Reicherter und Becker-Heidmann 2008b

Reicherter, K., Becker-Heidmann, P. (2008): Tsunamites in lagunas: remains of the 1522 Almería earthquake (western Mediterranean). – In: Mastronuzzi, G., Brückner, H., Sansò, P., Vött, A. (Hg.): Second International Tsunami Field-Symposium, Ostuny (Italy) – Ionian Islands (Greece), S. 127–129. GI²S Coast Research Publication 6, Bari, Italy

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Abstract:

Shallow drilling in the laguna and saline of the Cabo de Gata area proved sedimentary evidence for paleo-tsunamis along the Spanish Mediterranean coast. Several coarse grained intervals with finingup and thinning-up sequences, rip-off clasts, broken shells of lamellibranchs and planktic foraminifera show erosive bases are interpreted as tsunamites. The coarse-grained intervals show up to three sequences divided from each other by a silty layer, as so-called mud drape. These intervals are interpreted as a tsunami train and correspond to three individual waves.
Radiocarbon dating of sediment and shells reveal evidence that these layers can be ascribed to deposition during the 1522 Almería earthquake. The 1522 Almería earthquake (M > 6.5) affected large areas in the western Mediterranean and caused more than 1000 causalities. The epicentral area was probably offshore in the Gulf of Almería (southern Spain) along a 50 km long sinistral strikeslip fault, the Carboneras Fault Zone (CFZ). The potential for tsunami generation along a strike-slip fault is generally small, therefore we assume that seismic shaking triggered submarine slides in the Gulf of Almería and the Cabo de Gata spur, which then caused tsunami waves and due to the geometry of the Gulf also reflections. We focused our investigations on the Salinas of the Cabo de Gata. The laguna is situated at the foot of the San Miguel volcanic hills. Salt production facilities occupy 500 ha, 300 of which are flooded, favoring the entrance of the sea by gravity. The laguna has a depositional history of about 6000 years BP (Reicherter and Becker-Heidmann, 2008), the sediment logs allowed us to distinguish three periods of the evolution of the laguna. The initial stage is an alluvial fan phase, which commenced already during Pleistocene times, followed by an intermediate beach phase from approximately 5000 yr BP to 3000 yr BP, from than on a marsh laguna developed. The early period is characterized by alluvial fan deposits (Harvey et al., 1999), which consist of reddish unsorted coarsegrained gravels, sands and intercalated paleosols. Sedimentary source is the volcanic Cabo de Gata range, hence, clastic material is exclusively made up of volcanic rocks and some Neogene carbonates. The alluvial fan sequences are cyclic and show fining-up cycles, mostly terminated by a soil development. Upsection, sands with intercalated clays are developed, which point to open marine, and hence beach-like conditions between 5000 to 3000 yr BP. After approximately 3000 years BP a hyper-saline environment of typical for the laguna-stage several a developed with organic-rich clayey and evaporitic layers (i.e. gypsum). In these well-stratified cyclic deposits laguna sandy and coarse-grained layers are intercalated, partly well sorted sand layers are interpreted to be aeolian dunes. Several other coarse-grained intervals with fining-up and thinning-up sequences contain rip-off clasts, shells of lamellibranchs and 14 clasts, shells of lamellibranchs and foraminifera. AMS 14C- dating of the layer directly below provided an age of 680±30 yr BP, taken into account an erosive base and, hence, some missing (eroded) deposits, we interpret this layer being deposited during a tsunami event accompanying the 1522 earthquake of Almería. We have also found another intercalation of tsunamites downhole (850±35 yr BP, 92-121 cm depth), which are interpreted as either an expression of repeated earthquake activity or tsunami-like waves induced by submarine slides triggered by seismic shaking in the Gulf of Almería. Our evidence suggests a definite tsunami potential and hazard for offshore active and seismogenic faults in the western Mediterranean region.

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