Evidence for active retreat of a coastal cliff between 3.5 and 12 ka in Cassis (South East France)

This study on the Cap Canaille cliff (N 43.19 degrees, E 5.55 degrees Cassis, SE France) combines cosmic ray exposure (CRE) dating using two cosmogenic nuclides (in situ-produced Be-10 and Cl-36) and morphological analyses to gain a better understanding of a major coastal cliff collapse event. Morphological analysis reveals evidence (cliff morphology, presence of big collapsed blocks) of a possible major collapse of Cap Canaille in the past. Aerial pictures and GIS software allow estimation of a potential collapsed volume of at least 7 x 10(6) m(3), of which roughly 6 x 10(6) m(3) fell into the Mediterranean Sea. In situ-produced Be-10 and Cl-36 concentrations were measured in samples collected on collapsed block surfaces and in situ-produced Be-10 was measured along the cliff face to date the last major collapse event. Statistical analysis of the CRE ages calculated from cliff samples shows that these ages cluster around 3.5 and 6.7 ka, suggesting the existence of a two-step past major collapse. The older ages obtained (at 9 and 12 ka) coincides with the approach of present day sea level, implying a control of sea level on the cliff retreat. The CRE ages calculated from collapsed block samples are more scattered (toward younger ages) due to several geomorphological factors. The estimated collapsed volume associated with the last major collapse around 3.5 ka seems sufficient to have triggered a local tsunami in the Cassis Bay, if it fell at once into the sea. (C) 2009 Elsevier B.V. All rights reserved.