SEDIMENTARY EVIDENCE OF PALAEO-TSUNAMI DEPOSITS ALONG THE LOUKKOS ESTUARY (MOROCCAN ATLANTIC COAST)

Abstract

Analysis of the CARLA-11 core drilled along the lower Loukkos valley near Larache in northern Morocco shows a thin level of shelly sand at 465 to 482 cm depth, whose sedimentological features are those of a high-energy, certainly a tsunami deposit. The level can be subdivided into 3 subunits: Subunit 1 (6 cm) shows a sharp erosive base and comprises basal medium to coarse sands containing numerous marine shell fragments of bivalves, plant fragments and rip-up clasts of organic matter. Subunit 2 (7 cm) is a flame structure consisting of coarse sand containing a layer of organic matter and another one of greyish clay. Subunit 3 (4 cm) is similar to subunit 1 and consists of coarse sands containing numerous complete or broken shells of bivalves, plant fragments and dark organic matter. The deposit is mostly composed of subangular to subrounded quartz grains derived from nearby Miocene sandstones. Benthic and planctonic foraminifera are common within the samples. Magnetic susceptibility measurements show two major lows at ~350 cm, and especially at 477 cm within the high-energy deposit. Subunit 1 can be interpreted as the result of the first wave uprush of a tsunami, the fine mud level of subunit 2 capping subunit 1 can be interpreted as emplaced during a decantation phase, and subunit 2 probably corresponds to a second wave uprush, Subunit 3 might be interpreted as the result of the backwash (outflow phase). The age of this event can be roughly dated between 5 and 3 ky BP according to recent dating of nearby levels.

Keywords

• Loukkos
• Gulf of Cadiz
• sedimentology
• high-energy events