Coastal boulders in Martigues, French Mediterranean: Evidence for extreme storm waves during the Little Ice Age

Boulder accumulations occur along a stretch of rocky coast of about 1.5 km near the French Mediterranean city of Martigues. The boulders occur up to 100 m inland from the present shoreline and some contain marine bio-constructions that are proof of residence in a subtidal or intertidal setting. The setting, spatial distribution and morphologic characteristics of these boulders indicate that they were detached from the rocky shore platform and transported landward by high-energy waves. The size, position and distance from the shoreline of 1475 boulders were measured in order to determine their volume and mass, as well as the conditions under which they were transported landward to their present positions. The results were then statistically analyzed and confronted with hydrodynamic models commonly used to evaluate the charactestics of the transporting waves. The wave characteristics thus obtained were compared to recorded and modeled extreme waves in the region. Dating of the boulders shows age ranges that correspond to the Little Ice Age (LIA), thus suggesting a relationship between their deposition and the high storm frequency that characterized the LIA. The results also indicate little likelihood of a tsunami origin for these boulders, although there is historical evidence of tsunamis in this region. The study insists on the potential for stormrelated hazards on this heavily populated and industrialized part of the Mediterranean coast of France.