Assinaturas microtexturais em grãos de quartzo e foraminíferos em depósitos de tsunami da plataforma portuguesa

Abstract

This study presents results from two sediment cores collected on the southern Portuguese shelf attempting to, partially, fill the knowledge gap of the offshore record of high-energy events. The results were obtained based on lithostratigraphy, microtextural analysis of quartz grains and foraminiferal taphonomy. The lithostratigraphy, corresponding to late Holocene sedimentation, consist of silt to very fine sand, with intercalations of sand-rich in bioclastic fragments. Within this sequence a unit has been associated with backwash sedimentation based on a group of sedimentological criteria (grain-size, erosive basal contact, composition, etc.). In terms of microtextures, a high degree and presence of mechanical marks on the grains associated with tsunami deposition was observed in samples from this study and reflects the high-energy hydrodynamic processes. In compositional terms it was observed a higher presence of quartz grains in these units, when compared to over and underlying layers. This, also favours the increase of mechanical marks, because grain-to-grain contact is more intense compared to the impact with bioclasts. Additionally, the bathymetric and geomorphological setting of the coring sites determined the degree and type of mechanical microtextures observed. This is due to the presence of the Portimão canyon. The backwash flow along the Portimão canyon favours more violent collisions, as it was observed on core GeoB23512-02, located contiguous to this geomorphic structure. Furthermore, post-depositional changes and characteristics of the original sediment source contribute to explain the occurrence of dissolution in silt-rich units of GeoB23513-02. The foraminiferal taphonomy displayed a predominance of dissolution alteration in the tests surfaces that was more evident in the silty layers. On the other hand, similarly to quartz grains microtextural signature, the sandy high-energy units exhibit a slight predominance of physical processes (abrasion) despite the still strong presence of dissolution derived either from the original sedimentary source features or related with post-depositional processes. Furthermore, the sole presence of species (middle to outer shelf) in some units is an indication that there was little reworking of these specimens. Finally, the results obtained in this study show potential to recognize the microtextural signature of Holocene tsunami events in offshore environments.