Evidence of high-energy events in shelly layers interbedded in coastal Holocene sands in Cadiz Bay (south-west Spain)

摘要

In some present-day coastal areas, recent inactive deposits now outside the reach of sea agents can be observed. These deposits, although formed under different climatic and sea level conditions, often show similar facies to current littoral deposits. They are frequently interpreted as old dunes and beach ridges, or as abandoned spit bars, representative of previous dynamic stages. Nevertheless, in coastal areas which have been subjected to highly-energetic events similar deposits can also be found. When a dynamic event acts on several adjacent environments, the transported and re-deposited sediments can create homogeneous deposits with similar facies, that are easily confused. In this work, Shelly layers interbedded in relict littoral sands located in the La Algaida pinewood, on the edge of the Rio San Pedro tidal channel, in Cadiz Bay (south-west Spain) have been studied. The main constituents of the Shelly beds are Glycimeris valves, organisms which are no longer found as live specimens in the present-day sea bed of Cadiz Bay. From their appearance, the origin of these Shelly beds could be related to sea washovers generated by tidal or storm action, but their disposition and height over the present sea level implies that even higher energy agents were involved in their formation, such as major storms or tsunami waves. The most significant process was the mobilization of the sub-tidal and littoral sediments and their dispersal and re-sedimentation, both lengthways and widthways of the coast, giving way to homogeneous sandy deposits in all littoral environments some of which are now outside the reach of current sea agents. The exception is the present-day shore of the San Pedro tidal channel, where the sediments are being reworked by tidal and small wave action. If the proximity of the study zone to the limit of the African and Iberian plates is considered, where several historical earthquakes and tsunamis have taken place, it is possible to think that these deposits could be a consequence of sporadic and successive washovers, generated by tsunamis occurring between AD 800 and AD 1200. Sedimentological and historical data indicate an increase in seismic and tsunami activity during this period of time, while the Shelly layers would be the consequence of the most intense pulses occurring during these high-energy events.