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

Abstract

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 presentday 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. Figure 5. Corresponding section of the lowest shelly layers. (A) and (A′) Close-up of the outcrops (Station 5). (B) Overlying sandy deposit.Figure 6. Sands and upper shelly layer, corresponding to Station 3. (A) Image of the outcrop. (B) Section at Station 3. (C) Close-up of the shelly level. (D) Grain-size distribution of the sandy sediments in this stretch.

New sedimentary and geomorphic evidence of tsunami flooding related to an older events along the Tangier-Asilah coastal plain, Morocco

“…Erosion of the substratum by the tsunami flood waters is attested by scour-and-fill features and rip-up clasts (of soil for example). Rip-up clasts of clay or soil (from the substratum or mud lines) grade into stretched rip-ups, discontinuous laminae (Gutiérrez-Mas et al 2009;Pozo et al 2010). The abundance of rip-up clasts of organic matter decreases upwards.…”

Boulder deposition during major tsunami events

“…Here, abundant boulders of different shapes and sizes form a roughly triangular-shaped deposit containing both fluviatile and marine-derived debris, some of the latter exhibiting evidence of bioerosion and containing endolithic bivalve shells and extending c. 250 m inland. Corrochano et al (2000); 2, Andrade et al (2003); 3, Scheffers and Kelletat (2005); 4, Abrantes et al (2008), Abrantes et al (2005); 5, Andrade et al (1997), Kortekaas (2002), Kortekaas and Dawson (2007); 6, Dawson et al (1995), Hindson and Andrade (1999), Hindson et al (1996);7, Costa et al (2009);8, Schneider et al (2010); 9, Andrade (1992); 10, Morales et al (2008);11, Ruiz et al (2005); 12, Luque et al (2001), Gracia et al (2006), Gutiérrez-Mas et al (2009);13, Whelan and Kelletat (2005); 14, Reicherter et al (2010). This figure is available in colour online at wileyonlinelibrary.com/journal/espl…”

El registro de eventos energéticos marinos en la bahía de Cádiz durante épocas históricas

“…Los registros sedimentarios obtenidos recientemente en la zona sumergida próxima al noroeste de Cádiz y en el extremo sur del cordón H 3 (Gutiérrez-Mas et al, 2009ay b, Gutiérrez-Mas, 2011, han permitido identificar diferentes depósitos de alta energía datados entre 1200 y 700 BP. Su posición y distribución por el cordón (Figura 2) permiten afirmar que, con anterioridad a esta fecha, el actual río San Pedro nunca estuvo emplazado al oeste de H 3 , ya que de lo contrario estos depósitos habrían sido capturados por el canal mareal, como sucediera posteriormente en el caso del tsunami de 1755.…”

“…La migración del ramal meridional del río Guadalete hacia el sur permitiría, posteriormente, el desarrollo de las primeras unidades de H 4 , erosionando los restos de los antiguos cordones litorales romano-medievales de H 3 , ahora en la margen izquierda del río, conforme el canal migraba (Figura 4G). El sondeo S-2 (Figura 2), ubicado sobre el sector sur de la misma unidad progradante H 3 , contiene niveles de alta energía datados en 745 años BP, cuya génesis puede ponerse en relación con otro tsunami acaecido en el año 1033 según el catálogo sísmico, al que se deben asociar igualmente los depósitos estudiados por otros autores (Gutiérrez-Mas et al, 2009a), tanto en ambiente subaéreo, en este mismo sector de H 3 , como sumergido (Figura 4G).…”