Polygenic (tsunami and storm) deposits? A case study from Ushant Island, western France

RIVISTA: Zeitschrift für Geomorphologie

ANNO: 2010

AUTORI: Regnauld H., Oszwald J., Planchon O., Pignatelli C., Piscitelli A., Mastronuzzi G., Audevard A.

KEYWORD: Ushant, Brittany, storm, tsunami, blocks


Ushant Island is the westernmost island off shore western France. It is exposed to severe storms. The western peninsulas of the island are partly covered with boulders fields, the origin of which is an open question. On March 10, 2008 a very severe storm (973 hPa) was able to move some of these blocks and to raft them over several tens of meters. The heaviest block weighs about 62 tons. Some blocks were located in the intertidal zone, others were above the spring tide limit and were detached from the cliff during the storm. Most of the blocks had already existed before; thus, the event which actually caused their detachment was not the March 2008 storm. When applied to this case, the hydro-dynamical equations by Nott would suggest a storm with wave heights of 12 to 50 meters. New equations by the authors reduce these values to about 8 to 32 meters. Anyhow, these heights have never been recorded in Ushant. These results thus lead to the hypothesis that the blocks were not created by a storm but by a tsunami, possibly the one of 1755 (Lisbon Tsunami). The important point is that these blocks have a polygenetic origin: they were detached from their initial location by an event which cannot have been a storm, but subsequently they were moved by each severe storm. Boulder fields on Ushant Island, therefore, seem to be polygenetic coastal high energy event features.