Lithofacies and origin of a deposit of large boulders in the coast of Cadiz (SW, Spain). Evidence of an event of high energy during the Pleistocene.

Resum

A deposit of large boulders, interstratified in Pliocene and Pleistocene sediments has been studied in the coast of Cádiz (SW Spain). The deposit is constituted by conchiferous large boulders, medium and small size clasts and shell fragments. The facies indicate a marine origin of the deposit, and the re-worked character of the sediments and fossils, which were deposited in a high energy marine environment, previously to that they were mobilized and re-deposited by action of a high energy directional flow. The most appropriate depositional mechanism to explain the formation of this deposit, is the action of big waves, generated by a great energy marine event, probably a very strong storm or a tsunami. The tsunami action is probable since the area is located in a seismically active area, near to the limit of the African and Euro-Asian plates, at the same that several earthquakes and tsunamis are historically documented in the Cadiz coast. The event should happen during the early Pleistocene, coincident with a highstand stage. The isotopic analysis by means of the Method 87Sr/86Sr carried out, has established an age interval from 1.35 to 1.1 million years, corresponding to the Calabriense (Lower Pleistocene).