Sedimentological record of tsunamis on shallow-shelf areas: The case of the 1969 AD and 1755 AD tsunamis on the Portuguese Shelf off Lisbon

One of the aims of the SEDPORT project (ESF Euromargins EUROCORES program) was to investigate the depositional features on the shelf and upper slope of the Portuguese Margin — Tagus System, during the Holocene. To fulfill the objectives, two shallow box-cores and two sedimentary sequences were studied: PO287-26B and the spliced sequence of sites D13902, PO287-26G recovered on the Portuguese Continental Shelf SSW off the Tagus River mouth, and the box PO287-28B and gravity-core GeoB8903 collected towards the W of the river mouth.Magnetic susceptibility, grain-size and XRF-Fe data as well as 210Pb and AMS 14C dating of these sedimentary sequences have allowed the identification of an “instantaneous deposit” at about 20 cm in the box-cores. Downcore we could establish: a hiatus corresponding to 355 yr of sedimentation at both sites; a 39 cm “instantaneous deposit” of coarse material including carbonate broken shells on the SSW site; and a 1.5 m “instantaneous deposit” of fine material on the W site. Both hiatus and instantaneous deposits are considered to be of tsunamigenic origin. The box-cores' deposit has an estimated age of 1969 AD and it is attributed to the 1969 earthquake caused tsunami. The hiatus and deposits found downcore are, due to the age limits encountered, believed to represent the tsunami related to the 1755 Lisbon major earthquake. These data further indicate a sediment deposit similar in type and thickness on both SSW and W sites in 1969, while the 1755 AD record indicates a higher energy backwash along the SSW and a preferential deposition of fine suspended matter towards the W.