Paleogene mass-transport deposits (MTDs) from submarine landslides in the Julian Basin (Italy-Slovenia): mechanisms of transport, emplacement and tsunamigenic potential

Submarine landslides are significant geological events characterized by the rapid downslope movement of ocean-floor sediments, often triggered by seismic activity, oversteepened slopes, rapid sedimentation, or fluid overpressure. These processes produce substantial Mass Transport Deposits (MTDs) and Mass Transport Complexes (MTCs). Advances in seismic and acoustic imaging have revealed their widespread presence along continental margins, highlighting their environmental and geological importance. Studying MTDs provides insights into the emplacement mechanisms of deep-water sedimentary deposits, flow transformations during mass-wasting events, and triggers of such phenomena. They also serve as valuable stratigraphic markers, aiding in basin morphology delineation and analyses of syn-sedimentary tectonic movements.The Julian Basin, located between E-Italy and W-Slovenia, offers a unique geological setting to investigate ancient MTDs. The basin, characterized by mixed carbonate and terrigenous sedimentation, contains over 25 “megabeds” dating from the Late Paleocene to Early Eocene. These MTDs are embedded within carbonate, siliciclastic, and mixed turbidites and are interpreted as sourced from the Friuli-Adriatic Carbonate Platform, forming the basin's passive margin. The deposits are remarkable in scale, with some extending over 70 km, thicknesses exceeding 250 m, and volumes up to 25 km3.This study focuses on these MTDs as fossil counterparts of massive submarine landslide events. A multidisciplinary approach was employed, integrating large-scale remote sensing, geological mapping, and meso- to microscopic-scale sedimentological, stratigraphic, and structural analyses of rocks sampled directly from the MTDs. One of the outcomes is the development of a correlation scenario of the MTDs in the area between Iudrio and Isonzo rivers, extending towards the Bainsizza plateau. This work integrates geological mapping with remote sensing techniques, including processing of Digital Terrain Models (DTMs) and creation of 3D photogrammetric models of outcrops. These methods allowed correlation of geological features despite challenges posed by dense vegetation and tectonic overprinting.Analysis of 3D photogrammetric outcrop models revealed internal features of the MTDs, reflecting dynamics of transport and deposition. Notably, within Megabed n.3 (MB3), an internal erosive contact surface was identified, indicating resumption of flow within the mass-transport complex itself. This supports the hypothesis that MB3 represents a composite event with multiple sequential mass-transport processes. The calcarenitic portions of the MTDs, known as Pietra Piasentina, are exploited as ornamental stone, with major quarries located within MB3. Identification of internal erosive surfaces within MB3 may have practical implications for geological mapping in the mining sector.Furthermore, the study investigated the tsunamigenicity of the MTDs by focusing on the finer-grained topmost units to identify potential sedimentological and geochemical signatures indicative of tsunami-induced backwash deposition. In the Lastivnica megabed (Anhovo Quarry), fine-grained thin-bedded turbidites distinct from surrounding lithology were found. Elemental analyses using portable XRF and ICP-MS techniques revealed higher contents of siliciclastic mineral phases compared to the host rock, indicating a variation in the sediment source. SEM analyses of heavy minerals confirmed these observations. However, definitive evidence associating these deposits with a tsunamigenic trigger was not found. Despite this, analyses provided insights into the deposition processes within the uppermost unit and its organic carbon content. Thin sections exhibited traction structures within deposits apparently massive, suggesting possible ponding effects. Image analysis of organic carbon content, compared with TOC analyses, showed interesting responses and potential for future research.