Hydroacoustic expression of offshore tsunami deposits on the Algarve shelf, Portugal

Continental shelves are often affected by high-density sediment flows triggered by high-energy events such as floods, storms, submarine slope failures or tsunamis. The sedimentary imprints of these events are recorded as erosive or depositional features in the geological record of the shelves. Especially floods, storms and tsunamis can severely affect coastal societies and have an impact on local economies. Research focus on the sedimentary imprints of these events has relied essentially on onshore studies where their impacts are more noticeable and access is facilitated. However, investigations into the offshore domain become more important for palaeotsunami research as, in theory, their Holocene record can be better preserved there compared to shallower areas. The Algarve area, southern Portugal, provides conditions for a proof-of-concept study on offshore tsunami imprints as it was affected by the 1755 CE Lisbon tsunami and, probably, older events.

Here, we present sub-bottom profiles from an investigation into the shallow sedimentary cover of this area. Along with a coring campaign, the profiles were recorded in November 2018, within the scope of RV METEOR cruise M152, aiming to a) analyse the shelf’s Holocene sedimentary record in the most tsunamigenic earthquake-prone region in Atlantic Europe; and b) identify sedimentological features of offshore tsunami deposits. To support the results obtained from cruise M152 and to extend the collected profiles further towards the coast into shallower water depths, an additional hydroacoustic campaign was conducted in January 2020.

According to the profiles, the study area is generally marked by rough erosional seafloor and frequent exposure of bedrock. Sub-bottom profiles show different geomorphological conditions and sediment dynamics between the western and eastern parts of the study area. Already onboard METEOR during cruise M152, a strong reflector was noticed in the sub-bottom profiles of the western study area down to a water depth of 75 m and about 1.20-1.55 m below the seafloor. This reflector correlates with a conspicuous sediment layer in the cores of this transect which we interpret as a tsunami (backwash) deposit based on a multi-proxy approach applying various well-established and innovative analytical techniques. This tsunami deposit is unique in Portugal and dates to ca. 3400 cal. years BP. The January 2020 survey was also able to find the same strong reflector and trace it into shallower water depths. With a detailed analysis of the obtained profiles, we aim to investigate the lateral extent of this tsunami deposit, to better understand offshore tsunami processes, related sediment transport and the prevailing sediment dynamics of the Algarve shelf in general.