Abstract
The high coastal cliffs of Sidi Moussa reveal a complex, reliably dated coastal series of Late Pleistocene age. The lagoon, beach and dunes sequence shows repeated seismically induced deformation. These seismites range from the classic hydroplastic load-casts and convolute laminations to a new type of seismite, which corresponds to an oblique fracturation with respect to the horizontal substratum, parallel to the leeside bedding (avalanche foresets) of the overlying dune sequence. Three major earthquakes are thus identified in two high sea-level deposits (lagoonal and paleocliff foot beach), both related to Eemian sea-level peaks (Marine Isotope Stages, MIS 5.53; 5.51), associated with shoreline dune complexes (MIS 5.52; 5.54). The most destructive historical earthquake to affect this Moroccan coast is the so-called Lisbon, 1755 megaseism , the 350 km eastward Atlantic origin of which must be excluded because stratigraphic analysis locally demonstrates repeated co-seismic subsidence. Rather, the tectonic history of the Gharb foreland basin (basement faults reactivation) accounts for the location of major seismicity changing from the Neogene to Pleistocene to the recent times activity.
This example of paleo-earthquake reconstruction demonstrates the need for precise chronology and environmental interpretation of the host sedimentary sequence. The discovery of a new structure-type also exemplifies the unbounded diversity of syn-diagenetic seismites, including those found in early lithified eolianites.
Geologic and Tectonic History of the Gulf of California