epomeo uplift. (C) 2010 Elsevier B.V. All rights reserved.