AD 1755 Lisbon tsunami deposits – geophysical, sedimentological and organic geochemical analysis (Conil de la Frontera, Spain)

On the 1^st November AD 1755, the tsunami, triggered by the 8.5 to 9 M_W 1755 Lisbon earthquake, caused major inundations with sediment transport along the coastline of the Gulf of Cadiz. The study area, Conil de la Frontera (El Palmar de Vejer), located at the Gulf of Cadiz in southwestern Spain, was severely stuck by the AD 1755 Lisbon tsunami. Witness of the destruction and power of the tsunami inundation are the walls of Torre de Castilnovo, close to the study area, which got heavily destroyed. El Palmar de Vejer was chosen as a study area due to the topographical setting, characterized by the flat alluvial flood plain. With these peculiarities, the area presents good preconditions as a sedimentological archive for potential deposits of the AD 1755 tsunami.

First, geophysical methods were used to identify potential sandy layers attributed to the AD 1755 tsunami. Ground-penetrating radar (270 MHz antenna) was used to systematically scan the ground to a depth of ca. 3 m. The evaluation of these radargrams were taken into account for the selection of GeoSlicer drilling locations. Based on the samples obtained, granulometric analyses were carried out (1) to identify the potential sandy tsunami deposit; (2) to analyze the different sedimentological depositional environments before, during and after the tsunami; (3) to detect tsunami sublayers deriving from different waves within the wave-train of  the AD 1755 Lisbon tsunami, since 3 waves were reported.

Furthermore, both inorganic and organic geochemical investigations were performed on the samples. With the help of inorganic geochemical analysis of major elements (Si, Sr, Ti, Ca, N, S) as well as elemental ratios can identify a distinction between marine and terrestrial depositional environments and accumulate more information about the deposit facies. By the use of organic geochemistry for the analysis of biomarker, several different natural compounds were detected (e.g., n-alkanes, n-aldehydes). Biomarker results suggest a distinct differentiation between the AD 1755 tsunami deposit and the surrounding background sediment layers above and below. The tsunami deposits contrasts to the post and pre-tsunami layers by different concentrations of biomarkers and deviant occurrence of specific compounds. The n-alkanes are manifesting the difference of marine and terrestrial sources of the different layers. Results of this study analyzing the Iberian sedimentary archives at Conil de la Frontera present strong evidence that a multi-proxy approach with the inclusion of geochemical applications can confidently detect tsunami deposits, distinguish them from surrounding background sediments and subsequently characterize the internal structure and composition of the tsunami deposit.