Microtextural signatures in quartz grains and foraminifera from tsunami deposits of the Portuguese shelf

Abstract

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 description of cores, microtextural analysis of quartz grains and foraminiferal taphonomy. The lithostratigraphy, corresponding to late Holocene sedimentation, with intercalations of medium sand-rich in bioclastic fragments with erosive basal contact. In terms of microtextures, a high degree of mechanical marks on the grains associated with tsunami deposition was observed and reflects the high-energy hydrodynamic processes. In compositional terms the higher presence of quartz grains in these units favours the increase of mechanical marks, because grain-to-grain contact is more intense. Additionally, the geomorphological setting of the coring sites determined the degree and type of mechanical microtextures observed. Furthermore, post-depositional changes and characteristics of the original sediment source contribute to explain the occurrence of dissolution in 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 despite the still strong presence of dissolution. 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.