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Résumé/Sommaire :

Background : Despite a position along the passive margin of Africa, the Moroccan Atlantic coast is under the influence of the tsunami threat from earthquakes triggered along the Nubia-Eurasia plate boundary. Along Tangier, Asilah, Sale and Mazagao’s coasts, tsunami have been described since historic times. The 1755 Lisbon quake triggered a tsunami that struck the shores of Morocco 60 min after the tremor, the waves reaching from 2 to 15 m in amplitude. The coastal sedimentary record, together with other proxies, is now being experienced to discriminate and reconstruct evidence of prehistoric inundation events.
Results : Recently, we investigated the southwestern coast of Tangier (Tahaddart estuary) and found out morphologies and sedimentary deposits evidencing a strong energy-flooding event. A combination of sedimentological (grain size, sorting, AMS) and microplaentological analyses combine to provide a primary reconstruction of uprush and backwash phases of tsunami deposition and flow dynamics. This event that we attribute to the 1755 tsunami, left noticeable traces on a hilly dissected topography. In the proximal domain, no deposits were emplaced. The high turbulence of the flow allows a complete erosion of the soil. Landward, the decreasing of the energy induced a strong depositional process. The thick brownish sandy layer deposited contains at the base large angular rip-up clasts of dark soil. Marine origin of the deposits is attested by bioclasts and the presence of benthic and planctonic foraminifera. In the distal domain, the sudden loss of energy, due i) to a reverse slope, and to ii) a flow diffluence resulting from the presence of a pass between two dunes, resulted in an accumulation of a huge amount of unsorted marine bioclasts, rounded pebbles, sub-angular beach rock clasts, remains of microlithic industry, and pieces of pottery displaying blunt breaks. Reaching the proximal dune, beyond an undulating line, still perceptible in the landscape, the flow energy was insufficient to erode the soil, which remained unaffected. Application of the AMS technique to the sediments confirmed that the lower part of the deposits was emplaced by a landward flow spreading N91°-171° (uprush phase to the SE) while the upper part was oriented seaward N280°-325° with a topographical control (backwash phase to the NW). At the very base of the deposit, the first AMS measure gives an uprush direction of N105°.

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