A Glycymeris-rich unit as evidence of a late Pleistocene tsunami event from NW Algeria: A biostratigraphic, taphonomic, and sedimentological approach

Compared to the extensive research carried out on the Neogene deposits of the Lower Chelif Basin, the Pleistocene series is still poorly studied, with no detailed lithological succession published to date. This study focuses on the Glycymeris-rich Unit (GRU) along the coastal area of the Hachacha Plateau in Northwestern Algeria. This unit unconformably overlies Miocene, Pliocene, and Pleistocene basements. The latter was identified for the first time in this work using a biostratigraphic approach based on calcareous nannofossils and planktonic foraminifera. The GRU is interpreted as a tsunami-related deposit, formed in a coastal environment (foreshore/backshore) during the upper Pleistocene, corresponding to the Last Interglacial period, i.e., Marine Isotopic Substage 5e (MIS 5e). This interpretation provides a first multidisciplinary description of a tsunami deposit in Algeria that is supported by distinctive biotic, taphonomic, and sedimentological features. The deposits contain a mixture of marine organisms from different ecological zones (supralittoral to shallow circalittoral biocenoses), including molluscan assemblages such as the so-called Senegalese fauna (bivalves and gastropods), sponges, serpulids, coralline algae and corals. Occasionally, rare terrestrial snails are also found mixed with the marine fauna. Taphonomic analysis reveals low percentages of boring, absence of encrustation, and excellent shell preservation, suggesting that powerful waves eroded sediment masses and transported them inland from deeper areas beneath the taphonomic active zone. The predominance of sharp-edged fragmented shells, chaotic arrangements with oblique to vertical shell orientations and the good shell sorting, indicates transport by mass flows and rapid deposition during an extreme event – a tsunami –, distinguishing these deposits from those associated with gradual and oscillatory flows, such as storm events. Sedimentological characteristics, including irregular erosive base, lateral facies variations, wide grain size ranges (clay to boulders), normal and inverse grading, and diagnostic structures (both fragile and hard-rock rip-up clasts, high-energy flow features such as horizontal and oblique laminations, and hummocky cross-stratification, injection of sediment into the substrate, imbrication of large angular boulders and soft sediment deformation structures), combined with the active tectonic context of Northwestern Algeria support the interpretation as a seismically triggered tsunami and enhances the understanding of this type of deposits in similar coastal settings.